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Review

Palladium-Catalyzed Cross-Coupling Reactions of Borylated Alkenes for the Stereoselective Synthesis of Tetrasubstituted Double Bond

by
Tomáš Tobrman
* and
Sergej Mrkobrada
Department of Organic Chemistry, University of Chemistry and Technology, Technická 5, 166 28 Prague, Czech Republic
*
Author to whom correspondence should be addressed.
Organics 2022, 3(3), 210-239; https://doi.org/10.3390/org3030017
Submission received: 15 May 2022 / Revised: 24 June 2022 / Accepted: 4 July 2022 / Published: 20 July 2022
(This article belongs to the Collection Advanced Research Papers in Organics)
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Abstract

:
The stereoselective formation of tetrasubstituted alkenes remains one of the key goals of modern organic synthesis. In addition to other methods, the stereoselective synthesis of tetrasubstituted alkenes can be achieved by means of cross-coupling reactions of electrophilic and nucleophilic alkene templates. The use of electrophilic templates for the stereoselective synthesis of tetrasubstituted alkenes has previously been described. Therefore, the present review summarizes the procedures available for the stereoselective preparation of tetrasubstituted alkenes using stable and isolable nucleophilic templates.

1. Introduction

Alkenes are organic compounds with a double bond between two carbon atoms. Such compounds have undergone tumultuous development in terms of their preparation, reactivity, and applications in the fields of materials and medicinal chemistry. The applications of alkenes in organic synthesis include carbonyl olefin metathesis [1], alkene epoxidation [2], hydrogenation [3], and transition-metal-catalyzed functionalization [4,5]. Those alkenes with four different substituents represent a specific group of alkenes. They are usually referred to as tetrasubstituted alkenes, and they are considered attractive compounds due to their synthesis and application potential. An interesting feature of tetrasubstituted double bonds is the presence of four substituents, which allows for better modification of their properties via the introduction of different substituents. An example of a biologically relevant alkene is tamoxifen (F1–1). Variations of the substituents on the double bond can yield other biologically relevant derivatives of tamoxifen F1–2 and GDC-0810 (F1–3), which have received considerable attention from synthetic [6] and biological [7,8,9] perspectives (Figure 1).
The general procedures available for the preparation of tetrasubstituted alkenes S1–3 can be divided into several groups. The oldest procedures are arguably those involving elimination and olefination reactions, although it must be acknowledged that they are associated with a considerable disadvantage due to their low stereoselectivity in certain cases (Scheme 1) [10,11]. Carbometalation of alkynes [12,13,14,15,16] is a simple procedure for the preparation of tetrasubstituted alkenes. Yet, a typical drawback associated with the carbometallation of alkynes is the limited regioselectivity of the carbometallation step, especially for alkynes bearing substituents with similar electron and steric properties. The frequently used procedure for the stereoselective preparation of tetrasubstituted alkenes makes use of transition-metal-catalyzed cross-coupling reactions of electrophilic S1–1 templates. The popularity of such procedures can be attributed to the availability of the electrophilic templates S1–1 and organometallic compounds used for the cross-coupling reactions. This concept has been the subject of several reviews in previous years [17,18,19,20,21]. However, procedures that use nucleophilic alkene templates S1–2 for the stereoselective synthesis of alkenes are also widely used, although this concept has not previously been the subject of a review article.
Nucleophilic alkene templates represented by a borylated double bond S1–2 are an important alternative for the stereoselective preparation of tetrasubstituted alkenes for researchers involved in organic synthesis, materials, and medicinal chemistry. Therefore, the aim of this review is to summarize procedures developed for the stereoselective synthesis of tetrasubstituted alkenes based on the cross-coupling reactions of trisubstituted alkenylboronic acids and alkenylboronic acid esters over the last seven years. In addition, the procedures available for the preparation of alkenylboronic acids and alkenylboronates will also be briefly mentioned here, although it is not the aim of this review to provide a complete list of them, as prior work has already done so [21,22]. Although alkenylboronic acid derivatives have lower toxicity compared to alkenylstannanes, this review also briefly summarizes the most recent developments in the use of alkenylstannanes for the stereoselective preparation of tetrasubstituted alkenes.

2. Recent Developments in Stereoselective Synthesis of Tetrasubstituted Alkenes by Palladium-Catalyzed Cross-Coupling Reactions of Trisubstituted Alkenylstannanes and Alkenylsilanes

Depending on the properties of the organometallic compounds, a variety of procedures can be used to prepare metalated tetrasubstituted alkenes. The use of Grignard and organozinc reagents is limited by their low stability in acidic conditions. Tetrasubstituted alkenyl stannanes exhibit substantially better stability when compared with Grignard and organozinc reagents. Recent approaches to stannylated double bonds include Li–Sn exchange reactions. In this case, the organolithium compounds generated in situ reacted with trialkyltin chlorides (Scheme 2a) [23]. A different procedure for the preparation of the stannylated tetrasubstituted double bond makes use of the transition-metal-catalyzed carbostannylation of internal alkynes (Scheme 2b) [24,25]. Finally, alkenyl stannanes can be prepared by means of cyclization procedures, including the cobalt-catalyzed Pauson–Khand reaction [26], radical cyclization [27], or [4 + 2] cycloaddition [28] (Scheme 2c).
Stannylated tetrasubstituted alkenes have a wide range of applications, including the synthesis of natural compounds. Aside from the other natural products that can be synthesized by means of the Stille reaction [29,30,31,32,33], the organostannanes attached to the acyclic S3–2 and cyclic S3–7 double bond have been used in the preparation of trichoaurantianolides C and D and delavatine A (Scheme 3). The preparation of intermediate S3–3 during the synthesis of trichoaurantianolides C and D is accomplished via the cross-coupling reaction of allyl acetate with stannane S3–2 [34]. Analogously, the formation of delavatine A (S3–9) is completed via the Stille reaction of aryl triflate S3–6 with cyclic stannane S3–7 [35]. It is worth noting that cyclic stannane S3–7 is prepared from triflate S3–8 by means of the reaction with the corresponding cuprate. In addition to the above-mentioned applications of organostannanes with a cyclic and acyclic tetrasubstituted double bond, the Stille reactions of similar stannylated tetrasubstituted alkenes have been used to synthesize substrates for the thermal ring expansion of boroles [36], palladium-catalyzed carbocyclization [37], the cascade reaction [38], and other reactions [39,40,41,42,43].
An alternative method for the preparation of tetrasubstituted alkenes based on nucleophilic templates involves the use of a silylated double bond. This type of silane, as represented by the general structure S4–1 (Scheme 4a), is not often used for the preparation of tetrasubstituted alkenes by means of their transition-metal-catalyzed reaction with electrophiles. The most common means of using alkenyl silane S4–1 involves the substitution of the SiR3 group for the halogen via halogenation [44,45]. Then, the obtained organohalide S4–2 is subsequently used in cross-coupling reactions with organometallic reagents. In addition, the Hiyama reaction of silylated alkenes can be used for the preparation of tetrasubstituted alkenes. An example published in 2020 makes use of the syn-carbosilylation of oct-4-yne (S4–4), which is catalyzed by a palladium catalyst and a DrewPhos ligand to synthesize alkene S4–5 (Scheme 4b) [46]. Interestingly, the use of a JessePhos ligand prefers anti-carbosilylation. Then, the stereoselective synthesis of the tetrasubstituted alkene S4–6 is achieved via the Hiyama reaction under Denmark conditions [47].

3. Acyclic Boron-Based Tetrasubstituted Double Bond Templates

3.1. Synthetic Approaches to Acyclic Borylated Alkenes

Typically, the preparation of borylated tetrasubstituted alkenes begins with two readily available starting materials: alkyne S2–4 and alkenes S5–3 and S5–4. In this way, various types of borylated alkenes can be synthesized, including the mono-, di-, and triborylated templates S5–1, S5–2, S5–5, and S5–6 (Scheme 5).
The preparation of diborylated alkenes is a widespread process that makes use of alkynes as the starting materials. The transition-metal-free borylation of alkyne S6–1 can be performed in the presence of a catalytic amount of base, as described by Song (Scheme 6a) [48]. An optimization study determined that 10 mol% of potassium carbonate gave the best yields of borylated alkene S6–2. The reaction tolerates a minimal number of functional groups, and a reaction mechanism has been proposed based on experimental studies. This proposal involves the formation of an ate complex S6–A which reacts with the starting alkynes S6–1 to give the borylated alkenes S6–2. An analogous procedure for the preparation of diborylated alkenes is shown in Scheme 6b [49]. The formation of the borylated product is achieved through the reduction of alkyne S2–4 with sodium in the presence of trimethyl borate. The published reaction involves the preparation of 18 diborylated alkenes S6–3, while the proposed mechanism for the formation of the products S6–3 considers the formation of the intermediate S6–B by means of the reduction of the triple bond via a SET (single electron transfer) mechanism, followed by transmetallation to give the ate complex S6–A, which reacts with pinacol to provide the major reaction products. It is important to note that internal alkynes with aryl and alkyl substituents require three equivalents of sodium dispersion.
The fact that borylmetallation remains a popular method for the preparation of diborylated tetrasubstituted alkenes was demonstrated by Nakamura (Scheme 7) [50]. Here, the starting internal alkyne S2–4 is diborylated in the presence of a catalytic amount of iron (II) dichloride with limited scope and good functional group tolerance, which includes a substrate with a bromine atom or ester group. The authors also proposed and confirmed the mechanism behind the discovered reaction via quantum chemical calculations. The first step of the reaction involves transmetallation with lithium methoxide, and the resultant intermediate S7–A subsequently reacts with pinB–Bpin and then with alkynes S2–4 to form the alkenyl complexes S7–C. Finally, the borylferration product S7–C reacts with MeOBpin to form the main reaction product S7–1. In addition to the above-mentioned examples of alkyne diborylation, examples of cobalt-catalyzed di- and monoborylation of internal alkynes have previously been published. This borylation has mostly been studied on pinacolborane affording a trisubstituted double bond with the Bpin moiety. However, for a limited number of internal alkynes, pinB–Bpin has been used to provide diborylation products with limited stereoselectivity [51].
The original procedure for the preparation of the triborylated alkene S8–2 starts with alkyne S8–1 substituted with the Bmida moiety (Scheme 8). The subsequent Suzuki reaction of boronate S8–2 with aryliodides proceeds with different regioselectivity when compared with the Suzuki reaction given in Scheme 9. The reaction scope of the Suzuki reaction is limited to 12 examples, albeit with satisfactory functional group tolerance [52].
A slightly different procedure for the preparation of triborylated alkenes relies on the iridium-catalyzed C–H borylation of terminal alkynes to form the borylated alkynes S9–4, which are then further borylated in the presence of carbon monoxide (Scheme 9) [53,54]. The reaction is limited in terms of its scope and functional group tolerance (see unreactive alkyne S9–2a), although the selected example S9–2c illustrates how diborylated dienes can also be prepared via this procedure. In addition, the authors showed that borylated alkene S9–2b reacts in the Suzuki reaction with trans selectivity, thereby affording the diborylated alkene S9–3 in a 77% isolated yield.
A similar approach for the preparation of mono- and diborylated alkenes was published in 2020 (Scheme 10) [55]. The starting terminal alkyne S9–1 is triborylated in the presence of a catalytic amount of copper acetate and a tributylphosphine ligand. The developed borylation conditions tolerate a wide variety of functional groups, including –CN, –Cl, –F, or –CO2Me groups. The proposed mechanism involves the borylation of terminal alkyne followed by syn-cupraborylation. The triborylated alkene S9–2 can be used in a Suzuki reaction with trans regioselectivity or in a Bpin–halogen exchange reaction.
A rapid approach to the preparation of 1,1-diborylated alkenes by means of the copper-catalyzed dehydrogenative borylation of the terminal alkene S5–3 has been published by Yamaguchi (Scheme 11) [56]. Different reaction conditions allow for the formation of the mono- and diborylated products S11–2 and S11–1. However, the borylation of disubstituted terminal alkenes to the borylated products S11–1 is associated with a long reaction time and only moderate isolated yields of alkenes S11–1a and S11–1c. Other tetrasubstituted alkenes have not been synthesized. The authors proposed that 2-adamantanone and benzophenone are used as HBpin acceptors as can be seen from proposed mechanism for the formation of the alkenes S11–1.
The monoborylated tetrasubstituted alkene S12–2 can also be prepared via cooperative catalysis (Scheme 12) [57]. The starting materials are usually the symmetrical internal alkynes S2–4, which can be converted into monoborylated alkenes S12–2 by means of the reaction with aryl chlorides. The reaction proceeds well for symmetric alkenes, although unsymmetric alkynes with similar substituents give a stereoselective mixture of the regioisomers S12–2a and S12–3a. High regioselectivity is achieved only for alkynes with the aromatic and aliphatic substituents S12–2b and S12–2c. The proposed mechanism assumes the oxidative addition of Pd0 to the C–Cl bond, while the alkenylcopper reagent S12–C is used for the transmetallation step, which results in the formation of the borylcupration product S12–B. The final step is reductive elimination. A similar approach was published by Brown [58], although in that work the prepared monoborylated alkenes are oxidized to the corresponding ketones. In addition, Pd/Cu cooperative catalysis has been used to prepare symmetrical monoborylated tetrasubstituted alkenes from allenylic carbonates [59].

3.2. Applications of the Borylated Acyclic Double Bond for Stereoselective Synthesis of Tetrasubstituted Alkenes

In the previous section, various procedures for the synthesis of the borylated tetrasubstituted double bond were briefly introduced. This type of organometallic reagent can be used for the nonstereoselective preparation of tetrasubstituted fluoroalkenes [60] or corrphycene derivatives [61]. However, this review focuses on their conversion into tetrasubstituted alkenes. Therefore, in this section, the use of the borylated tetrasubstituted double bond for the stereoselective synthesis of tetrasubstituted alkenes is discussed. These applications go hand in hand with the preparation of the borylated double bond, which led to the inclusion of the synthesis of borylated alkenes in this paper.
The reaction of carbonyl compounds S13–1 with the C1 synthon S13–2 can directly afford the silylborylation products S13–3 and S13–4 (Scheme 13) [62]. The selected examples show that the reaction is sensitive to the structures of the starting ketones. Ketones with aliphatic and aromatic substituents undergo silylborylation with high regioselectivity. In contrast, ketones with two similar substituents give a mixture of both regioisomers. The preparation of (Z)-tamoxifen (F1–1) can be performed starting with the alkene S13–3d, which is iodinated before a double Suzuki reaction gives (Z)-tamoxifen as >93% Z. Thus, the above-mentioned examples of the stereoselective synthesis of (Z)-tamoxifen complement previous preparations of (Z)-tamoxifen via the cross-coupling reactions of borylated alkenes [63,64].
The synthesis of tetrasubstituted alkenes without the use of transition metal catalysis was published by Liu in 2021 (Scheme 14) [65]. The reaction starts with the isomerization of boronate S14–1 into S14–3 by means of Zweifel-type deprotonative olefination. Subsequently, the reaction of the allylboronate S14–3 with benzaldehyde resulted in the tetrasubstituted alkene S14–5 in a high yield and with high stereoselectivity. Alternatively, protodeborylation into the symmetrical alkene S14–4 can also be performed. The use of highly reactive organolithium reagents and resulting low functional group tolerance, as well as the limited scope of synthesized tetrasubstituted alkenes is a major drawback of this protocol.
A comprehensive solution for the preparation of the tetrasubstituted skipped diene S15–2 was published by Zhong (Scheme 15) [66]. The developed procedure makes use of copper-catalyzed borylation of internal and terminal alkynes. Terminal alkynes are suitable for the preparation of trisubstituted alkenes. In contrast, a tetrasubstituted double bond with a Bpin group is formed from the internal alkynes as the major stereo- and regioisomer S15–2. The published reaction was optimized to determine the influence of the structures of allyl phosphate S15–1 and alkyne S2–4 on the regio- and stereoselectivity of the carboboration reaction. The findings were then used for the regio- and stereoselective preparation of a wide variety of skipped dienes containing both trisubstituted and tetrasubstituted double bonds. Unfortunately, the scope of the reaction includes only simple functional groups (MeO, F, Cl, CF3, and TMS). In addition, chirality retention can be observed in relation to the borylation of the diphenylacetylene S2–4a with the chiral allyl phosphate S15–1a.
In 2016, Kanai et al. published a procedure for the preparation of compounds with tetraalkylated and monoarylated double bonds (Scheme 16) [67]. Borylated trialkylethylenes were prepared via the NQIMesCuCl-catalyzed [68] reaction of the internal alkynes S2–4 with the alkyl iodide S16–1 in the presence of pinB–Bpin with satisfactory regioselectivity. This method tolerates a variety of functional groups, including the ether (BnO, TBSO), ester (CO2Me, PivO), dialkyamino (Bn2N), and amide (NH2(O)C) groups. The priming of tetralkyl olefins is completed via the reaction of the borylated alkene S16–2a with dodecyl bromide or iodobenzene in the presence of a catalytic amount of a palladium complex and potassium hydroxide as a base.
Two reports describing the borylative cyclization of alkynyl bromides were published by Ito’s research group. In the first report, the copper-catalyzed silicon-tethered cyclization of the silylated alkyne S17–2 was shown to result in products with the borylated tetrasubstituted double bond S17–3 (Scheme 17) [69]. Starting alkynes S17–1 are readily available by means of the iridium-catalyzed hydrosilylation of alkynes, as reported by Rahaim’s group [70]. However, the method suffers from low functional group tolerance, where only alkynes with Cl, OTBS, OTHP, and OMe groups are used. The products of the carboborylation reaction can be used to prepare tetrasubstituted acyclic alkenes. This has been demonstrated through the preparation by means of stereoselective synthesis of the alkene S17–6 via the Suzuki reaction of borylated alkene S17–3a. Then, the opening of the silacyclopentane ring via the reaction of alkene S17–4 with bromine followed by the Suzuki reaction with 4-methoxyphenylboronic acid resulting in the alkene S17–5. The preparation of the tetrasubstituted alkene was completed by Tamao oxidation [71].
Subsequently, Ito et al. used copper-catalyzed borylation of the internal alkyne S18–1 to prepare tetrasubstituted alkenes (Scheme 18). [72] Similar to their previous work [69], the authors proposed that the cupraborylation of the triple bond forms the cuprate S18–2, which completes the stereoselective synthesis of the borylated alkene S18–3 by means of intramolecular alkylation. The reaction is also suitable for the preparation of trisubstituted double bonds. The obtained borylated alkene S18–3, in conjunction with the Suzuki reaction or the Diels–Alder reaction, allows for the preparation of the tetrasubstituted alkenes S18–4a and S18–4b or the cyclic boronate S18–5.
In addition, allenes can also be used for the stereoselective formation of tetrasubstituted alkenes via borylated intermediates (Scheme 19 [73] and Scheme 20 [74]). The borylated alkenes S19–2 and S20–3 are prepared via palladium-catalyzed hydroboration or oxidative borylation of allenes. These reactions are characterized by good functional group tolerance and a good reaction scope. Moreover, the oxidative borylation of allenes is sensitive to additives and the utilized oxidant. Benzoquinone (BQ) prefers the carbocyclization of the starting allenes into cyclobutene S20–2, while the formation of borylated tetrasubstituted alkenes was accomplished by means of 2,6-dimethylbenzoquinone (2,2-diMeBQ) (Scheme 20). The prepared borylated alkenes were used for the stereoselective formation of tetrasubstituted alkenes by means of the Suzuki reaction catalyzed by tetrakis(triphenylphosphine)palladium.
The above-mentioned reactions differ significantly in terms of the mechanism behind the formation of borylated alkenes (Scheme 21). Thus, the palladium-catalyzed hydroboration of allenes begins with the reduction of palladium acetate by pinB–Bpin, which is followed by the oxidative addition of palladium to the H–O bond of methanol or acetic acid to obtain the catalytic species HPdX [73]. The next step involves the coordination of the allene, followed by the hydropalladation of the allenic double bond to S19–B. The reaction product S19–2 is obtained via transmetallation and reductive elimination. In contrast, the oxidative borylation of allenes can be explained by the coordination of palladium acetate to the starting material and the subsequent isomerization to the vinyl intermediate S20–B. Next, transmetallation and reductive elimination complete the synthesis of the borylated alkene S20–3. The oxidation of Pd0 into PdII is achieved using 2,6-dimethylbenzoquinone [74].
An unusual trans borylation of propargyl alcohol S22–1 was reported by Fürstner in 2019 (Scheme 22) [75]. The starting material is converted into the ate complex S22–2 via the reaction with NaHMDS and pinB–Bpin. Subsequently, the borylated alkene S22–3 is formed in excellent isolated yields and with excellent reaction scopes via the palladium-catalyzed reaction with methyliodide or diaryliodonium triflate under different conditions. The selected examples illustrate how the methodology is suitable for the preparation of the conjugated diene S22–3c, the enyne S22–3d, and the tetrasubstituted double bond with a steroid skeleton S22–3b. In addition, the methodology can be used for the preparation of tetrasubstituted alkene S22–4 and the nonsteroidal estrogen receptor modulator iodoxifen (S22–5).
The diborylated tetrasubstituted double bond has also been used for the synthesis of (Z)-tamoxifen (Scheme 23). Nogi et al. extended the sodium-mediated preparation of cis diborylated alkenes [49] to the preparation of trans diborylated alkene S23–2 [76]. The authors experimentally verified that the dianion S23–1 acts as an intermediate in the formation of the alkene S23–2, which is formed by means of the oxidation of S23–1 with 2,3-dibromobutane. The formal synthesis of (Z)-tamoxifen was performed via the palladium-catalyzed ethylation of alkene S23–2a in a 56% isolated yield.
A similar iterative approach for the stereoselective preparation of tetrasubstituted alkenes was reported by Tsuchimoto (Scheme 24) [77]. This work builds on previous results concerning the preparation of borylated compounds reported by Tsuchimoto’s research group [78,79]. Thus, the starting triborylated alkene S24–2 is synthesized by means of the dehydrogenative coupling of the terminal alkyne S9–1 with 1,8-naphthalenediamineborane (HB(dan)) followed by the diborylation of the internal alkyne S24–1 (Scheme 24a). In a further step, the triborylated alkene is trans arylated with high regioselectivity to form the diborylated alkene S24–3 as the major product (Scheme 24b). The second introduction of aryl substituents was performed with only a limited amount of aryl iodides, as illustrated in Scheme 24c. Subsequently, the B(dan) group was esterified via the reaction with pinacol under acidic conditions, and the stereoselective synthesis of tetrasubstituted alkenes was completed by the Suzuki reaction with aryl iodides. In addition, a complementary series of (E)- and (Z)-tetraarylethylenes can be prepared using this procedure. The developed procedure can also be used for the formal synthesis of (Z)-tamoxifen from the triborylated alkene S24–2a. The starting compound was initially used in the Suzuki reaction with iodobenzene and vinyl bromide under optimized reaction conditions. The reduction of the vinyl group with hydrogen gives the B(dan) derivative S24–5b in a high yield (86% in three steps). The substitution of the dan group for pinacol provides the intermediate needed for the preparation of (Z)-tamoxifen in a 98% yield [63].
An exhaustive approach for the preparation of tetraarylated ethylenes was reported in 2020 (Scheme 25) [80]. In this work, various mono-, di-, tri-, and tetraborylated ethylenes were used in palladium-catalyzed Suzuki reactions to produce symmetrically and unsymmetrically substituted tetraarylethylenes. It is worth noting here that the triborylated alkene S9–2 coupled with aryl iodide to give trans diarylated alkene S25–4 (Scheme 25c). A wide reaction scope is typical for all starting alkenylboronic acid esters. In conclusion, the developed methodology enabled a Lego-based approach to the preparation of the tetraarylated alkenes S25–2, S25–3, S25–5, S25–7, and S25–9 from the easily available borylated double bond.
As mentioned in the introductory section, tetrasubstituted alkenes have a wide range of applications, including the field of materials chemistry. A detailed survey of the available literature has revealed that the symmetrical tetrasubstituted double bond is most commonly used for materials chemistry purposes. Examples of these borylated alkenes are given in Scheme 26. Miyaura borylation, [81,82,83], the Li–B exchange reaction [84], and platinum-catalyzed diborylation of internal alkynes [85,86,87,88] are commonly used for the synthesis of the borylated alkenes S26–2 and S26–5. The applications of borylated alkenes S26–2 and S26–5 include the Suzuki–Miyaura reaction, which is commonly catalyzed by the tetrakis(triphenylphosphine)palladium complex along with inorganic bases (Na2CO3 [89], K2CO3 [82,90,91,92]), organic solvents (THF, toluene, MeOH, 1,4-dioxane), and water.

4. Synthetic Approaches to and Applications of Cycloalkenylboronates

In the previous section, we showed that the acyclic tetrasubstituted double bond with a boryl group is frequently synthesized from alkynes by means of triple bond borylation. The situation is different with regard to tetrasubstituted alicyclic alkenylboronic acids and alkenylboronic acid esters. Cyclooctyne is an isolable compound [93], although four-, five-, six-, and seven-membered cycloalkynes are unstable compounds under normal conditions that can be stabilized via, for example, the formation of cobalt complexes [94]. Therefore, cycloalkynes are considered unsuitable substrates for use in the preparation of cycloalkenyl boronic acids and their esters. In principle, the modification of the tetrasubstituted cycloalkene S27–3 with a suitable functional group or the cyclization of the acyclic precursor S27–2 can be used for the synthesis of cyclic boronic acid esters (Scheme 27).
The [2 + 2]-cycloaddition of borylated acetylene can be used to prepare the cyclobutenylboronate S28–3 in a high yield (Scheme 28) [95]. The reaction is catalyzed by a cobalt complex and proceeds at room temperature. Unfortunately, the scope of the reaction is limited to the preparation of only the single cyclobutenylboronate S28–3. Based on mechanistic studies, it has been suggested that the reaction involves the formation of cobalt(III) metallacycle S28–4.
Cyclic boronates derived from cyclobutene can be used for the preparation of [4]dendralenes (Scheme 29) [96]. Starting neopentylglycol (npg) boronates are prepared via the reaction of cyclobutenyl lithium with triethylborate, which is followed by esterification with neopentylglycol. The Suzuki reaction of boronates S29–2a and S29–2b with diethyl 2-bromocyclobutenyl phosphate results in phosphates S29–3a and S29–3b. Subsequently, the phosphate group is replaced by means of the Negishi reaction with arylzinc chlorides in the presence of aluminum chloride, and [4]dendralenes are obtained via the thermal opening of cyclobutenes S29–4a and S29–4b in xylene at 150 °C.
Pinacol cyclobutenylboronic acid ester can be prepared via the Miyaura borylation of triflate S30–1 (Scheme 30) [97]. The subsequent Suzuki reaction of the borylated cyclobutene with aryl bromide (ArBr) gives 1,2-disubstituted cyclobutene S30–2 in a 52% isolated yield. Removal of the MOM-protecting group and hydrogenation of the cyclobutene double bond result in combretastatin analogue A-4 S30–3.
The cyclization procedure for the preparation of cyclopentenylboronic acid esters is shown in Scheme 31 [98]. The cobalt-mediated Pauson–Khand reaction is used to prepare the alicyclic boronate S31–2, which is subsequently used in a palladium-catalyzed Suzuki reaction. As the selected examples show, this procedure is useful for the preparation of tetrasubstituted cycloalkenes with a wide range of functional groups.
The further cyclization of alkynylboronic acid esters makes use of zirconium-mediated cyclization (Scheme 32) [99]. The zirconacyclopentene S32–1 is proposed as an intermediate for the preparation of cyclopentenylboronate S32–2. Although the reaction is limited in terms of its scope, the prepared pinacol boronic acid ester S32–2b can be used for the preparation of the 2,3-disubstituted cyclopentenone S32–3.
An application of the Suzuki reaction of cyclopentylboronate also involves the preparation of photoswitchable deoxyuridine nucleosides S33–3 under standard reaction conditions (Scheme 33) [100,101,102]. In addition, a catalytic system based on Pd2(dba)3, along with an AsPh3 ligand and silver oxide as a base, can be used for intramolecular Suzuki coupling in order to afford the tetrasubstituted double bond as a part of a tricyclic molecule [103,104].
The Suzuki reactions of cyclic borylated alkenes have been used to synthesize the natural compounds delavatine A (S34–5) and (−)-incarviatone A (S34–6) (Scheme 34) [105,106]. The key step in the synthesis of the natural compounds S34–5 and S34–6 is the site-selective oxidative addition of a palladium catalyst at 3,5-dibromo-2-pyrone (S34–1). Moreover, it has been experimentally verified that the formation of oxidative addition products depends on the reaction conditions [105]. In N,N-dimethylformamide (DMF) and in the presence of CuI, the oxidative addition product S34–3b is preferentially formed. In addition, it has been shown that S34–3a undergoes conversion into the oxidative addition product S34–3b during the subsequent transmetallation and reductive elimination step. Thus, the complex S34–3a is a kinetic oxidative addition adduct, while complex S34–3b is a thermodynamic oxidative addition adduct. A detailed mechanism has been proposed based on quantum chemical calculations. The observed reactivity was used for the preparation of the natural compounds delavatine A (S34–5) and (−)-incarviatone A (S34–6). The similar borylated alkene S34–8 can be used in the alternative total synthesis of (−)-incarviatone A (S34–6), although in this case the Suzuki reaction is performed under standard conditions [107]. The borylated alkenes S34–2 and S34–8 are prepared from the corresponding bromocyclopentenes by means of Miyaura borylation in the presence of a catalytic amount of Pd(dppf)Cl2 and pinB–Bpin, and AcOK as a base in 1,4-dioxane at 80 °C.
Typical examples of the preparation of alicyclic alkenylboronates are shown in Scheme 35a,b. The first example illustrates the iron-catalyzed borolysis of carbamate S35–1 in the presence of a bpy ligand and lithium methoxide as a base. The reaction leads to the preparation of different alkenyl and arylboronic acid esters as well as only two borylated tetrasubstituted alkenes S35–2a and S35–2b [108]. The second process relies on the iridium-catalyzed C–H borylation of the cyclohexenyl carboxylate S35–3 [109]. The reaction tolerates a variety of functional groups, as illustrated by selected examples S35–4a, S35–4b, S35–4c, and S35–4d, and the overall isolated yields of the boronates are high. The C–H borylation process can also be used to prepare acyclic tetrasubstituted vinylboronic acid esters, although the cyclopentenyl-, cycloheptenyl-, and cyclooctenylboronic acid esters are only obtained in moderate isolated yields.
Another approach for the preparation of alicyclic alkenylboronates makes use of the cyclization of acyclic precursors. An example of this approach is shown in Scheme 36 [110]. Initially, the brominated diene S36–1 is borylated, although alkene S36–2 cannot be isolated in its pure form, meaning that the subsequent ring-closing metathesis must be performed in a tandem reaction setup. Then, the borylated cycloalkenes S36–3a and S36–3b are obtained in high yields.
The preparation of the borylated dihydronaphthalene derivative S37–2 was performed via the trans borylative cyclization of the internal alkyne S37–1 by means of the reaction with boron trichloride and the subsequent conversion into the pinacol boronic acid ester S37–2 under various reaction conditions (Scheme 37) [111]. The selected examples illustrate how borylative cyclization is not limited to the formation of the dihydronaphthalene derivatives S37–2a and S37–2b, as depending on the structures of the starting materials, it is possible to obtain the tricyclic compounds S37–2c and S37–2d. In addition, oxidation of the dihydronaphthalene derivative S37–2f can yield the boronate S37–4, while the Suzuki reaction of boronate S37–2f allows for formal access to biologically relevant nafoxidine-derived compounds.
Transition-metal-catalyzed Miyaura borylation is a popular tool for the preparation of organoboronic acid esters [112,113,114]. Thus far, an interesting alternative to transition-metal-catalyzed borylation has been published by Ito et al. (Scheme 38) [115]. In this work, the borylation of aryl and vinyl halides was performed with a borylating reagent in the presence of sodium ethoxide. The authors mainly used the developed procedure for the preparation of arylboronic acid esters, as well as borylated mono- and disubstituted alkenes. In addition, only two examples of borylated trisubstituted double bond S38–2a and S38–2b were synthesized to expand the scope of the developed borylation protocol.
Aside from the modular and asymmetric preparation of baskets [116] and starting materials for the photoelectrocyclization of bis-aryl cycloalkenones [117,118], the synthesis of substituted cyclopentenones involving the benzofuran-ring opening reaction [119], and the oxidation of nonactivated anilines [120,121], cyclohexenylboronic acid esters can be used in natural product synthesis. Selected examples are shown in Scheme 39. The tricyclic cores of cyrneine B (S39–4) and glaucopine C (S39–5) are synthesized from the diene S39–3, which is prepared by means of the Suzuki reaction of the cyclohexenylboronic acid ester S39–2 with the cyclopentenyl triflate S39–1 [122]. By contrast, the Suzuki reaction of the boronic acid S39–6 is used to prepare the polyenes S39–9 and S39–10 during the preparation of the dihydroretinoic acids S39–11a and S39–11b [123].

5. Conclusions

In this work, we have shown how the borylated tetrasubstituted double bond has become an indispensable tool for the stereoselective preparation of tetrasubstituted alkenes. Alkenylboronic acids and alkenylboronates are the most commonly used tools for this purpose. These substances are mostly stable and easy to isolate compounds, which facilitates their use for the preparation of tetrasubstituted alkenes. In addition, a number of procedures have been developed for the stereoselective preparation of acyclic alkenylboronic acids from readily available compounds without the need for highly reactive organolithium and Grignard reagents. The situation is dramatically different for cycloalkenylboronic acids and cycloalkenylboronates. In this case, the stereoselectivity of the double bond is fixed, however, the limited stability and availability of cycloalkynes restricts their preparation to Li–B exchange reaction, Miyaura borylation, and cyclization procedures. The lack of suitable procedures for the preparation of cycloalkenylboronates is characteristic especially for the cyclization procedures. On the other hand, cyclic and acyclic vinylstannanes and vinylsilanes are less commonly used for the stereoselective preparation of tetrasubstituted alkenes, which leaves the door open for further research in this regard.

Author Contributions

T.T.: conceptualization, writing—original draft preparation. S.M.: writing—reviewing and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Structure of tamoxifen and related tetrasubstituted alkenes.
Figure 1. Structure of tamoxifen and related tetrasubstituted alkenes.
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Scheme 1. General approaches to tetrasubstituted alkenes.
Scheme 1. General approaches to tetrasubstituted alkenes.
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Scheme 2. Synthetic approaches to tetrasubstituted alkenyl stannanes (a–c).
Scheme 2. Synthetic approaches to tetrasubstituted alkenyl stannanes (a–c).
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Scheme 3. The use of tetrasubstituted alkenylstannanes in the total synthesis (a,b).
Scheme 3. The use of tetrasubstituted alkenylstannanes in the total synthesis (a,b).
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Scheme 4. An application of organosilanes for the stereoselective synthesis of tetrasubstituted alkenes (a,b).
Scheme 4. An application of organosilanes for the stereoselective synthesis of tetrasubstituted alkenes (a,b).
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Scheme 5. Frequently synthesized borylated acyclic tetrasubstituted alkenes (a,b).
Scheme 5. Frequently synthesized borylated acyclic tetrasubstituted alkenes (a,b).
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Scheme 6. Transition-metal-free route to diborylated alkenes by base-catalyzed borylation of internal alkynes (a) and sodium mediated borylation of internal alkynes (b).
Scheme 6. Transition-metal-free route to diborylated alkenes by base-catalyzed borylation of internal alkynes (a) and sodium mediated borylation of internal alkynes (b).
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Scheme 7. Iron-catalyzed diborylation of internal alkynes.
Scheme 7. Iron-catalyzed diborylation of internal alkynes.
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Scheme 8. Palladium-catalyzed diborylation of Bmida alkynes and subsequent Suzuki reaction.
Scheme 8. Palladium-catalyzed diborylation of Bmida alkynes and subsequent Suzuki reaction.
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Scheme 9. Synthesis of triborylated alkenes via C–H borylation of terminal alkenes followed by diborylation.
Scheme 9. Synthesis of triborylated alkenes via C–H borylation of terminal alkenes followed by diborylation.
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Scheme 10. Synthesis of 1,2-diborylated tetrasubstituted alkenes via the manipulation of triborylated alkenes.
Scheme 10. Synthesis of 1,2-diborylated tetrasubstituted alkenes via the manipulation of triborylated alkenes.
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Scheme 11. Alternative synthesis of 1,1-diborylated alkenes.
Scheme 11. Alternative synthesis of 1,1-diborylated alkenes.
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Scheme 12. Pd/Cu cooperative catalysis for the carboborylation of alkynes.
Scheme 12. Pd/Cu cooperative catalysis for the carboborylation of alkynes.
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Scheme 13. Synthesis of (Z)-tamoxifen by means of the Suzuki reaction of borylated alkenes.
Scheme 13. Synthesis of (Z)-tamoxifen by means of the Suzuki reaction of borylated alkenes.
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Scheme 14. Transition-metal-free synthesis of tetrasubstituted alkenes from allylboronates.
Scheme 14. Transition-metal-free synthesis of tetrasubstituted alkenes from allylboronates.
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Scheme 15. Stereoselective synthesis of skipped dienes by means of the substrate-controlled carboborylation of internal alkynes.
Scheme 15. Stereoselective synthesis of skipped dienes by means of the substrate-controlled carboborylation of internal alkynes.
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Scheme 16. Alkyne carboboration for the stereoselective synthesis of tetraalkylated alkenes.
Scheme 16. Alkyne carboboration for the stereoselective synthesis of tetraalkylated alkenes.
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Scheme 17. Silicon-tethered borylative cyclization of alkynyl bromides for the synthesis of tetrasubstituted alkenes.
Scheme 17. Silicon-tethered borylative cyclization of alkynyl bromides for the synthesis of tetrasubstituted alkenes.
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Scheme 18. Preparation of tetrasubstituted exocyclic double bond by means of the copper-catalyzed alkylative borylation of internal alkynes.
Scheme 18. Preparation of tetrasubstituted exocyclic double bond by means of the copper-catalyzed alkylative borylation of internal alkynes.
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Scheme 19. Palladium-catalyzed hydroboration of allenes en route to boralyted alkenes as well as stereoselective synthesis of tetrasubstituted alkenes.
Scheme 19. Palladium-catalyzed hydroboration of allenes en route to boralyted alkenes as well as stereoselective synthesis of tetrasubstituted alkenes.
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Scheme 20. Oxidative borylation of allenes for the synthesis of trienes containing a tetrasubstituted double bond.
Scheme 20. Oxidative borylation of allenes for the synthesis of trienes containing a tetrasubstituted double bond.
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Scheme 21. Proposed mechanisms for the synthesis of borylated alkenes.
Scheme 21. Proposed mechanisms for the synthesis of borylated alkenes.
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Scheme 22. Stereoselective trans diborylation of internal alkynes for the synthesis of allylic alcohols with tetrasubstituted double bonds.
Scheme 22. Stereoselective trans diborylation of internal alkynes for the synthesis of allylic alcohols with tetrasubstituted double bonds.
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Scheme 23. Borylation of internal alkynes en route to (Z)-tamoxifen.
Scheme 23. Borylation of internal alkynes en route to (Z)-tamoxifen.
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Scheme 24. Iterative synthesis of tetrasubstituted alkenes, including the formal stereoselective synthesis of (Z)-tamoxifen (a–d).
Scheme 24. Iterative synthesis of tetrasubstituted alkenes, including the formal stereoselective synthesis of (Z)-tamoxifen (a–d).
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Scheme 25. Lego-based approach to tetraarylated ethylenes (a–e).
Scheme 25. Lego-based approach to tetraarylated ethylenes (a–e).
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Scheme 26. Approaches to tetrasubstituted alkenes and examples of borylated alkenes used in materials chemistry.
Scheme 26. Approaches to tetrasubstituted alkenes and examples of borylated alkenes used in materials chemistry.
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Scheme 27. Synthetic approaches toward alicyclic boronic acid esters.
Scheme 27. Synthetic approaches toward alicyclic boronic acid esters.
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Scheme 28. [2 + 2]-Cycloaddition for the synthesis of borylated cyclobutene.
Scheme 28. [2 + 2]-Cycloaddition for the synthesis of borylated cyclobutene.
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Scheme 29. Synthesis of [4]dendralenes by means of the ring-opening reaction of biscyclobutenes.
Scheme 29. Synthesis of [4]dendralenes by means of the ring-opening reaction of biscyclobutenes.
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Scheme 30. Cyclobutenylboronic acid ester for the synthesis of the combretastatin A-4 analogue.
Scheme 30. Cyclobutenylboronic acid ester for the synthesis of the combretastatin A-4 analogue.
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Scheme 31. The Pauson–Khand reaction for the synthesis of borylated tetrasubstituted cycloalkenes.
Scheme 31. The Pauson–Khand reaction for the synthesis of borylated tetrasubstituted cycloalkenes.
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Scheme 32. Zirconium-mediated cyclization of borylated alkynes en route to disubstituted cyclopentenone.
Scheme 32. Zirconium-mediated cyclization of borylated alkynes en route to disubstituted cyclopentenone.
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Scheme 33. Suzuki reaction of cyclopentenylboronic acid esters for the preparation of photoswitchable deoxyuridine nucleosides.
Scheme 33. Suzuki reaction of cyclopentenylboronic acid esters for the preparation of photoswitchable deoxyuridine nucleosides.
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Scheme 34. Suzuki reaction of borylated tetrasubstituted alkenes en route to (−)-incarviatone A and delavatine A.
Scheme 34. Suzuki reaction of borylated tetrasubstituted alkenes en route to (−)-incarviatone A and delavatine A.
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Scheme 35. Synthesis of the borylated tetrasubstituted double bond via the iron-catalyzed cross-coupling of the activated C–O bond and iridium-catalyzed C–H borylation.
Scheme 35. Synthesis of the borylated tetrasubstituted double bond via the iron-catalyzed cross-coupling of the activated C–O bond and iridium-catalyzed C–H borylation.
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Scheme 36. Tandem Miayura borylation–RCM for the synthesis of alicyclic vinylboronates.
Scheme 36. Tandem Miayura borylation–RCM for the synthesis of alicyclic vinylboronates.
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Scheme 37. Transition-metal-free trans borylative cyclization for the synthesis of dihydronaphthalene derivatives.
Scheme 37. Transition-metal-free trans borylative cyclization for the synthesis of dihydronaphthalene derivatives.
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Scheme 38. Alternative synthesis of mono- and diborylated alkenes.
Scheme 38. Alternative synthesis of mono- and diborylated alkenes.
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Scheme 39. The use of borylated cyclohexenes for the synthesis of terpenoids.
Scheme 39. The use of borylated cyclohexenes for the synthesis of terpenoids.
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Tobrman, T.; Mrkobrada, S. Palladium-Catalyzed Cross-Coupling Reactions of Borylated Alkenes for the Stereoselective Synthesis of Tetrasubstituted Double Bond. Organics 2022, 3, 210-239. https://doi.org/10.3390/org3030017

AMA Style

Tobrman T, Mrkobrada S. Palladium-Catalyzed Cross-Coupling Reactions of Borylated Alkenes for the Stereoselective Synthesis of Tetrasubstituted Double Bond. Organics. 2022; 3(3):210-239. https://doi.org/10.3390/org3030017

Chicago/Turabian Style

Tobrman, Tomáš, and Sergej Mrkobrada. 2022. "Palladium-Catalyzed Cross-Coupling Reactions of Borylated Alkenes for the Stereoselective Synthesis of Tetrasubstituted Double Bond" Organics 3, no. 3: 210-239. https://doi.org/10.3390/org3030017

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