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Proceeding Paper

Current Trends in the Synthesis of Practically Important Five-Membered Boracarbocycles by Transmetalation of Aluminacarbocycles with Boron Halides †

by
Liliya I. Tulyabaeva
*,
Usein M. Dzhemilev
and
Tatyana V. Tyumkina
Institute of Petrochemistry and Catalysis, Ufa Federal Research Center, Russian Academy of Sciences, 141 Prospekt Oktyabrya, Ufa 450075, Russia
*
Author to whom correspondence should be addressed.
†
Presented at the 26th International Electronic Conference on Synthetic Organic Chemistry, 15–30 November 2022; Available online: https://sciforum.net/event/ecsoc-26.
Chem. Proc. 2022, 12(1), 67; https://doi.org/10.3390/ecsoc-26-13698
Published: 18 November 2022
(This article belongs to the Proceedings of The 26th International Electronic Conference on Synthetic Organic Chemistry)
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Abstract

:
The data obtained by the authors in the field of chemistry of substituted borolanes and 2,3-dihydro-1H-boroles are summarized. The authors developed a selective method for the synthesis of five-membered boracarbocycles via transmetalation of aluminacarbocycles, obtained by the catalytic cycloalumination of unsaturated compounds (terminal olefins or acetylenes) with AlEt3 in the presence of Cp2ZrCl2 as a catalyst by boron halides (BF3·Et2O, BCl3, and BBr3). Some examples of the use of this approach to modify steroid compounds (in particular, to introduce a borolan fragment into them) are described in this review.

1. Introduction

There are only a few methods and approaches to synthesize five-membered organoboron compounds. Borolane derivatives were prepared from 1,4-dienes via cyclic hydroboration (Brown reaction) using monochloroborane etherate [1] or isopinocampheylchloroborane etherate [2] and thermal disproportionation of trialkylboranes [3,4]. 2,5-Dihydro-1H-borole (borol-3-enes, 1-boracyclopent-3-enes) derivatives were originally prepared by Zweifel [5] from conjugated enynes in a two-step reaction sequence involving the hydroboration and subsequent photocyclization of dienylborane. Herberich [6,7] reported the preparation of 2,3- and 2,5-dihydro-1H-boroles via the reaction of the Cl2BNR2 substrate with an oligomeric “butadiene-magnesium” reagent. Examples were found where 2,3-dihydroborole systems are readily formed through treatment of dicyclopropylacetylene with the strongly electrophilic borane B(C6F5)3 [8].
Borolanes and 2,3-dihydroborole are also prepared by transmetalation of cyclic and acyclic organometallic compounds of transition or non-transition metals with various boron derivatives [9,10]. Prior to our studies, organoaluminum compounds were not used to obtain boracarbocycles.
This review summarizes and systematizes the results of the authors’ studies on the synthesis and study of the properties of five-membered cyclic organoboron compounds (borolanes and 2,3-dihydro-1H-boroles) via transmetalation of cyclic organoaluminum compounds with boron halides.

2. Results and Discussion

To date, we have accumulated significant experience in the development of effective one-pot methods for the synthesis of five-membered boracarbocycles via transmetalation of aluminacarbocycles, obtained by catalytic cycloalumination [11,12] of unsaturated compounds (olefins or acetylenes) with AlEt3 in the presence of Cp2ZrCl2 as a catalyst under solvent-free conditions, by boron halides (BF3·Et2O, BCl3, and BBr3).
So, in 2012, we developed an efficient one-pot method for the synthesis of 1-fluoro-substituted borolanes 2a–d through exchange reaction between aluminacyclopentanes 1a–d and the boron trifluoride–diethyl ether complex [13,14,15] (Scheme 1). The 1,3-disubstituted borolanes 2a–d were isolated as molecular complexes with EtBF2.
EtBF2 is formed as a result of the exchange reaction between EtAlF2 and BF3·Et2O, whereas EtAlF2 is the product of the transmetalation of aluminacyclopentane with BF3·Et2O (Scheme 2).
Continuing these works to study the exchange reactions between various aluminacyclanes and boron halides, we developed a one-pot method for the synthesis of unsaturated cyclic organoboron compounds, 2,3-dihydro-1H-boroles 4a–c, by the reaction between 1-ethyl-2,3-dialkylsubstituted aluminacyclopentenes 3a–c, synthesized by the cycloalumination of disubstituted acetylenes with Et3Al in the presence of a Cp2ZrCl2 as a catalyst [16,17] and BF3·Et2O [18,19] (Scheme 3).
It has been established that aluminacyclopent-2-enes obtained from unsymmetrical acetylenes, for example, butylphenylacetylene, can also be successfully converted into the corresponding substituted 2,3-dihydro-1H-boroles. So, 1-ethyl-2-phenyl-3-butylaluminacyclopent-2-ene 5 reacts with BF3∙Et2O to give 4-butyl-2,3-dihydro-5-phenyl-1-fluoro-1H-borole 6 in a yield of 52% (Scheme 3). The 2,3-dihydro-1H-boroles also form complexes with EtBF2. The yields of borolanes and 2,3-dihydro-1H-boroles (Scheme 1 and Scheme 3) correspond to the isolated products. However, the monitoring of the transmetalation reaction by 1H, 13C, 11B, 27Al, and 19F NMR spectroscopy showed that aluminacarbocycles were completely transformed into the corresponding boracyclanes. It should be noted that the signals of boron atoms for the obtained borolanes 2a–d and 2,3-dihydro-1H-boroles 4a–c in the 11B NMR spectra were recorded in a rather low-field region (80–93 ppm).
The aluminacyclopent-2-enes 7a,b [20] with nitrogen-containing substituents can also be successfully transmetalated with BF3·Et2O to the corresponding substituted 2,3-dihydro-1H-boroles 8a,b, which, in contrast to the above 2,3-dialkyl-2,3- dihydro-1H-boroles 4a–c, were isolated as individual compounds (Scheme 4). The values of δB in the 11B NMR spectrum for 2,3-dihydro-1H-boroles 8a,b are ~5.2 ppm (CDCl3) and correspond to the region of the tetracoordinated boron atom. Intra- or intermolecular interaction between heteroatoms B and N takes place in a non-polar solvent. There are no signals of EtBF2 in the NMR spectra [18].
The obtained results on the synthesis of boracarbocycles with BF3·Et2O stimulated the studies of B–Al exchange reactions in the series of aluminacyclopentan(en)es with BCl3 and BBr3 as the boron regents [21,22,23]. We found that boron trichloride and tribromide successfully interact with the aluminacyclopentanes 1a–c and aluminacyclopent-2-enes 3a–c to form 1-chloro(bromo)-substituted borolanes 9a–c and 10a–c and 2,3-dihydro-1H-boroles 11a–c and 12a–c (Scheme 5 and Scheme 6). However, 1-chloro(bromo)-substituted five-membered boracyclanes were isolated individually and do not form a complex with EtBHal2 (Hal = Cl, Br) under the reaction conditions.
The reaction with BBr3 requires special conditions: the cooling of the reaction mass to –60 °C followed by the addition of THF. This is due to the fact that 1-bromo-substituted boracarbocycles decompose during thermal distillation, but if THF is added, they form donor–acceptor molecular complexes (10a–c)·(THF)n and (12a–c)·(THF)n, which, in the conditions of thermal distillation, stabilize the borolanes 10a–c (Scheme 5) and 2,3-dihydro-1H-boroles 12a–c (Scheme 6), preventing decomposition [21,23].
The mechanism of Al−B exchange was studied, taking into account possible reaction routes by the quantum–chemical method PBE/3z (Priroda 6.0) [24,25,26]. It has been established that acyclic Al,B-adducts 14 and 15 (Scheme 7) are formed with equal probabilities at the first stage, because both reactions (1) and (2) are characterized by negative Gibbs energies (−9.0 kcal/mol) and similar energy barriers. Further, the target borolane 17 can be formed by two reaction routes: through the intermediate diboron derivative 16 according to the reaction (3) or through an alternative route (4) as a result of a one-step intramolecular cyclization. Computer simulation taking into account the solvation effect showed that the reactions proceed with a lower barrier. This was confirmed by NMR monitoring of the reaction—13C, and the 1H NMR spectra of the reaction mixture in THF correspond to the cyclic product 17, while in toluene, only diboron derivatives are formed. Obviously, the diborane derivative obtained in the nonpolar solvent (hexane or toluene) (Scheme 1), is transformed into cyclic borane during thermal distillation. At the same time, borolane was not isolated by distillation from a solution of tetrahydrofuran because of destruction of the cycle.
In 2013, the Cp2ZrCl2-catalyzed cycloalumination of cholestane derivatives (3’-methylene-spiro[(5α)-cholestane-3,1’-cyclobutane] 18 and 2’-methylidene-2’,3’-ethano-(5α)-cholestane) 20 with Et3Al to give spiroalumincarbocycles was performed for the first time. The latter were converted with BF3·Et2O into the corresponding boraspirocarbocycles 19,21 without preliminary isolation [27,28] (Scheme 8 and Scheme 9).
This method was extended to methylenecyclobutane terpene derivatives (D(+)camphor, (+)-camphene, β-pinene, and L(−) menthol)) to obtain the corresponding boron-containing terpenoids [29].

3. Conclusions

In summary, new effective one-pot methods for the synthesis of borolanes and 2,3-dihydro-1H-boroles by reaction of boron halides with aluminacyclopentanes and aluminacyclopent-2-enes obtained by the Cp2ZrCl2-catalyzed cycloalumination of unsaturated compounds with Et3Al (Dzhemilev reaction) were developed. This method can be used as an effective tool for the functionalization of terpenes, steroids, and their derivatives.

4. Patents

1. Dzhemilev, U.M.; Ibragimov, A.G.; Khafizova, L.O.; Khusainova L.I.; Gubaidullin R.R. The method for obtaining 1-ethyl-3-alkylaluminacyclopentanes. Patent RU 2433132 (10.11.2011, Bull. № 31).
2. Dzhemilev, U.M.; Khafizova, L.O.; Ibragimov, A.G.; Khusainova L.I.; Tyumkin T.V.; Khanov, V.Kh. The method for obtaining 1-fluoro-3-alkylboracyclopentanes. Patent RU 2440356 (20.01.2012, Bull. № 2).
3. Khafizova, L.O.; Khusainova L.I.; Tyumkina T.V.; Popodko N.R. Dzhemilev U.M. The method for obtaining 1-fluoro-2,3-dialkylborocyclopent-2-enes. Patent RU 2478641 (10.04.2013. Bull. № 10).
4. Dzhemilev, U.M.; Khafizova, L.O.; Khusainova L.I.; Tyumkina T.V. The method for obtaining 1-chloro-2,3-dialkylboracyclopent-2-enes. Patent RU 2507207 (20.02.2014. Bull. № 5).
5. Dzhemilev, U.M.; Khafizova, L.O.; Khusainova L.I.; Tyumkina T.V. The method for obtaining 1-bromo-3-alkylborolanes. Patent RU 2507208 (20.02.2014. Bull. № 5).
6. Dzhemilev, U.M.; Khafizova, L.O.; Khusainova L.I.; Tyumkina T.V. The method for obtaining pyridinates of 1-fluoro(chloro)-3-alkyl(aryl)borolanes. Patent RU 2532925 (16.09 2014. Bull. № 32).

Author Contributions

Conceptualization, U.M.D. and L.I.T.; methodology, validation, and execution of chemistry experiments, L.I.T. and T.V.T.; manuscript preparation, L.I.T. All authors have read and agreed to the published version of the manuscript.

Funding

The studies was carried out in accordance with the Federal Program No. FMRS-2022-0075.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data available on request.

Acknowledgments

The structural studies of compounds were performed on the equipment of «Agidel» Collective Usage Center of Ufa Federal Research Center at the Institute of Petrochemistry and Catalysis, Ufa Federal Research Center, Russian Academy of Sciences.

Conflicts of Interest

The authors declare no conflict of interest.

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Chemproc 12 00067 sch001 550
Scheme 1. The synthesis of 1-fluoro-substituted borolanes 2a–d via transmetalation of aluminacyclopentanes with BF3·Et2O (1 Isolated yield after distillation).
Scheme 1. The synthesis of 1-fluoro-substituted borolanes 2a–d via transmetalation of aluminacyclopentanes with BF3·Et2O (1 Isolated yield after distillation).
Chemproc 12 00067 sch001
Chemproc 12 00067 sch002 550
Scheme 2. The reaction between EtAlF2 and BF3·Et2O.
Scheme 2. The reaction between EtAlF2 and BF3·Et2O.
Chemproc 12 00067 sch002
Chemproc 12 00067 sch003 550
Scheme 3. The synthesis of 1-fluoro-substituted 2,3-dihydro-1H-boroles 4a–c, 6 via transmetalation of aluminacyclopent-2-enes with BF3·Et2O (1 Isolated yield after distillation).
Scheme 3. The synthesis of 1-fluoro-substituted 2,3-dihydro-1H-boroles 4a–c, 6 via transmetalation of aluminacyclopent-2-enes with BF3·Et2O (1 Isolated yield after distillation).
Chemproc 12 00067 sch003
Chemproc 12 00067 sch004 550
Scheme 4. The synthesis of (3-alkyl-1-fluoro-4,5-dihydro-1H-borol-2-yl)- N,N-dimethylmethanamines 8a,b.
Scheme 4. The synthesis of (3-alkyl-1-fluoro-4,5-dihydro-1H-borol-2-yl)- N,N-dimethylmethanamines 8a,b.
Chemproc 12 00067 sch004
Chemproc 12 00067 sch005 550
Scheme 5. The synthesis of 1-chloro(bromo)-substituted borolanes 9a–c and 10a–c via transmetalation of aluminacyclopentanes with BCl3 and BBr3 (1 Isolated yield after distillation).
Scheme 5. The synthesis of 1-chloro(bromo)-substituted borolanes 9a–c and 10a–c via transmetalation of aluminacyclopentanes with BCl3 and BBr3 (1 Isolated yield after distillation).
Chemproc 12 00067 sch005
Chemproc 12 00067 sch006 550
Scheme 6. The synthesis of 1-chloro(bromo)-substituted 2,3-dihydro-1H-boroles 11a-c and 12a-c via transmetalation of aluminacyclopent-2-enes with BCl3 and BBr3 (1 Isolated yield after distillation).
Scheme 6. The synthesis of 1-chloro(bromo)-substituted 2,3-dihydro-1H-boroles 11a-c and 12a-c via transmetalation of aluminacyclopent-2-enes with BCl3 and BBr3 (1 Isolated yield after distillation).
Chemproc 12 00067 sch006
Chemproc 12 00067 sch007 550
Scheme 7. The transformation of 1,3-diethylalumolane 13 into 1-fluoro-3-ethylborolane 17.
Scheme 7. The transformation of 1,3-diethylalumolane 13 into 1-fluoro-3-ethylborolane 17.
Chemproc 12 00067 sch007
Chemproc 12 00067 sch008 550
Scheme 8. The synthesis of borolane 19 based on (3′-methylene-spiro[(5α)-cholestane-3,1′-cyclobutane]. (1 Yields determined by weighing the oxidation reaction products, isolated by column chromatography).
Scheme 8. The synthesis of borolane 19 based on (3′-methylene-spiro[(5α)-cholestane-3,1′-cyclobutane]. (1 Yields determined by weighing the oxidation reaction products, isolated by column chromatography).
Chemproc 12 00067 sch008
Chemproc 12 00067 sch009 550
Scheme 9. The synthesis of borolane 21 based on 2′-methylidene-2′,3′-ethano-(5α)-cholestane (1 Yields determined by weighing the oxidation reaction products, isolated by column chromatography).
Scheme 9. The synthesis of borolane 21 based on 2′-methylidene-2′,3′-ethano-(5α)-cholestane (1 Yields determined by weighing the oxidation reaction products, isolated by column chromatography).
Chemproc 12 00067 sch009
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Tulyabaeva, L.I.; Dzhemilev, U.M.; Tyumkina, T.V. Current Trends in the Synthesis of Practically Important Five-Membered Boracarbocycles by Transmetalation of Aluminacarbocycles with Boron Halides. Chem. Proc. 2022, 12, 67. https://doi.org/10.3390/ecsoc-26-13698

AMA Style

Tulyabaeva LI, Dzhemilev UM, Tyumkina TV. Current Trends in the Synthesis of Practically Important Five-Membered Boracarbocycles by Transmetalation of Aluminacarbocycles with Boron Halides. Chemistry Proceedings. 2022; 12(1):67. https://doi.org/10.3390/ecsoc-26-13698

Chicago/Turabian Style

Tulyabaeva, Liliya I., Usein M. Dzhemilev, and Tatyana V. Tyumkina. 2022. "Current Trends in the Synthesis of Practically Important Five-Membered Boracarbocycles by Transmetalation of Aluminacarbocycles with Boron Halides" Chemistry Proceedings 12, no. 1: 67. https://doi.org/10.3390/ecsoc-26-13698

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