Boranes (Dimethylsulfide Borane, Borane-Tetrahydrofuran Complex)
Sandra Forbes
Product Manager
Diborane (B2H6), the simplest borane, is a useful reagent with many applications, but it is pyrophoric, gaseous and not convenient to handle. There is a wide range of boranes: Decaborane for example is not as reactive as diborane and is used as reducing agent too.
Borane-lewis base complexes are often found in literature. Borane-tetrahydrofuran (BTHF) and borane-dimethyl sulfide (BMS, DMSB) are often used as a borane source. Both reagents are available in solution (e.g. 1 M in THF), and are therefore easier to handle than diborane. Volatility and flammability are always a drawback. BMS is more stable than BTHF but has an unpleasant odor.
BTHF and BMS are used in hydroborations. They offer interesting applications in the reduction of various functional groups. The most interesting application is as a borane source for oxazeborolidine catalyzed asymmetric reductions of ketones (Corey-Bakshi-Shibata Reduction). Very interesting is also the direct reduction of carboxylic acids to alcohols (BMS) and amides to amines (BTHF).
The presence of complexing groups sometimes leads to borane adducts. Products may be liberated in acidic solutions or, very time consuming, in neutral solutions:
Some new methods have been developed:
M. Couturier, J. L. Tucker, B. M. Andresen, P. Dube, J. T. Negri, Org. Lett., 2001, 3, 465-467. DOI: 10.1021/ol006969+
Despite their relative stability as complexes, some aminoboranes such as diisopropylaminoborane, α-picoline-borane, H3N-BH3, Me3N-BH3, DMAP-BH3 or sodium aminodiboranate can be used as reducing agents too.
Name Reactions
Corey-Bakshi-Shibata Reduction
Recent Literature
A chiral oxazaborolidine catalyst prepared in situ from a chiral lactame and borane at room temperature catalyzed the enantioselective reduction of ketones with high enantioselectivity. The preparation of the catalyst is not time consuming and no expensive reagents are involved.
Y. Kawanami, S. Murao, T. Ohga, N. Kobayashi, Tetrahedron, 2003, 59, 8411-8414.
DOI: 10.1016/j.tet.2003.08.064
The catalytic asymmetric borane reduction of both electron-deficient and electron-rich ketones was achieved with high enantioselectivity with a C3-symmetric chiral tris(β-hydroxy phosphoramide) ligand .
D.-M. Du, T. Fang, J. Xu, S.-W. Zhang, Org. Lett., 2006, 8, 1327-1330.
DOI: 10.1021/ol0600584
Odorless Dod-S-Me and MMS are developed as efficient borane carriers. The yields of hydroborations and reductions with the borane complex of Dod-S-Me are very high. The recovery of Dod-S-Me after the reaction is quantitative.
P. K. Patra, K. Nishide, K. Fuji, M. Node, Synthesis, 2004, 1003-1006.
A triazole-based N-heterocyclic carbene borane (NHC-borane) enables an efficient and highly diastereoselective reduction of various of tert-butanesulfinyl ketimines. The reagent was more efficient than or comparable to commonly used reductive reagents such as NaBH4, NaBH3CN, l-selectride, a Ru catalyst, or BH3-THF.
T. Liu, L-y. Chen, Z. Sun, J. Org. Chem., 2015, 80, 11441-11446.
Borane-trimethylamine is an efficient reducing agent for the selective methylation and formylation of amines with 1 atm CO2 under metal-free conditions. 6-Amino-2-picoline serves as a highly efficient catalyst for the methylation of various secondary amines, whereas in its absence, the formylation of primary and secondary amines was achieved in high yield with high chemoselectivity.
Y. Zhang, H. Zhang, K. Gao, Org. Lett., 2021, 23, 8282-8286.
DOI: 10.1021/acs.orglett.1c03023
Borane-trimethylamine is an efficient reducing agent for the selective methylation and formylation of amines with 1 atm CO2 under metal-free conditions. 6-Amino-2-picoline serves as a highly efficient catalyst for the methylation of various secondary amines, whereas in its absence, the formylation of primary and secondary amines was achieved in high yield with high chemoselectivity.
Y. Zhang, H. Zhang, K. Gao, Org. Lett., 2021, 23, 8282-8286.
DOI: 10.1021/acs.orglett.1c03023
Highly regioselective ring opening of epoxides with poor nucleophiles such as indole, borane, O-trimethylsilylhydroxylamine, p-nitroaniline and sterically hindered tert-butylamine is a fast and convenient reaction in the presence of 5.0 M lithium perchlorate-Et2O solution (LPDE).
A. Heydari, M. Mehrdad, A. Maleki, N. Ahmadi, Synthesis, 2004, 1557-1558.
I2-catalyzed oxo-acyloxylation of alkenes and enol ethers with carboxylic acids provides α-acyloxyketones and esters in high yields. This unprecedented regioselective oxidative process employs TBHP and Et3N in stoichiometric amounts under metal-free conditions in DMSO as solvent. α-Acyloxyketones can be converted in situ to monoprotected diol derivatives in excellent yields upon treatment with BH3·SMe2.
R. N. Reddi, P. K. Prasad, A. Sudalai, Org. Lett., 2014, 16, 5674-5677.
DOI: 10.1021/ol5027393
P. K. Patra, K. Nishide, K. Fuji, M. Node, Synthesis, 2004, 1003-1006.
P. K. Patra, K. Nishide, K. Fuji, M. Node, Synthesis, 2004, 1003-1006.
The use of an oxazaborolidine-BH3 complex enables a simple and efficient copper-catalyzed selective transfer hydrogenation of nitriles to primary amine-boranes and secondary amines in good yields. The selectivity control was achieved under mild conditions by switching the solvent and the copper catalysts.
H. Song, Y. Xiao, Z. Zhang, W. Xiong, R. Wang, L. Guo, T. Zhou, J. Org. Chem., 2022, 87, 790-800.
1,2,3-triazole-bound cationic Au(I) catalysts possess much better thermal stability than literature-reported Au catalysts, including IPrAu•NTf2. By application of these catalysts, challenging intermolecular hydroaminations were achieved with less-reactive internal alkynes and unprotected aliphatic amines, giving excellent yields with low catalyst loading.
H. Duan, S. Sengupta, J. L. Petersen, N. G. Akhmedov, X. Shi, J. Am. Chem. Soc., 2009, 131, 12100-12102.
DOI: 10.1021/ja9041093
A simple and efficient method converts aldehydes into α,β-unsaturated aldehydes with a two-carbon homologation. Hydroboration of ethoxy acetylene with BH3 • SMe2 generates tris(ethoxyvinyl) borane. Transmetalation with diethylzinc, addition to aldehydes or ketones, and acidic workup affords enals or protected derivatives depending on the workup procedure.
P. Valenta, N. A. Drucker, J. W. Bode, P. J. Walsh, Org. Lett., 2009, 11, 2117-2119.
DOI: 10.1021/ol9005757
The selective aldimine cross-coupling represents a simple and flexible method for the synthesis of highly substituted unsymmetrical 1,2-diamines. In addition, either the syn- or anti-configurated vicinal diamine can be obtained, depending on the choice of the workup and reduction conditions.
C. Kison, N. Meyer, T. Opatz, Angew. Chem. Int. Ed., 2005, 44, 5662-5664.
Exposure of (functionalized) aryl chlorides to catalytic quantities of nickel-on-charcoal in the presence of stoichiometric amounts of Me2NH · BH3/K2CO3 in refluxing acetonitrile leads to high yields of the dehalogenated arenes.
B. H. Lipshutz, T. Tomioka, K. Sato, Synlett, 2001, 970-973.
DOI: 10.1055/s-2001-14636
Nickel-on-graphite is a very inexpensive, heterogeneous catalyst for the chemoselective reduction of aryl tosylates and mesylates. The catalyst can be used under conventional heating conditions or microwave irradiation and is recyclable without loss of activity.
B. H. Lipshutz, B. A. Frieman, T. Butler, V. Kogan, Angew. Chem. Int. Ed., 2006, 45, 800-803.
Borane-tetrahydrofuran complex or borane-methyl sulfide complex is used to generate triacyloxyboranes, which can be effectively reacted with various nucleophiles (alkylamines, arylamines, hydrazides, alcohols, phenols) at reflux in toluene to provide the corresponding amides and esters in excellent yield.
Z. Huang, J. R. Reilly, R. N. Buckle, Synlett, 2007, 1026-1030.
Z. Huang, J. R. Reilly, R. N. Buckle, Synlett, 2007, 1026-1030.
A highly stereoselective asymmetric 1,4-addition of diarylphosphines to α,β-unsaturated aldehydes catalyzed by a bis(phosphine) pincer-Pd complex enables the synthesis of chiral phosphines with excellent stereoselectivity under mild conditions.
Y.-R. Chen, W.-L. Duan, Org. Lett., 2011, 13, 5824-5826.
DOI: 10.1021/ol2024339
Palladium and Raney nickel catalyze the methanolysis of strongly complexed borane-amine adducts. The method is applicable to primary, secondary, tertiary, and aromatic amines, and the mildness of the reaction conditions allows preservation of otherwise labile functional groups.
M. Couturier, J. L. Tucker, B. M. Andresen, P. Dube, J. T. Negri, Org. Lett., 2001, 3, 465-467.
DOI: 10.1021/ol006969+
Quoted from: https://www.organic-chemistry.org/chemicals/reductions/boranes.shtm
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