Boranes (Dimethylsulfide Borane,
Borane-Tetrahydrofuran Complex)
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:
+ B(OMe)3 + 3 H2 |
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.
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.
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.
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.
Solid amidophosphine boranes were synthesized to replace more commonly used
borane reagents. These compounds demonstrated excellent reactivity and functional
group tolerance toward a wide variety of nitriles, alkynes, and carboxylic
acids, affording the corresponding ammonium salts, alkenes, and alcohols in good
yield.
R. Kumar, R. K. Meher, J. Sharma, A. Sau, T. K. Panda, Org. Lett., 2023, 25,
7923-7927.
Solid amidophosphine boranes were synthesized to replace more commonly used
borane reagents. These compounds demonstrated excellent reactivity and functional
group tolerance toward a wide variety of nitriles, alkynes, and carboxylic
acids, affording the corresponding ammonium salts, alkenes, and alcohols in good
yield.
R. Kumar, R. K. Meher, J. Sharma, A. Sau, T. K. Panda, Org. Lett., 2023, 25,
7923-7927.
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.
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.
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.
Solid amidophosphine boranes were synthesized to replace more commonly used
borane reagents. These compounds demonstrated excellent reactivity and functional
group tolerance toward a wide variety of nitriles, alkynes, and carboxylic
acids, affording the corresponding ammonium salts, alkenes, and alcohols in good
yield.
R. Kumar, R. K. Meher, J. Sharma, A. Sau, T. K. Panda, Org. Lett., 2023, 25,
7923-7927.
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.
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.
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.
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.
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.
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.
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.