Boranes (Dimethylsulfide Borane,
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:
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.
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.
A mild and efficient one-pot reductive amination of aldehydes and ketones with amines using α-picoline-borane as a reducing agent in the presence of small amounts of AcOH is described. The reaction has been carried out in MeOH, in H2O, and in neat conditions. This is the first successful reductive amination in water and in neat conditions.
S. Sato, T. Sakamoto, E. Miyazawa, Y. Kikugawa, Tetrahedron, 2004, 60, 7899-7906.
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.
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.