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.
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.
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.
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.
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.
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., 2005, 117, 5807-5809.
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.
M. Couturier, J. L. Tucker, B. M. Andresen, P. Dube, J. T. Negri, Org. Lett., 2001, 3, 465-467.
