Borane Complexes in Organic Synthesis

The application of boron reagents in organic synthesis led to Herbert C. Brown (1912-2004) being awarded the Nobel Prize in Chemistry in 1979 (Angew. Chem. Int. Ed. 2005, 44, 1438. DOI: 10.1002/anie.200500286), and since that time this relatively rare element has remained on the cutting-edge of modern synthetic chemistry. This brief survey highlights some interesting chemistry reported recently, in the period from 2004 to present.

1. Phosphine-borane complexes

Phosphine-borane complexes were first reported by Burg and Wagner (J. Am. Chem. Soc. 1953, 75, 3872.) and their broad application in synthesis has recently been reviewed (Synthesis 1998, 1391. DOI: 10.1055/s-1998-2166; Coord. Chem. Rev. 1998, 178-180, 665. DOI: 10.1016/S0010-8545(98)00072-1). Zhou and McNulty now report a simple, general, economical and high-yielding (80-100%) synthesis of phosphine-borane complexes 2 using solid sodium borohydride as the borane source (Tetrahedron Lett. 2004, 45, 407. DOI: 10.1016/j.tetlet.2004.11.106). These phosphine-borane complexes can be used in the synthesis of soluble hindered phosphine ligands and polymer-linked hindered phosphines.

Other phosphine-borane complexes, 3 and 4 for example, can be used in the synthesis of novel, biologically active phosphorus heterocycles using a ring closure enyne metathesis (RCEM) methodology (Chemical Reviews 2004, 104, 1317. DOI: 10.1021/cr020009e).

2. Amine-borane complexes

Kikugawa developed a green (water as solvent and/or solvent-free conditions) one-pot procedure for the reductive amination of aldehydes and ketones using α-picoline-borane 5 (Tetrahedron 2004, 60, 7899. DOI: 10.1016/j.tet.2004.06.045).

3. Other borane complexes

Asymmetric reductions are extensively used in organic chemistry, and borane-mediated methodologies play an important role in these transformations. Based on his own previous work and that of Wills and Buono (see cited references), Chandrashekar developed the new catalyst 6 containing a N–P=O structural framework with a proximal hydroxyl group, for use in the enantioselective reduction (59-96% ee) of prochiral ketones. The mechanism of this reduction is not clearly understood and is currently under study (Tetrahedron Asym. 2004, 15, 47. DOI: 10.1016/j.tetasy.2003.11.002).

Following the work of Itsuno (J. Chem. Soc., Chem. Commun. 1983, 469.) and Corey (J. Am. Chem. Soc. 1987, 109, 5551.), Pilli, Costa and coworkers reported the enantioselective reduction (43-95% ee) of aromatic and aliphatic ketones using the in situ prepared chiral oxazaborolidine catalyst 7 (Tetrahedron Lett. 2005, 46, 495. DOI: 10.1016/j.tetlet.2004.11.052).

Yamamoto and Futatsugi also developed chiral oxazaborolidines 8 which are useful precatalysts for enantioselective Diels-Alder reactions, using a Lewis acid assisted Lewis acid catalyst (LLA) system, where boron is a highly Lewis acidic centre.

The reactions with 8a proceed with good yield (69-95% yield, exo/endo= 73:27 to 77:23) and high enantioselectivity (80-87% ee for the exo adduct and 95-97% ee for the endo adduct), using small amounts of SnCl₂, even in the presence of water and other deactivating impurities. Precatalyst 8b was found to enhance the enantioselectivity of the reaction (up to 99% ee of an endo adduct with exo/endo= 1:99) (Angew. Chem. Int. Ed. 2005, 44, 1484 . DOI: 10.1002/anie.200461319).