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. ), 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. ; Coord. Chem. Rev. 1998, 178-180, 665. ). 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. ). 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. ).
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. ).
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. ).
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. ).
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 . ).