Dabbling with air-stable organoaluminum species*
Kallolmay Biswas, Alex Chapron, Thea Cooper, Paul K. Fraser, Andrew Novak, Oscar Prieto, and Simon Woodward
School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
Abstract: Highly stereoselective 1,2-additions of AlMe3 or its air-stable analog DABCO(AlMe3)2 to aldehydes are realized in the presence of a Ni(acac)2-derived catalyst using phosphoramidite ligands giving sec-alcohols in up to 95 % ee. Very high turnover number (TON) (>1500) and turnover frequency (TOF) (>350 h-1) values can be realized in these reactions. The substrate range, trials of various (DABCO)a(AlR3)b reagents (a = 0.1; b = 1.2; R = Me, Et, Bui), ligands, and molecular modeling studies are used to propose a working model for the catalytic cycle and the origin of the stereoselectivity. The phosphoramidite ligand is proposed to bind the nickel in an η2 manner via the P-donor and one of the C=C aryl bonds of the CHAr amine group. Preliminary studies indicate that DABCO(AlMe3)2 can also be used as a methyl source in Pd-catalyzed cross-coupling reactions of ArX (X = Br, I) species.
Keywords: 1,2-additions; stereoselective; DABCO(AlMe3)2; trimethylaluminum; catalytic asymmetric synthesis; asymmetric synthesis.
*Pure Appl. Chem. 78, 197-523. An issue of reviews and research papers based on lectures presented at the 13th IUPAC International Symposium on Organometallic Chemistry Directed Towards Organic Synthesis (OMCOS-13), Geneva, Switzerland, 17-21 July 2005.