67301-11-5Relevant articles and documents
Stereochemical aspects in the asymmetric Michael addition of chiral imines to substituted electrophilic alkenes
Cave, Christian,Desmaele, Didier,D'Angelo, Jean,Riche, Claude,Chiaroni, Angele
, p. 4361 - 4368 (2007/10/03)
The Michael-type addition of chiral imines, derived from racemic α-substituted cyclanones and optically active 1-phenylethylamine, to electrophilic alkenes, in neutral conditions, constitutes one of the most efficient methods for the stereocontrolled construction of quaternary carbon centers. In order to create an additional stereogenic center at the α- or β-position to the quaternary one, the behavior of a variety of α- and β-substituted alkenyl acceptors was examined. In general, these additions are highly regioselective, the alkylation taking place predominantly, if not exclusively, at the more substituted α-side of the imine function; however, in some cases (electrophilic alkenes 28 and 49), significant amounts (10-15%) of regioisomeric adducts were obtained. With the exception of methyl propiolate 52, a remarkable control of the absolute configuration of the adducts were always observed with these Michael acceptors. According to the general rule we have previously proposed, the alkylation process takes place preferentially on the less hindered π-face of the more substituted secondary enamine, in tautomeric equilibrium with the starting imine. An excellent diastereocontrol was always obtained by using the present α- and β-substituted alkenes. These stereochemical outcomes can be interpreted by invoking that the reaction proceeds through a compact approach of the reactants, the hydrogen atom at the nitrogen center of the enamine being transferred to the α-vinylic carbon atom of the acceptor, concertedly with the creation of the C-C bond. In this respect the "endo-approach" 58, in which the electron-withdrawing group of the acceptor faced to the nitrogen atom of the enamine (case of acceptors 10, methyl methacrylate, 24, 28, 43, 47, and 49) largely prevails over the "exo-approach" 59 (case of acceptor 38). This predominant "endo-preference" can be reasonably interpreted in terms of a cooperative effect between steric and stereoelectronic factors.
