Additions of carbon nucleophiles to acyclic imine complexes of the chiral rhenium Lewis acid [(eta(5)-C5H5)Re(NO)(PPh3)](+): controlling factors in 1,3-asymmetric induction and syntheses of non-racemic organic amines
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Reactions of the racemic or enantiomerically pure benzaldehyde-derived imine complex [(5-C5H5)Re(NO)(PPh 3)(1-N-(CH3)=C(H)C6H 5] +TfO- (1+TfO-; 95:5 E/Z N=C isomers) and RLi (THF, -100C) give amido complexes (5-C5H5)Re(NO)-(PPh 3)(N(CH3)CH(R)C6H5) (5) (R=a/CH3, b/CCSi(CH2CH3)3, c/CH2Si(CH3)3) in quantitative NMR yields as 74-72:26-28 mixtures of Re,C configurational diastereomers. These labile adducts and TfOH react to give amine complexes [(5-C5H5)Re(NO)(PPh 3)-(NH(CH3)CH(R)C6H5)] +TfO- (6+TfO-). Reaction of 6a+TfO- (prepared from (R)-1+TfO-) and Et4N+CN- yields the cyanide complex (R)-(5-C5H5)Re(NO)(PPh3)(CN) (85%, >98% ee) and amine (R)-NH(CH3)CH(CH3)C6H5 (81%, 46% ee). Reactions of 1+TfO- and CN- give similar amido complexes (85-84:15-16 Re,C diastereomers), but appear reversible, with additions of CH3OTf affording 1+TfO-. Reactions of the acetophenone-derived complex [(5-C5H5)Re(NO)(PPh 3)(1-N(CH3)C(CH3)C 6H5)] +TfO- (4+TfO-) with C6H5CH2MgCl and (CH3)3SiCH2Li give 41 and >99% deprotonation to the enamido complex (5-C5H5)Re-(NO)(PPh 3)(N(CH3)C(C6H5)=CH 2). Some addition occurs with C6H5CH2MgCl, and CN-. NMR spectra (-100C) show Re-N= rotamers of 1,4+TfO-, and mechanisms of 1,3-asymmetric induction are analyzed in detail. 1998 Elsevier Science S.A.
