Additions of carbon nucleophiles to acyclic imine complexes of the chiral rhenium Lewis acid [(η5-C5H5)Re(NO)(PPh3)] +: Controlling factors in 1,3-asymmetric induction and syntheses of non-racemic organic amines

Article abstract of DOI:10.1016/S0020-1693(97)05787-3

Reactions of the racemic or enantiomerically pure benzaldehyde-derived imine complex [(η⁵-C₅H₅)Re(NO)(PPh ₃)(η¹-N-(CH₃)=C(H)C₆H ₅] ⁺TfO^- (1⁺TfO^-; 95:5 E/Z N=C isomers) and RLi (THF, -100°C) give amido complexes (η⁵-C₅H₅)Re(NO)-(PPh ₃)(N(CH₃)CH(R)C₆H₅) (5) (R=a/CH₃, b/C≡CSi(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 [(η⁵-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)-(η⁵-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 [(η⁵-C₅H₅)Re(NO)(PPh ₃)(η¹-N(CH₃)C(CH₃)C ₆H₅)] ⁺TfO^- (4⁺TfO^-) with C₆H₅CH₂MgCl and (CH₃)₃SiCH₂Li give 41 and >99% deprotonation to the enamido complex (η⁵-C₅H₅)Re-(NO)(PPh ₃)(N(CH₃)C(C₆H₅)=CH ₂). Some addition occurs with C₆H₅CH₂MgCl, and CN^-. NMR spectra (-100°C) show Re-N= rotamers of 1,4⁺TfO^-, and mechanisms of 1,3-asymmetric induction are analyzed in detail.