Stereospecific Syntheses, Metal Configurational Stabilities, and Conformational Analyses of meso-(R,S)- and (R,R)-(η5-C5R5)Ti(CH3)2-N,N‘-bis(1-phenylethyl)acetamidinates for R = H and Me

Abstract

Insertion of optically pure (R,R)- and meso-(R,S)-1,3-bis(1-phenylethyl)carbodiimide into a Ti−CMe bond of (η5-C5R5)TiMe3 provides the title compounds (R,R)-3 (R = H), (R,R)-5 (R = Me), (R,S)-4 (R = H), and (R,S)-6 (R = Me), in high yield. Reaction between (R)-1-tert-butyl, 3-(1-phenylethyl)carbodiimide, and (η5-C5H5)TiMe3 yields compound 7 in a similar fashion. Variable-temperature 1H NMR studies unequivocally establish that a low-energy amidinate ring twisting pathway is the exclusive origin of configurational instability in this class of piano-stool complexes. Further, evidence for a dynamic process involving amidinate ring conformation is obtained for (R,S)-6 by a similar study. A conformational analysis of the 1-phenylethyl substituents observed in the solid-state structures of (R,R)-3, (R,R)-5, exo-(R,S)-4, and exo-(R,S)-6, as determined by single-crystal X-ray analysis, provides a rationale for the apparent preferred 1-phenylethyl conformations of these compounds and of the unprecedented amidinate ring conformation (interplane angle of 41.5°) observed for the exo isomer of the meso complex 6.