Configurational Equilibria in Amido and Lithioamido Complexes of Formulas (η⁵-C₅H₅)Re(NO)(PAr₃)(NHCHRR‘) and (η⁵-C₅H₅)Re(NO)(PAr₃)(NLiR‘‘): Epimerization Occurs at Rhenium via Phosphine Dissociation

Abstract

The diastereomerically and enantiomerically pure amido complex (SR)-(η5-C5H5)Re(NO)(PPh3)(N̈HCH(CH3)Ph) ((SR)-5) converts to (RR)-5 (inversion at rhenium, retention at carbon) in THF-d8 at 49.4 °C with k1 = 2.34 × 10-4 s-1 and k-1 = 0.90 × 10-4 s-1. Similarly, (SS)-5 converts to (RS)-5 with k1 = 0.90 × 10-4 s-1 and k-1 = 2.30 × 10-4 s-1. Both epimerizations give equilibrium ratios (RR/SR or SS/RS) of 70:30. Reactions with HOTf yield [(η5-C5H5)Re(NO)(PPh3)(NH2CH(CH3)Ph)]+TfO-, and subsequent additions of Et4N+CN- afford (η5-C5H5)Re(NO)(PPh3)(CN) and NH2CH(CH3)Ph (all steps with retention at rhenium and carbon). Enantiomeric purities and absolute configurations are assayed by chiral NMR shift reagents and (−)-menthyl chloroformate derivatives, respectively, establishing configurations of epimerized 5. Reaction of (SR)-5 and P(p-tol)3 in THF-d8 at 49.4 °C gives (η5-C5H5)Re(NO)(P(p-tol)3)(N̈HCH(CH3)Ph) (50:50 (t0) → 66:34 (t∞) RR/SR) with k = 4.6 × 10-4 s-1, twice that for the conversion of (SR)-5 to (RR)-5. Rate data for the latter at 32.3−59.1 °C give ΔH⧧ = 26 kcal/mol and ΔS⧧ = 6 eu. These results are best modeled by mechanisms involving initial and rate determining PPh3 dissociation, with anchimeric assistance by the amido lone pair, to give an intermediate that is trigonal planar at rhenium and combines with PAr3 without significant diastereoselectivity. Reactions of n-BuLi with 5 and related complexes give N̈LiR species from which PPh3 is lost at lower temperatures, and are presumed to be much less configurationally stable.