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

The diastereomerically and enantiomerically pure amido complex ( SR )-(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(N̈HCH(CH 3 )Ph) (( SR )- 5 ) converts to ( RR )- 5 (inversion at rhenium, retention at carbon) in THF- d 8 at 49.4 °C with k 1 = 2.34 × 10 - 4 s - 1 and k - 1 = 0.90 × 10 - 4 s - 1 . Similarly, ( SS )- 5 converts to ( RS )- 5 with k 1 = 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 [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(NH 2 CH(CH 3 )Ph)] + TfO -, and subsequent additions of Et 4 N + CN - afford (η 5 -C 5 H 5 )Re(NO)(PPh 3 )(CN) and NH 2 CH(CH 3 )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- d 8 at 49.4 °C gives (η 5 -C 5 H 5 )Re(NO)(P( p -tol) 3 )(N̈HCH(CH 3 )Ph) (50:50 ( t 0 ) → 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 Δ H ⧧ = 26 kcal/mol and Δ S ⧧ = 6 eu. These results are best modeled by mechanisms involving initial and rate determining PPh 3 dissociation, with anchimeric assistance by the amido lone pair, to give an intermediate that is trigonal planar at rhenium and combines with PAr 3 without significant diastereoselectivity. Reactions of n -BuLi with 5 and related complexes give N̈LiR species from which PPh 3 is lost at lower temperatures, and are presumed to be much less configurationally stable.