Configurational Equilibria in Amido and Lithioamido Complexes of Formulas (η<SUP>5</SUP>-C<INF>5</INF>H<INF>5</INF>)Re(NO)(PAr<INF>3</INF>)(&Numl;HCHRR‘) and (η<SUP>5</SUP>-C<INF>5</INF>H<INF>5</INF>)Re(NO)(PAr<INF>3</INF>)(&Numl;LiR‘‘):  Epimerization Occurs at Rhenium via Phosphine Dissociation

The diastereomerically and enantiomerically pure amido complex (SR)-(η⁵-C<INF>5</INF>H<INF>5</INF>)Re(NO)(PPh<INF>3</INF>)(&Numl;HCH(CH<INF>3</INF>)Ph) ((SR)-<BO>5</BO>) converts to (RR)-<BO>5</BO> (inversion at rhenium, retention at carbon) in THF-d<INF>8</INF> at 49.4 °C with k<INF>1</INF> = 2.34 × 10^-4 s^-1 and k<INF>-</INF><INF>1</INF> = 0.90 × 10^-4 s^-1. Similarly, (SS)-<BO>5</BO> converts to (RS)-<BO>5</BO> with k<INF>1</INF> = 0.90 × 10^-4 s^-1 and k<INF>-</INF><INF>1</INF> = 2.30 × 10^-4 s^-1. Both epimerizations give equilibrium ratios (RR/SR or SS/RS) of 70:30. Reactions with HOTf yield [(η⁵-C<INF>5</INF>H<INF>5</INF>)Re(NO)(PPh<INF>3</INF>)(NH<INF>2</INF>CH(CH<INF>3</INF>)Ph)]⁺TfO^-, and subsequent additions of Et<INF>4</INF>N⁺CN^- afford (η⁵-C<INF>5</INF>H<INF>5</INF>)Re(NO)(PPh<INF>3</INF>)(CN) and NH<INF>2</INF>CH(CH<INF>3</INF>)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 <BO>5</BO>. Reaction of (SR)-<BO>5</BO> and P(p-tol)<INF>3</INF> in THF-d<INF>8</INF> at 49.4 °C gives (η⁵-C<INF>5</INF>H<INF>5</INF>)Re(NO)(P(p-tol)<INF>3</INF>)(&Numl;HCH(CH<INF>3</INF>)Ph) (50:50 (t<INF>0</INF>) → 66:34 (t<INF>∞</INF>) RR/SR) with k = 4.6 × 10^-4 s^-1, twice that for the conversion of (SR)-<BO>5</BO> to (RR)-<BO>5</BO>. Rate data for the latter at 32.3−59.1 °C give ΔH^&thermod; = 26 kcal/mol and ΔS^&thermod; = 6 eu. These results are best modeled by mechanisms involving initial and rate determining PPh<INF>3</INF> dissociation, with anchimeric assistance by the amido lone pair, to give an intermediate that is trigonal planar at rhenium and combines with PAr<INF>3</INF> without significant diastereoselectivity. Reactions of n-BuLi with <BO>5</BO> and related complexes give &Numl;LiR species from which PPh<INF>3</INF> is lost at lower temperatures, and are presumed to be much less configurationally stable.