CONFIGURATIONAL EQUILIBRIA IN AMIDO AND LITHIOAMIDO COMPLEXES OF FORMULAS (ETA(5)-C5H5)RE(NO)(PAR3)(NHCHRR') AND (ETA(5)-C5H5)RE(NO)(PAR3)(NLIR'') - EPIMERIZATION OCCURS AT RHENIUM VIA PHOSPHINE DISSOCIATION

The diastereomerically and enantiomerically pure amido complex (SR)-(e ta 5-C5H5)Re(NO)-(PPh(3))(NHCH(CH3)Ph) ((SR)-5) converts to (RR)-5 (in version at rhenium, retention at carbon) in THF-d(8) at 49.4 degrees C with k(1) = 2.34 x 10(-4) s(-1) and k(-1) = 0.90 x 10(-4) s(-1). Simi larly, (SS)-5 converts to (RS)-5 with k(1) = 0.90 x 10(-4) s(-1) and k (-1) = 2.30 x 10(-4) s(-1). Both epimerizations give equilibrium ratio s (RR/SR or SS/RS) of 70:30. Reactions with HOTf yield ta(5)-C5H5)Re(N O)(PPh(3))(NH2CH(CH3)Ph)](+)TfO(-), and subsequent additions of Et(4)N (+)CN(-) afford eta(5)-C5H5>Re(NO)(PPh(3))(CN) and NH2CH(CH3)Ph (all s teps with retention at rhenium and carbon). Enantiomeric purities and absolute configurations are assayed by chiral NMR shift reagents and ( -)-menthyl chloroformate derivatives, respectively, establishing confi gurations of epimerized 5. Reaction of (SR)-5 and P(p-tol)(3) in THF-d (8) at 49.3 degrees C gives (eta(5)-C5H5)Re(NO)(P(p-tol)(3)(NHCH(CH3)P h) (50:50 (t(0)) --> 66:34 (t(infinity)) RR/SR) with k = 3.6 x 10(-4) s(-1), twice that for the conversion of (SR)-5 to (RR)-5. Rate data fo r the latter at 32.3-59.1 degrees C give Delta H-double dagger = 26 kc al/mol and Delta S-double dagger = 6 eu. These results are best modele d by mechanisms involving initial and rate determining PPh(3) dissocia tion, with anchimeric assistance by the amido lone pair, to give an in termediate that is trigonal planar at rhenium and combines with PAr3 w ithout significant diastereoselectivity. Reactions of n-BuLi with 5 an d related complexes give NLiR species from which PPh(3) is lost at low er temperatures, and are presumed to be much less configurationally st able.