MECHANISM OF EQUILIBRATION OF DIASTEREOMERIC CHIRAL RHENIUM ALKENE COMPLEXES OF THE FORMULA [(ETA-5-C5H5)RE(NO)(PPH3)(H2C=CHR)]+BF4- - THE METAL TRAVERSES BETWEEN ALKENE ENANTIOFACES WITHOUT DISSOCIATION

The (RS,SR)/(RR,SS) diastereomers of [(eta-5-C5H5)Re(NO)(PPh3)(H2C = CHR)]+BF4- (1: R = (a) CH3, (b) CH2CH2CH3, (c) CH2C6H5, (d) C6H5, (e) CH(CH3)2, (g) Si(CH3)3) differ in the alkene enantioface bound to rhenium, and interconvert in chlorocarbons at 95-100-degrees-C. Isomerization is nondissociative (no incorporation of deuterated alkenes or PPh3) and occurs with retention of configuration at rhenium and without scrambling of E/Z deuterium labels. The latter excludes mechanisms that involve intermediate carbocations ReCH2CHR+ and alkylidene complexes, and nucleophilic addition to the alkene. The isomerization of (RR,SS)-1d to (RS,SR)-1d proceeds (96.5-degrees-C) with k(H)/k(= CHD(E)) = 1.64, k(H)/k(= CHDz) = 1.07, and k(H)/k(= CDC6H5) = 1.15. Triethylamine promotes the isomerization of substrates that bear allylic protons via sigma-allyl complexes (eta-5-C5H5)Re(NO)(PPh3)(CH2CH = CHR'). However, rate data suggest that "conducted tour" mechanisms involving transient binding to RC = C substituents are unlikely. These results are best accommodated by a mechanism in which the rhenium moves through the pi-nodal plane of the alkene via a carbon-hydrogen "sigma-bond complex" involving H(E) and/or a vinyl hydride oxidative addition product.