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
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The (RS,SR)/(RR,SS) diastereomers of [(5-C5H5)Re(NO)(PPh3)(H2CCHR)]+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 95100 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 C) with k(H)/k(=CHDE) = 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 -allyl complexes (5-C5H5)Re(NO)(PPh3)(CH2CHCHR). However, rate data suggest that conducted tour mechanisms involving transient binding to RCC substituents are unlikely. These results are best accommodated by a mechanism in which the rhenium moves through the nodal plane of the alkene via a carbon-hydrogen bond complex involving HE and/or a vinyl hydride oxidative addition product. 1992, American Chemical Society. All rights reserved.