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 [(η5-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 °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 σ-allyl complexes (η5-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 π nodal plane of the alkene via a carbon-hydrogen “σ bond complex” involving HE and/or a vinyl hydride oxidative addition product. © 1992, American Chemical Society. All rights reserved.
author list (cited authors)
Peng, T. S., & Gladysz, J. A.