Mechanism of coupling of methylidene to ethylene at a homogeneous (triphenylphosphine)nitrosyl(.eta.-cyclopentadienyl)rhenium(+) [(.eta.-C5H5)Re(NO)(PPh3)]+ center. Remarkable enantiomer self-recognition
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The methylidene complex [(η-C5H5)Re(NO)(PPH3)(=CH2)]+ PF6- (1) couples to the ethylene complex [(η-C5H5)Re(NO)(PPH3)(H2C=CH2)]+ PF6- (2; ca. 50%) at 273-308 K in CH2Cl2. The byproduct [(η-C5H5)Re(NO)(PPH3)2] Pr6- (3; ca. 25%) or, in the presence of CH3CN, [(η-C5H5)Re(NO)(PPH3)(NCCH3)]+PF6- (5; ca. 50%) also forms. The rate of coupling is second order in 1 and not affected by the presence of 5-10 equiv of RCN. Data collected from 273 to 308 K give ΔH‡ = 9.8 ± 0.6 kcal/mol and ‡S‡ = -33.8 ± 1.0 eu. At 298 K, [formula-ommited] = 0.39 ± 0.03. Surprisingly, optically pure 1 couples 2.3 times faster than racemic 1. Crossover experiments show that no PPH3 dissociation or intermolecular =CH2 scrambling occurs prior to the rate-determining coupling step, and that the RR and SS transition states are greatly preferred over the RS transition state (enantiomer self-recognition). Experiments with optically pure 1 show that 2 is formed with >98% retention at rhenium. An X-ray crystal structure of (+)-(SS)-[(η-C5H5)Re(NO)(PPH3)(NCCH(C6H5)CH2CH3)]+PF6-((+)-(SS)-6) shows that RCN adducts also form with retention. These data are interpreted as evidence for the rate-determining formation of initial ReCH2ReCH2 (7a) or ReCH2CH2Re (7b) intermediates. Subsequent rapid conversion to primary monomeric products 2 and [η-C5H5)Re(NO)(PPH3)(S)]+PF6- (8; S = CH2Cl2 or vacant coordination site) is proposed. Comparisons are made to other homogeneous and heterogeneous =CH2 coupling reactions. © 1983, American Chemical Society. All rights reserved.
author list (cited authors)
Merrifield, J. H., Lin, G. Y., Kiel, W. A., & Gladysz, J. A.