MECHANISM OF COUPLING OF =CH2 TO H2C=CH2 AT A HOMOGENEOUS (ETA-C5H5)RE(NO)(PPH3)+ CENTER - REMARKABLE ENANTIOMER SELF-RECOGNITION
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The methylidene complex [(-C5H5)Re(NO)(PPH3)(=CH2)]+ PF6- (1) couples to the ethylene complex [(-C5H5)Re(NO)(PPH3)(H2C=CH2)]+ PF6- (2; ca. 50%) at 273-308 K in CH2Cl2. The byproduct [(-C5H5)Re(NO)(PPH3)2] Pr6- (3; ca. 25%) or, in the presence of CH3CN, [(-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 H = 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)-[(-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 [-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.
