REACTIONS OF RHENIUM CYCLOPENTADIENYL AND INDENYL DICHLOROMETHANE COMPLEXES OF THE FORMULA [(ETA(5)-CXHY)RE(NO)(PPH3)(CLCH2CL)]+BF4- WITH KETONES - SUBSTITUTION RATES AND MECHANISM
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abstract
Rates of reaction of the coordinatively saturated cyclopentadienyl and indenyl dichloromethane complexes [formula omitted] and cyclohexanone to give [formula omitted] are measured by 31P{1H} NMR under pseudo-first-order conditions (CH2C12, 70 to 30 C, 1060-fold excesses of cyclohexanone). These substitutions are first-order in both 1/2 and cyclohexanone, even at cyclohexanone concentrations of 1.03.4 M. Eyring plots of the second-order rate constants give H* of 15 1 kcal/mol and S* of 5 45 eu. There is only a modest kinetic indenyl ligand effect (factor of 6 at 60 to 50 C). Thus, CxHy ring slippage is viewed as unlikely during substitution. Faster reactions of 1 with tropone (H* 12 2 kcal/mol, AS* 17 11 eu) and methyl ethyl sulfide give similar rate data. These results and competition experiments with ethyl chloride exclude pathways involving initial dichloromethane dissociation. An associative mechanism that features a square pyramidal intermediate with a bent nitrosyl ligand is considered. Testable aspects of this model and stereochemical implications are discussed. 1993, American Chemical Society. All rights reserved.
