MECHANISM OF BENZENE LOSS FROM TP'RH(H)(PH)(CN-NEOPENTYL) IN THE PRESENCE OF NEOPENTYL ISOCYANIDE - EVIDENCE FOR AN ASSOCIATIVELY INDUCED REDUCTIVE ELIMINATION

Thermolysis of Tp'Rh(H)(Ph)(CN-neopentyl) (1) (Tp' = hydrotris(3,5-dimethylpyrazolyl)borate) in benzene in the presence of added isocyanide gives Tp'Rh(CN-neopentyl)2 (2). The rate of reaction increases with increasing isocyanide concentration, but the dependence is nonlinear and approaches zero order at high isocyanide concentrations. When the reaction is conducted with Tp'Rh(D)(d5-Ph)(CN-neopentyl) in benzene-d6, an inverse kinetic isotope effect of 0.85 +/- 0.02 is observed. Measurement of the temperature dependence of the rate of the reaction at a single isocyanide concentration gives DELTA-H double dragger = 15.2 +/- 1.2 kcal/mol and DELTA-S double-dagger = -36 +/- 3 eu. Thermolysis of 1 in the presence of added 2,6-xyly] isocyanide shows no evidence for the formation of TpRh(H)(Ph)(2,6-xylyl isocyanide) (3). Thermolysis of isotopically labeled Tp'Rh(H)(d5-Ph)(CN-neopentyl) at 60-degrees-C in either THF-d8 or toluene-d8 results in intramolecular migration of the metal hydride into all of the phenyl proton positions at the same time and rate. A mechanism is proposed to account for all of the above observations which involves initial reductive elimination of benzene from 1 to give an eta-2-benzene intermediate in which the rhodium atom can migrate around the benzene ring. Subsequent associative exchange of isocyanide for benzene occurs in a second step.