Physical organic chemistry of transition metal carbene complexes .10. Opposing effects of alpha-alkyl groups on the thermodynamic and kinetic acidities of (CO)(5)Cr=C(OMe)CHR'R''-type Fischer carbene complexes in aqueous acetonitrile. Analogy to the nitroalkane anomaly

The thermodynamic acidities of (CO)(5)Cr=C(OMe)CH2CH3 (3-Et), (CO)(5)Cr=C(OMe)CH2CH2CH2CH3 (3-Bu), and (CO)(5)Cr=C(OMe)CH(CH3)(2) (3-Pr) have been determined in 50% acetonitrile-50% water (v/v) at 25 degrees C; after applying statistical corrections, the pK(a) values are 12.62 for 3-Et, 12.84 for 3-Bu, and 12.27 for 3-Pr. These pK(a) values are all lower than pK(a) = 12.98 for (CO)(5)Cr=C(OMe)CH3 (3-Me) determined previously and give an acidity order of 3-Me < 3-Bu < 3-Et < 3-Pr. if it is assumed that the main resonance structure of the conjugate anions of the carbene complexes has the negative charge delocalized onto the (CO)(5)Cr moiety (e.g., 3-Et-: (CO)(5) (C) over bar rC(OMe)=CHCH3), the increased acidity with increasing alkyl substitution on the ct-carbon can be understood as reflecting the well-known stabilization of alkenes by alkyl groups. Rate constants for the deprotonation of 3-Et, 3-Bu, and 3-Pr by OH- and by piperidine are also reported and compared with the corresponding rate constants for 3-Me obtained previously. They follow the order 3-Me > 3-Et > 3-Bu > 3-Pr, essentially opposite to the order of the thermodynamic acidities. This leads to negative Bronsted a-values which are reminiscent of the nitroalkane anomaly. The observed kinetic order is the result of a lowering of the intrinsic rate constant by alkyl substitution. Four factors are identified that contribute to this lowering. (1) The disproportionately weak development of the C=C double bond at the transition state which prevents the latter from significantly benefiting from the alkene-stabilizing effect of the alkyl groups. (2) The disproportionately large negative charge on the a-carbon at the transition state that leads to a disproportionately strong destabilization of the transition state by the inductive/field effect of the alkyl groups relative to that of the product anion. (3) The destabilizing field effect of the alkyl groups on the partial negative charge on the hydroxide ion at the transition state. (4) Steric crowding at the transition state. Kinetic data on the hydrolysis of 3-Et, 3-Bu, and 3-Pr over a wide pH range are also reported. They support a previously proposed mechanism that involves rate limiting protonation of the conjugate anion of the carbene complex that is concerted with cleavage of the bond between the carbene carbon and the metal. With 3-Pr the hydrolysis is subject to catalysis by light.