Hydrolysis of Fischer carbene complexes. Steric effects of the pi-donor group

Rate constants for the hydrolysis of Fischer carbenes (CO)5Cr=C(OR)Ph (R = n-propyl, neopentyl, isopropyl, and menthyl) in 50% MeCN-50% water (v/v) at 25 degrees C are reported. The rate constants for the addition of -OH to the carbene carbon are 5.3, 3.7, 0.84, and 0.01 M(-1) s(-1), respectively. These rate constants give linear correlations with the corresponding rate constants for the hydrolysis of esters such as acetate, benzoate, and formiate. The slopes of the plots of the observed rate constants for the carbenes vs the rate constants for the esters are 1.4 and 1.2 for acetate and benzoate, respectively, indicating that the factors that decrease the reactivity of the two types of compounds are similar, but the carbenes show higher sensitivity. The rate constants are well correlated with several steric parameters giving a value of -3.84 for the Charton's psi parameter. The results show that the steric bulkiness of the R group is the main factor determining the reactivity differences for these carbene complexes.