Disilenes with phenols

Kinetic isotope effect measurements support this interpretation. A small kinetic isotope effect (kR/kD = 0.71) is observed for p-methoxyphenol in agreement with a ratedetermining nucleophilic attack, while a large kinetic isotope effect (kR/kR = 5.27), observed for p-trifluoromethylphenol, strongly supports a mechanism in which a phenolic H (or D) is transferred to 39 in the rate-determining step. Unfortunately, 39 is a symmetric disilene so that diastereoselectivity could not be determined. It will be interesting to examine whether the diastereoselectivity will be effected by the change in the addition mechanism from electrophilic to nucleophilic. 

Absolute kinetic data have been reported for four of the characteristic bimolecular reactions of disilenes 1,2-addition of alcohols and phenols (equation 72), [2 + 2]-cycloaddition of ketones (equation 73), [2 +4]-cycloaddition of aliphatic dienes (equation 74) and oxidation with molecular oxygen (equation 75). As with silenes, the addition of alcohols has been studied in greatest detail. 

Kinetic data for other characteristic bimolecular reactions of disilenes are much more limited than is the case with alcohol and phenol additions, and hence contribute little to the understanding that product studies have already provided in regards to reaction mechansims. The only absolute kinetic data known at the present time, for reaction of disilenes 103, 104 and 35 with 2,3-dimethyl-1,3-butadiene, oxygen and a few symmetric n-alkanones in hydrocarbon solution at room temperature, are listed in Table 19. Unfortunately, none of these reactions has been specifically characterized with product studies, as far as we know. The data indicate that the reactivity of relatively nonpolar disilenes toward these reagents decreases in the order ko2 > A r2c=o 1 EtOH >... 

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