Intermediates in the decomposition of aliphatic diazo-compounds. Part VIII. The mechanism of ether formation from diarylmethylenes and alcohols

Abstract

The thermal decomposition of diphenyldiazomethane and its 4,4′-dichloro-, 4,4′-dimethoxy-, and 4,4′-dimethyl analogues at 85 °C in acetonitrile containing methyl and t-butyl alcohols (ca. 1M) has been studied. Kinetic studies indicate that the major reaction product, the alkyl diarylmethyl ether, is formed by attack of intermediate diarylmethylene on the alcohol. The relative reactivities of methyl and t-butyl alcohols towards the series of diarylmethylenes have been determined: methyl alcohol is the more reactive in all cases, but the difference in reactivity between the two alcohols decreases with increasing electron release by the 4-substituents. By examining the radioactivity of the product ethers formed by reaction of diarylmethylenes with hydroxyl-tritiated alcohols, substantial kinetic isotope effects on ether formation have been detected. The isotope effects are in all cases greater for formation of t-butyl ethers than for methyl ethers. The compatibility of these observations with three possible mechanisms of ether formation has been assessed, and it is concluded that the most likely route is that in which the carbene attacks the oxygen atom of the alcohol to form an ylid which is subsequently transformed by a prototropic rearrangement into the ether.