Structure and isotope effects upon the thermal decomposition of carbamates of highly congested tertiary alcohols

Abstract

Rate constants have been measured for the thermal decomposition in diphenyl ether at 383–433 K of the carbamates of congested tertiary aliphatic alcohols (1)–(4) which are more reactive than any carbamates previously studied. The effects of structural change among (1)–(4) or of variation in substituent at the tertiary carbon atom are inconsistent with a predominantly carbocation type of mechanism. Kinetic deuterium isotope effects (k.i.e.) upon the methyl derivatives are unambiguously primary (2.2–2.6 at 400 K) and clearly indicate extensive C–H fission in the rate-determining step. These results contrast with the previous interpretation of the k.i.e in decompositions of CH₃C₆H₄NHCO₂Bu^t for which rate-determining C–O fission was proposed. Reinterpretation of the earlier data in terms of superimposed primary and secondary isotope effects leads to results in agreement with the new data and with the product isotope effects. Phenyl substituents, however, increase the carbocationic character of the transition state, but this mechanistic change does not cause the reactivity to be higher than that of the congested derivatives there is, therefore no relationship between the isotope effect and the reactivity.