Competitive 1,2- and 1,3-hydride shifts and the possible role of protonated and methylated cyclopropane intermediates in alkyl group rearrangements accompanying the thermal decomposition of saturated alkyl chloroformates in the liquid phase

Abstract

Deuterium labelling shows that the thermal decomposition of ethyl chloroformate in the liquid phase yields chloroethane without rearrangement of the ethyl group. Longer chain primary alkyl chloroformates yield rearrangement products which can be rationalised on the basis of two initial competitive pathways, one involving a 1,2-hydride shift and the other the formation of a corner-protonated cyclopropane intermediate. The latter may be captured to yield rearranged primary alkyl chloride (with considerable proton scrambling in the n-propyl case) or give rise to a 1,3-hydride shift in higher alkyl groups (Buⁿ, n-pentyl, 3-methylbutyl, n-octyl) where rearrangement to a s- or t-alkyl cation is possible. Successive 1,2-hydride shifts are of minor significance unless the second of these leads to a t-alkyl cation. The 3,3-dimethylbutyl group affords products derived from successive 1,2-shifts of hydrogen and methyl and also a 1,2-t-butyl shift which is thought to involve the intermediacy of a methylated cyclopropane. s-Alkyl cations derived directly from s-alkyl chloroformates show less tendency to rearrangement than those derived by rearrangement from primary alkyl chloroformates. Evidence is presented which suggests that alk-1-enes are formed from primary alkyl chloroformates via carbocation intermediates in which rearrangement by 1,2- and/or 1,3-hydride shift has occurred.