l+-Phenylalky1
)-1-tifluoromethylbenzyl cations and the a-CH,-substituted analogues were generated from the corresponding protonated benzyl alcohols by loss of H,O during chemical ionization (CI). The unimolecular reactions of these ions and of their derivatives specifically deuterated at the 1-phenyl group were studied using the CI (methane) and CI (isobutane) mass spectra and the mass-analysed ion kinetic energy spectra. The characteristic reaction giving rise to abundant ions in all the spectra is the elimination of C,H, initiated by an electrophilic attack of the benzyl cation on the remote phenyl group. The u-CH,-and a-CF,-substituted benzyl cations respond differently to the effect of internal excess energy on the intensity of the benzene elimination process and on the H-D exchange accompanying this process of the deuterated ions. This different reactivity of both series of ions demonstrates unambiguously that the a-CF,-substituted benzyl cations correspond to destabilized benzyl cations exhibiting the particular reactivity of 'hot' carbenium ions in the gas phase.
I N T R O D U C T I O N
Carbenium ions are important intermediates in many organic reactions, and their stability has a dominating effect on the ease and course of the reaction and its potential in organic synthesis. The stability of carbenium ions is determined by the substituents at the positively charged carbon atom, and in most cases electron donors are used to stabilize the carbenium ion and to facilitate its formation during the progress of the reaction. In fact, the detection of a stable triphenylmethyl cation by Gomberg and Walden in 1902' was the beginning of the still fascinating carbocation chemistry.2 However, it is now well known that there also exists a class of carbenium ions in solution3 and in the gas phase4 with an electron-withdrawing substituent attached to the carbenium centre. Typically, this electron-withdrawing substituent is an acyl, a nitro or a trifluoro group. The highly electron deficient centre of these destabilized carbenium ions creates a high reactivity, making these ions synthetically very effective intermediates.
Recently, it has been shown that certain n-aryl-1,1,1 -trifluoro-2-alkyl triflates and 2,n-diaryl-1,1,1trifluoroalkan-2-01s can be conveniently used to synthesize the corresponding trifluoromethyl-substituted benzocycloalkanes by a cyclization during solvolysis in CF,COOH.' This cyclization occurs either by nucleophilic assistance of the o-aryl group to the developing t Dedicated to Professor Dr H. Budzikiewicz on the occasion of his 60th birthday.