ELECTROPHILIC SUBSTITUTION OF METHANE REVISITED

Ab initio molecular orbital theory at the correlated level of MP4SDTQ/6-31G**//MP2/6-31G** has been applied to investigate the reactions of NO+ and NOH2+ with methane. Reinvestigation of the theoretical reaction of CH4 with NO+ showed that the reaction path involves attack on carbon instead of C-H bond insertion in accord with recent studies of Schreiner er al. This is, however, the consequence of the ambident electrophilic nature of NO+ and does not represent a general electrophilic reaction pathway for methane involving its highly energetic rearrangement (via flattening) and attack on the developing nonbonded electron pair as claimed by Schreiner et al. In contrast to NO+, with superelectrophilic NOH2+ methane preferentially reacts via proton transfer to give CH5+ or by hydride abstraction. The results of the reaction of methane with NO+ were compared with such strong electrophiles as D+, CH3+, C2H5+, OH+, and F+, and the consequences on the reaction mechanism are discussed. The studies reaffirm the general pattern of electrophilic substitution of methane with strong electrophiles to proceed via five coordinate carbocations involving 3c-2e C-H bond insertion as previously suggested by Olah.