GAS-PHASE CATIONIC BENZOYLATION OF AMBIDENT AROMATIC SUBSTRATES STUDIED WITH THE DECAY TECHNIQUE

Gas-phase benzoylation of typical ambident arom. substrates PhR (R = OH, OMe, NH2) was examd. by a combination of the decay technique and of FT ICR mass spectrometry. Labeled phenylium ions, from the decay of multiply tritiated C6H6, reacted with excess CO contg. traces of PhR at 90-650 Torr. Radiochem. gas-liq. chromatog. and HPLC of the tritiated products show 2 competitive reaction channels, i.e., phenylation and benzoylation of the arom. substrates. A sharp kinetic bias of the gaseous phenylium ions was obsd. for the arom. substrates, measured by an apparent kCO/kPh ratio of 0.12, 0.13 and 0.04, resp., in the systems at nearly atm. pressure. Gas-phase benzoylation displays a high intramol. selectivity, occurring exclusively at the n-type center of PhOH and PhNH2. In the case of PhOMe, appreciable ring benzoylation is obsd., characterized by a remarkably high (≤30:1) bias for the para position. The mechanistic features of the gas-phase benzoylation and phenylation processes, deduced from the decay and the ICR expts., are discussed and compared with those of related arom. acylation and alkylation reactions occurring in the dil. gaseous state.