Arylation of cyclic ethers by gaseous phenylium ions. Formation and behavior of phenoxenium ions in the gas phase

Abstract

SIMONETTA FORNARINI and MAUR~ZIO SPERANZA. Can. J. Chem. 66, 2506Chem. 66, (1988. Free, unsolvated phenylium ions formed by the spontaneous decay of 1,4-ditritiobenzene have been allowed to react with gaseous cyclic ethers (oxirane, propene oxide, and oxetane) and acetaldehyde in the pressure range 30-250 Torr and in the presence of a thermal radical scavenger (02, 4 Torr). The effects of a gaseous base (NMe3, 20 Ton) and of an energy moderator (He, 630-720 Torr) were also investigated. Phenylium ion confirms its considerable site selectivity, demonstrated by the distinct preference for the n-type center of the substrate, although appreciable insertion into the carbocyclic structure of propene oxide and oxetane is observed as well. The stability features of the ionic intermediates from addition of phenylium ion to selected substrates have been evaluated as well as their fragmentation and isomerization mechanisms. The behavior of gaseous phenylium ion toward cyclic ethers, in particular its ability to formally abstract an oxygen atom from the ether to give the phenoxenium ion, a reaction first observed in the present study, is discussed and compared with previous mechanistic investigations carried out in the gas phase and in solution. (He,. L'ion phCnylium confirme sa haute sClectivitC qui est dCmontrCe par sa prCfCrence marquCe pour le centre de type n du substrat m&me si on observe aussi une quantitC appreciable d'insertion dans la structure carbocyclique de l'oxyde de propene et de 1'oxCtanne. On a CvaluC les caracteristiques de stabilitC ainsi que les mCcanismes de fragmentation et dlisomCrisation des intermediaires ioniques obtenus par l'addition de l'ion phCnylium sur des substrats choisis. On discute du comportement de l'ion phCnylium gazeux vis-a-vis des Cthers cycliques, en particulier de sa facilitC d'arracher formellement un atome d'oxygene de 1'Cther pour donner l'ion phCnoxCnium, une reaction qui a CtC observee pour la premiere fois dans cette Ctude; on compare ce comportement avec celui observe dans des Ctudes mkcanistiques rapportCes antkrieurement et effectuees soit en phase gazeuse soit en solution.[Traduit par la revue]