INTERVENTION OF CARBONYL AND OXONIUM YLIDES IN REACTIONS OF [(ALKOXYCARBONYL)PHENYL]CARBENES IN THE GAS-PHASE FORMING 3-ALKYLPHTHALIDES AND 2-ALKOXY-1(2H)-BENZOCYCLOBUTENONE - C-13 AND O-18 LABELING STUDIES

The generation of isomeric [(methoxycarbonyl)phenyl]carbenes (6a-c) by flash vacuum pyrolysis (FVP, 350-350-degrees-C, 10(-5) Torr) of the corresponding diazomethanes (1a-c) produced 3-methylphthalide (2) and 2-methoxy-1(2H)-benzocyclobutenone (3), presumably as a result of carbene-carbene rearrangement followed by the interaction of the methoxycarbonyl group with the carbene center in the ortho isomer (6a). Labeling studies were carried out in order to determine the mechanism of the reaction. Thus, FVP of [o-[carboxy-C-13](methoxycarbonyl)phenyl]diazomethane produced 3-methyl[carbonyl-C-13]phthalide and 2-methoxy-1(2H)-[carbonyl-C-13]benzocyclobutenone, thereby indicating that 3 was formed via Stevens-type migration of the acyl group in an oxonium ylide intermediate (9). FVP of [p-[carbonyl-O-18](methoxycarbonyl)phenyl]diazomethane, on the other hand, produced 3-methyl[ether-O-18]phthalide, along with a small amount of 3-methyl[carbonyl-O-18]phthalide and 3-methoxy-1(2H)-[carbonyl-O-18]benzocyclobutenone, indicating that 2 was produced mainly via 1,5-methyl migration in a carbonyl ylide intermediate (7). Generation of other [(alkoxycarbonyl)phenyl]diazomethanes (16, where alkoxy = EtO and Pr(i)O under similar conditions also produced 3-alkylphthalides (17) and 2-alkoxybenzocyclobutenones (18), but phthalide 19, which was not detected in the FVP of 1, was also formed in this case, presumably as a result of alpha,beta-elimination of alkenes in the oxonium ylide. These results showing the intervention of the oxonium ylide of the ester group are in marked contrast to the observation that, at much lower temperatures in the liquid and solid phases, only the carbonyl ylide has been involved.