Kinetics and mechanism of gas-phase pyrolysis of ylides. Part 2. Analysis and comparison of reactivity of benzoyl-stabilized triphenylphosphonium and triphenylarsonium methylides

Abstract

Six triphenylphosphonium benzoylmethylides and four arsonium analogues were synthesized and characterized. The kinetics of gas-phase pyrolysis of these compounds was investigated over the temperature range 386-545 K for the phosponium ylides and 419-556 K for the arsonium ylides. The products of both sealed-tube pyrolysis and flash vacuum pyrolysis were analyzed and compared. The values of the Arrhenius log A and Ea are, respectively, 12.6 ± 1.4 s-1 and 134.5 ± 5.3 kJmol-1 for the phosphorane, and 10.4 ± 0.7 s-1 and 124.5 ± 17.8 kJmol-1 for the arsorane compounds. A mechanism involving the extrusion of Ph3PO in case of the phosponium ylides, and the extrusion of Ph3AsO or Ph3As in case of arsonium ylides is proposed and used to rationalize the molecular reactivities of the ylides under study, and to draw comparisons between the rate constants of pyrolysis at 500 K of the analogous phosphonium and arsonium ylides. © ARKAT USA, Inc.