Reactivity of aldehydes with semi-stabilised arsonium ylide anions: synthesis of terminal (e)-1,3-dienes.

A study of the reactivity of semi-stabilised arsonium ylide anions in olefination reactions is presented. The different ylide anions were generated by the addition of nBuLi to various arsonium halide derivatives: [Ph(2)As(R)R'](+)X(-), where R and R' are methyl, allyl, prenyl or benzyl groups. By using diallyldiphenylarsonium bromide (R=R'=allyl) an olefination protocol was optimised allowing the efficient transformation of aliphatic aldehydes into terminal 1,3-dienes with a high selectivity for the E isomer (E/Z ratios ranging from 90:10 to 97:3). The olefination reactions of aldehydes with dissymmetric arsonium halides (R not equal R') are very chemoselective; with arsonium ylide anions the benzyl moiety is more reactive than the allyl moiety which is much more reactive than prenyl and methyl groups. Based on the experimental results, a mechanism is proposed for the reaction.

Computational Investigation of Mechanisms for Ring-Opening Polymerization of ε-Caprolactone: Evidence for Bifunctional Catalysis by Alcohols.

Stepwise addition/elimination and concerted mechanisms for the methanolysis of ε-caprolactone, as a model for the initiation and propagation of ring-opening polymerization (ROP), have been investigated computationally using the B3LYP/6-31G* density functional method, with assistance from one or two ancillary methanol molecules. The effects of specific solvation by these extra methanols in cyclic hydrogen-bonded clusters are very significant, with barrier height reductions of about 50 kJ mol(-)(1). However, the effects of bulk solvation as treated by the polarized continuum model are almost negligible. Increasing the ring size lowers the barriers for both the addition and elimination steps of the stepwise mechanism but does not do so for the concerted mechanism; a stepwise mechanism is preferred for methanol-assisted ROP. The essential catalytic role of solvent molecules in this reaction is to avoid the unfavorable accumulation or separation of charges.