|
Trimethylenemethane cycloaddition is the formal () annulation of trimethylenemethane (TMM) derivatives to two-atom pi systems. Although TMM itself is too reactive and unstable to be stored, reagents which can generate TMM or TMM synthons ''in situ'' can be used to effect cycloaddition reactions with appropriate electron acceptors. Generally, electron-deficient pi bonds undergo cyclization with TMMs more easily than electron-rich pi bonds.〔Yamago, S.; Nakamura, E. ''Org. React.'' 2003, ''61'', 1. 〕 ==Introduction== Trimethylenemethane is a neutral, four-carbon molecule composed of four pi bonds; thus, it must be expressed either as a non-Kekulé molecule or a zwitterion. The orbital energy levels of TMM reveal that it possesses singlet and triplet states; generally, these states exhibit different reactivity and selectivity profiles.〔Baseman, R. J.; Pratt, D. W.; Chow, M.; Dowd, P. ''J. Am. Chem. Soc.'' 1976, ''98'', 5726.〕 A singlet () cycloaddition, when it is concerted, is believed to proceed under frontier orbital control. When electron-rich TMMs are involved, the ''A'' orbital serves as the HOMO (leading to fused products if the TMM is cyclic). When electron-poor (or unsubstituted) TMMs are involved, the ''S'' orbital serves as the HOMO (leading to bridged products if the TMM is cyclic). Cycloadditions involving the triplet state are stepwise, and usually result in configurational scrambling in the two-atom component.〔Platz, M. S.; Berson, J. A. ''J. Am. Chem. Soc.'' 1980, ''102'', 2358.〕 ''(1)'' The rapid closure of TMMs to methylenecyclopropenes (MCPs) is a general problem that affects the rate and yield of () cycloaddition reactions involving this class of reaction intermediates.〔Yamago, S.; Nakamura, E. ''J. Am. Chem. Soc.'' 1989, ''111'', 7285.〕 The problem is generally less severe for five-membered, cyclic TMMs due to ring strain in the corresponding MCPs. When ring closure and TMM dimerization can be controlled, () cycloaddition affords isomeric mixtures of methylenecyclopentanes. Three classes of compounds have been used to generate synthetically useful TMM intermediates: diazenes, silyl-substituted allylic acetates and methylenecyclopropenes. Transition metal catalysis can be used with the latter two classes, although polar MCPs may open under light or heat (see below). ''(2)'' 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Trimethylenemethane cycloaddition」の詳細全文を読む スポンサード リンク
|
