[3+2] Cycloaddition of Trimethylenemethane and its Synthetic Equivalents

摘要: Trimethylenemethane (TMM) is a non-Kekule molecule composed of four carbons, six hydrogens, and π-orbitals. Therefore, TMM can only be expressed as resonance structures involving 1,3-diyls zwitterions. The two degenerate nonbonding molecular orbitals in the Huckel orbital TTM indicate existence singlet triplet electronic configurations, which play significant roles chemistry. TMMs were imaginary molecules until late 1960's, parent was first isolated low-temperature matrix. It stable for several weeks at 77 K, ground-state with D3h-planar structure assigned by ESR spectroscopy. unique coupled complex states derivatives have been topics theoretical, mechanistic, synthetic studies. By analogy Diels-Alder reaction synthesis six-membered rings through [4 + 2] cycloadditions, use three-carbon unit [3 + 2] cycloadditions appears to represent viable possibility, because π-electron system theoretically suitable [4πs + 2πs] cycloaddition. As expected, however, rapid ring closure methylenecyclopropane (MCP) preferred cycloaddition 2π acceptor. use, tailor-made its equivalents need designed achieve reactions. The breakthrough TMMs reactions reported isopropylidenecyclopentane-1,3-diyl, has considerable lifetime reacts electron-deficient alkenes. Because gives many MCP undergo thermal isomerization, MCPs considered serve precursors TMMs. However, direct evidence formation from their rather scant recently. Since species are generally too short-lived reactive used organic synthesis, various attempts made stabilize them transition metal templates. Efficient catalyzed achieved using Ni(0) or Pd(0) catalysts. proceeds via opening form (methylene)metallacyclobutane-type intermediates. More recently, Pd(0)-catalyzed [2-(acetoxymethyl)allyl]trimethyl-silane acceptors emerged powerful method cyclopentane synthesis. This involve zwitterionic TMM-Pd intermediate. serves well acceptors, it applied cyclopentanoid natural products. This review surveys complexes acceptors. divided into five categories, defined types precursors: (1) free generated diazenes, (2) MCPs, (3) (4) silylated allylic acetates, (5) TMM-metal complexes. Transformations that accomplish net but require steps not covered. Various aspects TMMs, including physical chemistry, already reviewed. Keywords: cycloaddition; trimethylenemethane (TMM); structures; dipolar TMM; methylenecyclopropanes; transition reactions; silylated acetates; metal complexes; scope; limitations; diazenes; intramolecular reactions; intermolecular reactions; generation; regioselectivity; sterospecificity; stereoselectivity; periselectivity; synthetic applications; mechanisms; selectivities; comparison methods; experimental conditions; experimental procedures; tabular survey