text: 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. A singlet (3+2) 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.
  The rapid closure of TMMs to methylidenecyclopropanes is a general problem that affects the rate and yield of (3+2) cycloaddition reactions involving this class of reaction intermediates. 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, (3+2) 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).