Diazenes may extrude nitrogen to provide discrete TMM intermediates. Generally, bridged diazenes are used to avoid competitive closure to MCPs and dimerization reactions. In combination with an alkenic acceptor, cyclization to either fused or bridged products takes place. Fused products are generally favored, unless the diazene precursor is substituted with electron-donating groups at the methylene carbon atom. The configuration of the alkene is maintained as long as the reaction is proceeding through a singlet TMM.
When stabilizing groups are present, MCPs may open to the corresponding zwitterionic TMMs. Acetal 1 has been used in this context, and provides cyclopentanes with the acetal functionality exo to the newly formed ring with high selectivity. This reaction is also stereospecific with respect to alkene geometry, and exhibits high selectivity for endo products in most cases.
MCPs lacking stabilizing groups may generate TMM synthons in the presence of palladium(0) or nickel(0) catalysts. Formal insertion of the catalyst into either of the two chemically distinct cyclopropane bonds (called "distal" and "proximal" to reflect their distance from the double bond) has the potential to generate isomeric products. Generally, palladium catalysts cause formal distal bond cleavage. This process is believed to occur through direct attack of the distal bond on the coordinated alkene. The reaction is stepwise and lacks stereospecificity under both palladium and nickel catalysis.
Silylated allylic acetates, carbonates and other substituted allyl compounds may form TMM synthons under palladium catalysis. The reaction is highly regioselective, providing only the substitution pattern shown below regardless of the position of the R' group on the starting allylic acetate. However, cyclization takes place in a stepwise fashion and does not exhibit stereospecificity. Rapid racemization of chiral pi-allyl palladium complexes occurs, and only moderate diastereoselectivity is observed in reactions of chiral allylic acetates. Chiral non-racemic alkenes, however, may exhibit moderate to high diastereoselectivity.