4-Methylene-1-cyclopentenes from Methylenecyclopropanes and Alkynyltrimethylsilanes

Oxidation of Enones for Regioselective [3+2] Cycloaddition through γ-Enone Radical Intermediates

Herein, an oxidization reaction of enones with a Cu^II complex that leads to a new type of regioselective [3+2] cycloaddition is reported. Highly functionalized cyclopentenes and spirocyclic compounds are obtained in moderate-to-good yields. This cycloaddition reaction occurred through the formation of γ-enone radicals, providing a rarely explored reactivity pattern for enones.

Transition-metal-free coupling reaction of vinylcyclopropanes with aldehydes catalyzed by tin hydride

Donor-acceptor cyclopropanes are useful building blocks for catalytic cycloaddition reactions with a range of electrophiles to give various cyclic products. In contrast, relatively few methods are available for the synthesis of homoallylic alcohols through coupling of vinylcyclopropanes (VCPs) with aldehydes, even with transition-metal catalysts. Here, we report that the hydrostannation of vinylcyclopropanes (VCPs) was effectively promoted by dibutyliodotin hydride (Bu2 SnIH). The resultant allylic tin compounds reacted easily with aldehydes. Furthermore, the use of Bu2 SnIH was effectively catalytic in the presence of hydrosilane as a hydride source, which established a coupling reaction of VCPs with aldehydes for the synthesis of homoallylic alcohols without the use of transition-metal catalysts. In contrast to conventional catalytic reactions of VCPs, the presented method allowed the use of several VCPs in addition to conventional donor-acceptor cyclopropanes.

Palladium-catalyzed [3+2] intramolecular cycloaddition of alk-5-ynylidenecyclopropanes: a rapid, practical approach to bicyclo[3.3.0]octenes

Readily accessible hex-5-ynylidenecyclopropane derivatives cycloisomerize to bicyclic five-membered carbocycles upon heating with catalytic amounts of a palladium complex.

Theoretical study of the mechanisms of palladation of methylenecyclopropane and [3 + 2] cycloadditions

Palladation of methylenecyclopropane and [3 + 2] cycloaddition with alkenes have been studied theoretically. In palladium complexes of methylenecyclopropane or trimethylenemethane, the eta 2 forms have been shown to be more stable than the palladacyclobutane forms. However, the barrier heights to yield palladacyclobutanes are not very high and the methylene-bridged carbon can be converted to the metal-attached carbon on palladium via a ring-opened transition state, allowing the scrambling of hydrogens in methylenecyclopropane. With respect to the palladium-catalyzed [3 + 2] cycloaddition reaction of methylenecyclopropane with alkenes, the path via palladacyclobutanes has been demonstrated to be more favorable than the path via an eta 2 complex. These results are discussed by using the paired interaction orbital scheme.

Mechanisms of the Cycloaddition Reaction of Methylenecyclopropane-Palladium and Oxa- and Azatrimethylenemethane-Palladium Complexes with Olefins

Mechanisms of palladium-catalyzed cycloaddition reactions of methylenecyclopropane with olefins and those of oxatrimethylenemethane (OTMM) and azatrimethylenemethane (ATMM) complexes with olefins have been studied by applying the ab initio molecular orbital method. Assuming an intramolecular coupling mechanism, we have examined some model complexes, Pd(eta(2)-methylenecyclopropane)(ethylene)(PH(3)) and Pd(eta(3)-OTMM or eta(3)-ATMM)(ethylene)(PH(3)), to determine the transition state structures. The coupling of methylenecyclopropane, OTMM, and ATMM with ethylene on the metal center takes place in two steps. In the reaction of methylenecyclopropane, the first step involves an opening of the cyclopropane ring promoted by an attack of ethylene. The methylenecyclopropane moiety has a pi-allylic form at the first transition state which leads to a metallacyclic intermediate. The first transition state of the reactions of OTMM and ATMM complexes looks very similar to that of the methylenecyclopropane complex, having pi-allylic coordinations. The two paths are separated in the second step. A [3 + 2] addition product is obtained by a reductive elimination from the intermediate metallacycle, whereas a prototropic shift followed by a reductive elimination affords a [2 + 1] addition product. It is the energetics of the second stage starting from the metallacycles that differentiates the reactions of the OTMM and ATMM complexes from the reaction of the methylenecyclopropane complex. The relative stabilities of various isomeric forms of these complexes have also been studied.