Pd-catalyzed cycloisomerization to 1,2-dialkylidenecycloalkanes. 1

Article abstract of DOI:10.1021/ja00089a015

Enhancing synthetic efficiency requires the development of synthetic reactions that, to the extent possible, are simple additions wherein everything else is required only in catalytic amounts. The Alder ene reaction constitutes a classical reaction that meets this requirement that has much unrealized potential. A transition-metal-catalyzed version helps to increase that potential by permitting this reaction to proceed under mild conditions. A significant benefit of transition metal catalysis is the feasibility of diverting the reaction along pathways not feasible under thermal conditions. The synthesis of 1,3-dienes rather than 1,4-dienes is a very important diversion because of the utility of 1,3-dienes as reaction partners in the Diels-Alder reaction, another highly atom economical process. A catalyst derived from palladium acetate cycloisomerizes 1,6- and 1,7-enynes to dialkylidenecyclopentanes and -cyclohexanes. 1,3-Diene formation is favored over the Alder ene process by both steric and electronic effects. The reaction is highly chemoselective - tolerating a wide diversity of functionality including hydroxyl groups, ketones, esters, alkynyl and enol ethers, alkynyl and vinyl silanes, and enones. Many of the substrates are available by palladium-catalyzed alkylation reactions - highlighting the effectiveness of palladium catalyzed methodology in organic synthesis. The atom-economical nature of these reactions combined with the Diels-Alder reaction permit butadiene and dimethyl propargylmalonate to be molded into a polyhydro-as-indacene. The mechanism of this reaction may involve a tautomerization of an enyne-Pd(+2) complex to a pallada(+4)cyclopentene intermediate as a key step.