95124-00-8Relevant academic research and scientific papers
Pd-catalyzed cycloisomerization to 1,2-dialkylidenecycloalkanes. 2. Alternative catalyst system
Trost, Barry M.,Romero, Donna L.,Rise, Frode
, p. 4268 - 4278 (2007/10/02)
The mechanisms by which palladium complexes may catalyze the cycloisomerization of 1,6- and 1,7-enynes to dialkylidenecycloalkanes were probed by exploring a catalyst system different than a ligated palladium acetate which previously has proven to be successful. Although carboxylic acids showed no discernible interaction with palladium(0) complexes, this combination proved to be a powerful catalyst system to effect this cycloisomerization. The fact that the two catalyst systems do not have the same reactivity profile suggests this new catalyst system may operate by a different mechanism. Evidence supporting a pathway invoking formation of a hydridopalladium acetate followed by hydropalladation as initiation is presented. Steric and electronic effects direct the regioselectivity of the termination step to form either 1,3- or 1,4-diene products. The 1,3-diene products participate exceedingly well in Diels-Alder reactions, both inter- and intramolecularly. The presence of an oxygen substituent at the position allylic to the diene served as both a regiochemical control element for the palladium-catalyzed cycloisomerization and a diastereochemical control element for the Diels-Alder reaction. The net result of these two steps the first of which is a catalyzed isomerization and the second an addition, is a highly efficient approach to complex polycycles in terms of both selectivity and atom economy.
Pd-catalyzed cycloisomerization to 1,2-dialkylidenecycloalkanes. 1
Trost, Barry M.,Tanoury, Gerald J.,Lautens, Mark,Chan, Chuen,MacPherson, David T.
, p. 4255 - 4267 (2007/10/02)
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.
