METAL-PROMOTED HIGHER-ORDER CYCLOADDITION REACTIONS - STEREOCHEMICAL, REGIOCHEMICAL, AND MECHANISTIC ASPECTS OF THE [6-PI-PLUS4-PI] REACTION

Transition-metal-promoted higher-order cycloaddition reactions are described. (Eta6-1,3,5-cycloheptatriene)tricarbonylchromium(0) and various substituted derivatives undergo photochemically or thermally initiated [6pi + 4pi] cycloaddition with a range of substituted butadiene partners. Electron-rich and electron-deficient dienes participate equally well in the transformation, and in every case examined only the endo diastereomer was produced. The reaction displays little regioselectivity for 2- and 3-substituted cycloheptatriene complexes; however, most yields of products exceed 85%. Chiral auxiliaries appended to the diene partner can provide very good stereoinduction in the resultant bicyclo[4.4.1]undecane adducts. Tropone and 1-substituted tropone complexes also undergo photoinduced cycloadditions with hydrocarbon dienes but not with oxygen-substituted partners. Azepine and thiepin 1,1-dioxide complexes engage dienes to provide high yields of the corresponding heterocyclic adducts. Molybdenum- and tungsten-based complexes are inferior to Cr(0) complexes as 6pi partners in the [6 + 4] cycloaddition, and tungsten has been totally ineffective for promoting the cycloadditions studied to date. Possible mechanistic schemes are presented, and the implications of each are discussed with regard to the scope and limitations of the cycloaddition reaction. Optimization of the reaction conditions on the basis of the aspects of these mechanisms is also outlined. The [6 + 4] cycloaddition of cycloheptatrienechromium(0) complexes can also be effected by heating in various ether solvents, and the cycloadducts produced in this fashion are identical in all respects with those prepared photochemically. The [6 + 4] reaction can also be carried out with modest efficiency employing only catalytic quantities of chromium(0) catalysts.