Synthesis of substituted 2,2 '-bipyridines and 2,2 ': 6 ',2 ''-terpyridines by cobalt-catalyzed cycloaddition reactions of nitriles and alpha,omega-diynes with exclusive regioselectivity

A variety of substituted 2,2'-bipyridines were synthesized by a 1,2-bis(diphenylphosphino)ethane (dppe)/cobalt chloride hexahydrate (COCl2 center dot 6H(2)O)/zinc-catalyzed [2+2+2]cycloaddition reaction of diynes and nitrites, with all reactions exhibiting exclusive regioselectivity. Thus, symmetrical and unsymmetrical 1,6-diynes and 2-cyanopyridine reacted in the presence of 5 mol % of dppe, 5 mol % Of COCl2 center dot 6H(2)O and 10 mol % of zinc powder to provide the corresponding 2,2'-bipyridines. Under identical reaction conditions, 1-(2-pyridyl)-1,6-diynes and nitrites reacted smoothly with exclusive regioselectivity,to produce 2,2'-bipyridines in good yield. 2,2'-Bi- pyridines were also obtained by the double [2+2+ 2] cycloaddition reaction of 1,6,8,13-tetraynes with nitriles. Similarly, 2,2':6',2-terpyridines were synthesized from 1-(2-pyridyl)-1,6-diyne and 2-cyanopyridine. The regiochemistry observed can be explained by considering the electronic nature of cobaltacyclo-pentadiene intermediates and nitrites. A survey of the exclusive regiochemical trend gives reasonable credence to the synthetic potential of the present method.