[reaction: see text] A highly practical, general method for catalytic formation of substituted pyridines from a variety of unactivated nitriles and alpha,omega-diynes is given. The reactions which were catalyzed by 5 mol % of dppe/CoCl2-6H2O in the presence of Zn powder (10 mol %) could proceed at rt to approximately 50 degrees C with high functional compatibility and regioselectivity.

A variety of substituted 2,2'-bipyridines were synthesized by a 1,2-bis(diphenylphosphino)-ethane (dppe)/cobalt chloride hexahydrate (CoCl 2 ·6 H 2 O)/zinc-catalyzed [2 + 2 + 2] cycloaddition reaction of diynes and nitriles, 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 CoCl 2 ·6 H 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 nitriles reacted smoothly with exclusive regioselectivity to produce 2,2'-bipyridines in good yield. 2,2'-Bipyridines 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 cobaltacyclopentadiene intermediates and nitriles. A survey of the exclusive regiochemical trend gives reasonable credence to the synthetic potential of the present method.

Regioselectivity. -The Co-Zn-catalyzed cycloaddition of diynes (I), (IV) and tetraynes (VII) proceeds with excellent regioselectivity (> 99%). The regioselectivity for unsymmetrical diynes is determined by electronic rather than steric factors. The presented method allows for the synthesis of defined 2,2'-bipyridines and 2,2':6',2''-terpyridines. -(GOSWAMI, A.; OHTAKI, K.; KASE, K.; ITO, T.; OKAMOTO*, S.; Adv. Synth. Catal. 350 (2008) 1, 143-152; Dep. Mater. Life Chem., Kanagawa Univ., Kanagawa, Yokohama 221, Japan; Eng.) -Mischke 21-154