Ruthenium-mediated regio- and stereoselective alkenylation of pyridine

A novel alkenylation reaction of pyridine is developed. Heating a cationic ruthenium vinylidene complex [CpRu(C=C=HR)(PPh3)2]PF6 in pyridine at 100-125 °C for 24 h affords (E)-2-alkenylpyridine. Initially, pyridine coordinates to ruthenium by displacement of one of the phosphine ligands. Then, [2 + 2] heterocycloaddition occurs to form a four-membered ruthenacyclic complex. Deprotonation of the β-hydrogen affords a neutral π-azaallyl complex. Protonolysis furnishes the product. As a result, a vinylidene group is inserted into the α C-H bond of pyridine. The alkenylation reaction is made catalytic in ruthenium by the use of (alkyn-1-yl)silane as the vinylidene source. Treatment of (alkyn-1-yl)trimethylsilane with pyridine in the presence of a cationic ruthenium complex [CpRu(PPh3)2]PF6 affords the corresponding (E)-2-alkenylpyridine in good yield in a regio- and stereoselective manner. Copyright © 2003 American Chemical Society.