Titanium‐Catalyzed 2‐Selective C─H Alkenylation of Pyridines With Alkynes

ABSTRACT First examples of titanium‐catalyzed C─H‐addition reactions of pyridines to symmetrical and unsymmetrical internal alkynes, which lead to highly substituted alkenylpyridines, are described. The reactions take place exclusively at the 2‐position of the pyridine, and both aryl‐ and alkyl‐substituted alkynes are successfully converted. In most cases, the new reaction takes place with excellent stereoselectivity, leading to E ‐configurated 2‐alkenylpyridines exclusively. Only in two exceptional cases, products with the opposite configuration of the double bond are formed selectively. When unsymmetrical alkynes are used, both possible regioisomers are formed in ratios of approximately 1:1. The active catalyst is prepared in situ from the heteroleptic titanium complex IndTiBn 3 ( Ti3 ) and the Lewis acid Ph 3 C[B(C 6 F 5 ) 4 ].