Rhodium(Ⅰ) catalyzed regioselective alkylation of olefins and aromatic rings by C-H bond activation

Abstract

There has been great interest in transition metal mediated C-H bond activation. One goal is to develop new routes for selective C-C bond formation. Cyclometallation is well known as a good method for activation of olefinic and aromatic C-H bonds. We have found that 2-vinylpyridines reacted with olefins in the presence of rhodium(I) complex such as Wilkinson``s complex as catalyst to result in the alkylated vinylpyridines according to the anti-Markownikoff``s rule. The reaction of 6-methyl-2-vinylpyridine or 2-vinylpyridine with olefins produced the alkylated products together with dimeric products of the substrate. But, α-substituted vinylpyridines afforded the corresponding alkylated products in high yields. In the case of β-substituted vinylpyridines, four different alkylated products were obtained. A regioselective alkylation of the ortho position of various 2-phenylpyridines has been examined : 2-phenylpyridine reacted with olefins to give the ortho-alkylated product and doubly alkylated product (9:1). The meta or para alkylated products could not be detected. In the case of 2-(p-tolyl)pyridine, the ratio of the doubly alkylated product increased. On the other hand, 3-methyl-2-phenylpyridine gave mono ortho alkylated product exclusively under the same conditions. The regioselective alkylation of olefinic C-H bond of 2-(1-cyclohexenyl)pyridine was carried out. 2-(1-Cyclohexenyl)pyridine reacted with olefins in the presence of Rh (I) catalyst to give the alkylated product at the olefinic position regiospecifically. When the ortho alkylation of benzylideneamines was examined, the regioselective mono and bisalkylated products were obtained in low yields.