Chelation control through the coordination of an olefinic π-bond to Lewis acid

The reaction of a 1:1 mixture of ortho-alkenylbenzaldehydes 3 and their para-isomers 4 with Bu3SnH (1 equiv.) in the presence of Me3Al (1 equiv.) resulted in very high (>10:1) to good (3.4:1) chemoselective reduction of 3, giving the ortho-alkenyl benzyl alcohol 5 selectively. Similarly, the highly chemoselective allylation of 3d in the presence of 4d was observed when the mixture was treated with allyltributyltin/Me3Al. The chemoselectivities are most probably due to the bidentate chelation of the Lewis acid to an olefinic π-bond and a lone pair of aldehydes.