Chelation control through the coordination of an olefinic pi-bond to Lewisacid

The reaction of a 1:1 mixture of ortho-alkenylbenzaldehydes 3 and their par a-isomers 4 with Bu3SnH (1 equiv.) in the presence of Me3Al (1 equiv.) resu lted in very high (>10:1) to good (3.4:1) chemoselective reduction of 3, gi ving 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 ar e most probably due to the bidentate chelation of the Lewis acid to an olef inic pi -bond and a lone pair of aldehydes. (C) 2000 Published by Elsevier Science Ltd.