Novel catalytic reactions involving π-allylpalladium and -nickel as the key intermediates:: umpolung and β-decarbopalladation of π-allylpalladium and nickel-catalyzed homoallylation of carbonyl compounds with 1,3-dienes

Three subjects concerning novel reactions involving pi-allylpalladium and pi-allylnickel as intermediates are reviewed: the first is the umpolung of pi-allylpalladium, an alternation of the electrophilic nature of pi-allylpalladium species into the nucleophilic one of allylzinc species, which proceeds by treating the pi-allylpalladium with diethylzinc. Unique regio- and stereoselectivity of the thus-formed allylzincs toward carbonyl compounds is described. The second is the ring-opening reaction of 5-vinyl-2-oxa-1-palladacyclopentane intermediates, which furnishes aldehydes and ketones in good yields. The reaction proceeds via a novel C-C bond cleavage beta to the palladium (decarbopalladation reaction). Finally, the nickel-catalyzed homoallylation of carbonyl compounds with 1,3-dienes is discussed. The reaction is promoted by triethylborane or diethylzinc and tolerates carbonyl compounds (alkyl-, arylaldehydes and ketones) and 1,3-dienes, of a wide structural variety. The reaction provides 1,2- and 1,3-disubstituted 4-pentenols with high 1,2- and 1,3-diastereoselectivity. Mechanistically, this final reaction is closely related, in a reverse manner, to the decarbopalladation reaction (second subject). (C) 1999 Elsevier Science S.A. All rights reserved.