Bulky Monodentate Phosphoramidites in Palladium-Catalyzed Allylic Alkylation Reactions: Aspects of Regioselectivity and Enantioselectivity

A series of bulky monoden- tate phosphoramidite ligands, based on biphenol, BINOL and TADDOL back- bones, have been employed in the Pd- catalysed allylic alkylation reaction. Reaction of disodium diethyl 2-methyl malonate with monosubstituted allylic substrates in the presence of palladium complexes of the phosphoramidite li- gands proceeds smoothly at room tem- perature. The regioselectivities ob- served depend strongly on the leaving group and the geometry of the allylic starting compounds. Mono-coordina- tion occurs when these ligands are li- gated in (Pd(allyl)(X)) complexes (allyl = C3H5, 1-CH3C3H4, 1-C6H5C3H4, 1,3-(C6H5)2C3H3 ;X = Cl, OAc). The solid-state structure determined by X- ray diffraction of (Pd(C3H5)(1)(Cl)) re- veals a non-symmetric coordination of the allyl moiety, caused by the stronger trans influence of the phosphoramidite ligand relative to X. In all of these complexes, the syn,trans isomer is the major species present in solution. Be- cause of fast isomerisation and high re- activity of the syn,cis complex, the major product formed upon alkylation is the linear product, especially for monosubstituted phenylallyl substrates in the presence of halide counterions. In the case of biphenol- and BINOL- based phosphoramidites, however, a strong memory effect is observed when 1-phenyl-2-propenyl acetate is em- ployed as the substrate. In this case, nucleophilic attack competes effective- ly with the isomerisation of the transi- ent cinnamylpalladium complexes. The asymmetric allylic alkylation of 1,3-di- phenyl-2-propenyl acetate afforded the chiral product in up to 93 % ee. Sub- strates with smaller substituents gave lower enantioselectivities. The ob- served stereoselectivity is explained in terms of a preferential rotation mecha- nism, in which the product is formed by attack on one of the isomers of the intermediate (Pd{1,3-(C6H5)2C3H3}(L)- (OAc)) complex.