Diethylzinc-Mediated Allylation of Carbonyl Compounds Catalyzed by [(NHC)(PR3)PdX2] and [(NHC)Pd(η3-allyl)Cl] Complexes

[(NHC)(PR3)PdX2] complexes (NHC = N-heterocyclic carbene) are active precatalysts in the palladium-catalyzed allylation of carbonyl compounds with allylic acetates and diethylzinc. A comparative study examining the catalytic activity of a series of six of these complexes was carried out with allyl and cinnamyl acetates. [(IMesMe)(PPh3)PdI2] was found to be the most versatile precatalyst (IMesMe = 1-mesityl-3-methylimidazol-2-ylidene) and the scope of the reaction was investigated with this complex. [(IMesMe)(PPh3)PdI2] catalyzes the allylation of aromatic (except 4-nitrobenzaldehyde) and aliphatic aldehydes (including enolizable aldehydes) with cinnamyl acetate to give the corresponding homoallylic alcohols in 57–98 % yields and diastereoselectivities ranging from 70:30 to 92:8. The allylation of acetone also takes place under the same conditions, leading to the expected adduct in 63 % yield. The reaction with cyclohexenyl acetate proceeds at room temperature to afford the homoallylic alcohols in 40–78 % yields with excellent diastereoselectivities (>98:2), but is limited to aromatic aldehydes. An experimental study concerning the mechanism of the transformation was also carried out. We first demonstrated that the phosphane ligand was not essential for the reaction to take place. [(NHC)Pd(allyl)Cl] complexes are active precatalysts and lead to similar yields in the presence or in the absence of PPh3. Transmetalation of [(NHC)Pd(allyl)Cl] complexes with diethyl- or dimethylzinc, which is a determining step for the mechanism, was studied by 1H NMR spectroscopy. The reaction of [(IPr)Pd(allyl)Cl] with dimethylzinc affords rapidly [(IPr)Pd(η3-allyl)(Me)] but no detectable trace of allylzinc species [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. [(IPr)Pd(η3-allyl)(Me)] was found to be a nucleophilic species able to react smoothly at room temperature with an aldehyde in the absence of zinc to form the corresponding homoallylic alcohol. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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