Palladium- and Platinum-Catalyzed Addition of Aldehydes and Imines with Allylstannanes. Chemoselective Allylation of Imines in the Presence of Aldehydes

Abstract

The reaction of allylstannanes 1 with aldehydes 2 in THF was catalyzed by Pd(II) or Pt(II) complexes (10 mol %) either at room temperature or at reflux, giving the corresponding homoallylic alcohols 3 in high to good yields. Among the catalysts examined, PtCl2(PPh3)2 gave the best result. No only allyltribitylstannane but also methallyl- and crotyltributylstannane could be utilized in this transition metal catalyzed reaction. Detailed mechanistic studies of the Pd(II)-catalyzed allylation, using NMR spectroscopy, revealed that the bis-π-allylpalladium complex is a key intermediate for the catalytic cycle and it exhibits nucleophilic reactivity. The nucleophilic reactivity of the intermediate is in marked contrast to the electrophilic reactivity of ordinary π-allylpalladium complexes (π-allylPdX, X = OAc, halogen, OCO2R, etc.). The addition of allyl-, crotyl-, and methallylstannanes to various imines 4 proceeded very smoothly to give the corresponding allylated products (homoallylic amines 5) in good to high yields. The reactivities of allylstannanes to imines were higher than those to aldehydes under the catalytic conditions, although it is known that the reactivity of allylstannanes to aldehydes is higher than that to imines under the Lewis acid promoted condition. Imines were chemoselectively allylated in the presence of aldehydes and the highest selectivities were obtained using π-allylpalladium chloride dimer 11 as a catalyst.