Interactions of cationic palladium(II)- and platinum(II)-η3-allyl complexes with fluoride:: Is asymmetric allylic fluorination a viable reaction?

The complex cations [M(eta(3)-R(2)All)(PPFPz{3-tBuj)}](+) (M = PrIn, R(2)All = 1,3-diphenylallyl, 1,3-dicyclohexylallyl, indenyl; M = Pt-II, R(2)All = 1,3- diphenylallyl; PPFPz-{3-tBu} = 3-tert-butyl-1-{1-[2-diphenylphosphanyl- ferrocenyl]ethyl}-1H-pyrazole) have been prepared as salts with PF6- or SbF6-. They have been characterized by NMR spectroscopy in solution and by X-ray crystallography in the solid state. Their reactions with sources of nucleophilic and "naked" fluoride have been investigated by multinuclear NMR spectroscopy. The Pd-II complexes did not undergo any nucleophilic substitution with concomitant release of allyl fluorides. The dicyclohexylallyl fragment was released as a 1,3-diene by elimination, but with other allyl complexes nonspecific decomposition reactions predominated. The complex [Pt(eta(3)-1,3-Ph,C3H3)- (PPFPz{3-tBu})]PF6 underwent an anion exchange with Me4NF to give [Pt(1,3-Ph2C3H3)(PPFPz{3-tBu})]F which existed as a mixture of interconverting allyl isomers in solution at ambient temperature, For the bromide salt, [Pt(eta(3)-1,3-Ph2C3H3)(PPFPz{3-tBu})]Br, allyl isomerization was slow at ambient temperature. Precursors of Pt-0 reacted with bromo-1,3-diphenylprop-2-ene to give [Pt-2{mu-Br}(2)(eta(3)-1,3-Ph(2)All)(2)] and precursors of PdO underwent oxidative additions with bromo- and fluoro-1,3-diphenyl-2-propene to give 1,3-diphenylallyl complexes of Pd-II. Therefore, the nucleophilic attack of fluoride on the allyl fragment of Pd-II complexes is endergonic, and the high energy barrier of this step is difficult to overcome in a catalytic allylic fluorination reaction. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).