Interactions of Cationic Palladium(II)‐ and Platinum(II)‐η3‐Allyl Complexes with Fluoride: Is Asymmetric Allylic Fluorination a Viable Reaction?

Abstract

AbstractThe complex cations [M(η3‐R2All)(PPFPz{3‐tBu})]+ (M = PdII, R2All = 1,3‐diphenylallyl, 1,3‐dicyclohexylallyl, indenyl; M = PtII, R2All = 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 PdII 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(η3‐1,3‐Ph2C3H3)(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(η3‐1,3‐Ph2C3H3)(PPFPz{3‐tBu})]Br, allyl isomerization was slow at ambient temperature. Precursors of Pt0 reacted with bromo‐1,3‐diphenylprop‐2‐ene to give [Pt2(μ‐Br)2(η3‐1,3‐Ph2All)2] and precursors of Pd0 underwent oxidative additions with bromo‐ and fluoro‐1,3‐diphenyl‐2‐propene to give 1,3‐diphenylallyl complexes of PdII. Therefore, the nucleophilic attack of fluoride on the allyl fragment of PdII complexes is endergonic, and the high energy barrier of this step is difficult to overcome in a catalytic allylic fluorination reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)