Interactions of Cationic Palladium( II )‐ and Platinum( II )‐η 3 ‐Allyl Complexes with Fluoride: Is Asymmetric Allylic Fluorination a Viable Reaction?

The complex cations [M(η 3 ‐R 2 All)(PPFPz{3‐ t Bu})] + (M = Pd II , R 2 All = 1,3‐diphenylallyl, 1,3‐dicyclohexylallyl, indenyl; M = Pt II , R 2 All = 1,3‐diphenylallyl; PPFPz‐{3‐ t Bu} = 3‐ tert ‐butyl‐1‐{1‐[2‐diphenylphosphanyl‐ferrocenyl]ethyl}‐1 H ‐pyrazole)have been prepared as salts with PF 6 – or SbF 6 – . 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(η 3 ‐1,3‐Ph 2 C 3 H 3 )(PPFPz{3‐ t Bu})]PF 6 underwent an anion exchange with Me 4 NF to give [Pt(1,3‐Ph 2 C 3 H 3 )(PPFPz{3‐ t Bu})]F which existed as a mixture of interconverting allyl isomers in solution at ambient temperature. For the bromide salt, [Pt(η 3 ‐1,3‐Ph 2 C 3 H 3 )(PPFPz{3‐ t Bu})]Br, allyl isomerization was slow at ambient temperature. Precursors of Pt 0 reacted with bromo‐1,3‐diphenylprop‐2‐ene to give [Pt 2 (μ‐Br) 2 (η 3 ‐1,3‐Ph 2 All) 2 ] and precursors of Pd 0 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)