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

Abstract

The 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)

Preparation of 9α‐Fluorinated Sesquiterpenic Drimanes and Evaluation of Their Antifeedant Activities

“…[49] It is interesting to note that this reaction might involve a previous oxidative addition of the allyl fluoride moiety to Pd 0 , a reaction that has yet to be thoroughly studied. [60] It is likely in this case that the intermediate η 3 -allylpalladium(II) complex formed is stabilized by the oxygenated function at C-11, which favours the observed reaction. Although useless for the main objective of this work, the approach detailed in Scheme 8 could be of interest to elaborate the type of functionalization present in 42, which is not only a characteristic of some drimanes but also of other polycyclic terpenes.…”

Regio- and stereoselective synthesis of functionalized allylic fluorides via palladium-catalyzed three-component tandem carbofluorination of allenes

“…7 In addition, the relatively weak nucleophilicity of fluoride makes the bond formation event more challenging. 8 Nevertheless, nucleophilic fluorination of p-allyl metallic intermediates has recently attracted much attention, with rhodium, 9 iridium 10 and copper 11 catalyzed nucleophilic allylic fluorination of certain allylic precursors being documented within a short time span. Gouverneur 12 and Doyle 13 group recently demonstrated that nucleophilic attack of inorganic fluorides on pallyl palladium intermediates derived from various allylic compounds could deliver their respective allylic fluoride under certain conditions.…”

Transition Metal-Mediated and Metal-Catalyzed Carbon–Fluorine Bond Formation

“…An intrinsic challenge in allylic fluorination chemistry is the reactivity of the desired allylic fluoride. Togni illustrated the difficulty of this transformation in reporting that cationic allylpalladium complexes failed to yield allylic fluorides upon treatment with several fluoride sources [17].…”