On factors influencing insertion of allylic substrates in Pd–Caryl bonds

The main products of reactions between palladium(II) aryl complexes [PdArI(phen)] (1a, Ar = C6H5–; 2a, Ar = 4-MeO–C6H4–; 3a, Ar = 4-CF3–C6H4–; phen = 1,10-phenanthroline), [Pd(C6H5)Cl(phen)] (1b) or [PdAr(phen)(MeCN)]BF4 (1, Ar = C6H5–; 3, Ar = 4-CF3–C6H4–) with allylic substrates CH2double bond; length as m-dashCHCH2A, where A = OH, OR, OCOR, CN or View the MathML sourceNMe3+ (R = Et or C6H5), or 2,5-dihydrofuran were identified. The nature of the products affords information on mechanistic features of arylation of allylic substrates prompted by Pd(II), meanwhile allowing comparison with analogous Pt(II) reactivity. A most noteworthy difference concerns the fate of the early insertion product derived from cationic complexes, containing the moiety ArCH2CH(MII)CH2A. The product can be obtained, in case metal is platinum, only in the absence of a coordinating solvent, and evolves via CAr,H oxidative addition followed by H,A elimination and formation of a stable σ,π-chelated arylallyl moiety, thus avoiding β-elimination. The corresponding palladium species, again in the absence of coordinating species, affords a σ-derivative prone to β-elimination. The choice between the two possible elimination pathways is dictated by the relative stabilities of the end products (both retaining the A function) of the two chains of sequential equilibrium steps.