Phenylpalladium(IV) chemistry: Selectivity in reductive elimination from palladium(IV) complexes and alkyl halide transfer from palladium(IV) to palladium(II)

Article abstract of DOI:10.1021/om00017a071

Methyl iodide, benzyl bromide, and benzyl iodide react with PdMePh(bpy) (bpy = 2,2′-bipyridyl) in acetone at 0°C to form the isolable fac-triorganopalladium(IV) complexes PdIMe₂Ph(bpy) (3) and PdXMePh(CH₂Ph)(bpy) [X = Br (4), I (5)]. Complex 3 occurs as a mixture of isomers in a ca. 1:1 ratio, involving the phenyl group in a position trans either to bpy (3a) or to iodine (3b), while complexes 4 and 5 are obtained as one isomer which, most likely, has the benzyl group trans to the halogen. The selectivity of reductive elimination from a metal bonded to three different groups could be studied for the first time. The complexes undergo facile reductive elimination in (CD₃)₂CO at 0°C, in which PdIMe₂Ph(bpy) gives a mixture of ethane and toluene in a 4:1 molar ratio together with PdlR(bpy) (R = Ph, Me), whereas PdXMePh(CH₂Ph)(bpy) (X = Br, I) gives exclusively toluene and PdX(CH₂Ph)(bpy). The analogous tmeda complex, PdMePh(tmeda) (tmeda = N,N,N′,N′-tetramethylethylenediamine), reacts more slowly than PdMePh(bpy) with alkyl halides. Methyl iodide reacts cleanly with PdMePh(tmeda) at 0°C in (CD₃)₂CO to form ethane and PdIPh(tmeda), but the expected palladium(IV) intermediate could not be detected. Benzyl bromide does not react with PdMePh(tmeda) below the decomposition temperature of the latter under these conditions (50°C, (CD₃)₂CO), while benzyl iodide reacts at 40°C to give a complicated mixture of products of which ethane, diphenylmethane, ethylbenzene, toluene, and PdIR(tmeda) (R = Me, Ph) could be identified. Benzyl iodide reacts with PdMe2(tmeda) at -30°C in (CD₃)₂CO to form PdIMe₂(CH₂Ph)(tmeda), for which ¹H NMR spectra showed the benzyl group to be trans to one of the N-donor atoms. However, PdIMe₂(CH₂Ph)(tmeda) is unstable and undergoes facile reductive elimination to form ethane and PdI(CH₂Ph)(tmeda). Transfer of alkyl and halide groups from palladium(IV) to palladium(II) complexes occurs in (CD₃)₂CO at low temperatures for several reaction systems in which the resulting palladium(IV) complex is known to be more stable than the palladium(IV) reagent. There is a strong preference for benzyl group transfer from PdXMePh(CH₂Ph)(bpy) to PdMe₂(L₂) (X = Br, I; L₂ = bpy, phen). The mechanism of the transfer reactions is discussed in terms of the mechanism suggested earlier for alkyl halide transfer from palladium(IV) to platinum(II), palladium(II) to palladium(0), cobalt(III) to cobalt(I), and rhodium(III) to rhodium(I). These reaction systems involve nucleophilic attack by the lower oxidation state reagent at an alkyl group attached to the higher oxidation state reagent.