PHENYLPALLADIUM(IV) CHEMISTRY - SELECTIVITY IN REDUCTIVE ELIMINATION FROM PALLADIUM(IV) COMPLEXES AND ALKYL HALIDE TRANSFER FROM PALLADIUM(IV) TO PALLADIUM(II)

Methyl iodide, benzyl bromide, and benzyl iodide react with PdMePh(bpy ) (bpy = 2,2'-bipyridyl) in acetone at 0-degrees-C to form the isolabl e fac-triorganopalladium(IV) complexes PdIMe2Ph(bpy) (3) and PdXMePh(C H2Ph)(bpy) [X = Br (4), I (5)]. Complex 3 occurs as a mixture of isome rs 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 obta ined as one isomer which, most likely, has the benzyl group trans to t he halogen. The selectivity of reductive elimination from a metal bond ed to three different groups could be studied for the first time. The complexes undergo facile reductive elimination in (CD3)2CO at 0-degree s-C, in which PdIMe2Ph(bpy) gives a mixture of ethane and toluene in a 4:1 molar ratio together with PdIR(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'-tetram ethylethylenediamine), reacts more slowly than PdMePh(bpy) with alkyl halides. Methyl iodide reacts cleanly with PdMePh(tmeda) (tmeda) at 0- degrees-C in (CD3)2CO to form ethane and PdIPh(tmeda), but the expecte d palladium(IV) intermediate could not be detected. Benzyl bromide doe s not react with PdMePh(tmeda) below the decomposition temperature of the latter under these conditions (50-degrees-C, (CD3)2CO), while benz yl iodide reacts at 40-degrees-C to give a complicated mixture of prod ucts of which ethane, diphenylmethane, ethylbenzene, toluene, and PdIR (tmeda) (R = Me, Ph) could be identified. Benzyl iodide reacts with Pd Me2(tmeda) at -30-degrees-C in (CD3)2CO to form PdlMe2(CH2Ph)(tmeda), for which H-1 NMR spectra showed the benzyl group to be trans to one o f the N-donor atoms. However, PdIMe2(CH2Ph)(tmeda) is unstable and und ergoes facile reductive elimination to form ethane and PdI(CH2Ph)(tmed a). Transfer of alkyl and halide groups from palladium-(IV) to palladi um(II) complexes occurs in (CD3)2CO at low temperatures for several re action systems in which the resulting palladium(IV) complex is known t o be more stable than the palladium(IV) reagent. There is a strong pre ference 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 ha lide transfer from palladium(IV) to platinum(II), palladium(II) to pal ladium(0), cobalt(III) to cobalt(I), and rhodium(III) to rhodium(I). T hese reaction systems involve nucleophilic attack by the lower oxidati on state reagent at an alkyl group attached to tbe higher oxidation st ate reagent.