Cross-coupling versus homocoupling in the reactions of dimethyl(N,N,N′,N′-tetramethylethanediamine)palladium with organic halides

Article abstract of DOI:10.1021/om00119a019

Reaction of PdMe₂(tmeda) (tmeda = N,N,N′,N′-tetramethylethanediamine) with benzyl and allyl bromide at 20°C affords the homocoupled product ethane and the organometallic complexes PdBr(η¹-benzyl)(tmeda) and [Pd(η³-allyl)(tmeda)]Br, respectively. The reaction of PdMe₂(tmeda) with acetyl chloride did not give the acetyl complex PdCl(COMe)(tmeda). Instead, the cross-coupled product acetone and the organometallic product PdClMe(tmeda) were obtained. Acetyl complexes PdX(COMe) (tmeda) were easily obtained by reaction of PdXMe(tmeda) with carbon monoxide at ambient temperature and pressure. The reaction of PdMe₂(tmeda) with ethyl iodide is not selective and gives a 3:2 mixture of PdIMe(tmeda) and PdIEt(tmeda) together with a 3:2 mixture of propane and ethane. At 0°C, the reaction of PdMe₂(tmeda) with benzyl bromide and the reaction of PdMe(benzyl)(tmeda) with methyl bromide both afford the palladium(IV) species a-benzyl-b-bromo-cd-dimethyl-ef-(tmeda)palladium(IV) (4a), in which the benzyl group and the bromine atom are in a cis position. ¹H NMR spectroscopy in acetone-d₆ at 0°C shows that in the reaction of the platinum(IV) complex PtMe₂(tmeda) with benzyl bromide a trans oxidative-addition product, a-benzyl-f-bromo-bc-dimethyl-de-(tmeda)platinum(IV) (8a), is formed initially and that this complex rapidly isomerizes to a-benzyl-b-bromo-cd-dimethyl-ef-(tmeda)platinum(IV) (8b). We believe that similar addition/isomerization sequences occur in the reactions leading to 4a. The configuration of the thermodynamically stable palladium(IV) and platinum(IV) cis oxidative-addition products (4a and 8b) seems to be controlled by steric interaction between the benzyl group and the tmeda ligand.