Coupling of cationic olefin complexes of platinum(II) with potential ambident nucleophiles

Abstract

Cationic complexes [PtCl(η 2 -CH 2 CHR)(tmeda)] + (R = H, 1a , or Me, 1b ) have shown a different reactivity towards aromatic nucleophiles: 1a couples both with the heteroatom and a ring carbon, while, in the same experimental conditions, 1b undergoes only deprotonation. The reactivity of [PtCl(η 2 -CH 2 CHR)(tmeda)] + (R = H, 1a , or Me, 1b ; tmeda = N,N,N ′ ,N ′-tetramethyl-1,2-diaminoethane) towards some ambident nucleophiles like anilines and phenolate anion has been tested. The reaction of 1a with N -methylaniline gives immediately N -addition to the coordinated ethene ( 3a ), but, in the presence of an inorganic carbonate, a partial rearrangement, with the para carbon of the phenyl ring taking the place of nitrogen, is observed ( 4a and 5a ). Reaction with a tertiary aromatic amine, such as N,N -dimethylaniline, leads exclusively to the C -coupled species. The phenolate anion acts initially as an oxygen donor, however the resulting species ( 6a ), in contact with free phenol, rearranges to C -bonded species ( 7a ). For free phenol/ 6a ratios ⩾ 5 the rearranged product has an isomeric ortho / para ratio of ≈3. For lower free phenol/ 6a ratios (⩽ 1) oligomeric complexes, in which two or three platinum ethanide moieties are bound to the same phenol ring, are also formed. In the case of 1b , the above described reactivity has to compete with the base-induced deprotonation of propene, leading to formation of the allyl-bridged platinum dimer [{PtCl(tmeda)}(μ-η 1 :η 3 -CHCHCH 2 ){Pt(tmeda)}] + . The X-ray crystal structure of 1b has also been determined; the structural parameters are very similar to those previously reported for 1a . DFT calculations have shown a similar activation of the two complexes towards nucleophilic addition at the coordinated olefin, although in 1b the electrophilic character of the olefin is masked by the Brønsted acidity of the propene methyl protons.