Steric and electronic effects on E/Z composition of exocyclic cyclopalladated N-benzylideneamines

Exocyclic six-membered cyclopalladated dimers 2a-2c, [Pd(C-N)Br](2) have been prepared by reaction of palladium(II) acetate with N-benzylidene-2-phenylanilines 1a-1c, RCH = NC(6)H(4)-2-C(6)H(5) {1a, R = 2,6-Cl(2)C(6)H(3); 1b, R = 2,6-F(2)C(6)H(3) and 1c, R = 2,4,6-(CH(3)O)(3)C(6)H(2)}, and a subsequent metathesis reaction with LiBr. Reaction of 2 with PPh(3) produced mononuclear compounds 3, [Pd(C-N)Br(PPh(3))]. Analysis of the (1)H NMR methinic region of 2 and 3 revealed their E/Z composition. 2a and 2b present imines in E and Z configuration in an approximate ratio of 1:1, white 2c consists almost exclusively of imines in E configuration. Compounds 3 also present imines in E and Z configuration, the former are more abundant in all cases (greater than or equal to 4:1). The crystal structure of 3c with the imine in E configuration has been determined. It crystallizes in the monoclinic space group P2(1)/a with a = 19.258(3) Angstrom, b = 13.177(2) Angstrom, c = 14.660(2) Angstrom and beta = 113.78(2)degrees. Steric and electronic effects on the E/Z composition of 2 and 3 and related exocyclic cyclopalladated N-benzylidenebenzylamines suggest that, for compounds of this type, the E --> Z isomerization takes place via a rotation through the N = C bond which is assisted by pi back bonding of the palladium(II) center to the iminic function. (C) 1997 Elsevier Science S.A.