Abstract
The previously unknown reactions between phthalonitriles, 1,2-(CN)(2)(C6)(RRRR4)-R-1-R-2-R-3 1 (1a, R-1 = R-2 = R-3 = R-4 = H; 1b, R-1 = R-2 = R-4 = H, R-3 = CH3; 1c, R-3 = R-4 = H, R-2 = R-3 = Cl; 1d, R-1 = R-2 = R-3 = R-4 = Cl; 1e, R-1 = R-2 = R-3 = R-4 = F), and a cyclic nitrone, -O+ N=CHCH2CH2(CMe2 2, proceed under heating in a sealed tube to give phthalimides 3, 2-oxadiazolyl-benzonitriles 4 or ortho-bis(oxadiazolyl)tetrafluorobenzene 4e'. In the presence of palladium(II) chloride, phthalonitriles 1 react with 2 at room temperature, to give bis(pyrrolidin-2-ylidene)phthalamide Pd-II complexes 5 via metal-promoted rupture of the N-O bond of the oxadiazoline ring. The ketoimine ligands thus generated can be liberated from the metal by displacement with a diphosphine. Although the first [2+3] cycloaddition of 2 to 1 can occur in the absence of the metal to give the mono-cycloadducts 4, the second [2+3] coupling at the still-unreacted cyano group requires its activation by coordination to Pd-II, affording complexes 6 containing two ligated oxadiazolyl-benzonitriles. These ligands undergo either i) further cycloaddition with 2 to afford ultimately (upon rearrangement) the bis(pyrrolidinylidene)phthalamide complexes 5 or ii) N-O bond cleavage in the oxadiazoline ring with intramolecular attack of the imine nitrogen on the cyano carbon and bridging to a second Pd-II center to afford dimeric palladium(II) complexes 7, with chloride bridges, that bear a dihydropyrrolyl-iminoisoindolinone, a new type of ligand.
The compounds were characterized by IR, H-1, and (CNMR)-C-13 spectroscopy, ESI MS or FAB(+) MS, elemental analyses and, in the case of 4c, 5a, 5c, and 7c, also by X-ray diffraction analysis. Complexes 5a and 7c show high catalytic activity for the Suzuki-Miyaura cross-coupling reaction of bromobenzene and phenylboronic acid and give biphenyl in high yields with turnover frequencies (TOFs) of up to 9.0 x 10(5) h(-1).