Metal-free and PdII-promoted [2+3] cycloadditions of a cyclic nitrone to phthalonitriles: Syntheses of oxadiazolines as well as phthalamide-PdII and dihydropyrrolyl-iminoisoindolinone-Pd II complexes with high catalytic activity in suzuki-miyaura cross-coupling reactions

Article abstract of DOI:10.1002/chem.200800510

The previously unknown reactions between phthalonitriles, 1,2-(CN) ₂(C₆)R¹R²R³R⁴ 1 (1a, R¹ = R² = R³ = R⁴ = H; 1b, R¹ = R² = R⁴ = H, R³ = CH ₃; 1e, R¹ = R⁴ = H, R² = R ³ = Cl; 1d, R¹ = R² = R³ = R ⁴ = Cl; 1e, R¹ = R² = R³ = R ⁴ = F), and a cyclic nitrone, ^-O⁺ N=CHCH ₂CH₂CMe₂ 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 monocycloadducts 4, the second [2+3] coupling at the still-unreacted cyano group requires its activation by coordination to Pdc, 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, ¹H, and ¹³C NMR spectroscopy, ESI MS or FAB⁺ MS, elemental analyses and, in the case of 4c, 5 a, 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 × 10⁵ h^-1.