A Palladium(II) Center Activates Nitrile Ligands toward 1,3-Dipolar Cycloaddition of Nitrones Substantially More than the Corresponding Platinum(II) Center

Palladium­(II)-coordinated NCR1 (R1 = Et (1), NMe2 (2), Ph (3))
species react smoothly with acyclic nitrones such as the ketonitrones
Ph2CN­(O)­R4 (R4 = p-MeC6H4 (4), p-ClC6H4 (5)) and the
aldonitrone p-MeC6H4CHN­(O)­Me
(6) in the corresponding nitrile media. This reaction
proceeds as a consecutive two-step intermolecular cycloaddition to
give the mono- and bis-2,3-dihydro-1,2,4-oxadiazole complexes [PdCl2(R1CN)­{NaC­(R1)­ON­(R4)Cb(R2R3)}](a−b) (7a–13a; R2, R3 = Ph; R4 = C6H4Me-p, R1 = Et (7), NMe2 (8), Ph (9); R4 = C6H4Cl-p, R1 = Et (10), NMe2 (11), Ph (12); R2 = H, R3 = C6H4Me-p, R4 = Me, R1 = NMe2 (13)) and [PdCl2{NaC­(R1)­ON­(R4)­Cb(R2R3)}2](a−b) (7b–13b), respectively.
Inspection of the obtained data and their comparison with the previous
results indicate that the PdII centers provide substantially
greater activation of RCN ligands toward the 1,3-dipolar cycloaddition
than the relevant PtII centers. The palladium­(II)-mediated
1,3-dipolar cycloaddition of ketonitrones to nitriles is reversible.
All complexes were characterized by elemental analyses (C, H, N),
high-resolution ESI-MS, and IR and 1H and 13C­{1H} NMR spectroscopy. The structure of trans-7b was determined by single-crystal X-ray diffraction.
Metal-free 5-NR′2-2,3-dihydro-1,2,4-oxadiazoles
(7c–13c) were liberated from the
corresponding (2,3-dihydro-1,2,4-oxadiazole)2PdII complexes by treatment with 1,2-(diphenylphosphino)­ethane, and the
heterocycles were characterized by high-resolution ESI+-MS and 1H and 13C­{1H} spectroscopy.