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 NCR(1) (R(1) = Et (1), NMe(2) (2), Ph (3)) species react smoothly with acyclic nitrones such as the ketonitrones Ph(2)C═N(O)R(4) (R(4) = p-MeC(6)H(4) (4), p-ClC(6)H(4) (5)) and the aldonitrone p-MeC(6)H(4)CH═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 [PdCl(2)(R(1)CN){N(a)═C(R(1))ON(R(4))C(b)(R(2)R(3))}]((a-b)) (7a-13a; R(2), R(3) = Ph; R(4) = C(6)H(4)Me-p, R(1) = Et (7), NMe(2) (8), Ph (9); R(4) = C(6)H(4)Cl-p, R(1) = Et (10), NMe(2) (11), Ph (12); R(2) = H, R(3) = C(6)H(4)Me-p, R(4) = Me, R(1) = NMe(2) (13)) and [PdCl(2){N(a)═C(R(1))ON(R(4))C(b)(R(2)R(3))}(2)]((a-b)) (7b-13b), respectively. Inspection of the obtained data and their comparison with the previous results indicate that the Pd(II) centers provide substantially greater activation of RCN ligands toward the 1,3-dipolar cycloaddition than the relevant Pt(II) 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 (1)H and (13)C{(1)H} 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)(2)Pd(II) complexes by treatment with 1,2-(diphenylphosphino)ethane, and the heterocycles were characterized by high-resolution ESI(+)-MS and (1)H and (13)C{(1)H} spectroscopy.
Department of Chemistry, St. Petersburg State University, Universitetsky Pr. 26, 198504 Stary Petergof, Russian Federation.
Andreii S
AS
Kritchenkov
Nadezhda A
NA
Bokach
Galina L
GL
Starova
Vadim Yu
VY
Kukushkin