Selective Nucleophilic Oxygenation of Palladium-Bound Isocyanide Ligands: Route to Imine Complexes That Serve as Efficient Catalysts for Copper-/Phosphine-Free Sonogashira Reactions

Metal-mediated
reactions between cis-[PdCl2(CNR)2] (R = Xy (1), Cy (2), tBu (3), C6H3(Cl-2)­Me-6 (4)) and the ketonitrones Ph2CN­(O)­C6H4R′ (R′ = Me (5), Cl
(6)) proceed in a 1:1 molar ratio as selective nucleophilic
oxygenations and provide the imine–isocyanide complexes [PdCl2{N­(C6H4R′)CPh2}­(CNR)] (7–14: R′ = Me, R
= Xy (7), Cy (8), tBu (9), C6H3(Cl-2)­Me-6 (10);
R′ = Cl, R = Xy (11), Cy (12), tBu (13), C6H3(Cl-2)­Me-6
(14)) in excellent yields (90–94%), while the
reaction of the cis-[PdCl2(CNR)2] complexes with aldonitrones proceeds as 1,3-dipolar
cycloaddition, giving carbene adducts which then convert to the imine
complexes. Theoretical calculations at the DFT level indicate that,
in the case of aldonitrones, formation of the imine complexes occurs
preferably via a cycloaddition/splitting pathway, including the generation
of a cycloadduct, while, in the case of ketonitrones, both the cycloaddition/splitting
route and the direct oxygen atom transfer pathway are equally plausible
from a kinetic viewpoint. Complexes 7–14 were characterized by elemental analyses (C, H, N), by high-resolution
ESI+-MS, IR, and 1H and 13C­{1H} NMR spectroscopy, and also by X-ray diffraction (for 8). The catalytic activity study conducted for 7–14, taken as the catalysts in the Cu-/phosphine-free
Sonogashira reaction, was evaluated for a typical model system comprising
4-methoxyiodobenzene and phenylacetylene and affording 1-methoxy-4-(phenylethynyl)­benzene.
The obtained data indicate that 7–14 exhibit a high catalytic activity (yields up to 95%, TONs up to
15000), and these catalysts are among the best studied so far.