Platinum(II) complexes with novel diisocyanide ligands: Catalysts in alkyne hydroarylation

A series of novel diisocyanide ligands (o-CNC6H 4O)2Y (diNC-1: Y = P(O)Ph; diNC-2: Y = o-C(O)C 6H4C(O); diNC-3: Y = m-C(O)C6H4C(O); diNC-4: Y = C(O)C2H4C(O); diNC-5: Y = trans-C(O)C 2H2C(O)) was successfully synthesized by reaction of lithium 2-isocyanophenate (generated in situ from benzoxazole and n-BuLi) and a diacylic or phosphonic dichloride. The corresponding platinum(II) complexes of general formula [PtX2(diNC)]1,2 (X = Cl, Me; diNC = diisocyanide ligand) were isolated by simple substitution of 1,5-cyclooctadiene in the starting [PtX2(COD)] complexes. The structure of the complexes, mononuclear or dinuclear, was confirmed by single-crystal X-ray analysis. A dinuclear complex of formula {(?-diNC)[cis-PtCl 2(PPh3)]2} could also be obtained with the diisocyanide ligand having a rigid fumaryl bridge and consequently the isocyanide moieties pointing in opposite directions. All the complexes were employed as catalysts in the hydroarylation of alkynes, showing generally good activity and selectivity toward the trans-hydroarylation product. With N-methylindole as aromatic substrate the major product was instead a heterocycle:alkyne 2:1 adduct.