Palladium complexes of a new type of N-heterocyclic carbene ligand derived from a tricyclic triazolooxazine framework

A new type of tricyclic triazolooxazine derived N-heterocyclic carbene precursors were developed by the alkylation reaction of a tricyclic triazolooxazine framework. In particular, the reaction of 5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazine with methyl iodide and ethyl iodide yielded the tricyclic triazolooxazine derived N-heterocyclic carbene precursors, (1-2)a, in 67 -84% yield. The tricyclic triazolooxazinium iodide salts, (1-2)a, underwent metallation in a straight forward manner upon treatment with PdCl2 in the presence of K2CO3 in pyridine to give the trans-{3-(R)-5a,6,7,8,9,9a-hexahydro-4H-benzo[b][1,2,3]triazolo[1,5-d][1,4]oxazin-4-ylidene} PdI2(pyridine) [R = Me (1b), Et (2b)] complexes in 23 -25% yield. The new tricyclic triazolooxazine derived N-heterocyclic carbene moiety, as stabilized upon binding to palladium in the (1-2)b complexes, was structurally characterized by the X-ray single crystal diffraction studies.