Palladium(II), silver(I), and gold(I) complexes of a new class of chiral bicyclic [1,2,3]-triazolooxazine derived N-heterocyclic carbenes (NHCs): Synthesis, structure and application studies

A new class of chiral bicyclic [1,2,3]-triazolooxazine derived N-heterocyclic carbene (NHC) ligands was synthesised in its enantiopure form from commercially available, cheap amino acid without undertaking any chiral resolution. In particular, the bicyclic N-heterocyclic carbene precursor, (S)-7-benzyl-6,6-(R¹)₂–2-R²-[1,2,3]-triazolooxazinium iodide [R¹ = R² = Me (1a); R¹ = H, R² = Me (2a); R¹ = H, R² = Et (3a)] salts were conveniently prepared by the N-alkylation reactions of the corresponding [1,2,3]- triazolooxazine derivatives with alkyl iodides in ca. 37–73% yields. The copper mediated [3 + 2] cycloaddition reaction of the PhCH₂CH(N₃)CR₂OH (R = H, Me) azido alcohol compounds and propargyl bromide gave the desired [1,2,3]-triazolooxazines. These chiral bicyclic [1,2,3]- triazolooxazine derived N-heterocyclic carbene ligands were characterised in the form of its silver (NHC)AgCl (1–3)b, gold (NHC)AuCl (1–3)c, and the palladium (NHC)₂PdCl₂ (1–3)d and the PEPPSI type (NHC)PdI₂(NC₅H₅) (1–3)e complexes (NHC = (S)-7-benzyl-6,6-(R¹)₂–2-R²-[1,2,3]-triazolooxazin-3-ylidene; R¹ = H, Me and R² = Me, Et; PEPPSI = Pyridine Enhanced Precatalyst Preparation Stabilisation and Initiation).