Synthesis, Reactivity and Demetallation of Tungsten–Azacyclic Carbeniums via Cycloalkenation of Tungsten–Alkynylamine Compounds

Treatment of tungsten–η1-α,δ-alkynylamine compounds with aldehyde and BF3·Et2O led to cycloalkenation reaction, giving good yields of tungsten–η1-pyrrolylidium salts. These azacyclic carbeniums provide a short synthesis of α-alkylidene-γ-lactam via oxidation with m-CPBA. In contrast with tungsten–η1-oxacyclic carbenium, this salt reacts with one molecule of organometallic reagents such as NaBH3CN, CH3MgBr and Me2CuLi to give tungsten–η1-4,5-dihydropyrrole complexes. An alternative use of this cyclialkenation is to provide a short synthesis of 3-vinyl-Δ2-pyrrolines.