Gold-catalyzed cycloisomerization of trifluoromethylated allenols: sustainability and mechanistic studies
Trifluoromethyl-substituted α-allenols are cyclized to the corresponding 2,5-dihydrofurans in the presence of neutral or cationic gold catalysts. The catalyst can be recycled if ionic liquids (in particular [BMIM][PF6]) are used as reaction medium. The allene forms droplets in the ionic liquid generating a heterogeneous two-phasic system. Kinetic studies in organic solvents using electronically different gold catalysts allowed to identify the protodeauration of a σ-gold-species as rate-determining step of typical allene cyclizations. Only if a CF3 group is present at C-5, π-complex formation is rate-limiting.