Tetrahydrofuran ring opening and related reactions with an N-heterocyclic carbene–boryl trifluoromethanesulfonate

Reaction of the triflate group in dipp-Imd-BH(2)OTf with nucleophiles generally occurs by direct substitution to give products dipp-Imd-BH(2)Nu. In contrast, reaction of this boryl triflate with aryloxides (ArO(-)) in THF resulted in insertion of THF with ring opening in between the NHC-boryl electrophile and the aryloxide to give products dipp-Imd-BH(2)O(CH(2))(4)OAr. The incorporation of THF was observed with other nucleophiles such as ethylthiolate and trimethylsilanolate whose conjugate acids have pK(a) values similar to that of phenol. The cleavage of ethyl acetate and oxetane is reported as well. A possible mechanism is proposed.