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

Abstract

Reaction of the triflate group in dipp-Imd–BH₂OTf with nucleophiles generally occurs by direct substitution to give products dipp-Imd–BH₂Nu. 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–BH₂O(CH₂)₄OAr. 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.