Unsaturated Vicinal Frustrated Lewis Pair Formation by Electrocyclic Ring Closure and Their Reaction with Nitric Oxide

Abstract

The Lewis acidic β-styryl-B(C6F5)2 reagent 6a undergoes a clean 1,1-carboboration reaction with 1-(PMes2)-2-cyclohexenyl acetylene 9 at 60 °C to give the vicinal P/B-substituted conjugated triene product 10a. At 80 °C this undergoes a stereoselective thermally induced disrotatory electrocyclic ring closure to give the cyclohexadiene-derived P/B system 11. Subsequent TEMPO oxidation gave the substituted phenylene-bridged P/B product 12. Both 11 and 12 are active phosphane/borane frustrated Lewis pairs (FLPs). The FLP 11 reacts in a typical way with phenylacetylene to give the phosphonium/alkynylborate product 13. Compound 12 cleaves dihydrogen at near ambient conditions to give the respective phosphonium/hydridoborate zwitterion 14. Both the FLPs 11 and 12 cooperatively add P/B to the nitrogen atom of nitric oxide (NO) within minutes at room temperature to give the persistent P/B FLPNO• radicals 19 and 21, respectively (both characterized by X-ray diffraction and by EPR spectroscopy). The FLPs 11 and 12 are thermally robust. At elevated temperatures (11: 75 °C, 12: 100 °C) they undergo a coupling reaction with dimethyl acetylenedicarboxylate with carbon–carbon bond activation at a P-mesityl substituent.