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

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.