Abnormal N-Heterocyclic-Carbene-Mediated Fixation of CO2 and N2O, and the Activation of Tetrahydro­furan and Tetrahydrothiophene under Ambient Conditions

Abstract

A strong σ-donor abnormal N-heterocyclic carbene (aNHC), 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimidazol-5-ylidene (L1), reacted with B(C₆F₅)₃ in the presence of CO₂ and N₂O to form the adducts [aNHC·CO₂·B(C₆F₅)₃] (1) and [aNHC·N₂O·B(C₆F₅)₃] (2), respectively, in high yields at room temperature within a very short period of time. Furthermore, this aNHC/B(C₆F₅)₃ reagent system was found to be very efficient for the ring-opening of tetrahydrofuran (thf) and tetrahydrothiophene (tht), affording 3 and 4, respectively, within 5–10 min at room temperature in good yields. Interestingly, this aNHC forms the air-stable Lewis acid/base adduct [aNHC·B(C₆F₅)₃] (5) with B(C₆F₅)₃ in the absence of any suitable substrate upon keeping this mixture in solution. DFT calculations showed that the Lewis acid/base adduct 5 is energetically less stable than compounds 1–4, and this may be the driving force for the formation of compounds 1–4 in the presence of small molecules such as CO₂, N₂O, thf and tht. All the new compounds were characterized spectroscopically (¹H, ¹³C, ¹⁹F and ¹¹B NMR, IR spectroscopy) in solution and the solid-state structures were unambiguously established by single-crystal X-ray diffraction analysis.

The abnormal N-heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimidazole-5-ylidene, reacts with B(C₆F₅)₃ in the presence of CO₂ or N₂O to form [aNHC·CO₂·B(C₆F₅)₃] or [aNHC·N₂O·B(C₆F₅)₃], respectively, in high yields at room temperature within a very short period of time.

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