Abnormal N-Heterocyclic-Carbene-Mediated Fixation of CO2and 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.