Experimental and computational study of metal-free Brønsted acidic ionic liquid catalyzed benzylic C(sp3)[sbnd]H bond activation and C[sbnd]N, C[sbnd]C cross couplings

Abstract

Metal and solvent free synthesis of tetrasubstituted imidazoles and tetrasubstituted pyrazines from 1,2-diketones and aliphatic amines is studied experimentally and computationally. Various ionic liquids (ILs) are taken as a catalyst in 10 mol% for the reaction. The best conversion to products is achieved in the presence of two ILs: [taH4][I4] and[deaH][HSO4]. The low-cost, simple Brønsted acidic IL [deaH][HSO4] catalyzed reaction yields 75% PRO1(1-benzyl-2,4,5-triphenyl-1H-imidazole) and 18% PRO2 (2,3,5,6-tetraphenylpyrazine) at 90 °C in 1 h. Further optimization with the IL shows that the products yield can be tuned with changing parameters: time, catalyst loading, and temperature. Optimization shows that PRO2 formation becomes predominant after 3 h. Density functional calculations show that PRO1 formation occurs via condensation to diimine, and further C(sp3)–H activated cyclization stages. An unusual hydrogen shift transition state (∆G‡ = 29.8 kcal) has a crucial role in the ring closure and subsequent annulation to the five- and six-membered rings (PRO1, PRO2).