H3O+SbF6− as a reagent in cyclotrimerization of nitriles

Cyclotrimerization of nitriles requires high temperatures and extreme pressures, [1] however use of catalysts greatly diminishes the need for extreme conditions. Most commonly used catalysts are SmI2/amine,[2] trifluoromethanesulfonic acid (TfOH)[3] and other Lewis acids.[4,5] While reaction conditions and yields are well documented, the mechanism of conversion is still relatively poorly investigated.[3,6] Focus of our research was the use of strong Lewis acids MF5 (M = As, Sb, Nb, Ta) and their products with different nitriles (R–CN). The system was investigated and reactive species were identified as well as different products. One of the obtained results was 2,4,6-trisubstituted-1,3,5-triazinium cation (1) which is a product of aforementioned cyclotrimerization. In order for reaction to proceed acidic proton (H+) is required in combination with weakly coordinating MF6 − moiety. Further optimization yielded easy-tohandle solid reagent H3O+MF6 − (M = As, Sb, Nb, Ta), which was found to produce equal results. Reaction takes place with liquid nitrile in inert conditions at room temperature, where nitrile also acts as a solvent. In addition, co-product competing to the process of 1,3,5-triazine formation was also prepared. Dissolution of 2,2-dimethylpropanenitrile (tBuCN) and H3O+SbF6 − in dichloromethane (DCM) yielded scarcely soluble 1-[(2,2-dimethylpropanoyl)amino]-2,2-dimethylpropaniminium cation (2) which is a dimer of two starting nitriles and corresponding anion. Our results will further elucidate the process of acidic activation of nitriles and formation of 2,4,6-trisubstituted-1,3,5-triazines.