Direct Synthesis of Nitriles from Alcohols with Trialkylsilyl Cyanide Using Brønsted Acid Montmorillonite Catalysts

We reported a simple protocol for nitrile synthesis by the direct cyanation of various secondary/tertiary benzylic and allylic alcohols with trialkylsilyl cyanide (TASCN) with moderate to excellent yields as well as high regioselectivities and turnover numbers/frequencies catalyzed by solid Brønsted acids of metal ion-exchanged montmorillonites (M-Mont; M = Sn and Ti). The easily prepared montmorillonites demonstrated a significant catalytic performance for the cyanation of various alcohols that is far superior to those by control catalysts, such as proton ion-exchanged zeolites, homogeneous Lewis acids, and homogeneous Brønsted acids. The water-tolerant character of the montmorillonites led to high catalytic activities, even in a crude solvent. In addition, they were easily recovered from the reaction mixtures by filtration and reused without any loss of activity. The corresponding trimethylsilyl (TMS) ethers of alcohols were also found transformable into the nitriles using TASCN in the presence of the montmorillonite catalysts. However, the conversions of alcohols with TASCN were faster than those of the corresponding TMS ethers with TASCN, indicating that the activity of the TMS ethers was lower than that of alcohols. It was determined that the cyanation of allylic alcohols having a conjugated polyene structure afforded a single isomer among the possible regioisomers. Interestingly, the use of tert-butyldimethylsilyl cyanide is superior in the cyanation of alcohols and their TMS ethers to that of TMSCN.