RhCl3-promoted hydroboration–oxidation reactions of representative alkenyl nitriles have been carried out. Carbanion of acetonitrile is formed from the carbon carbon heterolytic bond breaking in allyl cyanide during hydroboration. An unexpected process-trimerization of carbanion of acetonitrile has lead to the formation of 4-amino-2, 6-dimethylpyrimidine.