The Phosphinoboration of N-Heterocycles

The addition of phosphinoboronate ester Ph₂ PBpin (pin=1,2-O₂ C₂ Me₄ ) (1) to a number of different N-heterocycles has been investigated. Reaction of 1 with pyridine resulted in highly selective formation of the corresponding 1,4-addition product, with addition of the electron-deficient Bpin group to the pyridine nitrogen atom and the phosphido group to the para carbon atom. Conversely, reactions of para-substituted pyridine derivatives occurred predominately to afford 1,2-addition products while quinoline reacted to afford the 1,2-adduct which ultimately isomerized to afford the corresponding 1,4-addition product. Preliminary computational studies have been undertaken to explore possible pathways for these transformations including transfer of the PPh₂^- anion from [B(PPh₂ )₂ pin]^- to the 4-position of a borenium/boronium activated pyridine and concerted pathways for 1,2-addition via intramolecular nucleophilic attack of PPh₂ at C2 of a Ph₂ PBpin-coordinated pyridine via a four-centered transition state and intramolecular transfer of PPh₂ to the 2-position of a boron-activated pyridine in a phosphido-bridged dimer involving a six-centered transition state.