ABSTRACT
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.
ABSTRACT
The synthesis of phosphinoboronate esters containing a single P-B bond is reported, together with preliminary reactivity studies towards a range of organic substrates. These compounds add readily to aldehydes, ketones, aldimines, and α,ß-unsaturated enones to give primarily the corresponding 1,2-addition products containing a new P-C bond. The first examples of transition-metal-catalyzed phosphinoborations of C-C multiple bonds in which P-C and B-C bonds are formed in a single step are also disclosed; allenes react by a highly regioselective 1,2-addition whereas terminal alkynes undergo a formal 1,1-addition.
