Synthesis and stereochemistry of chiral aza-boraspirobifluorenes with tetrahedral boron-stereogenic centers.

We have synthesized chiral aza-boraspirobifluorenes and evaluated their structural and photophysical properties. Enantiomers were separated by chiral HPLC on a semi-preparative scale, and the absolute stereochemistry was determined by comparison of experimental circular dichroism (CD) spectra and theoretical electronic CD (ECD) spectra. A kinetic analysis combined with theoretical calculations revealed that the rate-determining step of the racemization involves the cleavage of the B-N bond.