Addition reactions to the intramolecular mesityl2P-CH2-CH2-B(C6F5)2 frustrated Lewis pair.

Hydroboration of dimesitylvinylphosphine with Piers' borane [HB(C6F5)2] gives the ethylene-bridged intramolecular frustrated P/B Lewis pair 1. It adds pyridine, tert-butyl isocyanide or pivalonitrile to the strongly electrophilic boron center to yield the respective adducts 5-7. Compound 1 undergoes a 1,1-phosphine/borane addition to the terminal nitrogen center of phenyl azide to yield the five-membered heterocycle 8, featuring a pendant N=N-Ph. This can be regarded as a boron-stabilized intermediate of a Staudinger reaction. Benzaldehyde and phenyl isocyanate undergo 1,2-P/B additions of 1 to their reactive carbonyl groups to yield the corresponding six-membered heterocycles 9 and 10, respectively. The P/B Lewis pair reacts with nitrosobenzene by 1,2-addition to the -N=O unit under P-N and O-B bond formation to give the six-membered heterocycle 11. The compounds 5-11 were characterized by X-ray crystal structure analyses.