Reactions of an Intramolecular Frustrated Lewis Pair with Unsaturated Substrates: Evidence for a Concerted Olefin Addition Reaction

The intramolecular frustrated Lewis pair (mesityl)(2)P-CH2-CH2-B(C6F5)(2) was generated in situ by HB(C6F5)(2) hydroboration of dimesitylvinylphosphine. The compound reacts with 1-pentyne by C-H bond cleavage. It undergoes a 1,2-addition to the carbonyl group of trans-cinnamic aldehyde to yield a zwitterionic six-membered heterocycle by B-O and P-C bond formation. The Lewis pair regioselectively adds to the electron-rich C=C double bond of ethyl vinyl ether, and it selectively undergoes an exo-cis-2,3-addition to norbornene. A combined experimental/theoretical study suggests that this reaction takes place in an asynchronous concerted fashion with the B-C bond being formed in slight preference to the P-C bond. The addition products were characterized by X-ray crystal structure analyses.