Addition reactions and diazomethane capture by the intramolecular P–O–B FLP: tBu2POBcat

Abstract

FLPs, R₂POBcat (R = tBu 1, Mes 2), are readily derived from the reactions of the corresponding phosphine oxides, nBuLi and ClBcat. Despite the poor Lewis acidity of boron, these species react with PhOH, CO₂, CS₂, PhNCO, MesCNO, O₂, 9,10-phenanthrenedione, and diazomethanes to effect FLP addition reactions affording a series of heterocycles. The reaction of 1 with EtO₂CCHN₂ gives the bicyclic product, EtO₂CCHN₂(tBu₂POBcat)₂. High-level DFT calculations reveal that these cyclization processes proceed via 1,1-addition to the terminal N of diazomethane followed by 1,2-boron-migration affording the five membered rings. The reaction of the EtO₂CCHN₂ derivative with the second equivalent of FLP 1 is attributed to less steric demands.