Phosphole formation by 1,1-carboboration – reactions of bis-alkynyl phosphanes with a frustrated P/B Lewis pair

Abstract

The arylbis(phenylethynyl)phosphanes 1a,b (aryl = mesityl, 2,4,6-triisopropylphenyl) react with the frustrated P/B Lewis pair (P/B FLP) mes₂PCH₂CH₂B(C₆F₅)₂ (4) to give mixtures of three products; the major products, the phosphole systems 2a,b, are formed by a sequence of 1,1-carboboration reactions. One of the minor compounds (6a,b) is formed by 1,1-carboboration followed by internal 1,2-FLP addition to the remaining CC triple bond. The other minor compound of the product mixture (5a,b) is obtained by 1,2-FLP addition to one alkynyl moiety of the starting material. The products 5a, 6b and a derivative of the phosphole 2a (formed by FLP reaction with a terminal alkyne) were characterized by X-ray diffraction. The reaction of the arylbis(pentynyl)phosphanes 1c,d with the FLP 4 selectively gave the respective –B(C₆F₅)₂/–CH₂CH₂–Pmes₂ substituted phospholes 2c,d which were isolated as orange solids in high yields. Rapid strongly temperature dependent equilibration between open and closed P⋯B FLP isomers was detected for both systems by NMR spectroscopy.