P/B Ketene Adduct Formation from Acyl Chlorides at a Vicinal Phosphane/Borane Frustrated Lewis Pair

The intramolecular frustrated phosphane/borane Lewis pair Mes2PCH2CH2B(C6F5)2 (3) reacts with acetyl chloride to give a 1 : 1 mixture of the FLP-ketene adduct, 6a, and the FLP-HCl addition product, 7. Similarly, the acyl chlorides phenylacetyl chloride and isobutyryl chloride react with 3 to give the respective substituted FLP ketene adducts, 6b and 6c, with HCl elimination. A series of aroyl chlorides reacts with 3 by formation of the respective aroyl-phosphonium/chloroborate zwitterions, 11a–d. The hetero-aromatic furoyl chloride reacts with 3 in a more complicated, but mechanistically probably closely related pathway, to form the 2 : 1 reaction product, 14. Most of the products were characterized by X-ray diffraction.