New synthetic route to polyphosphine ligands bearing 1,2,4,5-tetrakis(phosphino)benzene framework: structural characterizations of 1,4-(PPh 2) 2-2,5-(PR 2) 2-C 6F 2 (R = Ph, iPr, Et)

Abstract

Reactions of 1,4-dibromo-2,5-difluorobenzene with two equivalents of lithium diisopropylamide at low temperature ( T < −90 °C) followed by a quench with a slight excess of ClPPh 2 afford 1,4-dibromo-2,5-bis(diphenylphosphino)-3,6-difluorobenzene ( 1) in good yields. Reacting 1 with two equivalents of BuLi followed by a quench with a slight excess of ClPR 2 yield novel 1,2,4,5-tetrakis(phosphino)-3,6-difluorobenzenes 1,4-(PPh 2) 2-2,5-(PR 2) 2-C 6F 2 (R = Ph ( 2a); R = i Pr ( 2b); R = Et ( 2c)) in moderate yields. Compounds 1 and 2a– c were characterized by multinuclear NMR spectroscopy and elemental analyses. In addition, molecular structures of 2a– c have been determined by single crystal X-ray crystallography. Phosphorus atoms of PPh 2/PR 2 substituents in 2a– c are displaced from the plane of the central phenyl ring due to steric interactions with neighboring groups.