Conversion of dibenzoxaphosphinines into 2-hydroxybiphenyl-2′- ylphosphane ligands and their BH3 adducts: The O-H δ+⋯Hδ--B hydrogen-hydrogen bond

Abstract

Chlorodibenz[c,e][1,2]oxaphosphinine 1Cl reacted with bulky organolithium compounds R1Li (R1 = tBu, mesityl) under mild conditions to yield organodibenz[c,e][1,2]oxaphosphinines 1 R1, which underwent ring-opening metathesis with less bulky organolithium compounds R2Li. After O-trimethylsilylation, 2-(trimethylsilyloxy)biphenylylphosphanes 2 were isolated. Reaction of 1 Cl with R2Li followed by silylation provided symmetrically P-substituted compounds 2. The formation of compounds 3 and 4 demonstrated the facile hydrolysis and oxidation of 1R1, the formation of compound 5cox indicated the sensitivity of 2 to hydrolysis and air oxidation. Methanolysis of 2 provided the free hydroxybiphenylylphosphanes 5 (for further modification by O-acylation), which were converted into air-stable borane adducts 6. The latter were unable to undergo O-acylation. Acylation of 5 followed by borane protection afforded O-acyloxybiphenylylphosphane-borane adducts, as demonstrated by the formation of compounds 7a and 8a. X-ray crystal structure analysesrevealed intermolecular O-H⋯H-B hydrogen-hydrogen bonds; for compound 6b the average H⋯H distance is 1.90 à, whereas for 6e a three-centre O-H⋯H2B interaction (av. H⋯H 2.25 à) was observed, which is still shorter than twice the van der Waals radius for hydrogen (2.4 à). To the best of our knowledge this is the first report on hydrogen-hydrogen bonds in functionally substituted phosphane-boranes and may explain the low reactivity at the OH group. Only on stronger heating was hydrogen evolved. Chlorodibenzoxaphosphinine 1Cl reacted with bulky R1Li reagents to give organodibenzoxaphosphinines, which reacted with less bulky R2Li reagents to yield 2-oxybiphenylylphosphanes. Protection of the air-sensitive hydroxybiphenylylphosphanes with borane reduced their reactivity. X-ray analysis gave evidence of intermolecular hydrogen-hydrogen bonds between protic hydroxy and hydridic borane groups. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.