Phosphine promoted substituent redistribution reactions of B-chlorocatechol borane: molecular structures of ClBcat, BrBcat and L·ClBcat (cat = 1,2-O2C6H4; L = PMe3, PEt3, PBut3, PCy3, NEt3)

Abstract

In order to evaluate the potential for side reactions when using B-chlorocatechol borane (ClBcat) in stoichiometric or catalytic transformations involving metal phosphine complexes, we examined the interaction between ClBcat and a series of PR₃ compounds. Reactions of ClBcat with the basic phosphines PMe₃, PEt₃, PMe₂Ph and PBu^t₃, in exactly 1 ∶ 1 stoichiometry, all afforded crystalline adducts of the form R₃P·ClBcat, which have been characterised spectroscopically. All display broad singlets at room temperature in both ¹¹B{¹H} and ³¹P{¹H} NMR spectra with no B–P coupling observed. The low temperature ¹¹B{¹H} and ³¹P{¹H} NMR spectra, however, do show B–P coupling, suggesting rapid dissociation at room temperature. The compounds ClBcat, BrBcat, and the adducts between ClBcat and PMe₃, PEt₃ and PBu^t₃ have been characterised by single crystal X-ray diffraction. In ClBcat and BrBcat, the halogen contributes little π-bonding to boron as the B–O bond lengths are identical in both compounds. The reaction of ClBcat with a small excess of PCy₃ yielded Cy₃P·ClBcat, which has also been characterised by single crystal X-ray diffraction and shows considerable distortion. The reaction between ClBcat and the less basic phosphines PPh₃ and PPh₂Me yielded only the redistribution products R₃P·BCl₃ and B₂cat₃. In fact, reaction of all of the phosphines with an excess of ClBcat gave only redistribution products, although there was evidence for an intermediate in the ¹¹B{¹H} NMR spectra, with PMe₃ and PEt₃. The isolated R₃P·ClBcat adducts proved unstable, even at low temperature in the solid state, eventually leading to R₃P·BCl₃ and B₂cat₃. Reaction of ClBcat with NEt₃ afforded Et₃N·ClBcat, which was characterised spectroscopically and by single crystal X-ray diffraction. This adduct is stable both in solution and in the solid state.