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 PBut3, 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 PBut3 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.