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)

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 PBu3t, in exactly 1 & ratio;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 B-11{H-1} and P-31{H-1} NMR spectra with no B-P coupling observed. The low temperature B-11{H-1} and P-31{H-1} 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 PBu3t have been characterised by single crystal X-ray diffraction. In ClBcat and BrBcat, the halogen contributes little pi -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 B(2)cat(3). 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 B-11{H-1} 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 B(2)cat(3). 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.