Phosphine promoted substituent redistribution reactions of B-chlorocatechol borane: Molecular structures of ClBcat, BrBcat and L center dot ClBcat (cat=1,2-O2C6H4; L = PMe3, PEt3, PBu3t, PCy3, NEt3)

In order to evaluate the potential for side reactions when using B-chloroca techol borane (ClBcat) in stoichiometric or catalytic transformations invol ving metal phosphine complexes, we examined the interaction between ClBcat and a series of PR3 compounds. Reactions of ClBcat with the basic phosphine s PMe3, PEt3, PMe2Ph and PBu3t, in exactly 1∶1 stoichiometry, all aff orded crystalline adducts of the form R3P . ClBcat, which have been charact erised 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. Th e 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 ha ve been characterised by single crystal X-ray diffraction. In ClBcat and Br Bcat, the halogen contributes little pi -bonding to boron as the B-O bond l engths 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 re action between ClBcat and the less basic phosphines PPh3 and PPh2Me yielded only the redistribution products R3P . BCl3 and B(2)cat(3). In fact, react ion of all of the phosphines with an excess of ClBcat gave only redistribut ion 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 prov ed 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 st ate.