Chiral palladacycles based on resorcinol monophosphite ligands: the role of the meta-hydroxyl in ligand C-H activation and catalysis

The reactions of a range of chiral resorcinol monophosphite ligands with [PdCl2(NCMe)(2)] was investigated in order to establish whether the meta-hydroxyl function was involved in the orthometallation processes. These ligands underwent facile orthopalladation at room temperature in the presence of Et3N, whilst the equivalent hydroxyl-free analogues needed more forcing conditions to induce orthometallation. When the hydroxyl function was replaced by a similar sized methyl group no orthometallation occurred, even on heating. Furthermore the hydroxyl group influences both the structure and isomerism in the resultant palladacycles via hydrogen bonding to adjacent chloride ligands. Similarly, the hydroxyl function leads to higher enantiocontrol in the asymmetric allylation of benzaldehyde with allyl tributyltin. Representative examples of the ligands and the palladium complexes obtained were characterised by single crystal X-ray diffraction.