Oxidorhenium(v) complexes of apigenin: a rare example of catalysis with O,O-bidentate ligands obtained from a naturally renewable resource

Abstract

When Re(V) precursor [ReOCl₃(PPh₃)₂] (P1) is reacted with apigenin (HL1, Hapg), the flavone coordinates as an O,O-bidentate ligand via the deprotonated 5-OH group and its neighboring ketone to the Re center. Novel oxidorhenium(V) complex [ReOCl₂(PPh₃)(L1)] (1) was successfully synthesized in boiling acetone in sufficient yield. To increase the solubility of apigenin and its respective complexes, the other two –OH groups in the 7 and 4′-position of the apigenin structure were protected. To do so, well-established acetyl- and TBDMS- (tBuMe₂Si-) protecting groups were introduced to the apigenin frame, resulting in the novel ligands HL2 and HL3, respectively. Resulting oxidorhenium(V) complexes [ReOCl₂(PPh₃)(L2)] (2) and [ReOCl₂(PPh₃)(L3)] (3) showed much increased solubilities in standard organic solvents. All three complexes 1–3 were subsequently tested in catalytic olefin epoxidation, which is the first time for a Re complex with an O,O-bidentate ligand. All three complexes showed activity for substrate cyclooctene, with complex 2 giving a high activity for such complexes (TON = 300). The novel ligands HL2 and HL3 as well as all three novel complexes 1–3 were fully characterized by NMR and IR spectroscopy, MS spectrometry and elemental analysis. In addition, the solid-state structures of HL2, HL3 and 1–3 were determined by single-crystal X-ray diffraction analysis.