Oxorhenium(V)-picolinate complexes as catalyst for olefin epoxidation in cells

Olefins are commercially valuable materials commonly used to manufacture products like plastic, detergent, adhesive, rubber, and food packaging. Once present in cells, they are usually transferred to epoxides by P450 cytochrome, followed by chemical transformations through epoxide hydrolase enzymes (EHs) activity, or by forming conjugates with glutathione via glutathione S-transferase (GST) activities [1]. Over the last ten years, the chemistry of oxorhenium(V) complexes and their application in homogeneous catalytic olefin epoxidation have been developed. Metabolite of tryptophan, picolinic acid, as an N,O ligand, significantly contributed to the Re(V) complexes' olefin catalytic efficacy [2]. Five new picolinate-based Re(V) complexes have been synthesized and evaluated for their catalytic activity for the cyclooctane epoxidation as the model reaction. Complexes 1 - 4 were synthesized in the reaction of an equimolar amount of [ReOCl3(PPh3)2] and the corresponding ligand (HL1–HL5), using acetonitrile or DCM/methanol as solvent at 85 °C for 24h. Chemical structure was confirmed using standard analytical methods (IR, NMR, MS), and two of them were crystallized in the monocrystal form suitable for X-ray analysis. Complexes were tested in cyclooctene epoxidation using tert-butylhydroperoxide (TBHP, 5.5 M in decane) as the oxidant (CHCl3, 50 °C, 1 mol% catalyst loading, 3 equiv. TBHP). The cyclooctene conversion to epoxide was analyzed by GC/MS. All five complexes showed immediate activity without an induction period. Most of the cyclooctene gets converted within the first three hours, after which the productivity drops sharply and levels out. Also, in all five cases, the selectivity toward epoxide dropped over time, as various side-products, such as the ring-opened cyclooctane diol, were observed by GC/MS. After 24 hours, the 5-bromopicolinate complex reached the overall highest conversion of all five tested complexes of 85%, followed by 6-chloro, 6-bromo, 6-fluoro, and finally 5-hydroxo substituted complexes. In addition, especially 6-fluoro substituted complex showed a loss of selectivity over the 24 h reaction time, as the formed epoxide is converted again, mainly to the ring-opened diol.