Isomers in chlorido and alkoxido-substituted oxidorhenium() complexes: effects on catalytic epoxidation activity

The syntheses and characterizations of oxidorhenium( v ) complexes trans -dichlorido [ReOCl 2 (PPh 3 )( L1a )] ( trans - 2a ), cis -dichlorido [ReOCl 2 (PPh 3 )( L1b )] ( cis - 2b ) and ethoxido-complex [ReO(OEt)( L1b ) 2 ] ( 4b ), ligated with the dimethyloxazoline-phenol ligands H L1a and H L1b are described. The bidentate ligand H L1a (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol) is unsubstituted on the phenol ring; ligand H L1b (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol) contains a nitro group in para -position to the hydroxy group. In the reaction of precursor complex [ReOCl 3 (PPh 3 ) 2 ] and H L1a the two stereoisomers cis / trans - 2a , with respect to chlorido ligands, are formed. The solid state structures of both isomers cis - and trans - 2a were determined by single crystal X-ray diffraction analysis. In contrast, with ligand H L1b , only the cis -isomer cis - 2b was obtained. Ethoxido-complex 4b is exclusively obtained when precursor [ReOCl 3 (OPPh 3 )(SMe 2 )] is reacted with 2 equiv. of H L1b in ethanol in the presence of the base 2,6-dimethylpyridine (lutidine). If no lutidine is added, chlorido-complex [ReOCl( L1b ) 2 ] ( 3b ) is obtained. Complexes [ReOCl 2 (PPh 3 )( L1a )] ( cis / trans - 2a ), [ReOCl 2 (PPh 3 )( L1b )] ( cis - 2b ), [ReO(OMe)( L1a ) 2 ] ( 4a ) and [ReO(OEt)( L1b ) 2 ] ( 4b ) were tested as homogeneous catalysts in the benchmark reaction of cyclooctene epoxidation. The influence of isomerism and effects of ligand substitutions on catalytic activity was investigated. Based on the time-conversion plots it can be concluded that cis / trans -isomerism does not influence catalytic activity, but electron-withdrawing substituents, as in cis - 2b , 3b and 4b , show a beneficial effect. Pairs of stereoisomers and chlorido/alkoxido oxidorhenium( v ) complexes are described and allow for a systematic investigation of the influence on catalytic activity in standard cyclooctene epoxidation.