Ligand Design for Isomer-Selective Oxorhenium(V) Complex Synthesis

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

The syntheses and characterizations of oxidorhenium(v) complexes trans-dichlorido [ReOCl2(PPh3)(L1a)] (trans-2a), cis-dichlorido [ReOCl2(PPh3)(L1b)] (cis-2b) and ethoxido-complex [ReO(OEt)(L1b)2] (4b), ligated with the dimethyloxazoline-phenol ligands HL1a and HL1b are described. The bidentate ligand HL1a (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol) is unsubstituted on the phenol ring; ligand HL1b (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 [ReOCl3(PPh3)2] and HL1a 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 HL1b, only the cis-isomer cis-2b was obtained. Ethoxido-complex 4b is exclusively obtained when precursor [ReOCl3(OPPh3)(SMe2)] is reacted with 2 equiv. of HL1b 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 [ReOCl2(PPh3)(L1a)] (cis/trans-2a), [ReOCl2(PPh3)(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.

An efficient synthesis of oxazolines via a cascade reaction between azaoxyallyl cations and 1,2-benzisoxazoles

A formal [3 + 2] cycloaddition reaction between the C and O terminals of azaoxyallyl cations formed in situ and 1,2-benzisoxazoles has been realized. This one-pot cycloaddition method provided an effective and practical pathway to synthesize oxazoline in good yields under mild conditions. The title products exhibited unique fluorescence properties.

Stereoisomers and functional groups in oxidorhenium(v) complexes: effects on catalytic activity

The syntheses of oxidorhenium(v) complexes [ReOCl(L1a-c)₂] (3a-c), equipped with the bidentate, mono-anionic phenol-dimethyloxazoline ligands HL1a-c are described. Ligands HL1b-c contain functional groups on the phenol ring, compared to parent ligand 2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-phenol H1a; namely a methoxy group ortho to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-6-methoxyphenol, H1b), or a nitro group para to the hydroxyl position (2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)-4-nitrophenol, H1c). Furthermore, oxidorhenate(v) complexes (NBu₄)[ReOCl₃(L1a-b)] (2a-b) were synthesized for solid state structural comparisons to 3a-b. All novel complexes are fully characterized including NMR, IR and UV-Vis spectroscopy, MS spectrometry, X-ray crystallography, elemental analysis as well as cyclic voltammetry. The influence of functional groups (R = -H, -OMe and -NO₂) on the catalytic activity of 3a-c was investigated in two benchmark catalytic reactions, namely cyclooctene epoxidation and perchlorate reduction. In addition, the previously described oxidorhenium(v) complex [ReOCl(oz)₂] (4), employing the phenol-oxazoline ligand 2-(4,5-dihydro-2-oxazolyl)phenol Hoz, was included in these catalysis studies. Complex 4 is a rare case in oxidorhenium(v) chemistry where two stereoisomers could be separated and fully characterized. With respect to the position of the oxazoline nitrogen atoms on the rhenium atom, these two stereoisomers are referred to as N,N-cis and N,N-trans isomer. A potential correlation between spectroscopic and structural data to catalytic activity was evaluated.

Technetium and Rhenium Schiff Base Compounds for Nuclear Medicine: Syntheses of Rhenium Analogues to 99mTc-Furifosmin

Rhenium, the third-row congener of technetium, is often used to develop the macroscopic chemistry of potential ^99mTc diagnostic radiopharmaceuticals. The rhenium analogues to ^99mTc-furifosmin are being developed for potential radiotherapy of multidrug-resistant tumors. Complexes of the form trans-[M^III(PR₃)₂(N₂O₂-Schiff base)]⁺ are of interest for the potential imaging and treatment of multidrug-resistant tumors. Reaction of the tetradentate Schiff ligand 4,4'-[(1 E,1' E)-[ethane-1,2-diylbis(azanylylidene)]bis(methanylylidene)]bis(2,2,5,5-tetramethyl-2,5-dihydrofuran-3-ol) (tmf₂enH₂) with the M(V) starting materials ( nBu₄N)[TcOCl₄] and ( nBu₄N)[ReOCl₄] gave the monomeric products trans-[TcOCl(tmf₂en)] and trans-[ReOCl(tmf₂en)], respectively. Reduction of in situ formed trans-[ReOCl(tmf₂en)] by various tertiary phosphines yielded disubstitued Re(III) products of the general type trans-[Re^III(PR₃)₂(tmf₂en)]⁺. The rhenium(III) compounds were found to be water-soluble and stable in aqueous solution. Reversible Re^III/Re^IV and Re^III/Re^II redox processes were observed at about 0.8-0.9 and -0.65 to -0.8 V, respectively, for each of the rhenium(III) species. Reaction of in situ formed trans-TcOCl(tmf₂en) with triethylphosphine yielded the reduced, disubstituted trans-[Tc(PEt₃)₂(tmf₂en)]PF₆. A reversible Tc^III/Tc^II redox couple was observed for the technetium(III) species, about 200 mV less negative than their rhenium(III) analogues, in addition to an irreversible Tc^III/Tc^IV process. All compounds were characterized using conventional spectroscopic techniques, single-crystal X-ray crystallography, and cyclic voltammetry.