Sulfoxidation, MOFs

A values have been obtained for oxidation of benzenediols by [Fe(bipy)(CN)4], including the effect of pH, i.e., of protonation of the iron(III) complex, and the kinetics of [Fe(phen)(CN)4] oxidation of catechol and of 4-butylcatechol reported. Redox potentials of [Fe(bipy)2(CFQ7] and of [Fe(bipy)(CN)4] are available. The self-exchange rate constant for [Fe(phen)2(CN)2] has been estimated from kinetic data for electron transfer reactions involving, inter alios, catechol and hydroquinone as 2.8 2.5 x 10 dm moF s (in dimethyl sulfoxide). 

In a subsequent study, the synthesis of urea and thiourea derivatives of types 3a and 3b were undertaken in an attempt to obtain new hosts that might yield more stable complexes with glutamic acid (of types 4a and 4b) and that might, in fact, hold together in polar solvents. The rationale for the design was based on the previously documented observation that both 1,3-dimethylurea and 1,3-dimethyl-thiourea form stable host-guest complexes with acetate in deutero-dimethyl sulfoxide the K values for the latter are 45 dm moF and 340 dm moF , respectively. 

The effect of the temperature in the sulfoxidation reaction has been investigated over Ti-beta catalyst. BU2S has been chosen as the model substrate, because of its lower reactivity under standard conditions (303 K). No direct decomposition of hydrogen peroxide was observed at the various temperatures used 303, 313, 323 and 333 K respectively. A linear Arrhenius plot of the initial reaction rates (Figure 5) leading to a derived activation energy of 51 kJ.mof is obtained. 

Several other MOFs have also been used as oxidation catalysts. Kim and coworkers used [Zn2(BDC)(L-Lac)(DMF)] (DMF) as a heterogeneous catalyst for the oxidation of thioethers to sulfoxides by urea hydroperoxide (UHP) or hydrogen peroxide (H2O2) [26]. Its size selectivity was illustrated by the higher conversion of smaller sulfides over larger ones. Snejko and coworkers reported the use of ln2(OH)3(BDC)i,5 [27] and [Sc2(BDC)3] [28] as active catalysts for the oxidation of alkylphenylsulfides. 

Ferey and coworkers [106] have also demonstrated that the coordinatively unsaturated Cr(III) sites of the MIL-101 can catalyze the selective oxidation of aryl sulfides to aryl sulfoxides using H2O2 as the oxidant The activity of the catalyst in the oxidation of arylmethylsulfide compounds (having different ring substituents in the para position) was found to depend on the concentration of open Cr(III) sites in the MOF sample. A direct correlation was found between the intrinsic activity... 

Homochiral PCP has also been applied for the combined oxidation of prochiral thioethers and separation of chiral sulfoxides in semipreparative-scale chromatography [135]. Fedin and coworkers successfully obtained ee values reaching 60% using a zinc-lactate-based MOF when standard values are around 15-30%. 

One more example is related to the Unalool oxidation with hydrogen peroxide using a [Ln(OH)(H20)(naphthalenedisulfonate)], where Ln=Nd, Pr, La [104], though the nature of the active sites is not quite clear, since the MOF contains no transition metal ions, like Ti, Fe, Co, and so on. Similarly, oxidation of methylphenylsulfide to sulfoxide and sulfone with H O is catalyzed by scandium ions in a [Sc2(bdc)3] MOF [105]. Among other oxidation reactions, the use of a Cu-MOF ([Cu3(btc)J) in the conversion of phenols to higher phenols deserves attention and the nature of activity is not so elusive [106]. 

For the first time, a homochiral MOF membrane was reported by Wang et al. [167] for the enantioselective separation of chiral compounds, especially chiral drug intermediates. In this study, a homochiral MOF material [Zn2(bdc)(L-lac)(dmf)] (DMF) (ZnBLD) was used for the preparation of an MOF separation membrane through the solvothermal reaction of a metal cation, a chiral ligand, and an organic connector. Zn-BLD exhibits preferential adsorption ability to (S)-methyl phenyl sulfoxide (S-MPS) over R-MPS. They claimed that this membrane will allow potential development of a new, sustainable, and highly efficient chiral separation technique. 

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