Oxidative turnover number

Preliminary studies on catalytic osmylation were reported by Kokubo and co-workers who used bovine serum albumin, 0s04, and t-butylhydroperoxide as the oxidant. Turnover numbers up to 40 and an e.e. for 2-phenylpropene of 68% was achieved [23], Apparently the protein binds to osmium via nitrogen donors, but as different sites may be available this may lower the e.e. 

Alkene oxidation turnover numbers over TS-1 and Ti-beta, mol /mol Ti... 

More recently, Llobet and coworkers reported the anodic electropolymerization of A -substituted pyrroles as a convenient method of anchoring a redox-active dinuclear ruthenium catalyst onto conducting solid supports, like vitreous carbon sponges (VCS) and fluorine-doped tin oxide (FTO). In the presence of Ce(IV) as the sacrificial oxidant, turnover numbers up to 76 have been achieved. A major improvement of the system is accomplished by the copolymerization with a robust non active redox species, able to further separate the catalytically active species on the solid support, obtaining up to 250 catalytic cycles. 

The decarbonylation-dehydration of the fatty acid 887 catalyzed by PdCl2(Ph3P)2 fO.Ol mol%) was carried out by heating its mixture with acetic-anhydride at 250 C to afford the terminal alkene 888 with high selectivity and high catalyst turnover number (12 370). The reaction may proceed by the oxidative addition of Pd to the mixed anhydride[755]. 

Palladium(II) complexes provide convenient access into this class of catalysts. Some examples of complexes which have been found to be successful catalysts are shown in Scheme 11. They were able to get reasonable turnover numbers in the Heck reaction of aryl bromides and even aryl chlorides [22,190-195]. Mechanistic studies concentrated on the Heck reaction [195] or separated steps like the oxidative addition and reductive elimination [196-199]. Computational studies by DFT calculations indicated that the mechanism for NHC complexes is most likely the same as that for phosphine ligands [169], but also in this case there is a need for more data before a definitive answer can be given on the mechanism. 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

In respect of designing an economic production process, the stoichiometric cofactor required in carbonyl reductions or the respective oxidation reactions needs to be minimized that is, enabled by recycling of the cofactor. The measure for the efficiency of the recycling process is the total turnover number (TTN), which describes the moles of product synthesized in relation to the moles of cofactor needed. The different approaches in cofactor recycling were recently reviewed by Goldberg et at. [12]. 

Very recently a new kind of electrocatalyst has been propounded using the dinuclear quinone-containing complex of ruthenium (25).492,493 Controlled-potential electrolysis of the complex at 1.70 V vs. Ag AgCl in H20 + CF3CH2OH evolves dioxygen with a current efficiency of 91% (21 turnovers). The turnover number of 02 evolution increases up to 33,500 when the electrolysis is carried out in water (pH 4.0) with an indium-tin oxide(ITO) electrode to which the complex is bound. It has been suggested that the four-electron oxidation of water is achieved by redox reactions of not only the two Run/Ruin couples, but also the two semiquinone/quinone couples of the molecule. 

The catalyst efficiency of these hydroalumination varies from a turnover number (TON) of 20-91. It is possible that the catalyst is deactivated by the presence of oxygen and water. Examination of the 31P NMR spectrum of the catalyst indicates that the phosphine monoxide and dioxide are formed in the presence of nickel prior to the addition of the substrate. Rigorous exclusion of oxygen and water is necessary in all these reactions. The enantioselective nickel-catalyzed hydroalumination route to dihydronaphthalenols may prove to be particularly important. Only one other method has been reported for the enantioselective syntheses of these compounds microbial oxidation of dihydronaphthalene by Pseudomonas putida UV4 generates the dihydronaphthalenol in 60% yield and >95% ee.1... 

Washing for over 100 h in a solvent such as toluene before the reaction resulted in no significant loss of the catalytic activity, and recovery and reusability studies at high turnover number also indicated the catalyst stability. Same catalysts, H5[PV2Mo10O40] supported on fiber and fabric carbon materials, catalyzed 02-based oxidations of acetaldehyde and 1-propanethiols [113], This aerobic heterogeneous oxidation proceeded under mild reaction conditions. 

There is huge potential in the combination of biocatalysis and electrochemistry through reaction engineering as the linker. An example is a continuous electrochemical enzyme membrane reactor that showed a total turnover number of 260 000 for the enantioselective peroxidase catalyzed oxidation of a thioether into its sulfone by in situ cathodic generated hydrogen peroxide - much higher than achieved by conventional methods [52],... 

Sharpless stoichiometric asymmetric dihydroxylation of alkenes (AD) was converted into a catalytic reaction several years later when it was combined with the procedure of Upjohn involving reoxidation of the metal catalyst with the use of N-oxides [24] (N-methylmorpholine N-oxide). Reported turnover numbers were in the order of 200 (but can be raised to 50,000) and the e.e. for /rara-stilbene exceeded 95% (after isolation 88%). When dihydriquinidine (vide infra) was used the opposite enantiomer was obtained, again showing that quinine and quinidine react like a pair of enantiomers, rather than diastereomers. 

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