TUrn over numbers

The class C enzymes have Mr values of 39,000 and exhibit more uniform properties. They hydrolyse benzyl- and phenoxymethyl penicillin relatively well (turn-over numbers of 20-70 s-1), ampicillin and amoxicillin 10- to 20-fold less rapidly and extremely poorly the other penicillins (generally due to low k3 values). The early cephalosporins (cephalothin,... 

The loading of the activated monomeric catalyst could be decreased to a level of 0.04 mol% for certain substrates [turn over number 2,450], The developed process does not require the use of inert gas techniques and in most cases chromatographic purification was not necessary to obtain analytically pure products as no side products were formed and the catalyst could be separated by filtration. 

Figure 3.6. Turn over number (TON) displayed as function of time for the hydroformylation of 1-octene using a set-up for continuous processes with SCCO2 as mobile phase and supported catalyst 2...

Constant rates k (mol/s-gPd ) have been determined from the slope of the experimental curves plotting acetylenic molar concentration versus time. Turn over numbers (TONs = mol/s-atom Pd ) were calculated from the following equation ... 

Following the normal reactivity order, tertiary carbon atoms are more reactive than secondary ones, which in turn are far more reactive than primary ones [63-64,67]. Turn-over numbers in the oxidation of methyl cyclohexane with PhIO on FePcY decrease with increasing loadings of the phthallocyanine on the zeolite, as shown in Figure 4 [49-50,63-64,69]. This is due to pore blockage by the catalyst molecules themselves. 

The turn-over number of zeolite-encaged phthallocyanines is dependent on the nature of the metal complex as well as on the organic substrate. This is shown in Figure 5 for the oxidation of hexane and cyclohexane with iodosobenzene. The turn-over numbers obtained on Fet.BuPcY are about 4 times higher than those on FePcY, and those of cyclohexane are approximately 10 times higher than those of n-hexane [67]. 

The influence of reaction conditions upon activity was explored, revealing that 13 is 16 times more active than is 14, a fact that can perhaps be attributed to the enhanced steric bulk of the former. This property has previously been identified as fundamentally important in avoiding termination by the p-hydride elimination pathway. 71,172 The activity of the 13/MAO system was further probed, revealing a significant increase in turn-over number (TON) upon varying the pressure in the range 1.1 >30... 

The poor turn-over numbers (TON) in the hydrocyanation reactions are another limitation, because of degradation of the catalyst, which still has to be overcome. The maximum TON reached so far have been in the order of 500-750, which is extremely low compared with other homogenous catalytic reactions. 

Kinetic Studies. Reaction rates were determined by integration of selected H NMR resonances and are reported as turn-over numbers catalyst,). All reactions... 

Development of Tetraphosphine Rh(I) catalysts. The tetra(phosphine) rhodium(I) cations, prepared by adding stoichiometric amounts of a monophosphine to the ligand deficient dimers, were subsequently found to be very active hydrosilation catalysts. Although the addition of trisubstituted silanes was slow (1-2 days) and required elevated temperatures ( 55°C), high regioselectivity to the 1,2-hydrosilation products was obtained. Importantly, no products arising from catalyst deactivation in the form of trimeric rhodium(I) complexes were observed. More interestingly, these tetraphosphine rhodium complexes are extremely efficient catalysts for the 1,2-addition of disubstituted silanes to enolizable ketones. Turn-over numbers up to 105/hr at room temperature have been observed for a number of catalyst and ketone/silane combinations. 

The catalytic reduction of Cu2+ by hydrogen is faster on small platinum particles since the turn over number increases from 0.37 Cu2+ ions reduced per accessible platinum atom per hour to 3.0 when the platinum dispersion increases from 11% to 54% [8], Thus the catalytic hydrogenation of Cu2+ ions is a structure-sensitive reaction occurring preferentially on small platinum particles. 

As a test reaction for the catalytic activity the hydrogenation of dimethylitaconate was employed. No reaction takes place in the blank test, when the carrier Al-MCM-41 itself is used as catalyst. The immobilised rhodium complexes give enantioselectivities up to 92 % ee of dimethyl-(R)-methylsuccinate with a turn over number of 4000 for the S,S-Me-Duphos ligand. The corresponding supported catalysts with R,R-Diop and S,S-Chiraphos ligands lead to enantioselectivities of 34 % ee and 47 % ee with lower activities. With the (+)-Norphos ligand the favoured enantiomer is dimethyl-(S)-succinate which is formed with 47 % ee. 

After completion of the reaction, the reaction mixture was gas chromatographically analyzed to quantitatively determine the amounts of the unreacted starting material and the resultant desired product. As a result, the conversion of the starting material was 94%, the yield of the desired product was 83%, and the Pd turn-over number was 166. 

---