Notable turnover numbers

In chronological order, the next milestones in research were the studies by Fukuda and Utimoto on the addition of nucleophiles (water, alcohols and amines) to alkynes [13]. A decade later, Teles obtained notable turnover numbers (TONs) and turnover frequencies (TOFs) in the addition of alcohols to alkynes [14]. 

The first reported electroorganic synthesis of a sizeable amount of material at a modified electrode, in 1982, was the reduction of 1,2-dihaloalkanes at p-nitrostyrene coated platinum electrodes to give alkenes. The preparation of stilbene was conducted on a 20 pmol scale with reported turnover numbers approaching 1 x 10. The idea of mediated electrochemistry has more frequently been pursued for inorganic electrode reactions, notably the reduction of oxygen which is of eminent importance for fuel cell cathodes Almost 20 contributions on oxygen reduction at modified... 

Carbonic anhydrase is a metalloprotein with a co-ordinate bonded zinc atom immobilized at three histidine residues (His 94, His 96 and Hisl 19) close to the active site of the enzyme. The catalytic activity of the different isoenzymes varies but cytosolic CA II is notable for its very high turnover number (Kcat) of approximately 1.5 million reactions per second. 

Che has reported that both achiral and chiral rhodium catalysts function competently for intramolecular aziridination reactions of alkyl- and arylsulfonamides (Scheme 17.29) [59, 97]. Cyclized products 87 are isolated in 90% yield using 2 mol% catalyst, PhI(OAc)2, and AI2O3. Notably, reactions of this type can be performed with catalyst loadings as low as 0.02 mol% and display turnover numbers in excess of 1300. In addition, a number of chiral dimeric rhodium systems have been examined for this process, with some encouraging results. To date, the best data are obtained using Doyle s Rh2(MEOX)4 complex. At 10 mol% catalyst and with a slight excess of Phl=0, the iso-... 

Bycroft et al. [83] have reported a series of semisynthetic penicillin derivatives such as, 6-spiro-epoxypenicillins F, G (Fig. 8) possessing both (3-lactamase inhibitory and antibacterial activity (Fig. 8). It has been found that novel chlorinated 6-spiro-epoxypenicillins F are potent in vitro inhibitors of a range of chemically important (3-lactamases [84], whereas, 6-spirocyclopropylpenems, G, show a reduced level of (3-lactamase inhibitory activity. The significance of the five fold difference between the turnover numbers for F(a) and G(b) (differ only in their stereochemistry at one center) was found to be in close comparison with the turnover number of 20,000, reported for the established (3-lactamase inhibitor, sulbactam [140]. Thus, the notable (3-lactamase inhibitory and antibacterial properties of these spiro-(3-1 actams depend upon the substituents and the stereochemistry of the epoxide. 

Ruthenium complexes of chiral phosphines notably BINAP, 2,2 -bis(diphenyl-phosphino)-l,l -binaphthyl are very useful for industrial hydrogenations and H-transfer hydrogenation of alkenes and ketones. This is due to high turnover numbers and enantiomeric excesses (ee) of the products. A specific example41 of an... 

Much attention has been paid to metal-substituted alkaline phosphatases, notably Co" d-d spectra), Mn" (ESR) and Cd ( Cd NMR). The apoenzyme may be prepared by use of ammonium sulfate to remove zinc. After about five days the apoenzyme may be isolated having less than 3% of the original zinc. Furthermore, the apoenzyme is uncontaminated by chelating agents, which show a tendency to bind to the apoenzyme. A range of metalloalkaline phosphatases may be prepared from the apoenzyme. The binding of cadmium at three separate sites can be confirmed by the use of " Cd NMR, which shows " three separate resonances at 153, 72 and 3 p.p.m. in the phosphorylated dimer Cd"6AP. When all three sites are occupied by Cd , the enzyme has a very low turnover, at least 10 times slower than the native Zn" enzyme. This slow turnover number has made the Cd" enzyme particularly useful in NMR studies. 

Other catalysts (in Sect. 2) prepared in our laboratory were also employed for the hydrogenation of non-activated esters under mild conditions [30, 44]. The results are summarized in Fig. 5. Notably, higher turnover numbers (up to 3,284 TON) were obtained using complex 6 as a catalyst at 50 atm of dihydrogen for the hydrogenation of hexyl hexanoate (Fig. 5, entry 2). 

Both of these complexes can be used in ADMET polymerizations at temperatures up to approximately 55 °C, although decomposition certainly occurs over the time scale of a typical ADMET polymerization (days). A structure-reactivity study was performed on complexes 1 and 2 that revealed a number of features of these complexes [68]. Notably, 2 will polymerize dienes containing a terminal and a 1,1-disubstituted olefin, but never produces a tetrasubstituted olefin. One of the substituents of the 1,1-disubstituted olefin must be a methyl group. In contrast, complex 1 will not react with a 1,1-disubstituted olefin. The tungsten complex is more reactive towards internal olefins than external olefins [23, 63] indicating that secondary metathesis, or trans-metathesis, probably dominates the catalytic turnovers in ADMET with complex 1. 

A factor that can influence C02 hydration/dehydration reactivity is the overall coordination number of the zinc center and the coordination mode of a bicarbonate ligand (Fig. 7). It is reasonable to suggest that a unidentate coordinated HCO will be easier to displace, which could influence the rate of the overall hydration reaction. Data discussed below in terms of single turnover experiments supports the notion that bidentate bicarbonate coordination inhibits catalytic C02 hydration. Similarly, bidentate coordination of HCO could be expected to slow the dehydration reaction. Notably, X-ray crystallographic studies of bicarbonate-bound forms of a mutant CA-II, and a Co(II)-substituted form of the enzyme, have revealed both monodentate and bidentate coordination modes for the bicarbonate anion.28,32,45... 

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