Turnover Number TON

The turnover number specifies the maximum use that can be made of a catalyst for a special reaction under defined conditions by a number of molecular reactions or reaction cycles occuring at the reactive center up to the decay of activity. The rela-tionschip between TOF and TON is (Eq. 1-8)  

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Despite those challenges, both Johnson [161] and Grela [162] performed several cross metathesis reactions with vinylhalides using phosphine free catalysts. Turnover numbers (TON) above 20 were very few, while in many cases the TON stayed below ten. The diastereoselectivity of CMs with vinylhalides is shghtly in favour of the Z product which is similar to their acrolein-counterparts. 

Previous studies on the use of Anchored Homogeneous Catalysts (AHC s) have been concerned with studying the effect which different reaction variables had on the activity, selectivity and stability of these catalysts (1-9). These reactions were typically ran at relatively low substrate/catalyst ratios (turnover numbers-TON s), usually between 50 and 100. While these low TON reactions made it possible to obtain a great deal of information concerning the AHC s, in order to establish that these catalysts could be used in commercial applications it was necessary to apply them to reactions at much higher TON S and, also, to make direct comparisons with the corresponding homogeneous catalyst under the same reaction conditions. 

A catalytic reaction is one in which more than one turnover or event occurs per reaction center or catalytically active site (that is, the turnover number [TON] is greater than 1). Thus a reaction is not catalytic if it is stoichiometric or if its TON is less than 1. A reaction might indeed involve a true catalyst and under some circumstances be catalytic, but if one or fewer turnovers occur per active site, it is not a catalytic reaction. 

The most important progress in the last decade has been in the design and synthesis of [RuCl2(diphosphine)(l,2-diamine)] catalysts exploiting the metal-ligand bifunctional concept developed by Noyori and co-workers.29-31 The Noyori catalysts seem to possess all of the desired properties, such as high turnover number (TON), high turnover frequency (TOF), and operationally simple, safe, and environmentally friendly reaction conditions. 

Catalyst stability can be defined in terms of turnover number (TON). A textbook definition of this is ... 

Another area of high research intensity is the catalytic dehydrogenation of alkanes to yield industrially important olefin derivatives by a formally endothermic (ca. 35 kcal mol-1) loss of H2. Recent results have concentrated on pincer iridium complexes, which catalytically dehydrogenate cycloalkanes, in the presence of a hydrogen accepting (sacrificial) olefin, with turnover numbers (TONs) of >1000 (Equation (23)) (see, e.g., Ref 33,... 

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... 

In Group IV metal complexes, metallocene complexes are the main catalyst precursors for hydrogenation. Two major catalytic systems have been used 1) Cp2MR2 (R=H, Alkyl, Aryl) and 2) Cp2MX2 in combination with alkylating agent or an hydride (Table 6.1). The catalytic tests are typically run with 50 equiv. of substrate per metal, but in some cases turnover numbers (TONs) exceeding 1000 can be achieved [35]. 

The iridium complex [Ir(cod)(//2-,PrPCH2CH2OMe)]+BF4 (22) in dichloro-methane at 25 °C at 1 bar H2 is a particularly active catalyst for the hydrogenation of phenyl acetylene to styrene [29]. In a typical experiment, an average TOF of 50 mol mol-1 h-1 was obtained (calculated from a turnover number, TON, of 125) with a selectivity close to 100%. The mechanism of this reaction has been elucidated by a combination of kinetic, chemical and spectroscopic data (Scheme 14.10). 

This catalytic system was further studied by Strohmeier and Steigerwald, who performed reactions at 10 bar without solvent to achieve hydrogenation of a series of aldehydes (Table 15.1) [2]. Turnover numbers (TON) of up to 8000 were achieved in the case of the hydrogenation of benzaldehyde. The chemoselectivity of this catalyst towards carbonyl hydrogenation over alkene hydrogenation was... 

In order to facilitate the comparison of the effectiveness of the very diverse methods, turnover numbers (TON), and/or turnover frequencies (TOF) (if they were given by the author or could be calculated based on their data) are sum-... 

Whilst trying to be comprehensive, we have also intended to introduce a strong applied flavor to this summary. In the industrial case, catalyst performance is critically judged on overall efficiency, namely catalyst productivity and activity as well as enantioselectivity. As a result, turnover numbers (TONs) and turnover frequencies (TOFs) have been included or calculated whenever possible and meaningful. 

Zhang and colleagues [26] synthesized the Duanphos enantiomers 57 and 58, and reported on the Rh-Duanphos-catalyzed highly efficient hydrogenation of a series of /9-secondary-amino ketones with ee-values of up to >99%, and with turnover numbers (TONs) of more than 4500 (Table 33.7). This hydrogenation provides a potentially practical synthesis for key pharmaceutical intermediates. The y-secondary amino alcohols are of particular interest to synthetic chemists as they are key intermediates for the synthesis of an important class of antidepressants, 59-62 [32]. 

The mantiosdcctivity, expressed as enantiomeric excess (ee, %) of a catalyst should be >99% for pharmaceuticals if no purification is possible. This case is quite rare, and ee-values >90% are often acceptable. Chemosdectivity (or functional group tolerance) will be very important when multifunctional substrates are involved. The catalyst productivity, given as turnover number (TON mol product per mol catalyst) or as substrate catalyst ratio (SCR), determines catalyst costs. For hydrogenation reactions, TONs should be >1000 for high-value products and >50000 for large-scale or less-expensive products (catalyst re-use increases the productivity). 

The unmodified complex can be applied in very dilute concentrations allowing total turnover numbers (TONs), or a substrate (NAD(P)) to catalyst (rhodium complex) ratio of up to 400 [41]. This efficiency was due to the design of a three-dimensional electrode, which also resulted in an extraordinary space-time yield of the reduced cofactor of up to 1 kg IT1 per day. 

The Mo(CO)s(NEt3) complex isomerizes the allenyl ketone 111 to the furan 112 even the free hydroxyl group is tolerated, but with 50% of catalyst a 28% yield is not too (effective turnover number (TON) =0.5 Scheme 15.30) [69]. 

The "catalyst" may be added to the reactants in a different form, the catalyst precursor, which has to be brought into an active form ("activated"). During the catalytic cycle the catalyst may be present in several intermediate forms when we look more closely at the molecular level. An active catalyst will pass a number of times through this cycle of states in this sense the catalyst remains unaltered. The number of times that a catalyst goes through this cycle is the turnover number. The turnover number (TON) is the total number of substrate... 

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