Olefin turnover numbers

Several spiro-ketal-based phosphoramidites have been prepared by the same research group in multistep sequences (Figure 2.37) [18]. In the hydroformylation of terminal olefins, turnover numbers of up to 2.3 X10 and Ub ratios of 174.4 were stated. 

High enantioselectivities and regioselectivities have been obtained using both mono- and 1,2-disubstituted prochinal olefins employing chiral phosphine phosphite (33,34) modified rhodium catalysts. For example, i7j -2-butene ia the presence of rhodium and (12) (33) gave (3)-2-meth5ibutanal ia an optical yield of 82% at a turnover number of 9.84. ... 

The organometallic product of the reaction (1) catalyzes the hydromethylation of propene, although insertion of the olefin into the Sc-CH3 bond is slow and turnover numbers/turnover frequencies (TON and TOF) are low (Scheme 2). 

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,... 

Direct comparisons of the diamine system against the parent complex led to the conclusion that the effect of the diamine and KOH/i-PrOH activator decelerate olefin hydrogenation and in turn accelerate carbonyl hydrogenation. In the published report, there were no attempts to optimize turnover numbers or TOF for aldehyde hydrogenation. However, the catalyst has been shown to hydrogenate ketones with a SCR of 10000 at room temperature, which suggests that these catalysts represent the current state of the art in terms of activity and selectivity. 

The selectivity in favor of the desired monobenzylated product was found to be >99% and the immobilized Pt02 was found to be 4-5 times more active than the commercial Adams catalysts. In solution or in immobilized form, the PtOz colloid is effective in the hydrogenation of carbonyl compounds or of olefins. Recently, the heterogeneous catalytic amination of aryl bromides by immobilized Pd(0) particles has been reported [163], Secondary amines such as piperidine and diethyl amine are used in the amination of aryl bromides and the reaction proceeds with good turnover numbers and regio-control. The catalysts can be reused repeatedly without loss of activity or selectivity after filtration from the reaction mixture. 

It is likely that more silicon-carbon bonds are produced by the hydrosilylation of olefins than by any other method except the direct process. This deceptively simple addition of an Si-H bond to a C-C multiple bond can be promoted by a variety of means, but transition metal catalysis is by far the most significant. Two relatively old catalysts, H2PtCl6 ( Speier s catalyst ) and Pt2(Me2ViSiOSiMe2Vi)3 ( Karstedt s catalyst ), remain the most effective, and the remarkable rates and turnover numbers observed in these systems are among the most impressive in all of organometallic chemistry. The bulk of the literature on hydrosilylation falls outside the scope of this review, and readers are directed to the comprehensive work on hydrosilylation edited by Marciniec.93... 

A direct comparison of catalysis of olefin epoxidation with a homogeneous chemical catalyst (Mn salen), an enzyme (CPO), and an antibody resulted in sufficiently high enantioselectivity for all three catalysts, a higher turnover number for the enzyme, and a slightly higher substrate/catalyst ratio for the homogenous catalyst. Criteria for comparison should be quantitative and include catalyst lifetime as well as volumetric productivities, but have been found to depend on the different needs of laboratory synthetic chemists, who need a broadly specific catalyst quickly, versus those of process chemists, who often control catalyst availability and can allow narrow specificity (provided their substrate is accepted) but need high productivity. 

In spite of the remarkable improvement upon previously existing methodology, there is one disadvantage that remains. For the synthesis of an olefin, a second olefin has to be sacrificed. It is obvious that a process that would enable dehydrogenation to occur in the absence of sacrificial reagents would be highly desirable. Moreover, the selectivities that can be obtained at high turnovers are still too low for practical applications. Neither turnover frequencies nor turnover numbers of the catalysis are sufficient to be useful for industrial processes. These limitations are less of an issue in total synthesis, provided that the quality of the metal-mediated reaction justifies the use of stoichiometric processes. 

Ru-MeO-Biphep (Ru-96a) was used by Roche to reduce 98 to the corresponding P-hydroxy ester with >98% ee at 240-kg scale. Turnover numbers of 50,000 were achieved in this reduction. A process was developed by PPG-Sipsy to reduce 99 with Ru-MeO-Biphep for Pfizer in an approach to candoxatril. The olefin was reduced in >99% ee at 230-kg scale (S/C = 1000-2000).118 Although catalysts that contain DuPhos had been determined to be more effective based on overall yields and isomerization to an enol by-product, the Ru-MeO-Biphep catalyst was preferred as a result of catalyst availability at scale and more favorable licensing agreements.119... 

As stated in the introduction, chloramine-T (where T denotes three crystalline water molecules) is a commonly used nitrene precursor, which is commercially available and costs less than do most other nitrene sources. The benefit of a silver salt in nitrene transfer reactions with chloramine-T is surprisingly simple. Because silver chloride is insoluble in most solvents, substoichiometric amounts of silver salts (like silver nitrate) can be used to remove the chloride from chloramine to facilitate the release of a free nitrene radical, which can aziridinate olefins. Since the amount of silver is near stoichiometric, it should not be called silver-based catalysis, although turnover numbers (TONs) higher than 1 have been observed in some cases. 

For 1,3-butadiene hydrogenation, the toxicity of sulfur is 3 (Fig. 13). which is lower than the toxicity for olefin hydrogenation. The hydrogenation of 1-butyne has also been studied for various ratios of sulfur over palladium. As was already published (86), the 1-butyne hydrogenation rate increases with time. The same effect has been observed on sulfided palladium. The turnover number is consequently presented for 1-butyne hydrogenation versus the sulfur content for various 1-butyne conversions (see Fig. 14). During the first minutes of reaction (0-25% conversion), the toxicity of sulfur appears close to 1 the rates are proportional to the free surface. However, at higher conversion, the rate becomes independent from the sulfur ratio. The toxicity is zero. 

The catalytic activity for olefin polymerization was evaluated for complex 17a. High molecular weight addition-type polynorbornene (PNB) with a moderate molecular weight distribution (Mw = 106, Mw/Mn = 2.3-3.5) was obtained when 17a was activated with MAO. The activity was highest at 80 °C (107 g of PNB/(mol of Ni) h 1) resulting from an increase in the concentration of active catalyst centers at that temperature. However, further increases in temperature led to catalyst decomposition rather than higher turnover numbers. 

In 1980, Miller et al. [76] reported the first example of an intermolecular hydroacylation of an aldehyde with an olefin to give a ketone, during their studies of the mechanism of the rhodium-catalyzed intramolecular cyclization of 4-pentenal using ethylene-saturated chloroform as the solvent. Later James and Young [77] reported that the reaction of propionaldehyde with ethylene can be conducted in the presence of RuCl2(PPh3)3 as the catalyst without any solvent at 210 °C, resulting in the formation of 3-pentanone in 2-4% yield (turnover number of 230) (Eq. 49). 

The asymmetric oxidation of indene to the corresponding epoxide (Equation 24) is carried out commercially by Sepracor on a small scale. Chiral indene oxide is an intermediate in the synthesis of crixivan (an HIV protease inhibitor). Reaction is carried out at 5°C with moderately high turnover numbers in the presence of an exotic donor ligand ( P3NO , 3-phenylpropylpyridine N oxide) and sodium hypochlorite as the terminal oxidant. A similar epoxidation of a simple cis olefin (Equation 25) leads to an enantiomerically pure amino-alcohol used in the synthesis of taxol, a potent anticancer drug. 

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