Catalysts turnover

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]. 

Dehydrogenative Coupling of Hydride Functional Silanes. The autocouphng of dihydridosilanes was first observed usiag Wilkinson s catalyst (128). A considerable effort has been undertaken to enhance catalyst turnover and iacrease the molecular weight of polysilane products (129) because the materials have commercial potential ia ceramic, photoresist, and conductive polymer technology. 

One of the most significant developmental advances in the Jacobsen-Katsuki epoxidation reaction was the discovery that certain additives can have a profound and often beneficial effect on the reaction. Katsuki first discovered that iV-oxides were particularly beneficial additives. Since then it has become clear that the addition of iV-oxides such as 4-phenylpyridine-iV-oxide (4-PPNO) often increases catalyst turnovers, improves enantioselectivity, diastereoselectivity, and epoxides yields. Other additives that have been found to be especially beneficial under certain conditions are imidazole and cinchona alkaloid derived salts vide infra). 

Reaction conditions 0.014 mmol Ru, H2 lOOpsig, temp. 50"C, 30 ml EtOH, olar ratio of substrate to catalyst. Turnover frequency. 

To simplify the catalytic system further, Kodadek and Woo investigated the activity of [Fe(F2o-TPP)Cl] for alkene cyclopropanation with EDA in the absence of cobaltocene. These workers proposed that electron-deficient porphyrin would render the Fe(III) porphyrin more easily reduced by EDA. Indeed, [Fe(F2o-TPP)Cl] efficiently catalyzes alkene cyclopropanation with EDA with high catalyst turnover... 

Iron porphyrins display pronounced substrate preferences for alkene cyclopro-panation with EDA. In general, electron-rich terminal alkenes in conjunction with aromatic moiety or heteroatoms can efficiently undergo cyclopropanation with high catalyst turnover and selectivity. In contrast, 1,2-disubstituted alkenes cannot undergo cyclopropanation with diazoesters. Alkyl alkenes are poor substrates, giving cyclopropanated products in low yields. In both cases, the dimerization product diethyl maleate was obtained in high yield [53]. 

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. 

In 1977 Ford and co-workers showed that Ru3(CO)12 in the presence of a ca. fiftyfold excess of KOH catalyzes the shift reaction at 100°C/1 bar CO (79). The effectiveness of the system increased markedly as temperature was increased (rate of hydrogen formation approximately quadrupled on raising the temperature from 100° to 110°C), and over a 30-day period catalyst turnovers of 150 and 3 were found for Ru3(CO)12 and KOH, respectively. Neither methane nor methanol was detected in the reaction products. Although the nature of the active ruthenium species could not be unambiguously established, infrared data indicated that it is not Ru3(CO)12, and the complexity of the infrared spectrum in the... 

In this reaction, a rhodium atom complexed to a chiral diphosphine ligand ( P—P ) catalyzes the hydrogenation of a prochiral enamide, with essentially complete enan-tioselectivity and limiting kinetic rates exceeding hundreds of catalyst turnovers per second. While precious metals such as Ru, Rh, and Ir are notably effective for catalysis of hydrogenation reactions, many other transition-metal and lanthanide complexes exhibit similar potency. 

The first application of a heterocyclic carbenoid achiral ligand for hydrogenation of alkenes was reported in 2001 by Nolan and coworkers. Both ruthenium [36] and iridium [37] complexes proved to be active catalysts. Turnover frequency (TOF) values of up to 24000 b 1 (at 373 K) were measured for a ruthenium catalyst in the hydrogenation of 1-hexene. 

Complex (J )-140 serves as a chiral Lewis acid and coordinates to the aldehyde at the less hindered /J-face of 141. i e-side cyanation of (J )-141 and the subsequent cleavage of the alkoxide group give the product 142. Because at this stage the catalyst turnover is blocked, the reaction cannot be carried out in a catalytic manner. 

Chiral crown ethers such as 13 are suitable alternatives to the ammonium salts and not decomposed under alkaline conditions. They usually have higher catalyst turnover than the chiral ammonium salts, and the design of catalysts will be much easier. However, they are, in general, costly and difficult to prepare on large scale. Polyols (eg., (RR)-TADDOL14) also serve as phase transfer catalysts. 

Catalyst Turnover rate in pyridine (moles/moles atom catalyst per hour) Turnover rate in acetone (moles/moles atom catalyst per hour)... 

The impact of water on the differential behavior of the triflate and SbF6 catalysts has led to the postulate that catalyst turnover is facilitated by an open coordination site (apical) in four coordinate cationic Cu(II) complexes (200). Presumably, additional dienophile induces turnover through associative displacement of the neutral oxygen ligands (water or product). This rationale may explain why the triflate complex 266c is inactive as a catalyst prior to activation by dehydration. 

Although the selectivities are excellent, prolonged reaction times (2 1 days) are noted under these conditions. The addition of alcohols, particularly 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), was found to decrease reaction times (4 days to 36 h under identical conditions). In the presence of HFIP, Michael adducts are generated in comparable yields and selectivities suggesting that the principal role of the alcohol is catalyst turnover. 

Mukaiyama Michael reactions of alkylidene malonates and enolsilanes have also been examined (244). The stoichiometric reaction between enolsilane (342a) and alkylidene malonate (383) proceeds in high selectivity however, catalyst turnover is not observed under these conditions. The addition of HFIP effectively promotes catalyst turnover, presumably by protonation and silyl transfer from the putative copper malonyl enolate generated in this reaction. The reaction proved general for bulky P-substituents (aryl, branched alkyl), Eq. 209. 

Starting from the iron(III) complex, three reversible waves are observed in an aprotic solvent such as DMF (Figure 4.5a). The last one corresponds to generation of the iron(0) porphyrin. The latter does react with CO2, as attested to by the fact that the Fe(I)/Fe(0) wave becomes irreversible under 1 atm of C02 (Figure 4.5b). However, the reaction is sluggish and the current does not go beyond a two-electron-per-molecule stoichiometry. This is confirmed by preparative-sc ale electrolysis where catalyst turnover numbers are found not to be larger than a few units. 

The derivative, containing 10% of its residues alkylated with dodecyl groups and 15% with methyleneimidazole groups, brought about a rate enhancement of nearly 300 over imidazole for the hydrolysis of p-nitrophenylcaproate at pH 7.3 and 25° 44). Kinetic data indicated catalyst turnover and a two-step pathway. 

Figure 1.33 illustrates asymmetric hydrogenation of a functionalized imine with a XYLIPHOS-Ir catalyst, occurring with a catalyst turnover number of 2,000,000. The presence of I under acidic conditions is crucial to achieve high catalytic performance. (5)-Metolachlor, a herbicide, is industrially produced in a > 10,000-ton quantity per year by this reaction. 

These workers have also developed a Ru-salen system capable of desymmetriz-ing meio-diols in moderate enantiomeric excess [Eq. (10.47)] 7 It is of some interest that atmospheric oxygen suffices to induce catalyst turnover. Photolysis is presumably required to initiate nitrosyl loss from the Ru center ... 

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