Catalysts, Substrates, Conditions

Entry Catalyst Substrate Conditions TON TOF [h-1] Conv. [%] ee [%] Reference... 

Acetylenes have hijh synthetic utility, and hydrogenation of the triple bond occurs in many reaction sequences (7). Often the goal of this reduction is formation of the cis olefin, which usually can be achieved in very high yields (for an exception, see Ref. 10). Continued reduction gives the paraffin. Experimentally, both the relative and absolute rates of acetylene and olefin hydrogenation have been found to depend on the catalyst, substrate, solvent, reaction conditions, and hydrogen availability at the catalyst surface. Despite these complexities, high yields of desired product usually can be obtained without difficulty. 

Usually the benzyl ether is dissolved in EtOH and cyclohexene, and 20% Pd(OH)2/C is added (1 10 catalyst-substrate by weight) and stirred under reflux for the required period (thin layer chromotography monitoring).246 Without cyclohexene, the benzyl ether was usually recovered, signifying that the EtOH solvent is not a good hydrogen donor under these conditions. 

Notably, under catalytic conditions, reaction steps preceding catalyst-substrate-dihydrido formation are shown to be in rapid equilibrium. Therefore, stereoselec-... 

Catalytic studies and kinetic investigations of rhodium nanoparticles embedded in PVP in the hydrogenation of phenylacetylene were performed by Choukroun and Chaudret [90]. Nanoparticles of rhodium were used as heterogeneous catalysts (solventless conditions) at 60 °C under a hydrogen pressure of 7 bar with a [catalyst]/[substrate] ratio of 3800. Total hydrogenation to ethylbenzene was observed after 6 h of reaction, giving rise to a TOF of 630 h 1. The kinetics of the hydrogenation was found to be zero-order with respect to the al-kyne compound, while the reduction of styrene to ethylbenzene depended on the concentration of phenylacetylene still present in solution. Additional experi-... 

Interpretation of the reciprocals of the Michaelis constants allows the following conclusions to be made regarding hydrogenations under specified experimental conditions. In the case of the methyl and cyclohexyl ligand, the prevailing form of the catalyst in solution is the catalyst-substrate complex. However, for the other examples of first-order reactions, large Michaelis constants (or very... 

As explained earlier, the pre-equilibria are characterized by the limiting values of Michaelis-Menten kinetics. In the case of first-order reactions with respect to the substrate, we have Kfvl [S]0. Since the pre-equilibria are shifted to the side of educts during hydrogenation, only the solvent complex is detectable. In contrast, in the case of zero-order reactions only catalyst-substrate complexes are expected under stationary hydrogenation conditions in solution. These consequences resulting from Michaelis-Menten kinetics can easily be proven by var-... 

Entry Catalyst Substrate ee [°/o] TOF [h-1] Solvent Conditions Reference... 

Entry Catalyst Substrate TOF Selectivity Solvent and conditions Refer-... 

Thus, additional experimental and computational studies will be needed to draw definitive conclusions regarding the mechanism of Ir-catalyzed asymmetric hydrogenation. The Ir(I)-Ir(III) and Ir(III)-Ir(V) cycles seem to be similar in energy, so it may well be that depending on the catalyst, substrate, and the hydrogenation conditions, one or the other pathway will be preferred or both cycles could operate in parallel. 

Jencks (1972) has concluded that concerted bifunctional acid-base catalysis is rare or nonexistent because of the improbability of meeting simultaneously at two sites on reactant and catalyst the conditions of the rule which he has proposed for concerted reactions. The rule states that concerted general acid-base catalysis of complex reactions in aqueous solution can occur only (a) at sites that undergo a large change in pAT in the course of the reaction, and (b) when this change in pAf converts 2m unfavourable to a favourable proton transfer with respect to the catalyst, i.e., the pAT-value of the catalyst is intermediate between the initial and final pAf-vadues of the substrate site. 

Nickel-containing polyfluorometalated [Ni(H20)H2FeNaWi7055] was capable of catalytic activation of hydrogen peroxide for allylic alcohol oxidation in biphasic conditions (catalyst substrate ratio 1 1000). Homoallylic alcohols are significantly less reactive than allylic homologues and cis > trans reactivity is found also in this case. 

In a subsequent study of oxygen heterocychzation, Andersson et al. investigated various catalyst reoxidation conditions with the Pd(OAc)2/DMSO catalyst system (Eq. 27, Table 3) [ 150]. Several conditions result in high substrate conversion to the product, including the use of BQ, BQ with methanesulfonic acid, and molecular oxygen, with and without copper(II) salts as a cooxidant. Only the aerobic methods enable formation of the product 37 with high regio-selectivity. The presence of a copper cocatalyst enhances the rate but is not necessary for catalysis. 

The hydrogenation of aromatics (benzene, toluene, a-methylstyrene) can be carried out under very low (1 12,000) catalyst substrate ratio, and mild conditions on Rh and Ni organometallic complexes anchored to USY zeolites.474 A Rh complex anchored to functionalized MCM-41 exhibits excellent performance in the hydrogenation of arenes (benzene, toluene, p-xylene, mesitylene) under mild conditions (45°C and 1 atm) 475 A uniquely selective hydrogenation of acenaphthene and acenaphthylene was performed by using a triruthenium carbonyl cluster 476... 

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