Thermochemical studies hydrogen activation

A kinetic study was performed and thermodynamic factors were estimated for the activation energy of abstraction of hydrogen atoms from a disilacyclopentane <74izv6i>. In addition, a thermochemical study of the dissociative ionization of 2,3,3-trimethyl-l-thia-3-silacyclopentane has been reported <85JOM(288)27>. 

A likely intermediate in hydrogen activation reactions, including heterolytic splitting, is a hydrogen species where H2 is 7] -bonded to the metal center. The first stable dihydrogen molecule (and recognized as such) has been isolated by Kubas et al. in 1983, and some thermochemical studies on this type of molecules have appeared since then." " The enthalpy of reaction (57), AJP (57) = —30.5, —27.2, and —40.2 kJ moP for M=Cr, Mo, and W, respectively, measures DH° (M-H2) ... 

When we first contemplated thermochemical products available from Glu, a search of the literature revealed no studies expressly directed at hydrogenation to a specific product. Indeed, the major role that Glu plays in hydrogenation reactions is to act as an enantioselectivity enhancer (17,18). Glu (or a number of other optically active amino acids) is added to solutions containing Raney nickel, supported nickel, palladium, or ruthenium catalysts and forms stereoselective complexes on the catalyst surface, leading to enantioselective hydrogenation of keto-groups to optically active alcohols. Under the reaction conditions used, no hydrogenation of Glu takes place. 

The first lEA (Internationa) Energy Agency) thermochemical Round Robin was organised in 1988 as part of the lEA Voluntary Standards Activity led by BC Research (1). The main conclusions were the precision for carbon was excellent, while hydrogen, oxygen by difference and water were more variable, and oxygen by direct determination was poor. It was recottunended to use a wider variety of samples in the future studies. 

The thermochemical liquefaction of wood in water to produce fuel or chemical intermediates has been studied intensively over the last decade ( l). The earlier works used either sodium carbonate as soluble catalyst and carbon monoxide as reducing gas ( ) ( ), or nickel catalyst ( U) or palladium on activated charcoal ( 5) in the presence of hydrogen. Then it has been shown that the presence of a reducing gas was not necessary if iron powder was used as additive, with moderate heating rates ( ). When the wood suspended in water was rapidly heated to 350°C, and then quenched, no catalyst was necessary and a yield in acetone solubles as high as 50 wt was obtained (T ) ( ). 

Titanium. Catalyses of hydrogenation of alkenes, alkynes, carbonyl-, and nitro-compounds have been described. The effect of the nature of the ligand L and of the alkene to be reduced on reactivity in catalytic hydrogenation by Ti(7r-C5H5)2L2 has been quantitatively studied. The dependence of rate constants on solvent for reduction of decene in the presence of Ti(7r-C5H5)Me+ is interpreted in terms of electrostatic interaction between the active ionic species and the solvent. There is also a thermochemical report relevant here, and that is of a determination of the heats of mixing of cyclohexene and of hex-l-ene with titanium tetrachloride. The heats of mixing are close to zero, which implies very small heats of complex formation between these alkenes and titanium. ... 

Organometallic chemistry is interesting in part because it has applications to catalytic processes. Since the discovery of C-H bond activation and the homogeneous hydrogenation of olefins, the importance of organometallic complexes has been undisputed. Many experimental studies of organometallic catalysis have focused on catalyst and substrate structure, kinetics of transformations, mechanisms, thermochemical properties, turnover, selectivity, etc., and a massive quantity of experimental data has been accumulated. Molecular mechanics can be used to compile and analyze these data in order to direct the design of novel catalytic systems. 

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