Hydrogenation solvent-free systems

In solvent-free systems one usual component of the liquid reaction mixture is missing hence, all the interactions of this component with the other components in the system are also missing. At the beginning, the bulk phase consists only of the substrate or of several substrates during the reaction, hydrogenation products accumulate in the reaction mixture. At first glance the system seems to be simpler than the one with solvent, but consequences of the absence of the solvent must not be forgotten. 

In a competitive hydrogenation of two substrates in a solvent-free system the relative reactivity is again given by Eq. (8). On the other hand, however, the properties of the bulk phase vary with each change in the substrate, and this change may of course variously affect the rate constants and the adsorption coefficients. Moreover, the rate constant of A cannot be directly measured in the presence of an unsaturated compound B. As a consequence, substitution of /cah and knu into Eq. (8) instead of and k, yields the values Ka/Kb subjected to the same inaccuracy. In this case, therefore, the re-... 

Cerveny et al. (100 report an investigation of the hydrogenation of 12 olefinic substrates in the liquid state with 5% Pt on silica gel as catalyst under usual conditions and without solvents. The reaction rates related to 2,3-dimethyl-2-butene and the relative adsorption coefficients obtained in systems of various pairs of substrates and by recalculation using Eq. (25) to 2,3-dimethyl-2-butene are given in Table IV. Comparison between the measured reaction rates and the rates of hydrogenation in solvents (71 has revealed that relations existing between the rates of hydrogenation in a solvent-free system approximately correspond to those determined in... 

Relative Hydrogenation Rates and Adsorption Coefficients of Substrates on Pt in Solvent-Free Systems... 

The measured data also were used (700) in a quantitative representation of the effect of structure on the reactivity and adsorptivity of substrates by means of the Taft-Pavelich equation (22). The adsorption data suffered from a larger scatter than the rate data. No substrate or substituent could be detected that would fail to satisfy completely the correlation equations. In the correlation of the initial reaction rates and relative adsorption coefficients the parameter p was negative, while the parameter S was positive. In correlations of the reaction rates obtained by the hydrogenation of a similar series of substrates on the same catalyst in a number of solvents, the parameters p and had the same sign as in the hydrogenation in solvent-free systems, while in the correlation of the adsorption coefficients the signs of the parameters p and in systems with solvents were opposite to those in solvent-free systems. This clearly indicates that solvents considerably affect the influence of the structure of substrates on their reactivity. 

The investigation of the effect of olefins on the course of hydrogenation in solvent-free systems, similarly to the investigation of these relations in systems with solvents, demonstrated the necessity of a complex view of the whole system, because the behavior of substrates in hydrogenation may be affected by all the components present in the reaction mixture. 

Evidence of metal adducts in solvent-free systems was primarily obtained by phototolysis of Cr(CO)g in hydrogen-containing matrices [49], while the formation of both ri - and ri -Hj Pd(H2) moieties was proposed to occur in the reaction of 4d ° Pd atoms with Hj in rare gas matrices [50]. 

The Plaquevent group achieved a new and efficient method for the approach to both enantiomers of methyl dihydrojasmonate 47 by asymmetric Michael addition under solid-liquid phase-transfer catalysis. The main advantages of their procedure are the solvent-free system and the creation of two contiguous stereogenic centres in one step. The authors proposed a plausible mechanism with a transition state composed of substrate 45 and catalyst, quinine-, or quinidine-derived PTC catalyst (48a, 49a), in which hydrogen bonding ensures the proximity of the reactive centres and significantly stabilises the transition state (Scheme 16.14). ... 

Reductive dehalogenation of chlorinated phenols to phenol, cyclohexanol and other chlorine-free compounds takes place rapidly with hydrogen gas and Pd/C in an aqueous system or under solvent-free conditions. Thus, pentachloro phenol was able to be completely dechlorinated within 20 min (Scheme 4.45). This methodology enables a facile route for rapid and complete detoxification of highly toxic polychlorinated aromatic hydrocarbons and environmental remediation71,72. 

The reaction can be operated as a two-phase system in this case the process can be employed at the reflux temperature of the solvent. It should be noted that in some cases, no solvent need be used296 and this is especially efficacious where mono-brominated products are the target. An example of the system s versatility in solvent-free mode can be seen in Table 3.7. Here we see that at a molar ratio of 1 1.1 1 (hydrogen peroxide/hydrogen bromide/2-nitrotoluene) over 90% selectivity to the benzyl bromide is observed at a conversion of 76%. However, increasing the oxidant ratio to 3 3.2 1 (hydrogen peroxide/hydrogen bromide/2-nitrotoluene), a selectivity of 98% to the benzal bromide is afforded. Diphenylmethane can be fully converted to oxidized products with 96% selectivity to the benzophenone compound.297... 

The supported nanoparticle catalyst system was used for solvent-free hydrogenation reactions of cydohexene, 1-hexene, and 1,3-dicydohexadiene, respectively, and compared with a similar biphasic ionic liquid system and with a heterogeneous supported nanocatalyst (Table 5.6-6). 

---