Solvent-Free Catalytic Exchange

Solvent-free methods of hydrogen isotope exchange labeling usually utilize tritium gas as the isotope source, and the substrate is used either neat, as a solid, or as a component of a solid phase. Therefore in contrast to the methods of Sections 3.1—3.5 this class of methods are, at least at a gross level, gas-solid two-phase systems. 

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S. H., and Stitt, E.H. (2007) intensification of the solvent-free catalytic hydroformylation of cydododecatriene comparison of a stirred batch reactor and a heat-exchange reactor. Catal. Today, 128, 18-25. 

Figure 8.8 reprinted from Enache, D.I., Thiam, W., Dumas, D., EUwood, S., Hutchings, G.J., Taylor, S.H., Hawker, S. and Stitt, E.H. Intensification of the solvent-free catalytic hydroformylation of cyclododecatriene comparison of a stirred batch reactor and a heat-exchanger reactor. Catalysis Today, Vol. 128, pp. 18-25, 2007, with permission from Elsevier. 

Recent examples, for instance, of the catalytic application of the commercially available macroporous Amberlyst-15 include the Michael addition of pyrroles to a,P-unsaturated ketones (Scheme 10.4) [48]. In this process, the acid ion exchange resin (dry, 10% w/w) allows on to obtain mono and dialkylated pyrroles 5 and 6 in reasonable yields. Similarly, this catalyst (dry, 30% w/w) can catalyze the aza-Michael reaction of amines with a,P-unsaturated ketones, esters and nitriles to afford 7 in 75-95% yields under solvent-free conditions. Interestingly, yields were significantly lower using typical solvents such as DCM (dichloromethane), CH3CN, THF, DMF or EtOH [49], Recycling the catalyst is possible in both cases, but a smooth decrease in the yield is observed for each new run. 

The most commonly used method of solvent-free exchange labeling, called high-temperature solid-state catalytic isotope exchange (HSCE), came into use in the late 1980s. It involves exposing a mixture of substrate, support and a metal catalyst to... 

Cases are also known where a reaction product is firmly held to catalytic sites and diffuses out at a prohibitively low rate. The effect has been observed in several cases, including the esterification of alcohols, acting as reactant and solvent [16, 20]. It is thought that in these reactions water is the product which poisons the acidic sites of the sulphonic acid catalyst. A more straightforward case is the hydrolysis of amides where one of the reaction products, the amine, forms an ammonium salt with the resin acid and thus decreases the number of free acid groups [21]. Similarly, cationic reaction products (formed from cationic reactants) may be retained in the resin by virtue of its ion exchanger function. 

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