Catalysts, general polyfunctional

An interesting and very practical example of these phenomena concerns catalysts composed of both types of materials—called dual function, or bifunctional (in general, polyfunctional) catalysts. A lucid discussion is provided by Weisz [16], and a few examples indicate the importance of these concepts, not only to catalysis, but also to the kinetic behavior. Much of the reasoning is based on the concept of reaction sequences involving the surface intermediates. Consider the scheme where the species within the dashed box are the surface intermediates. 

It was found that sodium salt (II) can be easily and cleanly monoalkylated with a variety of polyfunctional organic halides under strictly anhydrous conditions. The reactions are generally carried out in refluxing benzene in the presence of 10 mol-% of tetra-n-butylammonium bromide (TBAB) as phase-transfer catalyst ... 

The knowledge currently available allows us to make useful predictions of which metals (pure or alloyed) and in which form (small or large particle size) have the best chance to be good catalysts for a new reaction with simple (monofunctional) molecules which have not yet been studied. However, much less can be predicted at the moment with regard to the polyfunctional molecules, in which (for example) a C=C bond stands in the neighbourhood of a C=0, C=N or other bond. The only general theory of selectivity in these reactions is that of Ballandin [86], but this theory does not seem to be satisfactory from the modern point of view. However, useful information is available for some individual reactions. For example, with regard to the a,(1-unsaturated aldehydes, of which acrolein is the simplest example. Let us describe this in more detail. 

Broad are the implications and application of the principles of polystep reactions on polyfunctional catalyst combinations (P. B. Weisz). Here we deal with reaction sequences in which two catalyst species X and Y (or more) participate in one set of reaction sequences. Some of the general principles combine thermodynamics and physical parameters to yield important information and criteria for such rate processes, generally whether they occur in hydrocarbon transformations, organic chemistry, in a petroleum plant or in a living cell. 

Generally, the polymerisation conditions for polyfunctional polyol alkoxylation, with KOH as catalyst, to rigid polyether polyols are ... 

Important reactions of rosin are summarized in Figure 8. Dimerization is generally carried out by a Lewis acid catalyst such as sulfuric acid, boron fluoride, or zinc chloride (14-16). A rosin dimer finds uses in resin preparation because it allows quick buildup of molecular weight through a polyfunctional alcohol or amine. 

The Baylis-Hillman (also called Morita-Baylis-Hillman) (MBH) reaction (see Sect. 10.2.4 and Scheme 10.17) is the base-catalyzed addition of keto compounds to acrylic derivatives. The catalyst is a cyclic tertiary amine such as 1,4-diazabicyclo[2.2.2]octane (DABCO). Due to the generally poor yields observed, the reaction has not received sufficient attention despite the great synthetic value of the polyfunctional adducts. Among the various methods proposed to activate the reaction, pressure 107] and hydrophobic effects [70] have been used. Table 10.31 presents the results obtained in some Baylis-Hillman reactions carried out under pressure in aqueous solution [108]. 

Mechanisms A and B represent the general basic catalysis. Cooperative interaction between two neutral imidazole molecules enhances the nucleophilic attack on the substrate. Mechanism C is a variety of the general acid catalysis caused by the nucleophilic attack. To prove the polyfunctional character of the catalysis, let us compare activation parameters of polymers with their low-molecular weight analogues. For instance, for a polymeric catalyst the change in enthalpy (AH) is 15.5 kJ/mole, whereas for imidazole this value is 29.4kJ/mole [23]. Additional entropy is obtained from the formation of a transition-state complex in which catalytic and reactive groups are oriented with respect to each other. Besides, with the transition from a low-molecular... 

The reaction with amine derivatives such as 4-hydroxybenzeneamine 20 and 4,4 -methylenebis-benzeneamine 22 is used to produce the tri- and telrafunctional epoxies N,N,0-tris(2,3-epoxypropyl)-4-hydroxybenzeneamine 21 and A,A,iV, iV -telrakis (2,3-epoxypropyl)-4,4 -methylenebisbenzeneamine 23, respectively. However, the polyfunctional epoxies that combine the most attractive properties for electronic applications are the resins produced by epoxi-dation of the phenol novolac 24 and cresol novolac 26. Novolac resins are obtained by the condensation of a phenol with formaldehyde in the presence of acid catalysts in such conditions that the degree of polycondensation is in the range of 3—5. The epoxy novolacs 25 and 26 are produced by the reaction of epichlorhydrin with the corresponding phenol novolac and ortho-cresol novolac resins. Epoxy resins are generally characterized by their dynamic viscosity (77) at 25 °C, expressed in millipascal second (mPa s). 

---