Promoter cations

Calcium sources, such as gypsum and lime, promote cation exchange from sodium clay to a less-sweUing calcium clay. Calcium concentrations ate normally low (<1000 mg/L) and osmotic swelling is only reduced if other salts are present. Calcium chloride has been used infrequently for this purpose but systems are available that allow high calcium chloride levels to be carried in the mud system (98). 

Closely related to the polyepoxide cascade procedure for the synthesis of polycyclic systems is Corey s biomimetic-type, nonenzymatic, oxirane-initiated (Lewis acid-promoted) cation-olefin polyannulation. By this strategy, compound 96, containing the tetracyclic core of scalarenedial, was constructed by exposure of the acyclic epoxy triene precursor 95 to MeAlCl2-promoted cyclization reaction conditions (Scheme 8.25) [45]. 

Selenoglycosides are attractive donor species owing to the wide range of reagents that can promote cation- and radical-cation-based processes for their activation and subsequent O-glycosylation (Scheme 4.102) [223,512],... 

The studies performed over promoted manganese molybdate catalysts have shown significant changes in catalytic behavior due to presence of the promoter. The preliminary results suggest that the pronounced differences observed in selectivity and activity may be related to the effect of the promoter cations on the reactivity of the lattice oxygen and the availability of adsorbed oxygen. 

This reaction exhibits strong analogy with the silver-promoted cationic aza-Cope rearrangement described by Overman et al. As a new route toward alkaloids, these authors showed thatcyanomethylamines carrying unsaturated chain led to pyrrolidine or piperidine derivatives on treatment with silver salts depending on the substituent (Scheme 3.38) 59 60... 

Free radical promoted, cationic polymerization also occurs upon irradiation of pyridinium salts in the presence of acylphosphine oxides. But phosphonyl radicals formed are not oxidized even by much stronger oxidants such as iodonium ions as was demonstrated by laser flash photolysis studies [51, 52]. The electron donor radical generating process involves either hydrogen abstraction or the addition of phosphorus centered or benzoyl radicals to vinyl ether monomers [53]. Typical reactions for the photoinitiated cationic polymerization of butyl vinyl ether by using acylphosphine oxide-pyridinium salt combination are shown in Scheme 10. 

UV irradiation of the resulting prepolymers caused a-scission, and benzoyl and polymer bound electron donating radicals are formed in the same manner as described for the low-molar mass analogues. Electron donating polymeric radicals thus formed may conveniently be oxidized to polymeric carboeations to promote cationic polymerization of cyclic ethers. It was demonstrated that irradiation of benzoin terminated polymers in conjuction with pyridinium salts as oxidants in the presence of cyclohexene oxide makes it possible to synthesize block copolymers of monomers with different chemical natures [75] (Scheme 19). 

Free radical promoted cationic polymerization was successfully employed [77] for the preparation of new classes of liquid crystalline (LC) block copolymers comprising a semicrystalline block, poly(cyclohexene oxide), and LC block of different structures ... 

On metals that produce substantial yields of oxygenates, such as rhodium, promotion by reducible oxides (e.g., molybdenum or vanadium oxides) appears beneficial 76—78). The key property of the cation of the promoting oxide is a weak affinity for CO combined with a strong affinity for oxygen. An aftractive hypothesis to explain the role of the promoting cations is that the reducible cation provides a site for the O atom generated upon CO dissociation, whereas CO interacts only weakly with the promoting cations. 

Electrdysis of moncaner sdulions in the presence of supporting electrolytes can produce cation radicals which will promote cationic polymerisation. Unfortunately, the contribution of this technique to a better understanding of the fundamentals of initiation and propagation has been disappointingly meagre, as wiU be shown in Chap. Vll. 

The photolytic excitation of charge-transfer complexes is another recent addition to the available physical expedients to promote cationic polymerisation. The cation radicals generated by the photolysis have been characterised in some systems. More recent still is the use of ultraviolet radiation to induce the photolysis of substances whose photoproducts are initiators of cationic polymerisation. These processes will be discussed in Chap. Vin. 

Uranium solubility is increased even more in the nitrate microcosms, and one possible explanation is the conversion of acetate to CO2, coupled with nitrate reduction, which would give higher dissolved carbonate concentrations in the nitrate microcosms. However, PHREEQE modelling showed that the higher CO concentration would not greatly affect uranium speciation in solution and is therefore unlikely to account for the enhanced solubility. Alternatively, as nitrate is reduced to ammonium (NH/), which promotes cation exchange, this could lead to displacement of U02 from surface complexes, which are the predominant uranyl species on mineral surfaces. " ... 

The concentration of the promoter cation and its ability to activate the equilibrium between methyl acetate and methyl iodide greatly influence the reaction rate (eq. (17)). 

Cation or anion adducts will cause the same undesirable effects as clusters or multimers, although this type of adduct may not be as easily removed. Certain types of molecules are more efficiently cationized than protonated, and cations may be deliberately added to the sample preparation to promote cationization. Some types of sample cleanup techniques will help minimize ion adducts but, unfortunately, cannot completely eliminate them. 

This process is usually termed as the free radical promoted cationic polymerization. This so-called free radical promoted cationic polymerization is an excellent and fairly flexible type of indirect initiation of cationic polymerization. 

S. Denizligil, Y. Yagci, and C. McArdle, Photochemically and thermally-induced radical promoted cationic polymerization using an allylic sulfonium salt. Polymer 1995,36(16), 3093-3098. 

Y. Yagci and W. Schnabel, New aspects on the photoinitiated free-radical promoted cationic polymerization. Makromol. Chem. Macromol. Symp. 1992, 60, 133-143. 

Scheme 4. Synthesis of block copolymer by combination of ATRP and Free Radical Promoted Cationic Polymerization.

Summary on Sn-Ru/AkO Catalysts. - In the last decade supported tin-ruthenium catalysts have been extensively studied because of their activity and high selectivity in the hydrogenation of unsaturated aldehydes, esters or acids into the corresponding alcohols. The increased performance of metal catalysts with the addition of promoters such as tin is attributed to the presence of promoter-cations on the surface or at the periphery of metal nanocluster, which activate the C=0 bond through the interaction with the lone electron pair of the carbonyl group oxygen. 

Although SET photochemistry is often a practical method to promote cation radical reactions, such a variant had not been probed until we recently discovered the SET-photoinduced degenerate methylenecyclopropane rearrangement of 2,2-diaryl-1-methylenecyclo-propanes." Below, we describe in detail the results of our studies of this process including the chemical and spectroscopic identification... 

More recently, iodonium salts have been widely used as photoinitiators in the polymerization studies of various monomeric precursors, such as copolymerization of butyl vinyl ether and methyl methacrylate by combination of radical and radical promoted cationic mechanisms [22], thermal and photopolymerization of divinyl ethers [23], photopolymerization of vinyl ether networks using an iodonium initiator [24,25], dual photo- and thermally-initiated cationic polymerization of epoxy monomers [26], preparation and properties of elastomers based on a cycloaliphatic diepoxide and poly(tetrahydrofuran) [27], photoinduced crosslinking of divinyl ethers [28], cationic photopolymerization of l,2-epoxy-6-(9-carbazolyl)-4-oxahexane [29], preparation of interpenetrating polymer network hydrogels based on 2-hydroxyethyl methacrylate and N-vinyl-2-pyrrolidone [30], photopolymerization of unsaturated cyclic ethers [31] and many other works. 

Carbocations generated in this way can add directly to appropriate monomers (e.g., tetrahydrofuran, cyclohexene oxide, n-butyl vinyl ether) or can form Bronsted acids by abstracting hydrogen from surrounding molecules. This method, which is commonly referred to as free-radical-promoted cationic polymerization, is quite versatile, because the user may rely on a large variety of radical sources. Some of them are compiled in Table 10.9. 

Table 10.9 Free radicals that may be employed in free-radical-promoted cationic polymerizations.

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