Cationic catalysts

Figure 1 shows the mechanistic picture developed by C. M. Starks (1,2) for Hquid—Hquid PTC in a graphical form. The catalyst cation extracts the more hpholilic anion Y from the aqueous to the nonpolar organic phase where it is present in the form of a poorly solvated ion pair Y ]. This then reacts rapidly with RX, and the newly formed ion pair X ] returns to the aqueous phase for another exchange process X — Y . In practice most catalyst cations used are rather lipophilic and do not extract strongly into the aqueous phase so that the anions are exchanged at the phase boundary. 

Catalyst Cation. The logarithms of extraction constants for symmetrical tetra- -alkylammonium salts (log rise by ca 0.54 per added C atom. Although absolute numerical values for extraction coefficients are vastly different in various solvents and for various anions, this relation holds as a first approximation for most solvent—water combinations tested and for many anions. It is important to note, however, that the lipophilicity of phenyl and benzyl groups carrying ammonium salts is much lower than the number of C atoms might suggest. Benzyl is extracted between / -propyl and -butyl. The extraction constants of tetra- -butylammonium salts are about 140 times larger than the constants for tetra- -propylammonium salts of the same anion in the same solvent—water system. 

Selectivity, Steering of reaction directions by the type of catalyst cation, eg, O- vs C-alkylation (7), substitution vs dibalocarbene addition (8), as weU as enantioselective alkylations by optical active catalysts (9) have been achieved in some systems. Extensive development is necessary, however, to generate satisfactorily large effects. 

Competitive Extraetion of Anions. The successful extraction of the necessary anion into the organic phase is cmcial for PTC. Often three anions compete for the catalyst cation the one that is to react, the one formed in the reaction, and the one brought in originally with the catalyst. Table 1 hsts the widely differing values of tetra-rr-butylammonium salts. The big difference in the halide series is noteworthy and preparatively important. Hydroxide is 10 times mote difficult to extract than chloride (11) and the divalent and trivalent anions and PO " are stiU more hydrophilic. Thus... 

It is not surprising that for most solvent-water combinations and for many anions, the rate of extraction increases with the number of C atoms of the catalyst cation. To a first... 

The strong acid, HBF4, acts as the catalyst. Cationic polymerisation is used for coatings and printing inks. 

However, the observation that very organophilic ammonium salts are also very active phase transfer agents suggests that the catalyst cation need not actually enter the aqueous phase, and the reaction can occur entirely at the interface, as depicted in Scheme 5.7 [43], This process is known as the interfacial mechanism. 

At a lower, ca. 5 % base concentration the reaction is substantially slower and enables the isolation of monobenzylated products in much higher yields than corresponds to statistical product composition [105]. Supposing the alkoxide formed from the alcohol in the aqueous phase forms an ion-pair with the catalyst cation which is then... 

Membrane is the catalyst Cation exchange membranes for esterification reactions Palladium membranes for hydrogenation/dehydrogenation reactions... 

As the cyclotrimerization catalysts, cationic (SnCl4, ZnCl2), anionic (triethylamine, pyridine, triphenylphosphine) and coordination-ionic (Cr+3 acetylacetonate, with water as cocatalyst) ones can be used [4] A1C13 was used as well [5]. 

We might be able to nse chlorine NQR to identify whether snch an anion showed secondary bonding interactions with catalyst cations, or whether it interacted with them only by long-range (and therefore weak) interionic attractions. Such an anion probably would show secondary interactions with soft cations such as Ag+, which would result in low halogen NQR frequencies for the coordinated chlorines (and, if measurable, high asymmetry parameters). However, we would hope for no such results for hard-acid cations that are similar in charge and size to Ag+ (e.g., Na+ or K+). 

Surface Analysis Using Neutral Radicals as Probe Molecules 5.2.4. Metal-related Signals in Supported Catalysts Cation Dimers... 

In most Diels-Alder reactions, no catalyst is needed, but Lewis acids are effective catalysts in many cases, particularly those in which Z in the dienophile is a C=0 or C=N group. A Lewis acid catalyst usually increases both the regioselec-tivity of the reaction (in the sense given above) and the extent of endo addition, °° and, in the case of enantioselective reactions, the extent of enantioselectivity. It has been shown that InCla is an effective catalyst for aqueous Diels-Alder reactions,which is suitable for ionic Diels-Alder reactions, and there are other Lewis acid catalysts that are effective in water. °° Brpnsted acids have also been used to accelerate the rate of the Diels-Alder reaction.Lanthanum triflate [La(OTf)3] has been reported as a reusable catalyst ° ° and Me3SiNTf2 has been used as a green Lewis acid catalyst. ° Cationic Diels-Alder catalysts have been developed, particularly oxazaborolidine catalysts. ° Some Diels-Alder reactions can also be catalyzed by the addition of a stable cation radical, for... 

The catalytic activity of quarternary ammonium salt usually depends on the corresponding catalyst cation and counter anion[2]. For a series of tetraalkylammonium chlorides, the activity increased in the order of TP AC < TBAC < TOAC. Bulky quaternary salts, having longer distance between cation and anion, are generally known to exhibit higher activity in activating anions[3]. This explains why they are more effective in nucleophilic attack of the anion to oxirane ring of GVE. Table 1 also shows that the rate constant with different halide anions of the quaternary ammonium salts decreases in the order of Cf > Bf > T. This is consistent with the nucleophilicity of the halide anions. 

The lactones, the cyclic esters of hydroxy acids have been polymerised with various catalysts anionic catalysts, cationic catalysts and organometallic catalysts [28, 29, 39, 45, 54]. Five member lactone rings are unpolymerisable. Four, seven or eight member lactones are polymerisable [28, 29, 39, 45, 54]. 

Step 1 is eq. (9.91) describes the competitive extraction of reactant and product anions between the aqueous and organic phases in the presence of a catalyst cation. The rate constants of this step includes the effects of mass transfer across the interfacial region and depends on the mass transfer. 

Selectivity of multiphase reactions catalysed by phase transfer catalysts can be greatly improved by the use of the so called capsule membrane - PTC (CM-PTC) technique. We report here the theoretical and experimental analysis of the CM-PTC and Inverse CM-PTC for exclusively selective formation of benzyl alcohol and benzaldehyde from the alkaline hydrolysis and oxidation of benzyl chloride, respectively. The theoretical analysis shows that it is possible to simultaneously measure rate constant and equilibrium constant under certain conditions. The effects of speed of agitation, catalyst concentration, substrate concentration, nature of catalyst cation, membrane structure, nucleophile concentration, surface area for mass transfer and temperature on the rate of reaction are discussed. 

All these aspects contribute to the character of a metallocene catalyst. Many attempts have been made in the past to design catalysts by molecular modeling. The results were not very satisfying because there are still too many open parameters that must be considered such as the degree of activation of the catalyst precursor, the interaction between catalyst cation and cocatalyst anion, or the role of the solvent. Because of this situation, we preferred the empirical way. The author s group synthesized over 650 met-... 

---