Lewis acid salt

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

A reaction in which an electrophile participates in het-erolytic substitution of another molecular entity that supplies both of the bonding electrons. In the case of aromatic electrophilic substitution (AES), one electrophile (typically a proton) is substituted by another electron-deficient species. AES reactions include halogenation (which is often catalyzed by the presence of a Lewis acid salt such as ferric chloride or aluminum chloride), nitration, and so-called Friedel-Crafts acylation and alkylation reactions. On the basis of the extensive literature on AES reactions, one can readily rationalize how this process leads to the synthesis of many substituted aromatic compounds. This is accomplished by considering how the transition states structurally resemble the carbonium ion intermediates in an AES reaction. 

The solubility of a Lewis acidic salt in an ionic liquid is dependent on the nature of both the salt and the ionic liquid. Hydrophobic ionic liquids have lower solubilities for such salts. For example, metal triflates such as Sc(TfO)3 readily dissolve... 

The benzobisazole family of rigid-rod polymers is soluble in acidic solvents such as PPA, methanesulfonic acid, chlorosulfonic acid, 100% sulfuric acid and Lewis acid salts such as antimony trichloride and bismuth trichloride. More recently, PBZT has been reported [22] to form liquid crystalline solutions in nitromethane containing aluminum trichloride or gallium trichloride. Since the glass transition temperature of these materials is above their decomposition temperature, they must be processed from solution. 

As expected, enol silyl ethers do react with XeF2 to form fluoroketones322. A similar but somewhat unusual reaction was used to convert the same starting ketone into a fluoroolefin via the vinyl tin derivative, which reacts with XeF2 in the presence of strong Lewis acid salts (equation 190)323. 

A significant amount of development is currently occurring relative to latent catalysts because of interest in their long shelf life, high reactivity, and single-component adhesive formulations. Present technologies involve absorption of acidic or basic catalysts in molecular sieves, formation of Lewis acid salts or other amine salts, microencapsulation of amines, and other novel segregation methods. 

Lewis acid anion Lewis acid Salt (ppt)... 

The catalysed crosslinking follows the Arrhenius equation, so that as a rule of thumb a temperature change of 10 °C causes a factor of 2 change in the condensation time as shown in Table 5.5. The most common catalysts are Lewis acid salts... 

The catalysts for cationic polymerization can be strong anhydrous acids, Lewis acids, salts of primary and secondary amines, carboxylic acids, and salts of amines with carboxylic acids that split off water at elevated temperatures. The initiators react by coordinating with and fonrung rapid pre-equilibrium lactam cations. These cations are the reactive species in the polymerizations. Initiations of this type are also possible with weakly acidic compound, but such compounds ate not able to transfer protons to the lactam. They are capable, however, of forming hydrogen bonds with the lactams. The high reactivity of the lactam cations may be attributed to the decreased electron density at the carbonyl carbon atoms. This makes them more subject to nucleophilic attacks. Protonations of the amides occur at the oxygens, but small fractions of N-protonated amides are also presumed to exist in tautomeric equilibrium. To simplify the illustrations, all lactams will be shown in this section as... 

Unlike resoles, which show a definite preference for methylolation and condensation at the para position, ring positions in novolacs are less differentiated. The normal ratio of o,p-, and /j,p-linkages in a novolac will be 1 2 1. This may be affected by the choice of catalyst, and much work has been done to control this aspect of novolac synthesis, with the emphasis on producing highly or/Zio-Iinked resins. In some cases, the judicious choice of protic acid may lead to the desired result. More commonly, a Lewis acid salt is chosen as the catalyst. These are usually divalent metal salts of acetates or similar small carboxylates. Zinc acetate is probably the most common example. Often resins made using these salts cannot be cleanly characterized as resole or novolac. They may have a resole molar ratio and a novolac pH or they may be made near neutral conditions. As mentioned before, commercial phenolic polymers showing 85% ortho linkage are available. Solvent choices may also be important to determination of substitution patterns. 

It should be noted that addition of Lewis acidic salts, such as MgBt2, is critical in order to achieve an effective catalytic transformation when using arylzinc compounds. This observation indicates that the difficult step of the catalytic cycle is the transmetaUation of the aryl group from the zinc reagent to the catalytically active iron complex [42]. While the involvement of an intermediate radical species or a single electron-transfer process is suspected, mechanistic details of these iron-catalyzed cross-coupling reactions remain unclear. 

Similar to aluminum, the synthetic application of metallic gallium or its salts in the aldol reaction produced gallium enolate and its Lewis acidic salt (161). The first example of the gallium enolate aldol reaction was reported by Han and Huang in 1998 (162). The gallium enolates could be formed easily from a-bromoketones and unmodified ketones with gallium triiodide and triethylgal-lium, respectively (163). 

Another efficient method to prepare chiral propargylamines 42 using a multicomponent process is by alkylation of in situ formed propargyl imines from alkynals 40 and o-phenoxy aniline (11c) by dialkylzinc derivatives 41 in the presence of a chiral ligand, for instance a dipeptide, and a Lewis acid salt, as depicted in Scheme 11.16 [48], Furthermore, the synthesis of A-aryl propargyl amines can be also performed by the alkynylation using dimethylzinc and terminal acetylenes of several aldehydes and o-methoxyaniline catalyzed by (l/ ,25)-A-bis(p-methoxybenzyl)norephedrine and phenylacetylene (52-93%, 79-97% ee) [49],... 

Epoxy B in Table 3 is cured with a Lewis acid salt, the monoethylamine salt of boron trifluoride (BF3MEA). The first gold- and silver-filled one-component epoxy die attach adhesives, which were sold in the U.S. in the early 1970s, contained this curing agent. When heated to 150 C, BF3MEA liberates BF3 gas, which serves as an acid catalyst to cure the epoxy. 

Jain SK (1978) Refractive index of molten Lewis acid salt hydrates mixtures of chromium(III) nitrate nonahydrate -i- calcium nitrate tetrahydrate. J Chem Eng Data 23 216-218... 

The basic mechanism for the polymeric or cyclic carbonate formation involves initiation of the reaction by the opening of the epoxide to give an alkoxo linkage. This is shown in Figure 4.14 by the conversion of 4.58 to 4.59 where an attack by the nucleophile, e.g., a halide, opens the epoxide ring. The source of the nucleophile could be the metal complex itself or a cocatalyst such as a Lewis acid salt A X". 

---