Lewis acids binary systems

With a few exceptions [26, 27], living cationic polymerization is initiated by the initiator/coinitiator (Lewis acid) binary system. Selection of an initiating system for a given monomer is of crucial importance, as there are no universal initiators such as organoHthiums in anionic polymerization. For example, while weak Lewis acids such as zinc halides may be necessary to effect living polymerization of the more reactive vinyl ethers, they are not effective for the living polymerization of the less reactive monomers, such as IB and St Detailed inventories of initiating systems for various monomers are well described in recent publications [25, 28, 29). 

The nickel/Lewis acid binary catalyst system can be used with carbonyl cyanides. It effectively catalyzes intermolecular addition reactions that are difficult to execute using a palladium catalyst (Eq. (6.22)) [61]. 

In principle the investigation of ternary systems in the solid state thus presents the opportunity of measuring the I.R. spectra of proton addition complexes. In the case of strongly basic aromatic substances it is furthermore also possible to demonstrate the existence of a-complexes in the binary system aromatic substance-Lewis acid without the participation of protons (Perkampus et al., 1964c). 

Perchloryl fluoride shows no tendency to form adducts with either strong Lewis acids or bases. This behavior has been rationalized in Section II, D. The binary systems of FCIO3 with BFa, PF5, AsFs, SbFj, or SO3 were studied by Lang 167), at Pennsalt 224), and by Nikitina and Rosolovskii 209). Similarly, at Pennsalt 224) no evidence was found for complexing of FCIO3 with either CsF or FNO2. 

Most coordination catalysts have been reported to be formed in binary or ternary component systems consisting of an alkylmetal compound and a protic compound. Catalysts formed in such systems contain associated multinuclear species with a metal (Mt)-heteroatom (X) active bond ( >Mt X Mt—X > or — Mt—X—Mt—X— Mt = Al, Zn, Cd and X = 0, S, N most frequently) or non-associated mononuclear species with an Mt X active bond (Mt = Al, Zn and X = C1, O, S most frequently). Metal alkyls, such as triethylaluminium, diethylzinc and diethylcadmium, without pretreatment with protic compounds, have also been reported as coordination polymerisation catalysts. In such a case, the metal heteroatom bond active in the propagation step is formed by the reaction of the metal-carbon bond with the coordinating monomer. Some coordination catalysts, such as those with metal alkoxide or phenoxide moieties, can be prepared in other ways, without using metal alkyls. There are also catalysts consisting of a metal alkoxide or related compound and a Lewis acid [1]. 

Chlorostannate and chloroferrate [110] systems have been characterized but these metals are of little use for electrodeposition and hence no concerted studies have been made of their electrochemical properties. The electrochemical windows of the Lewis acidic mixtures of FeCh and SnCh have been characterized with ChCl (both in a 2 1 molar ratio) and it was found that the potential windows were similar to those predicted from the standard aqueous reduction potentials [110]. The ferric chloride system was studied by Katayama et al. for battery application [111], The redox reaction between divalent and trivalent iron species in binary and ternary molten salt systems consisting of 1-ethyl-3-methylimidazolium chloride ([EMIMJC1) with iron chlorides, FeCb and FeCl j, was investigated as possible half-cell reactions for novel rechargeable redox batteries. A reversible one-electron redox reaction was observed on a platinum electrode at 130 °C. 

This is no reliable evidence of any solute acting as a Bronsted acid in HSO3F. So enhancement of acidity has always been achieved by addition of Lewis acids. The first potential acids investigated in HSO3F were binary fluorides. As discussed immediately below in Sec. 11.3.2.1, addition of S03 to fluorides dissolved in HSO3F was subsequently found to enhance acidity due to insertion of S03 into the metal-fluorine bond. More recently, Lewis acids of the HS03F system, i.e. binary fluorosulfates, have been studied. 

In addition to protonic acids, Lewis acids are the most common initiators of carbocationic polymerizations. Two mechanisms are possible. Direct initiation is rare and usually slow. The more prevalent mechanism is by cocatalysis in binary systems, with the Lewis acid acting as a coinitiator or catalyst rather than as initiator. Cationating or protonating species are the true initiators, which are therefore the species incorporated at the polymer s end group. The most common initiator is adventitious water in insufficiently dried systems. Thus, mechanistic studies should be performed under stringently dry conditions or in the presence of proton traps such as hindered pyridines. In addition to water, the protonating reagent may be an alcohol, carboxylic acid, amine, or amide [Eq. (28)]. 

Other Lewis acids have been proposed to initiate polymerization after self-ionization. However, most Lewis acids do not initiate polymerization under stringently dry conditions, which strongly indicates that the initiating systems are binary. 

The electrophilic reactivity of sulfenic esters strongly increases after addition of suitable Lewis acids and such binary systems effectively promote not only simple addition to a C—C double bond, but also induce cationic arene-alkene cyclization in a wide range of substrates29,30. 

The cocatalyst made by the combination of an Al-alkyl and a Lewis base is actually not a simple binary system since the two components interact chemically giving rise to new products. In the case of carboxylic esters, it has been known for a long time 56) that the reaction with Al-alkyl yields Al-alkoxides from which alcohols are obtained by hydrolysis. Based on the numerous studies conducted in the last twenty years 57 72), it is known that the reaction occurs through the preliminary formation of an acid-base complex which, thereafter, undergoes rearrangement which is more or less rapid, depending on the reaction conditions. 

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