Cation chain mechanism

The 9,10-dicyanoanthracene sensitized irradiation of c/i-stilbene results in nearly quantitative isomerization (>98%) to the trans isomer with quantum yields greater than unity. Therefore, the isomerization was formulated as a free radical cation chain mechanism with two key features (1) rearrangement of the c/i-stilbene radical cation and (2) electron transfer from the unreacted cis-olefin to the rearranged (trans-) radical cation. 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

This type of reaction can be induced also by radiolysis [133,134] or by chemical oxidation, particularly with tris-(p-bromophenyl)aminium salts (cation radical catalyzed Diels-Alder reaction) [10]. The scope of this reaction and its synthetic utility have been delineated in detail. The results unambiguously support a free radical cation chain mechanism [10]. 

A similar radical cation chain mechanism was suggested for the formation of 3,3,6,6-tetraphenyl-l,2-dioxane (16) upon chemical or photoinduced one-electron oxidation of 1,1-diphenylethylene [155, 156]. 

The general features of the isomerization are compatible with a free radical cation chain mechanism, featuring electron transfer from unreacted olefin to rearranged radical cation. This chain mechanism was firmly established in several other isomerizations by the observation of quantum yields greater than unity. Thus, the dicyanoanthracene sensitized irradiation of m-stilbene results in nearly quantitative isomerization (> 98%) to the trans-isomer. In this system, the quantum yield increases with increased ds-stilbene concentration, solvent polarity, salt concentration, as well as decreasing light intensity [159]. 

On the basis of these observations, a radical cation chain mechanism was proposed for this 2 + 2 cycloaddition (Scheme 5). 

As described above, upon photoexcitation, stilbene undergoes a two-way isomerization. However, upon 9,10-dicyanoanthracene (DCA) sensitization in acetonitrile, cis-stilbenes undergo essentially one-way isomerization via a radical cation chain mechanism. Either a unimolecular cis - - trans - or a bimolecular process involving addition of cis - to the neutral stilbene may lead to trans isomers [139-156]. 

Metzger and coworkers have studied reactive intermediates of chemical reactions in solution by using a microreactor coupled to an ESI mass spectrometer. The highly stereo- and regioselective dimerization of trows-anethole to give the head-to-head trans, anti, trans-cydobutane initiated by aminium salt proceeds by a radical cation chain mechanism (Scheme 4.4) and this method was further used to study the transient radical cations intermediates in electron transfer-initiated D-A reactions [12-14]. 

Maruyama T, Mizuno Y, Shimizu I, Suga S, Yoshida J-I (2007) Reactions of a N-acyliminium ion pool with benzylsilanes. Implication of a radical/cation/ radical cation chain mechanism involving oxidative C-Si bond cleavage. J Am Chem Soc 129 1902-1903... 

The distannane mediated organic radical addition to A -acyliminium ions [13], and the benzylic radical addition to A/ -acyliminium ions which proceeds via radical/cation/radical cation chain mechanism [14] show that the cation pool can be utilized as good nucleophilic radical acceptors because of their strong electrophilic character. Iterative molecular assembly based on the cation pool method lead to the efficient formation of dendritic molecules [15]. The manipulation of the cation pool in the microflow system realized an efficient... 

Electrocatalysis of ligand substitution proceeding by a cationic chain mechanism has been documented/ The application of a slight anodic current through a MeCN solution of [(Tj -C5H4Me)Mn(CO)2L] (L = MeCN or py) and nucleophile L (PPhs or t-BuNC) induces rapid ligand substitution according to equation (9)/ No reaction occurs in the absence... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

Epichlorohydrin Elastomers without AGE. Polymerization on a commercial scale is done as either a solution or slurry process at 40—130°C in an aromatic, ahphatic, or ether solvent. Typical solvents are toluene, benzene, heptane, and diethyl ether. Trialkylaluniinum-water and triaLkylaluminum—water—acetylacetone catalysts are employed. A cationic, coordination mechanism is proposed for chain propagation. The product is isolated by steam coagulation. Polymerization is done as a continuous process in which the solvent, catalyst, and monomer are fed to a back-mixed reactor. Pinal product composition of ECH—EO is determined by careful control of the unreacted, or background, monomer in the reactor. In the manufacture of copolymers, the relative reactivity ratios must be considered. The reactivity ratio of EO to ECH has been estimated to be approximately 7 (35—37). 

Kinetically indistinguishable chain mechanisms can be characterized by different ionic strength profiles, as was apparently first demonstrated in a study this author conducted with D. A. Ryan on the reaction of (aqua)-2-propylchromium cation with oxygen.17 This reaction was presented in Chapter 7. Two schemes that are consistent with the rate law are as follows ... 

The heats of formation are less suited to characterizing the stability and/or reactivity of carbocations as models of cationic chain ends in cationic polymerizations71). Model reactions closely connected to the cationic polymerization mechanism are better suited to this characterization, for example ... 

Figure 25 Overall cationic chain polymerisation mechanism of isobutylene.

The most industrially significant polymerizations involving the cationic chain growth mechanism are the various polymerizations and copolymerizations of isobutylene. In fact, about 500 million pounds of butyl rubber, a copolymer of isobutylene with small amounts of isoprene, are produced annually in the United States via cationic polymerization [126]. The necessity of using toxic chlorinated hydrocarbon solvents such as dichloromethane or methyl chloride as well as the need to conduct these polymerizations at very low temperatures constitute two major drawbacks to the current industrial method for polymerizing isobutylene which may be solved through the use of C02 as the continuous phase. 

Radical addition to an Af-acyliminium ion is also an interesting feature of the cation pool chemistry. We found that an alkyl iodide reacted with an N-acyliminium ion pool in the presence of hexabutyldistannane to give coupling product 19.24 A chain mechanism shown in Scheme 10, which involves the addition of the alkyl radical to the N-acyliminium ion to form the corresponding radical cation, seems to be reasonable. The present reaction opens a new possibility for radical-cation crossover mediated carbon-carbon bond formation. 

Fractional orders such as 3/2 often hint at a chain mechanism. The autoxidation of (H20)5CrCH(CH3) + leads to a number of products. Log (inititd rate) vs log (initial concentration of organochromium cation) plots give a 3/2 slope. The rate is independent of [H+] and [O2] and the rate law is therefore... 

Hence, cation-radical copolymerization leads to the formation of a polymer having a lower molecular weight and polydispersity index than the polymer got by cation-radical polymerization— homocyclobutanation. Nevertheless, copolymerization occnrs nnder very mild conditions and is regio-and stereospecihc (Bauld et al. 1998a). This reaction appears to occnr by a step-growth mechanism, rather than the more efficient cation-radical chain mechanism proposed for poly(cyclobutanation). As the authors concluded, the apparent suppression of the chain mechanism is viewed as an inherent problem with the copolymerization format of cation-radical Diels-Alder polymerization. ... 

The sterically hindered base 2,6-bis(tert-butyl)pyridine does not inhibit cyclization triaryl-amine retards this reaction photosensibilized one-electron oxidation of a diene leads to the same products, which are formed in the presence of ammoniumyl salt. As shown, in majority of cases, only the cation-radical chain mechanism of the diene-diene cyclization is feasible (Bauld et al. 1987). Meanwhile, cyclodimerizations of 2,4-dimethylpenta-l,3-diene (Gassman and Singleton 1984) and l,4-dimethylcyclohexa-l,3- or -1,4-diene (Davies et al. 1985) proceed through both mechanisms. 

As pointed out in Section 7.4.1, the head-to-head cyclodimerization is typical for phenylvinyl ether cation-radical (see Scheme 7.19). The anion-radical of phenylvinyl sulfone undergoes the same dimerization. The reaction is initiated electrochemically, develops according to the chain mechanism, and also leads to the trans-cyclic product (Bergamini et al. 2004). 

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