Radical 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... 

Allyl)Fp complexes are also subject to attack, at C-3, by radicals. The mechanism of allylic transposition of ()] -allyl)Fp complexes, as well as the mechanism of phosphite substitution for CO, has been ascribed to attack by Cp(CO)(L)Fe- on the original Fp-aUyl. The reaction of (12) with CCI4 proceeds by a radical chain mechanism, ultimately between CCI3 and the Fp-allyl. The substitution of a-halo ketones and esters most likely proceeds similarly. A radical cation coupling mechanism has been proposed for the dimerization of (jj -allyl)Fp and (j7 -propargyl)Fp complexes. ... 

Lucas and coworkers extended that investigation to the possibility to get such polymers by an electrochemical method. They used magnesium porphin as monomer, allowing the formation of long chains of porphin subunits connected through direct meso-meso bonds (Fig. 24) [172]. Continuous electrolysis electropolymerization was possible by applying a potential which allowed the generation of the porphin radical cation. The mechanism is then the same as that... 

The 9,10-dicyanoanthracene (DCA)-sensitized c/s,tram-photoisomerization of highly electron-rich 1,2-diarylcyclopropanes and diarylmethylenecyclopropanes efficiently proceeds via a radical cation chain transfer mechanism.The photoisomerization of electron-rich l-diarylvinylidene-c/s-2,3-dimethylcy-clopropane (c-2d) in acetonitrile was also sensitized by DCA via photoinduced electron transfer. The photoisomerization efficiently takes place to give a 3 7 PSS mixture of c-2d and t-2d. However, the less electron-rich 1-diarylvinylidenecydopropanes c-2a-c under the same reaction conditions slowly isomerized... 

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. 

Two pathways for the reaction of sulfate radical anion with monomers have been described (Scheme 3.81).252 These are (A) direct addition to the double bond or (B) electron transfer to generate a radical cation. The radical cation may also be formed by an addition-elimination sequence. It has been postulated that the radical cation can propagate by either cationic or a radical mechanism (both mechanisms may occur simultaneously). However, in aqueous media the cation is likely to hydrate rapidly to give a hydroxyelhyl chain end. 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

The localization of the HOMO is also important for another reason. Since it describes the distribution of a hole in a radical cation, it relates to the hindrance that a positive charge will encounter as it propagates along the chain. There is indeed experimental evidence (9) that the hole states of the polysilane chain are localized and that they move by a hopping mechanism. 

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. 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

In the recent study on the anodic oxidation of enaminones which possess an unsaturated chain susceptible to react intramolecularly with an electrogenerated radical cation, the evidence for an intramolecular reaction was provided on the basis of the dEp/dlogv slope of 30 mV and one-electron behavior of the voltametric wave [48], The reaction could involve similar mechanistic pathways as shown in Scheme 4 (e-c-dimerization and following chemical reactions). However, the authors were not able to isolate the products after preparative oxidation in order to confirm the possible mechanism. 

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