Radical cations reaction mechanisms

The mechanistic aspects of aromatic121 and alkene122 radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy.123 A review of the reactivity of divalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics.124 The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms.122125... 

Conversion increases from toluene to p-xylene to p-methoxytoluene due to the electron-donating effects of the substituents. The increased number of electron-donating substituents creates a positive charge on the reaction center and activates the ring for increased oxidation rates. This effect is in agreement with the radical cation oxidation mechanism proposed by Som-rani et al. (1995). Electron density on the reaction center was shown to play an important role in determining oxidation rates of toluenes. 

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]. 

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. ... 

Many radical cations derived from cyclopropane (or cyclobutane) systems undergo bond formation with nucleophiles, typically neutralizing the positive charge and generating addition products via free-radical intermediates [140, 147). In one sense, these reactions are akin to the well known nucleophilic capture of carbocations, which is the second step of nucleophilic substitution via an Sn 1 mechanism. The capture of cyclopropane radical cations has the special feature that an sp -hybridized carbon center serves as an (intramolecular) leaving group, which changes the reaction, in essence, to a second-order substitution. Whereas the SnI reaction involves two electrons and an empty p-orbital and the Sn2 reaction occurs with redistribution of four electrons, the related radical cation reaction involves three electrons. 

For example, the quantum yield for a CdS-, TiOj-, or ZnO-mediated valence isomerization of hexamethyl-dewar benzene to hexamethylbenzene is greater than xmity [106]. A cation radical chain reaction mechanism accounts for this observation (Fig. 2). 

Pol5unerization is initiated by monomer oxidation to yield the radical cation. The mechanism (55) is believed then to involve either radical-cation/radical-cation coupling or reaction of a radical-cation with a neutral monomer. The mechanism of electropolymerization of five-membered heterocycles with radical-cation/radical-cation conpling is shown in Fig. 6. 

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... 

Rate constants for the [Fe(phen)3] reactions show Marcus behavior indicative of an outer-sphere process in which reorganization of the metal alkyl is the same regardless of the steric bulk of the R groups. In contrast a marked rate dependence on steric bulk is found for [IrCl6] reactions and the presence of an alternative inner-sphere mechanism is confirmed by the fate of the radical cation [reaction (11)]. In general the rates of these reactions are considerably in excess of Marcus values and a chloride-bridged intermediate (1) is proposed. Detection of such... 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

This is an exothermic reaction, and both homogeneous (radical or cationic) and heterogeneous (soHd catalyst) initiators are used. The products range in molecular weight from below 1000 to a few million (see Olefin polymers). Reaction mechanisms and reactor designs have been extensively discussed (10-12). 

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