Cationic radical initiators

An analogous stepwise mechanism was also proposed by Wohrle [36] for the cation-radical-initiated cycloaddition of electron-rich allenes with pentamethyl-cyclopentadiene in the presence of tris (p-tolyl) aminium hexafluoroantimonate (TTA SbF6 ) (Equation 1.15). 

Studies on carotenoid autoxidation have been performed with metals. Gao and Kispert proposed a mechanism by which P-carotene is transformed into 5,8-per-oxide-P Carotene, identified by LC-MS and H NMR, when it is in presence of ferric iron (0.2 eq) and air in methylene chloride. The P-carotene disappeared after 10 min of reaction and the mechanism implies oxidation of the carotenoid with ferric iron to produce the carotenoid radical cation and ferrous iron followed by the reaction of molecular oxygen on the carotenoid radical cation. Radical-initiated autoxidations of carotenoids have also been studied using either radical generators like or NBS.35... 

Nucleophiles, when they are used in the presence of cupric perchlorate, capture the cation-radicals initially formed. Instead of benzidines, the para-substituted dialkylanilines were obtained. In this a manner, A,A-dialkylanilines with halo or thiocyanato moieties in para positions were prepared in good yields under the same (simple) conditions. Scheme 7.13 illustrates the sequence of the transformations observed. The products are useful intermediates in the synthesis of dyes, drugs, and color cinema formulations. 

It is true that conventional diene condensations (without the cation-radical initiation) also proceed with the predominant formation of endo adducts. In the cation-radical version, endoselectivity increases sharply. This is quite understandable. The cation-radical acts as an independent particle, and the role of the suitable MO of the cation-radical turns out to be a determinant. 

The simplest example is represented by pairs in which one-electron oxidation of a dienophile proceeds easier than that of a diene (Jia et al. 2003, Zhon et al. 2005). If cation-radicals of both the diene and dienophile can be formed on the action of a cation-radical initiator, some kind of separation operates. Each of these cation-radicals can exchange an electron with any participant in the reaction. However, since only the diene cation-radical is consnmed, the equilibrinm of the electron transfer is gradually shifted toward this particnlar cation-radical. The diene depicted in Scheme 7.22 enters the reaction in its i-cis form. If the diene cation-radical is in i -trans form, a cylobutane product forms (Remolds et al. 1989, Botzem et al. 1998). 

Although the 1,6 cyclization takes place in the thermal process, the cation-radical initiation leads to the 1,5 cyclization (Ramkumar et al. 1996). Chemical oxidation of o-diethynylbenzene bearing two terminal phenyl groups by tris(p-bromophenyl)ammoniumyl hexachloroantimonate as the catalytic oxidizing agent in the presence of oxygen yields 3-benzoyl-2-phenylindenone in 70% yield (Scheme 7.26). 

The intercalated cationic radical initiator was synthesized from 2,2 -azobis[2-methyl-N-(2-hydroxyethyl) propionamide] by esterification with bromoacetyl bromide, and subsequent quaternization with tributylamine. Sodium montmorillonite was suspended in deionized water. The cationic radical initiator was bound onto the montmorillonite by ion exchange. 

P. Uthirakumar, Y.B. Hahn, K.S. Nahm, and Y.-S. Lee, Exfoliated high-impact polystyrene/mmt nanocomposites prepared using anchored cationic radical initiator-mmt hybrid, Eur. Polym. /., 41(7) 1582-1588, July 2005. 

If cation radicals of both the diene and the dienophile can be formed upon the action of the cation radical initiator, some kind of separation operates. Each of these cation radicals can exchange an electron with any participant in the reaction. However, since only the diene cation radical is consumed, the equilibrium of the electron transfer is gradually shifted toward this particular cation radical. 

The enhanced reaction rates and regioselectivities (head-to-head) in the alkene dimerisation (cyclobutane formation e.g. 18-19) via radical cation catalysed reactions have led numerous studies in this direction [5,10, 36-39]. The head-to-head stereochemistry of these dimerisations have been explained in terms of the addition of the radical cation to a neutral molecule giving the stabilised 1,4-radical cation. An interesting application of the cation-radical initiated (2 + 2)-cyclodimerisation strategy was reported by Mizuno et al. [40] for the synthesis of macrocyclic 2-m-dioxabicyclo (m-1,2,0) ring systems (21) from the PET reactions of 20 (Scheme 6). 

Bulk polymerization initiated via more novel methods have also been used to form bulk PS-MMT nanocomposites. Zhang et al. [19] used gamma irradiation to initiate the polymerization of PS-MMT nanocomposites with different surface modifications (3, 34) and successfully prepared exfohated morphologies when reactive clay modifications were used. Uthirakumar et al. [51-54] modified MMT with a cationic radical initiator which was used to initiate the bulk polymerization of styrene. Because the polymerization was initiated from the clay surface and the monomer and the clay were suitably compatible, exfoliated morphologies were formed. 

Based on this same general idea, colloidal dispersions of nanocomposite particles made from silica cores and polymeric overlayers have been successfully prepared using appropriate cationic radical initiators, as described in a recent Japanese patent [100]. Recently, Luna-Xavier et al. also demonstrated the successful formation of nanosize siHca/PMMA composite colloids using AIB A as cationic initiator and a nonionic polyoxyethylenic surfactant (NP30) [63,91,101]. Composite particles made from silica beads surrounded by small heterocoagulated PMMA latexes or a thin polymer layer were produced, depending on the size of the silica beads (Fig. 4.11). 

The polymerization of benzene with aluminum chloride-cupric chloride has been suggested to proceed by oxidative cationic polymerization [57]. However, a cation radical initiation followed by a cationic propagation mechanism has generally been accepted [62]. A unique mechanism of propagation by coordination of benzene cation radical with benzene nuclei has been presented [63],... 

Figure 4. Wavepackec dynamics of photoexcitadon, shown as snapshots of the density (wavepacket amplitude squared) at various times. The model is a 2D model based on a single, uncoupled, state of the butatriene redical cation. The initial structure represents the neutral ground-state vibronic wave function vertically excited onto the A state of the radical cation.

Some of the target molecules gain so much excess internal energy in a short space of time that they lose an electron and become ions. These are the molecular cation-radicals found in mass spectrometry by the direct absorption of radiation. However, these initial ions may react with accompanying neutral molecules, as in chemical ionization, to produce protonated molecules. 

VEs such as MVE polymerize slowly in the presence of free-radical initiators to form low mol wt products of no commercial importance (9). Examples of anionic polymerization are unknown, whereas cationic initiation promotes rapid polymerization to high mol wt polymers in excellent yield and has been extensively studied (10). 

Vinyl ethers can also be formulated with acryHc and unsaturated polyesters containing maleate or fumarate functionaHty. Because of their abiHty to form alternating copolymers by a free-radical polymeri2ation mechanism, such formulations can be cured using free-radical photoinitiators. With acryHc monomers and oligomers, a hybrid approach has been taken using both simultaneous cationic and free-radical initiation. A summary of these approaches can be found in Table 9. 

Delaire et al. [124] have reported that laser photolysis of an acidic solution (pH 2.8) containing PMAvDPA (23) and MV2 + allows the formation of surprisingly long-lived MV + - and DPA cation radicals with a very high charge escape quantum yield. The content of the DPA chromophores in PMAvDPA is as low as less than 1/1000 in the molar ratio DPA/MAA. Figure 20 shows a decay profile of the transient absorption due to MV + monitored at 610 nm [124]. The absorption persists for several milliseconds. As depicted in Fig. 20, the decay obeys second-order kinetics, which yields kb = 3.5 x 10s M 1 s. From the initial optical density measured at 610 nm, the quantum yield for charge escape was estimated to be 0.72 at 0.2 M MV2 +. ... 

If the full molecular symmetry is assumed, the ground states of the cation radical of fulvalene and the anion radical of heptafulvalene are both predicted to be of symmetry by using the semiempirical open-shell SCF MO method The lowest excited states of both radicals are of symmetry and are predicted to be very close to the ground states in the framework of the Hiickel approximation these states are degenerate in both cases (Fig. 4). Therefore, it is expected that in both these radicals the ground state interacts strongly with the lowest excited state through the nuclear deformation of symmetry ( — with the result that the initially assumed molecular... 

Purified ligninase H8 produced by P. chrysosporium in stationary cultures oxidized pyrene to pyrene-1,6- and pyrene-l,8-quinones in high yield, and experiments with showed that both quinone oxygen atoms originated in water (Figure 8.25). It was suggested that initial one-electron abstraction produced cation radicals at the 1 and 6 or 8-positions (Hammel et al. 1986), whereas in... 

Copolymers of a cationic monomer and a vinyl alkoxysilane may be prepared by conventional vinyl polymerization techniques. These techniques include solution polymerization in water and emulsion polymerization with either free radical initiators or redox initiators. 

The difference in stabilities of cation radicals located on G, GG, and GGG sequences was initially investigated by Sugiyama and Saito [14], who employed ab initio methods to calculate the gas phase ionization potentials of nucleobases stacked in B-DNA geometries. Their results indicated large differences in potential for holes on G vs GG (0.47 eV) and GGG (0.68 eV) sequences. A similar G vs GG difference was calculated by Prat et al. [62]. These values suggest that GG and GGG are, in fact, deep hole traps and they have been widely cited as evidence to that effect [54, 63]. 

Mechanistically quite different from the process described in (19) is the process of methyl elimination from the cation radicals of 93 and its metal para isomers 9b40 . In the case of 93 methyl loss is initiated by a hydrogen transfer from the benzylic site to the ester function, 93- 94, thus forming a reactive intermediate 94 from... 

The majority of commercial polystyrene molecules consist of a backbone of carbon atoms with phenyl groups attached to half of the carbon atoms, as shown in Fig. 21.1. Free radical initiator residues terminate each end of the chain. Minor variants include chains terminated by anionic or cationic initiator residues. All commercial polystyrene products are atactic that is, the placement of the phenyl groups on either side of the chain is essentially random, as illustrated in Fig. 21.2. 

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