Atom-transfer-chain mechanism

Cyclizable radical-probe experiments have been extensively used in ET versus Spj2 investigations (see Ashby, 1988, and references cited therein). Attention has, however, been recently drawn to causes of possible misinterpretation, particularly in the case of iodides, where an iodine-atom-transfer chain mechanism is able to convert most of the starting linear iodide into the cyclized iodide, even if only a minute amount of linear-chain radical is present in 7-8 2 reactions (Newcomb and Curran, 1988). Rather puzzling results were found in the reaction of (CH3)3Sn ions with secondary bromides, which should not be involved in atom-exchange chain reactions... 

An atom transfer chain mechanism very similar to that in Scheme 6 is thought to occur in some photoinitiated CO substitution reactions of organometallic halide complexes. For example, [Cp2Mo2(CO)6] promotes the photosubstitution of CO by PR3 in [CpMo(CO)3X] (X = Br, I),"9 and... 

In 1989 Curran and co-workers reported on a photocatalytically induced free-radical cyclization leading to various cyclic, bi-, or polycyclic carbocycles (fused and spiro) via isomerization of unsaturated iodides (alkenes, alkynes) [63]. This corresponds to the nonreductive variant of the tin hydride method. Under sunlight irradiation and in the presence of 10 mol% hexabutylditin, a-iodo esters, ketones, and malonates are efficiently transformed via an iodide atom transfer chain mechanism (eq. (4)). 

Brown has shown that metal-carbonyl-hydride complexes such as [Re(H)(CO)s] can nndergo CO substitntion by a phosphine (PBus) according to the H-atom-transfer-chain mechanism A classic type of initiation to introduce the radical species into the chain is to photolyze the metal-carbonyl dimer, which generates the reactive 17-electron metal-carbonyl monomer ... 

Electron-transfer-chain and atom-transfer-chain mechanisms.127... 

Likewise, concerning the reactions of the type [MFl] -1- RX — [MX] -1- RH, it is often difficult to distinguish between the mechanisms (a) hydride transfer, (b) electron transfer followed by H atom transfer (non chain) and (c) atom-transfer chain (ATC, see Chap. 4). 

The reversible activation reactions in the most successful LRPs currently known may mechanistically be classified into three types (Schemes 7.lb-7. Id), which are the dissociation-combination (DC), degenerative chain transfer (DT), and atom transfer (AT) mechanisms. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

It has been found from MD simulations that friction of SAMs on diamond decreases with the increasing chain length of hydrocarbon molecules, but it remains relatively constant when the number of carbon atoms in the molecule chain exceeds a certain threshold [44], which confirmed the experimental observations. In simulations of sliding friction of L-B films, Glosli and McClelland [45] identified two different mechanisms of energy dissipation, namely, the viscous mechanism, similar to that in viscous liquid under shear, and the plucking mechanism related to the system instability that transfers the mechanical energy into heat, similar to that proposed in the Tomlinson model (see Chapter 9). On the basis of a series work of simulations performed in the similar... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

The functionalization of H—Si(l 11) surfaces has been extended to the reaction with aldehydes. Indeed, H—Si(lll) reacts thermally (16 h at 85 °C) with decanal to form the corresponding Si—OCH2R monolayer that has been characterized by ATR-FTIR, XPS and atomic force microscopy (AFM) [63]. The reaction is thought to proceed either by a radical chain mechanism via adventitious radical initiation or by nucleophilic addition/hydride transfer. On the other hand, the reaction of H—Si(lll) with octadecanal activated by irradiation with a 150W mercury vapour lamp (21 h at 20-50 °C) afforded a R... 

This study indicates that the oxidation of dihydroanthracene in a basic medium involves the formation of a monocarbanion, which is then converted to a free radical by a one-electron transfer step. It is postulated that the free radical reacts with oxygen to form a peroxy free radical, which then attacks a hydrogen atom at the 10-position by an intramolecular reaction. The reaction then proceeds by a free-radical chain mechanism. This mechanism has been used as a basis for optimizing the yield of anthraquinone and minimizing the formation of anthracene. 

Synergistic behavior by two antioxidants is not confined to compounds which inhibit by entirely different mechanisms—for example, two chain-breaking phenolic antioxidants may synergize one another. This homosynergism is caused by the suppression of the unfavorable chain propagation reactions of one phenoxy radical by a hydrogen atom transfer from the second phenol. 

A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. 

Some radical reactions occur under the control of transition metal templates. The first example of asymmetric creation of an asymmetric carbon with a halogen atom is shown by the a DIOP-Rh(I) complex-catalyzed addition of bromotrichloromethane to styrene, which occurs with 32% enantioselectivity (Scheme 99) (233). Ru(II) complexes with DIOP or BINAP ligands promote addition of arenesulfonyl chlorides to afford the products in 25-40% ee (234). A reaction mechanism involving radical redox transfer chain process has been proposed. 

At the present time there is no experimental or theoretical evidence which would support a photon chain mechanism. However, if reaction (30) does occur, the spectral and photochemical evidence is that the next reaction should be the transfer to 3nu or some other repulsive state, as discussed in Part II. Two O(3P) atoms would then be formed by predissociation. The reaction (28) would then decompose two additional ozone molecules. A quantum yield of four could then be achieved without invoking either a photon or energy chain. In view of the uncertainty of the quantum yields greater than two and the maximum reported yield of six, this mechanism is perhaps the simplest necessary to explain the reasonably acceptable experimental evidence. 

In ionic polymerization a hydride (H-) transfer or a proton transfer are the analogues of the hydrogen atom transfer in radical polymerization. A hydride (H-) ion transfer is observed in many isomerizations and dimerizations of hydrocarbons which proceed via carbonium-ion mechanism. A similar process is responsible for chain transfer ip some carbonium-ion polymerizations. The transfer of negative ions like Cl- is also common, e.g. triphenyl methyl chloride is an efficient transfer agent in such a polymerization. Transfer of a proton is, on the other hand, a very common mode of termination of anionic polymerization. Indeed, this mode of termination was discussed previously in connection with branching reactions, and it was postulated in the earliest studies of anionic poly-... 

The FVP of angular [3]phenylene and bis(2-ethynylphenyl)ethyne has been found to produce benz[g/z]fluoranthene and chrysene,54 respectively. A non-chain stepwise radical mechanism which is initiated by hydrogen-atom transfer from the donor to azulene has been proposed55 for the mechanism of the uncatalysed transfer-... 

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