Reaction mechanisms hydrogen chain transfer steps

The mechanism suggested for the chain-transfer step is disdiKtly different from that of other /S-hydrogen abstraction reactions from transition-metal alkyls. Several such reactions have been reported (f5) they evidently proceed without the assistance of coordinated monomer. 

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. 

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. 

Nickel,40 41 like almost all metal catalysts (e.g., Ti and Zr) used for alkene dimerization, effects the reaction by a three-step mechanism.12 Initiation yields an organometallic intermediate via insertion of the alkene into the metal-hydrogen bond followed by propagation via insertion into the metal-carbon bond [Eq. (13.8)]. Intermediate 11 either reacts further by repeated insertion [Eq. (13.9)] or undergoes chain transfer to yield the product and regenerate the metal hydride catalyst through p-hydrogen transfer [Eq. (13.10)] ... 

The observations that these reactions are inhibited by nitrobenzene (a free radical inhibitor), no hydrogenated by-products are formed and that CF BrCl gives only a-CFjCl carbonyl compounds, led the authors to propose a radical chain mechanism for these reactions (Scheme 1). The chain initiation step is the formation of XFiC radical and enamine radical cation by electron transfer from the enamine to BrCFjX. The addition of this perhaloalkyl radical to the enamine generates a RjNC R R" type radical which is known to have an unusually low oxidation potential with 1/2 in the range of — 1 V (sce). An electron transfer from this radical to another molecule of perhaloalkane then takes place to form the iminium salt and another perhaloalkyl radical which continues the chain. A similar mechanism operates in the case of Rp. ... 

The CCT technique is based upon the fact that certain Co(II) complexes such as cobaltoximes catalyze the chain transfer to monomer reaction. The mechanism is believed to consist of two consecutive steps [66] (Scheme 10). First, a growing polymeric radical Rn undergoes a hydrogen transfer reac-... 

The mechanism of thermal degradation of plastics proceeds through a radical chain reaction pathway with hydrogen transfer steps. In secondary reactions, branched products were only formed as a result of the interaction between two radicals without any rearrangement reactions [48]. As a consequence, thermal cracking of polyolefins leads toward a broad distribution of hydrocarbons up to waxy products. More than 500 °C temperatures are needed to receive more oily products. In contrast, catalytic cracking takes place at lower temperatures and leads to the formation of smaller branched hydrocarbons. This catalytic cracking can potentially lower the costs and increase the yields of valuable products. 

As an important example of the value of direct quantum mechanical calculations on the reactions that include electron transfer, let us consider some results of a theoretical analysis, done by Salem et al. [13] of the chain mechanism of the reaction of Eq. (9.3) whose idea was first put forward by Kornblum and Russell [10-12,22]. The simplest compounds susceptible to this reaction are 2-chloro-2-nitropropane (R = Me2CN02, X = Cl) and 2-nitropropyl anion (Y = Me2CN02). In the example under consideration the methyl groups were replaced by hydrogen atoms and the overall reaction of Eq. (9.3) could be represented as a sequence of the corresponding steps ... 

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