Double hydrogen atom transfer process

The two N-H protons inside the porphyrin free base migrate along N-H tau-tomerization between the four nitrogen sites (double hydrogen atom transfer). The mechanism of this inner N-H tautomerization process has been proven to be the... 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

It was found that most synthetic processes that are employed to prepare the diimide reagent generate trans-diimide, but ds-diimide undergoes faster hydrogen atom transfer to a double bond than does the trans isomer. It follows that a fast trans-cis isomerization precedes reduction. The transfer of hydrogen atoms takes place in a synchronous process188 via the transition state 19 ... 

Beckwith pioneered the use of a Thiol-Oxygen-Co-Oxidation (TOCO) process for the transformation of 1,4-dienes and 1,3,6-trienes to 1,2-dioxolanes [90], As illustrated by the example in Scheme 51 [90a], this process involves phenylthio radical addition to the least substituted double bond, oxygen entrapment, peroxyl radical cyclization, oxygen entrapment and hydrogen atom transfer from the thiol. In accord with the Beckwith-Houk transition state model [91, 92], cyclization provides preferentially the c/s-3,5-disubstituted 1,2-dioxolanes. 

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... 

Consequently, it is I ICo(CN)s3 that functions as a catalyst in hydrogenation processes. In the first step of the process shown in Figure 22.9, the alkene coordinates to HCo(CN)s3 as one hydrogen atom is added to the molecule so that only one double bond remains. The monoene is bonded to the cobalt in rf fashion. In the second step, another HCo(CN)53- transfers hydrogen to the alkene, which undergoes reductive elimination and leaves, having been converted to 1-butene. 

This stabilization may also be interpreted in terms of oxygen anions, which, due, to the vacancy, are initially double bonded to Mo. One electron is transferred to the catalyst in this reaction step. To form acrolein, a second hydrogen atom is transferred (to form water) and an oxygen atom is bonded to the allyl radical. In this (rather complex) process, another three electrons are transferred to the catalysts and doubtless distributed over several Mo ions. Reoxidation takes place at the bismuth cations, where oxygen molecules are attracted by the free electron pair. The intermediate result is a surface bismuth with an oxygen coordination similar to that in the bulk, viz. 

A further possibility to interpret cis addition is the so-called cA-concerted mechanism74,145. It assumes that the addition of the two hydrogen atoms takes place in a single step in a concerted fashion on a single 3M site possessing three coordinative unsaturations. The transfer of the two hydrogens to the double bond through a concerted process, where the interaction with the catalyst removes the symmetry restrictions imposed by the Woodward-Hoffman rales, leads directly to alkane formation. 

As it becomes clear from Fig. 15, a diazene-like conformation can be induced through energy transfer of about 300 kJ/mol. It should thus be possible to induce this conformational change photochemically. The diazene-like conformation corresponds to a double-bonded N2 moiety with two lone pairs, which can play an important role as hydrogen atom acceptors in the reduction process. This appears to be most important for a kinetic activation of the first endothermic reduction step. 

Chain termination can occur by radical coupling, as shown in Mechanism 30.1. Chain termination can also occur by disproportionation, a process in which a hydrogen atom is transferred from one polymer radical to another, forming a new C-H bond on one polymer chain, and a double bond on the other. 

In the spectrum of cyclobuxine-D, the 3-methylamino group is characterized by the peak at m/e 44. The fragmentation process (d) probably commences by the fission of the 2,3-linkage and transfer of the hydrogen atom from position 1 (16). Thus, the presence of the exomethylene double bond influences the course of the fragmentation more strongly than does the cyclopropane ring. 

Of special interest for the topic of the present chapter is the observation of Weaver that while the double-layer-corrected AS quantities are ligand sensitive, they are found to be independent of potential. This is not the case for the atom and electron transfer process involved in the hydrogen evolution reaction at Hg studied by Conway, et where an appreciable potential dependence of AS is observed, corresponding to conventionally anomalous variation of the Tafel slope with temperature. Unfortunately, in the work with the ionic redox reactions, as studied by Weaver, it is only possible to evaluate the variation of the transfer coefficient or symmetry factor with temperature with a limited variety of redox pairs since Tafel slopes, corresponding to any appreciable logarithmic range of current densities, are not always easily measurable. Alternatively, evaluation of a or /3 from reaction-order determination requires detailed double-layer studies over a range of temperatures. 

Selective reduction to the alkene without overreduction requires that the alkyne be adsorbed to the catalyst more tightly than the alkene. To selectively generate a cis alkene also requires that double bond migration and cis-trans isomerization of the alkene product be minimized. Palladium is usually a poor choice since both of these processes are faster than transfer of a hydrogen atom to the alkene. An order has been established for... 

Modification of Ring a.—Some of the modifications mentioned above (Scheme 11) are common to many organisms.The complete structure of A -3-keto-steroid isomerase (EC 5.3.3.1) has been determined. It has three sub-units, each of 125 amino-acids. This enzyme transfers the 4/9-hydrogen atom to the 6j3-position as the A -double bond is moved into conjugation. Starfish convert cholesterol into 5a-cholest-7-enol by a similar process. The A -double bond is introduced only after reduction of the A -3-one. ... 

The hydrogen transfer preferably takes place coplanar to the q -allyl and the alkyl groups of the Cm chain, with the shifted hydrogen atom residing in square-planar conformation together with the two groups. The process occurs with the displacement of the inner olefinic double bond, which is coordinated in 5, from the immediate proximity of the nickel atom by the shifted hydrogen... 

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