Additional reaction pathway

SCHEME 2.16 Additional reaction pathway for the generation of the quinone methide in the gas phase oxidation of 2-methylphenyl radical, investigated by the hybrid functional MPW1K (reproduced from Ref. [23] with permission from American Chemical Society). 

The authors also considered two additional reaction pathways for the o-QM decarbonylation to fulvene, both via ring opening (Scheme 2.19).23... 

To understand the effect of the protein on this modeled reaction mechanism, we selected the first reaction step, H2O2 reduction by a glutathione molecule for further investigations using the ONIOM (QM MM) method [28], The computational setup was similar to the structural study, but the effects of the additional water molecules were added from the active-site model. It is assumed that the reaction coordinate is the same as in the active-site study and no additional reaction pathways were investigated. An important point of the present ONIOM study is the full optimization of QM MM transition states using the novel ONIOM algorithms [9],... 

Theoretically Based Conjugate Addition Reaction Pathway... 

Finally, an additional reaction pathway exists and this does not seem to be operative with SAPO-34 and Beta under regular processing conditions. This path seems to be operative with ZSM-5 and that may involve successive methylations of propene, followed by cracking to yield higher alkenes [111]. A similar mechanism that involves successive methylations of ethylene followed by cracking to yield higher alkenes over ZSM-5 does not seem to be as important [125]. It is conceivable that this mechanism may be partly operative during the MTO experiments over SAPO-34 described above that used co-fed ethylene or co-fed propylene [126]. 

The chemistry outlined in Schemes 8.32 and 8.34 illustrates the complexity of reactions that occur between thiocarbonyl compounds and diazo compounds. Heimgartner and co-workers (214-217) observed a similar reactivity pattern when they combined l,3-thiazol-5(477)-thiones (153) with diazoalkanes. When ethyl diazoacetate was used, additional reaction pathways occurred giving rise to a complex mixture of products (218). An interesting aspect of this chemistry involves... 

For compounds like TCA, for which both high temperature and room temperature hydrolysis rates have been measured and contaminant plumes have been observed over a considerable length of time, extrapolation using the Arrhenius parameters appears valid. In addition, apparent augmentation of reactivity in real environmental situations often can be rationalized in terms of additional reaction pathways, so that it is not necessary to invoke the non-validity of the extrapolation process. 

The yields of SO2 and DMS02 were, on a molar basis, 60 10% and approximatly 30%, respectively. Because of a series of difficulties in calibration, wall loss, and aerosol formation it is not possible to indicate whether the observed yield of DMSO is being over- or underestimated. As stated above the observed products snow that both abstraction and addition reaction pathways are operative,... 

Since an addition reaction pathway appears to play a role in both the mechanisms of the reactions of OH and NO3 with DMSO it is surprising that the reactivity of NO3 towards DMSO is less than that towards DMS, whereas, the opposite is true for OH. This could reflect a difference in fate of the NO3-DMSO and OH-DMSO adducts. The former adduct can probably only decompose via reaction (12), whereas, the OH-DMSO adduct could react with 02 (reaction (10)) which would probably considerably shorten the lifetime of the adduct and lead to a faster overall rate constant for the reaction. This is only speculation and as mentioned in section 3.1 the possibily that secondary chemistry is effecting the OH rate determination needs to be elimated before meaningful comparisons can be made. 

In the reaction of SO with halobenzenes the correlation of the rate constants with the Hammett parameters gave values of -1.2 and -1.6 with halotoluenes. Studies on product distribution in substituted toluenes have shown that H atom abstraction from -CH3 group is an additional reaction pathway, especially in toluenes containing substituent at tw-positions (e.g. 4-chlorotoluene and 3,4- dichloro toluene). 

In addition to these atom transfer reactions, electron transfer reactions can occur. Reduction of 17 e MLn to 18 e M L anions is a common reaction. Oxidation of 19 e MLn(A-B) adducts to 18 e ML (A-B)+ cations is also frequently observed. Thus, an additional reaction pathway of the intermediate L M (A-B)ML is disproportionation to an ionic compound L M (A-B)]+ M L . The detailed mechanisms of these reactions are more complex than what is shown in Scheme 10.1. Many of the reactions are reversible and, in addition, they can couple to other reactions. 

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

The presence of oxygen can open up a number of additional reaction pathways that can control the actual surface chemistry. Madix has demonstrated that adsorbed atomic oxygen can behave as a nucleophillic center and attack surface bound hydrocarbon intermediates or as a Brqnsted base for hydrogen transfer reactions [63]. Chemisorbed atomic oxygen can also act as a poison on different transition metal surfaces. 

At higher energies, several additional reaction pathways open. These are shown in reactions (6) to (11). These reactions are endothermic in all cases, and their cross sections can be analyzed to provide thermodynamic information regarding the products. A particularly interesting aspect of reactions (6) to (8) is that both ionic and radical silicon hydrides are formed such that coupled information about the cations and neutrals can be obtained from these data. This is discussed in more detail in Section III.E. 

Although the mechanism shown in Scheme 2 was satisfactory to explain the radical cation mediated dimerization of a variety of arylalkenes and vinyl ethers, other studies provided mechanistic evidence for additional reaction pathways. For example, early studies reported the formation of a [2 + 4] dimer, 1,1,4-triphenyl-1,2,3,4-tetrahydronaphthalene, in the ET-sensitized dimerization of 1,1-diphenylethylene and postulated a mechanism involving 1,6-cyclization of an initial 1,4-acyclic radical cation. Later work demonstrated that dimerization of this alkene... 

Two groups (Becker, Sander) have presented some evidence for the need to complete the generally accepted Criegee mechanism (l)-(4) by additional reaction pathways ... 

The reasons for these measured effects are not presently known. The non ideality of the mixture or an additional reaction pathway, via formic acid are among the possibilities. 

The kinetic analysis of toluene ammoxidation reveals an additional reaction pathway. This reaction pathway is dominant at intermediate to low partial... 

In the case of educts which contain other functionalities of nucleophilic character (OH/OR, 8H/8R, NHj/NHR/NRj with R = alkyl, acyl, alkyl- or arylsul-fonyl, etc.) besides the leaving group (LG), additional reaction pathways are... 

Formation of the (RSSR) radical cations can be achieved by oxidation of disulfides with OH radicals [157, 158]. As already indicated in Section 1.1 on thiyl radicals, this reaction enters into at least two different pathways. In the case of simple aliphatic disulfides the yields of (RSSR) amount to about 50% but appear to drop for more functionalized disulfides such as cysteine, cysteamine, and others. First, these lower yields may not be the true yields since buffer effects could have distorted the time-resolved conductivity signals by which these yields have been determined. But more importantly, additional reaction pathways may be entered. For several of these compounds weak absorptions at 374 nm have been observed indicating the possible formation of perthiyl radicals [159]. This is corroborated by the fact that the decay of this presumed RSS absorption is not accelerated by OH" addition as is the case for the (RSSR) band. 

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