Stepwise mechanism/process

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

Tetraene 4 (Scheme 1.3), when treated with 40 mol % of triflic acid in methylene chloride at -23 °C for 1 h, gives the adducts 5 and 6 in a 1 1 ratio as the main reaction products. The formation of these adducts has been justified [21] by a stepwise mechanism that requires an initial reversible protonation of 4 to produce the allyl cation 7, which then cyclizes to 8 and 9 in a non-reversible process. Deprotonation of 8 and 9 gives 5 and 6, respectively. 

Considering the stepwise mechanism, different possibilities arise depending on the relative values for the rate constants ki, k i and k2- The application of the steady state condition to the carbanion yields equation (6) for the rate constant of the basic catalysis process. 

Fig. 3 Potential energy profiles for the concerted and the stepwise mechanism in the case of a thermal reductive process. E is the electrode potential for an electrochemical reaction and the standard potential of the electron donor for a homogeneous reaction. For an oxidative process, change - into + and donor into acceptor.

Both thermodynamic and kinetic factors are involved in the competition between concerted and stepwise mechanisms. The passage from the stepwise to the concerted situation is expected to arise when the ion radical cleavage becomes faster and faster. Under these conditions, the rate-determining step of the stepwise process tends to become the initial electron transfer. Then thermodynamics will favor one or the other mechanism according to equation (18). AG )eav is also the standard free energy of cleavage of the ion radical. 

Interaction of simple acyclic dications with acetylenes proceeds anomalously. Instead of unsaturated disulfonium salts, the reaction leads to the corresponding vinyltriflates 96.107 It was shown earlier108 that interaction of trifluoromethylsulfonyloxysulfonium triflate (the dimethylsulfide ditriflate complex) with certain acetylenes leads to analogous vinyl triflates. The drastic differences between the reactivity of cyclic and acyclic dications toward acetylenes provide another evidence for a stepwise mechanism of this process (Scheme 37). 

Af-Acyliminium ions are known to serve as electron-deficient 4n components and undergo [4+2] cycloaddition with alkenes and alkynes.15 The reaction has been utilized as a useftil method for the construction of heterocycles and acyclic amino alcohols. The reaction can be explained in terms of an inverse electron demand Diels-Alder type process that involves an electron-deficient hetero-diene with an electron-rich dienophile. Af-Acyliminium ions generated by the cation pool method were also found to undergo [4+2] cycloaddition reaction to give adduct 7 as shown in Scheme 7.16 The reaction with an aliphatic olefin seems to proceed by a concerted mechanism, whereas the reaction with styrene derivatives seems to proceed by a stepwise mechanism. In the latter case, significant amounts of polymeric products were obtained as byproducts. The formation of polymeric byproducts can be suppressed by micromixing. 

While the concerted pathway has been preferred in early publications on the subject, evidence for a stepwise mechanism involving distonic ion intermediates is presented in more recent work taking kinetic isotope effects into account. [90] This is also in agreement with the postulation that reactions involving multiple bonds are generally stepwise processes. [37,91] Nevertheless, this question is still a matter of debate. [90]... 

The thermolysis of a variety of 1,2,4-trioxanes in methanol has been followed by mass spectrometry and provided evidence of the corresponding products. A smdy of the thermal decomposition of 3,6-diphenyl-1,2,4,5-tetroxane in toluene and methanol revealed a significant solvent effect that supported a homolytic stepwise mechanism instead of a concerted process. ... 

In the above-described displacement reactions (Sh2) of sulfur- and selenium-containing compounds, the mechanism could either be a synchronous or a stepwise process (Scheme 4.2). Thus, in the Sh2 stepwise mechanism an intermediate sulfuranyl or selenyl radical is formed, followed by a-cleavage [56]. Based on competitive studies, a stepwise process was suggested to occur in the reaction of (TMS)3Si radical with -decylphenylselenide [43]. On the other hand, ab initio calculations supported the synchronous process and the same observations were explained in terms of an overall reversible reaction [57]. 

Extension of this line of reasoning suggests that there should be a stepwise mechanism based on the SN2 configuration. Thus, just as a concerted E2/E1 pathway finally turns into a stepwise El pathway, the E2H/E2C pathway is predicted, in the limit, to lead to a radical anion intermediate which then undergoes elimination. We have termed this, as yet undiscovered pathway, the E2C-I (I = intermediate) pathway (Pross and Shaik, 1982a). The E2C-I pathway constitutes the missing link in the chain of potential elimination processes based on three intermediate configurations (Fig. 26g). 

A recent theoretical study has nicely addressed the question of mechanism on the silicon surface. Minary and Tuckerman carried out an ab initio molecular dynamics (MD) study of the [4 + 2] cycloaddition reaction on Si(100)-2 x 1 [251]. Because the previously reported ab initio DFT models were static , these were not able to address in detail the mechanisms by which the [4 + 2] product was formed. The results of the MD study indicate that rather than being concerted, the dominant mechanism is a stepwise zwitterionic process in which an initial nucleophilic attack of one of the C=C bonds by the down atom of the dimer leads to a carbocation. This carbocation exists for up to 1-2 ps, stabilized by resonance, and depending on which positively charged carbon atom reacts with which Si surface atom, can form... 

The retention of olefin geometry in the oxidation of cis and trans alkenes408 suggests a one-step concerted [2 + 2]-cycloaddition process, suprafacial in the alkene, and antarafacial in singlet oxygen.363 The strong solvent dependence, however, observed in many cases, points to a stepwise mechanism involving a perepoxide intermediate 403,409... 

Fragmentation of quinuclidine derivatives,50-57 in contrast to the same process with aliphatic and bicyclic compounds without nitrogen, gives only one product. The solvolysis of 2-bromo-2-(quinuclidin-3-yl)propane (6) has a stepwise mechanism,50-57 and the degradation of 4-bromoquinuclidine (7) is synchronous.50 53... 

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