Stepwise reactions involving

Benzylideneeyclopropane (156) and diphenylmethylenecyclopropane (157) reacted rapidly with TCNE to afford 159 and 160, respectively, in good yields. Since the reaction rate is highly dependent on the solvent polarity, the cycloadditions of 156 and 157 with TCNE were rationalized as stepwise reactions involving the dipolar ions 158 (Scheme 23) [37],... 

The reaction of the sterically crowded thiocarbonyl ylide 69 with highly electron-deficient alkenes such as 2,3-dicyano fumarate and maleate, tetracya-noethene, a-cyano cinnamates, and l,2-bis(trifluoromethyl)ethene-l,2-dicarboni-trile occurred in a nonstereospecific manner (27,89,96,97,136-138). The formation of a mixture of cis/trans tetrahydrothiophenes of type 82 is the result of a stepwise reaction involving zwitterionic intermediates of type 81 (Scheme 5.29). Ylide 69 fulfills the fundamental requirements for a two-step reaction with electron-deficient alkenes. This species corresponds to an electron-rich 1,3-dipole that also contains a bulky substituent at one terminus (89). 

Figure 13-1 Schematic representation of energy versus reaction coordinate for (A) a concerted reaction and (B) a stepwise reaction involving formation of an unstable intermediate. Curve B has been drawn to have the highest energy point (the transition state) come before the intermediate is formed. For many processes the highest energy point comes after the intermediate is formed. If the highest energy point comes after the intermediate is formed, then the intermediate will be more or less in equilibrium with the reactants.

A stepwise reaction involving a biradical intermediate accounts for the formation of both 45 and 46. In the biradical mechanism the first step is formation of just one C-C bond between the reactants, and this could occur in two different ways to give 47 or 48ab ... 

Propadiene also appears to have the potential for much easier formation of a biradical than does ethene, as you will see if you work Exercise 21-27. Not all [2 + 2] cycloadditions proceed by biradical mechanisms, some clearly occur by stepwise reactions involving ionic intermediates (see Exercise 21-43). 

A concerted rather than a stepwise reaction involving a tetrahedral intermediate is supported for die papain-catalysed hydrolysis of amides. The TS for die hydrolysis has been determined by using a hybrid quantum mechanical/molecular mechanical potential, QM(AM1)/MM.216... 

Since concerted 2+2->-4 additions are forbidden by symmetry rules 123>, the adducts 153 and 154 may be considered as further, albeit unusual, examples of a stepwise reaction involving an initial facile addition to the reactive carbonyl group (see Section 4.1.1—4.1.3). 

The controversy about the timing of bond formation in cycloadditions continues. Although stepwise reactions involving zwitterionic intermediates can be detected more or less reliably by solvent polarity effects on rates, the distinction between mechanisms involving diradical intermediates or no intermediates at all (concerted pathways) is a more subtle one. Whereas the articles of debate were formerly experimental data, the discussion has now expanded into the theoretical realm. 

The [2 + 2] cycloaddition represents the most general and direct pathway for the formation of a cyclobutane structure from two alkene moieties, as outlined in Scheme 2.126. This process may occur as a concerted reaction via a cyclic transition state (mechanism a), as a stepwise reaction involving the formation of an acyclic biradical (mechanism b), or through bipolar (mechanism c) intermediates. Depending upon the structure of the reactants, cycloaddition may occur by any of these mechanisms. 

A stepwise reaction involves more than one step. A starting material is first converted to an unstable intermediate, called a reactive intermediate, which then goes on to form the product. 

Kawasaki et al. [91] looked at the oxidation kinetics of large photolytic silver clusters in redox buffer solutions. The process was analyzed in terms of a series of stepwise reactions involving electron transfer and silver ion elimination. It was shown that the single-step oxidation ... 

Under thermal conditions, methylenecyclopropanes can undergo rearrangement reactions in which cyclic and exocyclic methylene groups change places. This process, which can be considered as either a concerted [1,3] sigmatropic shift of a carbon or a stepwise reaction involving a 1,3-diradical intermediate, has been studied in order to elucidate the stereochemical course of the reaction. 

In in elcinemary reaction, reading molecules are transformed into products without going through intermediates. A stepwise reaction involves consecutive elementary reactions where the intermcdiate(s produced in the first elementary reaction react furl her in subsequent elentenla ry reactions. 

A stepwise reaction involves more than one elementary reaction. Intermediates are produced which react further to give products, often involving many steps. 

The d2/d3 redox pair [WF2(Tp )(PhC=CPh)]z z = + 1 or 0), has been obtained by a series of stepwise reactions involving sequential one-electron oxidation followed by ligand substitution. These octahedral compounds have been investigated by CV, EPR, and X-ray diffraction studies.176... 

One question that needs to be addressed is why are the activation volumes of pericyclic cycloadditions smaller (more negative) than those of the corresponding stepwise reactions involving diradical intermediates In the past it was assumed that the simultaneous formation of two new n bonds in a pericyclic [4 - - 2] cycloaddition leads to a larger contraction of volume than the formation of one bond in the stepwise process. The interpretation presented [28] is limited by the scope of Eyring transition state theory where the activation volume is related to the transition state volume, as mentioned above, and does not incorporate dynamic effects related to pressure-induced changes in viscosity [41]. An extensive discussion of reaction rates in highly viscous solvents can be found in Chapter 3. 

With this terminology we may not only describe a fully concerted, one-step Sn2 reaction (AnDn), and a stepwise SnI reaction involving intermediate ions that diffuse apart (Dn -H An), but we may also concisely represent a stepwise reaction involving a transient ion pair (E An). The lUPAC nomenclature system can also be used to describe other substitution reactions. Among them are the SnI (substitution nucleophilic unimolecular with rearrangement) reaction, equation 1.8, which is denoted by lUPAC as an (1/Dn -I- 3/An) reaction. The numbers before the slash symbols indicate atoms involved in the dissociation and association steps. Thus, 1/Dn means that the nucleofuge dissociates from one atom (1), while the 3/An term means that the nucleophile associates at an allylic position (3). 

As mentioned earlier (Chapter 9.2.1.1), a formal [3 + 2] approach toward the synthesis of benzofurans 57 and related compounds involving a coupling-cydization reaction between ortho-halophenol 54 and copper(I) acetylides 55 was first established by Castro and coworkers (Scheme 9.21) [75-77, 111]. It was proposed that the formation of the benzofuran 57 is a stepwise reaction involving formation of the ortho-alkynylphenol intermediate 56 [77]. The authors demonstrated that a variety of substituted benzofurans could be synthesized in generally high yields via this stoichiometric approach. 

Collman et al. " summarized and discussed the competition between epoxide formation and N-alkylation and the numerous mechanistic possibilities. They concluded in part that "Mechanisms ranging from a fully concerted reaction to a stepwise reaction involving an initial electron transfer may all be possible depending on the nature of the system." The theoretical results, summarize in a recent review, reflect this mechanistic multiplicity by suggesting a concerted mechanism for the low-spin doublet state and a stepwise process for the quartet state, just as was proposed for the hydroxylation of alkanes. 

Such chentical reactions, also called stepwise reactions, involve two or more one-step chanical reactions. For example, a hypothetical irreversible multistep chemical reaction is represented as... 

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