Reactive intermediate generation

You have already had considerable experience with carbanionic compounds and their applications in synthetic organic chemistry The first was acetyhde ion m Chapter 9 followed m Chapter 14 by organometallic compounds—Grignard reagents for example—that act as sources of negatively polarized carbon In Chapter 18 you learned that enolate ions—reactive intermediates generated from aldehydes and ketones—are nucleophilic and that this property can be used to advantage as a method for carbon-carbon bond formation... 

Thermal reactions of 1,4,2-dioxa-, 1,4,2-oxathia- and 1,4,2-dithia-azoles are summarized in Scheme 1. The reactive intermediates generated in these thermolyses can often be trapped, e.g. the nitrile sulfide dipole with DMAD. 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

This chapter will cover only reactions in which the isomerization to the allene starts from a stable molecule and not from a reactive intermediate generated in situ by reactions which are not isomerizations, such as the Doering-Moore-Skattebol reaction or free carbenes. Metallotropic rearrangements also will not be covered many of these reactions can be found in Chapter 9. Furthermore, the allene should be the final product of the reaction and not only a transient species leading to other products (see, for example, Chapters 6 and 20). 

The data of Table IV strongly suggest that water and SCN- are reacting in a competitive fashion, with a reactive intermediate generated in or after the ratedetermining step of the acid-catalyzed aquation. Equations 19, 20, and 21 represent a plausible mechanism for generation of the reactive intermediate, assumed to be Co(CN)5-2, and its competitive reactions with water and SCN . 

One of the present authors has investigated the importance of the nature of the electrode/electrolyte interface for the yields and selectivities of some anodic electrosynthesis reactions. A series of four successive reviews reports on the gathered information and improved understanding of the chemical kinetics of reactive intermediates generated at the interface carbon elec-trode/nonaqueous solvent (208-212) and citations of detailed investigations therein. 

The success of carbocation chemistry lies in the stabilization of carbocations in a medium of low nucleophilicity. Superelectrophiles, in turn, are reactive intermediates generated by further protonation (protosolvation). This second protonation increases electron deficiency, induces destabilization, and, consequently, results in a profound increase in reactivity. In particular, charge-charge repulsive interactions6 play a crucial role in the enhanced reactivity of dicationic and tricationic superelectrophilic intermediates. As various examples of acidity dependence studies show, without an appropriate acidity level, transformations may occur at much lower rate or even do not take place at all. In addition to numerous examples of superacid catalyzed reactions, various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are de facto observed in the condensed state, are also included in this chapter. 

In principle, the arrangement of reactive intermediates generated by electron - hole pair capture by two redox couples on the semiconductor surface may allow for divergent reaction paths when the same reactive intermediates are generated on the irradiated surface and in an isotropic environment. If a particular reactive intermediate is quite stable, the overall chemistry observed may be... 

Following are some examples of specific reactive intermediates generated by low-energy electrons NIRAM approach (Kajdas, 1994 and 2001) ... 

In addition to DNA adducts that occur as a result of covalent binding of reactive intermediates generated by oxidation or conjugation of parent compounds to DNA, reactive oxygen species produced during xenobiotic metabolism can also react with nucleophilic biomolecules. 

Jayalekshmy and Mazur illustrated the reduced mobility of polymer-bound reagents by delayed trapping of reactive intermediates generated within a polymer matrix 139). 

Transient spectroscopy A technique for the spectroscopic observation of transient species (excited-state molecular entities or reactive intermediates) generated by a pulse of short duration. 

The study of reactive intermediates generated at electrodes is most often referred to as organic electrochemistry. This is somewhat misleading and can give the impression that reactions of intermediates studied in this manner should differ significantly from those in which the intermediate is generated by non-electrochemical means due to the influence of the electrode. It is therefore of interest to examine the role played by the electrode in these reactions. In order to do this, it is necessary to distinguish between volume and surface reactions. 

In a retrospective cohort study in 83 patients with primary brain tumors who were treated with procarbazine, 20 patients had procarbazine hypersensitivity reactions (4). There was a significant association between exposure to antiepileptic drugs and the development of procarbazine hypersensitivity reactions. The authors suggested that this association may have been due to a reactive intermediate generated by induction of CYP3A. 

The reactive intermediate generated during the course of these reactions can be applied to annulation reactions if the cyclopropane ring contains the appropriate functionalities. A suitable element is the penta-2,4-dienyl group which is attacked at the terminal position by the electrophilic cyclopropyl carbon leading to octalinone derivatives 48 and 50, respectively. ... 

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