Reactive intermediates carbanions

Unlike most other chain reaction polymerizations involving low concentrations of transient, reactive intermediates, carbanions and organometallic species are often stable enough to be prepared and characterized independently of the polymerization process. 

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... 

The cyclization of aryl 3-chloropropyl sulfones by potassium t-butoxide in t-butyl alcohol at 30 °C (equation 20) has a p value of 2.32 for substituents in Ar202. This is considered by Bird and Stirling to indicate the formation of an intermediate carbanion which is essentially in equilibrium with the reactants. A recent review by Stirling203 deals with structure-reactivity aspects of many sulfonyl promoted reactions of this type. 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

Organic cations (carbocations and onium ions) are important reactive intermediates in organic synthesis. From an experimental point of view, it is noteworthy that the manner in which we carry out reactions of organic cations is different from that for carbanions (Scheme 1). Usually, carbanions are generated and accumulated in a solution in the absence of electrophiles. After the generation process is complete, an electrophile is added to the solution of the pre-formed carbanion to achieve a desired transformation. In contrast, organic cations are usually generated in the presence of nucleophiles. This is probably... 

So far, the formation of cyclic alkoxyallenes bearing an exocydic or endocyclic allenyl unit has been less developed. Several examples with this structural feature are described as unstable compounds or highly reactive intermediates [32-35]. However, in the 1990s, Lavoisier-Gallo and Rodriguez demonstrated a useful one-pot protocol for the synthesis of 2-vinylidenedihydrofurans such as 29 involving a tandem C-O cycloalkylation of stabilized carbanion intermediates 28 as crucial step (Scheme 8.10) [36, 37]. 

There is a large variety of polar and radical reactions, transition metal-catalyzed and pericyclic conversions, that have been carefully developed with regard to scope, selectivity, and yield. They are compiled in large compendia, for example, in [16-19], and in series, for example [20, 21], and are continuously improved and extended in timely research papers. This literature should be consulted in parallel with suggestions taken from electrosynthesis. Electrosynthesis is a clear alternative to chemical synthesis, when reactive intermediates (see Sect. 3.3) such as radical ions, radicals, carbanions, or carboca-tions are involved. The more advantageous are summarized in the following sections. 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

This chapter will begin with a brief overview of the development of carbanion chemistry followed by a section devoted to the structure and stability of carbanions. Methods of measuring carbon acidity and systematic trends in carbanion stability will be key elements in this chapter. Next, processes in which carbanions appear as transient, reactive intermediates will be presented and typical carbanion mechanisms will be outlined. Finally, some new developments in the field will be described. Although the synthetic utility of carbanions will be alluded to many times in this chapter, specific uses of carbanion-like reagents in synthesis will not be explored. This topic is exceptionally broad and well beyond the scope of this chapter. 

Closed-shell ions are among the most important intermediates in solution chemistry, and no treatise on reactive intermediates (including the present one) would be complete without extensive sections on carbocations and carbanions, if not also on heteroanalogues of the above species. Nevertheless, closed-shell ions are conspicuously absent from matrix isolation studies, apart from a few cases where such species were coincidentally formed in discharges, or where charged species were deliberately isolated by mass spectrometry (cf. Section 6.4). The reason for... 

Reactive Intermediate Chemistry is an attempt to provide an updated survey and analysis of the field. We have adopted a three-dimensional approach. Reactive Intermediates are considered by type (e.g., carbocations, radicals, carbanions, car-benes, nitrenes, arynes, etc.) they are examined according to the kinetic realms that... 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

The very important reactive intermediate, the enolate ion, is an example of a Z-substituted carbanion. The charge distribution and HOMO obtained by SHMO calculation are shown below ... 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

Carbon um ions are familiar intermediates in many simple reactions (s 1, E1, etc,) and we have met many of them in tbis program. Carbanions are another matter. Though there are some exceptions, such as Br3C, it is a good general rule to say that simple carbanions do not occur as reactive intermediates. 

Most photochromic systems are not reversible indefinitely. However, very little careful analytical data have been accumulated to characterize the nature of the degradation products or to specify the degree of quantitative reversibility. The reasons for side reactions are inherent in the high photochemical reactivities of the systems. First of all, there must be an excited state formed by absorption this state is then transformed into other excited states or reactive species. The latter may include triplet states, carbonium ions, carbanions, free radicals, or other highly reactive intermediates. Certain of these are oxygen sensitive so that exclusion of the atmosphere and other potential sources of contaminants during irradiation is necessary. A second major route of degradation involves the excited state of the colored form which may already be... 

Apparently, these results implied an inverse relationship between reactivity and selectivity, with the reactivity of the carbocation measured by the inverse of the rate constant for solvolysis. This indeed was not unexpected in the context of a general perception that highly reactive reagents, especially reactive intermediates such as carbocations, carbanions, or carbenes are unselective in their reactions.257 259 Such a relationship is consistent with a natural inference from the Hammond postulate258 and Bell-Evans-Polanyi relationship,260 and is illustrated experimentally by the dependence of the Bronsted exponent for base catalysis of the enolization of ketones upon the reactivity of the ketone,261,262 and other examples21,263 including Richard s careful study of the hydration of a-methoxystyrenes.229... 

These reactions likely proceed vi a the formation of an intermediate carban-ion. Indeed, the carbanion 496 generated by treatment of 491 and 492 with lithium di isopropyl amide gave 81% of 491 (H=D) and 19% of 492 (H=0). The preferential formation of 491 can be explained on the basis of stereoelectronic effects which influence the reactivity of the intermediate carbanion... 

Alkylation of allylnitroalkanes.9 Cyclic allylic nitro compounds react with stabilized carbanions or amines in the presence of Pd(0) at the allylic position without allylic transposition. The reactive intermediate is presumably an allylpalladium(Il) species. 

The representative reaction system applied in asymmetric phase-transfer catalysis is the biphasic system composed of an organic phase containing an acidic methylene or methine compound and an electrophile, and an aqueous or solid phase of inorganic base such as alkaline metal (Na, K, Cs) hydroxide or carbonate. The key reactive intermediate in this type of reaction is the onium carbanion species, mostly onium enolate or nitronate, which reacts with the electrophile in the organic phase to afford the product. 

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