System, delocalized carbanion

The chemical reactivities of the alkali metal organometallic compounds (RM) vary widely depending on metal M, basicity of the solvent systems used, and steric and electronic properties of the organic group R. In many reactions an important factor is the stabilization resulting from formation of a delocalized carbanion system as in the polymerization of dienes or aromatic substituted ethylenes, and in Reactions 3, 4, 5, and 10 in Table I. It is primarily with these delocalized carbanion systems that this review is concerned although saturated organolithium compounds are discussed briefly. 

Considerable theoretical and experimental effort has been expended to predict the position of the metal atom relative to the delocalized carbanion system in ir-carbanion complexes. One early effort was made by... 

An interesting case are the a,/i-unsaturated ketones, which form carbanions, in which the negative charge is delocalized in a 5-centre-6-electron system. Alkylation, however, only occurs at the central, most nucleophilic position. This regioselectivity has been utilized by Woodward (R.B. Woodward, 1957 B.F. Mundy, 1972) in the synthesis of 4-dialkylated steroids. This reaction has been carried out at high temperature in a protic solvent. Therefore it yields the product, which is formed from the most stable anion (thermodynamic control). In conjugated enones a proton adjacent to the carbonyl group, however, is removed much faster than a y-proton. If the same alkylation, therefore, is carried out in an aprotic solvent, which does not catalyze tautomerizations, and if the temperature is kept low, the steroid is mono- or dimethylated at C-2 in comparable yield (L. Nedelec, 1974). 

Extensive theoretical studies have been carried out to probe the nature of the allyl anion. These studies supplement and extend the experimental results. Allyl anion is of special interest because it is the simplest 7r-delocalized carbanion with 4 electrons and 3 Pjr-centers. Much recent theoretical discussion has concerned the role of resonance in the stabilization of such conjugated systems, a stabilization defined as the enthalpy difference between the localized double-bonded system and its conjugated state. The stabilization of allyl anion has generally been attributed to the delocalization of charge associated... 

On substitution of allyllithium with methyl groups, the structures are distorted tt complexes becoming more jj -like. The previously described allyllithiums are contact ion pairs (CIP) whose dissociation is too low to permit study of the free carbanion. However, this is not the case for a more delocalized system such as 1,3-diphenylallyl whose lithium salts can exist as solvent separated ion pairs (SSIP) in ethereal solutions for which the organic moiety could be treated essentially as a free carbanion55 Boche and coworkers studied the effect of substitution at C(2) in their 1,3-diphenylallyl lithiums on the rotational barriers... 

High-level ab initio calculations have provided more precise structural details, and relative stability estimates, for members of the 7-norbornyl anion series (12-15). Far from being classical carbanions, each of the ions is stabilized by delocalization of the negative charge into accessible LUMOs of anti-parallel C—C bonds of the molecular framework and each is more stable than methyl carbanion. Consequently, it is unlikely that solution studies of the unsaturated systems will reveal any bishomo-antiaromatic character. 

Base-catalyzed transformations can be carried out elsewhere on a complex molecule in the presence of such protected -dicarbonyl magnesium chelate. For example, the chelated magnesium enolate of a /3-ketoester such as 71 prevents the carbonyl keto group becoming an acceptor in aldol condensations. However, in the presence of excess of magnesium methanolate, exchange of the acetyl methyl protons can occur via a carbanion 72 stabilized by delocalization into the adjacent chelate system (equation 99). 

The chemistries of the benzyiic and allylic positions are very similar. Intermediate carbocations, free radicals and carbanions formed at these positions are stabilized by delocalization with the adjacent ir system, the benzene ring in the case of the benzyiic position. Another aspect of arene chemistry is the enhanced stability of unsaturated arenes having double bonds conjugated with the benzene ring. This property is akin to the stability of conjugated di- and polyenes. 

There are other anions for which the claim of homoaromatic delocalization has been made. Work on these systems is relatively old and has been reviewed extensively14 21. Overall, it is not clear there are any good examples of anions which are homoaromatic. Perhaps, with futher work, 133 will be demonstrated to be an example however, it is clear that homoaromatic delocalization is not generally going to be an important phenomenon in carbanions. 

The stabilization of the ylidic centers with Si-Si systems can be explained by a delocalization of the carbanion-charge in the Si—Si system. 

A rationale for the regiochemistries observed for the polychloroarene radical anions may be developed by considering the transition states for the two competing processes (Scheme 8). The loss of chloride ion in route (a) generates phenyl radical. The transition state for this process would, therefore, be expected to exhibit some radical localization at C-l. The shape of the transition state might be expected to be bent rather than planar, since heterolytic fission of the carbon-chlorine bond in a coplanar transition state would lead to an excited state (a phenyl cation with an extra electron in the n molecular orbital), while heterolytic fission of a bent system (such as C) could lead directly to a phenyl radical. Thus, the transition state for route (a) might very well possess some of the character of a delocalized anion with a bent localized radical center (C), while the transition state for chlorine atom loss, by a similar argument, would resemble a delocalized radical with a bent localized carbanionic center (D). 

In organic reactions there is abundant evidence for transient carbonium ions (R3C+), carbanions (R3C ), and carbenes ( CR2). Some stable carbonium ions like Ph3C+ and carbanions like C(CN)3 can be isolated as well as radicals like Ph3C In most of these cases the charge on the electron must be delocalized over the entire system for stability. Transition metal complexes with carbene or carbyne ligands, L M=CR2 and L M=CR, are discussed in Chapters 16 and 21. 

This mechanism has several disadvantages when compared to those given previously. Intermediate carbanion 7-50 is relatively unstable because its negative charge is localized. In contrast, the carbanions of the other mechanisms are stabilized by delocalization. Another intermediate of this mechanism, the bicyclo[2.2.0]hexenyl system, is not very stable because of high strain energy. 

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