Carbanions rearrangement reactions

It is generally felt that other Kharasch reactions involve radical intermediates, since 2,3-dimethyl-2,3-diphenylbutane is formed when cumene is present, and rearrangement products are observed in the neophyl chloride system, although some of the latter might arise from the carbanion rearrangement reactions described in Section II, D. 

Just as in its carbonium ion twin, the carbanion rearrangement takes place with retention of configuration of the migrating group. This proves at once that the reaction is intramolecular and that the displacement takes place at the front side.428... 

A reaction related to the pure carbanion rearrangement, but one that eventually involves an external displacing agent as well, is the Favorskii reaction. It has been shown with the aid of isotopic tracers that the mechanism is 429... 

Other reactions involving the addition of carbanions are reactions like Perkin s reaction, Claisen condensation, benzilic acid rearrangement and Michael addition. 

Carbanions play critical roles in a wide variety of reaction pathways. As stated in the Introduction, this chapter will not focus on the synthetic utility of carbanions, but will instead focus on their mechanistic significance. In this section, a sample of important reaction mechanisms that involve transient or relatively short-lived car-banion intermediates will be introduced. As you will see, the key element in these mechanisms is the ability to form a carbanion that is reasonably stable, and often the kinetics of the reactions are dominated by carbanion stability. The role of carbanion intermediates in elimination reactions will be presented in some detail as a way to illustrate some of the methods that have been developed to probe for carbanion intermediates in reaction mechanisms. Other processes including additions and rearrangement reactions will be presented in less detail, but the role of carbanion stability in these reactions will be outlined. 

Reviews have featured epoxidation, cyclopropanation, aziridination, olefination, and rearrangement reactions of asymmetric ylides 66 non-phosphorus stabilized carbanions in alkene synthesis 67 phosphorus ylides and related compounds 68 the Wittig reaction 69,70 and [2,3]-Wittig rearrangement of a-phosphonylated sulfonium and ammonium ylides.71 Reactions of carbanions with electrophilic reagents, including alkylation and Wittig-Homer olefination reactions, have been discussed with reference to Hammett per correlations.72... 

These rearrangement reactions are interpretable in terms of [2.3] sigmatropic shifts of the intermediate ylides. A number of such rearrangements of open-chain systems have been described, involving sulfonium ylides [43] [44] [45], ammonium ylides [46] [57], anions in a-position to oxygen (Wittig rearrangement) [48] [49], and fluorenyl carbanions [50]. 

We studied both carbanions and carbocations using the stereochemical fate of stereogenic centers as a probe of reaction mechanism and solvation phenomena. These studies involved substitution, elimination, and rearrangement reactions, most of which involved carbanions and carbocations as short-lived reaction intermediates. The chiral systems were designed and synthesized, the kinetics of the reactions were examined, and the... 

If) Hydroxyalkvl. Mercaptoalkvl. and related Acids.- The rearrangement of a carbanion following reaction between the epoxides (117) and a trace of EtO may well play an important role in the formation of the (3-hydroxy-l-propenyl)phosphonic acid esters... 

This result was confirmed when semibullvalene 71 was reacted with potassium or Na/K alloy. Even at —78 °C this reaction resulted only in the rearranged 73 56 c>. A common intermediate should be the dipotassium compound 72 (and/or its dimers 74). The different behavior of the lithium and potassium species 72 (and/or 74) illustrates again the problem of carbanion rearrangements changing the gegenion from lithium to potassium leads to species with rather different properties. 

Bromoaryloxy(chloromethyl)diorganosilanes (150) react with sodium or n-butyllithium via a metalation, [l,3]-carbanionic rearrangement, cyclization sequence to afford 1,3-benzoxasilolenes (151) in a one-pot-reaction (Equation (28)) <83JOM(243)l,92PS25,9lEGP296280>. 

The thrust of this chapter deals with reactions of Fischer carbene complexes that have been the subject of kinetic and/or thermodynamic studies. The number of these is relatively limited. They include the reactions of equations (1) and (4), proton transfers that generate carbanions such as 5 (see equation 2), reactions at the metal center such as the loss or exchange of ligands as well as rearrangement reactions. 

Close analogies to the rearrangement reaction described by Curtin may be found in the more conventional rearrangements of carbanions involving lithium gegenions. After a preliminary study of the 1,2-shift rearrangements induced in l,l,l-triphenyl-2-chloroethane on treatment with sodium, Zimmerman (35) turned to the following reactions of 2,2-diphenylpropyl-lithium ... 

Radicals are very reactive due to their unpaired electron. Once formed, they typically react very quickly with organic molecules in addition, substitution, or rearrangement reactions. If radicals react with neutral molecules, new radicals form and the reaction can quickly propagate as low energy barriers are characteristic for this kind of radical reactions. Note that, as a consequence of their high reactivity, radicals react in most cases in a less selective manner than carbocations or carbanions. 

OC-All lation of Carbonyl Compounds and Derivatives. The organoborate iatermediates generated by the reaction of alkylboranes with carbanions derived from a-halocarbonyl compounds and a-halonitriles rearrange to give a-alkylated products. 

One of the more important approaches to 1-azirines involves a similar base-induced cycloelimination reaction of a suitably functionalized ketone derivative (route c. Scheme 1). This reaction is analogous to route (b) (Scheme 1) used for the synthesis of aziridines wherein displacement of the leaving group at nitrogen is initiated by a -carbanionic center. An example of this cycloelimination involves the Neber rearrangement of oxime tosylate esters (357 X = OTs) to 1-azirines and subsequently to a-aminoketones (358) (71AHC-(13)45). The reaction has been demonstrated to be configurationally indiscriminate both syn and anti ketoxime tosylate esters afforded the same product mixture of a-aminoketones... 

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