Allyl anions stabilization

Several reviews cover hetero-substituted allyllic anion reagents48-56. For the preparation of allylic anions, stabilized by M-substituents, potassium tm-butoxide57 in THF is recommended, since the liberated alcohol does not interfere with many metal exchange reagents. For the preparation of allylic anions from functionalized olefins of medium acidity (pKa 20-35) lithium diisopropylamide, dicyclohexylamide or bis(trimethylsilyl)amide applied in THF or diethyl ether are the standard bases with which to begin. Butyllithium may be applied advantageously after addition of one mole equivalent of TMEDA or 1,2-dimethoxyethane for activation when the functional groups permit it, and when the presence of secondary amines should be avoided. 

The requirement of extensive allyl anion stabilization as a prerequisite for cyclopropyl anion opening has only been overcome in the treatment of arylcyclopropanes such as 16 and 20 with Lochmann s base (BuLi//-BuOK) in refluxing hexane. Under these conditions poly-metalated 17 and dimetalated species 21, respectively, were formed. The ring opening results from the need for the charge in the benzylic cyclopropyl position to be further removed from... 

The strong base -BuLi generates an allylic anion stabilized by the phosphorous atom and the alkene. 

The TT-allylpalladium complexes 241 formed from the ally carbonates 240 bearing an anion-stabilizing EWG are converted into the Pd complexes of TMM (trimethylenemethane) as reactive, dipolar intermediates 242 by intramolecular deprotonation with the alkoxide anion, and undergo [3 + 2] cycloaddition to give five-membered ring compounds 244 by Michael addition to an electron-deficient double bond and subsequent intramolecular allylation of the generated carbanion 243. This cycloaddition proceeds under neutral conditions, yielding the functionalized methylenecyclopentanes 244[148], The syn-... 

Dipoles without a double bond but with internal octet stabilization, referred to as the allyl anion type, are shown in Table 3. A third group, 1,3-dipoles without octet stabilization such as vinyl carbenes, iminonitrenes, etc., is known, but these are all highly reactive intermediates with only transient existence. Reference is made to this type where appropriate and in Table 4 (p. 146). 

Table 3 1,3-Dipoles Without a Double Bond but With Internal Octet Stabilization Allyl Anion Type...

In the 1,3-dipolar cycloaddition reactions of especially allyl anion type 1,3-dipoles with alkenes the formation of diastereomers has to be considered. In reactions of nitrones with a terminal alkene the nitrone can approach the alkene in an endo or an exo fashion giving rise to two different diastereomers. The nomenclature endo and exo is well known from the Diels-Alder reaction [3]. The endo isomer arises from the reaction in which the nitrogen atom of the dipole points in the same direction as the substituent of the alkene as outlined in Scheme 6.7. However, compared with the Diels-Alder reaction in which the endo transition state is stabilized by secondary 7t-orbital interactions, the actual interaction of the N-nitrone p -orbital with a vicinal p -orbital on the alkene, and thus the stabilization, is small [25]. The endojexo selectivity in the 1,3-dipolar cycloaddition reaction is therefore primarily controlled by the structure of the substrates or by a catalyst. 

Commonly employed anion-stabilizing groups are those containing silicon (Table 5.4, Entries 1-5). Magnus et al. reported that epoxysilane 147 could be deproto-nated with t-BuLi, and that the lithiated epoxide 148 thus generated could be trapped with allyl bromide to give epoxysilane 149 in a synthetically useful yield (Scheme 5.34) [55], Iodomethane (88%) and chlorotrimethylsilane (60%) could also be trapped. 

If R1 differs from R2. the preparation may lead to both regioisomers. In these cases, a synthetic route which does not rely on allyl anion substitution is often the most advantageous one. Thus, the best results are recorded for allylboronates and -silanes which also possess the required constitutional and configurational stability. 

Phosphonamide-stabilized allylic anions react y-selectively and serve as homocnolate reagents86 in the reaction with aldehydes only moderate simple diastereoselectivity is observed. 

It has been contended that here too, as with the benzene ring (Ref 6), the geometry is forced upon allylic systems by the a framework, and not the 7t system Shaik, S.S. Hiberty, P.C. Ohanessian, G. Lefour, J. Nouv. J. Chim., 1985, 9, 385. It has also been suggested, on the basis of ab initio calculations, that while the allyl cation has significant resonance stabilization, the allyl anion has little stabilization Wiberg, K.B. Breneman, C.M. LePage, T.J. J. Am. Chem. Soc., 1990, 112, 61. 

Hexaaza-1,5-dienes RN=NNRNRN=NR, derivatives of 15 [96], are unusual high-energy molecules. Very recently, Cowley, Holland, and co-workers [101] fairly well stabilized the dianion RN R "16 as a ligand in a transition metal complex. These species are stabilized by such conjugations as those in allyl anions, which are special conjugations of the n-tr conjugations. 

Ammonium ylides can also be generated when one of the nitrogen substituents has an anion stabilizing group on the a-carbon. For example, quaternary salts of /V-allyl a-aminoesters readily rearrange to y,8-unsaturated a-aminoesters.286... 

Sigmatropic rearrangements of anions of (V-allyl amines have also been observed and are known as aza-Wittig rearrangements.291 The reaction requires anion stabilizing substituents and is favored by (V-benzyl and by silyl or sulfenyl substituents... 

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... 

Reaction of a Dithioacetal Derived from Indoxyl with Allyl and Stabilized Benzyl Anions... 

Even the allyl anion can be seen as an example of resonance-enhanced coordination. As shown in Section 4.9.2, r -CsHs- complexation is accompanied by a shift toward the localized H2C —CH=CH2 resonance structure that places maximum anionic character at the metal-coordinated carbon atom. In effect, the carbanionic lone pair nc is shared between intramolecular nc 7icc (allylic resonance) and intermolecular nc—>-n M (metal coordination) delocalizations, and the former can be diminished to promote the latter, if greater overall stabilization of the metal-ligand complex is achieved thereby. 

Additional evidence for the greater stability of the cis conformation of allylic anions is provided by other base catalyzed isomerization studies of 1-butene and 1-pentene. It was found that the thermodynamically less stable cis isomers of 2-butene and 2-pentene were the major products of the reaction182-186). Furthermore, m-2-butene isomerizes, under the same conditions, faster than the tram isomer to give 1-butene. 

The third mechanism starts with addition of the AT-allylamine 103 to the cumulated acceptor system of an allene carbonester 108 (Acc=CHC02Me) to form an intermediate iV-allyl ammonium amide enolate 109 (allene carbonester Claisen rearrangement). The anion stabilizing group is exclusively placed... 

Resonance stabilization is also responsible for the increased acidity of a C-H group situated adjacent to a carbonyl group. The anion is stabilized through delocalization of charge, similar to that seen with the allyl anion derived from propene but this system is even more favourable, in that delocalization allows... 

As in the case of double bond isomerization of butenes, the double bond isomerization of VBH is considered to be initiated by abstraction of an allylic proton from a tertiary carbon atom to give an allylic anion that may be stabilized by metal ions, yielding the E- and Z-EBH isomers. 

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