Resonance allylic anion

To account for 1,2-addition in polar solvents, Szwarc (1959) proposed an ion-pair as the reactive species. This was visualized as a metal cation and a resonating allyl anion (XLVIII), the latter existing in the more favorable trans conformation. Since a secondary carbanion is more reactive than a primary one, 1,2-ad-... 

The 1,3-dipolar molecules are isoelectronic with the allyl anion and have four electrons in a n system encompassing the 1,3-dipole. Some typical 1,3-dipolar species are shown in Scheme 11.4. It should be noted that all have one or more resonance structures showing the characteristic 1,3-dipole. The dipolarophiles are typically alkenes or alkynes, but all that is essential is a tc bond. The reactivity of dipolarophiles depends both on the substituents present on the n bond and on the nature of the 1,3-dipole involved in the reaction. Because of the wide range of structures that can serve either as a 1,3-dipole or as a dipolarophile, the 1,3-dipolar cycloaddition is a very useful reaction for the construction of five-membered heterocyclic rings. 

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. 

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... 

An example of a more strongly delocalized species is the allyl anion, which is conventionally described in terms of two resonance structures ... 

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. 

The reaction of methylenesulphones with allyl halides in the presence of quaternary ammonium salts produces the 1-allyl derivatives [52], unlike the corresponding reaction in the absence of the catalyst in which the SN- product is formed (Scheme 6.5). In contrast, alkylation of resonance stabilized anions derived from allyl sulphones produces complex mixtures [51] (Scheme 6.6). Encumbered allyl sulphones (e.g. 2-methylprop-2-enyl sulphones) tend to give the normal monoalkyl-ated products. Methylene groups, which are activated by two benzenesulphonyl substituents, are readily monoalkylated hydride reduction leads to the dithioacetal and subsequent hydrolysis affords the aldehyde [61]. 

The substitution reaction of CP with methyl chloride, 2-chloroethyl radical, and allyl chloride has been treated by several different ab initio theoretical models. Depending on the method, the intrinsic barrier for the 5ivr2 process in allyl chloride is 7-11 kcalmoP higher than the barrier for the 5ivr2 reaction of methyl chloride. The reaction of CP with the 2-chloroethyl radical involves an intermediate complex, which is best described as an ethylene fragment flanked by a resonating chloride anion-chloride radical pair. There are many other points of interest. 

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... 

In Chapter 11 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as a type of concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for the synthesis of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have n-electron systems that are isoelectronic with allyl anion, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a... 

We may perform the same analysis for the allyl radical and the allyl anion, respectively, by adding the energy of 4>2 to the cation with each successive addition of an electron, i.e., H (allyl radical) = 2(a + V2/3) + a and Hn allyl anion) = 2(a + s/2f) + 2a. In the hypothetical fully 7T-localized non-interacting system, each new electron would go into the non-interacting p orbital, also contributing each time a factor of a to the energy (by definition of o ). Thus, the Hiickel resonance energies of the allyl radical and the allyl anion are the same as for the allyl cation, namely, 0.83/1. 

Molecules isoelectronic with the allyl anion but which are neutral and have at least one resonance form with formal positive and negative changes in a 1,3 relationship are called 1,3 dipoles. 

Shaik and Bar102 demonstrated that allyl anion has a distortive jr-component but at the same time exhibits a rotational barrier. This analysis was reaffirmed later for allyl radical.5 Subsequently, Gobbi and Frenking93 pointed out that the total distortion energy of allylic species is very small because it reflects the balance of jr-distortivity opposed by the a-symmetrizing propensity. They further argued that along with this jr-distortivity, the allylic species enjoys resonance stabilization which is the source of the rotational barrier. A detailed VB analysis by Mo et al.149 established the same tendency. 

The resonances due to unsaturated 29Si nuclei of cyclotetrasilenylium cation 70 appear at + 77.3 (terminal Si) and +315.7 ppm (central Si). The chemical shifts are independent of the solvent such as dichloromethane, benzene, and toluene, implying no covalent interaction between the cation part and solvent molecules. Rather unusually, the central silicon in the cation part is more deshielded than the terminal silicons.42 The terminal and central 29Si resonances of anion 71 are -31.5 and + 273.0 ppm in toluene-terminal silicon atoms are equivalent, indicating that the lithium cation is fluxional and 71 adopts an allylic anion-type structure in solution. Similarly to allyllithium, the central nucleus of 71 is deshielded, while the terminal nuclei are highly shielded. 

In the catalytic presence of tetrabutylammonium fluoride, a trimethylsilyl group is cleaved from AKtrimethylsilyl)rnethylbenzylimine to form the resonance-stabilized 2-aza-allyl anion which undergoes a Michael addition reaction with, for example, methyl acrylate, giving y-aminoesters.333 These types of aminoesters serve as a starting material for the elaboration of diversely substituted pyrrolidones.334... 

Remember that no resonance form has an independent existence A compound has characteristics of all its resonance forms at the same time, but it does not resonate among them. The p orbitals of all three carbon atoms must be parallel to have simultaneous pi bonding overlap between Cl and C2 and between C2 and C3. The geometric structure of the allyl system is shown in Figure 15-10. The allyl cation, the allyl radical, and the allyl anion all have this same geometric structure, differing only in the number of pi electrons. 

This molecular orbital representation of the allyl anion is consistent with the resonance forms shown earlier, with a negative charge and a lone pair of nonbonding electrons evenly divided between Cl and C3. 

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