Allyl anion resonance structures

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. 

X, Y, and Z may all be carbon atoms, as in the case of an allylic carbocation (resonance structures A and B), or they may be heteroatoms, as in the case of the acetate anion (resonance structures C and D). The atom Z bonded to the multiple bond can be charged (a net positive or negative charge) or neutral (having zero, one, or two nonbonded electrons). The two resonance structures differ in the location of the double bond, and either the charge, the radical, or the lone pair, generalized by [ ]. 

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. 

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses 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 it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

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. 

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. 

We have already drawn resonance structures for the acetate anion (Section 2.5C) and the aUyl radical (Section 15.10). The conjugated allyl carbocation is another example of a species for which two resonance structures can be drawn. Drawing resonance structures for the allyl carbocation is a way to use Lewis stmctures to illustrate how conjugation delocalizes electrons. 

The effect of the cyclopropene double bond on acidity of the allylic (C3-H) protons is striking in comparison to the situation for the vinyl (Cj-H) protons. As the data in Table 1 reveal, cyclopropene is at least 10 pX units less acidic than cyclopropane. On classical grounds, resonance stabilization of the cyclopropenyl anion (D31, structure. Scheme 1) should provide an acid-strengthening effect however, increased ring strain associated with planarization of the final ring carbon could offset this stabilization. If 7c-conjugative effects are considered unimportant, then by analogy to the cyclopropyl anion the nonplanar Q... 

Resonance structures (top) and resonance hybrid (bottom) for (a) allyl cation, (b) allyl radical, and (c) allyl anion. 

Allyl and benzyl cations are the prototype delocalized carbenium ions (look at Figures 1.21 B and 1.22). Conventional it delocalization does not convert a carbenium ion to a carbo-nium ion in this case. No hypervalent atoms are involved in any of the resonance structures for allyl or benzyl. The same orbitals that carry the negative charge in allyl and benzyl anion carry the positive charge in allyl and benzyl cation. 

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