Allyl anions theoretical studies

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... 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

The theoretical study of the structure of propene was then used as a model to calculate the effect of the structure on the proton affinity, and later to predict the acidity of similar systems such as cycloalkenes46. Deformation of the CCC angle as a function of the stability of the anion was probed, and the results were in agreement with the acidities of the hydrogens of propene. The allylic protons were found to be more acidic than the vinylic ones, which is in contrast to the results of Grundler47. 

A 1,3-dipole as shown in Schemes 6-5 and 6-6 corresponds to a system with three parallel atomic p-orbitals, i.e., to an allyl anion, but without net charge. It is, therefore, called an allyl-type 1,3-dipole. The system may contain, however, an additional 7i-bond in the plane perpendicular to the allyl anion type molecular oribtal, and then belongs to the propargyl - allenyl type. Normally, 1,3-dipoles of this type are linear, whereas those of the allyl type are bent. The term 1,3 relates to the reactivity in these positions, not to formal charges. A series of theoretical studies (e. g., by Hiberty and Leforestier, 1978 Yamaguchi et al., 1980 see review of Houk and Yamaguchi, 1984) clearly show, however, that some of these 1,3-dipoles have considerable biradical character (e.g., O3 53% and CH2N2 28% in ab initio calculations at the 4-3IG level). We will return to biradicals in the mechanistic discussion of Sect. 6.3. 

Let us examine in this section the mesomerism in the three allylic systems the cation, radical and anion. Such molecules have served as model systems in numerous theoretical studies on n delocalization [25-29]. 

The 5n2 reactions of y-substituted allyl chlorides where the nucleophile is a free anion and an ion pair were studied theoretically at the HF/6-31+G level of theory.10 The calculations showed that the free ion, S N2 reaction proceeds through a transition state (7) with significant positive charge on Ca and little conjugation with the n-system. Placing substituents on Cy of the free ion reactions gave computed Hammett p values of -3.3 and -4.6 for the trans- and ds-conformation reactions, respectively. The transition states (8) for the ion pair reactions, on the other hand, have considerable positive charge on both Ca and Cy and the computed Hammett p value is +18.8. 

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