Allyl anions, calculations

Extend the PPP-SCE calculation from ethylene to the allyl anion, CTIf,. 

Note that agreement with Pariser and Parr s empirical value is better for Y13 than for Yn ) Use Salem s values to calculate election densities on the three carbon atoms of the allyl anion for one iteration beyond the initial Huckel values, as was done in Exercise 8.9.1. Comment on the results you get, as to the qualitative picture of the anion, the influence of election repulsion on the charge densities, and agreement or lack of agreement with the results already obtained with the Pariser and Parr parameters. 

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. 

FIGURE 2. Structures of calculated allyl cations, 10, and allyl anions, 11... 

The rotational barriers increase from sodium to cesium to yield an estimate of the free allyl anion barrier to rotation. The calculated barrier is higher than that determined experimentally. Hommes and colleagues proposed that the decrease could be due to solvation or dimerization. Considering both dimerization and solvation, the calculated barrier decreases by 5.5 and 0.5 kcalmol-1, respectively. 

In bis-7T-allylnickel 2, Ni(II) has 8e and the two allyl anions supply four electons each 8 + (4 x 2) = 16. The following calculation is also possible if Ni(0) supplies lOe and the two allyl radicals supply three electrons each, the total number is 10 + (3 x 2) = 16. Therefore, this complex is coordinatively unsaturated. 

Theoretical calculations of the reactions of CH3SSR (R = H or CH3) with fluoride, hydroxide or allyl anion in the gas phase have been performed to determine the mechanism for both elimination and substitution reactions.6 The elimination reactions have... 

Molecular orbital calculations by Hofmann (162) indicated that an if-allyl anion complex with a noncomplexed butadiene group is more stable than an -butadiene complex, in which the allyl anion portion of the seven-carbon ring is not bonded to the metal. We have now been able to confirm this result by an X-ray structure analysis (see Fig. 8) on [Ph4As][C7H7Fe(CO)3] (163). 

Nonetheless, some useful information regarding methylation has been obtained from molecular orbital calculations. One CNDO/2 study on the allyl and pentadienyl anions indicated that in all cases the methyl group would serve to withdraw net electron density from the anions196. For the allyl anion, a 1-methyl substituent would withdraw a substantial 0.102 electrons, while at the formally uncharged 2 position the effect was consider-... 

Inspection of Table 7 shows that rates of rotation in allyllithium compound 32 and the associated barriers depend strongly on the nature of lithium solvation34. Thus the rotational process cannot involve a free allylic anion alone. Rather, the process must be driven by a chemical mechanism. This has been proposed by calculations to involve the development of a degree of Li—C (terminal) covalence in the transition state accompanied by some change in the solvation around lithium. It is demonstrated for compound 32 in Figure 17. 

Whereas the allyl anion, with a plane of symmetry through the central atom, has a node at that atom in j/ j, amides, esters, enamines, enol ethers and enolate ions do not have a node precisely on the central atom. Taking planar A V-dimethylviny-lamine and the enolate of acetaldehyde as examples, simple Hiickel calculations give the n orbitals in Fig. 2.15, which includes the allyl anion for comparison. 

Fig. 2.15 n Orbital energies and coefficients from simple Hiickel calculations of the allyl anion, enamine and enolate ion (orbital energies in kJ moP1 relative to a)... 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

While the isolated allyl anion is calculated to have a planar structure 42), interaction with lithium results in movement of the three of the hydrogens markedly out-of-plane (43-45). The ab initio (3-21G basis set) allyllithium geometry is shown in 25c the central hydrogen is bent toward and the two inner hydrogens at C(l) and C(3) are bent away from the lithium. 

The generally slow conrotatory ring-opening reaction of cyclopropyl anions as compared to the fast isomerization of the corresponding allyl anions has been supported by means of MO calculations with the parent anions (Table 31) . 

These results have been attributed to an increased ionicity of the carbon-lithium bond in the case of 331 and 340 as compared to 342 and 343 . This conclusion is supported by electrochemical measurements and MNDO calculations A similar conclusion had been reached earlier by Boche and Martens for thermal cyclopropyl-allyl anion transformations. 

Why is it that the predicted modes of rearrangement were first confirmed by means of these two systems and not with a real cyclopropyl-allyl anion system The long history of the cyclopropyl-allyl anion rearrangement shows that this is because of several problems 10b). The first of these, which is also indicated from the results with 1 and 4, and which is confirmed by MO calculations, is the slow thermal conrota-tion of a cyclopropyl anion to give the corresponding allyl anion , as compared to the fast isomerization of the allyl anion to give the most stable isomer. 

Table 1 gives the results of several MO calculations for the conrotatory ring-opening of the cyclopropyl anion to give the allyl anion, and of the isomerization of the allyl anion 13). 

It was shown that the ring-opening reaction e.g. of 10 to give 13 (and/or 11 which is not clear from the available data) is about 1500 times slower than the isomerization of the allyl anion 13 (and/or 11) to give the more stable 12. A similar situation occurs when one starts from the cyclopropyl anion 1418). These experimental data thus nicely confirm the results of the theoretical calculations discussed in connection with Table 1. 

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