Allylic systems, cations

Next, let us look at the allyl system cation, free radical, and anion. Regardless... 

MOs of the allylic system (cation, anion and radical) and their symmetry characteristics... 

We first show in Sect. 13.3.1 that the space-based HL-P and energy-based HL-CI methods give comparable results in the description of the resonance of the allylic systems (cation, radical and anion), except for the radical case, where the HL-P method is more reliable as it respects symmetry. 

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. 

Unsaturation at the p Carbon. The SnI rates are increased when there is a double bond in the P position, so that allylic and benzylic substrates react rapidly (Table 10.5). The reason is that allylic (p. 221) and benzylic (p. 222) cations are stabilized by resonance. As shown in Table 10.5, a second and a third phenyl group increase the rate still more, because these carbocations are more stable yet. It should be remembered that allylic rearrangements are possible with allylic systems. 

X = Br, in 50% aqueous ethanol. The observed solvent w =. 44 value for the allenyl system is comparable to the. 455 m value of the allylic system. No products were observed, as neither the expected propargyl alcohol nor acrolein was stable under the reaction conditions. In analogy with the solvolysis of trisubstituted haloallenes (203, 204) these results were interpreted in terms of an SnI mechanism and ionization to an allenyl cation. However, an alternative mechanism involving the unsaturated carbene, C=C=C , cannot be completely ruled out in the case of the parent system. Such a mechanism has been unambiguously established by a number of investigators (206-209) for the solvolysis of R2C=C=CHX or HC C—C(R)2X in aqueous solvents in the presence of a variety of bases. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

The energy level diagram including electron populations for the allyl radical, cation, and anion can be shown as illustrated in Figure 5.16. The orbital diagram and energy levels for the allyl system is shown in Figure 5.17. 

The reactions of the vinylcarbenes 7 and 15 with methanol clearly involve delocalized intermediates. However, the product distributions deviate from those of free (solvated) allyl cations. Competition of the various reaction paths outlined in Scheme 5 could be invoked to explain the results. On the other hand, the effect of charge delocalization in allylic systems may be partially offset by ion pairing. Proton transfer from alcohols to carbenes will give rise to carbocation-alkoxide ion pairs that is, the counterion will be closer to the carbene-derived carbon than to any other site. Unless the paired ions are rapidly separated by solvent molecules, collapse of the ion pair will mimic a concerted O-H insertion reaction. 

Although both Sn2 and SnI mechanisms might be formnlated for such reactions, all the available evidence favours an Sn 1 process. This is rationalized in terms of formation of a favourable resonance-stabilized allylic cation by loss of the leaving gronp. In the majority of natnral prodnct structures, the nucleophile has attacked the allylic system on the same carbon that loses the diphosphate, bnt there are certainly examples of nncleophilic attack on the alternative tertiary carbon. 

Answer. The Lewis acidity depends on the interaction energy ( ) from the interaction of the LUMO of the acid with the HOMO of the nucleophile. The interaction is of a type, with the base HOMO (usually a nonbonded p or spn hybrid) interacting end on with the LUMO, which for the methyl cation is a single 2p orbital and for the allyl system is a linear combination of 2p orbitals. The LUMOs of the two systems are shown below. 

Figure 4.2 Hiickel MOs for the allyl system. One pc orbital per atom defines the basis set. Combinations of these 3 AOs create the 3 MOs shown. The electron occupation illustrated corresponds to the allyl cation. One additional electron in

Unfortunately, while it is clear that the allyl cation, radical, and anion all enjoy some degree of resonance stabilization, neither experiment, in the form of measured rotational barriers, nor higher levels of theory support the notion that in all three cases the magnitude is the same (see, for instance, Gobbi and Frenking 1994 Mo el al. 1996). So, what aspects of Hiickel theory render it incapable of accurately distinguishing between these three allyl systems ... 

The ring opening of cyclopropyl cations (pp. 345, 1076) is an electrocyclic reaction and is governed by the orbital symmetry rules.389 For this case we invoke the rule that the o bond opens in such a way that the resulting/ orbitals have the symmetry of the highest occupied orbital of the product, in this case, an allylic cation. We may recall that an allylic system has three molecular orbitals (p. 32). For the cation, with only two electrons, the highest occupied orbital is the one of the lowest energy (A). Thus, the cyclopropyl cation must... 

Another example is the allylic system. The allyl cation (8), anion (9), and... 

A second difficulty arises from consideration of allylic systems. Because the resulting cationic center is stabilized by interaction with the -it electrons, allylic halides ionize readily to produce the delocalized allylic ion, 2. The free ipn theoiy predicts that isomeric allyli halides that give the samp intermediate,-upon ionization should yield a product distribution independent of the isomeric origin of thp ion Scheme 2 illustrates the argument. The prediction is sometimes, but... 

The trimethylsilyl substituent in 2-(trimethylsilyl)allyl cations is enforced into a perpendicular conformation and thus fi-C—Si hyperconjugative stabilization is essentially not possible. The carbocationic centre Cl (<513C = 295.0 ppm) and the terminal methylene carbon C3 of the allyl system (<513C = 166.6 ppm) in 348a are deshielded by 11.5 and 1.2 ppm, respectively, compared with the silicon-free analogue 348 (613C = Cl 283.5 C3... 

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