Conjugated systems allylic cations

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

Delocalization (Section 10.5) A spreading out of electron density over a conjugated tt electron system. For example, allylic cations and allylic anions are delocalized because their charges are spread out over the entire 77 electron system. 

With unsymmetrical dienes (74a and 74b) and unsymmetrical adducts, the problem of orientation of addition (cf. p. 184) arises. Initial attack will still be on a terminal carbon atom of the conjugated system so that a delocalised allylic intermediate is obtained, but preferential attack will be on the terminal carbon that will yield the more stable of the two possible cations i.e. (75) rather than (76), and (77) rather than (78) ... 

Figure 13.3 The n molecular orbitals of the allyl cation. The allyl cation, like the allyl radical, is a conjugated unsaturated system. The shapes of molecular orbitals for the allyl cation calculated using quantum mechanical principles are shown alongside the schematic orbitals. 

The reaction of carbenes with alcohols can proceed by various pathways, which are most readily distinguished if the divalent carbon is conjugated to a tt system (Scheme 5). Both the ylide mechanism (a) and concerted O-H insertion (b) introduce the alkoxy group at the originally divalent site. On the other hand, carbene protonation (c) gives rise to allylic cations, which will accept nucleophiles at C-l and C-3 to give mixtures of isomeric ethers. In the case of R1 = R2, deuterated alcohols will afford mixtures of isotopomers. 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

At the same time, delocalization of unpaired spin in the free-radical product appears to be important for the course of the substitution reaction. For example hydrogen shift in sabinene radical cation 39a leads to a conjugated system (40 ) nucleophilic attack on l-aryl-2-alkylcyclopropane radical cations 43 or 47 produces benzylic radicals nucleophilic attack on 39a generates an allylic species and attack on the tricyclane radical cations 55 or 56 forms tertiary radicals. Apparently, formation of delocalized or otherwise stabilized free radicals is preferred. 

All the other cycloadditions, such as the [4+2] cycloadditions of allyl cations and anions, and the [8+2] and [6+4] cycloadditions of longer conjugated systems, have also been found to be suprafacial on both components, wherever it has been possible to test them. Thus the trans phenyl groups on the cyclopentene 2.65 show that the two new bonds were formed suprafacially on the rrans-stilbene. The tricyclic adducts 2.61, 2.77, 2.79, and 2.83, and the tetracyclic adduct 2.82, show that both components in each case have reacted suprafacially, although only suprafacial reactions are possible in cases like these, since the products from antarafacial attack on either component would have been prohibitively strained. Nevertheless, the fact that they have undergone cycloaddition is important, for it is the failure of thermal [2+2], [4+4] and [6+6], and photochemical [4+2], [8+2] and [6+4] pericyclic cycloadditions to take place, even when all-suprafacial options are open to them, that is significant. 

There are other stereochemical features which have nothing to do with the symmetry of the orbitals, and are much less powerfully controlled. In many cycloadditions, there are two possible all-suprafacial approaches one having what is called the extended transition structure 2.102, in which the conjugated systems keep well apart, and the other called the compressed 2.103, where they lie one above the other. Both are equally allowed by the rules that we shall see in Chapter 3, but one will usually be faster than the other. This type of stereochemistry applies only when the conjugated systems have at least three atoms in each component it is therefore only rarely a consideration. It shows up in the cycloadditions of allyl cations to dienes, where the two adducts 2.56 and 2.57 on p. 13 are the result of the compressed transition structure 2.104 and the extended 2.105, respectively, with the former evidently lower in energy. 

Fig. 4.2 illustrates the first few members of the series of equilibria of conjugated ions. In cations, they are the equilibria between the allyl 4.11 and the cyclopropyl cation 4.12, the pentadienyl 4.13 and the cyclopentenyl cation 4.14, and the heptatrienyl 4.15 and cycloheptadienyl cation 4.16, In anions, they are between the allyl 4.17 and the cyclopropyl anion 4.18, the pentadienyl 4.19 and the cyclopentenyl anion 4.20, and the heptatrienyl 4.21 and cycloheptadienyl anion 4.22. There are heteroatom-containing analogues, with nitrogen and oxygen lone pairs rather than a carbanion centre, and the systems can again have substituents and fused rings. 

Related compounds, cyclo[2.2.3]azine and l,2,3,4-dibenzocyclo[2.2.3]azine, also give ion radicals with peripheral 77-clcctron conjugate systems (Gerson et al. 1973 Mat-sumoto et al. 1996). The difference in conjugation between neutral molecules and their ion radicals can be additionally traced in the case of keto-enol tautomerizm. As a rule, enols are usually less stable than ketones. Under equilibrium conditions, enols exist only at very low concentration. However, the situation is different in the corresponding cation radicals, where gas-phase experiments have shown that enol cation radicals are usually more stable than their keto tautomers. This is due to the fact that enol cation radicals profit from allylic resonance stabilization that is not available to ketones (see Bednarek et al. 2001 and references therein) see Scheme 3-58 ... 

Conjugated compounds undergo a variety of reactions, many of which involve intermediates that retain some of the resonance stabilization of the conjugated system. Common intermediates include allylic systems, particularly allylic cations and radicals. Allylic cations and radicals are stabilized by delocalization. First, we consider some reactions involving allylic cations and radicals, then (Section 15-8) we derive the molecular orbital picture of their bonding. 

The energetic effects of conjugation are largest when empty or half-empty p-orbitals interact with a 7r-system. Typical examples include allyl cations or allyl radicals, respectively. In these cases, the allylic stabilization was estimated to be 20 kcal/mol.26 In comparison, the effect on neutral, closed-shell molecules is relatively small. The conjugative effect on the rotation of 1,3-butadiene 2 is, for example, with 3 kcal/mol much smaller. 

The energies for the molecular orbitals for these two extremes are shown in Fig. 2.3. The true orbital energy for the orbitals of acrolein must be in between those of the corresponding orbitals of the allyl cation and butadiene. We can perhaps expect the true structure to be more like the butadiene system than the allyl cation system (for the same reason that we prefer to draw it as 2.1 rather than 2.2). We can see that the effect of having a Z-substituent conjugated with the double bond of... 

The products obtained from addition to conjugated dienes arc always consistent with the formation of the most stable intermediate carbonium ion an allyl cation. This requires the first step to be addition to one oj the ends of the conjugated system. 

Similar to the allyl cation the stabilization in the allenyl cation 1 occurs by overlap of the incipient vacant p orbital with the allenyl 7r-system as shown in 5. The allylic TT-orbitals are geometrically constrained to the most favored geometry for overlap with the p orbital due to the orthogonality of the two double bonds and the conjugation is not accompanied by any loss of ground state conjugation. 

The Allyl System. Another conjugated system we shall need later on is that of the allyl cation (24), allyl radical (25), and allyl anion (26). These three reactive intermediates all have the same orbitals, but different numbers... 

The similar order of magnitude of the reactivities of methyl-substituted 1,3-dienes (Table 4) which depended on the number but not on the position of the substituent was strong evidence that allyl cations serve as reaction intermediates in these reactions. The rate decrease with increase in the ring size of the cycloalkadienes was attributed to the increased deviation of the jr-system from planarity. The reactivities of 1,3-dienes deviated markedly from the roughly linear relationship between the rates of proton and carbenium ion additions to alkenes. These deviations were ascribed to abnonnally low reactivity of the conjugated jr-systems. although this interpretation was inconsistent with the similar behavior of alkenes and dienes in the structure-reactivity relationship for hydration . 

Alternatively, interconversion between the stereoisomeric allyl cations can take place by capture of a nucleophile at either end, followed by rotation about the more or less normal single bond, and then regeneration of the cation by ionisation. Interconversion between the corresponding anions can take place similarly by cr coordination (771) to a metal at one end or the other. Because of the availability of these pathways, experimental measurements of the barrier to rotation have confirmed that it is less than the very approximate theoretical value of 116 kJ mol-1 (28 kcal mol ). Furthermore, measurements have generally been made on significantly more substituted systems. Such substitution can stabilise the filled, half-filled or empty p orbital, or the double bond, even when these components are no longer conjugated, and so appropriate substituents lower the barrier to rotation. 

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