Allyls stabilization

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]

From this value and known C—H bond dissociation energies, pK values can be calculated. Early application of these methods gave estimates of the p/Ts of toluene and propene of about 45 and 48, respectively. Methane was estimated to have a pAT in the range of 52-62. Electrochemical measurements in DMF have given the results shown in Table 7.3. These measurements put the pK of methane at about 48, with benzylic and allylic stabilization leading to values of 39 and 38 for toluene and propene, respectively. The electrochemical values overlap with the pATdmso scale for compounds such as diphenyl-methane and triphenylmethane. 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

At 210° C the equilibrium constant is 1-3. This isomerization may take place by a concerted mechanism or via the intermediate formation of an allylically stabilized species as shown below ... 

The similarity of the reaction leading to 1-methylcyclohexene to that of the isomerization of isopropenylcyclopropane to 1-methylcyclopentene suggests that analogous transition complexes are involved, i.e. an allylically stabilized biradical. Ring closure of the biradical yields the... 

While there is a small discrepancy between the two determinations of the energy of activation, it is clear that the value cannot be far from that for the decomposition of methylcyclobutane. Thus the presence of the double bond has little significant effect on the energetics of the process. It might have been expected that, since an intermediate biradical could be allylically stabilized, the decomposition should proceed through a relatively low-energy path, viz. ... 

The thermal isomerizations of other bicyclic systems containing a cyclobutane ring appear not to have been investigated kinetically in detail, with the exception of a-and )3-pinene. These isomerizations all probably proceed through allylically stabilized biradicals, but the systems are complex and the studies were carried out well before the advent of modem analjrtical techniques of gas analysis. It is doubtful therefore whether a detailed discussion is worth while before more precise data are available (see Trotman-Dickenson, 1955). 

This selectivity presumably reflects several circumstances. Both carbonyl oxygens are presumably complexed by aluminum. The allylic stabilization of the y-deprotonation product can then lead to kinetic selectivity in the deprotonation. Selectivity for / -attack by the dienolate is accentuated by the steric bulk near the a. position. 

For the thiophene case the combination of both allylic stabilization as well as electron donation from the sulfur atom is strong enough to favor (17) strongly over (18), in which only donation of electrons from the heteroatom will be a stabilizing factor. 

Among the earliest examples of symmetrical bifunctional radical cations, the distonic trimethylene species (103) invoked by Williams and coworkers [293, 296, 297] are stabilized solely by hyperconjugation. The main rationale for their formation would be the relief of ring strain. On the other hand, the non-vertical radical cations 137 derived from cyclopentadiene dimers [386-389] are favored by two elements of allylic stabilization. This radical cation has three eonformat-... 

For all of them [2+2] addition is the predominant route and endoperoxide 113 is formed only starting from the E,Z-isomer. The results have been rationalized in terms of an allylic-stabilized open zwitterionic intermediates whose rotation can compete with closure to dioxetanes [125a]. 

Ans. Methyl cyclohex-2-en l-one and methyl cyclohex-2-en l-ol. Allylic stabilization of R. ... 

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. 

That this mechanistic pathway is followed is of some interest since the 1-tri-quinacenyl cation (420) has been shown on the basis of semiempirical calculations to be a twofold allyl-stabilized bridgehead ion.389 This cation, as well as 421 and 422, could be generated by allowing the respective chlorides to react with antimony pentafluoride in S02C1F at —78 °C.388 The and 13C NMR spectra have been recorded and suggest that almost planar divinyl carbonium ion units are present in... 

Similarly, the isomerization of vinylcyclopropane has a lower activation energy than the reactions of saturated cyclopropanes (Table 4) a similar result is obtained with other substituted vinylcyclopropanes . Moreover, these reactions have frequency factors nearly two orders of magnitude lower than the cis-trans isomeriza-tions of saturated cyclopropanes. These results can be explained by the formation of an allylically stabilized biradical as the intermediate, viz. 

The mechanism involves either the intermediacy of a chemically activated, allylically stabilized biradical or a direct pathway without an intermediateThe thermal C3 - C4 ring expansion of methylenespiropentane leading to 1,2- and 1,3-dimethylenecyclobutane has been reported (cf. Section II.E, equation 21). 

The remaining p orbitals in the product are parallel to the newly formed bond. It is less clear in the reverse reaction, which follows the same path but in the opposite direction, that the lone pair orbitals of O and L must be aligned parallel to each other and to the breaking bond at the transition state. The orbitals are then lined up so that they can easily become the allylic pi system of the ester. If one of the lone pairs were not lined up, the allylic system could not be established at the transition state, and that transition state would be much higher in energy. Allylic stabilization is about 14 to 25 kcal/mol. 

From the kinetic data the authors also deduced that the magnitude of the allylic stabilization in the vinyIsilyl radical is less than 7 kcal mol"1. This can be compared to the benzylic stabilization energy of about 2 kcal mol 1 which was derived for the phenylsilyl radical previously studied191. 

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