Pericyclic reactions allyl system

The four-electron system including an alkene Jt-bond and an allylic C-H o-bond can participate in a pericyclic reaction in which the double bond shifts and new C-H and C-C o-bonds are formed. This allylic system reacts similarly to a diene in a Diels-Alder Reaction, while in this case the other partner is called an enophile, analogous to the dienophile in the Diels-Alder. The Alder-Ene Reaction requires higher temperatures because of the higher... 

The ene reaction is another pericyclic reaction which involves the combination of an alkene with an allylic system and a hydrogen atom transfer (Scheme 3.20). 

As is the case for other pericyclic reactions, the selection rules for a thermal [i, ] sigmatropic reaction are reversed for the photochemical reaction. If irradiation of a 1,5-hexadiene produces the electronically excited state of one and only one of the two allyl components, then the HOMO of one component is (/f3, and the HOMO of ihe other component is suprafacial-suprafacial reaction (Figure 11.46) is forbidden (as is the antarafacial-antar-afacial pathway), but the antarafacial-suprafacial and suprafacial-antarafacial pathways are allowed (Figure 11.47). Analysis of higher sigmatropic reactions shows that the selection rules also reverse with the addition of a carbon-carbon double bond to either of the n systems. Thus, the [3,5] sigmatropic reaction is thermally allowed to be suprafacial-antarafacial or antarafacial-suprafacial and photochemically allowed to be suprafacial- suprafacial or antarafacial-antarafacial. Two of these reaction modes are illustrated in Figure 11.48. 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

Aromatic k systems are well-known participants in various pericyclic reactions, and aromaticity of substrates is often transiently disrupted in these transformations (e.g., Claisen rearrangement of O-allyl phenol). In most cases, a pathway for rearomatization exists (such as tautomerization) that ultimately leads to substituted aromatic products. In other instances, dearomatization upon pericyclic rearrangement results in formation of stable ahcyclic products, and this tactic has been successfully employed in several synthetic studies. 

The ene reaction is an important pericyclic reaction that is in some ways difficult to classify (Eq. 15.46). The reaction involves the migration of an allylic C-H bond to one end of an olefin, while the other end of the olefin forms a C-C bond to the opposite end of the allylic system. There are several different ways to think about the ene reaction. It resembles a sig-matropic shift, where the hydrogen shifts through space to the enophile (the hydrogen acceptor). If we draw "virtual" bonds between the partners (shown as dotted lines in the two reactions of Eq. 15.47), then we can view the reaction as either a [1,5] or a [3,3] shift. Alternatively, the reaction can be viewed as a cycloaddition, in which the allylic C-H bond plays the role of the second double bond in the diene. Either way, it is a six-electron cyclic process that can be drawn to conform to any of the theories given in this chapter. 

The reactant and product in a reaction of this type are isomers formed by alternative possible modes of combination of two conjugated systems. Thus the reactant and product in equation (5.304) can both be formed by combination of (R- -h allyl ) and those in equation (5.305) by alternative modes of dimerization of allyl. We will first consider the case where both the systems in question are odd, so that the reactant and transition state are even. Pericyclic reactions of this kind will be of EEj type. 

The studies of such closely related structures also ruled out any possible steric effects and the driving influence for reaction rate enhancement has to be seen in the oxygen atom in the y-allylic position (C6 of the Claisen system. Scheme 14). In previous reports [23], Carpenter and Burrows had developed a model to predict the influence of substituents on various pericyclic reactions based on Hiickel orbital energy calculations. According to this approach, a n-... 

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. 

Cycloaddition reactions involve the combination of two molecules to form a new ring. Concerted pericyclic cycloadditions involve reorganization of the Tr-electron systems of the reactants to form two new a bonds. Examples might include cyclodimerization of alkenes, cycloaddition of allyl cation to an alkene, and the addition reaction between alkenes and dienes (Diels-Alder reaction). 

So far we have considered only reactions in which the pericyclic ring contains an even number of atoms. Reactions of this kind are, however, known in which an odd-numbered ring is involved. A simple example is the Diels-Alder-like addition of 2-methylallyl cation (148) to cyclopentadiene (149) to form the methylbicyclooctyl cation (150). The transition state for this reaction is easily seen to be of Hiickel type (151) and so isoconjugate with tropylium. Since the allyl cation contains only two n electrons, we are dealing here with a six-electron system isoconjugate with the tropylium cation (147) and hence aromatic. In reactions of this kind, both the reactants and the transition state are odd. The reactions are therefore of 001 type. Since, moreover, the aromaticity or antiaromaticity of the transition state is again unrelated to the structures of the reactants or products, the reactions are of anti-BEP type and are consequently classed as 00 J. 

---