Alkenes pentadienyl cations

The oxidative electrochemistry of phenols has been developed elegantly [51, 52]. The key intermediate is often the pentadienyl cation (96) that is formed after the loss of two electrons and one proton from (95) (Scheme 22). It can be intercepted by alkenes, at the terminal carbons of the pentadienyl array, to achieve a [5 + 2] cycloaddition (e.g. (96) to 97), or by a nucleophilic solvent such as methanol, leading to a conjugated diene (99). The... 

Another rare kind of 6-electron ionic cycloaddition is that between a pentadienyl cation and an alkene. A telling example is the key step 2.66 — 2.67 in a synthesis of gymnomitrol 2.68, where the nature of the pericyclic step is heavily disguised, but all the more remarkable for that. Ionization of the acetal gives the cationic quinone system 2.66. That this is a pentadienyl cation can be seen in the drawing of a canonical structure on the left, with the components of the pericyclic cycloaddition emphasized in bold. Intramolecular [4+2] cycloaddition takes place, with the pentadienyl cation as the 4-electron component and the cyclopentene as the 2-electron component. Th is reaction is an excellent example of how a reaction can become embedded in so much framework that its pericyclic nature is obscured. 

Diels-Alder reactions are classified as [4 + 2] cycloadditions, and the reaction giving the cyclobutane would be a [2 + 2] cycloaddition. This classification is based on the number of electrons involved. Diels-Alder reactions are not the only [4 + 2] cycloadditions. Conjugated ions like allyl cations, allyl anions and pentadienyl cations are all capable of cycloadditions. Thus, an allyl cation can be a 2-electron component in a [4 + 2] cycloaddition, as in the reaction of the methallyl cation 6.2 derived from its iodide 6.1, with cyclo-pentadiene giving a seven-membered ring cation 6.3. The diene is the 4-electron component. The product eventually isolated is the alkene 6.4, as the result of the loss of the neighbouring proton, the usual fate of a tertiary cation. This cycloaddition is also called a [4 + 3] cycloaddition if you were to count the atoms, but this is a structural feature not an electronic feature. In this chapter it is the number of electrons that counts. 

Yet another [4 + 2] cycloaddition, rather rare, is that between a pentadienyl cation and an alkene. The best known example is the perezone-pipitzol transformation 6.9 —> 6.11, where it is heavily disguised. It can be understood as beginning with an intramolecular proton transfer to give the intermediate 6.10, which can then undergo an intramolecular [4 + 2] cycloaddition with the pentadienyl cation, emphasised in bold, acting as the 4-electron component and the pendant alkene, also bold, as the 2-electron component. 

The Woodward-Hoffman rules also predict that in a given cyclization mode a permutation of alkene geometry must be reflected in the configuration of the products. This test is precluded under the normal reaction conditions (acid, light) which would isomerize the dienone double bonds. However, Corey recently reported the formation of a c -disubstituted cyclopentenone from a (Z, )-precursor, derived fiom an allene oxide, which cyclizes via the 2-oxido pentadienylic cation (Section 6.3.8). ... 

Protonation of dienol complexes (244) with strong acids (e.g. HBF4, HPFe, or HCIO4) generates the cisoid ( -pentadienyl)iron cations (248) (Scheme 69). This reaction is believed to proceed via initial formation of the transoid pentadienyl cation, followed by isomerization to the more stable cisoid form of the cation. Computational methods on the unsubstituted cationic complex (248, R = Nu = H) suggest that the cisoid form is 9.2 kcal mol more stable. Protonation of an alkene that is adjacent to a diene complex can also lead to the formation of ( -pentadienyl)iron cations. Additionally, hydride abstraction using the trityl cation also produces (jj -pentadienyl)iron cations the rich chemistry of these species will be described later (Section 7.1). 

Electrocyclic reactions are not limited to neutral polyenes. The cyclization of a pentadienyl cation to a cyclopentenyl cation offers a useful entry to five-membered carbocycUc compounds. One such reaction is the Nazarov cyclization of divinyl ketones. Protonation or Lewis acid complexation of the oxygen atom of the carbonyl group of a divinyl ketone generates a pentadienyl cation. This cation undergoes electrocyclization to give an allyl cation within a cyclopentane ring. The allyl cation can lose a proton or be trapped, for example by a nucleophile. Proton loss occurs to give the thermodynamically more stable alkene and subsequent keto-enol tautomerism leads to the typical Nazarov product, a cyclopentenone (3.220). 

The pericycHc process comes next and it is a Nazarov reaction (p. 927 of the textbook), a conrotatory electrocychc closure of a pentadienyl cation to give a cyclopentenyl cation. There is no stereochemistry and the only regiochemistry is the position of the alkene at the end of the reaction. It prefers the more substituted side of the ring. 

It is important to note that the selection rule in Table 11.1 refers to the number of electrons in the systems undergoing pericyclic change, not for the number of orbitals. Thus, the addition of an allyl anion to an alkene, the addition of an allyl anion to an allyl cation, and the addition of a pentadienyl cation to an alkene are all [4 -I- 2] cycloadditions (Figure 11.71). ... 

In iron reactions where the reagent was equivalent to C, described in Section 8.9.A, the iron moiety was used as an auxiliary. Iron can also stabilize cations, which then react with nucleophiles to generate new carbon-carbon bonds. 08 xhese cations are formed as iron-alkene complexes, usually by reaction of cyclo-pentadienyl dicarbonyl ferrate anion (478) with an allylic halide such as 3-chloro-2-methyl-l-propene. The... 

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