Electrocyclic reactions pentadienyl-cyclopentenyl cation

A pentadienyl cation has the same number of ji-electrons as the allyl anion, and its electrocyclic reactions will be conrotatory. In terms of the Woodward-Hoffmann rule, it can be drawn 4.82 as an allowed [K4a] process. It has been shown to be fully stereospecific, with the stereo isomeric pentadienyl cations 4.83 and 4.85 giving the stereoisomeric cyclopentenyl cations 4.84 and 4.86 in conrotatory reactions, followed in their NMR spectra. 

An electrocyclic reaction is the formation of a new o-bond across the ends of a conjugated 7T-system or the reverse. They thus lead to the creation or destruction of one a-bond. Hexatrienes 1 can cyclise to six-membered rings 2 in a disrotatory fashion but we shall be more interested in versions of the conrotatory cyclisation of pentadienyl cations 3 to give cyclopentenyl cations 4. The different stereochemistry results from the different number of rt-electrons involved.1... 

The Nazarov cyclization is an example of a 47r-electrocyclic closure of a pentadienylic cation. The evidence in support of this idea is primarily stereochemical. The basic tenets of the theory of electrocyclic reactions make very clear predictions about the relative configuration of the substituents on the newly formed bond of the five-membered ring. Because the formation of a cyclopentenone often destroys one of the newly created centers, special substrates must be constructed to aUow this relationship to be preserved. Prior to the enunciation of the theory of conservation of orbital symmetry, Deno and Sorensen had observed the facile thermal cyclization of pentadienylic cations and subsequent rearrangements of the resulting cyclopentenyl cations. Unfortunately, these secondary rearrangements thwarted early attempts to verify the stereochemical predictions of orbital symmetry control. Subsequent studies with Ae pentamethyl derivative were successful. - The most convincing evidence for a pericyclic mechanism came from Woodward, Lehr and Kurland, who documented the complementary rotatory pathways for the thermal (conrotatory) and photochemical (disrotatoiy) cyclizations, precisely as predicted by the conservation of orbital symmetry (Scheme 5). 

The pericyclic process comes next and it is a Nazarov reaction (p. 962), a conrotator electrocyclic closure of a pentadienyl cation to give a cyclopentenyl cation. There is r stereochemistry and the only regiochemistry is the position of the double bond at the end of th -. reaction. Here it prefers the more substituted side of the ring. 

Pentadienyl cations can undergo electrocyclization to cyclopentenyl cations. As this is a four Tt-electron system, it should occur by conrotation. Based on gas phase ion stability data, the reaction is exothermic by 18 kcal/mol. ... 

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 conversion of a pentadienyl to a cyclopentenyl cation is an electrocyclic reaction of a system containing loury-electrons delocalized over five atomic orbitals, and should therefore be a csorotatory process. In the example just described, the stereochemistry of the cyclization reaction is masked by subsequent rearrangement of (alkj l groups, which ultimately affords the most stable carbonium ion. The stereochemistry of the cyclization of divinyl ketones under acidic conditions, however, is conrotatory, as predicted ... 

For the protic acid catalyzed reaction, the cyclization commences with protonation of the divinyl ketone 9 and formation of a pentadienyl cation 10. An analogous process is operational in the case of Lewis acid-catalyzed reactions. The pentadienyl cation 10 then undergoes a 47t electrocyclic closure to give a cyclopentenylic cation 11. This cyclization is a pericyclic reaction and is governed by the rules for conservation of orbital symmetry. Namely, this means the cyclization occurs stereospecifically in a conrotatory fashion, with predictable relative configurations of the substituents (i.e., the R groups in 11 are anti). Elimination of a proton, followed by tautomerization gives product 13. ... 

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