Cyclopentenyl cations, formation from pentadienyl

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). 

Pentadienyl cations (3) cyclise thermally to cyclopentenyl cations (4) in a conrotatory fashion. The most important example of this reaaion is the formation of cyclopentenones (8) from dienones (5) via cations (4) and (7). 

The cationic reaction pathway also allows for predictable effects on differential substrate substitution. In the rate-limiting step, the distribution of charge (marked by ) changes from C(l), C(3) and C(5), in pentadienylic cation 18, to C(2) and C(4), in cyclopentenylic cation 19. Therefore, substituents that stabilize positive charge (electron-donating groups), accelerate the reaction in the a-position (R ), or decelerate the reaction in the p-position (R ). Classically, under rather vigorous conditions, the reactions are under thermodynamic control and result in the formation of product 20, where the double bond occupies the most substituted position (Saytzeff s Rule). 

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