Anion Radical Chain Cycloaddition of Tethered Enones:  Intramolecular Cyclobutanation and Diels−Alder Cycloaddition

Abstract

[reaction: see text] The anion radicals of certain bis(enones), generated by cathodic reduction, are observed to participate in intramolecular cyclobutanation, yielding bicyclo[3.2.0]heptane derivatives through an anion radical chain mechanism. Evidence for stepwise cycloaddition involving distonic anion radical intermediates is presented. In addition to the novel anion radical cyclobutanations, an unprecedented intramolecular anion radical Diels-Alder product is observed. Parallel trends in substrate scope vis-à-vis the Co-catalyzed bis(enone) cyclobutanation are discussed.

Theoretical study of intramolecular anion radical cycloaddition of the phenyl-substituted bis(enone)

“…The EAs of cis-P, trans-P, and da-P are predicted to be 53.0, 55.8, and 56.7 kcalmol À1 , respectively, which are 7.4, 4.6, and 3.7 kcal mol À1 smaller than that of trans-R and 7.8, 5.0, and 4.1 kcal mol À1 smaller than that of cis-R, respectively, and 1.6-18.0 kcal mol À1 larger than those of the C 10 H 8 , C 14 H 10 , and C 18 H 12 molecules. Therefore, the electron transfer from the cis-P À , trans-P À , and da-P À anion radicals to the trans-R and cis-R neutral molecules is feasible energetically but to the C 10 H 8 , C 14 H 10 , and C 18 H 12 molecules is unfeasible energetically, which supports the chain mechanism of the cycloaddition of bis(enone) suggested by experimentalists [3].…”

“…2) and the formation of the C 1 -C 2 and C 3 -C 4 r bonds. Therefore, there exist three possible reaction paths: (I) a stepwise mechanism via a distonic anion radical intermediate (an intermediate with five-member ring) suggested by the experimentalists [3][4][5][6], (II) a stepwise mechanism via an intermediate with seven-member ring, and (III) a concerted reaction mechanism (forming the product only by one step). In order to determine the possibilities of above three kinds of reaction mechanisms, we selected isomers 1 and 2 (both having the smaller r 1 and r 2 values) of the R À anion radical as the reactants, respectively, and performed the B3LYP/6-31G(d,p) partial geometry optimization calculations along the three reaction paths.…”

“…In recent years, intramolecular anion radical cycloaddition reactions of a variety of bis(enones), as novel reactions, were reported from some laboratories [1][2][3][4][5][6] using metal-catalyzed [1,2], electrochemically promoted [3][4][5], and chemically induced [6] methods, respectively. These reactions are of special interest because they represent rare examples of stereoselective intramolecular anion radical cycloaddition, rather than the more commonplace cation radical cycloaddition.…”

“…Roh et al [3] and Felton and Bauld [4], using electrochemically promoted method, observed that the phenyl-substituted bis(enone) participates in intramolecular cycloaddition reactions, yielding the cis-cyclobutane (both benzoyl moieties being above the four-member ring), trans-cyclobutane (one benzoyl moiety being above the four-member ring and the other being the opposite), and Diels-Alder cycloaddition products, and they suggested a chain reaction mechanism for these pericyclic processes. According to their mechanism (see Fig.…”

Reticulated vitreous carbon as an electrode material

“…However, the overall yield was 76% with other polymeric products accounting for the mass balance. The authors proposed a mechanism involving the cycloaddition of an anion radical as an intermediate [62].…”