Transitions of Electron Demand in Pericyclic Reactions: Normal, Neutral, and Inverse Diels–Alder Reactions of Polyhalogenated Cyclopentadienes

Abstract

Normal, neutral, and inverse-type Diels-Alder reactions can be observed in [4+2) cycloadditions of polyhalogenated cyclopentadienes la-l i with various aryl-substituted cyclic and open-chain dienophiles, and characteristic gradations of reactivity associated with the different mechanistic types are often displayed. Reaction rate constants and semiempirically derived HOMO and LUMO values for the reactants suggest that neutral [4+2] cycloadditions should be regarded as a transition between normal and inverse Diels-Alder reactions.

Diels–Alder reaction of fused pyran-2-ones with ethyl vinyl ether

“…OEt ( ) n X NHCOPh the diene system (both ends in 1h bear carbonyl substituents), thus making the cycloaddition more like those with neutral electron demand (which are generally known to proceed poorly) [51].…”

Scandium(III) Zeolites as New Heterogeneous Catalysts: [4+2]Cyclocondensation of in situ Generated Aryl Imines with Alkenes

“…[19d, 31,34] However, evidence for a stepwise cationic mechanism has been reported. [35] Organic reactions performed in zeolites could occur with mechanisms similar or different from the same reaction in homogeneous conditions, as observed for Cu I -zeolite-catalyzed reactions.…”

Readily Accessible Ambiphilic Cyclopentadienes for Bioorthogonal Labeling

“…14,15 Sauer’s detailed reports on the synthesis and reactivities of substituted cyclopentadienes, however, have gone relatively unrecognized. 39,40 The highly reactive, self-orthogonal, and ambiphilic properties of the tetrachlorocyclopentadiene ketal (TCK) shown in Scheme 3 and described by Sauer attracted our attention to it as a potential bioorthogonal diene. 39,40 Ambiphilic molecules can react as electrophiles or nucleophiles, depending on the reaction partner.…”

“…39,40 The highly reactive, self-orthogonal, and ambiphilic properties of the tetrachlorocyclopentadiene ketal (TCK) shown in Scheme 3 and described by Sauer attracted our attention to it as a potential bioorthogonal diene. 39,40 Ambiphilic molecules can react as electrophiles or nucleophiles, depending on the reaction partner. TCK, being ambiphilic, is able to react with both electron-deficient dienophiles, such as maleic anhydride, and electronrich dienophiles, such as (1Z,5Z)-cycloocta-1,5-diene.…”