Cycloadditions with Polar Intermediates

Abstract

is frequently offset by the disadvantage that they form stable nitrogen addition products, thus forestalling a C reaction. It is often difficult, on the other hand, to find agents of sufficiently high electrophilic potential to react with enol ethers without initiating polymerization. Thus, the two classes of compounds complement each other well in their reactions as derivatives of ketones and aldehydes. C-Substitution reactions with phosgene, oxalyl chloride, and sulfonyl isocyanates, for example, which proceed very smoothly with enol ethers, are impracticable with enamines because of the formation of comparatively stable Nreaction products. Halogenation, and, in general, reactions tending to proceed via free-radical rather than polar mechanisms are also easier to carry out with en01 ethers than with enamines. Since, in addition to the electrophilic and free-radical reactions discussed here, enol ethers may also react with nucleophiles, there is a wide field for their preparative use.We are grateful to the Deutsche Forschungsgerneinschaft and the Fonds der Chemischen Industrie for the support of our work on enol ethers.

Synthesis of 1,2‐Dioxetanes as Thermochemiluminescent Labels for Ultrasensitive Bioassays: Rational Prediction of Olefin Photooxygenation Outcome by Using a Chemometric Approach

“…It is known that the formal [2 + 2] cycloaddition of singlet oxygen to alkenes takes place efficientlyo ne lectronrich systems. [29] However, the mechanism of this process has been extensively debated in the literature (Scheme 3), [30] and the most reliable one can vary case by case, depending on the molecular structure of the substrate, [30a,d-i,k] the solvation energy, [30a,e,i] and also the sensitizer employed. [30b,c,j,l] As ac onsequence, the natureo ft he olefin substituents (electron-donating ability,p resence of hydrogen-bond acceptors and/or donors, geometry,a nd steric hindrance) not only affects the successo fs inglet oxygen addition, but it also determines the reactionpathway.…”

A Mechanistic Spectrum of Chemical Reactions

“…2 , gives stable 1,4-dipolar compounds [18]. Some enamines react with TCNE to generate stable 1,4-dipoles at −40 °C which lose HCN at 20 °C, giving tricyanobutadiene [19].…”

Tropone Is a Mere Ketone for Cycloadditions to Ketenes

“…While the frontier-molecular-orbital (FMO) theory predicts the concerted addition path, the kinetic measurement suggested that the reaction between 1 and diphenyl ketene (2b) proceeds in a stepwise process including a zwitterionic intermediate 4b (Scheme 2) [8]. Thus, there is a dilemma concerning the reaction mechanism: either the symmetry-allowed concerted cycloaddition (Scheme 1) or the stepwise path including a zwitterionic intermediate (Scheme 2) operates.…”

“…The term [8 2] CC denotes a head-to-head addition path where the CC bond of 2 provides two electrons. For R Ph, the product 3b was identified first by Gompper and co-workers [7] [8]. In the first work on the reaction between 1 and 2b, structure I was assigned to the [2 2] cycloadduct through the C(2)ÀC(3) bond of 1 [9].…”

“…Thus, tropone (1) acts as a mere ketone for ketenes, which represents an unprecedented reaction in tropone chemistry [17]. As illustrated in Scheme 2, the formation of a zwitterionic intermediate has been suggested by kinetics [8]. But the intermediate species is the b-lactone 5, and the zwitterion is a transient species in the course of the isomerization 5 3 3.…”