On the mechanism of the [4+2] cycloaddition of 1Ο2 with 1,3-dienes: Identifying the putative biradical/dipolar intermediate by employing the gem-diphenylcyclopropyl group as a mechanistic probe

Abstract

respectively, through a photoinduced electron transfer mechanism, co-sensitized by methylene blue (MB) and thiourea. The structures of (Ε)-13 and (Ε,Ε)-14 were supported by experimental and theoretical NMR studies.

Evidence that Additions of Grignard Reagents to Aliphatic Aldehydes Do Not Involve Single-Electron-Transfer Processes

“…Our approach to discovering whether radical intermediates were present during the carbon–carbon bond-forming step of the addition of an allylmagnesium reagent to a nonaromatic aldehyde involved the use of a radical clock. The 2,2-diphenylcyclopropylcarbinyl system (Scheme ), one of the fastest radical clocks known, was chosen to maximize the chance of observing products derived from single-electron-transfer reactions. , The 2,2-diphenylcyclopropylcarbinyl radical undergoes ring opening at a rate of 5 × 10 11 s –1 , − and oxygen-containing substituents have been shown to have little effect on this rate. , Consequently, if a radical intermediate were formed, it should undergo ring opening at a rate that is competitive with the rate of geminate radical pairs undergoing recombination. , Cyclopropane-derived radical clocks have been used to provide evidence for radical intermediates. , These radical clocks have been used to identify ketyl radicals as intermediates in reactions of reagents such as SmI 2 and tributyltin hydride with aldehydes, ,, ketones, − esters, − amides, and carboxylic acids …”

“…If the addition of a Grignard reagent to an alkyl aldehyde proceeded by one-electron reduction of the aldehyde to form a ketyl radical and an alkyl radical and then recombination of these two radicals (Scheme ), the rate constant of the two radicals recombining would need to be faster than the rate of the particularly fast ring-opening rearrangement (Scheme ). , In this situation, because the second step is so fast, the stepwise reaction becomes effectively concerted. − …”

New Photochromic α-Methylchalcones Are Highly Photostable, Even under Singlet Oxygen Conditions: Breaking the α-Methyl Michael-System Reactivity by Reversible Peroxybiradical Formation

“…Because it is such an easily available oxidation and oxygenation (O versus O 2 -transfer) reagent, 1 O 2 has found numerous applications in organic synthesis [ 5 , 6 , 7 , 8 , 9 ]. Besides heteroatom oxidation, the following pericyclic reactions with unsaturated organic substrates are especially relevant: 2 + 2-cycloaddition with electron-rich monoalkenes, 4 + 2-cycloaddition with 1,3-dienes, and ene-reaction with monoalkenes or polyenes carrying allylic hydrogens [ 10 , 11 , 12 ]. Especially, the latter process has huge synthetic potential because allylic oxidation is one of the synthetically most relevant steps that allows the generation of allylic hydroperoxides, alcohols, epoxides, and many more [ 13 ].…”

Fundamental Studies of the Singlet Oxygen Reactions with the Potent Marine Toxin Domoic Acid

“…1 O 2 is continuously generated during daylight hours in natural surface waters, due to the presence of UV- and visible-light-absorbing components and dissolved O 2 . 1 O 2 has the potential as an oxidant for a solar-based water decontamination and disinfection processes , and is also used in a variety of processes from photodynamic cancer treatments to regio or stereoselective steps in the synthesis of natural products. − 1 O 2 reacts with dienes via [4 + 2] cycloaddition, ene, and/or [2 + 2] cycloaddition reactions . The common [4 + 2] cycloaddition of 1 O 2 yields modestly stable endoperoxides .…”

“…The ene reaction involves the abstraction of an allylic hydrogen by 1 O 2 to form the corresponding hydroperoxide, while the [2 + 2] reaction produces dioxetanes, which are unstable and typically collapse into their corresponding carbonyl compounds . While there is extensive literature on the products and general mechanisms of 1 O 2 reactions in organic solvents, − detailed product studies in aqueous media are limited. , …”