Lactone-Derived Carbon-Centered Radicals:  Formation and Reactivity with Oxygen

Abstract

[reaction: see text] Several lactones were examined to test the reactivity of carbon-centered radicals toward oxygen. Notably, the radical derived from 2-coumaranone (4) is unreactive toward oxygen, while 2-cuomaranone itself shows enhanced reactivity toward hydrogen abstraction by alkoxyl radicals. We propose that five parameters influence diminished reactivity toward oxygen, i.e., (a) benzylic resonance stabilization, (b) unpaired spin delocalization on oxygen, (c) favorable stereoelectronic effects, (d) electron-withdrawing effects, and (e) steric effects.

Stability of carbon‐centered radicals: Effect of functional groups on the energetics of addition of molecular oxygen

“…Using categories defined in previous work, particularly by Scaiano et al, 7,[9][10][11] we are able to make the following generalizations: (1) vinyl is a more powerful functional group than phenyl, because allyl radical distributes spin density more effectively than benzyl, where it tends to concentrate on the benzylic (exocyclic) carbon; (4) simple ortho-, meta-, and para-effects using the À ÀNO 2 group are minor, when considering stabilization of the benzyl radical; (5) lactones and to a lesser extent isolactones are important stabilizing groups, but benzolactones offer only a small improvement over lactones; (6) captodative stabilization provides a very strong radical stabilizing force, and the carbonyl acceptor seems to be special in this respect; (7) starting from cyclohexadienyl radical, spin delocalization onto a heteroatom is a useful design strategy; (8) the improvement in desired properties by using stereoelectronic effects in our examples (i.e., fluorene) is small, relative to unconstrained phenyl groups. Thus, stereoelectronic effects using the fluorenyl group are weak, although introduction of an oxygen into a six-membered ring adjacent to benzenes results in somewhat better stabilization; (9) considering all of the aforementioned effects, we have tabulated Gibbs free energies of addition for a variety of molecules, approximately half of which will be resistant to the addition of molecular oxygen.…”

“…9 Category A consists of simple substituents surrounding a carbon radical. B has conjugative delocalizing groups and their (oxygen) heteroatom analogues.…”

Mechanophores with a Reversible Radical System and Freezing‐Induced Mechanochemistry in Polymer Solutions and Gels

“…Thework of Cheng and co-workers supported this observation because the molar mass of polystyrene calculated from SEC decreased by freezing the solution but recovered by heating to 100 8 8C. [37][38][39] Because of these distinctive properties,t his radical system enables in situ quantitative evaluation of polymer chain scission through electron paramagnetic spectroscopic (EPR) studies,u nlike previously reported mechanophores. [31,32] These studies suggest that freezing polymer solutions does not generate polymer chain scission but conformational changes.T herefore,w hether freezing polymer solutions actually induces mechanical force along the polymer chains and if so,t he magnitude of the force generated, remains unclear.…”

Supramolecular photochemistry in zeolites: From catalysts to sunscreens

“…The probe was also used to study the formation and percolation of carbon-centered radicals produced by thermolysis of the size-excluded dimer, 3,3'-diphenyl-3H,3H'-[3,3']bisbenzofuranyl-2,2'-dione (Scheme 9). The resulting 3-phenyl-2-coumaranone radicals (PC • ) are able to penetrate the zeolite pores, and the documented persistency of these radicals ensures that they will survive to migrate through the zeolite pore structure [122][123][124][125][126]. A fluorescence increase over time was observed as PC • radicals penetrate the zeolite pores, and encounter and couple with the embedded prefluorescent probe.…”