On the Formation of “Free Radicals” from Alkylcobalt Complexes

Abstract

flections may have suffered a phase change so that the information in the appropriate frequency range is transferred with reversed contrast. The second stage in image analysis involves image reconstruction, where the electron micrograph now serves as object. Filtering of Fourier frequencies in reciprocal space (secondary diffraction pattern) is performed generally with the aid of a computer. The various stages of frequency filtering are very complex and must be treated with careful consideration of the physics involved. Otherwise, artifacts can easily be introduced. The safest way to guard against this is to compare the original image (which must be of very high quality) with the computer-treated image, and to ensure that defects have not been introduced which were not in the original. Details regarding these procedures will be published elsewhere [16].

Transition Metal-Catalyzed Organic Reactions under Visible Light: Recent Developments and Future Perspectives

“…Cobalt­(III)–carbon bonds have relatively low bond energy compared with other TM–carbon bonds, resulting in facile photohomolysis to generate a carbon radical and reduced Co­(II) complex under irradiation . In 1990, Scheffold used vitamin B 12a as catalyst for the synthesis of the prostaglandin F 2α core 144 through visible-light-induced homolysis of an alkyl Co­(III) intermediate to deliver an alkyl radical .…”

Reactions of free radicals with η3-allylpalladium(II) complexes: cyclohexyl radicals

“…Cobaloximes, RCo(DMG) 2 (L), generally have rather weak Co-C bonds which undergo homolytic cleavage on irradiation by visible light to form alkyl radicals R Å and the cobalt(II) compound Co(DMG) 2 (L). Thus cyclohexyl radicals are formed by visible light photolysis of the cobaloxime, D [15], and have been shown to exhibit chemical properties identical to those of cyclohexyl radicals generated from other methods of radical production [25].…”

Mechanism of Action of Vitamin B12. Ultrafast Radical Clocks Provide No Evidence for Radical Intermediates in Cyclopropane Models for the Methylmalonyl-CoA to Succinyl-CoA Carbon Skeleton Rearrangement

“…An alternative pathway is that part or all of the organocobalt intermediate is formed by direct S N 2 substitution of 10 with B 12s to give 14 in competition with dissociative electron transfer. Direct substitution is the expected pathway for normal primary halides, but neopentyl halides such as 10 probably react through the electron-transfer route 5 Reactions of the B 12 model. …”

“…There is some evidence in the literature to suggest that organocobalt species 15 may not be the product of the 1,2-migration. Related cobalt intermediates with simpler ligands are proposed in cobalt-mediated additions of radicals to alkenes . These intermediates tend to undergo competing protonation and disproportionation in ratios that are pH dependent.…”