Peroxyl Radicals in Synthesis

Abstract

This chapter covers organic compounds prepared via intermediates which are alkyl peroxyl radicals (ROO') generated either by addition of ground state (triplet) oxygen to carbon-centered radicals, or by hydrogen atom abstraction from the corresponding hydroperoxides (ROOH). While some earlier work is briefly mentioned, the focus is primarily on new developments, published over the past decade or so. The emphasis is heavily on synthetic applications rather than physical, biological and environmental aspects of peroxyl radical chemistry. A book exclusively devoted to the latter topics has recently been published [l]. Some preparatively relevant pathways involving peroxyl radicals have been previously discussed in the context of reviews on the oxidation of organic molecules [2-41 and the synthesis of organic peroxides [5][6][7][8]. The present survey is clearly distinguished in scope; it is intended to cover a more diverse array of peroxyl radical pathways and to highlight recent applications in the field of natural product synthesis. The aim is to alert not only the peroxide specialist, but also the synthetic chemist in general, since the first-formed peroxidic products can be used as relay intermediates in the synthesis of nonperoxidic compounds. Autoxidation of HydrocarbonsThe oxidation of hydrocarbons and other organic molecules with oxygen or air is usually referred to as autoxidation [4, 7a, 91. This entirely atom-economical process (Eq. 1, Scheme 1) is ca. 18-24 kcal/mol exothermic and irreversible at temperatures below 250°C. Although the initiation step (Eq. 2) is not well understood [2, lo], it is clear that a chain radical process is involved, with addition of molecular oxygen to R as the initial propagation step (Eq. 3 ) . At oxygen pressures above 100 mm Hg, the reaction of R' with 0 2 proceeds with rate constants approaching the diffusioncontrol limit [7a, 9a]. The second propagation step involves hydrogen atom abstrac- Radicals in Organic Synthesis Edited

Dual Behavior of Cationic Palladium Pentafluorophenyl Complexes as Catalysts for the Homopolymerization of Acrylates and of Nonpolar Olefins

“…In the absence of MA, both 5 and 7 decompose slowly at room temperature to give the β-H elimination product 6, H-transfer product PfCH subsequent reaction of the resulting radical with oxygen. 21 These decomposition routes were reported for the neutral complex 4 10 and support a radical chain growth for the polymerization of MA with these catalysts.…”

“…Intermediates Formed by Reaction of Methyl Acrylate with Brookhart's Catalysts subsequent reaction of the resulting radical with oxygen. 21 These decomposition routes were reported for the neutral complex 4 10 and support a radical chain growth for the polymerization of MA with these catalysts.…”

Synthesis of Cyclic Peroxides

“…The original reports by Porter and coworkers 152,153 and Pryor and Stanley 154 in 1975 on the homolytic cyclization of alkenyl hydroperoxides provided one of the most useful and highly utilized methods for the synthesis of endoperoxides 52,155,156 . This method is illustrated herein through the conversion of alkenyl hydroperoxide 152 into 3-hydroperoxyalkyl-1,2-dioxolane 156 (Scheme 37) 157 .…”

“…Since hydroperoxides cannot serve as good hydrogen atom donors to carbon-centered radicals, a mediating chain transfer agent is required. As found in many studies that were discussed in previous reviews, triplet oxygen is particularly useful for this purpose and was widely applied in synthesis 155,156 . Application of this methodology to hydroperoxide 152, that contains an allylic and a homoallylic tether, afforded the 3,5-cis-disubstituted 1,2-dioxolane hydroperoxide 156 in 20% isolated yield (Scheme 37) 157 .…”

Cobalt Hydride Oxidase Catalysis Enabled Hydroamination of Unactivated Olefins

“…This is due to the ground state triplet character of dioxygen, which leads to dominating oxygenation with rate constants approaching the diffusioncontrol limit. 6,7 A prominent example in the context of transition metal catalysis is the well-known Drago-Mukaiyama oxidation reaction. 8,9 Hydrogen atom transfer (HAT) from a putative cobalt hydride species to an alkene affords a nascent alkyl radical, which is rapidly intercepted by dioxygen and eventually furnishes ketones or alcohols in high yields.…”