Radical Processes Carbon-Heteroatom Bond Formation

Radical Processes Carbon-Heteroatom Bond Formation... 

Photocycloaddition and photoaddition can be utilized for new carbon-carbon and carbon-heteroatom bond formation under mild conditions from synthetic viewpoints. In last three decades, a large number of these photoreactions between electron-donating and electron-accepting molecules have been appeared and discussed in the literature, reviews, and books [1-10]. In these photoreactions, a variety of reactive intermediates such as excimers, exciplexes, triplexes, radical ion pairs, and free-radical ions have been postulated and some of them have been detected as transient species to understand the reaction mechanism. Most of reactive species in solution have been already characterized by laser flash photolysis techniques, but still the prediction for the photochemical process is hard to visualize. In preparative organic photochemistry, the dilemma that the transient species including emission are hardly observed in the reaction system giving high chemical yields remains in most cases [11,12]. 

Our review of the use of organoboron compounds in radical chemistry will concentrate on applications where the organoborane is used as an initiator, as a direct source of carbon-centered radicals, as a chain transfer reagent and finally as a radical reducing agent. The simple formation of carbon-heteroatom bonds via a radical process is not treated in this review since it has been treated in previous review articles [3,9]. 

The photocatalytic generation of aryl radicals was also successfully applied to the formation of carbon heteroatom bonds. The aryl pinacolboronates 141 can be easily achieved by visible light irradiation of a solution of aryl diazonium salts 142 and diboron pinacol ester 143 containing 5mol% of eosin (Scheme 29.24) [89]. The proposed meehanism involves the addition of aryl radical 144 to the complex 145 that is generated by interaction of tetrafluoroborate anion and diboran pinacol ester 143. This process leads to the formation of the aryl pinacolboronates 141 and the radical anion intermediate 146. The oxidation of this intermediate by eosin radical cation completes the catalytic cycle. 

In this article we provide a broad overview of the application of radical methods in carbohydrate chemistry, including typical examples classified by the type of bond formed. The factors controlling the stereoselectivity of inter- and intramolecular C-C bond formation are now well understood and have been exploited in the synthesis of C-glycosides [2]. Intramolecular C C bond formation using carbohydrate-based chiral templates also provides a powerful route to branched-chain sugars [3] and carbocycles [4]. Finally, we include synthetically useful processes involving key carbon-heteroatom and C-H bond formation. 

Since the pioneering studies of Bunnett [3], the scope of the unimolecular radical nucleophilic substitution (SrnI) reaction has increased considerably, and today this approach is well established for the formation of aryl-carbon and aryl-heteroatom bonds. The SrnI reaction is a chain process which includes radicals and radical anions as intermediates the reaction mechanism is depicted in Scheme 13.1 [1]. 

A weak C—X bond is required to facilitate the initiation step, but the success of the reaction depends on the subsequently formed C-heteroatom bond being stronger than the one broken in the initial reactant. In other words, to ensure the chain process, the initially generated radical should be more stable than the one formed after the addition. It is also critical that the halogen atom-transfer step be suffi-ciendy rapid to propagate the chain. The usual processes in ATR reactions involve a-carbonyl radicals leading to the formation of lactones, lactams, or cycloalkanones, which embody a new carbon-halogen bond after the transfer step. 

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