Radicals group transfer reactions

Radical combinations become important for chemi-excitation only when the product can undergo reactions other than stabilization and reformation of the reactants. The alternative reactions require that weaker bonds be present in the new molecule than in the one formed in the combination. The alkyl halides are useful in this respect. Elimination of HX is a competitive secondary reaction106 -110. Our final example of the use of chemi-excitation is drawn from this field111. It shows the application of three techniques group transfer reactions, radical combination, and methylene insertion... 

Mixed aryl selenides have also proven to be excellent ree ents for group transfer reactions.Photolysis of selenides in an inert solvent such as benzene can initiate chain reactions. Substituted radicals can be generated in this manner, from a-selenoe-... 

Intramolecular group-transfer reactions can involve either direct displacements or addition-fragmentation reactions that are discussed in Section 4.2.4. A direct displacement is shown in the reaction of aryl radical 1, produced from the corresponding bromide. Cyclization of 1 is fast enough to compete efficiently... 

Whereas additions of carbon radicals to alkene moieties are the best characterized homolytic additions, carbon radicals are known to add to a wide range of unsaturated systems. These include polyenes, alkynes, arenes, heteroarenes, carbon monoxide,isonitriles, °° ° nitriles, ° imines and derivatives, ° ° aldehydes,nitrones, and thiones. ° Many of these reactions, such as addition of an alkyl radical to a carbonyl group, ° are thermodynamically unfavorable and readily reversible, and they form the basis of composite group-transfer reactions discussed below. 

Propagation steps are the heart of any chain and generally fall into two classes atom or group transfer reactions and addition reactions to tr-bonds (or the reverse elimination). The rate of the chain transfer step is especially important in synthetic planning because, by fixing the maximum lifetime that radicals can exist, it determines what reactions will (or will not) be permitted. Termination steps are generally undesirable but are naturally minimized during chain reactions because initiation events are relatively uncommon. 

Almost all of the reactions of radicals can be grouped into three classes redox reactions, atom (or group) transfer reactions and addition reactions. A detailed discussion of these reactions is beyond the scope of this chapter, but a summary of some important features (with references to more in-depth discussions) is essential. Although addition reactions will receive the most attention, redox and atom transfer reactions are important because nearly all radicals formed by addition reactions will be removed from the radical pool to give nonradical products by one of these methods. 

A new radical chain group transfer reaction which does not involve tin reagents has been reported. The reaction proceeds by a photosensitized electron transfer reductive activation of PhSeSiR.3 using 1,5-dimethoxynaphthalene as the sensitizer [95ACIE2669]. In contrast to the tellurium transfer described above, the selenium transfer reaction gave higher diastereoselectivity (4 1 vs 2 1). 

Arenesulfonyl iodide and bromide are rather unstable compounds because of low bond dissociation energies of their S02-I and S02-Br. Therefore, treatment of p-tosyl bromide (47) with alkene or allene (48) produces 1,2-adduct (49) through the addition of the formed p-tosyl radical onto the allene as shown in eq. 4.19 [52]. Here, the p-tosyl radical attacks the central sp carbon of the allene group to generate the stable allylic radical, and then it reacts with p-tosyl bromide to give 1,2-adduct (49) and a p-tosyl radical again, i.e., chain pathway. So, this is also an atom(group)-transfer reaction. 

Renaud, P. and Abazi, S. (1996) Use of 0,Se-acetals for radical-mediated phenylseleno group transfer reactions. 

To achieve low radical concentrations, most radical reactions are traditionally performed as chain reactions. Atom or group transfer reactions are one of the two basic chain modes. In this process the atom or group X is the chain carrier. A metal complex can promote such chain reactions in two ways. On one hand, the catalyst acts only to initiate the chain process by generating the initial radical 29A from substrate 29 (Fig. 10). This intermediate undergoes the typical radical reactions, such as additions or cyclizations leading to radical 29B, which stabilizes to product 30 by abstracting the group X from 29. A typical example is the use of catalytic amounts of cobalt(II) salts in oxidative radical reactions catalyzed by /V-hydroxyphthalimide (NHPI), which is the chain carrier [102]. 

Selenium-containing molecules have also been used as precursors for radical seleno group transfer reactions. This is a very powerful method for radical additions to alkenes and alkynes it is especially interesting from an atom economy point of view since all atoms remain in the product molecule. The free-radical addition of selenosulfonates 146 can be initiated either photochemically or thermally using AIBN. The addition of 146 not only to alkynes 147,255-257 km also to alkenes258-261 or allenes,261 has been reported and the products such as 148 are versatile building blocks for subsequent reactions (Scheme 39). For example, vinyl selenides 148 can be easily transformed into allenes. 

The radical addition of selenomalonate 149 or the corresponding malononitriles are excellent substrates for phenylseleno group transfer reactions to alkenes and alkynes.263 Malononitrile 150 can be used for annulation and cyclization reactions (Scheme 40) 264,265... 

A photosensitized activation of carbon-selenium bonds was also used for performing phenylseleno group transfer reactions. This process involves a photosensitized electron transfer (PET) as the initial step in the reaction sequence. Fragmentation affords a radical and phenylselenolate, which is oxidized to diphenyl diselenide in the presence of oxygen. The cyclized radical is then trapped by diphenyl diselenide to afford the final product. This process is quite general for intramolecular radical reactions.70,266... 

The polar pathways are formally equivalent to a discrete electron-transfer step, that is, a pure SET step that is followed by a chemical step. If a hypothetical SET step is followed by coupling of a radical pair that is produced in the SET step, the overall reaction is the equivalent of a polar-group coupling reaction (Scheme 14(b)). If the coupling is accompanied by the elimination of a leaving group, a polar-group transfer reaction results (Scheme 14(a)). 

Although Bronsted proton transfer reactions appear to belong to a unique category not described by Scheme 14, they are examples of polar-group transfer reactions and are not different in principle from nucleophilic displacement reactions. Deprotonation by hydroxide ion can be regarded as the shift of an electron from HO to the Bronsted acid synchronously with the transfer of a hydrogen atom from the Bronsted acid to the incipient HO- radical, with the reaction driven by covalent bond formation between the HO- radical and the H- atom to form water (equation 161). 

The reaction of superoxide ion (02 )> radical anion, with water also can be viewed as a polar-group transfer reaction (equation 168). The product HOO- is a radical that reacts btmolecularly to form hydrogen peroxide and dioxygen (equation 169). 

Organoselenium compounds are very versatile radical precm-sors which are widely used. Due to their stability and ease of preparation, they offer imique advantages over organic halides as radical precm-sors. They can be utilized in tin mediated radical reactions as weU as in group transfer reactions for the formation of carbon-carbon bonds and carbon-heteroatom bonds. Selenols and diselenides have found applications as reducing agents and radical traps, respectively. A survey of these different reaction types wiU be given. Information about new reactions based on electron and photoelectron transfers wiU also be provided. 

Keywoids Radicals, Group transfer reactions. Photoelectron transfers, Selenosulfonation, Acyl radicals, Azidoselenenylation, Selenides, Selenol... 

Other selenoacetals have been prepared by means of radical phenylseleno group transfer reactions and by trapping of radicals with diphenyl diselenide, these procedures will be discussed later in this chapter (Sects. 3.1 and 5.2). 

Fig.l. Radical precursors for phenylseleno group transfer reactions... 

Nucleophilic substitution reactions. The view that substitution or displacement reactions that involve hydroxide ion are examples of polar-group-transfer reactions (with a single-electron shift) is probably the least iconoclastic proposal. Most accept the view that many nucleophilic displacement reactions occur by a SET mechanism.22 In a number of cases free-radical intermediates have been identified, which is consistent with a discrete SET step. Only a slight extension of this concept is required to encompass all nucleophilic reactions within the categories described in Scheme 8-1. 

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