Addition-fragmentation reactions

Chart 10.6 Allylic compounds employed in addition-fragmentation reactions. 

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Benzo[Z)]thiophene reacts with dimethyl l,2,4,5-tetrazine-3,6-dicarboxylate in a cyclo-addition-fragmentation reaction to yield (143), whereas benzo[A]furan and N- methylindole yield products (144) arising from ring opening and recyclization (76AP679). 

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... 

G. E. Keck and A. M. Tafesh, Free-radical addition fragmentation reactions in synthesis A second generation synthesis of (+)-pseudomonic acid C, J. Org. Chem. 54 5845 (1989). 

Coordination of the oxazolidinone 9 with the zinc complex activated the electrophi-licity of the alkene moiety toward addition of the nucleophilic radicals, but the stereodetermining step was the subsequent addition-fragmentation reaction of the intermediate radical with an allyltin reagent. A transition state XVIII similar to FV was proposed for the bis(oxazoline)-Mg complex-catalyzed Diels-Alder reaction reported by Corey [13], As the conformation of the bound a-amidyl radical formed by reaction with tert-butyl radical is s-cis [29a], the back face of the prostereogenic radical in XVni is shielded by one of the phenyl substituents on the oxazoline rings. So, the addition reaction occurred from the front face to the radical intermediate XVIII to give the (/ ) product from the (R,R) ligand 12. 

The attack of base represented in V is particularly instructive since it illustrates another limitation of the synthetic method The addition-fragmentation reaction is not as clearly defined when carbonyl groupings other than 7-lactones are present. For instance, the fragmentation of VIII with sodium ethoxide does not furnish the corresponding enone. Instead, it yields ethyl benzoate. 

Independently, Yamada and Otsu found a similar addition-fragmentation reaction of a-bromomethylacrylates, which produces polymers with bromo-methyl and vinylidene groups at both ends 126,127... 

Polymerizations initiated via addition-fragmentation reactions can also be classified as an initiation process involving radicalic species. The principle of this class of reactions consists in the reaction of a photolytically formed radical with an allyl-onium salt generating a radical cation intermediate. These reactive species undergo a fragmentation giving rise to the formation of initiating cations. 

I. Reetz, V. Bacak, and Y. Yagci, Addition-fragmentation reactions for cationic polymerization using a novel allyloxy-picolinium salt. Polym. Int. 1997, 43(1), 27-32. 

Y. Yagci and I. Reetz, Addition-fragmentation reactions in polymer chemistry. React. 

Radical Addition/Fragmentation Reactions the Fate of Imidoyl Radicals... 

Shimizu et al. prepared the cyclic ( )-vinyl ether (270) in moderate yield by an addition-fragmentation reaction of the diol (271) by treatment with sodium hydride in THF overnight. The conjugate addition of the alkoxide to the vinyl selenone generates (272), then (273), leading to bond rupture and the expulsion of phenylselenone to form (270) (Scheme 31) <84JOC1230>. 

Secolongifolene diol is one of the metabolites of the fungus Helminthosporium sativum (725). Recently, a simple synthesis of its optical antipode (193) has been achieved by exploiting the electrophilic addition-fragmentation reaction (726) of homoallylic alcohols. The homoallylic alcohol (191), which can be obtained from longicyclene (44) by cyclo-... 

Initially, a thermooxidative process takes place, due to the high oxygen content of phenolic resins. Conley and coworkers proposed that the first step, Eq. (24), is the formation of a hydroperoxide (36) which later fragments into phenols containing aldehyde and carboxyl groups.This is followed by a decarboxylation reaction which starts around 300°C and, finally, by a decarbonylation reaction which begins around 500°C. In addition, fragmentation reactions may occur above about 400°C, Scheme 1. 

Other allylic compounds which participate in addition fragmentation reactions and yield telecheUcs analogous to allylic sulfides include allylic bromides, allylic phosphonate, allyl peroxides, and allylic stannane and vinyl ethers (62). 

Macromonomers have also been used in the synthesis of telechelic polymers using benzyl mercaptan dimer in the synthesis of a,Q)-dicarboxyl telechelic PMMA. This reaction proceeds through an addition-fragmentation reaction whereby the macromonomer dimer splits in half One-half forms the co-terminal end of each macromolecule, while the other is released as a fiinclional initiating radical. 

Abstract Macromonomers of structure 4. prepared by the free radical polymerization of MMA using cobalt complexes as chain transfer agents, become incorporated into polymer chains by copolymerization but also act as efficient chain transfer agents by an addition-fragmentation reaction. Macromonomers of general structure 1 have been prepared by utilizing appropriately substituted allylic sulfides as chain transfer agents in free radical polymerizations. 

Vinyl Ethers. Chain transfer reactions involving suitably constituted vinyl ethers [64,65] also proceeded via addition-fragmentation reactions, as illustrated in Scheme 21. Benzyl and benzoyl groups were thus incorporated into polystyrene and poly(methyl methacrylate). 

Scheme 5 Synthesis of highly pure block copolymers by combination of RAFT polymerization, azide-alkyne click reaction, and de-grafting, in which cleavage of grafted chains from a silica surface can be achieved by either aminolysis or radical-induced addition-fragmentation reactions (RIAFR). Adapted from Zhao et al. [20]...

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