Addition-Fragmentation Process

Scheme 17. Danishefsky s radical addition/fragmentation process in a synthesis of (+)-3-demethoxyery-thratidinone [( )-93].

Stansbury and Bailey. A review by Colombam on addition-fragmentation processes is also relevant. Monomers used in ring-opening are typically vinyl (e.g. vinylcyclopropane - Scheme 4.20 Section 4.4.2.1) or methylene substituted cyclic compounds (e.g. ketene acetals - Section 4.4.2.2) where addition to the double bond is followed by p-scission. 

Actually, a two-step 1,2-alkyl shift has to be drawn, because Ph groups do not undergo concerted 1,2-shifts instead their n bonds participate in an addition-fragmentation process. 

Olefin synthesis. Condensation of aldehyde tosylhydrazones with stabilized carbanions affords alkenes by an addition-fragmentation process. The overall sequence is formulated in equation (I). Functional groups such as sulfides, sulfones, thioacetals, hemithioacetals, and nitriles can serve as Y, the anion stabilizing group in 2 and the leaving group in 3. When Y = SO2R, the anions 1 and 2 are conveniently prepared by treatment of a mixture of the tosylhydrazone and sulfone with LDA in... 

Kilburn has extensively studied radical cascades centered on the use of methylenecyclopropane derivatives. An addition-fragmentation process ap-parented to pathway c opened new routes for the synthesis of carbocycles. In a recent work, a Sml2-promoted cascade of propargyl ether 73 has been used to give bicyclic ether 78 with good diastereoselectivity (Scheme 24), thus providing a short route to the monoterpenoid paeonilactone B. The observed stereoselectivity in the 74 75 cyclization step was shown to be critically dependent on the presence of HMPA [84]. 

The mass spectrum (MS) 70 eV) (65JA4569) of uracil shows a molecular ion at mlz 112, which expels HNCO (43 mass units) and produces a peak at mlz 69 (C3H3NO+) and a metastable peak at mlz 42.5 (112 - 69). The additional fragmentation processes have been studied in detail. Protonation deprotonation sites have been discussed [see 76AHC(S1)71]. 

A two-step addition-fragmentation process involving a nine-electron Se atom intermediate could be drawn in place of the atom abstraction reaction. 

Scheme 8 Free-radical addition-fragmentation processes...

Few monomers have been studied in addition-fragmentation polymerization. Mainly styrene, acrylate, and methacrylates have been used so far in addition-fragmentation to obtain telechelic oligomers. As an example, styrene and methyl methacrylate (MMA) [60,61] were polymerized through an addition-fragmentation process, using allylic sulfides as CTAs (entries 12... 

Scheme 9 Synthesis of a, >-difunctional oligomers through addition-fragmentation processes. CTA chain transfer agent...

Various macromonomers made from an addition-fragmentation process have been employed as precursors of graft copolymers [292,297-300]. But Krstina et al. [301] also characterized the use of such macromonomers in the synthesis of block copolymers. They explained that for macromonomers based on methacrylic monomers (Scheme 60,1), fragmentation of the adduct (Scheme 60, 2) (formed by addition of the methacrylate monomer onto the methacrylate macromonomer) always dominates over reaction with the monomer. This fragmentation leads to block copolymers and graft copolymerization does not occur. 

The synthesis of CTAs is often very complex. It involves many reaction steps, which certainly limits the use of the addition-fragmentation process (Scheme 61). CCT may provide an answer to this drawback. 

Radical addition-fragmentation processes have been exploited in synthetic organic chemistry since the early 1970 s. Ally transfer reactions with allyl stannancs and the Barton-McCombic deoxygenation process with xanthates arc two examples of reactions known to involve a S[j2 mechanism. However, the first reports of addition-fragmentation transfer agents in polymerization appeared in the... 

Chain equilibrium (Main equilibrium). The active chain Pm is able to go through the addition-fragmentation process resulting in equilibrium between the active (radical) and dormant (bound to the CTA) states. This is the main equilibrium that constitutes the fimdamental step in the Raft process. It traps majority of the actrive propagating species into the dormant CTA compound, thereby limiting the possibility of conventional radical-radical termination reactions. While one polymer chain (P ) is in the dormant stage, the other (P ) is active in polymerization. 

ACE-containing oligomers were also made by a stepwise radical addition-fragmentation process in our laboratories (Chen et al.). 

Radical addition-fragmentation processes have been exploited in synthetic organic chemistry since the early 1970s. Allyl transfer reartions with allyl stannanes and... 

The first reports of radical addition-fragmentation processes appeared in the synthetic organic chemistry literature in the early 1970s. Well-known examples of processes that involve a reaction step with an Sh2 mechanism include allyl transfer reactions with allyl sulfides and stannanes (the Keck... 

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