Sulfides, allyl radical addition reactions

Ono and coworkers have extended the radical elimination of v/c-dinitro compounds to P-nitro sulfones151 and P-nitro sulfides.138,152 As P-nitro sulfides are readily prepared by the Michael addition of thiols to nitroalkenes, radical elimination of P-nitrosulfides provides a useful method for olefin synthesis. For example, cyclohexanone is converted into allyl alcohol by the reaction shown in Eq. 7.110. Treatment of cyclohexanone with a mixture of nitromethane, PhSH, 35%-HCHO, TMG (0.1 equiv) in acetonitrile gives ahydroxymethylated-P-nitro sulfide in 68% yield, which is converted into the corresponding allyl alcohol in 86% yield by the reaction with Bu3SnH.138 Nitro-aldol and the Michael addition reactions take place sequentially to give the required P-nitro sulfides in one pot. 

Plourde [17] has described the radical addition of alkyl, substituted alkyl and benzyl thiols to polymer-supported cychtol allyl ethers. Treatment of immobilized allyl ether 117 with benzyl thiol in the presence of AIBN provided, after base-induced cleavage from the support and purification, sulfide 119 in high yield and purity (Scheme 25). Similar reactions with hydroxyl- and carboxyl-substituted alkyl thiols also resulted in good yields of products as single regioisomers [17]. 

Barton and Crich reported the first examples of the uses of 2-substituted allylic sulfur compounds [53]. Their initial experiments with additions of simple alkyl radicals to allyl sulfides, sulfoxides and sulfones were relatively unsuccessful. This failure was largely due to the fact that the nucleophilic alkyl radicals, which were generated by photolysis of the corresponding Barton ester, underwent addition to a second equivalent of Barton ester faster than they added to the allyl transfer agent. Reactions were much more successful with the electron-deficient acrylate reagent 93 (Fig. 4). Crich was later able to show that this same reagent underwent addition reactions with an acyl radical derived from an acyl phenyl telluride [54]. 

A seven-membered cyclic allyl sulfide shown in Scheme 18 is a monomer that was designed based on the high reactivity of allyl sulfide in various radical reaction systems. It undergoes ROP selectively to afford the corresponding polysulfide. The first step is the addition reaction of the radical to the oxo-methylene group of the monomer, leading to the formation of a radical at the p-position of the sulfur atom. This cyclic... 

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... 

In the case of ACSO it was found also that N20 addition reduces the yield of S-allyl-L-cysteine (ACS), indicating that this product is formed by eaq - but not by OH radicals. As a result it can be expected that KBr addition will not reduce the ACS yield. It was found that KBr not only does not reduce the yield of ACS, but it rather increases i ts formation. This is explained as due to ACS formation by reaction of eaq" with ACSO, and its disappearance by reaction with OH radicals to give back ACSO as it is known for the reaction with sulfides. The authors suggest the same reactions for PCSO and PCS (propyl-L-cysteine) although the yield of PCS was not determined. 

In an analogous process, the reactions of unsubstituted and 2-substituted allyl phenyl sulfides with (TMSlsSiH give a facile entry to allyl fns(trimethylsilyl) silanes in high yields (Reaction 26). In this case, the addition of (TMSlsSi radical to the double bond is followed by the S-scission with ejection of a thiyl radical, thus affording the transposed double bond. Hydrogen abstraction from (TMSlsSiH by PhS radical completes the cycle of these chain reactions. ... 

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

The mechanism of this transformation is outlined in Scheme 38 and each step has important features. In step 1, the tributyltin radical abstracts the radical precursor X. A possible side reaction, the addition of the tributyltin radical to the allylstannane, is much slower than comparable additions to activated alkenes. Even if this addition occurs, the stannyl radical is simply eliminated to regenerate the starting materials. Thus, for symmetric allylstannanes, this reaction is of no consequence. As a result, the range of precursors X that can be used in allylation is more extensive than in the tin hydride method. Even relatively unreactive precursors like chlorides and phenyl sulfides can be used if they are activated by adjacent radical-stabilizing groups. 

Fragmentation reactions can also be executed within the framework of the thiohydroxamate method, as illustrated in Scheme 48. Addition of acid chloride (36) to a mixture of activated allyl sulfide (37) and sodium salt (31) provides allylated product (38) in 75% yield. The activating group insures that the initial radical adds to the acceptor more rapidly than to the thiohydroxamate. Rapid fragmentation of the resulting intermediate provides the product (38) and the thiobutyl radical. This last radical propagates the chain by addition to the starting thiohydroxamate. 

Acyl radical sources, other than aldehydes, are also available. The group transfer addition of an acyl radical has been reported by Zard and co-workers, where S-acyl xanthates serve as acyl radical sources [44]. Crich and co-workers reported that an acyl radical, generated from an aromatic acyl telluride by photolysis, adds to an allylic sulfide which contains an ethoxycarbonyl group to form the corre-.sponding y-unsaturated ketones [45]. The addition pathway involves Sh2 type reaction with extrusion of a /ert-butylthiyl radical. 

The additions of thiyl radical to alkenes and alkynes can also occur intra-molecularly, thus leading to sulfur heterocycles. This subject has been explored by Surzur s group [32-33]. Based on steric and stereoelectronic factors, the Baldwin-Beckwith rules [34] predict the 5-exo ring closure as the favored pathway. Scheme 6 shows that the pent-4-enylthiyl radical, generated from the allyl sulfide, yields both five- and six-membered rings, in the ratio of 1 19. The predominance of the larger cycle, in contrast with the above-mentioned rules, is due to the reversibility of the reaction. The product distribution reflects a thermodynamic control. 

Addition-elimination reactions are not restricted to allylic stannanes or sulfides. The vinyl stannane 45 acts as a suitable radical acceptor, leading to a,p-vmsaturated... 

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. 

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