Sulfide transfer agents

Scheme 11 Mechanism of addition-fragmentation chain transfer with an allyl sulfide transfer agent. Reproduced from Moad, G. Rizzardo, E. Thang, S. H., Radical addition-fragmentation chemistry in polymer synthesis. Polymer 2008, 49,1079-1131." ...

The following sections detail the chemistry undergone by specific transfer agents that react by atom or group transfer by a homolytic substitution mechanism. Thiols, disulfides, and sulfides arc covered in Sections 6.2.2.1,6.2.2.2 and 6.2.2.3 respectively, halocarbons in Section 6.2.2.4, and solvents and other agents in Section 6.2.2.5. The transfer constant data provided have not been critically... 

Some typical transfer constants for allyl sulfides are given in Table 6.7. The values of Clt for these reagents are less dependent on the particular monomer than those for halocarbons (Table 6.2) or thiol transfer agents (Table 6.4). The low transfer constant of 32 demonstrates the importance of the activating group Z (cf. 11). 

Depending on the choice of transfer agent, mono- or di-cnd-functional polymers may be produced. Addition-fragmentation transfer agents such as functional allyl sulfides (Scheme 7.16), benzyl ethers and macromonomers have application in this context (Section 6.2.3).212 216 The synthesis of PEG-block copolymers by making use of PEO functional allyl peroxides (and other transfer agents has been described by Businelli et al. Boutevin et al. have described the telomerization of unsaturated alcohols with mercaptoethanol or dithiols to produce telechelic diols in high yield. 

Tung et al21> have reported on the use of a polymeric thiol transfer agent for use in block copolymer production. Various methods have been used for the anion thiol conversion. Near quantitative yields of thiol arc reported to have been obtained by terminating anionic polymerization with ethylene sulfide and derivatives (Scheme 7.27). Transfer constants for the polymeric thiols are reported to be similar to those of analogous low molecular weight compounds.273... 

Complexes of tetravalent zirconium with organic acids, such as citric, tartaric, malic, and lactic acids, and a complex of aluminum and citric acid have been claimed to be active as dispersants. The dispersant is especially useful in dispersing bentonite suspensions [288]. Polymers with amine sulfide terminal moieties are synthesized by using aminethiols as chain transfer agents in aqueous addition polymerizations. The polymers are useful as mineral dispersants [1182]. 

The use of other phosphorus-sulfur reagents for heterocyclic synthesis appears rare. It would be interesting to investigate in more detail the reaction of compounds, such as the phosphine sulfides, with organic substrates. Triphenylphosphine sulfide is an effective sulfur transfer agent, as it converts oxiranes into thiiranes in good yield. The reaction proceeds with retention of configuration.128... 

The versatility of permanganate as an oxidant has been greatly enhanced in the past decade by the observation that it can be solubilized in nonaqueous solvents with the aid of phase transfer agents (1). The literature contains descriptions for the use of this procedure for the oxidation of alkenes (2-13), alkynes (13-18), aldehydes (19), alcohols (20), phenols (21,22), ethers (23), sulfides (24,25), and amines (20,26). The dehydrogenation of triazolines has also been achieved by the use of permanganate and a phase transfer agent (27). ... 

Schmidt KH, Flan P, Bartels DM (1995) Radiolytic yields of the hydrated electron from transient conductivity improved calculation of the hydrated electron diffusion coefficient and analysis of some diffusion-limited (e )aq reaction rates. J Phys Chem 99 10530-10539 Schoneich C, Aced A, Asmus K-D (1991) Halogenated peroxyl radicals as two-electron-transfer agents. Oxidation of organic sulfides to sulfoxides. J Am Chem Soc 113 375-376 Schuchmann Fl-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann Fl-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217-221 Schuchmann Fl-P, von Sonntag C (1997) Heteroatom peroxyl radicals. In Alfassi ZB (ed) Peroxyl radicals. Wiley, Chichester, pp 439-455... 

Iron is the most abundant metal on earth and the commonest electron transfer agents involve iron complexes. Life is thought to have evolved in reductive conditions, in which the dominant form of iron would be as iron sulfide, not iron oxide. The simplest forms of electron transfer agents (found in plants and bacteria) involve iron with thiolate ligands. Some simple electron transfer proteins, such as rubredoxin, contain a single iron centre in an S4 donor environment within a protein (Fig. 10-7). 

Schoneich C, Aced A, Asmus KD. (1991) Halogenated peroxyl radicals as two-electron transfer agents. Oxidation of organic sulfides to sulfoxides. Chem Soc 113 375-376. 

Dimethyldioxirane (52) and other methyldioxiranes are easily generated peroxides which have proved to be very useful oxygen atom transfer agents alkenes, sulfides and amines can all be oxidized and dimethyldioxirane is reduced to acetone. ... 

Transition-metal/sulfide sites, especially those containing iron, are present in all forms of life and are found at the active centers of a wide variety of redox and catalytic proteins. These proteins include simple soluble electron-transfer agents (the ferredoxins), membrane-bound components of electron-transfer chains, and some of the most complex metalloenzymes, such as nitrogenase, hydrogenase, and xanthine oxidase. 

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]. 

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