Radical sulfanyl

According to results from laser flash photolysis, the p-(methoxyphenyl) sulfanyl radical adds exclusively to the central atom in of 2,4-dimethylpenta-2,3-diene (If) with a rate constant of 1.1 x 10s M-1 s-1 (23 1 °C) (Scheme 11.6) [45], A correlation between the measured rate constants for addition of para-substituted arylsulfanyl radicals to allene If was feasible using Brown and Okomoto s o+ constant [46], The p+ value of 1.83, which was obtained from this analysis, was interpreted in terms of a polar transition state for C-S bond formation with the sulfanyl radical being the electrophilic part [45]. This observation is in agreement with an increase in relative rate constant for phenylsulfanyl radical addition to 1-substituted allene in the series of methoxyallene lg, via dimethylallene Id, to phenylsulfanylallene lh, to ester-substituted 1,2-diene li (Table 11.2). 

Detailed kinetic studies reveal that the radical addition of the phenylsulfanyl radical to the central bond of bicyclobutanes is determined by the steric and electronic effects of the substituents. Most importantly, electron-withdrawing substituents at the corresponding 1-position retard the attacking rate of the sulfanyl radical, while electron donors show the opposite... 

Polymerization of the oxathiocinone 771 proceeded, in benzene at 40-70 °C, with complete ring-opening up to ca. 25% conversion. A two-step mechanism was involved addition of sulfanyl radical onto monomer exocyclic double bond to form the intermediate carbon centered radical 782 fragmentation of this latter yielding a new propagating sulfanyl radical and polymer backbone double bond (Scheme 152) <2005ASC1811>. 

A similar mechanism was proposed when 1,5-dithiocin 838g underwent polymerizations with methyl methacrylate (MMA) and styrene (STY). The activated double bound of 838g was found to have a profound affect on reactivity. In fact, co-polymerization of 838g with MMA at 70 °C the 5-terminated sulfanyl radicals preferred to undergo homopropagation, while cross-propagation is favored for MMA-terminated radicals. Both monomers possessed an electron-deficient acrylate double bond with similar possibilities for conjugative stabilization of the adduct radical by the ester functionality, which would explain the apparent equal reactivity of the MMA radical to either monomer. 

In co-polymerization of 838g with STY at 80 °C, the cross-propagation is favored, consistent with electrophilic sulfanyl radicals adding rapidly to electron-rich STY, and nucleophilic styryl radicals adding rapidly to electron-deficient acrylate double bond (Scheme 169) <2006MI2475>. 

The EPR spectra of the radical cations derived from pyrrole solutions can all be simulated by electronic structures in which the unpaired electron is located in an orbital with the nodal plane on the nitrogen and (2) showing large coupling constant values at the 2- and 5-positions and small values at the 3- and 4-positions <2000J(P2)905>. The EPR parameters of the radical cations from 2,5-dimethyl-l-phenylpyrroles and 3,4-bis(alkylthio)-2,5-dimethyl-l-phenylpyrroles (Table 30) denote a marked stabilization of the radical cations by the sulfanyl groups through mesomeric effects. 

Zard has developed the use of N-amidyl radicals. The precursors of the radical intermediates are 0-benzoyl hydroxyamines such as 37. Addition of a tributylstannyl radical to the carbonyl group of the benzoate moiety is followed by the cleavage of the weak N - O bond. A subsequent 5-exol6-endo tandem cyclization takes place to yield the skeleton of the natural product deoxyserratine (Scheme 12) [49]. Later, the same group disclosed a tin-free source of amidyl radicals that relies on the use of M-(0-ethyl thiocarbonyl-sulfanyl) amides and lauryl peroxide as initiator. Examples of polycyclization were also given [50]. On the occasion of a model study toward the synthesis of kirkine, the use of thiosemicarbazide precursors gave access to the tetracyclic structure of the natural product [51]. 

Purine radical cations are relatively stable, so that the highest intensity ion in the mass spectrum is due to the molecular ion. Subsequent fragmentation depends on the nature of the substituents. The only common fragmentation reaction to all purines appears to be the elimination of HCN from the molecular or fragment ions. Multiple expulsions of HCN also occur as major processes in the mass spectra of a number of purine analogs, including 6-sulfanyl-purine, 7-deazaadenine, or purine itself. ... 

Two routes have been shown to produce sulfanyl- and selanyldifluoromethyl-phosphonates (342). Generation of phosphonodifluoromethyl radicals (343) from such precursors and their addition reactions with alkenes represents a... 

Lequeux, T., Lebouc, E. Lopin, C., Yang, H., Gouhier, G., and Piettre, S.R., Sulfanyl- and selanyldi-fluoromethylphosphonates as a source of phosphonodifluoromethyl radicals and their addition onto alkenes, Org. Lett., 3. 185, 2001. 

The chemistry for RAFT is illustrated in Scheme 3. The RAFT process is the newest of the living-radical processes and is reported not to have the limitations of the two previously described systems [45], It is essentially a degenerative transfer process in which a polymer chain (P ), initiated with an azo or peroxy initiator, reacts with a (thiocarbonyl)sulfanyl compound, S=C(Z)-S-R, to release R, an alkyl radical which can go on to initiate another polymer chain. Another propagating chain (Pm ) can subsequently react with P -S-C(Z)=S to release P which can go on to add more monomer. This cycle then repeats itself to produce polymer. 

Analogous reactions leading to heterocyclic compounds were carried out in the same years by Tundo and coworkers by reacting aromatic isonitriles with alkyl and sulfanyl radicals bearing a cyano-substituted side-chain. In the first example [17],... 

Analogous [4-1-1] annulations were also obtained starting from isonitriles and f-cyano-substituted sulfanyl radicals, generated either by hydrogen abstraction from aliphatic thiols or (more profitably) through photolysis of aromatic disulfides [18]. The reactions afforded thieno- (24) and benzothienoquinoxalines (25), respectively (Scheme 10). 

Scheme 10. [4+1] Annulations with isonitriles and sulfanyl radicals...

With this substrate (28) the ring closure can be conveniently accomplished with either stannyl or sulfanyl radicals with no concomitant formation of the six-membered-cyclization quinoline product, which is present instead, or is predominantly formed, in all of the reaction mixtures obtained with group other than TMS. At the same time, both radical precursors, that is, stannane and thiol, can serve as nucleophiles for the intermediate indolenines 29 and 31, which are trapped to give the final substituted indoles 30 and 32 with high efficiency. 

Analogous cyclizations were performed on the silylated alkynyl isonitriles 37 (R = TBDPS or TBDMS), which required higher temperatures (Scheme 15), and allylsulfides 38, which underwent an interesting cyclization-isomerization process mediated by catalytic amounts of sulfanyl radicals (Scheme 16). 

Scheme 16. Sulfanyl-radical-catalyzed cyclization-isomerisation of (isocyano)allylsulfides...

Finally, a novel three-component radical cascade reaction involving isonitriles has just been published [6]. In this paper, aromatic disulfides, alkynes, and isonitriles have been reported to react under photolytic conditions to afford -arylthio-substituted acrylamides 49 or acrylonitriles 50 in fair yields as mixtures of the E and Z geometric isomers (Scheme 21). The procedure entails addition of a sulfanyl radical to the alkyne followed by attack of the resulting vinyl radical on the isonitrile. A fast reaction, for example, scavenging by a nitro-derivative (route a) or f-fragmentation (route b), is necessary in order to trap the final imidoyl radical, since addition of vinyl radicals to isonitriles seems to be a reversible process. The reaction provides very easy access to potentially useful poly-functionalized alkenes through a very selective tandem addition sequence. 

G. (2010) Switching from (R)- to (S)-selective chemoenzymatic DKR of amines involving sulfanyl radical-mediated racemization. Org. Biomol. Chem., 8 (18), 4165-4168. 

Hydrogen sulfide H2S is emitted to the atmosphere either from natural sources, e.g. volcanos, soils, biomass burning, and marine biology, or from anthropogenic sources, mainly industries. The reaction of H2S with OH yields HS (sulfanyl radical) by abstraction reaction. 

The addition of radicals to enazides was only rarely investigated. Recently, it has been shown that sulfanyl radicals generated from thiols add to the P carbon of alkenyl azides to give a mixture of the corresponding imines and the tantomeric )3-sulfanylated enantines. ... 

Scheme 2.85. The Peterson reaction ofa-silyl carbanions generated by reduction of a sulfanyl group with aromatic radical anions.

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