Thiols, sulfuration reactions

The relatively high e.e. s obtained in most of the quinine-catalyzed thiol addition reactions, the wide range of enones and thiols potentially amenable to this reaction, and the versatility of the sulfur (and carbonyl) functionality combine to make this reaction useful in many ways. 

Thus, the thiol sulfur may act as either a leaving group in car-tap or as an electrophile in bensultap on route to the formation of nereistoxin. Therefore, the number of nereistoxin analogs which may be prepared for insecticidal evaluation is very large. This includes the possibility of derivatives formed from the reaction between 2-dimethylamino-propane-l,3-dithiol and the chlorosulfenyl and chlorosulfinyl intermediates described in Figure 2. 

Figure 5-79. Attempts to prepare the ligand of Figure 5-78 in a metal-free reaction of biacetyl with 2-aminoethanethiol are unsuccessful. The thiol sulfur is too nucleophilic, and attacks the imine in an intramolecular process.

Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide structure. Several representative substitution reactions are illustrated in Fig. 1.5. At moderate temperatures ( 100°C) and under mildly alkaline conditions, benzylic hydroxyl groups in phenolic units are converted to thiols by reaction with bisulfide (Q, Fig. 1.5). At higher temperatures and alkalinities, e.g., under kraft pulping conditions, the mercaptide group undergoes a series of transformations in which the sulfur is ultimately eliminated. 

Reaction of 228 with TsOH or TMSOTf in dichloromethane gave a thiiranium ion 229, which was captured by a thiol sulfur atom to give cyclic sulfide 230 in quantitive yield (Scheme 60) <1996TL707>. 

The high affinity of bismuth for charged sulfur ligands results in the ready generation of bismuth thiolates from a range of bismuth salts and an appropriate thiol, although reaction of Na(Li)SR with Bids or of BiPh3 with RSH have also been Examples are... 

The preparation of thiols involves nucleophilic substitution of the Sn2 type on alkyl halides and uses the reagent thiourea as the source of sulfur. Reaction of the alkyl halide with thiourea gives a compound known as an isothiouronium salt in the first step. Hydrolysis of the isothiouronium salt in base gives the desired thiol (along with urea) ... 

Substituent EfFects. The eflFect on the rate of a variety of electron-donating and -withdrawing para substituents on thiophenol were examined. Pseudo-second order analysis similar to those shown in Figure 1 were carried out but for only one thiol sulfur concentration ratio. The rate data obtained under specified conditions are shown in Table II. The thiols have been listed in order of increasing Hammett a value of the substituents—i.e., most eflFective electron donor to most eflFective withdrawer. Examination of these data shows that there is a marked substituent eflFect on the reaction. The electron-donating para substituents, methoxy and methyl, increased the rate while the electron-withdrawing groups, fiuoro, chloro, and bromo, decreased the rate. Two substituents, p-amino and p-nitro, are anomalous in that the reaction rates do not obey the Hammett... 

Figure 9 shows the characteristic curves for the accumulation of these species in the reaction between thiophenol and liquid elemental sulfur at thiol-sulfur ratios of 1 18.8, 1 8, and 1 3 at I30°C. Several facts are qualitatively apparent from Figures 8 and 9. Irrespective of the relative amounts of HS -H and SxH the concentration of HSjH becomes dominant compared with the other intermediate species HSJi when the relative concentration of SH to sulfur is increased in the reaction mixture. The buildup to a maximum concentration of intermediates is associated with the induction period. Similar behavior was observed in the NMR spectra of reaction mixtures of other thiophenols with sulfur. However, such intermediates were not observed to form in the cases of p-amino and p-nitrothiophenol, further suggesting that these thiols react by a different mechanism. 

From the cofactor structure, we now know that the thiol sulfurs are not directly connected to the pterin ring rather, they are appended from the pyran ring. Therefore, formation of urothione from the molybdenum cofactor must involve a cyclization step, and a similar process may also be involved in the formation of form B (1). Such reactions have been modeled through oxidation of quinoxaline dithiolene ligand (Scheme 2.29) as well as in complexes (see Scheme 2.20 above). ... 

Few examples have been reported for the organocatalytic asymmetric conjugate addition of sulfur nucleophiles other than thiols. The reaction of thiocarboxylic acids to cyclohex-2-enones [390] and a,p-nnsatnrated esters [391] was initially studied by Wynberg et al. employing Cinchona alkaloid catalysts with limited success in terms of selectivity (up to 54% ee). Slightly better enantioselectivities have been recently obtained by Wang et al. in the 1,4-addition of thioacetic acid to P-nitrostyrenes (up to 78% ee) [392] and trani-chalcones (up to 65% ee) [393], using Takemoto s thiourea 142 as catalyst (2-10 mol%). 

Tautomerism of the A-2-thiazoline-5-thiones has not been investigated intensively. A recent report shows that 2-phenylthiazo e-5-thiols exist in the thiol form in both polar and nonpolar solvents (563). This behavior is in contrast with that of corresponding thiazolones. Addition reactions involve only the exocyclic sulfur atom, and thiazole-5-thiols behave as typical heteroaromatic thiols towards unsaturated systems, giving sulfides (1533) (Scheme 80) (563),... 

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