Thiols arylthiols

Thiols (RSH) behave in a similar way to alcohols but are not so deshielded, as you would expect from the smaller electronegativity of sulfur (phenols are all about 5.0 p.p.m., PhSH is at 3.4l p.p.in.). Alkane thiols appear at about 2 p.p.m. and arylthiols at about 4 p.p.m. Amines and amides show a big variation, as you would expect for the variety of functional groups involved, and are summarized below. Amides are slightly acidic, as you saw in Chapter 8, and amide protons resonate at quite low fields. Pyrroles are special—the aromaticity of the ring makes the NH proton unusually acidic and they appear at about 10 p.p.m. 

A new alternative has also been described. The furan is oxidatively meth-oxylated, and the product condensed with an alkyl or arylthiol by means of an acid catalyst in acetonitrile at ordinary temperatures it is presumed that a carbocation is intermediate. Although the method makes available aryl thioesters as well as alkyl thioethers, heterocyclic thiols and thiourea fail. The furylthioethers are not very stable and decompose if the reaction is prolonged.202... 

Arylthiols were converted to mixed arylphenylsulfides by reaction with pentaphenylbismuth (4) or with tetraphenylbismuthonium trifluoroacetate (6). Oxidation products were also formed in minor amounts (<15%). On the other hand, thiols were readily converted to the corresponding disulfides by treatment either with triphenylbismuth dichloride (3) or with triphenylbismuth carbonate (5) in the presence of sodium hydride.2533... 

Treatment of alkylsulfenyl methyl thiocarbonates derived from alkyl thiols and Scm-Cl with triphenylphosphine at 0 °C in benzene generates 5 alkyl methyl thiocarbonates in good yield (78% for alkyl = benzyl) (eq 13). Arylsulfenyl methyl thiocarbonates do not parallel this behavior vs. triphenylphosphine but they react cleanly with tributylphosphine (e.g., in dioxane-water) to generate the free arylthiol (see below).The reductive cleavage of alkylsulfenyl methyl thiocarbonates with trialkylphosphines (McsP, EtsP, BU3P) can be achieved with practically quantitative yields in protic solvents (THF-water) in the presence of acids... 

In 2013, Liu et al. [147] reported the application of MnCl2 for the arylation of aryl and alkyl thiols with aryl iodides. The specific conditions that were used were MnClj (20 mol%), and phen ligand (20 mol%) with CSjCOj (1.5 equiv) in toluene or dioxane at 148 °C for 48 h. Good to excellent yields of the arylthiols were obtained, and good functional group tolerance, enabling steric2illy hindered iodides to couple with thiols, was evident. 

Keywords Arylthiols, diaryhodonium salts, cesium carbOTiate, toluene, room temperature, direct C-S coupling from thiols and diaryhodonium salts, aryl sulfides... 

Kinetic studies of thiolate substitution in alkyl (R = ethyl or tert-butyl) tetrameres dianions with arylthiols (R = XC6H4SH with X = P-NH2, P-CH3, P-NO2, 0-NO2) indicate that the rates are overall second order, first order in each tetramere and thiol. The most interesting feature of these kinetic studies is the dependence of rate constants on thiol acidity. This has led to the postulation of the mechanism illustrated in the scheme on Fig. 5.8. Here the slow protonation step should be followed by fast separation of the alkylthiol from and subsequent capturing of the generated arylthiolate anion by the metal center. 

The synthesis of thioesters through the coupling aldehydes with thiols has been achieved using common iron salts as catalysts (Scheme 5.33) [54], The reactions required an oxidant, and tert-butyl hydroperoxide was found to be effective tmder the reaction conditions. The reactions were carried out in pure water and afforded moderate to excellent yields of the thioesters. The scope of the chemistry was quite broad as both alkyl and aryl aldehydes as well as alkyl- and arylthiols were successfully coupled as well as several heteroaromatic aldehydes. Overall, this is a practical approach to the preparation of thioesters. 

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