Thiol/thiolate aromatic thiols

Thiolates are strong ligands and a rich coordination chemistry with cobalt(III) exists. Normally, thiols are employed directly in synthesis, and this is the usual entry into their coordination chemistry. However, alternative approaches do appear from time to time, and recently the cobalt-mediated direct thiolation of an aromatic ring in bidentate phenyl(2-pyridyl)diazene using dithiocarbamate ion to produce a coordinated tridentate thiolate has been reported.1034... 

The overall degradation of (103) assisted by the cluster [(Cp )2 M o2Co2S3(CO)4] (Cp = CH3C5H4) is the model reaction that best resembles the heterogeneous counterparts, particularly those classified as Co/Mo/S phase,158 in terms of both structural motif and HDS activity.229 Morever, the Co/Mo/S cluster has successfully been employed to show that the C—S bond scission in the desulfurization of aromatic and aliphatic thiols occurs in homolytic fashion at 35 °C and that thiolate and sulfido groups can move over the face of the cluster as they are supposed to do over the surface of heterogeneous catalysts.230... 

Similar behavior of other aromatic disulfides and thiols on gold electrodes has been described based on the SERS experiments [167]. Adsorption of benzenethiol, benzenemethanethiol, p-cyanobenzenemethanethiol, diphenyl sulfide, and dibenzyl sulfide was studied on the roughened gold electrode. All these species adsorb dissociatively as the corresponding thiolates. Monolayers formed from symmetric disulfides were exactly like those formed from the corresponding thiols. These monolayers were stable in a wide potential window from -1-800 to —1000 mV (versus SCE), which was limited by the oxidation of the Au surface from the positive side and hydrogen evolution at —1000 to —1200 mV at the negative side. 

Formation of a symmetrical sulphide (a) (e.g. dipropyl sulphide, Expt 5.204), is conveniently effected by boiling an alkyl halide (the source of carbocations) with sodium sulphide in ethanolic solution. Mixed sulphides (b) are prepared by alkylation of a thiolate salt (a mercaptide) with an alkyl halide (cf. Williamson s ether synthesis, Section 5.6.2, p. 583). In the case of an alkyl aryl sulphide (R-S Ar) where the aromatic ring contains activating nitro groups (see Section 6.5.3, p. 900), the aryl halide is used with the alkyl thiolate salt. The alternative alkylation of a substituted thiophenol is described in Section 8.3.4, p. 1160. The former procedure is illustrated by the preparation of isobutyl 2,4-dinitrophenyl sulphide (Expt 5.205) from l-chloro-2,4-dinitrobenzene and 2-methylpropane-1-thiol. 

Aromatic and aliphatic thioles RSH (R = 2-, 3-, or 4-tolyl, 2-hydroxyethyl, 2-ethoxyethyl, 4-chlorphenyl, etc.) in the presence of a base yield anionic dithiolatorhodium(III) porphyrins (path j) which show the so-called hyperporphyrin spectra and are susceptible to autoxidation yielding hyperoxide ions. Although the formation of the latter ones is formulated via a nucleophilic exchange of coordinated OJ with thiolate, it could well be that an outer-sphere electron transfer between he anionic bis(thiolato) complex and molecular dioxygen initiates the observed formation of disulfides RSSR. 

The coordination chemistry of palladium and platinum with large aromatic thiolates has been restricted to some studies with PFTPH, although there is an extensive reported chemistry of oligomeric complexes with smaller thiols. Halide complexes of Pd and Pt were shown to react with Tl(PFTP) to give the presumably monomeric, square planar anionic M(II) complexes [M(PFTP)4] (83). 

The deployment of sterically hindered thiolate ligands has considerably increased the scope of metal sulfur coordination chemistry over the past 10 years. The primary emphasis has been on aromatic thiols... 

Thiol esters of aromatic acids, ArC(0)SR, are cathodically reduced, with fission of the S-CO bond generating the arylcarbonyl radical and thiolate as in Eq. (25) [101]. The thiol may be obtained in good yield [102]—for example, L-cysteine in 83% yield from S-benzoyl-L-cysteine at a mercury cathode in DMF-water-tetramethylammonium chloride. The electroreduction of aryl thioesters [containing -C(0)SR, -C(S)OR, and -C(S)SR functions] has been studied, and some interesting products have been formed [103-105], as in Eqs. (26) through (29). Trithiocarbonates are reduced to the tetraalkyltetrathioethylenes [106]. 

No successful attempts to observe the spectrum of the thiophenoxy radical or its unhindered substituted analogues have been reported in solution studies. Smentowski (1963) found that the p-chlorobenzene-thiolate anion reacts with nitrosobenzene to give the spectrum of the latter s radical-anion (flow system) and a high yield of the disulphide thep-chlorothiophenoxy radical is apparently formed as an intermediate but dimerizes too rapidly for spectroscopic detection. The spectra of both aromatic and aliphatic thiol radicals have, however, been observed when the species, generated by ultraviolet irradiation of the corresponding disulphides, are trapped in the solid state (Smissman and Sorensen, 1965 Windle et al., 1964). 

Now for the first inversion. It turns out that nucleophilic attack by chloride ion occurs next to the electron-rich aromatic ring rather than next to the ester. The thiol 66 was used so that the thiolate anion could be made with a weak base, and the required regio- and stereochemistry is in place 67. The rest of the synthesis11 is in the Workbook. We shall be seeing other syntheses of diltiazem later in this chapter that use other asymmetric methods. 

A number of perfluoroalkylations of thiolates from aliphatic, aromatic, and heteroaromatic thiols were reported by Boiko [ 12 ], where the substitution with primary and secondary perfluoroalkyl iodides afforded the desired products in 60-85% yield, but no reaction occurred with tert-Rf-I [12c]. Some of the related reactions are shown in Schemes 2.35 [13] and 2.36 [14]. 

Of the many other methods we can mention the preparation of biospecific adsorbents utilizing trichloro-5-triazine [120], coupling via thiol-disulfide interchange with activated thiolated support [121] or via azide formation [122]. The attachment of affinants containing the aromatic residues can be carried out by means of diazonium groups [87,123]. The coupling of the affinant onto the silinized glass after the reactions with p-nitrobenzoylchloride followed by reduction with sodium dithionite was described by Weetall and Filbert [124]. 

Figure 12. Solution and solid-state Hg chemical shifts of Hg(II)-thiolate complexes. = aromatic thiols O = aliphatic thiols = solid-state chemical shifts A = Hg(02CCH,)2.

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