Sulphenyl thiocyanates

Naphthylsulphenyl thiocyanate, 2-Cjt)H7 S SCN, is readily converted into di-(2-naphthyl) disulphide with Fe(CO)6-THF at — 70 °C, and has been used for the conversion of JV-benzyloxycarbonyl-selenocysteine diphenylmethyl ester into the 5e-(2-naphthylsulphenyl) derivative, the seleno-sulphide R SSeR , though this disproportionates readily, especially in basic media, and diselenides and disulphides appear in the product mixture for this reason. The Bunte salt, PhCHa-S-SOg" Na+, gives similar results when used in place of the sulphenyl thiocyanate in these studies, and this general route to disulphides - and diselenides, and its variant BuS-S03"Na+ + TolSCl-> BuSSBu + TolSSTol, have been explored more widely. 

The direct conversion of thioethers to unsymmetrical disulphides with thiocyanogen or sulphenyl thiocyanates has been studied in some detail. Early invest ations established that the S-trityl thioethers could be selectively oxidized in the presence of S-benzhydryl or S-benzyl thioethers [95] as well as in the presence of one or more disulfide bonds [100]. Molecules containing two 5-trityl thioethers can also be cyclized [104, 105] in good yield. The reaction of thiocyanogen with thiols... 

In contrast to the iodine-methanol procedure, the sulphenyl-thiocyanate method does not provide statistical mixtures of symmetrical and unsymmetrical disulphides. Thus the reaction of thiols or S-trityl thioethers with sulphenyl thiocyanates appears to be relatively specific and rapid (e.g., (31) from the S-trityl thioether). In situations when the reaction is slow (e.g., (31) from S-benzhydryl thioether with no acid catalysis) disproportionation of the sulphenyl thiocyanate seems to occur (as with the sulphenyl iodides) and symmetrical disulphide can be isolated. Thus as pointed out by Kamber [92] the iodine oxidation of thioethers appears useful in situations where the... 

The formation of unsymmetrical disulphides by the reaction of S-sulphonates with thiols is analogous to a number of other sulphenyl derivatives. In general terms the reaction may be represented as shown in eq. 7.14. A number of intermediates in addition to the sulphenyl iodides [92], sulphenyl thiocyanates [108] and sulphenyl sulfonates [175] have been utilized to advantage in the synthesis of unsymmetrical disulphides. These include the monodisulphide dioxides (RSO2—), studied in detail by Field and his colleagues [178], the disulphide monoxides (RSO—) [178, 179] and more recently the S-monothiocar-bonates (ROCS--) [180] and sulphenamides [186]. 

An alternative preparative route to the unstable phosphorus thiocyanates uses McsSiCN in reaction with sulphenyl chlorides R R P(0)SC1, where R = R = Me3CCH20 or Me2CHO, and R = Me C, R = Ph. Because of steric effects, these products are more stable than those previously isolated. 

The first member of the previously unknown class of phosphorus thiocyanates has been obtained by reaction between a sulphenyl chloride (Me3CCH20)2P(0)SCl and silver cyanide at —5 The compound has a sharp i.r. band at 2170 cm but isomerizes after a few minutes at 25 C to the more stable isothiocyanate (broad band at 2010 cm i). 

Reactions of Thiocyanates and Isothiocyanates.—Although these functional groups are structiurally dissimilar from many points of view, a thiocyanate behaving as a sulphenyl cyanide, while an isothiocyanate shows the reactions expected of it on the basis of its heterocumulene structure, there are several examples now of isomerization reactions — S—C=N —N=C=S, partly justifying a unified treatment of their reactions. 

The combination of alkyl- or aryl-sulphenyl chlorides and trimethylsilyl cyanide in acetonitrile results in complete conversion into the corresponding thiocyanates within one hour at room temperature. This procedure then represents a new synthesis of alkyl or aryl thiocyanates free from isomeric isothiocyanates, from thiols, or from disulphides (Scheme 56). 

With Acids, Bases. The S-sulphonates appear to be relatively stable in the pH range 1-9 [160] although they are readily decomposed in O.lN-sodium hydroxide. According to Swan, S-sulphonates are relatively stable to N- or 6N-hydrochloric acid and are converted to thiol with hot dilute acid. These studies indicate that S-sulphonates are somewhat more stable than unsymmetrical disulphides and considerably more stable than the sulphenyl iodides or thiocyanates. 

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