Aryl thiocyanates

The carbon-nitrogen triple bond of aryl thiocyanates acts as a dipolarophile in 1,3-dipolar cycloadditions. Reactions with nitrile oxides yield 5-arylthio-1,2,4-oxadiazoles 227 (X = O Y = S). Aryl selenocyanates behave similarly forming 5-arylseleno-l,2,4-oxadiazoles 227 (X = 0 Y = Se). Reactions of 5-aryl-... 

Activated haloarenes react with potassium thiocyanate under the influence of a quaternary ammonium salt to form the corresponding aryl thiocyanates [61]. Aliquat is preferred over tetra-n-butylammonium bromide for the reactions of fluoro- and iodoarenes but, in all cases, yields are extremely high. 

Trifluoromethyl thioethers are produced in a fluoride-catalysed one-pot reaction of alkyl or aryl thiocyanates with trifluoromethyl silanes [37]. The reaction is initiated by fluoride ion displacement of the trifluoromethyl anion from the silane the thioether is formed from the thiocyanate by displacement by the trifluoromethyl anion of the cyanide ion, which then perpetuates the reaction. Trifluoromethyl selenoethers are obtained by an analogous route. In a similar manner, disulphides can be converted into trifluoromethyl thio- or selenoethers [38],... 

In the unconventional synthesis of thioethers (Scheme 4.11), cyanide ion is displaced from thiocyanates by carbanions [52, 53], which have been generated under phase-transfer catalytic conditions (cf. 4.1.12). Thiocyanates are readily obtained by a standard catalysed nucleophilic substitution reaction [4, 54-58] (see Table 4.19). Aryl thiocyanates are obtained from activated aryl halides [4, 57] (see Chapter 2). 

Alkyl thiocyanates were reduced with lithium aluminum hydride to mercap-tans (yield 81%), and aryl thiocyanates to thiophenols (yield 94%) [680]. 

Aqueous sodium sulfide reduced the aryl thiocyanate 85, prepared from ethidium bromide, affording the air-sensitive dithiol 86 in 63% yield381 (equation 92). Also, Nicolaou s group has reported on the cleavage of dithiatopazine 87, a stable 1,2-dithietane system with loss of one sulfur atom and formation of a rearranged thioketone 88382 (equation 93). 

ThiocyanatesSodium arenesulfinates, ArS02Na, are converted into aryl thiocyanates, ArSCN in about 60-75% yield by reaction with diethyl phosphorocyanidate in THF, This reaction is also possible with benzyl and adamantyl sulfinates but yields are only about 40%. Alkyl thiocyanates cannot be obtained in this way. 

Other methods that can be used to prepare thiol esters from carboxylic acids include the use of aryl thiocyanates,12 thiopyridyl chloroformate,13 2-fIuoro- V-methylpyridinium tosylate,14 1-hydroxybenzotriazole, 5 and boron thiolate.16 Direct conversion of 0-esters to 5-esters can also be effected via aluminum and boron reagents.17 However, the applicability of these 1217 and other more recent methods18 to the selective thiol ester formation discussed above has not been clearly defined. 

Aryl thiocyanates, ArSCN, are formed by direct thiocyanation of the aromatic nucleus or by treating diazonium salts with metallic thiocyanates. Methods for the thiocyanation of organic compounds have been reviewed. ... 

The action of concentrated sulfuric acid followed by treatment with ice water serves to transform aryl-thiocyanates into the corresponding thiocarbamates. 

Aryl thiocyanates react with primary alcohols in the presence of triphenyl-phosphine to give alkyl aryl sulphides in high yield. It has been suggested that the products arise from the collapse of a triphenyloxythiophosphorane (90) resulting from attack of the alcohol on the initially formed arylthiotriphenylphosphonium cyanide. ... 

The sodium salts of aryl sulfinic acids react rapidly with DECP in refluxing THF to give aryl thiocyanates in good yields, and even hindered substrates give the corresponding thiocyanates. ... 

There are fewer reports on the trimerization of aryl thiocyanates, although they are used in mixed trimerization reactions in the presence of other nitriles (vide infra).1 7... 

DEPC serves as a coupling agent for the synthesis of simple amides, esters, a-aminonitriles, and aryl thiocyanates it is also used as an efficient coupling agent for racemization-free peptide synthesis. Amides of various types can be obtained by the simple mixing of carboxylic acids and amines with DEPC in the presence of triethylamine. Both aromatic and aliphatic acids easily react with aromatic and aliphatic... 

When a trialkyl phosphite is heated with an alkyl or an aryl thiocyanate, an exothermic reaction occurs furnishing a nitrile and a phos-phorothioate ester (224,288). 

An interesting application of the electrochemical oxidation of thiocyanate ion is the preparation of alkyl and aryl thiocyanates via anodically generated thiocyanogen. Alcohols have been converted to the corresponding thiocyanates by constant current electrolysis of NaSCN in CH2CI2 containing triphenylphosphite and 2,6-lutidinium perchlorate. The yields were fair to good for the primary and secondary alcohols, but no thiocyanate formation was observed with tertiary ones. Similarly, various aromatic amines and phenols were thiocyanated in a two-step procedure, namely electrochemical preparation of (SCN)2 and subsequent reaction with the substrates k... 

This reaction was first reported by Riemschneider in 1949. It is an acidic transformation of alkyl or aryl thiocyanates into thiocarbamates and is known as the Riemschneider reaction. In this reaction, when thiocyanates are treated with concentrated sulfuric acid, the corresponding thiocarbamates are obtained. In parallel, when thiocyanates are treated with concentrated sulfuric acid in the presence of alcohol or olefin, the corresponding A -substituted carbamates are yielded. The alcohols or olefins that are stable in the presence of sulfuric acid, including isopropyl alcohol, cc-butyl alcohol, r-butyl alcohol, cyclohex-anol, isobutylene, and camphene aU are suitable for this reaction, whereas low-order alcohols such as methanol and ethanol do not react with thiocyanate to give the corresponding A -substituted thiocarbamates. hi addition, the benzyl thiocyante and some ortho-substituted phenyl thiocyanates cannot be transformed into the corresponding thiocarbamates under these conditions because they are sensitive to sulfuric acid. Some unsuitable thiocyanates are o-carboxyl, o-methoxy, o-nitrophenyl thiocyanates, and o-methyM-amino phenyl thiocyanate. ... 

Indole and its derivatives undergo smooth thiocyanation with ammonium thiocyanate in the presence of a mixture of Al203-MeS03H (AMA) under mild conditions without the use of any organic solvents to afford aryl thiocyanates in excellent yields and with high selectivity at room temperature (Hosseini-Sarvari and Tavakolian 2008) (Scheme 2.5). 

Hosseini-Sarvari, M. and Tavakohan, M. 2008. Synthesis of aryl thiocyanates using AI2O3/ MeSOjH (AMA) as a novel heterogeneous system. J. Chem. Res. (6) 318-321. 

Aryl thiocyanates are formed in high yields through pyrolysis of A-aryl-dithiocarbamates. ... 

Infrared, Raman, and Microwave Spectra.—Brief details of studies which provide information on conformations and electron distributions are summarized at the end of this section. A comparison of the polarized Raman spectra for (C Hj)2S02 with far-i.r. data for Mc2S02 and polarized i.r. spectra for aryl thiocyanates and selenocyanates represent less routine studies. 

A full paper detailing the conversion of primary alkylamines into the corresponding thiocyanates has appeared,and this important contribution to thiocyanate synthesis has now been extended to include aromatic derivatives. Thus, the readily prepared (56) reacts, in refluxing ethanol, with arylamines to give (57), which are smoothly pyrolysed to aryl thiocyanates in preparatively useful yields (Scheme 55). 

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