Thiocyanates reaction with alkyl halides

Pyrrolethiols, readily obtained from the corresponding thiocyanates by reduction or treatment with alkali, rapidly oxidize to the corresponding disulfides. They are converted into thioethers by reaction with alkyl halides in the presence of base. Both furan- and thiophene-thiols exist predominantly as such rather than in tautomeric thione forms. 

Relative Rates for 8 2 Reactions of Chloride, Azide, and Thiocyanate Ions with Alkyl Halides, Methyl Tosylate, Aryl Halides and a Platinum Complex in Methanol (M) and in DMF (D), at 25°C... 

Reaction between alkyl halides, ethanol, and thiocyanate ion 0-72 Cleavage of tertiary amines with CICOOPh... 

As has already been pointed out, the Finkelstein reaction can be conducted in situ in the absence of solvents. For example, alkylations of purine and pyrimidine bases with alkyl halides and dimethyl sulfate have been carried out by solid/liquid phase-transfer catalysis in the absence of any additional solvent [48], as have cyanation of haloalkanes [49] and / -eliminations [50]. Noteworthy is the synthesis of glycosyl isothiocyanates by the reaction of potassium thiocyanate with molten glycosyl bromide at 190 °C [51]. 

Yoneda et a/.243 have investigated the reaction between alkyl halides and thiocyanate ion in a large number of solvents including NMA. While the rate of reaction in NMA was greater than in many of the solvents tested (e.g., formamide), the rate of product formation was still slower than in several solvents, particularly the NJM-dimethylamides and DMSO. Smiley244 has suggested that the reaction of dimethyl sulfate with thiocyanate ion in NMA proceeds by two different pathways. Dimethyl sulfate can react initially with the solvent to form an ionic intermediate which then reacts with the thiocyanate ion. Dimethyl sulfate also can react directly with the thiocyanate ion. 

Alkyl thiocyanates have also been prepared from thioethers with cyanogen bromide and via the reaction of polymer-based quaternary ammonium salts with alkyl halides. Enethiocyanates have been prepared by Markovnikov addition of in situ generated thiocyanic acid to alk-l-ynes, and via the reaction of epoxy-ketones with Ph3P(SCN)2. ... 

Reaction of alkyl halides with benzyltrimethylammonium cyanide in water proceeds rapidly furnishing an improved method for the prepn. of nitriles. In place of benzyltrimethylammonium cyanide the chloride and alkali cyanide can be used. With the thiocyanate in place of the cyanide good yields of alkyl thiocyanates can be obtained.—E ... 

Disulfides can be prepared by treatment of alkyl halides with disulfide ions and also indirectly by the reaction of Bunte salts (see 10-41) with acid solutions of iodide, thiocyanate ion, or thiourea, or by pyrolysis or treatment with hydrogen peroxide. Alkyl halides also give disulfides when refluxed with sulfur and NaOH, and with piperidinium tetrathiotungstate or piperidinium tetrathiomolybdate. ... 

When the reagent is the thiocyanate ion, S-alkylation is an important side reaction (10-43), but the cyanate ion practically always gives exclusive N-alkylation. ° Primary alkyl halides have been converted to isocyanates by treatment with sodium nitrocyanamide (NaNCNN02) and m-chloroperoxybenzoic acid, followed by heating of the initially produced RN(N02)CN. ° When alkyl halides are treated with NCO in the presence of ethanol, carbamates can be prepared directly (see 16-7). ° Acyl halides give the corresponding acyl isocyanates and isothiocyanates. For the formation of isocyanides, see 10-111. 

The thiocyanate 4.24 is the kinetically preferred product in alkylations by alkyl halides undergoing SN2 reactions and with carbocationic electrophiles in SN1... 

Alkyl thiocyanates (39) are prepared by heating alkyl halides ortosylates with sodium or potassium thiocyanate (Scheme 21). The reaction may be performed in acetone, ethanol or water, but goes best in polar aprotic solvents like DMF or DMSO. The thiocyanate ion is an ambidentate nucleophile owing to resonance (Figure 1).4 In principle, therefore, the reaction shown in Scheme 21 may yield... 

Just as with the thiocyanate ion 73, the cyanide ion 76 reacts with soft alkyl halides in SN2 fashion and with hard carbocations in SN1 fashion to give, almost always, the nitrile 77, which is thermodynamically more stable than the corresponding isonitrile 78. The isonitrile product is formed along with the nitrile product when (a) the cation is so very reactive that the rate of reaction reaches d iIf u s ion -con tro 11 ed limit and (b) the reversible reaction that equilibrates the isonitrile and nitrile products is very slow. Since the reaction of a cyanide ion with a carbocation falls in the domain of ion-to-ion reaction, it is indeed very fast. For such a barrierless combination of ions, the kinetic factors associated with the HSAB principle are not applicable. 

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