Isomerizations of allylic thiocyanates

Allyl thiocyanate isomerizes at similar rates in toluene, nitrobenzene, dimethylformamide, cyclohexane, and acetonitrile The reaction is slowest in the two most polar solvents, dimethylformamide and acetonitrile, and its rate is not appreciably affected by dissolved potassium thiocyanate. 

Allyl and -methylallyl thiocyanates isomerize at almost the same rate . In cyclohexane solution, y-methylallyl thiocyanate isomerizes about 10 times faster, and y,y-dimethylallyl thiocyanate about 30 times faster than allyl thiocyanate The effect of y-methyl substitution on isomerization rate is somewhat larger in acetonitrile than in cyclohexane solutions. The substituent effects are several orders of magnitude smaller than would be expected for SnT anionotropic isomerizations. 

The insensitivity of isomerization rate to solvent and structural changes suggests that isomerizations of most allylic thiocyanates are intramolecular and involve cyclic transition states in which there is little charge separation. This view is supported by the fact that isomerizations of allyl and /3-methyl-allyl thiocyanates in toluene solution have substantial negative entropies of activation (— 9.4 eu and — 8,7 eu, respectively)  

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Experimental and theoretical studies on the isomerization of allyl thiocyanate to aUyl isothiocyanate have indicated that the transformation proceeds by a [3,3]-sigmatropic rearrangement involving a cychc transition state. A novel tandem [3,3]-sigmatropic rearrangement of allylic thiocyanates (129) followed by a stereo-controlled intramolecular addition of the amino function to the developing isothiocyanate group, has... 

R. J. Ferrier and N. Vethaviyasar, Isomerization of allylic-4-azides and 4-thiocyanates and the subsequent synthesis of 2-acetamido-2-deoxyhexopyranoside derivatives, J. Chem. Soc. (C), (1971) 1907-1913. 

Another route to sugar isothiocyanates involves thermal isomerization of the corresponding thiocyanates. Ferrier and Vethaviyasar20,21 and, later, Guthrie and Williams22-23 reported the allylic rearrangement of unsaturated thiocyanates to isothiocyanates see formulas 9-14. 

Organic selenocyanates similarly undergo facile isomerizations but very little information on this is available. Selenocyanates isomerize somewhat faster than thiocyanates and the rate is insensitive to solvent polarity for the allyl compound, but very sensitive to solvent and structure for benzhydryl compounds. It is considered that the mechanisms of rearrangement are closely parallel to those of the thiocyanates. 

The u-alkenyl isothiocyanates, except for allyl isothiocyanate, were prepared by the isomerization of the corresponding w-alkenyl thiocyanates 17). The u-alkenyl isothiocyanates were converted to the corresponding w-methylthioalkyl isothiocyanates 18). The other isothiocyanates were purchased from commercial sources and purified by vacuum distillation. 

A number of allylic compounds, including allyl vinyl ethers, allyJic azides, allylic thiocyanates, and allylic sulfinates, undergo unimolecular thermal isomerization reactions. Most of these reactions are concerted, four-center processes and in most of them the allylic substituent, as well as the allylic carbon skeleton, undergoes alteration. 

Allylic rearrangement of thiocyanates to isothiocyanates has been demonstrated for monosaccharide derivatives, " - " opening up methods for sugar transformations but especially for s mthesizing amino-sugars. " For example, (165) gives (166) by way of the isothiocyanate resulting from thermal isomerization. " The allylic O-methanesulphonyl substituent in... 

A study has been made of the kinetics of epoxidation of allylphosphinates and of the addition of C-benzoyl-iV-phenylnitrone to vinyl- and allyl-phosphonates. Boron trifluoride stabilizes phosphorus(v) thiocyanates, preventing their isomerization to isothiocyanates. Cleavage of the phosphinothioic methanesulphonic anhydride (97) with aluminium halides yields the phosphinic halide with retained chirality. ... 

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