5- Methyl xanthates, Chugaev elimination

The Chugaev reaction of S methyl xanthates is a synthetically useful p5u olysiss process. The corresponding electron impact induced elimination is a prominent feature of xanthate mass spectra, (45). > The electron impact induced cleavage is at least partially cfs-stereospecific as the thermal reaction is... 

Although other xanthates have been used, methyl xanthates (R3 = Me) are by far the most commonly employed in the Chugaev elimination. For the preparation of the xanthate, a variety of bases have been used including sodium hydride, sodium hydroxide, sodium amide, and sodium/potassium metal. In the case of pure chiral alcohol stereoisomers, epimerisation of the alcohol stereocentre, under the basic conditions, can occur (with the corresponding xanthates leading to different products). Purification of the xanthate, prior to pyrolysis, is often a problem and it is usual to pyrolyze the crude xanthate directly. Pyrolysis of the xanthate is often carried out by distillation. Depending on the pyrolysis temperature, pressure, and the boiling point of the olefin, the product will either distill with the other products (COS, thiol) or remain in the reaction flask. 

Multiple Chugaev eliminations of several xanthates within the same molecule are possible. For example, triol 28 was smoothly converted to the corresponding methyl xanthates and subsequently heated to yield ellacene (29), a precursor to the polyquinenes.21 The Chugaev elimination of xanthates derived from 1,2-diols, however, often yields other products such as cyclic thionocarbonates.2... 

The behaviour of tertiary alcohols in the Chugaev elimination mirrors that of primary and secondary systems however, relatively few examples have been reported. The 5-methyl xanthate of dimethylcyclopropylcarbinol 30 was pyrolyzed to give the olefin in 24%... 

Cram demonstrated that the Chugaev reaction is a syn elimination by showing that the pyrolysis of the S-methyl xanthate of diastereomeric 3-phenyl-2-butanols is stereospecific, as shown in equations 10.72 and 10.73.As noted, the eliminations also produced 3-phenyl-l-butene (in 30-40% yield). 

The Chugaev elimination of tertiary alcohols is improved by use of the potassium xanthate rather than the 5-methyl xanthate this method cannot be extended to primary or secondary alcohols. Secondary alcohols are dehydrated on heating in hexamethylphosphoramide the transition state (209) is proposed. 

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