Pyrolysis of xanthates

An instructive example on how stereochemical features influence the stereochemical outcome of the elimination is the pyrolysis of xanthates from erythro-and t/zrco-l,2-diphenyl-l-propanol. The erythro-dlcohoX 8 is converted into fi-methylstilbene 9 only, and the threo-dlcohoX 10 is converted into the corresponding Z-isomer 11 only. These results support the assumption of a syn-elimination process through a cyclic transition state ... 

The pyrolysis of xanthates (84)—the Chugaev reaction—and of carboxylic esters (85) differ from the above in proceeding via six-membered, cyclic transition states, e.g. (86) and (87), respectively ... 

Chugaev discovered the formation of olefins from the pyrolysis of xanthates in 1899,5 in connection with his studies on the optical properties of xanthates6 and other compounds.7 He subsequently employed the reaction in his structural investigations of terpenes and steroids, demonstrating its utility as an olefin-forming reaction and as a tool for structural determination. For example, he converted cholesterol into methyl cholesteryl xanthate (4)... 

Examples involving the pyrolysis of xanthates of primary alcohols are relatively few in number. The preparation of small molecular weight alkenes, for example, I -pentene and isopropylethylene, using the Chugaev elimination has been demonstrated.14 More recently, the heating of xanthate 8 furnished derivative 9, containing an exocyclic double bond. Compound 9 was subsequently converted to the insect-antifeedant dihydroclerodin. 

The Preparation of Olefins by the Pyrolysis of Xanthates. The Chugaev Reaction Harold R. Nace... 

For the related Chugaev reaction, which involves the pyrolysis of xanthate esters (0-alkyl-5 -methyl xanthates), conditions are much milder (100—200 °C) (Scheme 6.20). 

Xanthate ester SCHEME 6.20 Pyrolysis of xanthate esters. 

Pyrolysis of xanthate esters known as Chugaev reaction requires temperatures of 150-250 °C. For example, the pyrolysis of xanthates 4 and 5 gives more substituted olefins (Saytzeff products) as major products. 

Casadevall s groups has investigated the pyrolysis of xanthates of stereo-specifically deuteriated alcohols in the bicyclo[4,2,0]octane series. Xanthates were prepared without epimerization. Xanthate (411) pyrolyses with no loss of deuterium (mass spectroscopy) in a stereospecific syn-eUmination to give (414)... 

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