Olefination Chugaev elimination

The Chugaev elimination is of synthetic value, because it proceeds without rearrangement of the carbon skeleton. Other non-thermolytic elimination procedures often lead to rearranged products, when applied to the same substrates. However applicability of the Chugaev reaction is limited if the elimination is possible in more than one direction, and if a /3-carbon has more than one hydrogen. Complex mixtures of isomeric olefins may then be obtained. For example the thermolysis of xanthate 12, derived from 3-hexanol yields 28% S-hex-3-ene 13, 13% Z-hex-3-ene 14, 29% -hex-2-ene 15 and 13% Z-hex-2-ene 16 ... 

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. 

The utility of the Chugaev elimination for the formation of olefins without rearrangement of the carbon skeleton can be observed upon pyrolysis of the xanthate of alcohol 14.17 The desired vinyl cyclopropane (IS) was isolated in reasonable yield (42%) along with a small amount of the rearranged xanthate. Alternatively, acid-mediated dehydration with sulphuric acid yielded a variety of rearranged products in low yield. Ester pyrolysis (of the acetate of 14) also furnished a variety of compounds, with the major product being cyclopentene 16. 

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%... 

The heating of xanthates usually results in /3-elimination and formation of olefins (Chugaev reaction ). However, when there is no /3-hydrogen at the alcohol moiety, rearrangement takes place. A kinetic study of the influence of substituents on the reaction rate of a series of diaryl xanthates to diaryl dithiocarbonates (81->82) again indicates a four-membered cyclic transition state . A similar transition state is indicated, by the same... 

Chugaev reaction. Formation of olefins from alcohols without rearrangement through pyrolysis of the corresponding xanthates via cis elimination. 

Acid-sensitive t-alcohols can be dehydrated by a process that is related to the Chugaev reaction but which takes place at much lower temperatures (refluxing THF) (Scheme 9). u/c-Diols react with iodoform, triphenylphosphine, and imidazole to give the corresponding olefin, probably via reductive elimination from a di-iodo intermediate this method is particularly useful for preparing unsaturated sugars. 

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