Eliminations Chugaev

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 ... 

The superfluous carbonyl oxygen atom was removed by carbonyl reduction to provide the alcohol 171, subsequent Chugaev elimination (via 172 to 173) and double bond hydrogenation with in situ generated diimide (Scheme 27) [94]. The isopropenyl double bond was finally re-established by reductive cleavage of the a-bromo ether unit in 173 to afford the fully functionalized enantiomerically pure A-ring building block (162). 

In Figure 4.14, we learned about the Chugaev elimination in connection with the synthesis of alkenes. The second (primary) product of this reaction is the dithiocarbonic acid 5-methyl ester (A). It equilibrates with the zwitterion B, which decomposes into carbon oxysulfide (a heterocumulene) and methanethiol (a heteroatom nucleophile). 

The mechanism involves a stereospecific syn-elimination via ion-pair formation from the intermediate sulfamate ester (comparable to the Chugaev elimination of xanthate esters). Kinetic and spectroscopical data are consistent with an initial rate-limiting formation of an ion-pair followed by a fast c s- 3-proton transfer to the departing anion. 

CHUGAEV ELIMINATION REACTION (XANTHATE ESTER PYROLYSIS)... 

In the late stages of the total synthesis of dihydroclerodin, A. Groot and co-workers used the Chugaev elimination reaction to install an exocyclic double bond on ring Before employing the xanthate ester pyrolysis, the authors tried several methods that failed to convert the primary alcohol to the exocyclic methylene functionality. The corresponding xanthate ester was prepared followed by heating to 216 °C in n-dodecane for 2 days to afford the desired alkene in 74% yield. 

J.M. Cook and co-workers accomplished the total synthesis of ellacene (1,10-cyclododecanotriquinancene) by utilizing the Weiss reaction and the Chugaev elimination as key steps.The elimination of the fris-xanthate was performed in HMPA at 220-230 °C in very high yield. This pyrolysis was superior to the elimination conducted under neat conditions. 

The Cope elimination is a stereoseiective syn eiimination and the mechanism invoives a pianar 5-membered cyciic transition state. There is strong resembiance to the mechanism of ester pyrolysis and the Chugaev elimination. The first evidence of the stereochemistry of the eiimination was the thermai decomposition of the threo and erythro derivatives of A/,A/-dimethyi-2-amino-3-phenyibutane. The erythro isomer gives predominantiy the (Z)-aikene (20 1), whiie the threo isomer forms the ( )-oiefin aimost exciusiveiy (400 1). Two decades iater deuterium-iabeiing evidence confirmed the mechanism of the Cope elimination to be 100% syn. ... 

Chugaev elimination Thermal syn elimination of xanthate esters to form alkenes. 82... 

Related reactions Chugaev elimination, Cope elimination, Hofmann elimination ... 

Alkyl xanthates 2 are prepared by treatment of an alcohol 1 with a base and carbon disulfide, followed by exposure of the resulting sodium salt with an alkylating agent.1 Pyrolysis of the xanthate 2 to give an alkene 3 is called the Chugaev elimination. 

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 mechanism of the Chugaev elimination involves a cyclic, concerted transition state as originally proposed by Barton11 and Cram.12... 

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 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. 

As mentioned previously, the Chugaev elimination has found particular use in the field of terpenoid chemistry, not only in structural studies, but also in synthesis. For example, a tandem Claisen rearrangement-ene reaction of geraniol derivative 23 gave... 

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%... 

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