Thermal eliminations via cyclic transition states

Under the mild conditions of the reaction, there is no equilibration of the alkenes, so the product composition is governed by the relative stabilities of the various transition states. Usually, more of the trans olefin than of its cis isomer is formed, presumably because partial eclipsing raises the energy of the transition state leading to cis olefin. The selectivity is not high, however, since the ratio of trans cis from some simple cases is in the range 3 1 to 2 1. In cyclic systems, conformational effects and the requirement for a cyclic transition state are the most 

Selenoxides are even more reactive than amine oxides. In fact, many types decompose spontaneously on generation at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and elimination. We have already discussed examples of these procedures in Chapter 4 where the synthesis of a,/ -unsaturated carbonyl compounds (Section 4.7) was considered. In addition to the electrophilic addition of selenenyl halides and related compounds to alkenes and enolate selenenation, which was discussed in Section 4.5, selenides can be readily formed by nucleophilic displacement on halides, tosylates, or epoxides.Selenium is also capable of stabilizing an adjacent carbanion so that a-selenenylcarbanions are accessible carbon nucleophiles. One versatile procedure involves conversion of ketone to fe/5-selenoketals which are then cleaved by butyllithium. These carbanions 

Alcohols can be converted to o-nitrophenylselenides by reaction with o nitrophenyl selenocyanate and tri(rt-butylphosphine). Several oxidants have been employed to convert selenides to selenoxides and bring about elimination. Hydrogen peroxide, sodium metaperiodate, peroxycarboxylic acids, tert bnty hydroperoxide, and ozone have been used most frequently. 

Selenoxide eliminations usually favor formation of the (E) isomer in acyclic systems. This stereoselectivity reflects the fact that the cyclic transition state which minimizes steric interferences corresponds to E- alkene geometry. In cyclic systems 

That the elimination is syn has been established by use of deuterium labels. Deuterium was introduced stereospecifically by LiAlD4 reduction of cis and stilbene oxide. The product of the subsequent ester pyrolysis is trans-stilbene because of eclipsing effects in the transition state. The syn elimination is demonstrated by retention of deuterium in the olefin from a trans-siilhent oxide and its absence in the olefin from c/5-stilbene oxide.An alternative view of the mechanism has been presented. Although recognizing the existence of the concerted cyclic 

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Scheme 6.17. Thermal Eliminations via Cyclic Transition States... 

Alkenes are formed by the thermal decomposition of esters, xanthates, amine oxides, sulfoxides, and selenoxides that contain at least one (3-hydrogen atom. These elimination reactions require a cw-configuration of the eliminated group and hydrogen and proceed by a concerted process. If more than one (3-hydrogen is present, mixtures of alkenes are generally formed. Since these reactions proceed via cyclic transition states, conformational effects play an important role in determining the composition of the alkene product. 

This chapter is concerned with a group of thermally induced elimination reactions widely used for the introduction of carbon-carbon double Irands into complex molecules. These reactions form a discrete group of elimination reactions in that they proceed with syn stereochemistry via concerted cyclic transition states. Related syn elimination processes are believed to be involved in other elimination reactions, e.g. alcohol dehydration using the Burgess reagent, but are not discussed here. One of the advantages of the syn elimination reactions discussed in this chapter is that they do not require the use of... 

In some cases, eliminations occur in non-ionizing solvents and without the addition of any base. In these cases the reactant itself has an internal base and a cyclic transition state leads to elimination. The symbolism for the reactions is Ei, standing for elimination, intramolecular. Only heat is required to induce the reaction, and hence these reactions are called thermal eliminations (the term pyrolysis is also sometimes used). Thioesters, xanthates, selenoxides, and N-oxides are common in these reactions. The Cope elimination involves the formation of an N-oxide and subsequent elimination via the pathway shown in Eq. 10.91, and the Chugaev elimination involves xanthate esters [ROC(S)SR]. The Chugaev elimination was shown to follow a syn elimination pathway based on the stereospecific nature of the reaction (Eqs. 10.92 and 10.93). 

Tosyl arenecarboxaldoximes, ArCH=NOTs, undergo unimolecular pyrolysis at 500 °C via a cyclic six-membered transition state to yield toluenesulfonic acid and the corresponding nitrile the oximes are ca 10" -fold more reactive than hydrazone counterparts which undergo an analogous thermal elimination. ... 

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