SolutionsPyrolytic syn eliminations
Exercise 1
The S-methyl xanthate of trans-2-methylcyclohexanol is heated to 200 °C. Which alkene is formed mainly? And if the cis isomer is used?
The elimination is syn: only a β hydrogen cis to the xanthate can leave. In the trans isomer, the H on C2 (the one bearing the methyl) is cis to the leaving group, but so is one of the H atoms on C6; both alkenes form, 1-methylcyclohexene and 3-methylcyclohexene, the latter in substantial proportion. In the cis isomer, the H on C2 is trans and cannot be eliminated, so only 3-methylcyclohexene, the less substituted alkene, is formed.
Exercise 2
Propose a synthesis of styrene and N,N-dimethylhydroxylamine from N,N-dimethyl-2-phenylethylamine. What advantage does this reaction have over the Hofmann elimination?
Cope elimination: the tertiary amine is oxidized with H2O2 (or m-CPBA) to the N-oxide, and heating (100–150 °C) brings about the intramolecular syn elimination, giving styrene and Me2NOH. Compared with the Hofmann elimination there is no need to prepare the quaternary ammonium salt with MeI or to use Ag2O and a strong base at high temperature; the conditions are neutral and milder.