Unimolecular Syn-Eliminations
E2 elimination reactions are commonly bimolecular and prefer an anti-coplanar transition state. This important class of functional transformations is complimented by a small group of thermal, unimolecular syn-eliminations, described in the following table. The syn or suprafacial character of these eliminations is enforced by the 5- or 6-membered cyclic transition states (A & B) by which they take place.
The temperature variations noted in the table suggest that these eliminations are facilitated by a
negative charge on the O or Z atom and a low C–Y bond energy. Amine oxides have a full negative
charge on the oxygen, and the
Cope elimination proceeds well at
temperatures near or slightly above 100 °C. Together with the Hofmann elimination, Cope
eliminations have proven useful for removing a permethylated amino group from a larger molecule.
Sulfoxides are eliminated to sulfenic acids at roughly similar temperatures as the amine oxides.
Here, oxygen charge neutralization by p-d bonding to the positive sulfur atom is balanced by the
weaker C–S bond. Selenoxides eliminate rapidly at low temperature, reflecting a greater charge on
oxygen due to poorer p-d bonding (selenium is much larger than oxygen), and a weak C–Se bond.
Although
a six-membered transition state is relatively unstrained, esters and thioesters of alcohols
require higher temperatures for elimination. This is expected because of the stronger C–O bond and
the lower polarity of C=Z. The thioester function of xanthate derivatives of alcohols undergoes
elimination at much lower temperatures than carboxylic esters, probably reflecting a favorable
bond energy change from O–C=S in the xanthate to S–C=O in the eliminated fragment.
Some examples of these syn-thermal eliminations are given in the following diagram. The ester pyrolysis in equation # 4 demonstrates the importance of a cis-alignment of the eliminating groups, in this case the acetate ester and the vicinal hydrogen atom. Xanthate ester pyrolysis (equation # 5) is known as the Chugaev (or Tschugaev) reaction. Finally, the conversion of 1°-alcohols to aryl selenium ethers prior to selenoxide elimination, as in example # 3, is carried out via a hypervalent phosphorus species similar to that involved in the Mitsunobu reaction. The preferred aryl group in the selenocyanate reagent is o-nitrophenyl.
