6.19 : E1 反応: 立体化学と位置選択性

Just like E2 reactions, E1 eliminations are regioselective and form more than one regioisomers.

In this example, water functions as a weak, non-bulky base, and the reaction is heated to favor elimination over substitution, forming two alkenes.

Recall that the stability of alkenes increases with the number of alkyl groups across the double bond. E1 reactions favor the Zaitsev product since it is more substituted and more stable than the Hofmann product.

Additionally, the transition state leading to the Zaitsev product has a trisubstituted partial double bond, which is lower in energy than the disubstituted counterpart. Therefore, not only is the Zaitsev product thermodynamically stable but it is also formed faster.

Unlike E2 reactions, E1 mechanisms are independent of the nature of the base. Consequently, the regioselectivity of E1 eliminations cannot be controlled using sterically hindered bases.

For example, the same reaction with a weak, bulky base like isopropyl alcohol still favors the Zaitsev over the Hofmann product.

In some E1 reactions, the expected alkene is not the major product because E1 reactions proceed via a carbocation intermediate.

In this example, the secondary carbocation can undergo a 1,2-hydride shift into a more stable tertiary carbocation to give the tetrasubstituted alkene as the major product.

E1 reactions are also stereoselective, favoring the E or trans alkene over the Z or cis isomer. However, unlike E2 reactions, they are not stereospecific and do not require the hydrogen and halogen to be anti-coplanar.

Instead, the vacant p orbital on the positively charged carbon and the adjacent carbon-hydrogen σ bond are required to be parallel for optimal overlap to form the new π bond.

The carbocation can adopt two configurations satisfying this requirement. One is the less stable, sterically strained syn conformation, and the other is the more stable anti conformation with the bulky groups farther apart.

The syn conformation forms the less stable Z-alkene as the minor product, whereas the anti conformation yields the less hindered and more stable E-alkene as the major product.