6.19: E1 Reaction: Stereochemistry and Regiochemistry
One of the critical aspects of the E1 reaction mechanism, as also observed in E2, is the regiochemistry, with multiple regioisomers obtained as products. In the example discussed, the presence of water as a weak base favors elimination over substitution to generate two alkenes. Given that alkenes’ stability increases with the number of alkyl groups across the double bond, typically, E1 reactions lead to the Zaitsev product, for this is more substituted and stable than the Hofmann product. Further, the transition state intermediate in the Zaitsev product pathway has lower energy, confirming that this Zaitsev product is both thermodynamically stable and kinetically favored.
The E1 mechanism is independent of the nature of the base; hence, the regioselectivity of E1 eliminations is not tailorable using sterically hindered bases. An instance of this is the formation of Zaitsev products irrespective of using a bulky base like potassium tert-butoxide. However, at times, the expected alkene is not obtained as the primary product, owing to the E1 mechanism of a carbocation intermediate where a 1,2-hydride shift can occur. This leads to the more stable tertiary carbocation, generating a tetrasubstituted alkene instead.
In general, the E1 reactions are stereoselective, as they favor the formation of the E or trans alkene over the Z or cis isomer. However, they are not stereospecific like E2 reactions and do not factor in the planarity of the hydrogen and halogen. Here, it depends on the orientation of the neighboring vacant p orbital on the positively charged carbon and its adjacent carbon–hydrogen σ bond that ought to be parallel for forming an optimal π bond. The intermediate carbocation in the mechanism of E1 satisfies this requirement in two configurations: (a) the less stable syn conformation, which is sterically strained, and (b) the more stable anti conformation, where the bulky groups are farther apart. Consequently, the syn conformation leads to the minor product of the Z-alkene, which is less stable, and the anti conformation yields the more stable E-alkene with less steric hindrance as the primary product.