Chemistry Reference
In-Depth Information
Scheme 6.6 trans-Periplanar relationships lead to direct E2 eliminations.
A practical example demonstrating the importance of the trans-periplanar relationship
between protons and leaving groups is illustrated in Scheme 6.5. As shown, when treated
with base, the 1,2-cis-substituted cyclohexane analog rapidly converts to the illustrated
cyclohexene. However, the same reaction conditions applied to the 1,2-trans analog
results in conversion to the cyclohexene analog at a much slower rate. These observations
are mechanistically explained in Schemes 6.6 and 6.7. As shown in Scheme 6.6, the 1,2-cis
analog, possessing a trans-periplanar relationship, reacts through a direct E2 elimination
mechanism. However, as shown in Scheme 6.7, the 1,2-trans analog must first proceed
through deprotonation followed by delocalization of the resulting anion into the ester
Scheme 6.7 E2 eliminations can proceed in the absence of a trans-periplanar relationship in the
reaction substrate if reaction intermediates can obtain conformations that are favorable for elimination
reactions to occur.
