Level 3 - Case 34
Helicenophanes and their Racemization
Key point: Activation parameters
Helicenophanes and their Racemization
Helicenes are a fascinating class of compounds. They are prototypes of overcrowded molecules adopting helical configurations, and hence they are chiral. As
a group, they share a very high specific rotation (for example [a]n
25 = 9620 for
[ l3]helicene), and have been found to racemize thermally (Scheme 34.1 ). These
facts make them archetypical compounds to study the physico-chemistry of the racemization process by polarimetry. The accepted mechanism to account for this
racemization takes place through a conformational change, with the necessary
bond defonnations being spread over much of the molecular skeleton.
1
Scheme 34.1
It is known that alkoxy-substituted helicenes racemize more slowly than their
unsubstituted congeners. For example helicene 2 has a t 112 more than twice as long
as that ofthe unsubstituted parent helicene 1 (Figure 34.1). 1 To study the influence
1 t 1 " 2 is the halflife of the reaction, that is, the time required for the amount or concentration of reactant to decrease by 50%.