Rhodium(II)-Catalyzed Asymmetric Cyclopropanation and Desymmetrization of [2.2]Paracyclophanes

Chiral [2.2]paracyclophane derivatives are of considerable
interest
because of their potential in asymmetric catalysis and the development
of chiral materials. This study describes the scope of rhodium-catalyzed
reactions of aryldiazoacetates with [2.2]paracyclophanes. The reaction
with the parent [2.2]paracyclophane resulted in cyclopropanation at
two positions, the ratio of which is catalyst-controlled. Because
of the strain in the system, one of the cyclopropanes exists primarily
as the norcaradiene structure, whereas the other preferentially exists
as the cycloheptatriene conformer. In contrast, the reaction with
[3.3]paracyclophane results in benzylic C–H functionalization.
The reactions with substituted [2.2]paracyclophanes using chiral catalysts
can result in either kinetic resolution or desymmetrization. The Rh2(S-p-PhTPCP)]4-catalyzed reaction of monosubstituted paracyclophanes results in
kinetic resolution with a selectivity (s) factor
of up to 20, whereas reactions on C2v-symmetric disubstituted
[2.2]paracyclophanes with Rh2(S-TPPTTL)4 [TPPTTL = 2-(1,3-dioxo-4,5,6,7-tetraphenylisoindolin-2-yl)-3,3-dimethylbutanoate]
results
in effective desymmetrization to form cycloheptatriene-incorporated
paracyclophanes in 78–98% ee.