Comparison of 1,2-Diarylcyclopropanecarboxylates with 1,2,2-Triarylcyclopropanecarboxylates as Chiral Ligands for Dirhodium-Catalyzed Cyclopropanation and C–H Functionalization

Dirhodium
triarylcyclopropanecarboxylate catalysts (Rh2TPCP4) are sterically demanding and capable of controlling
the site selectivity of C–H functionalization by means of C–H
insertion with donor/acceptor carbenes. This study compares the structures
and reactivity profiles of dirhodium triarylcyclopropanecarboxylates
with dirhodium diarylcyclopropanecarboxylates. The absence of the
third aryl group makes the catalysts less sterically demanding and
lacks a well-defined preferred conformation. The catalysts have a
greater tendency for inducing C–H functionalization at tertiary
C–H bonds versus their triaryl counterparts but are generally
not capable of achieving high levels of asymmetric induction. These
studies confirm the critical requirement of having at least three
substituents on the cyclopropanecarboxylate ligands to have well-defined
sterically demanding catalysts capable of high levels of asymmetric
induction.