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.