Comparison of Reactivity and Enantioselectivity between Chiral Bimetallic Catalysts: Bismuth–Rhodium- and Dirhodium-Catalyzed Carbene Chemistry

This
paper describes the influence of replacement of one of the
rhodium atoms by bismuth in a chiral dirhodium tetracarboxylate catalyst
on asymmetric induction in the cyclopropanation and C–H functionalization
chemistry of trichloroethyl aryldiazoacetates. The chiral ligand used
in this study is S-tert-butylsulphonylprolinate
(S-TBSP), which was used in the first highly enantioselective
dirhodium tetracarboxylate catalyst. Even though the replacement of
a Rh atom with a Bi atom has changed the electronic properties of
the system, the RhBi complexes have several similarities to the corresponding
Rh2 catalysts, with regard to their reactions with donor/acceptor
carbenes. The asymmetric induction with RhBi­(S-TBSP)4 is very similar to that achieved with Rh2(S-TBSP)4. The major differences between the two
systems are the rates of reactions with the Rh2 complexes
reacting much faster, and the scope of the C–H functionalization
with Rh2 complexes capable of catalyzing reactions with
a much wider range of substrates. An unexpected structural feature
of the RhBi­(S-TBSP)4 catalyst is the arrangement
of the arylsulfonyl groups in the periphery of the catalyst, leading
to a C4 symmetric structure.