Diruthenium Tetracarboxylate-Catalyzed Enantioselective Cyclopropanation with Aryldiazoacetates

A series of chiral bowl-shaped diruthenium(II,III) tetracarboxylatecatalysts were prepared and evaluated in asymmetric cyclopropanationswith donor/acceptor carbenes derived from aryldiazoacetates. The dirutheniumcatalysts self-assembled to generate C (4)-symmetric bowl-shaped structures in an analogous manner to theirdirhodium counterparts. The optimum catalyst was found to be Ru-2(S-TPPTTL)(4)& BULL;BArF [S-TPPTTL = (S)-2-(1,3-dioxo-4,5,6,7-tetraphenylisoindolin-2-yl)-3,3-dimethylbutanoate,BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate],which resulted in the cyclopropanation of a range of substrates inup to 94% ee. Synthesis and evaluation of first-row transition-metalcongeners [Cu(II/II) and Co(II/II)] invariably resulted in catalyststhat afforded little to no asymmetric induction. Computational studiesindicate that the carbene complexes of these dicopper and dicobaltcomplexes, unlike the dirhodium and diruthenium systems, are proneto the loss of carboxylate ligands, which would destroy the bowl-shapedstructure critical for asymmetric induction.