LIGAND EFFECTS ON DIRHODIUM(II) CARBENE REACTIVITIES - HIGHLY EFFECTIVE SWITCHING BETWEEN COMPETITIVE CARBENOID TRANSFORMATIONS

Carboxylate and carboxamide ligands of dirhodium(II) catalysts control chemoselectivity in competitive metal carbene transformations of diazo compounds. For competitive intramolecular cyclopropanation versus intramolecular aromatic substitution with 1-diazo-3-aryl-5-hexen-2-ones, use of Rh2(OAc)4 results in the products from both transformations in nearly equal amounts, but dirhodium(II) perfluorobutyrate (Rh2(pfb)4) provides only the aromatic substitution product while dirbodium(II) caprolactamate (Rh2(cap)4) gives only the cyclopropanation product. Similar results are obtained from dirhodium(II) catalysts in competitive intramolecular cyclopropanation versus tertiary C-H insertion, aromatic cycloaddition versus C-H insertion, cyclopropanation versus aromatic cycloaddition, and C-H insertion versus aromatic substitution. The order of reactivity for metal carbenes generated from Rh2(pfb)4 is aromatic substitution > tertiary C-H insertion > cyclopropanation is similar to aromatic cycloaddition > secondary C-H insertion, and the rate differences between them are as much as 100-fold. For Rh2(cap)4 the order of reactivity is cyclopropanation > tertiary C-H insertion > secondary C-H insertion > aromatic cycloaddition with aromatic substitution not observed as a competing process for the diazo compounds examined. Control of chemoselectivity through charge and/or frontier molecular orbital properties of the intermediate metal carbene has been evaluated. Competitive product formation from dirhodium(II) caprolactamate catalyzed reactions of N-tert-butyl-N-benzyldiazoacetoacetamide is temperature dependent over a narrow 15-deg range. The effect of carbene substituents other than the ligated dirhodium(II) on chemoselectivity is described and discussed.