Ligand effects on dirhodium(II) carbene reactivities. Highly effective switching between competitive carbenoid transformations

Article abstract of DOI:10.1021/ja00072a021

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 dirhodium(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 ～ 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.