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 diaz o compounds. For competitive intramolecular cyclopropanation versus in tramolecular 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) c aprolactamate (Rh2(cap)4) gives only the cyclopropanation product. Sim ilar results are obtained from dirhodium(II) catalysts in competitive intramolecular cyclopropanation versus tertiary C-H insertion, aromati c cycloaddition versus C-H insertion, cyclopropanation versus aromatic cycloaddition, and C-H insertion versus aromatic substitution. The or der of reactivity for metal carbenes generated from Rh2(pfb)4 is aroma tic substitution > tertiary C-H insertion > cyclopropanation is simila r to aromatic cycloaddition > secondary C-H insertion, and the rate di fferences between them are as much as 100-fold. For Rh2(cap)4 the orde r of reactivity is cyclopropanation > tertiary C-H insertion > seconda ry C-H insertion > aromatic cycloaddition with aromatic substitution n ot observed as a competing process for the diazo compounds examined. C ontrol of chemoselectivity through charge and/or frontier molecular or bital properties of the intermediate metal carbene has been evaluated. Competitive product formation from dirhodium(II) caprolactamate catal yzed reactions of N-tert-butyl-N-benzyldiazoacetoacetamide is temperat ure dependent over a narrow 15-deg range. The effect of carbene substi tuents other than the ligated dirhodium(II) on chemoselectivity is des cribed and discussed.