Allylic C–H bond activation and functionalization mediated by tris(oxazolinyl)borato rhodium(i) and iridium(i) compounds

Abstract

Allylic C–H bond oxidative addition reactions, mediated by tris(oxazolinyl)borato rhodium(I) and iridium(I) species, provide the first step in a hydrocarbon functionalization sequence. The bond activation products To^MMH(η³-C₈H₁₃) (M = Rh (1), Ir (2)), To^MMH(η³-C₃H₅) (M = Rh (3), Ir (4)), and To^MRhH(η³-C₃H₄Ph) (5) (To^M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) are synthesized by reaction of Tl[To^M] and the corresponding metal olefin chloride dimers. Characterization of these group 9 allyl hydride complexes includes ¹H-¹⁵N heteronuclear correlation NMR experiments that reveal through-metal magnetization transfer between metal hydride and the trans-coordinated oxazoline nitrogen. Furthermore, the oxazoline ¹⁵N NMR chemical shifts are affected by the trans ligand, with the resonances for the grouptrans to hydride typically downfield of those trans to η³-allyl and tosylamide. These group 9 oxazolinylborate compounds have been studied to develop approaches for allylic functionalization. However, this possibility is generally limited by the tendency of the allyl hydride compounds to undergo olefin reductive elimination. Reductive elimination products are formed upon addition of ligands such as CO and CN^tBu. Also, To^MRhH(η³-C₈H₁₃) and acetic acid react to give To^MRhH(κ²-O₂CMe) (8) and cyclooctene. In contrast, treatment of To^MRhH(η³-C₃H₅) with TsN₃ (Ts = SO₂C₆H₄Me) gives the complex To^MRh(η³-C₃H₅)NHTs (10). Interestingly, the reaction of To^MRhH(η³-C₈H₁₃) and TsN₃ yields To^MRh(NHTs)(H)OH₂ (11) and 1,3-cyclooctadieneviaβ-hydride elimination and Rh–H bond amination. Ligand-induced reductive elimination of To^MRh(η³-C₃H₅)NHTs provides HN(CH₂CHCH₂)Ts; these steps combine to give a propene C–H activation/functionalization sequence.