3 years ago

Structure and Reactivity of Half-Sandwich Rh(+3) and Ir(+3) Carbene Complexes. Catalytic Metathesis of Azobenzene Derivatives

Structure
and Reactivity of Half-Sandwich Rh(+3) and
Ir(+3) Carbene Complexes. Catalytic Metathesis of Azobenzene Derivatives
Alois Fürstner, Christophe Werlé, Daniel J. Tindall, Christophe Farès, Petra Philipps, Richard Goddard
Traditional rhodium carbene chemistry relies on the controlled decomposition of diazo derivatives with [Rh2(OAc)4] or related dinuclear Rh(+2) complexes, whereas the use of other rhodium sources is much less developed. It is now shown that half-sandwich carbene species derived from [Cp*MX2]2 (M = Rh, Ir; X = Cl, Br, I, Cp* = pentamethylcyclopentadienyl) also exhibit favorable application profiles. Interestingly, the anionic ligand X proved to be a critical determinant of reactivity in the case of cyclopropanation, epoxide formation and the previously unknown catalytic metathesis of azobenzene derivatives, whereas the nature of X does not play any significant role in −OH insertion reactions. This perplexing disparity can be explained on the basis of spectral and crystallographic data of a representative set of carbene complexes of this type, which could be isolated despite their pronounced electrophilicity. Specifically, the donor/acceptor carbene 10a derived from ArC(═N2)COOMe and [Cp*RhCl2]2 undergoes spontaneous 1,2-migratory insertion of the emerging carbene unit into the Rh–Cl bond with formation of the C-metalated rhodium enolate 11. In contrast, the analogous complexes 10b,c derived from [Cp*RhX2]2 (X = Br, I) as well as the iridium species 13 and 14 derived from [Cp*IrCl2]2 are sufficiently stable and allow true carbene reactivity to be harnessed. These complexes are competent intermediates for the catalytic metathesis of azobenzene derivatives, which provides access to α-imino esters that would be difficult to make otherwise. Rather than involving metal nitrenes, the reaction proceeds via aza-ylides that evolve into diaziridines; a metastable compound of this type has been fully characterized.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b12673

DOI: 10.1021/jacs.7b12673

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