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

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.