Preparation, Aromaticity, and Bromination of Spiro Iridafurans

Iridium centers of [Ir(μ-Cl)(C8H14)2]2 (1) activate the
Cβ(sp2)–H bond of benzylideneacetone
to give [Ir(μ-Cl){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (2), which is the starting point
for the preparation
of the spiro iridafurans IrCl{κ2-C,O-[C(Ph)CHC(Me)O]}2(PiPr3) (3), [Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2(MeCN)2]BF4 (4), [Ir(μ–OH){κ2-C,O-[C(Ph)CHC(Me)O]}2]2 (5), Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-C,N-[C6MeH3-py]} (6), and Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acac]} (7). The five-membered rings are orthogonally arranged with the oxygen
atoms in trans in an octahedral environment of the
iridium atom. Spiro iridafurans are aromatic. The degree of aromaticity
and the negative charge of the CH-carbon of the rings depend on ligand trans to the carbon directly attached to the metal. Aromaticity
has been experimentally confirmed by bromination of iridafurans with N-bromosuccinimide (NBS). Reactions are sensitive to the
degree of aromaticity of the ring and the negative charge of the attacked
CH-carbon. Iridafurans can be selectively brominated, when different
ligands lie trans to metalated carbons. Bromination
of 3 occurs in the ring with the metalated carbon trans to chloride, whereas the bromination of 6 takes place in the ring with the metalated carbon trans to pyridyl. The first gives IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}(PiPr3) (8), which reacts
with more NBS to form IrCl{κ2-C,O-[C(Ph)CBrC(Me)O]}2(PiPr3) (9). The second yields Ir{κ2-C,O-[C(Ph)CBrC(Me)O]}{κ2-C,O-[C(Ph)CHC(Me)O]}{κ2-C,N-[C6MeH3-py]} (10). The origin
of the selectivity is kinetic, with the rate-determining step of the
reaction being the NBS attack. The activation energy depends on the
negative charge of the attacked atom; a higher negative charge allows
for a lower activation energy. Accordingly, complex 7 undergoes bromination in the acetylacetonate ligand, giving Ir{κ2-C,O-[C(Ph)CHC(Me)O]}2{κ2-O,O-[acacBr]} (11).