Synthesis, Characterization, Properties, and Asymmetric Catalytic Diels−Alder Reactions of Chiral-at-Metal Imino−Iridium(III) Complexes

The synthesis and characterization of optically active
imino complexes (RIr,RC)-
and (SIr,RC)-[(η5-C5Me5)IrCl(imine)][SbF6]
(imine = Ln =
N-(2-pyridylmethylene)−(R)-1-phenylethylamine
(L1; 1a,a‘),
N-(2-pyridylmethylene)-(R)-1-naphthylethylamine
(L2; 2a,a‘),
N-(2-quinolylmethylene)−(R)-1-naphthylethylamine (L3;
3a,a‘),
N-(6-methyl-2-pyridylmethylene)−(R)-1-naphthylethylamine (L4; 4a,a‘),
N-(2-pyridylmethylene)−(R)-1-cyclohexylethylamine
(L5;
5a,a‘),
N-(2-pyridylmethylene)−(1R,2S,4R)-1-bornylamine)
(L6; 6a,a‘)),
(RIr,RC)- and
(SIr,RC)-[(η5-C5Me5)IrCl(L2)][A]
(A = Cl (7a,a‘), BF4
(8a,a‘), PF6
(9a,a‘),
(1S)-camphor-10-sulfonate
(R*SO3) (10a,a‘)) and
(RIr,RC)- and
(SIr,RC)-[(η5-C5Me5)(L6)(H2O)][SbF6]2
(11a,a‘)) are reported.
The absolute crystal structures of
(RIr,RC)-1a,
(RIr,RC)-2a,
and
(RIr,RC)-3a
were determined
by X-ray analysis. All three complexes show the chiral metal
center in a pseudo-octahedral
environment, being bonded to an
η5-C5Me5 ring, to a terminal
chloride, and, in a chelate
fashion, to the two nitrogen atoms of the imine ligands. For the
chloride compounds 1−6,
1H NMR solution data reveal conformational differences
between the crystal and solution
structures. At room temperature, in acetone, complexes
1−11 are configurationally stable.
At 62 °C, in methanol, the more labile chloro complex
5a epimerizes at Ir with a half-life of
68.0 min (activation parameters, ΔH⧧ =
97.7 ± 13.2 kJ mol-1 and
ΔS⧧= −26.3 ± 6.2 J
K-1mol-1;
equilibrium constant, 5a‘/5a = 4.26
± 1.65). Dichloromethane/acetone solutions
of the solvate complexes
[(η5-C5Me5)Ir(imine)S][SbF6]2,
prepared in situ by treating complexes
1−6 with equimolar amounts of
AgSbF6, are active catalysts for the Diels−Alder
reaction
between methacrolein and cyclopentadiene. The reaction occurs
rapidly at room temperature
with good exo:endo selectivity (84:16 to 95:5) and moderate
enantioselectivities (up to 46%).