Bis(pyridylimino)isoindolato-iridium complexes as epoxidation catalysts for alkenes

Article abstract of DOI:10.1021/om1010116

The reaction of the sodium salts of ligands 1a,b (1a = 1,3-bis(2-(5-(3,5- xylyl)pyridyl)imino)-5,6-dimethylisoindole, 1b = 1,3-bis(2-(4-tert-butylpyridyl) imino)-5,6-dimethylisoindole) with [Ir(μ-Cl)(COD)]₂ (COD = cyclooctadiene) and [Ir(μ-Cl)(C₂H₄)₂] ₂ afforded the corresponding isoindolato complexes [{BPI(1a,b)}Ir^I(COD)] (2a,b) and [{BPI(1a,b)}Ir^I(C ₂H₄)₂] (3a,b), respectively. The catalytic activity of the complexes 2a,b was tested in the epoxidation of a wide range of non-electron-rich olefins, using PPO (PPO = 3-phenyl-2-(phenylsulfonyl)-1,2- oxaziridine) as oxidizing agent, giving the corresponding epoxides in moderate to high yields.

Full text of DOI:10.1021/om1010116

Organometallics 2011, 30, 379–382 379
DOI: 10.1021/om1010116
Bis(pyridylimino)isoindolato-Iridium Complexes as
Epoxidation Catalysts for Alkenes
ꢀ
Jose A. Camerano, Christoph Samann, Hubert Wadepohl, and Lutz H. Gade*
€
€
Anorganisch-Chemisches Institut, Universitat Heidelberg, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany
Received October 25, 2010
Summary: The reaction of the sodium salts of ligands 1a,b (1a =
development of efficient Ir-catalyzed allylic substitutions,6
C-C and C-X couplings,7 cycloadditions,8 and hydrosilyla-
tions9 as well as alkane dehydrogenations10 indicate the rapid
expansion of this field. In contrast, very little is known about
iridium in oxidation catalysis. The use of iridium complexes
as epoxidation catalysts has been barely developed,11 prob-
ably due to the low activity in this kind of catalysis as repor-
ted in some early papers.12
The choice of the appropriate ancillary ligands is particu-
larly crucial in the development of oxidation catalysts, since
they have to withstand the reactivity of the oxidants employed.
Monoanionic meridionally coordinating tridentate (“pincer”)
ligand systems have proved to give rise to molecular catalysts
1,3-bis(2-(5-(3,5-xylyl)pyridyl)imino)-5,6-dimethylisoindole, 1b =
1,3-bis(2-(4-tert-butylpyridyl)imino)-5,6-dimethylisoindole)
with [Ir(μ-Cl)(COD)]2 (COD = cyclooctadiene) and [Ir(μ-Cl)-
(C2H4)2]2 afforded the corresponding isoindolato complexes
[{BPI(1a,b)}IrI(COD)] (2a,b) and [{BPI(1a,b)}IrI(C2H4)2]
(3a,b), respectively. The catalytic activity of the complexes
2a,b was tested in the epoxidation of a wide range of non-
electron-rich olefins, using PPO (PPO = 3-phenyl-2-(phenyl-
sulfonyl)-1,2-oxaziridine) as oxidizing agent, giving the corre-
sponding epoxides in moderate to high yields.
Whereas the applications of rhodium in molecular cata-
lysis are now innumerable,1 the development of catalytic
processes involving its heavier congener iridium has pro-
gressed more slowly.2 The latter has been extensively em-
ployed in the quest for reactive and structural models of
intermediates in the mechanistic cycles of 4d platinum metals
and, in particular, rhodium.3 However, iridium complexes
have proved to be active and selective catalysts in the pro-
duction of both bulk chemicals, such as the carbonylation of
methanol,4 as well as fine chemicals, as exemplified by an
ever-growing number of enantioselective hydrogenations of
CdC and CdX bonds in prochiral substrates.5 The recent
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*To whom correspondence should be addressed. Fax: þ49-6221-
545609. E-mail: lutz.gade@uni-hd.de.
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2011 American Chemical Society
Published on Web 01/04/2011
pubs.acs.org/Organometallics

380 Organometallics, Vol. 30, No. 3, 2011
Camerano et al.
Scheme 1. Synthesis of the Iridium(I) Complexes 2a,b and 3a,b
center and overall molecular C2v symmetry, the structures of
the COD complexes 2a,b were found to be less symmetrical.
The 1H NMR spectra of 2a,b exhibit different sets of signals
for the pyridyl and isoindolate protons and two singlets, at
1.34 and 1.28 ppm for 2b, assigned to two nonequivalent tert-
butyl groups. This indicates that the deprotonated isoindoles
1a,b coordinate the iridium atom in a κ2 fashion to form 2a,b
and thus the CdN double bond of the uncoordinated pyridine
arm is isomerized to an E conformation. The resulting per-
pendicular orientation of the pyridine ring with respect to
one of the protons of the isoindole backbone has a large
influence on its chemical shift (observed at 6.09 ppm for 2a
and 5.84 ppm for 2b), since it is located above the anisotropic
cone of the pyridine ring. The olefinic protons of the cyclo-
octadiene ligand resonate at 5.64 and 3.66 ppm for 2a and at
5.66 and 3.60 ppm for 2b as broad singlets, which is also
consistent with the nonsymmetric structure of the complex.17
The 13C NMR spectra of 2a,b are in agreement with the non-
symmetric geometry of the complex. The COD ligand gives
rise to five 13C NMR signals, unambiguously assigned on the
basis of a 1H-13C HSQC experiment. Two singlets appear at
67.2 and 63.0 ppm for 2a and at 67.9 and 62.9 ppm for 2b,
representing the olefinic carbons coordinated to the iridium
center, and at 31.7 and 30.6 ppm for 2a and at 32.0 and 30.4
ppm for 2b, the methylene 13C nuclei.
A single-crystal X-ray study of complex 2b confirmed the
general structural assignment based on the spectroscopic data
and established a slightly distorted square planar structure
with the BPI ligand coordinated to the iridium atom by N1
and N3 in a κ2 fashion (Figure 1).18 The amido-like N(3)-Ir
bondlengthis shorter thanthe interatomic distanceassociated
of great stability.13 In particular, the readily accessible mod-
ular bis(pyridylimino)isoindolato (BPI) derivatives14 have
been employed in oxidation catalysis with cobalt and iron.15
In this paper, we report the preparation of (BPI)IrI(COD)
complexes and the exploration of their catalytic activity in
epoxidations of substituted alkenes using PPO (PPO = 3-phenyl-
2-(phenylsulfonyl)-1,2-oxaziridine) as oxidizing agent16 is
discussed.
˚
with the coordinated pyridyl arm N(1)-Ir (by ∼0.05 A) as
usually observed in BPI metal complexes.19 The arrange-
ment of the dangling, uncoordinated pyridyl unit is char-
acterized by the torsion angle C(18)-N(4)-C(1)-N(1)
of -164.3(3)°. This orientation explains the shift to higher
field of the proton of the isoindole group at C(5) observed in
1
the H NMR spectrum as discussed above. The distances
The iridium complexes 2a,b and 3a,b were prepared by
reaction of the in situ generated sodium salts of 1a,b and sub-
sequent addition of [Ir(μ-Cl)(COD)]2 or [Ir(μ-Cl)(C2H4)2]2 to
the resulting suspension. After the reaction mixtures were
stirred at ambient temperature for 4 h, the corresponding
complexes [{BPI(1a,b)}IrI(COD)] (2a,b) and [{BPI(1a,b)}IrI-
(C2H4)2] (3a,b) were isolated in good yields (Scheme 1).
The formulation of complexes 2a,b as [{BPI(1a,b)}IrI-
(COD)] as well as of the corresponding ethylene compounds
3a,b was established by elemental analyses and their mass spectra
(FABþ), which were dominated by the respective molecular
ion and the complex fragment {(BPI)Ir}þ. However, the 1H
and 13C NMR spectra of the four Ir complexes indicated a
significant difference of the structures on going from COD to
the bis-ethylene derivatives. Whereas the latter displays resonance
patterns consistent with 5-fold coordination at the metal
€
(17) Wilbuer, A.; Vlecken, D. H.; Schmitz, D. J.; Kraling, K.; Harms,
K.; Bagowski, C. P.; Meggers, E. Angew. Chem., Int. Ed. 2010, 49, 3839.
(18) Crystal data and data collection and structure refinement details
for complex 2b: C36H44IrN5, monoclinic, space group P21/c, a =
˚
˚
˚
13.165(7) A, b = 10.728(5) A, c = 22.186(10) A, β = 101.711(6)°,
V = 3068(3) A , Z = 4, μ = 4.386 mm-1, F000 = 1488. T = 100(2) K, θ
3
˚
range 2.1-31.5°, index ranges h, k, l (indep set) -19 to þ18, 0-15, 0-32,
77 512 reflections measured, 10 141 independent reflection (Rint
0.0687), 7698 observed reflections (I > 2σ(I)), final R indices (Fo
=
>
4σ(Fo)) R(F) = 0.0292, Rw(F2) = 0.0515, GOF = 1.017, intensity data
collection Bruker AXS Smart 1000 CCD diffractometer, Mo KR radia-
˚
tion, graphite monochromator, λ = 0.710 73 A, Lorentz, polarization,
and numerical absorption correction,18b structure solution by direct
methods,18c,d refinement by full-matrix least-squares methods based on
F2,18d,e all non-hydrogen atoms anisotropic, hydrogen atoms at calcu-
lated positions (refined riding), except those of the cod methyne groups
(refined with d(CH) restrained equal). (b) Sheldrick, G. M. SADABS;
Bruker AXS, 2004-2008. (c) Sheldrick, G. M. SHELXS-97; University of
€
€
Gottingen, Gottingen, Germany, 1997. (d) Sheldrick, G. M. Acta Crystal-
logr. 2008, A64, 112. (e) Sheldrick, G. M. SHELXL-97; University of
€
€
Gottingen, Gottingen, Germany, 1997.
(19) (a) Hui-Min, W.; Yu-Hui, W.; Yang, F.; Li-Yi, Z.; Chang-Neng,
C.; Zhong-Ning, C. Inorg. Chem. 2010, 49, 2210. (b) Langlotz, B. K.;
Wadepohl, H.; Gade, L. H. Angew. Chem., Int. Ed. 2008, 47, 4670.
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(b) Meder, M. B.; Siggelkow, B. A.; Gade, L. H. Z. Anorg. Allg. Chem.
2004, 630, 1962.
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(c) Broring, M.; Kleeberg, C; Consul-Tejero, E. Eur. J. Inorg. Chem.
2007, 3208.
(20) (a) Tejel, C.; del Rio, M. P.; Ciriano, M. A.; Reijerse, E. J.; Hartl,
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(16) (a) Horner, L.; Jurgens, E. Chem. Ber. 1957, 90, 2184. (b) Davis,
F. A.; Lamendola, J., Jr.; Nadir, U. K.; Kluger, E. V.; Sedergran, T. C.;
Panunto, T. W.; Billmers, R., Jr.; Turchi, I. J.; Watson, W. H.; Chen, J. S.;
Kimura, M. J. Am. Chem. Soc. 1980, 102, 2000. (c) Davis, F. A.; Sheppard,
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F.; Zalis, S.; Hetterscheid, D. G. H.; Tsichlis i Spithas, N.; de Bruin, B.
Chem. Eur. J. 2009, 15, 11878. (b) Mc Bee, J. L.; Escalada, J.; Tilley, D. T.
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Communication
Organometallics, Vol. 30, No. 3, 2011 381
Table 1. Epoxidation Catalysis of Complexes 2a,bd
˚
Figure 1. Molecular structure of 2b. Selected bond lengths (A)
and angles (deg): Ir-N(1) = 2.087(2), N(2)-C(4) = 1.291(4),
Ir-C(33) = 2.122(3), N(2)-C(9) = 1.362(4), Ir-C(29) =
2.124(3), N(3)-C(9) = 1.361(4), Ir-N(3) = 2.134(3), N(3)-
C(13) = 1.363(4), Ir-C(30) = 2.125(3), N(4)-C(1) = 1.274(4),
Ir-C(34) = 2.143(3), N(4)-C(18) = 1.404(4), N(1)-C(4) =
1.380(4), N(5)-C(18) = 1.332(4), N(1)-C(1) = 1.418(4), N(5)-
C(22) = 1.342(4); N(1)-Ir-C(33) = 92.0(1), N(3)-Ir-C(29) =
91.5(1), Ir-N(1)-C(1) = 131.1(2), N(1)-Ir-C(34) = 95.6(1),
N(3)-Ir-C(30) = 91.6(1), Ir-N(1)-C(4) = 121.6(2), N(1)-
Ir-N(3) = 88.82(9), Ir-N(3)-C(13) = 119.8(2), C(33)-Ir-
C(30) = 80.5(1), N(1)-C(1)-N(4) = 122.3(3), N(1)-C(4)-
N(2) =132.0(3),C(29)-Ir-C(34) =79.5(1),C(1)-N(4)-C(18) =
121.7(3), C(4)-N(2)-C(9) = 126.8(3).
between the iridium atom and the olefinic carbons are similar
to the values found in the literature.20
The robust nature of the BPI ligands, which contain three
hard nitrogen donors, makes them of interest for oxidation
catalysis. In general, complexes 2a,b and 3a,b behaved in a
very similar way in these studies. However, due to the greater
thermal stability of the precatalysts 2a,b, only these com-
pounds were studied systematically. In an initial screening of
the appropriate oxygenation reagents and reaction conditions,
cis-cyclooctene was chosen as the test substrate. Hydrogen
peroxide, iodosobenzene, and PPO (PPO = 3-phenyl-2-(phenyl-
sulfonyl)-1,2-oxaziridine) were tested as oxygen transfer agents
in the presence of the iridium complexes in reaction mixtures
using dichloromethane, chloroform, benzene, thf, acetone,
and acetonitrile as solvents. All our attempts using hydrogen
peroxide as oxidant led to a rapid deactivation of the cata-
lyst, due to the oxidative degradation of the Ir complex. While
iodosobenzene gave rise to moderate activity in the epoxida-
tion of the test substrate (49% conversion after 48 h with a
catalyst/substrate/oxidant ratio of 1/10/40), PPO proved to
be the epoxidizing agent of choice, probably due to its greater
solubility in most organic solvents favoring the transfer of
oxygen. Initial studies (see the Supporting Information) esta-
blished the optimized catalyst/substrate/oxidant ratio of
1/100/150 used in all subsequent catalytic tests, which were
carried out for a wide range of non-electron-rich olefins. Blank
reactions showed that no significant amount of epoxide was
formed in the absence of catalyst, corroborating results pre-
viously described in the literature.21 Results obtained for the
a Conversion into the corresponding epoxides; determined by 1H
NMR integral analysis. In brackets diastereomeric excess determined
by 1H NMR integral analysis. b After the time indicated no further
conversion was observed. c The isolated yields were determined using
complex 2b as catalyst and the experimental conditions described above.
d Experimental conditions for all experiments: 1 equiv of catalyst, 100
equiv of substrate, 150 equiv of PPO in dichloromethane; concentration
of the catalyst in the reaction mixture 15 mM.
different substrates, which were converted to their corre-
sponding epoxides, are given in Table 1.
As displayed in entries 1-10, olefins with different sub-
stitutional patterns were epoxidized, giving moderate to high
yields. As indicated above, 3-phenyl-2-(phenylsulfonyl)-1,2-
oxaziridine is a good epoxidizing agent by itself for very
electron rich alkenes such as 2,3-dimethyl-2-butene but is
much less reactive toward less electron rich olefins. For
example, trans-stilbene was epoxidized in high yield in the
presence 1 mol % of 2a,b as catalyst within 24 h (entry 12),
while cis-stilbene required 48 h (entry 11). In this case,
virtually no formation of trans-stilbene oxide was obtained
(compared to 36% of trans epoxide formation observed
(21) Yuan, L. C.; Bruice, T. C. J. Chem. Soc., Chem. Commun. 1985,
868.

382 Organometallics, Vol. 30, No. 3, 2011
Camerano et al.
previously when the reaction was performed at 60 °C without
catalyst).22 The comparison of entries 14 and 15 shows a
clear effect of the catalytic activity of the iridium complexes:
in entry 14 the conversion of trans-R-methylstilbene in the
epoxide is 99% (within 48 h) for complexes 2a,b; however,
with triphenylethylene as substrate 88% (2a) or 90% (2b)
was observed within 22 h (entry 15). In this case the less
nucleophilic olefin (entry 15) is epoxidized more rapidly than
the more electron rich substrate (entry 14), which is contrary
to the observed noncatalyzed behavior.
(and in 93% yield and 97% de using 2b as catalyst), the
epoxidation of R-pinene led to a conversion of 94% with
64% de using 2a as catalyst (and 99% (70% de) for 2b). The
observation that the two catalysts have relatively similar
activities indicates that a difference in the substitution pat-
terns of the ancillary BPI ligands does not significantly affect
the catalyst performance. The activities of complexes 2a,b in
the epoxidation of alkenes with PPO are significantly higher
than those reported using this oxidant and manganese por-
phyrin complexes as catalyst,23 while being comparable with
the results obtained for other oxidants and metal catalysts.
Bicyclic monoterpenes were also tested as substrates (entries
17 and 18) for the epoxidation, giving good conversion with
high diastereomeric excesses in both cases. Whereas β-pinene
was epoxidized in 94% yield and 85% de using 2a as catalyst
Acknowledgment. We thank the Deutsche Forschungs-
gemeinschaft for funding (SFB 623) and the Ministerio
~
ꢀ
Espanol de Ciencia e Innovacion for a postdoctoral
(22) Davis, F. A.; Abdul-Malik, N. F.; Awad, S. B.; Harakal., M. E.
Tetrahedron Lett. 1981, 917.
(23) (a) Murali, M.; Mayilmurugan, R.; Paliniandavar, M. Eur. J.
fellowship (to J.A.C.).
Supporting Information Available: Text, figures, and a table
giving synthesis and characterization data of all new compounds
and CIF files giving crystallographic information. This material
is available free of charge via the Internet at http://pubs.acs.org.
ꢀ
Inorg. Chem. 2009, 22, 3238. (b) Roy, P.; Nandi, M.; Manassero, N.; Ricco,
M.; Mazzani, M.; Bhaumik, A.; Banerjee, P. Dalton Trans. 2009, 9543.
(c) McDonald, A. R.; Franssen, N.; van Klink, G. P. M.; van Koten, G.
J. Organomet. Chem. 2009, 14, 2153.