Flexible, bowl-shaped n-heterocyclic carbene ligands: Substrate specificity in indium-catalyzed ketone hydrosilylation

Article abstract of DOI:10.1021/om800878m

A series of benzimidazolium chlorides was synthesized as precursors to N-heterocyclic carbene ligands, with N-substituents varying in size from 3,5-xylyl (la) to first-generation dendritic 3,5-bis(3,5-di-tert-butylphenyl) phenyl (lb), to the second-generation 3,5-bis[3,5-bis(3,5-di-terf-butylphenyl) phenyl]phenyl (1c). The dendritic side groups of lb and 1c form a flexible, bowl-like cavity. Iridium complexes of 1a-c were synthesized and were shown to be catalytically active for the hydrosilylation of aryl methyl ketones. The dendritic ligands lb and 1c effect a moderate level of substrate specificity in the competitive hydrosilylation of ketones of varying size. In the competitive hydrosilylation of acetophenone versus 3-(3,5-di-tert-butylphenyl)acetophenone, acetophenone is consumed approximately 3.7 times more quickly using the second-generation ligand 1c. Using the control ligand la, this ratio is 1.8.

Full text of DOI:10.1021/om800878m

Organometallics 2009, 28, 465–472
465
Flexible, Bowl-Shaped N-Heterocyclic Carbene Ligands: Substrate
Specificity in Iridium-Catalyzed Ketone Hydrosilylation
Anthony R. Chianese,* Allen Mo, and Dibyadeep Datta
Department of Chemistry, Colgate UniVersity, 13 Oak DriVe, Hamilton, New York 13346
ReceiVed September 9, 2008
A series of benzimidazolium chlorides was synthesized as precursors to N-heterocyclic carbene ligands,
with N-substituents varying in size from 3,5-xylyl (1a) to first-generation dendritic 3,5-bis(3,5-di-tert-
butylphenyl)phenyl (1b), to the second-generation 3,5-bis[3,5-bis(3,5-di-tert-butylphenyl)phenyl]phenyl
(1c). The dendritic side groups of 1b and 1c form a flexible, bowl-like cavity. Iridium complexes of
1a-c were synthesized and were shown to be catalytically active for the hydrosilylation of aryl methyl
ketones. The dendritic ligands 1b and 1c effect a moderate level of substrate specificity in the competitive
hydrosilylation of ketones of varying size. In the competitive hydrosilylation of acetophenone versus
3-(3,5-di-tert-butylphenyl)acetophenone, acetophenone is consumed approximately 3.7 times more quickly
using the second-generation ligand 1c. Using the control ligand 1a, this ratio is 1.8.
mond, Bergman, et al.12,13 have demonstrated selective catalysis
inside enclosed self-assembled containers. Mirkin and co-
workers14,15 have designed allosterically regulated catalysts.
One strategy that mimics some features of enzyme catalysis
is to enclose a catalyst in the core of a dendrimer.16-22 With
higher generations, the reaction microenvironment is defined
increasingly by the dendrimer structure rather than the solvent.
Interesting and potentially useful effects have been observed,
including increased catalyst activity,23,24 varied selectivity,25-27
and substrate specificity.28,29
Introduction
A central goal in synthetic chemistry is the development of
organic transformations that are simultaneously general and
specific. The most widely useful methods offer predictable
selectivity for a well-defined class of substrates, ideally tolerating
variations in substrate auxiliary functionality and steric size.
Homogeneous catalysis is at the center of this effort, as rational
and empirical variation of the catalyst structure provides a
versatile, cost-effective manifold for optimizing the reaction
outcome. Enzymes are known for their often extreme substrate
specificity, but Nature has also evolved catalysts with syntheti-
cally useful generality, as evidenced by the utility of lipases in
kinetic resolutions.1
This report describes the synthesis of dendritic N-heterocyclic
carbene (NHC)30 ligands expected to exhibit a bowl-like
topology and their application to iridium-catalyzed ketone
hydrosilylation, with an emphasis on discrimination between
Efforts at bridging the gap between traditional synthetic
catalysis and the exquisite, superstructure-directed selectivity
exhibited by enzymes are challenging, but many successes2
motivate continued study. For example, Nolte et al.,3 Gibson
and Rebek,4 and Crabtree, Brudvig, et al.5 have observed
substrate recognition in transition metal catalysis using rationally
designed, host-like ligands. Breit and co-workers6-8 and Reek
and co-workers9 have employed recognition in ligand self-
assembly. Nguyen, Hupp, et al.10 and Wa¨rnmark et al.11 have
employed self-assembled catalysts for specificity, while Ray-
(12) Fiedler, D.; Leung, D. H.; Bergman, R. G.; Raymond, K. N. Acc.
Chem. Res. 2005, 38, 349–358.
(13) Fiedler, D.; Bergman, R. G.; Raymond, K. N. Angew. Chem., Int.
Ed. 2004, 43, 6748–6751.
(14) Gianneschi, N. C.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc.
2005, 127, 1644–1645.
(15) Gianneschi, N. C.; Bertin, P. A.; Nguyen, S. T.; Mirkin, C. A.;
Zakharov, L. N.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 10508–
10509.
(16) Brunner, H. J. Organomet. Chem. 1995, 500, 39–46.
(17) Smith, D. K.; Diederich, F. Chem.-Eur. J. 1998, 4, 1353–1361.
(18) Astruc, D.; Chardac, F. Chem. ReV. 2001, 101, 2991–3023.
(19) Oosterom, G. E.; Reek, J. N. H.; Kamer, P. C. J.; van Leeuwen, P.
Angew. Chem., Int. Ed. 2001, 40, 1828–1849.
(20) Twyman, L. J.; King, A. S. H.; Martin, I. K. Chem. Soc. ReV. 2002,
31, 69–82.
* To whom correspondence should be addressed. E-mail: achianese@
colgate.edu.
(1) Turner, N. J. Curr. Opin. Chem. Biol. 2004, 8, 114–119.
(2) Das, S.; Brudvig, G. W.; Crabtree, R. H. Chem. Commun. 2008,
413–424.
(21) Helms, B.; Fre´chet, J. M. J. AdV. Synth. Catal. 2006, 348, 1125–
1148.
(3) Coolen, H. K. A. C.; Meeuwis, J. A. M.; Van Leeuwen, P. W. M. N.;
Nolte, R. J. M. J. Am. Chem. Soc. 1995, 117, 11906–11913.
(4) Gibson, C.; Rebek, J. Org. Lett. 2002, 4, 1887–1890.
(5) Das, S.; Incarvito, C. D.; Crabtree, R. H.; Brudvig, G. W. Science
2006, 312, 1941–1943.
(6) Weis, M.; Waloch, C.; Seiche, W.; Breit, B. J. Am. Chem. Soc. 2006,
128, 4188–4189.
(22) Me´ry, D.; Astruc, D. Coord. Chem. ReV. 2006, 250, 1965–1979.
(23) Fujihara, T.; Obora, Y.; Tokunaga, M.; Sato, H.; Tsuji, Y. Chem.
Commun. 2005, 4526–4528.
(24) Mu¨ller, C.; Ackerman, L. J.; Reek, J. N. H.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M. J. Am. Chem. Soc. 2004, 126, 14960–14963.
(25) Ooe, M.; Murata, M.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am.
Chem. Soc. 2004, 126, 1604–1605.
(7) Breit, B.; Seiche, W. Angew. Chem., Int. Ed. 2005, 44, 1640–1643.
(8) Breit, B.; Seiche, W. J. Am. Chem. Soc. 2003, 125, 6608–6609.
(9) Kuil, M.; Soltner, T.; van Leeuwen, P.; Reek, J. N. H. J. Am. Chem.
Soc. 2006, 128, 11344–11345.
(10) Merlau, M. L.; Mejia, M. D. P.; Nguyen, S. T.; Hupp, J. T. Angew.
Chem., Int. Ed. 2001, 40, 4239–4242.
(26) Oosterom, G. E.; van Haaren, R. J.; Reek, J. N. H.; Kamer, P. C. J.;
van Leeuwen, P. Chem. Commun. 1999, 1119–1120.
(27) Sato, H.; Fujihara, T.; Obora, Y.; Tokunaga, M.; Kiyosu, J.; Tsuji,
Y. Chem. Commun. 2007, 269–271.
(28) Bhyrappa, P.; Young, J. K.; Moore, J. S.; Suslick, K. S. J. Am.
Chem. Soc. 1996, 118, 5708–5711.
(11) Jo´nsson, S.; Odille, F. G. J.; Norrby, P. O.; Wa¨rnmark, K. Chem.
Commun. 2005, 549–551.
(29) Chow, H. F.; Mak, C. C. J. Org. Chem. 1997, 62, 5116–5127.
(30) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.
10.1021/om800878m CCC: $40.75
2009 American Chemical Society
Publication on Web 12/19/2008

466 Organometallics, Vol. 28, No. 2, 2009
Chianese et al.
Figure 1. N-Heterocyclic carbene ligands with m-phenylene dendritic side groups.
aryl methyl ketone substrates of varying size. Substrate specific-
ity has been explored using competition experiments continu-
ously monitored by NMR spectroscopy. The largest ligand
employed, 1c, exhibits modest specificity for smaller ketone
substrates over larger ones, as compared to smaller control
ligands 1a and 1b. The smallest and largest iridium-NHC
precatalysts have been structurally characterized by X-ray
crystallography.
co-workers have employed phosphine ligands of this nature in
palladium-catalyzed Suzuki-Miyaura coupling33 and rhodium-
catalyzed ketone hydrosilylation. A significant rate enhancement
was observed for the latter reaction, caused by enhanced
formation of monophosphine-rhodium species.34,35 Ding and co-
workers have shown that achiral bowl-shaped phosphine ligands
are also useful for Ru-catalyzed asymmetric hydrogenation of
ketones.36 Goto and Kawashima have also reported the synthesis
of an NHC with meta-terphenyl side groups; a derived Pd0
complex exhibited unique reactivity toward CO2 and O2.37
Benzimidazolium precursors to 1a-c were synthesized as
shown in Scheme 1. The simplest ligand precursor, 1a · HCl,
was prepared via Buchwald-Hartwig amination38,39 of 1,2-
Results and Discussion
Ligand Design and Synthesis. Inspection of molecular
models indicates that for NHC ligands with dendritic side groups
based on a 1,3,5-substituted aromatic monomer unit as shown
in Figure 1, the carbene lone pair (or bound metal atom) rests
at the bottom of a roughly bowl-shaped cavity. The structure
may be highly rigid or semiflexible, depending on hindrance
of rotation about the aryl-aryl linkages. Goto and Kawashima
have used the same meta-terphenyl framework in the synthesis
of bowl-shaped tertiary phosphine ligands, which have an
extremely large cone angle, estimated at 206°.31,32 Tsuji and
(33) Ohta, H.; Tokunaga, M.; Obora, Y.; Iwai, T.; Iwasawa, T.; Fujihara,
T.; Tsuji, Y. Org. Lett. 2007, 9, 89–92.
(34) Niyomura, O.; Iwasawa, T.; Sawada, N.; Tokunaga, M.; Obora,
Y.; Tsuji, Y. Organometallics 2005, 24, 3468–3475.
(35) Niyomura, O.; Tokunaga, M.; Obora, Y.; Iwasawa, T.; Tsuji, Y.
Angew. Chem., Int. Ed. 2003, 42, 1287–1289.
(36) Jing, Q.; Zhang, X.; Sun, H.; Ding, K. L. AdV. Synth. Catal. 2005,
347, 1193–1197.
(37) Yamashita, M.; Goto, K.; Kawashima, T. J. Am. Chem. Soc. 2005,
127, 7294–7295.
(31) Ohzu, Y.; Goto, K.; Kawashima, T. Angew. Chem., Int. Ed. 2003,
42, 5714–5717.
(32) Ohzu, Y.; Goto, K.; Sato, H.; Kawashima, T. J. Organomet. Chem.
2005, 690, 4175–4183.
(38) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.;
Alcazar-Roman, L. M. J. Org. Chem. 1999, 64, 5575–5580.
(39) Sadighi, J. P.; Harris, M. C.; Buchwald, S. L. Tetrahedron Lett.
1998, 39, 5327–5330.

Flexible, Bowl-Shaped NHC Ligands
Organometallics, Vol. 28, No. 2, 2009 467
Scheme 1. Synthesis of Benzimidazolium Salts 1a-c · HCl43
dibromobenzene with 3,5-dimethylaniline to give 3, followed
by treatment with triethyl orthoformate and HCl.40 To prepare
the first-generation dendrimer 1b · HCl, nitroarene 4 was
synthesized by the Suzuki coupling41 of 3,5-di-tert-butylben-
zeneboronic acid with 3,5-dibromonitrobenzene. Reduction of
the nitro group42 then gave the aniline 5, from which benzimi-
dazolium salt 1b · HCl was acquired analogously to 1a · HCl.
To synthesize the second-generation 1c · HCl, Suginome’s
procedure for boronic acid masking was employed.44 First, 3,5-
dibromobenzeneboronic acid was protected with 1,8-diaminon-
aphthalene to give 6. Reaction with 3,5-di-tert-butylbenzenebo-
ronic acid under Buchwald’s optimized conditions for Suzuki
coupling41 gave 7 in high yield, and deprotection with aqueous
acid in THF44 yielded boronic acid 8. Coupling with 3,5-
dibromonitrobenzene gave 9, and nitro-group reduction gave
10. Palladium-catalyzed amination of 1,2-dibromobenzene fol-
lowed by treatment with triethyl orthoformate and HCl then
gave the desired benzimidazolium chloride 1c · HCl.
Synthesis and Characterization of Iridium Complexes.
Neutral iridium(I) complexes of ligands 1a-c were synthesized
by Lin’s method45 of transmetalation from intermediate silver(I)
complexes 1a-c · AgCl (Scheme 2). Metalation of the benz-
imidazolium salts 1a-c · HCl using Ag2O was monitored by
1H NMR spectroscopy, but the silver complexes were not
isolated as described by Lin and co-workers. Instead, [Ir(cod)Cl]2
(cod ) 1,5-cyclooctadiene) was added directly to the dichlo-
(40) Khramov, D. M.; Boydston, A. J.; Bielawski, C. W. Org. Lett. 2006,
8, 1831–1834.
(41) Walker, S. D.; Barder, T. E.; Martinelli, J. R.; Buchwald, S. L.
Angew. Chem., Int. Ed. 2004, 43, 1871–1876.
(42) McDonald, I. M.; Black, J. W.; Buck, I. M.; Dunstone, D. J.; Griffin,
E. P.; Harper, E. A.; Hull, R. A. D.; Kalindjian, S. B.; Lilley, E. J.; Linney,
I. D.; Pether, M. J.; Roberts, S. P.; Shaxted, M. E.; Spencer, J.; Steel,
K. I. M.; Sykes, D. A.; Walker, M. K.; Watt, G. F.; Wright, L.; Wright,
P. T.; Xun, W. J. Med. Chem. 2007, 50, 3101–3112.
(43) Abbreviations: rac-BINAP ) rac-2,2′-bis(diphenylphosphino)-1,1′-
binaphthyl; SPHOS ) 2-dicyclohexylphosphino-2′,6′-dimethoxy-1,1′-bi-
phenyl; DPEPHOS ) bis(2-diphenylphosphinophenyl) ether.
(44) Noguchi, H.; Hojo, K.; Suginome, M. J. Am. Chem. Soc. 2007,
129, 758–759.
(45) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972–975.

468 Organometallics, Vol. 28, No. 2, 2009
Chianese et al.
Scheme 2. Synthesis of Iridium Complexes 2a-c
romethane solutions of 1a-c · AgCl. Precipitation of silver
chloride was observed immediately, but the mixtures were
allowed to stir at room temperature for 30 min before workup.
Iridium complexes 2a-c were isolated by silica gel chroma-
tography in 36-58% yield.
1
The H and 13C NMR spectra for 2a-c are as expected for
complexes of the general formula Ir(NHC)(cod)Cl.46-50 Cy-
clooctadiene vinylic hydrogens appear in the range 2.50-2.74
ppm for the alkene moiety trans to chloride; those trans to the
NHC ligand appear at 4.49-4.61 ppm, indicative of the larger
trans influence of the NHC ligand. The carbene carbon resonates
at 192.0-193.3 ppm. For 2a, the four methyl groups are
Figure 2. ORTEP diagram of 2a, showing 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
1
chemically equivalent by H and 13C NMR; the same is true
for the ortho-CH groups and the meta-ipso carbons. This
indicates that rotation about the N-C single bonds is fast on
the NMR time scale. For 2b and 2c, a broadened resonance is
observed for the four hydrogens closest to the benzimidazole
1
ring in the H NMR spectrum; the associated carbon signal is
also broad in the 13C NMR spectrum. This is consistent with
rotation about the N-C bonds near the fast exchange limit.
Complexes 2a and 2c were structurally characterized by X-ray
crystallography. Figure 2 shows the structure of 2a. Figure 3
shows the full structure of 2c, and Figure 4 shows a close-up
view of the organometallic fragment. Relevant distances and
angles are given in Table 1 (2a) and Table 2 (2c). The structural
parameters for 2a and 2c are consistent with previously
characterized related complexes.47,51,52 The Ir-CNHC distances
of 2.007 Å (2a) and 2.008 Å (2c) are within the expected range.
The unsymmetrical binding of cyclooctadiene reflects the larger
trans influence of the NHC ligand as compared to chloride: the
distance between Ir and the alkene centroid trans to the NHC
ligand is 2.063 Å for 2a and 2.063 for 2c, while the distance to
the alkene centroid trans to chloride is 1.981 Å for 2a and 1.995
for 2c. As is generally observed for monodentate NHC ligands,
the benzimidazol-2-ylidine ring plane is orthogonal to the
iridium coordination plane.
Figure 3. ORTEP diagram of 2c, showing 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
The crystal structure of 2c confirms what is predicted by
molecular models: that NHC ligand 1c forms a loose pocket
around the iridium center, which is expected to impose an
(46) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J.
Chem.-Eur. J. 1996, 2, 772–780.
(47) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.; Crabtree,
R. H. Organometallics 2003, 22, 1663–1667.
(48) Stylianides, N.; Danopoulos, A. A.; Tsoureas, N. J. Organomet.
Chem. 2005, 690, 5948–5958.
(49) Kelly, R. A.; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.;
Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P.
Organometallics 2008, 27, 202–210.
(50) Zanardi, A.; Peris, E.; Mata, J. A. New J. Chem. 2008, 32, 120–
126.
Figure 4. ORTEP diagram of 2c, showing 50% probability
ellipsoids. Side groups attached to C(81) and C(123) are excluded,
and hydrogen atoms are omitted for clarity.
(51) Seo, H.; Kim, B. Y.; Lee, J. H.; Park, H. J.; Son, S. U.; Chung,
Y. K. Organometallics 2003, 22, 4783–4791.
(52) Herrmann, W. A.; Baskakov, D.; Herdtweck, E.; Hoffmann, S. D.;
Bunlaksananusorn, T.; Rampf, F.; Rodefeld, L. Organometallics 2006, 25,
2449–2456.
energetic cost as the side groups rearrange to accommodate a
large incoming substrate.
Ketone Hydrosilylation. The hydrosilylation of carbonyl
compounds to give silyl ethers is a useful transformation, as

Flexible, Bowl-Shaped NHC Ligands
Organometallics, Vol. 28, No. 2, 2009 469
Table 1. Selected Bond Lengths and Angles for 2a
Bond Lengths (Å)
Ir(1)sC(1)
Ir(1)sCl(1)
Ir(1)sC(24)
2.007(4)
2.384(1)
2.101(4)
Ir(1)sC(25)
Ir(1)sC(28)
Ir(1)sC(29)
2.106(4)
2.189(5)
2.165(5)
Bond Angles (deg)
N(1)sC(1)sN(2)
N(1)sC(1)sIr(1)
105.2(3)
126.1(3)
N(2)sC(1)sIr(1)
C(1)sIr(1)sCl(1)
128.5(3)
92.65(10)
Torsion Angles (deg)
N(1)sC(1)sIr(1)sCl(1)
N(2)sC(1)sIr(1)sCl(1)
-92.2(3)
93.7(3)
Table 2. Selected Bond Lengths and Angles for 2c
Bond Lengths (Å)
Ir(1)sC(1)
Ir(1)sCl(1)
Ir(1)sC(74)
2.008(3)
2.3654(8)
2.169(3)
Ir(1)sC(75)
Ir(1)sC(78)
Ir(1)sC(79)
2.179(4)
2.108(4)
2.124(3)
Bond Angles (deg)
N(1)sC(1)sN(2)
N(1)sC(1)sIr(1)
105.4(2)
127.20(19)
N(2)sC(1)sIr(1)
C(1)sIr(1)sCl(1)
127.3(2)
92.20(8)
Torsion Angles (deg)
N(1)sC(1)sIr(1)sCl(1)
N(2)sC(1)sIr(1)sCl(1)
-96.1(2)
87.5(2)
Table 3. Hydrosilylation of Acetophenone Using Catalysts 2a-ca
protected alcohols can be produced directly from ketone or
aldehyde precursors. Catalyst systems employing a wide variety
of transition metal complexes have been developed, including
several highly enantioselective systems for the asymmetric reduc-
tion of prochiral ketones.53,54 Rhodium complexes are often
employed, but the use of less expensive iridium is increasingly
common,55-62inonecasegivingusefullydistinctenantioselectivity.63,64
Rhodium and iridium complexes of N-heterocyclic carbenes are
well known as catalysts for ketone hydrosilylation,23,60,65-78 so
this reaction is a potentially useful testbed for comparing the steric
influence of ligands 1a-c in catalysis. Iridium complexes 2a-c
proved to be active precatalysts for the hydrosilylation of aryl
methyl ketones using diphenylsilane at room temperature. Hy-
drosilylation of acetophenone 11a, performed in benzene-d6 with
3.0 equiv of silane and 3 mol % catalyst, reaches full conversion
in 3-17 h, depending on the ligand (Table 3). In each case, the
major product is the silyl ether 12a, while 4% of the enol silyl
ether 13a, a product of dehydrogenative silylation, is formed. A
slight decrease in the yield (NMR) of 12a with increasing size of
the NHC ligand is observed.
entry
catalyst
time (h)
NMR yield 12a
NMR yield 13a
1
2
3
2a
2b
2c
3
16
17
92%
87%
84%
4%
4%
4%
a Initial concentration: 0.074 M 11a in benzene-d6. Yields calculated
by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal
standard.
5 was employed. Internal competition experiments were per-
formed for each catalyst 2a-c, comparing the reactivity of
acetophenone (11a) with larger ketones 11b-d of varying size
and shape. For each experiment, the conversion of both
1
substrates was monitored continuously by H NMR. For each
catalyst and pair of substrates, Figure 6 shows the ratio of
To assess whether ligands that form a bowl-shaped cavity
such as 1b and 1c can promote substrate specificity based on
size and shape, the series of aryl methyl ketones shown in Figure
(65) Lappert, M. F.; Maskell, R. K. J. Organomet. Chem. 1984, 264,
217–28.
(66) Herrmann, W. A.; Goossen, L. J.; Ko¨cher, C.; Artus, G. R. J. Angew.
Chem., Int. Ed. Engl. 1996, 35, 2805–2807.
(67) Enders, D.; Gielen, H.; Breuer, K. Tetrahedron: Asymmetry 1997,
8, 3571–3574.
(68) Enders, D.; Gielen, H.; Runsink, J.; Breuer, K.; Brode, S.; Boehn,
K. Eur. J. Inorg. Chem. 1998, 913–919.
(69) Duan, W. L.; Shi, M.; Rong, G. B. Chem. Commun. 2003, 2916–
2917.
(53) Diez-Gonzalez, S.; Nolan, S. P. Org. Prep. Proced. Int. 2007, 39,
523–559.
(54) Carpentier, J. F.; Bette, V. Curr. Org. Chem. 2002, 6, 913–936.
(55) Kinting, A.; Kreuzfeld, H. J.; Abicht, H. P. J. Organomet. Chem.
1989, 370, 343–349.
(56) Faller, J. W.; Chase, K. J. Organometallics 1994, 13, 989–992.
(57) Nishibayashi, Y.; Singh, J. D.; Segawa, K.; Fukuzawa, S.; Uemura,
S. J. Chem. Soc., Chem. Commun. 1994, 1375–1376.
(58) Nishibayashi, Y.; Segawa, K.; Singh, J. D.; Fukuzawa, S.; Ohe,
K.; Uemura, S. Organometallics 1996, 15, 370–379.
(59) Karame, I.; Tommasino, M. L.; Lemaire, M. J. Mol. Catal. A: Chem.
2003, 196, 137–143.
(60) Chianese, A. R.; Crabtree, R. H. Organometallics 2005, 24, 4432–
4436.
(61) Cuervo, D.; D´ıez, J.; Gamasa, M. P.; Gimeno, J.; Paredes, P. Eur.
J. Inorg. Chem. 2006, 599–608.
(70) Rivera, G.; Crabtree, R. H. J. Mol. Catal. A: Chem. 2004, 222,
59–73.
(71) Cesar, V.; Bellemin-Laponnaz, S.; Wadepohl, H.; Gade, L. H.
Chem.-Eur. J. 2005, 11, 2862–2873.
¨
(72) Yig˘it, M.; Oezdemir, I.; C¸ etinkaya, B.; C¸ etinkaya, E. J. Mol. Catal.
A: Chem. 2005, 241, 88–92.
(73) Yuan, Y.; Raabe, G.; Bolm, C. J. Organomet. Chem. 2005, 690,
5747–5752.
(74) Chen, T.; Liu, X. G.; Shi, M. Tetrahedron 2007, 63, 4874–4880.
¨
(75) Ozdemir, I.; Yig˘it, M.; Yig˘it, B.; C¸ etinkaya, B.; C¸ etinkaya, E. J.
Coord. Chem. 2007, 60, 2377–2384.
(62) Go´mez, M.; Jansat, S.; Muller, G.; Bonnet, M. C.; Breuzard, J. A. J.;
Lemaire, M. J. Organomet. Chem. 2002, 659, 186–195.
(63) Nishibayashi, Y.; Segawa, K.; Ohe, K.; Uemura, S. Organometallics
1995, 14, 5486–5487.
(64) Nishibayashi, Y.; Segawa, K.; Takada, H.; Ohe, K.; Uemura, S.
Chem. Commun. 1996, 847–848.
(76) Sato, H.; Fujihara, T.; Obora, Y.; Tokunaga, M.; Kiyosu, J.; Tsuji,
Y. Chem. Commun. 2007, 269–271.
(77) Wolf, J.; Labande, A.; Daran, J. C.; Poli, R. Eur. J. Inorg. Chem.
2007, 5069–5079.
(78) Nonnenmacher, M.; Kunz, D.; Rominger, F. Organometallics 2008,
27, 1561–1568.

470 Organometallics, Vol. 28, No. 2, 2009
Chianese et al.
effects a moderate level of substrate specificity in the competi-
tive hydrosilylation of ketones of varying size and shape. Current
efforts are directed at the development of ligands with confor-
mationally more rigid dendritic side groups, which may improve
the level of discrimination between substrates or between
different carbonyl groups in the same substrate molecule.
Experimental Section
Figure 5. Aryl methyl ketones of varying size and shape.
General Methods. [Ir(cod)Cl]2 was prepared as previously
described.80 All other materials were commercially available and
were used as received, unless otherwise noted. All solvents were
reagent grade. Synthesis was performed using solvents that were
sparged with argon, then passed through columns of activated
alumina. Catalytic experiments were performed in benzene-d6,
which was dried over activated 3.5 Å molecular sieves, but was
not purified further. Isolated yields are given for all products. NMR
spectra were recorded on a Bruker spectrometer operating at 400
MHz (1H NMR) and 100 MHz (13C NMR) and referenced to the
residual solvent resonance (δ in parts per million, and J in Hz).
NMR spectra were recorded at room temperature. Elemental
analyses were performed by Robertson Microlit, Madison, NJ.
Iridium Complex 2a. Benzimidazolium salt 1a · HCl (160 mg,
0.44 mmol) and Ag2O (56 mg, 0.24 mmol) were combined in a 25
mL Schlenk flask, and 10 mL of dry, degassed dichloromethane
was added under argon. After stirring for 30 min, the flask was
opened, and [Ir(cod)Cl]2 (147 mg, 0.22 mmol) was added. The
mixture was stirred for an additional 30 min, opened, and filtered
through Celite, washing with dichloromethane. The crude product
was purified by flash chromatography on silica gel, first washing
with dichloromethane, then eluting the product with 1:1 ethyl
acetate/dichloromethane. The product was recrystallized from
Figure 6. Results of internal competitive hydrosilylation of 11a
versus 11b-d.The ratio of conversion 11a/11b-d is reported at
25% conversion of 11a.
1
CH2Cl2/pentane. Yield: 170 mg, 58%. H NMR (CDCl3): δ 7.69
3
4
(s, 4H); 7.35 (AA′BB′, 2H, JHH ) 6.2 Hz, JHH ) 3.3 Hz); 7.23
3
4
(AA′BB′, 2H, JHH ) 6.2 Hz, JHH ) 3.3); 7.14 (s, 2H); 4.49 (m,
2H); 2.50 (m, 2H); 2.46 (s, 12H); 1.74 (m, 2H); 1.45 (m, 4H);
1.25 (m, 2H). 13C NMR (CDCl3): δ 192.0; 138.6; 137.6; 135.7;
130.0; 125.3; 123.2; 111.2; 84.7; 52.1; 33.1; 29.2; 21.5. Anal. Calcd
for C31H34ClIrN2: C, 56.22; H, 5.17; N, 4.23. Found: C, 56.22; H,
5.18; N, 4.20.
conversion 11a/11b-d, when 11a is 25% consumed.79 As one
might expect, no discrimination is observed between 11a and
11b using any catalyst, the only difference being the para tert-
butyl group of 11b. For the competitive hydrosilylation of 11a
versus 11c, which has a large 3,5-di-tert-butylphenyl group at
the para position, catalyst 2c exhibits a slight discrimination,
while catalysts 2a and 2b still show none. For 11a versus 11d,
where the bulky di-tert-butylphenyl group is installed at the meta
position, the unsubstituted 11a is consumed more rapidly using
any catalyst, but the second-generation catalyst 2c shows the
largest effect, as 11a is consumed approximately 3.7 times more
quickly than 11d. Figure 7 shows the time course of competition
experiments between 11a and 11d.79 It is apparent that both
substrates are smoothly consumed throughout the course of the
experiment and that the discrimination between 11a and 11d
increases as the size of the NHC ligand is increased. It is also
noteworthy that catalyst initiation and/or decomposition do not
appear to pose a problem under these conditions.
Iridium Complex 2b. Benzimidazolium salt 1b · HCl (0.146
mmol, 150 mg) and Ag2O (0.081 mmol, 18.8 mg) were combined
in a 25 mL Schlenk flask, and 4 mL of dry, degassed dichlo-
romethane was added under argon. After stirring for 30 min, the
flask was opened, and [Ir(cod)Cl]2 (0.073 mmol, 48.8 mg) was
added; a white precipitate was observed immediately. The mixture
was stirred for an additional 30 min and then filtered through Celite.
The solvent was evaporated, and the residue was purified by flash
chromatography on silica gel, using 2% EtOAc in hexanes as eluent.
The product was recrystallized by slow evaporation of a CH2Cl2/
1
benzene solution. Yield: 72 mg, 36%. H NMR (CDCl3): δ 8.33
(4H, br s), δ 8.03 (2H, s), δ 7.70 (8H, s), δ 7.52 (6H, m), δ 7.32
(2H, aa′bb′, 3JHH ) 6.1 Hz, 4JHH ) 3.2 Hz), δ 4.58 (2H, m), δ 2.6
(2H,m), δ 1.43 (72H, s), δ 1.66-1.16 (8H, m). 13C NMR (CDCl3):
193.3, 151.7, 143.6, 139.9, 138.3, 135.9, 126.6, 125.6, 123.5, 122.1,
122.1, 111.1, 85.8, 52.7, 35.3, 33.1, 31.7, 29.2. The coincidence at
122.1 ppm was verified by HMQC. Anal. Calcd for C83H106ClIrN2:
C, 73.33; H, 7.86; N, 2.06. Found: C, 73.27; H, 8.01; N, 1.96.
Iridium Complex 2c. Compound 1c · HCl (77.2 µmol) was
dissolved in 5 mL of CH2Cl2, and silver(I) oxide (18 mg, 77
µmol) was added. The mixture was stirred under argon for 1 h,
and [Ir(cod)Cl]2 (26 mg, 39 µmol) was added. After stirring for
30 min, the mixture was filtered through Celite. The volatiles
were removed, and the residue was purified by flash chroma-
Conclusion
A series of benzimidazole-based N-heterocyclic carbene
ligands was synthesized as the hydrochloride salts, with N-
substituents varying in size from a simple 3,5-xylyl group to
first- and second-generation meta-phenylene-linked dendrimers
that form a flexible, bowl-like cavity. Iridium complexes were
synthesized and were shown to be catalytically active for the
hydrosilylation of aryl methyl ketones. The largest ligand, 1c,
(79) See Supporting Information for conversion data at additional
timepoints and additional time course plots.
(80) Lin, Y.; Nomiya, K.; Finke, R. G. Inorg. Chem. 1993, 32, 6040–
6045.

Flexible, Bowl-Shaped NHC Ligands
Organometallics, Vol. 28, No. 2, 2009 471
Figure 7. Time course for competition experiments between substrate 11a and 11d.
tography on silica gel, eluting with 1% ethyl acetate in hexanes.
Table 4. Crystal Data and Structure Refinement for 2a and 2c
Some residual hexane was observed even after several days under
vacuum at room temperature. Yield: 82 mg, 44%. 1H NMR
2a
2c
4
color, shape
empirical formula
fw
yellow, block
C31H34ClIrN2
662.25
yellow, block
C171H222ClIrN2O2
2565.32
(CDCl3): δ 8.51 (br d, 4H); 8.19 (br t, 2H); 7.95 (d, 8H, JHH
)
)
4
3
1.3 Hz); 7.81 (t, 4H, JHH ) 1.3 Hz); 7.58 (aa′bb′, 2H, JHH
4
4
6.1 Hz, JHH ) 3.2 Hz); 7.52 (d, 16H, JHH ) 1.6 Hz); 7.48 (t,
8H, 4JHH ) 1.6 Hz); 7.29 (aa′bb′, 2H, 3JHH ) 6.1 Hz, 4JHH ) 3.2
Hz); 4.61 (m, 2H, CHcod); 2.74 (m, 2H, CHcod); 1.7-1.25 (m,
8H, CH2-cod); 1.38 (s, 144H). 13C NMR (CDCl3): δ 193.2, 151.4,
144.2, 143.0, 141.2, 140.9, 138.9, 135.9, 127.2, 126.9, 125.8,
125.6, 123.6, 122.2, 121.9, 111.2, 86.2, 52.9, 35.2, 31.7, 29.9,
22.8. Anal. Calcd for C163H202ClIrN2: C, 81.00; H, 8.42; N, 1.16.
Found: C, 80.88; H, 8.40; N, 1.09.
radiation
temp (K)
cryst syst
space group
unit cell dimens
a (Å)
Mo KR, 0.71073
Mo KR, 0.71073
100
100
triclinic
triclinic
j
j
P1 (No. 2)
P1 (No. 2)
7.9618(18)
11.220(3)
16.253(4)
81.661(3)
83.367(3)
70.772(3)
1352.9(6)
2
20.670(2)
21.111(2)
21.924(2)
105.182(1)
91.662(1)
112.457(1)
8442.8(14)
2
b (Å)
c (Å)
R (deg)
X-ray Structure Determination of 2a. A yellow plate, obtained
by slow evaporation of a dichloromethane/pentane solution, was
mounted on a Cryoloop with Paratone oil and transferred to a
Bruker CCD platform diffractometer. Unit cell determination,
data collection, and multiscan absorption correction were
performed using the APEX2 program suite. Subsequent calcula-
tions were carried out using the SHELXTL81 program package.
The structure was solved by a Patterson projection and refined
on F2 by full-matrix least-squares techniques. All non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were
placed in calculated positions using a riding model. Details of
data collection and refinement are given in Table 4.
X-ray Structure Determination of 2c. A yellow plate, grown
by slow evaporation of an ethanol/diethyl ether solution, was
mounted on a Cryoloop with Paratone oil and transferred to a
Bruker CCD platform diffractometer. Unit cell determination,
data collection, and multiscan absorption correction were
performed using the APEX2 program suite. Subsequent calcula-
tions were carried out using the SHELXTL81 program package.
The structure was solved by direct methods (SHELXS) and
refined on F2 by full-matrix least-squares techniques (SHELXL).
In the main residue, a tert-butyl group was disordered over two
conformations, C(8A)-C(12A) and C(8B)-C(12B); these frag-
ments were restrained using SAME and were refined isotropi-
cally. Two diethyl ether molecules were located and included
in the model. Correction for residual density of additional
disordered solvent was performed using the option SQUEEZE
in the program package PLATON.82 A total of 79 electrons per
unit cell were removed, from a total potential solvent-accessible
void of 1441.2 Å3. In the final Fourier difference map, a positive
residual density peak of 5.35 e/Å3 was observed, 1.295 Å from
C(78), 1.224 Å from C(79), and 2.135 Å from Ir(1). Attempts
to account for this density were unsuccessful. All remaining non-
hydrogen atoms were refined anisotropically, and hydrogen atoms
were placed in calculated positions using a riding model. Details
of data collection and refinement are given in Table 4.
ꢀ (deg)
ꢁ (deg)
V (Å3)
Z
Dcalc (g cm-3
)
1.626
5.055
1.009
0.852
µ(Mo KR) (mm-1
)
cryst size (mm)
0.30 × 0.20 × 0.20 0.54 × 0.51 × 0.33
θ range for data coll. (deg) 1.93 < θ < 28.76
total, unique no. of reflns
Rint
1.55 < θ < 28.36
121 184, 38 383
0.483
9466, 6156
0.339
no. of params, restraints
321, 0
1645, 64
2
R, Rw for all data
0.0334, 0.0799
0.0312, 0.0780
1.074
0.0619, 0.1358
0.0492, 0.1293
1.079
2
R, Rw for I > 2σ
GOF
resid density (e Å-3
)
-2.66 < 1.73
-0.80 < 5.35
Ketone Hydrosilylation, Table 3. Iridium complex 2a, 2b, or
2c (1.1 µmol) and 1,3,5-trimethoxybenzene (internal standard,
3.1 mg, 18.6 µmol) were weighed into a small vial. C6D6 (0.48
mL) was added, followed by diphenylsilane (21 µL, 0.112 mmol)
and acetophenone (4.3 µL, 37.2 µmol). The tube was capped
and inverted several times, and the reaction was monitored by
1H NMR spectroscopy until acetophenone was fully consumed.
The yield of silyl ether 12a was calculated by comparing the
integration values of the CH3 doublet and the CH quartet with
that of the OCH3 singlet from the internal standard, 1,3,5-
trimethoxybenzene. The yield of the enol silyl ether 13a was
calculated by comparing the integration values of the two vinylic
CH doublets with that of the OCH3 singlet from the internal
standard, 1,3,5-trimethoxybenzene.
Competition Experiments, Table 4. Iridium complex 2a, 2b,
or 2c (1.1 µmol) and 1,3,5-trimethoxybenzene (internal standard,
3.1 mg, 18.6 µmol) were weighed into a small vial. About 0.3
mL of C6D6 was added, followed by diphenylsilane (69 µL, 0.372
mmol). The solution was transferred to an NMR tube, and C6D6
was added to bring the total volume to 0.5 mL. The tube was
capped and incubated at room temperature for 2 h, during which
time a small amount of H2 was evolved as adventitious water
was consumed by iridium-catalyzed silane hydrolysis.83 Ac-
etophenone (11a) and ketone 11b, 11c, and 11d were added (37.2
µmol of each), the NMR tube was inverted several times, and
the reaction was monitored by 1H NMR at 23 °C for ap-
(81) Sheldrick, G. M. SHELXTL, Version 6.14; Bruker AXS Inc.:
Madison, WI, 2004.
(82) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.

472 Organometallics, Vol. 28, No. 2, 2009
Chianese et al.
proximately 12 h. Conversion of the starting ketones was
measured using the -COCH3 resonance. In all experiments, silyl
ethers 12a-d were the major products; less than 5% yield
(NMR) of silyl enol ether from dehydrogenative silylation was
observed. Products 12a-d decompose upon attempts at chro-
helpful advice on X-ray crystallography, and the Crystal-
lography Summer School hosted by Arnold Rheingold for
instruction and use of instrumentation.
Supporting Information Available: Details of the synthesis
and characterization of new compounds, including images of
1H and 13C NMR spectra. Complete data for catalytic experi-
ments. CIF files giving X-ray diffraction data, atomic coordi-
nates, thermal parameters, and complete bond distances and
angles. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
matographic purification and were identified by their H NMR
spectra, as observed in individual (noncompetition) hydrosily-
lation experiments using 2a as catalyst. The identities of silyl
ethers 12c,d were confirmed by hydrolysis of the siloxy group
and full characterization of the resulting alcohols 13c,d, as
described in the Supporting Information. Silyl ethers 12a,b were
identified by hydrolysis and comparison with the known phen-
ethyl alcohols.84
OM800878M
Acknowledgment. This work was supported by generous
funding from the ACS Petroleum Research Fund (46754-
GB3) and Colgate University. The authors would like to
thank Bruce Foxman, Paul Williard, and James Golen for
(83) Lee, Y.; Seomoon, D.; Kim, S.; Han, H.; Chang, S.; Lee, P. H. J.
Org. Chem. 2004, 69, 1741–1743.
(84) Hu, A. G.; Ngo, H. L.; Lin, W. B. J. Am. Chem. Soc. 2003, 125,
11490–11491.