Ligand-metal cooperation in pcppincer complexes: Rational design and catalytic activity in acceptorless dehydrogenation of alcohols

Article abstract of DOI:10.1002/anie.201007367

A helping hand: The dibenzobarrelene-based PCsp3Ppincer complex 1 was designed as a bifunctional catalyst for the acceptorless dehydrogenation of alcohols. The mechanism of the H₂ release involves interaction between the hydride ligand and the acidic sidearm in the intermediate 2 (red O, yellow Cl, green P, blue Ir). The feasibility of the complete catalytic cycle was studied using a stoichiometric model and the reaction was subsequently realized in a catalytic fashion. Copyright

Full text of DOI:10.1002/anie.201007367

DOI: 10.1002/anie.201007367
Homogeneous Catalysis
Ligand–Metal Cooperation in PCPPincer Complexes: Rational Design
and Catalytic Activity in Acceptorless Dehydrogenation of Alcohols**
Sanaa Musa, Irina Shaposhnikov, Shmuel Cohen, and Dmitri Gelman*
Cooperative ligands are non-innocent ligands that actively
participate in reversible structural transformations of cata-
lytic species over the course of a catalytic cycle. The ligand–
metal cooperation often brings about unusual and exciting
reactivity and plays a very important role in natural[1] and
artificial systems.[2] Among others, this concept is of great
interest for the design of new catalytic bond-breaking and
bond-forming processes, as it offers a non-oxidative mecha-
nistic alternative to the classical oxidative addition/reductive
elimination sequence.[3]
The initial interest in this concept has been translated into
practice, and efficient catalytic processes, which exploit
conceptually different cooperating systems, have been dis-
covered since the prominent reports on bifunctional hydro-
genation catalysts by Noyori and co-workers (1; Figure 1)[4]
PNP/iridium cooperative systems were employed as catalysts
in the transfer hydrogenation of ketones and imines, as well as
in the dehydrogenation of ammonia-borane adducts.[9] Each
of these systems operate by a unique ligand–metal coopera-
tion mechanism.[10] It is, thus, obvious that the discovery of
novel cooperation modes, as well as the rational design of new
cooperative ligands, is of key importance for the development
of better performing catalysts and new reaction schemes.[11–13]
We now describe the design and synthesis of a new
bifunctional dibenzobarrelene-based PCsp3Ppincer ligand and
its excellent performance in the acceptorless dehydroge-
nation of alcohols. We believe that the unique topology of this
family of compounds (see Figure 2) and the flexibility of their
synthesis offer essentially unlimited opportunities for the
fine-tuning of the catalytic activity in this and related
transformations. Just as important is the mechanism of the
ligand–metal cooperation which is unprecedented for “coor-
dinatively rigid” carbometalated pincer complexes,[14] and,
therefore, opens new avenues in the chemistry and applica-
tions of these powerful compounds.
Recently, we introduced a family of Csp3-metalated pincer
compounds that were based on the dibenzobarrelene scaffold
(Figure 2, left).[15] Despite the structural complexity, the
synthesis of the ligands and of their transition-metal com-
Figure 1. Examples of practical cooperating catalysts. Ts=p-toluenesul-
fonyl.
and Shvo and Czarkie (2).[5] For example, Yamaguchi, Fujita,
and Tanino reported on an interesting 2-hydroxypyridine/
Cp*Ir catalyst (3) for the acceptorless dehydrogenation of
secondary alcohols.[6] In a series of papers, Milstein and co-
workers described sophisticated PNP- and PNN-type ruthe-
nium pincer catalysts (4 and 5) for the dehydrogenative
synthesis of esters, acetals, imines, and amides from alcohols,
and vice versa.[7,8] Structurally simpler PNP/ruthenium (6) or
Figure 2. Csp3-metalated pincer compounds designed by our group.
plexes can be readily accomplished using a reliable and
straightforward [4+2] cycloaddition strategy. Unlike more
traditional pincer complexes, these compounds are three
dimensional. In principle, they can be furnished with any
functional group that is capable of interacting with the
catalytic site, thus making them ideal candidates for catalytic
applications in general, and for the design of cooperative
systems in particular. To explore this possibility, we synthe-
sized the iridium hydride pincer complex 7, which possesses
an acidic sidearm (Figure 2, right).
[*] S. Musa, I. Shaposhnikov, Dr. S. Cohen, Prof. D. Gelman
Institute of Chemistry, The Hebrew University
Edmond Safra Campus
Givat Ram, 91904 Jerusalem (Israel)
Fax: (+972)2-658-5279
E-mail: dgelman@chem.ch.huji.ac.il
Homepage: http://chem.ch.huji.ac.il/~dgelman/
The complex 7 was prepared in three stages: 1) the
quantitative Diels–Alder cycloaddition of the known 1,8-bis-
(diphenylphosphino)anthracene and dimethyl fumarate;
2) the reduction of the diester adduct (these two steps may
[**] This research was supported by the Israel Science Foundation. We
thank Prof. I. Willner for the help in detecting the H2 evolved.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201007367.
Angew. Chem. Int. Ed. 2011, 50, 3533 –3537
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3533

Communications
be performed as a one-pot operation and without isolation of
torless dehydrogenation of alcohols[13,20–23] may proceed, thus
following this sequence of mechanistic events (Scheme 2):
a) H2-forming step, leading to the formation of the arm-closed
iridium species 8; b) ligand exchange step, leading to the arm-
the intermediate); 3) an essentially quantitative reaction of
the resulting phosphine with [{IrCl(cod)}2]. 31P{1H} NMR
analysis of 7 shows the expected set of doublets at around d =
26 ppm, which are due to the presence of two different
phosphine groups (JP-P = 10 Hz), whilst the hydride signal
appears as a deceptively simple triplet at d = ꢀ19.86 ppm
(JP-H=12 Hz) in the 1H NMR spectrum. The remarkably small
JP-P and JP-H coupling constants suggest a cisoid arrangement
of the three ligands around the metal center, which is atypical
for the traditional PCP complexes.[16] In addition, a room
1
temperature H-NOESY experiment displayed a clear cross-
ꢀ
peak between the Ir H and the methylene hydrogen atoms
(Ha in 7; Figure 2), which is consistent with the expected close
intramolecular contact between the hydride ligand and the
hydroxymethylene sidearm.[17] These NMR data indicate
trigonal bipyramid-like geometry around the metal center
with equatorial hydride and phosphine groups, as depicted in
the Figure 2.
Scheme 2. Possible mechanism for dehydrogenation of alcohols by 7
or 8.
Our attempts to grow single crystals of 7 for more detailed
structural investigations revealed that it is moderately stable
in solution and gradually, but selectively, transforms into a
new compound, which features no hydride signals in the
1H NMR spectrum and a different set of doublets at d = 15.8
and ꢀ1.6 ppm (JP-P = 13 Hz) in the 31P{1H} NMR spectrum.
The X-ray analysis of the product led to the conclusion that
7[18] decomposes by extrusion of molecular hydrogen, which
apparently originates from the intramolecular hydride–
proton interactions. This decomposition gives rise to the
formation of the arm-closed species 8, which features a
strongly distorted trigonal bipyramidal geometry around the
iridium center (Scheme 1).[19] The presence of H2 in the
headspace above the heated sample of 7 in [D6]DMSO was
unequivocally detected using GC (TCD) and IR-MS tech-
niques. More interestingly, addition of isopropyl alcohol to
the resulting [D6]DMSO solution of 8 recovers the parent 7 as
open iridium alkoxide species 9; and c) regeneration of the Ir-
H catalyst 7 by b-hydride elimination with subsequent
formation of the oxidized product.
Indeed, this fascinating transformation was realized under
catalytic conditions. Thus, oxidation of 1-phenylethanol under
acid- or base-free conditions in the presence of 0.1 mol% of 7
in p-xylene, with heating under reflux and in a N2 atmosphere
led to the formation of the desired acetophenone as the sole
product after 10 hours. Similar activity was essentially dem-
onstrated by the complex 8 (Table 1, entry 1), whilst none of
the iridium hydride complexes that lack acidic sidearms,
which were reported by us earlier,[15] were found to be active
under these reaction conditions.
Further optimization revealed that the employment of a
catalytic base improves the performance of our catalyst and
the best results were obtained using 5 mol% of Cs2CO3 to
give the corresponding ketones in excellent yield after only
6 hours (Table 1, entries 3 and 6); K3PO4, K2CO3, KOH and
Et3N were found to be much less, if at all, effective. We
speculated that the superior acceleration induced by Cs2CO3
may indicate that the formation of the arm-opened species 9
(Scheme 2, step b) is difficult and, therefore, benefits from the
formation of a more nucleophilic cesium alkoxide species.[24]
Alternatively, the positive effect of the base could originate
from a competing mechanism that involves interaction of the
cesium alkoxide with the axial chloride ligand in 7 leading to
the formation of the cisoid dihydride species capable of the
thermal H2 loss.[21,25] However, when the stoichiometric
experiment (Scheme 1) was repeated in the presence of
cesium isopropoxide the formation of a new hydride species
was not detected and, the base had essentially no effect on the
hydrogen-forming step (7!8). In contrast, the reverse
reaction (8!7) proceeded with almost 10-fold acceleration;
this behavior is more consistent with the first mechanistic
scenario.
1
the original H and 13P{1H} NMR spectroscopy patterns are
observed.
Remarkably, this simple stoichiometric experiment points
to a hypothetical catalytic cycle through which the accep-
Scheme 1. ORTEP drawing with the ellipsoids shown at 50% proba-
bility. Hydrogen atoms and solvent molecules are omitted for clarity.
Selected bond lengths [ꢀ] and angles [deg.]: C1–Ir1 (2.037(7)), Ir1–P1
(2.365(2)), Ir1–P2 (2.203(2)), Ir1–O1 (2.236(6)); P1-Ir1-Cl1 (97.14(7)),
C1-Ir1-P1 (84.0(2)), O1-Ir1-P1 (88.58(16)), P2-Ir1-O1 (156.15(16)), P2-
Ir1-P1 (109.83(8)), C1-Ir1-Cl1(175.0(2)).
Nevertheless, the presence of the catalytic base does not
represent a drawback; both aromatic and aliphatic alcohols
(Table 1, entries 1–6) react cleanly leading to the correspond-
ing ketones that remain stable under the described reaction
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Angew. Chem. Int. Ed. 2011, 50, 3533 –3537

Table 1: Representative results in acceptorless dehydrogenation of 28 and 18 alcohols by 7 and 8.
Entry Alcohol[d]
Catalyst (mol%)/Addi-
tive
t [h] Product
Yield[e] [%]
1[a]
2[a]
3[b]
4[a]
5[a]
6[b]
7[a]
8[b]
1-phenylethanol
7 (0.1 mol%)/none
8 (0.1 mol%)/none
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/none
7 (0.1 mol%)/none
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/none
7 (0.1 mol%)/Cs2CO3
10 acetophenone
10 acetophenone
94
91
94
93
92
87
84[f]
99
1-phenylethanol
1-phenylethanol[6]
diphenylmethanol
octan-2-ol
6
acetophenone
12 benzophenone
10 octan-2-one
octan-2-ol[6]
l-carveol
6
octan-2-one
12 dihydrocarvones (1:1:1 mixture)[h]
1,2-phenylenedimetha-
12 benzofuran-2(3H)-one
nol[21,29]
9[b]
butane-1,4-diol[21,29]
7 (0.1 mol%)/Cs2CO3
7 (0.01 mol%)/Cs2CO3
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/Cs2CO3
7 (0.1 mol%)/Cs2CO3
12 g-butyrolactone
48 g-butyrolactone
12 1,4-dioxan-2-one
36 benzyl benzoate
96
10[b] butane-1,4-diol
36[g]
96
11[b] diethylene glycol
12[b] benzyl alcohol
98
13[c] benzyl alcohol[7]
18 benzyl benzoate
96[g]
92
14[b] 4-chlorobenzyl alcohol
15[b] 2-chlorobenzyl alcohol
16[b] 4-methylbenzyl alcohol
17[b] 4-methoxybenzyl alcohol
36 4-chlorobenzyl 4’-chlorobenzoate
36 2-chlorobenzyl 2’-chlorobenzoate
36 4-methylbenzyl 4’-methylbenzoate/4-methylbenzaldehyde (4:1)
36 4-methoxybenzyl 4’-methoxybenzoate/4-methoxybenzaldehyde
(3:1)
88
94[f]
97[f]
[a] Reaction conditions: alcohol (1 mmol) and 7 (0.001 mmol) in p-xylene (1.5 mL), reflux; [b] Reaction conditions: alcohol (1 mmol), 7 (0.001 mmol)
and Cs2CO3 (0.05 mmol), p-xylene (1.5 mL), reflux; [c] In the presence of 1 equivalent of Cs2CO3; [d] The given references may be used for the direct
face-value comparison with the reported systems; [e] Yield of the isolated product at full conversion of the starting alcohol (average of 2 runs) if not
stated otherwise; [f] Isolated as a mixture; [g] Yield determined by NMR spectroscopy; [h] No H2 formation was detected in this reaction, thus the
reaction must be defined as a transfer hydrogenation.
conditions (any loss of product occurs during the isolation).
Notably, the dehydrogenation of l-carveol (Table 1, entry 7)
proceeds smoothly, but the release of hydrogen is intercepted
by the hydrogenation of the double bonds. The sensitivity of
our catalyst to the presence of the potential hydrogen
accepting groups is not surprising and is a limitation of the
method.[11,26]
The formation of H2 was not monitored routinely.
However, the amount of dihydrogen evolved upon dehydro-
genation of 1-phenylethanol to acetophenone in a sealed tube
correlates well to the theoretical yield. Interestingly, no
significant rate retardation was observed in the closed system
which demonstrates a low sensitivity of our catalyst to the
produced hydrogen.
The acceptorless dehydrogenation of primary alcohols
and diols could also be catalyzed using 7, but led to the
formation of the corresponding esters and lactones (Table 1,
entries 8–17). Mechanistically, formation of these Tischenko
products can be rationalized, as in previously published
works,[7,27] by tandem dehydrogenation/hemiacetalyzation/
dehydrogenation reactions. This scenario may explain why
some electron-rich substrates produce mixtures of the corre-
sponding ester and aldehyde upon full conversion of the
starting alcohols (Table 1, entries 16–17); obviously, electron-
releasing substituents must have the opposite effects on the
rates of the subsequent dehydrogenation and acetalyzation.
However, in all other cases, our catalyst demonstrated
remarkably high selectivity and excellent yields. Finally,
employment of 7 on a homeopathic scale of 0.01 mol%
showed that deactivation of the catalyst takes place after
approximately 3600 turnovers (Table 1, entry 10).[28]
Notably, we found that, unlike the secondary substrates,
dehydrogenation of the primary alcohols is more strongly
affected by the presence of a base. For example, the use of
Cs2CO3 (5 mol%) was necessary to drive the intramolecular
reaction to completion after 12 hours (Table 1, entries 8–9
and 11) and intermolecular reactions after 36 hours (Table 1,
entries 12 and 14–17). Further acceleration may be achieved
when Cs2CO3 is used in a stoichiometric fashion (Table 1,
entry 13). Here again, we speculate that the arm-opening step
(Scheme 2, step b) is particularly problematic when sterically
more demanding hemiacetals are involved and, therefore,
requires the base assistance.
Nevertheless, the results tabulated in Table 1 lead to the
conclusion that the performance of our first-generation
catalysts in the acceptorless dehydrogenation of alcohols is
more than satisfactory and is comparable to or exceeds the
state-of-the-art catalysts that operate under neutral condi-
tions.[23,30] For example, with respect to the dehydrogenation
of the secondary substrates, the use of our catalyst generally
results in the selective synthesis of ketones under lower
catalyst loading and shorter reaction times,[6,22,31] although in
some cases under higher reaction temperatures, than the
state-of-the-art catalysts.[32] Also, the use of our catalyst is
advantageous[21,29,33] for the synthesis of esters and lac-
tones;[34] only catalyst 4[7] gives better results. We are
convinced that further improvements and variations in the
catalyst activity can be achieved by facile modification of the
structure of the cooperative ligands and the nature of the
cooperative functional groups. Studies aimed at the develop-
ment of a more general understanding of structure–reactivity
relationships in these novel pincer catalysts are underway.
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Communications
Lokshin, P. V. Petrovskii, A. S. Peregudov, Organometallics
2010, 29, 4360.
To conclude, we described herein the rational design and
catalytic activity of a new bifunctional PCsp3Ppincer catalyst
for the acceptorless dehydrogenation of the primary and
secondary alcohols to give carbonylic and carboxylic com-
pounds. The mechanism of the H2 formation involves intra-
molecular cooperation between the structurally remote
functionality and the metal center, which is unprecedented
for PCPpincer complexes. It is, therefore, another important
ramification of our work because coordinational diversity in
“coordinatively rigid” carbometalated pincer complexes
provides new reactivity and offers new reaction patterns to
this powerful family of organometallic compounds. Thus,
further mechanistic, synthetic and catalytic studies, which
include the employment of chiral complexes in this and
related transformations,[35] are a topic of active investigation.
[4] a) K.-J. Haack, S. Hashiguchi, A. Fujii, T. Ikariya, R. Noyori,
Angew. Chem. 1997, 109, 297; Angew. Chem. Int. Ed. Engl. 1997,
36, 285; b) R. Noyori, T. Okhuma, Angew. Chem. 2001, 113, 40;
Angew. Chem. Int. Ed. 2001, 40, 40; c) R. Noyori, M. Yamakawa,
S. Hashiguchi, J. Org. Chem. 2001, 66, 7931; d) T. Ikariya, A. J.
Blacker, Acc. Chem. Res. 2007, 40, 1300; e) T. Ikariya, I. D.
Gridnev, Top. Catal. 2010, 53, 894.
[5] a) Y. Shvo, D. Czarkie, J. Organomet. Chem. 1986, 315, C25;
b) R. Karvembu, R. Prabhakaran, K. Natarajan, Coord. Chem.
Rev. 2005, 249, 911.
[6] K.-i. Fujita, N. Tanino, R. Yamaguchi, Org. Lett. 2007, 9, 109.
[7] J. Zhang, G. Leitus, Y. Ben-David, D. Milstein, J. Am. Chem. Soc.
2005, 127, 10840.
[8] a) C. Gunanathan, D. Milstein, Angew. Chem. 2008, 120, 8789;
Angew. Chem. Int. Ed. 2008, 47, 8661; b) C. Gunanathan, L. J. W.
Shimon, D. Milstein, J. Am. Chem. Soc. 2009, 131, 3146; c) B.
Gnanaprakasam, J. Zhang, D. Milstein, Angew. Chem. 2010, 122,
1510; Angew. Chem. Int. Ed. 2010, 49, 1468.
[9] a) Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J.
Lough, K. Abdur-Rashid, Organometallics 2006, 25, 4113; b) M.
Kꢂß, A. Friedrich, M. Drees, S. Schneider, Angew. Chem. 2009,
121, 922; Angew. Chem. Int. Ed. 2009, 48, 905; c) N. Andrushko,
V. Andrushko, P. Roose, K. Moonen, A. Boerner, Chem-
CatChem 2010, 2, 640.
[10] a) K. Abdur-Rashid, S. E. Clapham, A. Hadzovic, J. N. Harvey,
A. J. Lough, R. H. Morris, J. Am. Chem. Soc. 2002, 124, 15104;
b) C. A. Sandoval, T. Ohkuma, K. Muniz, R. Noyori, J. Am.
Chem. Soc. 2003, 125, 13490; c) R. Abbel, K. Abdur-Rashid, M.
Faatz, A. Hadzovic, A. J. Lough, R. H. Morris, J. Am. Chem. Soc.
2005, 127, 1870; d) R. Yamaguchi, C. Ikeda, Y. Takahashi, K.-i.
Fujita, J. Am. Chem. Soc. 2009, 131, 8410; e) C. Gunanathan, B.
Gnanaprakasam, M. A. Iron, L. J. W. Shimon, D. Milstein, J.
Am. Chem. Soc. 2010, 132, 14763.
Experimental Section
General procedure for acceptorless dehydrogenation of alcohols: A
10 ml round-bottomed flask equipped with a double surface con-
denser was charged with 7 (5 mg, 5.5 ꢀ 10ꢀ3 mmol, 0.1 mol%) and
Cs2CO3 (89.3 mg, 0.27 mmol, 5 mol%) in p-xylene (1 ml) under Ar.
Alcohol (5.5 mmol) was injected and the mixture was heated under
reflux with stirring for the specified time. The solvent was removed
under reduced pressure and the product was isolated by filtration
through a pad of silica.
Received: November 23, 2010
Published online: March 15, 2011
Keywords: cooperative effects · dehydrogenation ·
homogeneous catalysis · iridium · pincer complexes
.
[11] A. Prades, R. Corberan, M. Poyatos, E. Peris, Chem. Eur. J. 2008,
14, 11474.
[12] a) F. W. Patureau, S. de Boer, M. Kuil, J. Meeuwissen, P.-A. R.
Breuil, M. A. Siegler, A. L. Spek, A. J. Sandee, B. de Bruin,
J. N. H. Reek, J. Am. Chem. Soc. 2009, 131, 6683; b) M. Ito, Y.
Endo, N. Tejima, T. Ikariya, Organometallics 2010, 29, 2397.
[13] D. Milstein, Top. Catal. 2010, 53, 915.
[14] a) M. Albrecht, G. van Koten, Angew. Chem. 2001, 113, 3866;
Angew. Chem. Int. Ed. 2001, 40, 3750; b) M. E. van der Boom, D.
Milstein, Chem. Rev. 2003, 103, 1759; c) W. Leis, H. A. Mayer,
W. C. Kaska, Coord. Chem. Rev. 2008, 252, 1787; d) M. Hꢃlscher,
W. Leitner, Chem. Eur. J. 2010, 16, 14266.
[15] C. Azerraf, D. Gelman, Chem. Eur. J. 2008, 14, 10364; C. Azerraf,
A. Shpruhman, D. Gelman, Chem. Commun. 2009, 466; C.
Azerraf, D. Gelman, Organometallics 2009, 28, 6578.
[16] a) H. E. Selnau, J. S. Merola, Organometallics 1993, 12, 3800;
b) R. Ciganda, M. A. Garralda, L. Ibarlucea, C. Mendicute, E.
Pinilla, M. R. Torres, Eur. J. Inorg. Chem. 2010, 3167; c) J.-Y.
Hung, C.-H. Lin, Y. Chi, M.-W. Chung, Y.-J. Chen, G.-H. Lee, P.-
T. Chou, C.-C. Chen, C.-C. Wu, J. Mater. Chem. 2010, 20, 7682.
[17] We have previously shown that synthesis of PCsp3P iridium (III)
complexes using this method leads to the formation of only one
stereoisomer. For an example, see Ref. [15c].
[1] a) D. J. Evans, C. J. Pickett, Chem. Soc. Rev. 2003, 32, 268;
b) J. W. Whittaker, Arch. Biochem. Biophys. 2005, 433, 227.
[2] a) C. B. Pamplin, P. Legzdins, Acc. Chem. Res. 2003, 36, 223; b) P.
Gamez, I. A. Koval, J. Reedijk, Dalton Trans. 2004, 4079; c) S. E.
Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev. 2004,
248, 2201; d) P. Chaudhuri, K. Wieghardt, T. Weyhermueller,
T. K. Paine, S. Mukherjee, C. Mukherjee, Biol. Chem. 2005, 386,
1023; e) T. Ikariya, K. Murata, R. Noyori, Org. Biomol. Chem.
2006, 4, 393; f) A. Choualeb, A. J. Lough, D. G. Gusev, Organo-
metallics 2007, 26, 5224; g) M. Livieri, F. Manicin, G. Saielli, J.
Chin, U. Tonellato, Chem. Eur. J. 2007, 13, 2246; h) H.
Grꢁtzmacher, Angew. Chem. 2008, 120, 1838; Angew. Chem.
Int. Ed. 2008, 47, 1814; i) J. I. van der Vlugt, J. N. H. Reek,
Angew. Chem. 2009, 121, 8990; Angew. Chem. Int. Ed. 2009, 48,
8832; j) A. Friedrich, M. Drees, M. Kaess, E. Herdtweck, S.
Schneider, Inorg. Chem. 2010, 49, 5482; k) B. L. Conley, M. K.
Pennington-Boggio, E. Boz, T. J. Williams, Chem. Rev. 2010, 110,
2294.
[3] a) M. D. Fryzuk, P. A. MacNeil, S. J. Rettig, J. Am. Chem. Soc.
1987, 109, 2803; b) T. R. Cundari, T. R. Klinckman, P. T.
Wolczanski, J. Am. Chem. Soc. 2002, 124, 1481; c) P. Maire, T.
Buttner, F. Breher, F. P. Le, H. Grutzmacher, Angew. Chem.
2005, 117, 6477; Angew. Chem. Int. Ed. 2005, 44, 6318; d) D. J.
Mindiola, Acc. Chem. Res. 2006, 39, 813; e) A. D. Phillips, G.
Laurenczy, R. Scopelliti, P. J. Dyson, Organometallics 2007, 26,
1120; f) L. C. Gregor, C.-H. Chen, C. M. Fafard, L. Fan, C. Guo,
B. M. Foxman, D. G. Gusev, O. V. Ozerov, Dalton Trans. 2010,
39, 3195; g) A. A. Koridze, A. V. Polezhaev, S. V. Safronov,
A. M. Sheloumov, F. M. Dolgushin, M. G. Ezernitskaya, B. V.
[18] Our multiple attempts to grow single crystals of 7 did not
succeed because the less soluble 8 always crystallizes upon its
spontaneous formation.
[19] CCDC 800655 (8) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[20] a) A. Dobson, S. D. Robinson, J. Organomet. Chem. 1975, 87,
C52; b) C. W. Jung, P. E. Garrou, Organometallics 1982, 1, 658;
3536
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c) R. A. Sheldon, I. W. C. E. Arends, B. G.-J. Ten, A. Dijksman,
Acc. Chem. Res. 2002, 35, 774; d) J. Zhang, M. Gandelman,
L. J. W. Shimon, H. Rozenberg, D. Milstein, Organometallics
2004, 23, 4026.
[28] One of the possible deactivation pathways may include decar-
bonylation of the produced aldehydes leading to the catalytically
inactive carbonylated iridium species (see for example
Ref. [29]). Indeed, when 7 was pretreated with CO prior to the
reaction diminished activity was seen.
[29] J. Zhao, J. F. Hartwig, Organometallics 2005, 24, 2441.
[30] A. Friedrich, S. Schneider, ChemCatChem 2009, 1, 72.
[31] T. Mitsudome, Y. Mikami, K. Ebata, T. Mizugaki, K. Jitsukawa,
K. Kaneda, Chem. Commun. 2008, 4804.
[21] S. Murahashi, T. Naota, K. Ito, Y. Maeda, H. Taki, J. Org. Chem.
1987, 52, 4319.
[22] H. Junge, B. Loges, M. Beller, Chem. Commun. 2007, 522.
[23] G. E. Dobereiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681.
[24] a) D. N. Reinhoudt, F. De Jong, H. P. M. Tomassen, Tetrahedron
Lett. 1979, 20, 2067; b) W. H. Kruizinga, R. M. Kellogg, J. Am.
Chem. Soc. 1981, 103, 5183.
[32] For example, entries 3 and 6 in Table 1 versus 0.2–0.33 mol% of
catalyst 3 over 20–50 h at 1158C (Ref. [6]).
[25] D. Morton, D. J. Cole-Hamilton, J. Chem. Soc. Chem. Commun.
1988, 1154.
[33] Y. Blum, Y. Shvo, J. Organomet. Chem. 1985, 282, C7.
[34] Based on the direct comparison of entries 8, 9 and 13 with
catalytic systems reported in Ref. [7, 21] and [29].
[35] E. Balaraman, B. Gnanaprakasam, L. J. W. Shimon, D. Milstein,
J. Am. Chem. Soc. 2010, 132, 16756.
[26] a) N. Menashe, Y. Shvo, Organometallics 1991, 10, 3885; b) S. J.
Pridmore, J. M. J. Williams, Tetrahedron Lett. 2008, 49, 7413.
[27] a) S. Murahashi, K. Ito, T. Naota, Y. Maeda, Tetrahedron Lett.
1981, 22, 5327; b) T. Ito, H. Horino, Y. Koshiro, A. Yamamoto,
Bull. Chem. Soc. Jpn. 1982, 55, 504.
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