Exploring the reactivity of C(sp3)-cyclometalated Ir III compounds in hydrogen transfer reactions

Article abstract of DOI:10.1002/chem.200801728

The manuscript describes the synthesis and full characterization of a new PC(sp³)P-based cyclometalated Ir^III complex that manifests an exceptional thermal stability, as well as outstanding reactivity in hydrogen transfer reactions. The described compound represents the first example of a new family of stable C-(sp³)-metalated compounds.

Full text of DOI:10.1002/chem.200801728

DOI: 10.1002/chem.200801728
3
III
Exploring the Reactivity of C
Hydrogen Transfer Reactions
ACHTUNGTRENUNNG( sp )-Cyclometalated Ir Compounds in
[a]
Clarite Azerraf and Dmitri Gelman*
Dedicated to Professor Jochanan Blum on the occasion of his 75th birthday
Abstract: The manuscript describes the synthesis and full characterization of a
3
III
new PC
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(sp )P-based cyclometalated Ir complex that manifests an exceptional
Keywords: cyclometalation · hydro-
genation · iridium · P ligands ·
pincer complexes
thermal stability, as well as outstanding reactivity in hydrogen transfer reactions.
The described compound represents the first example of a new family of stable C-
(sp )-metalated compounds.
3
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
Introduction
After more than 30 years of extensive research, the chemis-
try and applications of PCP pincer-like complexes are well-
[1]
documented. These compounds have been widely studied
and used as homogeneous catalysts and stoichiometric pro-
[1a–d]
moters for challenging chemical transformations,
as well
as building blocks for the construction of advanced materi-
[
1e–f]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
als.
However, research activity in this field has mainly fo-
Figure 1. Left: Equilibrium of carbometalated compound bearing all-ali-
2
3
cused on sp -carbon- rather than sp -carbon-based com-
3
phatic ligands. Right: PC
hydrogen atoms.
A
H
U
G
E
N
N
(sp )P-based metal complex bearing no available
pounds. This imbalance originates from the greater thermal,
2
conformational and chemical stability of C
A
H
U
G
R
N
N
(sp ) compared to
3
[2]
C ACHTUNGTRENNUNG( sp ), which is especially true of complexes bearing all-
aliphatic ligands. In these cases, the carbometalated com-
pounds often coexist in equilibrium with isomeric olefinic or
carbenic species, as a result of facile a- or b-hydride elimina-
scaffold, that manifests an exceptional stability and out-
standing reactivity in hydrogen transfer reactions (Figure 1,
right).
[2f–h]
3
tion (Figure 1, left).
The reactivity of C
ACHTUNGTRENNGNU( sp )- versus C-
2
ACHTUNGTRENNUNG( sp )-based compounds differs significantly, owing to elec-
tronic factors, such as a stronger trans influence by the meta-
Results and Discussion
lated carbon and higher nucleophilicity of the metal
[2a,3]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
center.
Herein, we wish to introduce a new class of PC
based iridium(III) complexes, based on a robust triptycene
Synthesis and structure: Recently, we studied the coordina-
tion preferences of potentially trans-chelating ligands based
3
AHCTUNGTRENNUNG( sp )P-
[4]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
on a triptycene scaffold. Although one of the implied fea-
tures of these ligands was their chemical inertness toward
possible carbometalation (resulting from the low acidity of
the methine hydrogen), this process is apparently unavoida-
ble, as trans-chelating ligands are structurally related to
[
a] C. Azerraf, D. Gelman
Institute of Chemistry, The Hebrew University of Jerusalem
Edmund Safra Campus
Givat Ram, 91904 Jerusalem (Israel)
Fax : (+972)2-6585279
[5]
pincer ligands. Thus, an attempted synthesis of a trans-
I
chlorocarbonyl Ir Vaska-like complex from IrCl
3A
H
U
G
E
N
N
(H O) and
2
n
1
,8-bis(diisopropylphosphino)triptycene (1) in boiling DMF
E-mail: dgelman@chem.ch.huji.ac.il
III
3
led to the isolation of the unusual Ir PC ACHUTNGRNEGUN( sp )P-type com-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801728.
pound 2 in 71% yield (Scheme 1).
10364
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 10364 – 10368

FULL PAPER
In principle, this extreme distortion may indicate “hemila-
3
bility” of the C
A
H
U
G
E
N
N
(sp )ÀIr bond. However, the rigidity and
chemical inertness of the polycyclic backbone, in combina-
tion with the absence of easily abstractable b-hydrogens, ap-
parently compensates for this “weak” bond. Indeed, we
found that 2 decomposes above 2608C, and no structural
mutations were observed after a prolonged heating in
DMSO near to its boiling point. It is also worth noting that,
although the synthesis of carbonylated transition-metal com-
plexes by DMF decarbonylation is very common, the forma-
III
3
Scheme 1. Synthesis of Ir PCACHTGNUTRENNUNG
III
tion of Ir carbonyl species, such as 2, is less typical. How-
ever, it can be rationalized as depicted in Scheme 2.
3
The existence of the CACHTUNGTRENNUNG
1
cated by NMR spectroscopic analysis. H NMR spectra of
all nonmetalated complexes bearing triptycene-based li-
gands show a very characteristic low-field resonance (9-
[
4]
1
0 ppm) for the central methine hydrogen. This signal
1
[6]
does not appear in the H NMR spectrum of 2. X-ray crys-
tallographic analysis unequivocally proved the structural ar-
[7]
rangement of 2 (Figure 2). The iridium center adopted a
slightly distorted octahedral coordination environment, with
two nonequivalent chlorine atoms located in trans and cis
positions with respect to the metalated carbon and to the
carbon monoxide ligand. Comparison of the iridium–chlor-
ine bond lengths, cis and trans to C1 confirmed a strong
III
Scheme 2. Formation of carbonylated Ir complex 2 by DMF decarbony-
lation
3
+
trans-influence exerted by the sp carbon, indicating a possi-
We believe that cationic [IrCl2
A
H
U
G
R
N
N
(DMF) ] , formed upon
n
[9]
ble lability of the trans chlorine. However, the most interest-
ing structural feature of the new compound is an abnormal
distortion of the metalated carbon C1 from its natural tetra-
hedral geometry. For example, the C15-C1-Ir angle was
dissolution of the iridium precursor in DMF, coordinates
to the bidentate ligand 1 to produce an ideal environment
for abstraction of the only available proximal methine hy-
drogen. Conversely, steric congestion around the metal
center suppresses further reduction processes under the
given conditions.
Catalysis: With compound 2 in hand, we investigated its
catalytic activity in hydrogen-transfer reactions. Although
the transformation is well-studied and many efficient
3
found to be 1278 (compared to 1098, as is normal for sp
carbon). However, the C1ÀIr bond length was within the
[8]
usual range.
[10]
(
mainly ruthenium-based) catalysts are available, there is
still much room for improvement in developing better cata-
lytic systems. In addition, the reactivity of iridium PCP com-
pounds in transfer hydrogenation of ketones is underex-
plored and, even more importantly, can be extrapolated
[11]
onto other related transformations.
Our initial experiments indicated that 2 is a highly effi-
cient precatalyst for transfer hydrogenation of ketones using
isopropanol as the hydrogen source [Eq. (1)]. The injection
of 2’-chloroacetophenone into a preheated solution of 2
(
0.05 mol%) and NaOtBu (5 mol%) in isopropanol results
in its rapid (<30 sec) and essentially quantitative (>99%)
conversion into the corresponding alcohol under air. The
turnover frequency (TOF) calculated for this run at 50%
Figure 2. ORTEP representation of 2. Thermal ellipsoids are shown at
0% probability. Hydrogen atoms have been removed for clarity. Select-
5
ed bond lengths [ꢂ] and angles [8]: IrÀC1=2.154, IrÀCl1=2.389, IrÀ
6
À1
Cl2=2.463, Ir
ÀC33=1.839, IrÀP1=2.365, IrÀP2=2.364; P1-Ir-P2=
conversion corresponds to 3.6ꢁ10 h and is among the
[12]
1
64.74, C1-Ir-Cl2=173.64, C33-Ir-Cl1=173.36, C15-C1-Ir=127.38.
highest such values reported.
Chem. Eur. J. 2008, 14, 10364 – 10368
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10365

D. Gelman and C. Azerraf
The catalyst was also active at lower catalyst/substrate (C/
S) ratios. For example, with the reduction in the concentra-
tion of 2 to 0.01 mol%, complete conversion was detected
after 5 min with halogenated acetophenone substrates
equilibrium, representing a potential scale-up obstacle in de-
[14]
signing industrially relevant processes. Unlike the afore-
mentioned transfer-hydrogenation catalysts, the reactivity of
2 is essentially insensitive to the presence of acetone and
the hydrogenation can be carried out in a closed flask in
concentrated solutions (up to 4m).
(
Table 1, runs 9-11). However, the reduction of simple ace-
Clearly, compound 2 is not a true catalyst, but is in fact a
precatalyst. As postulated for transition-metal-catalyzed
transfer-hydrogenation chemistry, the mechanism of the
ketone reduction mainly follows either “mono- or dihydri-
Table 1. Caption: Transfer hydrogenation of representative ketones cata-
lyzed by 2.
[
e]
Run Ketone
S/C
Time TOF
Conv. Yield
[f]
[g]
[%]
[%]
[15]
[
[
a]
a]
6
6
dic” routes. However, the direct MPV (Meerwein-Ponn-
dorf-Verley)-type hydride transfer pathway cannot be com-
pletely ruled out. Higher reduction rates were detected for
reactions involving halogenated ketones (Table 1), which
may be consistent with a classical MPV case, in which a ÀI
1
2
2’-chloroaceto
2’-bromoaceto-
phenone
4’-bromoaceto-
phenone
3’-bromoaceto-
phenone
acetophenone
2-acetonaphthone
4,4’-dichlorobenzo-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
phenone
2000:1 5 sec 3.6ꢁ10
2000:1 5 sec 3.6ꢁ10
99
99
98
98
ACHTUNGTRENNUNG
[
[
a]
a]
6
4
3
4
2000:1 5 sec 3.6ꢁ10
2000:1 30 min 1.2ꢁ10
96
99
95
98
ACHTUNGTRENNUNG
[16]
effect facilitates ketone reduction. However, other results
suggest a hydride-involving mechanism: 1) In accord with
the general trends of the “hydridic” route, essentially no
ACHTUNGTRENNUNG
[
[
[
a]
a]
a]
4
4
4
5
6
7
2000:1 30 min 1.2ꢁ10
2000:1 30 min 1.2ꢁ10
2000:1 30 min 1.2ꢁ10
94
95
97
93
94
96
[17]
conversion was detected under base-free conditions.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
phenone
4-methylbenzo-
phenone
2’-chloroacetophenone 10000:1 5 min
[
a]
4
2) The conditions obviate the need for acetone-removal
over the course of the reaction to achieve complete conver-
sion, even with a C/S ratio of 1:100000. This fact explains
why the forward reaction (reduction) is much faster than
the reverse (oxidation), which is not typical MPV behav-
8
2000:1 30 min 1.2ꢁ10
98
98
ACHTUNGTRENNUNG
[
[
b]
b]
5
5
9
0
9ꢁ10
9ꢁ10
99
99
99
99
1
1
2’-bromoaceto-
phenone
4’-bromoaceto-
ACHTUNGTRENNUNGp henone
10000:1 5 min
ACHTUNGTRENNUNG
[
[
b]
b]
5
1
2
10000:1 5 min
9ꢁ10
96
95
[18]
ior.
3) An attempted hydrogenation of acetophenone,
1
1
acetophenone
acetophenone
acetophenone
10000:1 12 h
100000:1 48 h
100:1 12 h
N/D
N/D
N/D
96
94
45
95
93
N/D
with sodium formate as the hydride source in acetonitrile/
water medium, led to the, admittedly less efficient, forma-
tion of hydrogenated product [Table 1, run 14 and Eq. (2)].
[
c]
3
[
d]
1
4
[
8
[
a] Ratio of catalyst/substrate/base (C/S/B)=1:2000:100, iPrOH (1m) at
28C under air. [b] C/S/B=1:10000:500, iPrOH (4m) at 88C under air.
c] C/S/B=1:100000:5000, iPrOH (4m) at 88C under air. [d] C/S/
HCO Na=1:100:500, CH CN/H O (2:1) at 828C. [e] Determined at ap-
proximately 50% conversion. [f] Conversion determined by GC analysis.
g] Isolated yields of at least 95% pure compound (average of two runs).
2
3
2
[
tophenone proceeded rather slowly and complete conver-
[
13]
sion was achieved after 12 h. On the other hand, despite a
lower TOF, the catalyst remains active and the reaction goes
to completion even under low catalyst loading conditions at
This result is inconsistent with the direct MPV-type mecha-
nism; 4) An attempted hydrogenation of allyl phenyl ether
with 2 under the described reaction conditions yielded parti-
ally isomerized product (although no hydrogenation oc-
curred, [Eq. (3)]). This process is also unlikely to proceed
1
5 g scale (C/S=1:100000) (Table 1, runs 12–13).
Further experimentation showed that a number of alkyl,
aryl, or diaryl ketones can be converted, in high yields, into
the corresponding alcohols within minutes at 828C and with
a base/catalyst/substrate ratio of 100:1:2000 or higher
(
Table 1, runs 1-10). It is worth noting that the turnover fre-
quencies calculated at ꢀ50% conversion vary in the range
4
6
À1
of 1.2ꢁ10 –3.6ꢁ10 h depending on the stereoelectronic
parameters of substrates.
For each run, essentially identical reactivity was detected
under air and under air-free conditions, indicating the ro-
bustness of 2. Similarly, there was no difference in reactivity
between experiments conducted in anhydrous and commer-
cial iPrOH.
Notably, most known transfer-hydrogenation catalysts re-
quire relatively high-dilution reaction conditions and open
reactors, in order to remove by-product acetone from the re-
action mixture and consequently shift the substrate/product
under hydride-free conditions. 5) Finally, the experiment
conducted in deuterium-labeled iPrOH led to the formation
of approximately 90% monodeuterated product in 5 min (in
[15]
accord with the accepted monohydride mechanism). Even
more prolonged reaction times (1 h) did not change the
degree of deuteration [Eq. (4)]. This result rules out the
[19]
possibility of the dihydridic route, indicating that the cata-
lytic cycle is similar to the previously suggested monohy-
[15]
dride route. Such steps as ketone insertion into the IrÀH
10366
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Chem. Eur. J. 2008, 14, 10364 – 10368

3
III
C ACHTUNGTRENNUNG( sp )-Cyclometalated Ir Compounds
FULL PAPER
atmospheres. Although the active catalytic species are still
3
unknown, the reactivity of the C
A
H
U
G
R
N
U
G
2
and different from well-studied C ACHUTNGRNENUG( sp )-metalated com-
pounds. Currently, mechanistic studies and the syntheses of
more sophisticated complexes, as well as enantiopure chiral
compounds of this type, are in progress.
bond, and displacement of the product from Ir by the iso-
propoxide ligand might be greatly facilitated by a strong
3
[20]
trans-influence imposed by the C AHCUTNGRNENUG( sp ) ligand (Scheme 3).
Experimental Section
General considerations: Iridium chloride hydrate (98%), iPrOH (HPLC
grade), and all starting ketones were purchased from Aldrich. All re-
agents were used without further purification. 1,8-bis(diisopropylphosphi-
[
5]
no)triptycene was synthesized according to the published procedure.
All reagents were weighed and handled in air. Flash column chromatog-
raphy was performed with Merck ultra pure silica gel (230–400 mesh)
purchased from Fluka. All reactions were carried out under air in single-
1
13
use screw-capped tubes. H NMR and C NMR spectra were recorded
on a Bruker 400 MHz instrument with chemical shifts reported in ppm
relative to the residual deuterated solvent or the internal standard tetra-
methylsilane. IR spectra were recorded using Bruker Vector 22 instru-
ment. Gas chromatography analyses were performed on a Hewlett Pack-
ard 5890 instrument with a FID detector and a Hewlett Packard 25 mꢁ
0.2 mm internal diameter Supelcowax–10 capillary column. Yields refer
to isolated compounds of greater than 95% purity, as determined by
Scheme 3. Proposed catalytic cycle for 2.
Another interesting point to be addressed in our future
studies is the possible “hemilability” of the PC ACHUTNGRENNUG( sp )P ligand.
At this point, we can only speculate about the reasons for
the high catalytic activity of 2, but if the ligand is indeed
3
1
H NMR analysis. Yields reported in Table 1 are an average of two runs.
Compound 2: A solution of IrCl
3 2
·H O (0.2 g, 0.67 mmol) and 1,8-bis(di-
“hemilabile”, an alternative mechanism involving a dissocia-
AHCTUNGTREiNNNUG sopropylphosphino)triptycene (1, 0.65 g, 1.34 mmol) in N,N-dimethylfor-
tion–association sequence could play a role in the catalytic
cycle (Scheme 4). Indeed, hemilabile PNP pincer-type cata-
mamide (DMF, 10 mL) was heated at reflux for 48 h under nitrogen.
DMF was removed from the reaction mixture by distillation under re-
[21]
duced pressure, and the off-white residue was recrystallized three times
lysts were recently reported.
1
from methanol, affording
2
(0.37 g, 0.48 mmol, 71%). H NMR
(
400 MHz, CDCl3): d=1.02 (6H, dd, J=7.0 Hz, J=8.0 Hz), 1.29 (6H,
dd, J=7.0 Hz, J=8.0 Hz), 1.71 (12H,
m), 2.52 (H, m), 3.85 (2H, m), 5.35
(
(
1H, s), 6.81 (1H, t, J=6.5 Hz), 6.88
1H, t, J=7.3), 7.08 (2H, t, J=
8
(
.6 Hz), 7.17 (1H, d, J=6.5 Hz), 7.28
2H, d), 7.30 (2H, t, J=7.3 Hz),
.40 ppm (1H, t, J=7.3 Hz);
C NMR, (400 MHz, CDCl ): d=0.0
d, J=30.8 Hz), 20.80, 21.86, 26.63 (t,
7
1
3
3
(
J=14.7 Hz), 28.29 (t, J=14.7 Hz),
4.81, 52.03, 54.66, 123.03, 124.3 (d,
J=24.1 Hz), 125.03, 125.6 (d, J=
3
1
2
1
1
6.1 Hz), 128.53, 130.70 (t, J=
6.2 Hz), 144.39 (t, J=7.0 Hz), 149.02,
51.70,
164.3
(t,
J=12.1 Hz),
3
1
67.27 ppm;
P NMR
(100 MHz,
CDCl
3
): d=34.38 ppm. IR (neat): n˜ =
À1
2
020 cm (C=O); elemental analysis
2 2
calcd (%) for C33H39Cl IrOP : C 51.03,
H 5.06; found: C 50.71, H 5.23.
Scheme 4. An alternative catalytic cycle for 2.
General procedure for catalytic trans-
fer hydrogenation of ketones: In a typ-
ical catalytic transfer-hydrogenation
procedure, in
capped reaction tube, as solution of
the catalyst and NaOtBu in iPrOH (0.5 mL) was preheated to 828C. A
solution of the substrate in 4m iPrOH was then injected into the reaction
mixture. After equilibrium was attained, the solvent was removed by
evaporation under reduced pressure. Products were isolated after a stan-
a
single-use screw-
Conclusions
III
To conclude, we reported the synthesis of a robust Ir PC-
ACHTUNGTRENNUNG( sp )P compound, based on a 1,8-bis(diisopropylphosphino)-
3
triptycene scaffold, that demonstrated fascinating coordina-
tion chemistry, a robust nature and exceptional catalytic ac-
tivity in hydrogen transfer reactions, even under non-inert
dard workup and purified, if needed, by flash chromatography.
1
1
-Phenylethanol (CAS Registry No.: 98-85-1): H NMR (400 MHz,
3
CDCl ): d=1.5 (3H, d, J=6.5 Hz), 2.02 (1H, s), 4.89 (1H, q, J=6.5 Hz),
Chem. Eur. J. 2008, 14, 10364 – 10368
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10367

D. Gelman and C. Azerraf
1
3
7
1
.29–7.38 ppm
27.4, 128.4, 145.9 ppm.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(5H, m); C NMR (400 MHz, CDCl
3
): d=25.1, 70.3, 125.4,
[4] a) C. Azerraf, S. Cohen, D. Gelman, Inorg. Chem. 2006, 45, 7010–
7017; b) C. Azerraf, O. Grossman, D. Gelman, J. Organomet. Chem.
2007, 692, 761–767.
1
1
(
-(2-Naphthyl)ethanol (CAS Registry No.: 7228-47-9): H NMR
[
5] Z. Freixa, P. W. N. M. van Leeuwen, Coord. Chem. Rev. 2008, 252,
1755–1786.
400 MHz, CDCl
q, J=6.5 Hz), 7.49–7.55 (3H, m), 7.84–7.87 ppm (3H, m); C NMR
400 MHz, CDCl3): d=70.5, 123.8, 125.8, 126.1, 127.6, 127.9, 128.3, 133.0,
3
): d=1.61 (3H, d, J=6.5 Hz), 1.97 (1H, br), 5.10 (1H,
1
3
[
6] Other spectral data is in agreement with the proposed structure:
The P NMR spectrum shows a single resonance frequency at d=
(
1
3
1
33.3, 143.1 ppm.
-(4-Bromophenyl)ethanol (CAS Registry No.: 5391-88-8): H NMR
): d=1.46 (3H, d, J=6.5 Hz), 1.78 (1H, br), 4.86 (1H,
1
34.4 ppm. The splitting of the phosphorus-coupled carbon signals
1
1
3
into 1:2:1 triplets was observed in the C NMR spectrum of 2 (see
supporting information). Similar triplets have been observed in
spectra for numerous compounds, forming AXXꢃ spin systems
where the two phosphorus nuclei couple to each other with a large
coupling constant, characteristic of trans-coordinated compounds
(
400 MHz,CDCl
3
1
3
q, J=6.5 Hz), 7.23–7.26 (2H, m),7.44–7.46 pm
A
H
U
G
E
N
N
C NMR
(
400 MHz, CDCl
3
1
1
-(2-Chlorophenyl)ethanol (CAS Registry No.: 13524-04-4): H NMR
): d=1.49 (3H, d, J=6.5 Hz), 1.94 (1H, br), 5.29 (1H,
q, J=6.5 Hz), 7.19 (1H, td, J=1.5, J=7.5 Hz), 7.27–7.33 (2H, m),
.19 ppm(1H, dd, J=1.7, J=7.8 Hz); C NMR (400 MHz, CDCl ): d=
3
3.5, 66.8, 126.4, 127.2, 128.3, 129.3, 131.6, 143.1 ppm.
(
400 MHz, CDCl
3
(
for example, see ref. [5]).
2
3
[
7] CCDC-683153 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.
2
3
13
7
2
1
1
-(2-Bromophenyl)ethanol (CAS Registry No.: 5411-56-3): H NMR
(
500 MHz, CDCl
3
): d=1.51 (3H, d, J=6.5 Hz), 2.00 (1H, br), 5.26 (1H,
[8] A. M. Winter, K. Eichele, H.-G. Mack, W. C. Kaska, H. A. Mayer,
Organometallics 2005, 24, 1837–1844.
[9] P. Serp, M. Hernandez, B. Richard, P. Kalck, Eur. J. Inorg. Chem.
2001, 2327–2336.
[10] a) H.-U. Blaser, C. Malan, B. Pugin, F. Spindler, H. Steiner, M.
Studer, Adv. Synth. Catal. 2003, 345, 103–151; b) M. Kitamura, R.
Noyori, in Ruthenium in Organic Synthesis (Ed: S.-I. Murahashi)
Wiley-VCH, Weinheim, 2004, pp. 3–52; c) T. Ikariya, A. J. Blacker,
Acc. Chem. Res. 2007, 40, 1300–1308; d) A. Del Zotto, W. Baratta,
M. Ballico, E. Herdtweck, P. Rigo, Organometallics 2007, 26, 5636–
q, J=6.5 Hz), 7.15 (1H, t, J=8 Hz), 7.38 (1H, t, J=8 Hz), 7.54 (1H, t,
J=8 Hz), 7.62 ppm(1H, t, J=8 Hz); C NMR (400 MHz, CDCl
1
3
3
): d=
2
3.5, 69.1, 121.7, 126.7, 127.8, 128.7, 132.6, 144.7 ppm.
-(3-Bromophenyl)ethanol (CAS Registry No.: 52780-14-0): H NMR
): d=1.51 (3H, d, J=6.5 Hz), 1.81 (1H, br), 4.9 (1H, q,
J=6.5 Hz), 7.24 (1H, t, J=7.0 Hz), 7.31 (1H, d, J=7.0 Hz), 7.42 (1H, t,
1
1
(
500 MHz, CDCl
3
1
3
J=7 Hz), 7.56 pm
1
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(1H, s). C NMR (400 MHz, CDCl
3
): d=25.2, 69.6,
22.5, 124.0, 128.5, 130.1, 130.4, 148.1 ppm
1
Bis(4-chlorophenyl)methanol (CAS Registry No.: 90-97-1): H NMR
5
642; e) W. Baratta, G. Chelucci, E. Herdtweck, S. Magnolia, K.
(
400 MHz, CDCl
3
): d=2.08 (1H, s), 5.81 (1H, s), 7.29–7.34 ppm
C NMR (400 MHz, CDCl ): d=74.9, 127.8, 128.5, 133.5, 141.8 ppm.
-Methylbenzhydrol (CAS Registry No.: 5472-13-9): H NMR (400 MHz,
ACHTUNGTRNENUNG( 8H,m);
1
3
Siega, P. Rigo, Angew. Chem. 2007, 119, 7795–7798; Angew. Chem.
Int. Ed. 2007, 46, 7651–7654; f) W. Baratta, K. Siega, P. Rigo, Chem.
Eur. J. 2007, 13, 7479–7486.
3
1
2
1
3
3
CDCl ): d=2.35 (3H, s), 5.84 (1H, s), 7.16–7.37 (9H, m); C NMR
(
1
[
11] a) M. W. Haenel, S. Oevers, K. Angermund, W. C. Kaska, H. J. Fan,
M. B. Hall, Angew. Chem. 2001, 113, 3708–3712; Angew. Chem. Int.
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41.0, 144.0 ppm.
3
): d=21.1, 75.9, 126.5, 126.6, 127.4, 128.4, 129.2, 137.2,
1
3044–13053.
[
12] a) P. Dani, T. Karlen, R. A. Gossage, S. Gladiali, G. van Koten,
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6
À1
[
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[20] Unfortunately, our attempts to convert 2 into an unambiguously
identifiable catalytic species are yet to succeed. When 2 was allowed
to react with iPrOH in the NMR tube, with or without stoichiomet-
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Received: August 20, 2008
Published online: October 8, 2008
10368
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Chem. Eur. J. 2008, 14, 10364 – 10368