Alcohol amination with aminoacidato Cp*Ir(III)-complexes as catalysts: Dissociation of the chelating ligand during initiation

Article abstract of DOI:10.1021/cs4009418

The use of aminoacidato Cp*Ir(III)-complexes in catalytic alcohol amination reactions of primary and secondary alcohols with amines permits to carry out these transformations at very mild reaction conditions without the use of an additional base. Herein we discuss the fate of the chelating aminoacidato ligands upon initiation of Cp*Ir(III)-complexes from a mechanistic perspective. Catalyst initiation has been followed by NMR using isotopically labeled ¹³C,¹⁵N-glycinato complexes.

Full text of DOI:10.1021/cs4009418

Research Article
pubs.acs.org/acscatalysis
Alcohol Amination with Aminoacidato Cp*Ir(III)-Complexes as
Catalysts: Dissociation of the Chelating Ligand during Initiation
†
†,‡
§
§
∥
Simone Wockel, Philipp Plessow, Mathias Schelwies, Marion K. Brinks, Frank Rominger,
̈
†
,∥
,†,§
Peter Hofmann, and Michael Limbach*
†
CaRLa (Catalysis Research Laboratory), Im Neuenheimer Feld 584, 69120 Heidelberg, Germany
‡
§
Quantum Chemistry and Synthesis & Homogenous Catalysis, BASF SE, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany
∥
̈
Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
*
S Supporting Information
ABSTRACT: The use of aminoacidato Cp*Ir(III)-complexes
in catalytic alcohol amination reactions of primary and
secondary alcohols with amines permits to carry out these
transformations at very mild reaction conditions without the
use of an additional base. Herein we discuss the fate of the
chelating aminoacidato ligands upon initiation of Cp*Ir(III)-
complexes from a mechanistic perspective. Catalyst initiation
13
15
has been followed by NMR using isotopically labeled C, N-
glycinato complexes.
KEYWORDS: alcohol amination, hydrogen borrowing, iridium, sandwich complexes, amino acids
1
1
INTRODUCTION
the need of basic additives. [Cp*Ir(Pro)Cl] (4) and similar
■
12
Cp*Ir(III)-complexes bearing chelating aminoacidato ligands
The catalytic amination of alcohols is an atom-economical
synthetic route to primary, secondary, or tertiary amines from
readily available starting materials. State-of-the-art catalysts for
do not require bases, either, and allow for even milder reaction
conditions than literature paragons 1 and 2 (T < 100 °C).
Their substrate and functional group tolerance in organic
solvents as well as in water is broad. Surprisingly, although
1
the synthesis of amines from ammonia and alcohols on a large
scale are Brønstedt-acidic and heterogeneous in nature.
Homogenous catalysts supplement the heterogeneous catalysts
in a useful way since they work at milder reaction conditions
with respect to temperature. Additionally, an intrinsically
different reaction pathway, that is, a hydrogen borrowing
mechanism (Scheme 1) as opposed to nucleophilic sub-
stitution, allows a more selective access to more complicated
amines. In industrial process development in the area of fine-
13
iridium(III) half-sandwich complexes bearing α-, β-amino-
14
15
acidato, or even peptide-derived ligands have been known
for decades, their application in catalysis, for example, alcohol
amination or asymmetric transfer hydrogenation, is just
12,16
emerging.
catalysts are far less explored.
investigate how complexes with aminoacidato ligands such as
initiate and what the fate and relevance of the aminoacidato
From the mechanistic point of view, those
6,11
This motivated us to
2
3
chemicals besides complexes based on ruthenium and other
4
4
5
transition metals, especially iridium complexes turned out to
be highly active and selective catalysts to generate secondary or
tertiary amines from an alcohol and an amine source. Examples
ligand is under catalytic conditions. Consequently, we
synthesized aminoacidato Cp*Ir(III)-complexes 4 to 11
(Scheme 2) to investigate the role of their aminoacidato
ligands under reaction conditions using NMR spectroscopy. To
further investigate the activity determining structural features of
the catalyst, we also studied the influence of the halide ligand at
the iridium center on catalytic activity and selectivity.
6
are Fujita’s and Yamaguchi’s [Cp*IrCl ] (1) and [Cp*Ir-
2
2
7
(
NH ) ]·2I (2), iridium(III)-NHC-complexes of Crabtree and
3 3
8
Peris (e.g. 3, cf. Scheme 1) or just [Ir(COD)Cl]2 in
9
combination with a phosphine-ligand.
Especially the air-stable tris(ammine) complexes [Cp*Ir-
(
NH ) ]X (X = I, Br, Cl) reported by Fujita and Yamaguchi
3 3 2
enable the selective formation of secondary or tertiary amines
in water. The activity of 2 is superior to its halide derivatives,
and all complexes tolerate a broad scope of substrates without
Received: October 17, 2013
Revised: November 28, 2013
Published: December 3, 2013
1
0
©
2013 American Chemical Society
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Scheme 1. Proposed Hydrogen Borrowing Mechanism and Pre-Catalysts 1−4
Scheme 2. Synthesis of Complexes 4 and 6−11
RESULTS AND DISCUSSION
X-ray structure analysis revealed the saturation of the vacant
coordination side at iridium by the exocyclic oxygen atom of
the aminoacidato ligand of a second iridalactone (Figure 1).
Such trimeric metallalactones with other aminoacidato ligands
■
Synthesis of Complexes 4 and 6−11. A suspension of
1
7
18
[
Cp*IrCl ] (1) or [Cp*IrI ] (5), the appropriate amino
2 2 2 2
acids glycine (Gly), (S)-proline (Pro) or aminoisobutyric acid
Aib) and potassium tert-butoxide (KOtBu) in methanol was
16
than glycinato have been already reported by Carmona et al.
(
1
3d,19
and others.
When 6 and AgOTf were reacted in a
reacted at room temperature for 2 h. After a workup sequence
of filtration and recrystallization from CH Cl layered with
excess pentane, complexes 4 and 6−9 were isolated in good
yields as yellow or orange crystals (64−89%, Scheme 2).
Surprisingly, when we aimed to synthesize a complex without
a direct halide substituent on the iridium center of the catalyst
using silver triflate (AgOTf) to remove the halide, trimer
coordinating solvent such as acetonitrile (MeCN), [Cp*Ir-
2
2
(
Gly)MeCN]OTf (11) was formed and isolated in 71% yield.
Crystals of [Cp*Ir(Gly)I] (7) as well as the acetonitrile
complex [Cp*Ir(Gly)MeCN]OTf (11), both complexes
2
bearing the achiral κ -N,O-glycinato ligand, were obtained
either by slow crystallization from CH Cl layered with pentane
2
2
[
Cp*Ir(Gly)] (OTf) (10) was obtained in 83% yield from 6.
or from MeCN/Et O (Figure 1). Complex 7 crystallized with
3
3
2
1
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Figure 1. Solid state (X-ray) molecular structures of [Cp*Ir(Gly)I] (7), [Cp*Ir(Aib)Cl] (9), [Cp*Ir(Gly)] (OTf) (10), and [Cp*Ir(Gly)-
3
3
MeCN]OTf (11) with ellipsoids drawn at the 50% probability level. H atoms are omitted for clarity. Selected bond lengths and angles: 7 (two
independent molecules): Ir−I 2.7182(6)/2.7015(8) Å, Ir−O 2.115(5)/2.115(6) Å, Ir−N 2.122(6)/2.134(6) Å, I−Ir−O 86.97(15)/87.07(16)°, I−
Ir−N 86.89(17)/85.84(18)°, O−Ir−N 78.1(2)/78.0(2)°. 9 (two independent molecules): Ir−Cl 2.4100(14)/2.4174(15) Å, Ir−O 2.086(3)/
2
.088(4) Å, Ir−N 2.115(4)/2.126(4) Å, Cl−Ir−O 85.30(11)/85.53(12)°, Cl−Ir−N 82.56(13)/82.89(14)°, O−Ir−N 78.37(14)/77.89(15)°. 10:
Ir1−O6 2.116(6) Å, Ir1−O1 2.127(6) Å, Ir1−N1 2.110(8) Å, Ir2−O2 2.120(7) Å, Ir2−O3 2.125(5) Å, Ir2−N2 2.113(7) Å, Ir3−O4 2.146(6) Å,
Ir3−O5 2.125(6) Å, Ir3−N3 2.124(7) Å, O1−Ir1−O6 84.3(3)°, O1−Ir1−N1 77.9(3)°, O6−Ir1−N1 80.2(3)°, O2−Ir2−O3 82.9(2)°, O2−Ir2−N2
7
9.1(3)°, O3−Ir2−N2 77.4(3)°, O4−Ir3−O5 82.8(2)°, O4−Ir3−N3 79.9(3)°, O5−Ir3−N3 77.5(3)°. 11: Ir−N20 2.076(3) Å, Ir−O 2.105(3) Å,
Ir−N1 2.117(3) Å, N20−Ir−O 85.02(12)°, N−Ir−N 85.23(13)°, O−Ir−N1 77.81(11)°.
Figure 2. 13C NMR spectra in D O of [Cp*Ir( C, N-Gly)Cl] (8) (in red), [Cp*Ir(NH ) ]·2Cl (in green), 8 and NH (in blue), and C, N-
13
15
13 15
2
3
3
3
glycine and 30 equiv of NH (in purple).
3
two independent molecules per unit cell, as did its chloro
derivative [Cp*Ir(Gly)Cl] (6). Because of its bigger size, the
iodo ligand significantly elongated the metal-halide bond
and 6) turned out to be sensitive to the substituent at the α-
position and the N-terminus of the aminoacidato ligand. This
is why we investigated catalyst initiation and the influence of
12
12
(
6
(
2.7182(6)/2.7015(8) Å for 7 vs. 2.4177(6)/2.4143(7) for
), which goes along with a slight elongation of the Ir−O bond
2.115(5)/2.115(6) for 7 vs. 2.0992(18)/2.0998(18) for 6).
the aminoacidato ligand on reactivity more closely. For
mechanistic studies complex 8 was used, the 1 C, N-labeled
glycinato ligand which provided an excellent spectroscopic
handle to follow the course of the reaction and the fate of the
aminoacidato ligand by NMR spectroscopy. Although a
3
15
Change of the chloro ligand to the neutral donor acetonitrile
does not significantly influence the geometry around iridium in
1
3
15
[
Cp*Ir(Gly)MeCN]OTf (11).
stoichiometric experiment using precatalyst [Cp*Ir( C, N-
Gly)Cl] (8), 1-octanol and gaseous NH3 (ca. 60 equiv)
demonstrated the poor solubility of 8 in the organic solvent d8-
Mechanistic Studies. Although structurally very similar,
the activity of complexes bearing aminoacidato ligands (cf. 7
1
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Scheme 3. Reaction of 9 with a Slight Excess of aq. NH Gives 12, whereas the Resting-State of Pre-Catalysts 1, 4, and 13 in the
3
Presence of a Large Excess of NH is 13
3
Figure 3. Solid state (X-ray) molecular structures of [Cp*Ir(Aib)NH ]Cl (12), [Cp*Ir(NH ) ]Cl (13), and N-ethyl-aminoisobutyric acid (14),
3
3
3
2
with ellipsoids drawn at the 50% probability level. Most H atoms are omitted for clarity. Selected bond lengths and angles: 12: Ir−N2 2.123(3) Å,
Ir−O1 2.110(2) Å, Ir−N1 2.133(3) Å, N2−Ir−O1 82.18(10)°, N2−Ir−N1 82.99(10)°, O1−Ir−N1 77.12 (10)°.
toluene even after heating the reaction mixture to 65 °C for 24
and washing the residue with Et O. Four species were detected
2
h, the color of the suspended solid changed from yellow to pale
by HR-FAB MS based on the fragments at m/z = 402, 470, 509,
+
+
yellow. After decantation of the supernatant liquid, washing of
and 764, that were assigned to be [M-Cl] , [M-Gly+BnNH ] ,
2
1
+
+
the precipitate with Et O and a drying step, the H NMR
2
[M-Cl+BnNH ] , and [M-Gly+3BnNH ] , respectively (cf.
2 2
spectrum of this precipitate in d -MeOD revealed a singlet at δ
4
Supporting Information; M = [Cp*Ir(Gly)Cl]). The assump-
tion that not only the chloro but also the aminoacidato ligand
dissociates upon catalyst initiation became obvious when a
solution of 8 in D O was reacted with 75 equiv of gaseous NH
=
1.74 ppm, which fits the shift of the Cp*-signals reported
earlier by Yamaguchi et al. for the tris(ammine)-ligated
10
complex [Cp*Ir(NH ) ]·2Cl.
3
3
2
3
Based on the stoichiometric reaction of 2 and benzyl alcohol
13
and the reaction was followed by C NMR spectroscopy
Figure 2): The signal of the coordinated amino acid at δ =
86.8 ppm moved to δ = 180.7 ppm, which corresponds to the
carboxylate carbon of free C, N-Gly. At the same time, the
diagnostic signals of the Cp* ligand shifted from δ = 85.2 ppm
for 8 to 86.1 ppm, which is close to the corresponding
(
BnOH) in D O at 140 °C, Yamaguchi et al. reported as the
2
(
1
initial steps of the catalytic cycle (cf. Scheme 1) the formation
of iridium-alkoxide species A, which readily eliminates a β-
hydrogen of the alkoxide moiety and already after 30 min
formed benzaldehyde (18%) and iridium-hydride B. The latter
13
15
1
10
was identified by its H NMR shift at δ = −15.49 ppm. When
1
5
13
15
resonance at δ = 86.2 ppm for [Cp*Ir(NH ) ]·2Cl. The N-
[
Cp*Ir( C, N-Gly)Cl] (8) was reacted with BnOH in D O at
3 3
2
spectra in D O point to the same conclusion as the signals of
9
5 °C no benzaldehyde signal was observed (see Supporting
Information, Figure S25), whereas the reaction of [Cp*Ir-
Gly)Cl] (6) with an 10-fold excess of BnNH in MeCN (1 h
2
the coordinated amino acid shift from δ = −16.6 ppm to 28.1
ppm, which corresponds to the deprotonated, uncoordinated
aminoacid anion.
(
2
at 20 °C) yielded a pale yellow solid after removal of volatiles
1
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1
Figure 4. H NMR spectra of the solutions of residues in d -MeOD obtained after the reactions of NH with EtOH using 1 and KOtBu (in purple),
4
3
1
3 (in blue), 4 (in green), and 9 (in red).
2
0
To prove this hypothesis [Cp*Ir(Aib)Cl] (9) was treated
TZVP and M06/def2-TZVP give basically identical results.
With geometries obtained at the BP86/def2-SV(P) level of
with 6 equiv of aq. NH in MeOH (Scheme 3). Again, the color
3
21
of the solution turned from bright to pale yellow. Removal of
the solvent after 1 h at 20 °C and recrystallization of the residue
theory using the RI approximation and suitable auxiliary basis
22
sets, we computed gas-phase free energies at p = 1 bar and T
= 298.15 K within the usual rigid-rotator, harmonic oscillator,
free translation approximation at the B3LYP-D3/def2-TZVP//
BP86/def2-SV(P) level of theory with TURBOMOLE. Gibbs
free energies in water were obtained by adding free energies of
solvation for water, ΔGsolv, resulting in free energies for infinite
dilution at a reference concentration of 1 mol/L. The solvation
free energies were computed with the SMD (M06/def2-TZVP)
from MeOH/Et O yielded single crystals of 12. Here, the
2
aminoacidato backbone still coordinates to iridium but
obviously a ligand exchange of chloro to the neutral ammonia
ligand has occurred (cf. Figure 3).
23
The bond lengths of 12 and 11, both cationic complexes with
a neutral N-donor ligand, turned out to be very similar, whereas
the angles formed by the N-donor, iridium, and the O-terminus
of the aminoacidato ligand are slightly smaller for 12 (N−Ir−O
24
and COSMO-RS (BP86/def-TZVP) model. The largest
8
2.18(10)°) vs. 11 (N−Ir−O 85.02(12)°).
deviations between the solvation free energies computed this
Independent of the starting complex, that is, [Cp*IrCl ] ,
way were found for Cl− (ΔΔG
2
2
= 96 kJ/mol) and the
solv
[
Cp*Ir(NH ) ]Cl , [Cp*Ir(Pro)Cl], or [Cp*Ir(Aib)Cl], the
3 3 2
glycinato ligand (ΔΔG = 66 kJ/mol). An experimental value
solv
reaction with NH3 and EtOH (48 h at 95 °C) yielded
is available for Cl− (ΔG
25
= −304 kJ/mol), a good
solv,exp
Yamaguchi’s catalyst 13 as the final product (Scheme 3). It was
estimate for the anionic glycinato ligand (ΔG
= −316 kJ/
1
solv,exp
unambiguously identified by X-ray analysis (Figure 3) or H
mol) is available from its experimental acidity in solution and in
NMR spectroscopy (Figure 4) after removal of volatiles from
the crude reaction mixture under reduced pressure and
+
the gas-phase, ΔG
of H as well as a reasonably accurate
solv,exp
calculation for ΔG of glycine to either the neutral or the
solv
crystallization of the residue from MeOH/Et O. Furthermore,
2
zwitterionic form (cf. Supporting Information for details). For
these two examples, the values obtained (mainly) from
experiment are bracketed by the computed ones (SMD/
when 9 was used as catalyst, crystals of the N-alkylated
aminoacid Aib were obtained. We consider these results as the
ultimate proof that upon exposure to ammonia Yamaguchi’s
catalyst forms as a catalyst resting state.
COSMO-RS). An experimental value is also available for NH
ΔGsolv,exp = −10 kJ/mol). These three experimental values
for ΔG have been employed in our calculations, all others
have been obtained with COSMO-RS or SMD. The agreement
of SMD and COSMO-RS for the neutral and positively charged
metal complexes is fair; the deviation for Yamaguchi’s complex
13 is only 5 kJ/mol. The largest remaining error of ΔGsolv is
found for 6, which is 34 kJ/mol more stable with COSMO-RS.
We will discuss free energies at the B3LYP-D3/def2-TZVP//
3
26
(
Our experimental findings suggest a simple, thermodynami-
cally favored ligand exchange upon catalyst initiation. Never-
theless, we decided to take a closer look at the initiation
mechanism leading from complex [Cp*Ir(Gly)Cl] (6) to
solv
2
+
Yamaguchi’s catalyst [Cp*Ir(NH ) ] (13) by quantum
3
3
chemistry using the density functional theory (DFT) method-
ology. The main uncertainty of such a computational
investigation is the influence of solvation, since the reaction
involves two charge separations. The influence of the method
for electronic energies was found to be small: B3LYP-D3/def2-
BP86/def2-SV(P) level in H O (c = 1 mol/L) with solvation
2
free energies obtained as described above.
1
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Scheme 4. Ligand Exchange Reactions Yielding Yamaguchi’s Complex 13 at the B3LYP-D3/def2-TZVP//BP86/def2-SV(P)
a
Level in Water
a
The values in parentheses have been obtained with free energies of solvation computed with COSMO-RS and the others have been obtained with
free energies of solvation computed with SMD. All free energies are given relative to 13.
Initially, it is instructive to get an estimate of the
experimental free reaction energy for 13. An equilibrium ratio
of 13 to 6 of 99:1 or higher means a maximal value of ΔG ≤
geometry of the 16-electron complexes is flat (the plane defined
by Ir and the two remaining “legs” of the piano stool is
perpendicular to the Cp* ring-plane). This is in agreement with
27
+
12 kJ/mol (concentration of 6 of 0.06 mol/L, 60 equiv of
previous theoretical studies on this type of complexes.
NH ). The computed results for the ligand-exchange reactions
Catalysis. With the above-mentioned findings in mind and
being aware of the significant differences in activity and
selectivity for reactions catalyzed by Cp*Ir-complexes with
3
are presented in Scheme 4. We discuss all free energies relative
to 13. This way a high agreement between the two values for
the computed solvation free energy with SMD or COSMO-RS
has been achieved. In the backward reaction from 13 to 6, the
agreement of SMD/COSMO-RS is good for most of the
reaction paths, and there are no high-energy intermediates that
would hinder the reaction from occurring at room temperature.
The most striking difference of the solvation models is the
different stability of the precursor 6 (SMD: −4 kJ/mol and
COSMO-RS: −34 kJ/mol). Both numbers bracket the value
expected from experiment (≥−12 kJ/mol). The highest energy
intermediate in the equilibrium is 18 (SMD: 62 kJ/mol;
COSMO-RS: 73 kJ/mol).
1
2,28
different aminoacidato ligands in organic solvents,
compared the performance of [Cp*Ir(Pro)Cl] (4) and
Cp*Ir(NH ) ]·2Cl (13) in the alkylation of benzylamine
we
[
(
3 3
BnNH ) with benzyl alcohol (BnOH) to form dibenzylamine
2
in toluene (Scheme 5, Table 1). The conversion obtained with
a
Scheme 5. Preparation of Dibenzylamine in Toluene
Taking into account that SMD and COSMO-RS usually
bracket the true value (where it is known) and that 6 is
probably somewhat too low in free energy, we conclude that
the dissociative ligand exchange pathway, cf. Scheme 4,
accounts for the observed equilibrium between Ir-complexes
with different ligands. Although those calculations do not allow
to define a clear preference as to which ligand dissociates first
starting from 6 via the coordinatively unsaturated (16-electron)
a
For details, cf. Table 1.
4
was indeed significantly higher than that obtained with 13 (98
vs. 63%) at comparable selectivity. Furthermore, it turned out
that 13 was not entirely soluble in toluene and solid material
remained.
To ensure the complete solubility of the precomplexes in the
1
3g,h,27
complexes 15, 18, or 21 of flat geometry,
our
reaction medium, we ran the reaction of BnNH and 1,6-
2
experimental findings based on the isolation of 12 point to
an initial loss of the chloro ligand via 16. The coordination
hexanediol in water to form N-benzylazepane (Scheme 6, Table
2). As expected from our findings concerning catalyst initiation,
1
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Table 1. Preparation of Dibenzylamine in Toluene
Table 2. Preparation of 1-Benzylazepane with Catalysts 2, 4,
6, 7, 10, 11, and 13 in Water
a
b
c
catalyst
conv. (%)
sel. (%)
a
b
c
catalyst
conv. (%)
sel. (%)
4
98
63
99
99
1
3
2
4
6
97
99
86
93
93
90
93
85
83
89
a
Conditions: BnNH (1 mmol), BnOH (1 mmol), toluene (0.3 mL),
2
cat. (2 mol %), 95 °C, 24 h, diglyme (0.35 mmol as standard).
100
100
96
b
d
Conversion and selectivity determined by GC analysis. Conversion in
24
c
BnNH . Selectivity in dibenzylamine.
7
2
1
1
1
0
1
3
99
100
97
very similar conversion and selectivity was found for
precomplexes 2, 4, 6, 7 and 10, 11, 13. The nature of the
counterion (i.e., chloride, triflate) or halide ligand (i.e., chloro
or iodo) had no effect on conversion and selectivity in water:
a
Conditions: BnNH (2.5 mmol), 1,6-hexanediol (2.5 mmol), water
2
(0.25 mL), cat. (2 mol %), 95 °C, 24 h, diglyme (0.5 mmol as
standard). Conversion and selectivity determined by GC analysis.
10
b
c
d
This is in contrast to reports by Yamaguchi and Fujita. When
Conversion in BnNH . Selectivity in 1-benzylazepane. BnNH and
2
2
the Yamaguchi-type tris(benzylamino) complex 24 was formed
6 were stirred for 3 h at 20 °C prior to addition of 1,6-hexanediol to
ensure the formation of Cp*Ir-amine adduct 24.
in situ by stirring 6 with BnNH in water for 3 h at 20 °C prior
2
to addition of 1,6-hexanediol (Table 2 and Supporting
Information), comparable conversion and selectivity to N-
benzylazepane was observed.
spectrometry was measured at the Mass Spectrometry Facility
(Institute of the Organic Chemistry, University Heidelberg).
Gas chromatography was performed on an Agilent 6890N
modular GC base equipped with a split-mode capillary injection
system and a flame ionization detector using a DB-1 capillary
column (Agilent 122−1033; 30 m × 0.25 mm ×1.00 μm; He
flow 1.0 mL/min, program: initial 70 °C for 5 min, ramp 5 °C/
min, 300 °C for 10 min). Starting materials and products had
In conclusion, we have shown that the aminoacidato ligand is
cleaved from the corresponding precomplexes in catalytic
1
13
alcohol amination reaction as confirmed by kinetic H and C
NMR experiments as well as by X-ray characterization of
intermediates and the catalyst resting state. We additionally
confirmed that the different catalytic activities in organic
solvents such as toluene as a function of the ligand or
counterion reported recently, most probably originate from the
different solubility of the catalysts in the organic medium. As
expected, those differences are not present in media, which
entirely dissolve the complexes, such as water. Despite of the
decoordination of the aminoacidato ligands, we consider our
precomplexes to be useful catalysts for alcohol amination
because of the good solubility of some representatives both in
organic solvents and in aqueous media, the mild reaction
conditions, and the absence of activating additives.
following retention times: benzyl alcohol (t = 17.64 min),
R
benzylamine (t = 16.93 min), dibenzylamine (t = 38.98 min),
R
R
1,6-hexanediol (t = 22.89 min), 1-benzylazepane (t = 33.69
R
R
min). Elemental analyses were performed in the “Mikroanaly-
tisches Laboratorium der Chemischen Institute der Universitat
Heidelberg”. For X-ray crystal structure analyses suitable
crystals were mounted with a perfluorinated polyether oil on
nylon loops or micro mounts and flash-frozen in a nitrogen
stream at 200(2) K on the goniometer head. Data collection
̈
was performed with Mo−K radiation (λ = 0.71073 Å) from a
α
Incoatec IμS microsource on a Bruker APEX-II diffractometer
(7, 9, 11, 12) or a sealed tube on a Bruker APEX diffractometer
(10, 13, 14). 0.5° ω-scans were carried out to collect the
intensity data in all cases. Intensities were corrected for Lorentz
and polarization effects, and an empirical absorption correction
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried
out in air using p. A. quality solvents, if not otherwise noted.
Moisture and air sensitive reactions were performed under an
anaerobic atmosphere of argon using standard Schlenk
29
was applied using SADABS based on the Laue symmetry of
the reciprocal space. The structures were solved by direct
1
13
techniques. H and C NMR spectra were recorded on a
1
2
Bruker Avance 200 MHz, 400 or 600 MHz spectrometer. H
methods and refined against F with a Full-matrix least-squares
13
and C chemical shifts are reported relative to residual solvent
signals of CD Cl (5.32 ppm and 54.0 ppm), D O (4.79 ppm)
algorithm using the SHELXTL (Version 2008/4) software
30
package. Hydrogen atoms were treated using appropriate
riding models, unless otherwise noted for the particular
structures.
2
2
2
or d -MeOD (3.31 ppm and 49.0 ppm). Multiplicities are
4
reported as follows: s = singlet, d = doublet, t = triplet, q =
1
3
quartet, m = multiplet. The C NMR data was assigned by
General Procedure for the Synthesis of Complexes 4 and
HSQC and HMBC spectra. FAB, ESI, and HR mass
6−9. [Cp*IrCl ] (1, 100 mg, 126 μmol) and amino acid (252
2
2
a
Scheme 6. Preparation of 1-Benzylazepane with Catalysts 2, 4, 6, 7, 10, 11, and 13 in Water
a
For details, cf. Table 2.
1
58
dx.doi.org/10.1021/cs4009418 | ACS Catal. 2014, 4, 152−161

ACS Catalysis
Research Article
μmol, 2 equiv) were suspended in MeOH (15 mL). To this
mixture was slowly added KOtBu (265 μmol, 2.1 equiv) in
MeOH (5 mL). The orange suspension turned into a
homogeneous yellow solution at room temperature upon
stirring (15−120 min). The solution was filtered, and the
solvent was removed under reduced pressure. The yellow
residue was retaken in acetone (15 mL), and the resulting
suspension was filtered to remove KCl. The filtrate was again
with a mean redundancy of 3.49 and a completeness of 99.9%
to a resolution of 0.84 Å, 22826 reflections measured, 6889
unique (R(int) = 0.0475), 5235 observed (I >2σ(I)). μ = 9.44
−1
mm , T
= 0.04, Tmax = 0.76, 380 parameters refined,
min
goodness of fit 1.05 for observed reflections, final residual
2
values R1(F) = 0.043, wR(F ) = 0.072 for observed reflections,
−3
residual electron density −1.61−1.09 e Å .
13
15
[Cp*Ir( C, N-Gly)Cl] (8). 98.5 mg (224 μmol, 89%
1
concentrated to dryness and the residue taken up in CH Cl2
yield). − mp =234 °C (decomp.); H NMR (d -MeOD, 600
2
4
1
3
(
2−3 mL), filtered over a pad of Celite, and the filter cake
MHz): δ = 1.70 (s, 15H, CH ), 3.35−3.43 (m, 2H, CH ); C
3
2
washed with CH Cl (1−2 mL). The combined filtrates were
NMR (d -MeOD, 150 MHz): δ = 8.9 (CH ), 45.7 (dd, CH ),
2
2
4
3
2
+
layered with pentane (10−30 mL). After 1−2 d the yellow to
orange crystals were collected by filtration (or decanting the
solvent), washed with pentane, and dried in vacuum.
85.5 (C ), 186.9 (d, C ); MS (FAB): m/z = 440.1 [M+H] ;
q q
Elemental analysis: Calcd. (%) for C11 CH ClIr NO ·1/
2CH Cl ·H O: C 31.09, H 4.39, N 2.97. Found: C 31.25, H
13
15
19
2
2
2
2
General Procedure for Mechanistic Studies. A high-
4.32, N 2.99.
1
3
15
pressure NMR tube was charged with [Cp*Ir( C, N-Gly)Cl]
[Cp*Ir(Aib)Cl] (9). 101.5 mg (218 μmol, 87% yield). − mp
1
(
8, 13.3 mg, 30 μmol) in d -toluene (0.5 mL). After measuring
>260 °C (decomp.); H NMR (CD Cl , 400 MHz): δ = 1.33
8
2
2
a blank sample (pure complex), 1-octanol (1 equiv, 4.75 μL, 30
μmol) was added, and the tube was pressurized with gaseous
(s, 3H, CH ), 1.46 (s, 3H, CH ), 1.69 (s, 15H, CH ), 3.96 (d,
3 3 3
1H, J = 10.5 Hz, NH ), 4.72 (d, 1H, J = 10.1 Hz, NH );
H,H
2
H,H
2
13
NH (60 equiv). The tube was heated to 65 °C in an oil bath
C NMR (CD Cl , 100 MHz): δ = 9.6 (CH ), 27.5 (CH ),
3
2 2 3 3
and regularly shaken to ensure proper mixing of the reaction
mixture. NMR measurements were performed in regular
intervals.
29.4 (CH ), 58.1 (C ), 84.6 (C ), 184.8 (C ); MS (FAB): m/z
3 q q q
+
= 466.1 [M+H] ; Elemental analysis: Calcd. (%) for
C H ClIrNO : C 36.16, H 4.99, N 3.01. Found: C 36.23,
14
23
2
A high-pressure NMR tube was charged with [Cp*Ir-
H 5.06, N 2.90.
Yellow crystal (needle), dimensions 0.16 × 0.05 × 0.04 mm ,
crystal system triclinic, space group P1, Z = 4, a = 9.1576(6) Å,
(13
C, N-Gly)Cl] (8, 15.0 mg, 30 μmol) in D O (0.5 mL).
15
3
2
After measuring a blank sample (pure complex), the tube was
̅
pressurized with gaseous NH (10−60 equiv). NMR measure-
b = 11.2916(8) Å, c = 16.2687(11) Å, α = 99.653(1)°, β =
3
3
ments were performed in regular intervals (TMS as external
standard).
91.159(1)°, γ = 91.704(1)°, V = 1657.2(2) Å , ρ = 1.864 g/
3
cm , Θ = 31.06°, covering the asymmetric unit in reciprocal
max
1
2
[
Cp*Ir(Pro)Cl] (4). 76.5 mg (160 μmol, 64% yield). − mp
space with a mean redundancy of 3.83 and a completeness of
90.4% to a resolution of 0.69 Å, 36815 reflections measured,
9611 unique (R(int) = 0.0382), 7511 observed (I > 2σ(I)), μ =
1
=
(
187 °C (decomp.); H NMR (CD Cl , 600 MHz): δ = 1.65
2 2
s, 15H, CH ), 1.66−1.68 (m, 1H, CH ), 1.89−1.97 (m, 2H,
3
2
−1
CH ), 2.17−2.23 (m, 1H, CH ), 2.90−2.97 (m, 1H, CH ),
8.22 mm , T = 0.35, Tmax = 0.73, 358 parameters refined,
2
2
2
min
3
1
.55−3.59 (m, 1H, CH ), 3.89−3.93 (m, 1H, CH ), 4.38 (bs,
goodness of fit 1.03 for observed reflections, final residual
2
2
13
2
H, NH); C NMR (CD Cl , 150 MHz): δ = 9.5 (CH ), 27.7
values R1(F) = 0.034, wR(F ) = 0.061 for observed reflections,
2
2
3
−3
(
(
CH ), 29.5 (CH ), 55.3 (CH ), 62.8 (CH), 84.7 (C ), 184.1
residual electron density −1.16−2.74 e Å .
2
2
2
q
Cq).
Cp*Ir(Gly)Cl] (6). 74.6 mg (171 μmol, 68% yield). − mp
[Cp*Ir(Gly)] (OTf) (10). [Cp*Ir(Gly)Cl] (6, 160 mg, 380
3
3
1
2
[
μmol) was dissolved in CH Cl (60 mL). To this solution was
2 2
1
=
1
236 °C (decomp.); H NMR (d -MeOD, 600 MHz): δ =
added AgOTf (97.6 mg, 380 μmol, 1.1 equiv), and the
suspension was stirred for 2 h at 20 °C in the dark. The
suspension was filtered over Celite, the solvent evaporated, and
the pale yellow residue was redissolved in CH Cl . Diffusion of
4
1
3
.70 (s, 15H, CH ), 3.33−3.45 (m, 2H, CH ); C NMR (d4-
3
2
MeOD, 150 MHz): δ = 8.9 (CH ), 45.7 (CH ), 85.5 (C ),
3
2
q
186.9 (Cq).
2
2
[
Cp*Ir(Gly)I] (7). [Cp*IrI ] (5, 145 mg, 125 μmol) and
Et O into the complex solution afforded 10 as colorless crystals
2
2
2
glycine (19 mg, 250 μmol, 2 equiv) were suspended in MeOH
40 mL). To this mixture was slowly added KOtBu (30 mg, 267
(167.3 mg, 101 μmol, 83% yield). − mp >260 °C (decomp.);
1
(
H NMR (d -MeOD, 200 MHz): δ = 1.77 (s, 45H, CH ), 3.31
4
3
1
3
μmol, 2.1 equiv) in MeOH (5 mL). The orange suspension was
stirred for 10 h at room temperature turning into a
homogeneous yellow solution. After filtration, the solvent was
evaporated and the residue redissolved in CH Cl (6 mL),
(d, 3H, CH ), 3.77 (d, 3H, CH , J = 17.5 Hz); C NMR
2 2 H,H
(CD Cl , 50 MHz): δ = 9.8 (CH ), 46.2 (CH ), 85.8 (C ),
2
2
3
2
q
+
190.5 (C ); MS (FAB): m/z = 402.0 [Cp*Ir(Gly)] ; Elemental
analysis: Calcd. (%) for C H F Ir N O S : C 28.36, H 3.48,
q
2
2
39 57
9
3
3
15 3
filtered over a pad of Celite, and layered with pentane (20−30
N 2.54. Found: C 28.35, H 3.76, N 2.35.
mL). A sequence of filtration and recrystallization from CH Cl
Colorless crystal (polyhedron), dimensions 0.23 × 0.19 ×
2
2
3
layered with excess pentane afforded 98.5 mg (186 μmol, 74%
0.14 mm , crystal system orthorhombic, space group P2 2 2 , Z
1
1 1
1
yield) of 7. − mp =198 °C (decomp.); H NMR (d -MeOD,
= 4, a = 12.8738(18) Å, b = 16.382(2) Å, c = 26.730(4) Å, α =
4
3
3
6
00 MHz): δ = 1.78 (s, 15H, CH ), 3.40−3.46 (m, 2H, CH );
C NMR (d -MeOD, 150 MHz): δ = 9.7 (CH ), 46.9 (CH ),
6.4 (C ), 186.8 (C ); MS (FAB+): m/z = 530.0 [M+H] ;
90°, β = 90°, γ = 90°, V = 5637.4(14) Å , ρ = 2.014 g/cm ,
max
3
2
1
3
4
3
2
Θ
= 29.51°, covering the asymmetric unit in reciprocal space
with a mean redundancy of 7.64 and a completeness of 99.0%
+
8
q
q
Elemental analysis: Calcd. (%) for C H IIrNO ·2CH Cl ·
to a resolution of 0.72 Å, 62044 reflections measured, 15484
1
2
19
2
2
2
H O: C 23.48, H 3.52, N 1.96. Found: C 23.28, H 3.57, N 2.09.
unique (R(int) = 0.1013), 13934 observed (I > 2σ(I)), μ = 7.27
2
3
−1
Yellow crystal (needle), dimensions 0.88 × 0.04 × 0.03 mm ,
crystal system monoclinic, space group P2 /c, Z = 8, a =
mm , T = 0.29, Tmax = 0.43, 735 parameters refined, Flack
min
absolute structure parameter −0.018(10), goodness of fit 1.02
1
9
=
.0412(4) Å, b = 31.3975(15) Å, c = 12.3426(6) Å, α = 90°, β
for observed reflections, final residual values R1(F) = 0.050,
3 3
2
96.3560(10)°, γ = 90°, V = 3482.2(3) Å , ρ = 2.176 g/cm ,
wR(F ) = 0.127 for observed reflections, residual electron
−3
Θmax = 27.25°, covering the asymmetric unit in reciprocal space
density −1.69−2.52 e Å .
1
59
dx.doi.org/10.1021/cs4009418 | ACS Catal. 2014, 4, 152−161

ACS Catalysis
Cp*Ir(Gly)MeCN]OTf (11). [Cp*Ir(Gly)Cl] (6, 114.7
mg, 260 μmol) was dissolved in MeCN (15 mL). To this
solution was added AgOTf (74.2 mg, 290 μmol, 1.1 equiv) and
the suspension stirred for 2 h at 20 °C in the dark. The
suspension was filtered over Celite, the solvent evaporated, and
the pale yellow residue was redissolved in MeCN. Diffusion of
Research Article
[
99.6% to a resolution of 0.79 Å, 20464 reflections measured,
6723 unique (R(int) = 0.0372), 5673 observed (I > 2σ(I)), μ =
8.36 mm , T = 0.44, Tmax = 0.55, 325 parameters refined,
goodness of fit 1.10 for observed reflections, final residual
values R1(F) = 0.038, wR(F ) = 0.073 for observed reflections,
−
1
min
2
−3
residual electron density −1.50−1.23 e Å .
Et O in to the complex solution afforded 11 as yellow crystals
2
(
110.3 mg, 186 μmol, 71% yield). − mp >260 °C (decomp.);
ASSOCIATED CONTENT
* Supporting Information
■
1
H NMR (d -MeOD, 600 MHz): δ = 1.72 (s, 3H, CH CN),
S
4
3
1
1
=
.77 (s, 15H, CH ), 3.28 (d, 1H, CH , J = 17.5 Hz), 3.77 (d,
NMR spectra of complexes, HR-FAB MS, and NMR data for
selected reactions, computational details. This material is
available free of charge via the Internet at http://pubs.acs.org.
CCDC 962946 (7), 962947 (9), 962948 (10), 962949 (11),
962950 (12), 962951 (13), and 962952 (14) contain 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.
3
2
H,H
1
3
H, CH , J = 17.5 Hz); C NMR (d -MeOD, 150 MHz): δ
2
H,H
4
9.1 (CH CN), 9.8 (CH ), 46.2 (CH ), 85.7 (C ), 118.6,
3
3
2
q
120.7, 122.9, 125.0 (SO CF ), 190.5 (C ); the CN resonance is
3 3 q
missing and might be hidden under the SO CF -resonance; MS
3
3
+
(
ESI, MeCN): m/z = 402.0 [M-MeCN-OTf] ; Elemental
analysis: Calcd. (%) for C H F IrN O S: C 30.45, H 3.75, N
1
5
22
3
2
5
4
.74. Found: C 30.36, H 3.85, N 4.85.
Yellow crystal (polyhedron), dimensions 0.11 × 0.10 × 0.05
3
mm , crystal system monoclinic, space group C2/c, Z = 8, a =
2
1
=
6.456(2) Å, b = 8.5874(7) Å, c = 21.4517(18) Å, α = 90°, β =
AUTHOR INFORMATION
Corresponding Author
E-mail: michael.limbach@basf.com. Phone: +49-6216048957.
Fax: +49-621606648957.
■
*
3
3
21.888(1)°, γ = 90°, V = 4138.1(6) Å , ρ = 1.899 g/cm , Θ
max
29.63°, covering the asymmetric unit in reciprocal space with
a mean redundancy of 4.66 and a completeness of 100.0% to a
resolution of 0.72 Å, 27855 reflections measured, 5827 unique
−1
Notes
(R(int) = 0.0458), 4618 observed (I > 2σ(I)), μ = 6.61 mm ,
The authors declare no competing financial interest.
Tmin = 0.53, Tmax = 0.73, 250 parameters refined, goodness of fit
1
0
.04 for observed reflections, final residual values R1(F) =
ACKNOWLEDGMENTS
S.W., P.P., P.H., and M.L. work at CaRLa of Heidelberg
University, being cofinanced by Heidelberg University, the state
of Baden-Wurttemberg, and BASF SE. Support from these
̈
institutions is gratefully acknowledged.
2
■
.031, wR(F ) = 0.056 for observed reflections, residual
−3
electron density −0.66−1.29 e Å .
[
Cp*Ir(Aib)NH ]Cl (12). [Cp*Ir(Aib)Cl] (9, 32.7 mg, 70
3
μmol) was dissolved in MeOH (3 mL). To this solution was
added NH in water (15 M, 30 μL, 450 μmol, 6.4 equiv) and
3
the solution was stirred for 1 h at 20 °C. The solvent was
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■
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̈
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̈
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1
60
dx.doi.org/10.1021/cs4009418 | ACS Catal. 2014, 4, 152−161

ACS Catalysis
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