Acceptorless Alcohol Dehydrogenation: OH vs NH Effect in Bifunctional NHC-Ir(III) Complexes

Article abstract of DOI:10.1021/acs.organomet.7b00220

Bifunctional complexes bearing N-heterocyclic carbene (NHC) ligands functionalized with hydroxy or amine groups were synthesized to measure the beneficial effect of different modes of metal-ligand cooperation in the acceptorless dehydrogenation of alcohols. In comparison to complexes with an amine moiety, hydroxy-functionalized iridium catalysts showed superior activity. In contrast to alcohols, 1,4-diols underwent cyclization to give the corresponding tetrahydrofurans without involving dehydrogenation processes. Mechanistic investigations to rationalize the "OH effect" in these types of complexes have been undertaken.

Full text of DOI:10.1021/acs.organomet.7b00220

Article
pubs.acs.org/Organometallics
Acceptorless Alcohol Dehydrogenation: OH vs NH Effect in
Bifunctional NHC−Ir(III) Complexes
́ ́
Greco Gonzal Martín-Matute*
ez Miera,† Elisa Martínez-Castro, and Belen
†
Department of Organic Chemistry, Stockholm University, Stockholm SE-10691, Sweden
*
S Supporting Information
ABSTRACT: Bifunctional complexes bearing N-heterocyclic
carbene (NHC) ligands functionalized with hydroxy or amine
groups were synthesized to measure the beneficial effect of
different modes of metal−ligand cooperation in the accept-
orless dehydrogenation of alcohols. In comparison to
complexes with an amine moiety, hydroxy-functionalized
iridium catalysts showed superior activity. In contrast to
alcohols, 1,4-diols underwent cyclization to give the corresponding tetrahydrofurans without involving dehydrogenation
processes. Mechanistic investigations to rationalize the “OH effect” in these types of complexes have been undertaken.
INTRODUCTION
alkoxide moieties allows both inner- and outer-sphere
■
mechanisms as a result of the weaker metal−oxygen
Bifunctional transition-metal complexes, having two or more
active sites (i.e., a transition-metal center and a functional group
that can be engaged in protonation/deprotonation steps), have
given excellent results in terms of activity and selectivity in
11,12,14,15
interaction.
This opens up an alternative to the
coordinatively more rigid bidentate or pincer ligands that
show the “NH effect”.
NHC (N-heterocyclic carbene) ligands have played a
1
numerous catalytic transformations. Bifunctional ligands not
significant role on the design of bifunctional metal complexes
only influence the steric and electronic properties of transition-
metal complexes but also affect the mechanistic pathway
through chemical interaction with substrates and products.
There are two conceptually different families of bifunctional
complexes: those whose participating functional group is in the
19
for catalytic applications. In a recent review article, Peris
describes them as “smart” ligands, since they can be “switchable,
multifunctional, adaptable and tuneable”. As part of our current
research into the development of efficient catalytic processes,
we have also investigated the catalytic activity of a bifunctional
Ir(III) complex bearing an NHC ligand functionalized with a
1
a
first coordination sphere of the metal center, and those in
which this functional group is not directly bound to the metal
14a
2
hydroxy active site (1a; Scheme 1). The activity of 1a was
demonstrated in the alkylation of amines with alcohols under
mild conditions. Importantly, the addition of an external base
was not needed, since the bifunctional tethered alcohol
participated in the protonation/deprotonation steps, through
center. This latter type leads to so-called remote metal−ligand
3
−5
cooperation.
In both cases, the ligands participate in
reversible proton-transfer steps, while at the same time the
metal is involved in other elementary steps in the catalytic
transformation, commonly in hydride-transfer processes. The
synergistic effect of bifunctional complexes of the first type was
first demonstrated by Noyori with a ruthenium complex
bearing an ethylenediamine ligand.
1
14b
both inner- and outer-sphere pathways. A related complex
6
,7
Scheme 1. N-Alkylation of Amines with Alcohols Catalyzed
by a Bifunctional NHC−Ir(III) Catalyst
From these early
examples, many reports of this type of metal−ligand
cooperative catalysis have been published.
6
a,8,9
Interestingly,
the vast majority of these reports are based on ligands bearing
nitrogen donor atoms giving rise to an “NH effect” in
1c,2
bifunctional catalysis.
Examples where the cooperation relies on oxygen-containing
active sites in the first coordination sphere have been much less
4
−17
common.
This could be due to the fact that, for late
transition metals, coordination with hydroxy moieties is weaker
than coordination with amine functional groups. Amine-
containing bifunctional complexes have been used as catalysts
9g,18
in a vast number of hydrogen-transfer processes.
These
Special Issue: Organometallic Chemistry in Europe
Received: March 24, 2017
complexes usually follow a concerted outer-sphere mechanism
involving a six-membered transition state. On the other hand,
the hemilabile behavior of bifunctional complexes with alcohol/
1a
©
XXXX American Chemical Society
A
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Article
was also immobilized into a metal−organic framework (MOF),
and the beneficial cooperative behavior of the Ir−alcohol/
alkoxide complex was demonstrated here in the isomerization
of allylic alcohols, a reaction that also involves hydride- and
Scheme 3. (Top) Synthesis of Complexes 1b,c and (Bottom)
Structures of NHC−Ir(III) Complexes 1a−c
2
0
proton-transfer steps. The enhanced catalytic activity
observed for bifunctional complexes of this type, attributed to
the alcohol/alkoxide functionality, and so termed an “OH
effect”, may be dual in nature: (1) the oxidation of polar bonds
takes place through a concerted transfer of hydride and proton
to the metal center and the alkoxy functionality, respectively,
and (2) unlike bifunctional NH ligands, OH ligands are able to
provide available coordination sites on the metal center upon
decoordination.
The oxidation of alcohols (3) by dehydrogenation is a
21
conceptually different reaction (Scheme 2). This is an atom-
Scheme 2. Acceptorless Alcohol Dehydrogenation
economical oxidation that does not require hydrogen acceptors.
Instead, molecular hydrogen is released, adding a positive
22
entropic contribution to this endothermic process. The
reaction is currently significant, as it provides a source of liquid
organic hydrogen carriers that could be implemented using
liquid fuel infrastructure in the proposed incoming “hydrogen
economy”. Acceptorless alcohol dehydrogenation (AAD)
offers also significant advantages in organic chemistry for the
synthesis of carbonyl-containing building blocks (4) from the
corresponding alcohols, avoiding the use of oxidants and
facilitating purification procedures. AAD reactions have
imidazolium ring at δ 8.96 (1 H) and 7.65 (2 H) ppm and
resonances corresponding to the two alkyl chains on the
nitrogen substituents at δ 5.10 (2 OH), 4.13 (4 H), and 1.10
2
3
(
12 H) ppm. The two hydroxy proton signals at δ 5.10 ppm
disappeared upon treatment with D O. Reaction of 5 with
2
Ag O yielded a carbene silver complex (5′), which, upon
2
transmetalation with [Cp*IrCl ] , gave complex 1b in 57%
2
2
isolated yield. Imidazolium salt 8 was prepared from imidazole
2
1a,24
progressed significantly in recent years.
recent reports of AAD reactions, involving homogeneous
13
The number of
by a four-step synthetic route involving an Fe(III)-mediated
34
1
addition of sodium azide to alkene intermediate 6. The H
NMR spectrum of 8 also showed the characteristic resonances
at δ 9.29 (1 H) and at 7.79 (2 H) ppm corresponding to the
imidazolium ring and at δ 4.59 (2 H) and 1.48 (6 H) ppm due
to the amino alkyl chain (see the Supporting Information for
further details). The signals at δ 4.30 (2 H), 1.97 (2 H), 1.48 (2
H), and 0.99 (3 H) ppm confirmed the presence of the butyl
chain. Complex 1c was synthesized in 61% isolated yield from 8
using a synthetic procedure similar to that described for
complex 1b.
2
5
26
27
28
29
30
31
complexes of Ru, Ir, Rh, Os, Fe, Co, Cu, and Ni,
among others, and even heterogeneous Pd nanoparticles
32
supported in MOFs or Ru nanoparticles supported on
3
3
alumina, underlines the importance of research into atom-
economical processes that avoid the use of harmful oxidizing
agents.
In this paper, we report the catalytic activity of a family of
NHC−Ir(III) complexes in acceptorless alcohol dehydrogen-
ation reactions. Surprisingly, we found that complexes with
bifunctional ligands bearing weakly coordinating hydroxy active
sites showed activity significantly higher than those with amine
groups. The scope of the reaction to prepare carbonyl
compounds from alcohols has been investigated. Mechanistic
investigations to understand the “OH effect” of these types of
bifunctional catalysts in AAD processes were also undertaken.
NHC−iridium(III) complexes 1b,c (Scheme 3) were
characterized by NMR spectroscopy and mass spectrometry.
In addition, complex 1b was characterized by single-crystal X-
1
ray diffraction analysis. The H NMR spectrum of 1b in CDCl
3
features a symmetrical pattern. The carbene olefinic protons
appear at δ 7.36 (s, 2 H) ppm and the methylene protons on
the side chains as doublets at δ 5.17 (2 H) and 3.64 (2 H) ppm,
with a geminal coupling constant of 13.3 Hz. The methyl
substituents appear at δ 1.38 (6 H) and 1.31 (6 H) ppm, and
the Cp* group appears at δ 1.50 ppm. The fact that the
methylene protons appear with different chemical shifts with a
large geminal coupling constant and that different chemical
shifts are also observed for the methyl groups indicate that
there is hydrogen bonding between the hydroxy groups and the
chlorides. Single-crystal X-ray diffraction analysis of 1b (Figure
1 left, and Figure S1 in the Supporting Information) confirmed
RESULTS AND DISCUSSION
Synthesis of Iridium Complexes. Iridium(III) complex
a was synthesized according to our previous procedure.
■
1
14a
Related bis-hydroxy- and amine-functionalized analogues, 1b,c
respectively, were synthesized as shown in Scheme 3
The corresponding chloride-free complexes 2a−c were
prepared in situ upon treatment of 1a−c with AgBF (see
4
the Supporting Information).
Starting from imidazole, bis-hydroxy-functionalized imidazo-
1
35
lium salt 5 was prepared in 79% yield (Scheme 3). The H
the molecular structure of the complex. In the solid state,
NMR spectrum of 5 confirmed its symmetrical character and
showed characteristic resonances corresponding to the
intermolecular hydrogen bonds between the hydroxy groups
and chloride ligands of neighboring complexes are formed.
B
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center (see Figure S2 in the Supporting Information). At −25
°
C, this coordination swap is stopped, and the imidazole ring
resonances move from a broad singlet (2 H) at δ 7.69 ppm to
two doublets of 1 H each with a coupling constant of 1.8 Hz, at
δ 7.76 and 7.64 ppm. The methylene protons become well-
defined at −25 °C as two sets of two doublets: δ 4.69 and 4.27
(
1 H, J = 14.2 Hz) and δ 4.45 and 3.94 (1 H, J = 14.9 Hz) ppm,
and all four methyl groups on the tethers resonate at different
chemical shifts at δ 1.61, 1.49, 1.44, and 0.94 ppm (3 H each).
Therefore, VT NMR experiments clearly show that both OH-
functionalized N wingtips on the NHC can coordinate to Ir. In
contrast to 1b, the hydroxy group in 2b is coordinated to the
3
5
Figure 1. (left) Crystal structure of iridium complex 1b shown at
5
0% probability. Selected bond lengths (Å) and angles (deg): Ir(1)−
C(1) 2.049(4), Ir(1)−Cl(1) 2.4317(11), Ir(1)−Cl(2) 2.4210(12),
iridium center both in CDCl solution and in the solid state
3
N(1)−C(1) 1.371(5), N(2)−C(1) 1.369(5); C(1)−Ir(1)−Cl(1)
(Figure 1, right, and Figure S3 in the Supporting
37
9
8
4.41(11), C(1)−Ir(1)−Cl(2) 93.68(12), Cl(1)−Ir(1)−Cl(2)
Information). Single-crystal X-ray analysis of 2b shows an
intramolecular hydrogen bond between a labile water ligand
and the hydroxy functionality that is not tethered to the iridium
3.22(5). Intermolecular hydrogen bonds between the hydroxy
functionalities and the chloride ligands are observed: O(1) and
Cl(1) 3.249 Å and O(2) and Cl(2) 3.220 Å. Hydrogen atoms are not
center. 1
3
7
shown for clarity. (right) Crystal structure of iridium complex 2b
shown at 50% probability. Selected bond lengths (Å) and angles
deg): Ir(1)−C(1) 2.037(6), Ir(1)−O(1) 2.182(4), Ir(1)−O(3)
.151(4), N(1)−C(1) 1.339(9), N(2)−C(1) 1.365(8); C(1)−
Ir(1)−O(1) 85.2(2), C(1)−Ir(1)−O(3) 90.7(2), O(1)−Ir(1)−O(3)
1.31(2). A hydrogen bond (2.57 Å) between a labile water ligand
The H NMR spectrum of bis-cationic amine-functionalized
complex 2c was rather more similar to that of 1c, except for a
substantial shift from δ 6.00 to 4.33 ppm of one of the
(
2
resonances corresponding to the NH protons: in 1c there is an
2
8
interaction between one of these protons and the chloride,
which is absent in 2c.
(
O(3)) and the nontethered hydroxy functionality (O(2)) is shown.
For catalytic tests, chloride-free cationic complexes 2a−c
were prepared by addition of AgBF to the corresponding
In contrast to complex 1b, amine complex 1c was obtained as
4
chloride complexes 1a−c in CH Cl . AgCl was removed by
a cyclic structure with the nitrogen bonded to the iridium,
2
2
1
filtration, and the solvent was evaporated. The complexes thus
prepared were used without further purification.
leaving chloride as a counteranion. The H NMR spectrum
agrees well with the structure of a cyclic cationic complex,
which is expected as a result of the higher coordination ability
of the amine group. The multiplicity observed for the NCH2
protons in the butyl chain, at δ 4.29 ppm as a ddd (1 H) and δ
Catalytic Activity. The catalytic activity of 1a−c and of
2
a−c in the acceptorless dehydrogenation (Scheme 2) of 1-
phenylethanol (3a) was then evaluated (Table 1). In addition,
3
.88 ppm as a ddd (1 H), agreed with previous observations for
14
a related cyclic complex and confirmed the cyclic structure of
complex 1c, in which the nitrogen atom is directly attached to
the iridium center (Scheme 3). Additionally, the NH protons
Table 1. Catalytic Activity of Cp*Ir(NHC) Complexes 1a−d
a
and 2a−c
2
are observed as two distinct doublets at δ 6.00 and 3.31 ppm,
indicating a possible hydrogen bond between one of them and
a chloride. Crystals suitable for X-ray analysis of 1c were not
obtained. However, the crystal structure of a related amine-
functionalized NHC−Ir(III) complex further supports the
cationic cyclic structure proposed here for 1c.
yield (%)
36
a
b
entry
[Ir]
in toluene
in toluene/t-BuOH
1
2
3
4
5
6
7
8
1a
2a
1b
2b
1c
2c
1d
51
30
39
64
5
17
39
18
Following our previous report on the preparation of bis-
14a
cationic complex 2a, the analogous complexes 2b,c (Figure
) were obtained upon treatment of 1b,c, respectively, with
AgBF in CH Cl (see the Supporting Information).
2
c
>99
4
2
2
3
8
10
24
9
21
81
1d/AgBF4
a
b
Conditions: 3 mol % [Ir], toluene, reflux, 15 h. Conditions: 3 mol %
c
[
Ir], toluene/t-BuOH 3/1 (v/v), reflux, 15 h. Full conversion after 12
h.
Figure 2. Bis-cationic NHC−Ir(III) complexes 2a−c. L is a neutral
ligand.
we also compared their activity to that of nonbifunctional NHC
III
38
complex 1d, Cp*Ir [NHC(nBu )]. The reactions were
2
The synthesis and characterization of 2a was reported in our
previous work. Complexes 2b,c were characterized by H
NMR spectroscopy and also by single-crystal X-ray diffraction
initially carried out in toluene, a solvent that gave excellent
results in the alkylation of amines with alcohols catalyzed by
1
4
1
14
2a. The reactions were carried out at reflux in a Schlenk tube
fitted with a condenser (oil bath temperature 130 °C, reaction
time 15 h). Neutral complexes 1a−d gave moderate to low
conversions in toluene (Table 1, entries 1, 3, 5, and 7).
Furthermore, the yields were very similar or even lower when
1
analysis in the case of 2b. The variable-temperature (VT) H
NMR spectrum of 2b in acetone-d6 indicates a dynamic
equilibrium where cyclic structures are formed through an
alternative coordination of the hydroxy moieties to the iridium
C
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a
Table 2. Catalytic Acceptorless Alcohol Dehydrogenation of Alcohols and Diols by 2b
a
b
Conditions: alcohol (0.5 mmol), 2b (0.015 mmol, 3 mol %), toluene (1.3 mL), t-BuOH (0.5 mL), reflux. Isolated yields unless otherwise stated.
c
1
d
e
Yield determined by H NMR spectroscopy, using 1,3,5-trimethoxybenzene as an internal standard. From benzoin. From the corresponding diol.
f
g
1
From the corresponding 1,4-diols. Yields determined by H NMR spectroscopy and isolated yields in parentheses.
the bis-cationic complexes were used (Table 1, entries 2, 4, 6,
and 8). We next investigated the addition of a protic cosolvent
to the reaction mixture. The addition of t-BuOH to the
reactions catalyzed by complexes 1a−d resulted in lower yields
in comparison to those when toluene was used as the sole
solvent (entries 1, 3, 5, and 7). An interesting result was
observed, though, when the bis-cationic complexes were used in
the solvent mixture toluene/t-BuOH; 2a,c gave results similar
to those obtained in just toluene (entries 2 and 6), but a
substantial enhancement of the catalytic activity was observed
D
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for 2b, which gave a quantitative yield of 4a after only 12 h
dehydrogenated in 71% yield (entry 16). Even benzyl alcohol
reacted to give benzaldehyde in 67% yield after 72 h (entry 17).
Other primary benzylic alcohols gave moderate yields (entries
18−21). The bulky alcohol β-cholestanol gave the correspond-
ing carbonyl derivative 4v in very good yield (entry 22). When
complex 2b was tested on 1,4-diols 9a,b, instead of an expected
dehydrogenation reaction, these compounds were transformed
(
entry 4). This increase may be due to a proton-transfer or
proton-relay effect, in which a protic species facilitates the
dehydrogenation reaction (see the mechanistic investigation
3
c,39
below).
In a mechanism involving proton transfer steps, any
chemical species that can temporarily carry protons will
participate in the mechanism. Indeed, alcohol substrates 3a−v
can carry out that role. However, as the reaction proceeds the
concentration of alcohol substrate diminishes, and so does the
rate of the overall reaction. This is why a substantial increase of
reaction rate is observed upon addition of t-BuOH, which
guarantees a constant concentration of species that can shuttle
protons (see Tables S1 and S2 and reaction kinetics on page
S32 in the Supporting Information). Interestingly, a substantial
improvement in the catalytic activity was also obtained for 1d/
1
into cyclic ethers 10a,b (entries 23 and 24). Their H NMR
40,43
spectra indicated the presence of diastereomeric mixtures.
Mechanistic Studies. The first step in the AAD reaction
was investigated by subjecting enantiopure (S)-3a and (S)-3a-d
to the reaction conditions (Table 3). This experiment can shed
Table 3. Racemization and H/D Scrambling in the AAD of
a
(S)-3a-d0,1 by 2b
AgBF upon addition of t-BuOH (entry 8). Complex 1d forms
4
a bis-cationic complex upon reaction with AgBF , while bis-
4
cationic 2a,b easily evolve into mono-cationic complexes due to
facile deprotonation of the alkoxide side arms under the
14
reaction conditions. This agrees with the higher activity boost
for 1d/AgBF when t-BuOH is added to the reaction mixture in
3a (t = 0 h)
3a (t = 3 h)
4a (t = 3 h)
4
b
entry
1
ee (%)/D (%)
solvent
ee (%)/D (%)
(%)
comparison to those for 2a,b (entry 8 vs entries 2 and 4).
Nevertheless, the best catalytic activity is obtained through
combining the use of hemilabile bifunctional catalyst 2b and a
protic solvent such as t-BuOH (entry 4). Figure S4 in the
Supporting Information shows the conversion based on evolved
hydrogen volume in real time. In comparison to 1b, the
superior catalytic activity of 2b may be related to the active
participation of the second hydroxy moiety. The single-crystal
X-ray analysis of 2b (vide supra, Figure 1) shows that the
second hydroxy group is intramolecularly hydrogen bound to a
labile water molecule coordinated to iridium and thus placed in
a privileged position near the metal active site. This clearly
suggests that this second hydroxy-functionalized substituent on
the NHC is not a mere spectator but can participate in the
mechanism by hydrogen-bonding the alcohol substrates and
positioning them in close proximity to the iridium center.
c
c
(S)-3a, >98/0
toluene/
t-BuOH
0/0 (42/0)
64 (21)
2
3
(S)-3a, >98/0
(S)-3a-d,
toluene
toluene
0/0
0/0
59
54
>98/>95
4
(S)-3a-d,
>98/>95
toluene-d8
0/>95
31
5
6
(S)-3a, >98/0
(S)-3a-d,
toluene-d
toluene-d
t-BuOH
0/>95
0/95
38
56
8
/
8
>98/>95
a
Reaction conditions: alcohol (0.5 mmol), 2b (0.015 mmol, 3 mol %),
b
1
reflux, 24 h. Yield determined by H NMR spectroscopy, using 1,3,5-
trimethoxybenzene as an internal standard. Percent deuterium was
1
determined by H NMR spectroscopy, and ee was determined by
c
chiral GC. In parentheses are given data for reaction carried out at 80
°
C oil bath temperature.
The use of H O instead of t-BuOH gave similar results (see
2
Table S2 in the Supporting Information). However, t-BuOH
light onto the potential reversibility of the C−H bond-breaking
step and can also give information about the rate-determining
step (rds) of the reaction. When (S)-3a (>98% ee) was treated
with 2b in toluene/t-BuOH for 3 h under reflux, a 64% yield of
acetophenone 4a was obtained (Table 3, entry 1). Importantly,
the remaining starting material 3a had been racemized. This
indicates that the C−H bond-breaking step is reversible, since
the racemization process occurs through an oxidation/
reduction sequence. Even when the reaction was carried out
at significantly lower temperature (80 °C oil bath) for 3 h, the
ee of the starting material decreased to 42% (entry 1). Similar
results were obtained when toluene was used as the sole
solvent, albeit, as expected, with lower yield (entry 2). When
the same reaction was run with (S)-3a-d, a 54% yield of 4a was
obtained after 3 h. The remaining starting material had also
been racemized, but in addition, 3a had lost all deuterium
content (entry 3). This result indicates that D/H exchange
between (S)-3a-d and the solvent toluene occurs under the
reaction conditions. Indeed, when toluene was replaced by
was chosen over H O, as the reaction mixture was biphasic
2
when the latter was used.
Substrate Scope. The scope of the acceptorless dehydro-
genation of alcohols was studied using the best reaction
conditions found (i.e., bis-cationic complex 2b in a toluene/t-
BuOH mixture, Table 1, entry 4). Benzylic sec-alcohols
substituted with alkyl chains or aromatic rings, including
naphthyl derivatives, were oxidized in good to excellent yields
(Table 2, entries 1−8). A methoxy functionality at the para
position was not well tolerated and gave a low yield of the
expected ketone, together with symmetrical ethers and p-
methoxystyrene (entry 9). Electron-deficient benzylic sec-
alcohols were dehydrogenated in very good yields, although
they required longer reaction times. For practical purposes,
these substrates were allowed to react for 48 h (entries 10−12).
It is worth mentioning that for halogenated substrates, entries
10 and 11, carbon−halogen bond cleavage did not occur. High
yields were not obtained in the oxidation of benzoin (entry 13),
possibly due to the electron-poor nature of this substrate. On
the other hand, α-hydroxy-substituted alcohol 4n efficiently
underwent dehydrogenation (entry 14) selectively at the
secondary alcohol. Double dehydrogenation of benzylic diol
toluene-d , the results were comparable in terms of yield and of
8
ee, with the difference being that the deuterium content in the
starting material remained intact (entry 4 vs 3).
When nondeuterated substrate 3a was oxidized in toluene-d8,
the remaining unreacted starting material was completely
deuterated, confirming a fast H/D exchange between the
4o was achieved in good yield without increasing the catalyst
loading (entry 15). Aliphatic sec-cyclic alcohol 4p was
E
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substrates and the solvent (entry 5). The possible involvement
of t-BuOH in these H/D exchange processes was also ruled out,
since the deuterium content of (S)-3a-d did not change when
The Hammett plot under noncompetition conditions is
shown in Figure 3. A slight negative slope was obtained, with a
rather small absolute value of ρ of only 0.26.
41
the AAD was carried out in toluene-d in the presence of
8
nondeuterated t-BuOH (entry 6). This last experiment also
confirms that the proton (O−H) and the hydrogen (C−H)
keep their identities during the reaction.
Further support for the reversibility of the C−H bond-
breaking step was obtained by carrying out the AAD of 3k in
the presence of acetophenone 4a (3k/4a = 3/1) (Table 4). In
Table 4. Crossover Experiment: Reaction of Alcohol 3k in
a
the Presence of Ketone 4a
Figure 3. Hammett plot constructed from the relative rates obtained
from noncompetition experiments with para-functionalized 1-phenyl-
entry
t (h)
3k (%)
4a (%)
4k (%)
3a (%)
2
ethanol derivatives 3a,e,k,l (y = −0.26x (±0.03) + 0.01 (±0.01), R =
1
2
3
0
1
3
75
50
26
25
11
7
0
.98).
29
47
14
18
a
1
Regarding the effect of t-BuOH on the rate of the reaction, it
Yield determined by H NMR spectroscopy, using 1,3,5-trimethox-
ybenzene as an internal standard.
was observed that it increases substantially when small amounts
of this cosolvent were added, until ca. 9 mol %. A higher
concentration of t-BuOH did not have a substantially greater
effect on the rate of the reaction (Figure S5 in the Supporting
Information).
Mechanistic Discussions. A mechanistic proposal for the
AAD reaction based on the mechanistic investigations is
presented in Scheme 4. The active catalytic species is formed
upon deprotonation of the hydroxy group in one of the
this experiment, two competitive transformations can occur
simultaneously: transfer hydrogenation, producing alcohol 3a
and ketone 4k, as well as the expected AAD of both 3k and 3a
to give 4k and 4a, respectively. Table 4 shows the composition
of the reaction mixture at t = 0, 1, and 3 h. Alcohol 3a could be
formed by transfer hydrogenation with 3k, and this would
confirm the reversibility of the C−H breaking step in the
catalytic cycle. After 1 h, the reaction mixture contained 14% of
pendant arms of the NHC. Since the pK of this hydroxy group
a
should be significantly lowered upon coordination to the
1
4
iridium center, t-BuOH or the substrate alcohol are
sufficiently basic to carry out this deprotonation activation
step. This is followed by coordination of the alcohol substrate
3a to iridium to give intermediate I (step i). Deprotonation of
the coordinated substrate through a proton transfer or proton
relay mechanism involving t-BuOH gives alkoxide complex II
(step ii). β-Hydride elimination from coordinatively unsatu-
rated intermediate II results in alcohol oxidation with
concomitant formation of an iridium hydride species
(intermediate III, step iii). This step is reversible, as
demonstrated in the racemization and transfer-hydrogenation
experiments (vide supra, Tables 3 and 4, respectively).
Recoordination of the hydroxy group of one of the side arms
facilitates dissociation of the ketone product 4a (step iv). To
close the cycle, a tBuOH-assisted proton transfer facilitates the
formation of molecular dihydrogen and the regeneration of
alcohol complex I.
3
a, and after 3 h 18% of 3a was found. Note that alcohol 3a can
also undergo AAD to give 4a. The formation of 3a could also
be accounted for through a hydrogenation reaction (i.e.,
reduction of 4a by hydrogenation with the H produced in the
AAD reaction). This possibility was ruled out by studying the
reverse overall reaction: i.e., hydrogenation of acetophenone 4a
catalyzed by 2b. The reaction worked well in toluene/t-BuOH,
2
but only when 10 atm of H was used at 80 °C, and gave a 70%
2
yield of 3. Since a rather high pressure of H was needed, it can
2
be concluded that the reverse hydrogenation reaction is
thermodynamically disfavored under the AAD reaction
conditions. These results therefore confirm that formation of
3
a in the crossover experiment (Table 4) is due to a transfer-
hydrogenation process. This further confirms the reversibility of
the first oxidation step in the AAD.
Kinetic investigations were carried out by monitoring the
1
reactions by H NMR spectroscopy. The reaction rates were
The small magnitude of the KIE (2.0 ± 0.1) and the small
influence of the electronic properties of the substrates on the
reaction rate (vide supra, ρ = −0.26, Figure 3) seem to rule out
the possibility that the C−H bond-breaking step is the rds.
Instead, all the data suggest that the rds of the catalytic cycle
lower when the reactions were run in NMR tubes than when
the reflux-condenser setup described above for the substrate-
scope investigation (Table 2) was used. This is due to a less
efficient release of pressure. Due to H/D exchange with the
solvent toluene, the magnitude of the kinetic isotope effect
42
involves the step where the Ir−H bond is broken. Since the
rate is not completely independent of the electronic properties
of the alcohol substrates (vide supra, Figure 3 and Table 2), the
resting state of the catalytic cycle is probably intermediate II or
intermediate III, both of which contain the (modified) reaction
substrate. However, the negative slope of the Hammett
experiment, which indicates that electronic density is lost,
suggests that the rds corresponds to the process of passing from
(
KIE) under noncompetition conditions was estimated by
comparing the initial rates of the AAD of 3a in nondeuterated
toluene to that of 3a-d in toluene-d . Under these conditions,
8
only H/H or D/D exchange between substrates and solvent
occurs in each experiment, ensuring a constant hydrogen or
deuterium content in the starting alcohols. A KIE of 2.0 ± 0.1
was obtained (Figure S6 in the Supporting Information).
F
DOI: 10.1021/acs.organomet.7b00220
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 4. Proposed Catalytic Cycle for Solvent-Mediated Ligand-Assisted AAD Catalyzed by 2b
resting state II over the highest transition state TSIV-V, toward
the formation of dihydrogen intermediate V.
Mechanistic investigations showed that the rds of the
catalytic cycle involves breaking the Ir−H bond and that the
electronic properties of the substrates have a rather small
influence on the reaction rate. This explains the broad substrate
scope of the AAD reaction catalyzed by 2b.
The superior activity of the alcohol-/alkoxide-functionalized
catalyst (2b) in comparison with the related amine-/amide-
functionalized complex 2c is somewhat surprising. Bifunctional
catalysts bearing an amine/amide group have previously been
shown to have substantially higher activity in redox reactions
involving alcohol substrates in comparison to those where the
bifunctionality of the catalyst relies on an alcohol/alkoxide
moiety. With the results described in this paper, we have shown
that significantly better performance is obtained as the result of
an “OH effect” in the AAD mediated by Ir(III)−NHC
complexes.
The cyclization reaction of diols 9a,b does not involve an
overall dehydrogenation process. The mechanism may involve
iridium-catalyzed hydrogen transfer processes. An alternative
and straightforward mechanism would be an acid-mediated
pathway where a proton or the iridium complex would act as an
acid catalyst. The mechanism is currently under investigation
and will be communicated in due course.
CONCLUSIONS
■
We have given a comparative analysis of two modes of metal−
ligand cooperation in the acceptorless dehydrogenation of
alcohols. A family of bifunctional amine-functionalized and
hydroxy-functionalized Cp*Ir(NHC) complexes has been
investigated, and their activities have been compared to that
of a nonbifunctional Cp*Ir(NHC) complex. The screening
clearly showed the benefits of including a hemilabile alcohol/
alkoxide moiety in the catalyst structure, rather than an amine/
amide functionality or an spectator ligand. The best results
were obtained with catalyst 2b, which has two hydroxy side
chains on its NHC ligand. The activity is also dependent on the
reaction conditions and in particular on the solvent system
used. The presence of protic solvents enhanced the catalytic
activity of complexes of 2b, as these solvents facilitate proton-
transfer steps between the catalyst and the substrates or
products, thus improving the outcome of the AAD reaction
through a proton transfer or proton-relay effect.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00220.
Experimental details, 1H NMR and 13C NMR spectra,
kinetic data, and crystallographic data (PDF)
Accession Codes
CCDC 1509620 and 1531608−1531609 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12,
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Bis-cationic complex 2b showed a wide scope in the AAD of
numerous alcohols. A catalytic or stoichiometric base was not
needed. Although both secondary and primary alcohols could
be oxidized, catalyst 2b showed higher activity toward the
former, allowing the chemoselective oxidation of secondary
alcohols in substrates containing both types of alcohol
functionalities.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for B.M.-M.: belen.martin.matute@su.se.
G
DOI: 10.1021/acs.organomet.7b00220
Organometallics XXXX, XXX, XXX−XXX

Organometallics
ORCID
Article
Ghosh, A.; Vaccaro, L.; Ackermann, L.; Gelman, D. Adv. Synth. Catal.
2
(
015, 357, 2351−2357.
Greco Gonzal
́
ez Miera: 0000-0002-2856-5295
Martín-Matute: 0000-0002-7898-317X
Author Contributions
13) Chakraborty, S.; Piszel, P. E.; Brennessel, W. W.; Jones, W. D.
Belen
́
Organometallics 2015, 34, 5203−5206.
(14) (a) Bartoszewicz, A.; Marcos, R.; Sahoo, S.; Inge, A. K.; Zou, X.;
Martín-Matute, B. Chem. - Eur. J. 2012, 18, 14510−14519.
b) Bartoszewicz, A.; Gonzalez Miera, G.; Marcos, R.; Norrby, P.-
́
†
G.G.M. and E.M.-C. contributed equally.
(
Notes
O.; Martín-Matute, B. ACS Catal. 2015, 5, 3704−3716.
15) Chu, W.-Y.; Zhou, X.; Rauchfuss, T. B. Organometallics 2015,
4, 1619−1626.
16) Geri, J. B.; Szymczak, N. K. J. Am. Chem. Soc. 2015, 137, 12808−
2814.
17) Nakano, R.; Nozaki, K. J. Am. Chem. Soc. 2015, 137, 10934−
0937.
18) (a) Kanai, M. In Organic Chemistry−Breakthroughs and
The authors declare no competing financial interest.
(
3
(
1
(
1
(
ACKNOWLEDGMENTS
■
(
This project was supported by the Swedish Research Council
VR) and the Knut and Alice Wallenberg Foundation. We also
thank the Swedish Governmental Agency for Innovation
Systems (VINNOVA) for a VINNMER grant to B.M.-M.
E.M.-C. thanks the Wenner-Gren foundation for a postdoctoral
scholarship. Prof. Per-Ola Norrby is gratefully acknowledged
for fruitful mechanistic discussions, and. Dr A. Bartoszewicz and
Dr R. Marcos are thanked for discussions.
Perspectives; Ding, K., Dai, L. X., Eds.; Wiley-VCH: New York, 2012;
Chapter 11, p 385. (b) Askevold, B.; Roesky, H. W.; Schneider, S.
ChemCatChem 2012, 4, 307−320.
(
19) Peris, E. Chem. Rev. 2017, DOI: 10.1021/acs.chemrev.6b00695.
(20) Carson, F.; Martínez-Castro, E.; Marcos, R.; Gonzal
́
ez Miera,
G.; Jansson, K.; Zou, X.; Martín-Matute, B. Chem. Commun. 2015, 51,
10864−10867.
21) (a) Gunanathan, C.; Milstein, D. Science 2013, 341, 1229712.
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since we anticipated that H gas would not be produced and so a larger
2
head space would not be needed.
I
DOI: 10.1021/acs.organomet.7b00220
Organometallics XXXX, XXX, XXX−XXX