Acceptorless Dehydrogenative Oxidation of Secondary Alcohols Catalysed by Cp*IrIII–NHC Complexes

Article abstract of DOI:10.1002/chem.201601648

A series of new Ir^IIIcomplexes with carbene ligands that contain a range of benzyl wingtip groups have been prepared and fully characterised by NMR spectroscopy, HRMS, elemental analysis and X-ray diffraction. All the complexes were active in the acceptorless dehydrogenation of alcohol substrates in 2,2,2-trifluoroethanol to give the corresponding carbonyl compounds. The most active complex bore an electron-rich carbene ligand; this complex was used to catalyse the highly efficient and chemoselective dehydrogenation of a wide range of secondary alcohols to their respective ketones, with turnover numbers up to 1660. Mechanistic studies suggested that the turnover of the dehydrogenation reaction is limited by the H₂-formation step.

Full text of DOI:10.1002/chem.201601648

DOI: 10.1002/chem.201601648
Full Paper
&
Oxidation Reactions
Acceptorless Dehydrogenative Oxidation of Secondary Alcohols
Catalysed by Cp*IrIII–NHC Complexes**
Sꢀleyman Gꢀlcemal,[a, b] Derya Gꢀlcemal,*[a, b] George F. S. Whitehead,[a] and Jianliang Xiao*[a]
Abstract: A series of new IrIII complexes with carbene li-
gands that contain a range of benzyl wingtip groups have
been prepared and fully characterised by NMR spectroscopy,
HRMS, elemental analysis and X-ray diffraction. All the com-
plexes were active in the acceptorless dehydrogenation of
alcohol substrates in 2,2,2-trifluoroethanol to give the corre-
sponding carbonyl compounds. The most active complex
bore an electron-rich carbene ligand; this complex was used
to catalyse the highly efficient and chemoselective dehydro-
genation of a wide range of secondary alcohols to their re-
spective ketones, with turnover numbers up to 1660. Mecha-
nistic studies suggested that the turnover of the dehydro-
genation reaction is limited by the H2-formation step.
Introduction
actions are typically carried out by using stoichiometric- or
excess amounts of metal-based oxidants. Environmentally ac-
ceptable oxidants such as molecular oxygen,[7] hydrogen per-
oxide[8] and, less desirably, acetone[9] have been used in catalyt-
ic oxidation reactions. However, from an atom-efficiency and
environmental viewpoint, the oxidant-free AD reaction is more
desirable because H2 is released as a gas.[10]
N-Heterocyclic carbenes (NHCs) have emerged as a versatile
class of ligands in organometallic chemistry and catalysis.
NHCs often form stable complexes with transition metals irre-
spective of their oxidation states[1] Additionally, their tunable
character allows for easy control of the electronic and steric
properties at the metal centre. Recently, transition-metal/NHC
complexes have found applications in acceptorless dehydro-
genative b-alkylation,[2] amidation,[3] N-formylation[4] and imine
formation reactions.[5] However, there are only a few reported
examples of NHC-based complexes that catalyse the acceptor-
less dehydrogenative oxidation of alcohols to afford carbonyl
compounds.[6] In these reports, the acceptorless dehydrogena-
tion (AD) reactions are typically carried out at reflux tempera-
ture in high-boiling solvents, such as toluene, and full conver-
sion of the alcohol to the corresponding carbonyl compound
requires high catalytic loadings (2–5 mol%) and furnishes max-
imum turnover numbers (TONs) of only 50.
Herein, we report the synthesis and characterisation of
a range of Cp*IrIII–NHC complexes (Cp*=1,2,3,4,5-pentame-
thylcyclopentadienyl) with different benzyl substituents and
azole skeletons. These complexes are active catalysts for the
AD of secondary benzylic and aliphatic alcohols to ketones in
2,2,2-trifluoroethanol (TFE; b.p. 788C) at reflux temperature,
with TONs up to 1660. To the best of our knowledge, this is
one of the highest TONs obtained for the AD of secondary al-
cohols to the corresponding ketones promoted by an NHC-
containing catalyst, as well as the first example of Cp*IrIII–NHC
catalysts for these transformations. It is noted, however, that
transition-metal complexes bearing CÀN, NCN and PCP pincer
ligands have been reported to be efficient catalysts for the AD
of primary and secondary alcohols.[11–15]
Oxidation of alcohols to carbonyl compounds is one of the
most fundamental reactions in organic synthesis, both in aca-
demic laboratories and industrial processes to access chemi-
cals, fuels and pharmaceuticals. The removal of hydrogen from
a hydrogen-rich organic molecule is often a thermodynamically
unfavourable process. Thus, conventional dehydrogenation re-
Results and Discussion
Scheme 1 outlines the route used for the synthesis of
[Cp*Ir(NHC)Cl2] complexes 2a–f. The new Cp*IrIII–NHC com-
plexes 2a–f were prepared in high yields as air- and moisture-
stable yellow solids by a two-step process that involved trans-
metallation[16] from the Ag–NHC derivatives formed in situ,
(Scheme 1). Complexes 2a–f were characterised by NMR spec-
troscopy, HRMS and elemental analysis. Complexes 2a–f exhib-
it 13C NMR chemical shifts at d=156.5, 157.5, 158.7, 186.5,
172.0 and 161.9 ppm, respectively, for the characteristic IrÀ
Ccarbene carbon atom, and the values are comparable to those
of other reported Cp*IrIII–NHC complexes with an azole skele-
ton.[9c–e,17] Meanwhile, the characteristic downfield signals for
[a] Prof. S. Gꢀlcemal, Prof. D. Gꢀlcemal, Dr. G. F. S. Whitehead, Prof. J. Xiao
Department of Chemistry, University of Liverpool
Liverpool, L69 7ZD (UK)
E-mail: jxiao@liv.ac.uk
[b] Prof. S. Gꢀlcemal, Prof. D. Gꢀlcemal
Department of Chemistry, Ege University
35100 Bornova-Izmir (Turkey)
E-mail: suleyman.gulcemal@ege.edu.tr
[**] Cp*=1,2,3,4,5-pentamethylcyclopentadienyl, NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601648.
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Scheme 1. Synthesis of the Cp*IrIII–NHC complexes 2a–f used in this study.
the NCHN+ proton of azolium salts 1a–f disappeared in the
1H NMR spectrum. Finally, the molecular structure of all the
complexes was determined by single-crystal X-ray diffraction.
Single crystals were obtained by diffusion of pentane into con-
centrated solutions of the complexes in chloroform. Coordina-
tion of a single NHC to the iridium centre was confirmed (Fig-
ures 1–6; atom-numbering and selected bond lengths [ꢂ] and
angles [8] are shown). All the complexes exhibit piano-stool
type geometry. The IrÀCcarbene bond lengths (2.065(5)–
2.023(6) ꢂ) are in the expected range,[9c–e,17] and are slightly
longer with more electron-rich NHC ligands.
We started the investigation with 1-phenylethanol (3a) as
a model substrate and TFE as the solvent for the AD reaction
catalysed by 2a. TFE has been shown to be a most effective
solvent for the AD of heterocycles catalysed by cyclometallated
Cp*IrIII, possibly by promoting the dissociation of chloride from
the complex and, hence, promoting coordination of the sub-
strate to the metal centre and protonation of the intermediate
hydride to facilitate H2 formation.[18] The AD of 3a under vari-
ous conditions was examined first (Table 1). When a solution of
3a in TFE was heated at reflux for 2 h in the presence of 2a
(1.0 mol%), we observed the formation of acetophenone (4a)
and also, unexpectedly, fluorinated ether (5) (3a/4a/5=
66:20:14; Table 1, entry 1). Addition of NaBF4 or NH4BF4
(5.0 mol%) increased the amount of undesired product 5
Figure 1. Molecular structure of 2a with hydrogen atoms removed for clari-
ty. Selected bond lengths [ꢂ] and angles [8]: Ir(1)ÀC(1) 2.059(5), Ir(1)ÀCl(1)
2.4399(10), C(1)ÀN(1) 1.374(4), C(2)ÀN(1) 1.392(4); C(1)-Ir(1)-Cl(1) 91.38(11),
C(1)-N(1)-C(2) 111.2(3).
(Table 1, entries 2 and 3). This could result from the formation
of acidic HF, which would be expected to catalyse the etherisa-
tion of 3a by TFE.[19] Hence, the reaction was examined by in-
troducing different bases (5.0 mol%) to suppress etherisation
(Table 1, entries 4–8). Indeed, 5 was not detected when a base
was introduced, and pleasingly, in the presence of NaOAc, the
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Figure 2. Molecular structure of 2b with hydrogen atoms removed for clari-
ty. Selected bond lengths [ꢂ] and angles [8]: Ir(1)ÀC(1) 2.055(6), Ir(1)ÀCl(1)
2.4300(9), C(1)ÀN(1) 1.367(6), C(2)ÀN(1) 1.395(6); C(1)-Ir(1)-Cl(1) 92.09(11),
C(1)-N(1)-C(2) 111.6(4).
Figure 4. Molecular structure of 2d with hydrogen atoms removed for clari-
ty. Selected bond lengths [ꢂ] and angles [8]: Ir(1)ÀC(1) 2.065(6), Ir(1)ÀCl(1)
2.4182(12), C(1)ÀN(1) 1.345(5), C(2)ÀN(1) 1.468(6); C(1)-Ir(1)-Cl(1) 92.93(12),
C(1)-N(1)-C(2) 113.6(4).
Figure 3. Molecular structure of 2c with hydrogen atoms removed for clarity.
Selected bond lengths [ꢂ] and angles [8]: Ir(1)ÀC(1) 2.051(4), Ir(1)ÀCl(1)
2.4383(7), C(1)ÀN(1) 1.364(3), C(2)ÀN(1) 1.385(3); C(1)-Ir(1)-Cl(1) 91.36(8), C(1)-
N(1)-C(2) 111.5(2).
conversion to 4a increased to 82% (Table 1, entry 6>). Com-
plete conversion to 4a was achieved in the presence of 2a
(0.1 mol%) and NaOAc (5.0 mol%) after 20 h (Table 1, entry 9).
However, further decreasing the catalyst loading (0.05 mol%)
resulted in a lower conversion (77%) after 20 h (Table 1,
entry 10). The amount of NaOAc was also found to affect the
rate of the AD reaction (Table 1, entries 11–13): the highest
conversion to 4a was obtained in the presence of NaOAc
(2.5 mol%) (83%, TON=1660; Table 1, entry 12). By using this
lower amount of base, full conversion was obtained with
a lower catalyst loading (0.1 mol%; Table 1, entry 14).
Figure 5. Molecular structure of 2e with hydrogen atoms and chloroform
solvent removed for clarity. Selected bond lengths [ꢂ] and angles [8]: Ir(1)À
C(1) 2.029(3), Ir(1)ÀCl(2) 2.4054(9), C(1)ÀN(1) 1.373(4), C(2)ÀN(1) 1.397(5);
C(1)-Ir(1)-Cl(2) 93.23(10), C(1)-N(1)-C(2) 110.8(3).
NHC skeleton are important for the AD reaction. Comparison
of the conversions obtained with sterically similar complexes
2a–c indicates that the more electron rich the iridium centre
is, the higher the catalytic activity. In contrast, only 3% conver-
sion was obtained with [IrCl2Cp*]2 under the same conditions,
which shows the importance of the NHC ligand for the AD re-
action (Table 1, entry 20). A control experiment was carried out
without any catalyst and no product formation was observed
(Table 1, entry 21). Notably, when the AD reaction was carried
out in a closed system only 62% yield of 4a was achieved
(Table 1, entry 22), which indicates that the AD reaction is re-
Other iridium complexes with different NHC ligands (2b–e)
were subsequently evaluated under the same reaction condi-
tions. Complex 2a, which bore a 1,3-bis(4-methoxybenzyl)-imi-
dazol-2-ylidene ligand, provided the highest conversion
(Table 1, entries 14–19). It is clear that both the electronic
effect of the wingtip (p-OMe, p-H and p-CF3) and the type of
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sion (87%; Table 1, entry 23). This could be due to the inter-
mediate iridium–hydride species being unstable toward O2.[21]
The use of TFE is critical for the reaction to proceed; very little
or no reaction was observed with the other solvents tested
(Table 1, entries 24 and 25). This is reminiscent of the observa-
tions made for iridicycles in the AD of N-heterocycles and sug-
gests that the formation of H2, which is facilitated by acidic
TFE (pKa =12.5), may be turnover limiting,.[18] Consistent with
this, the AD reaction was slightly faster when an alcohol of
lower acidity was used as the solvent, for example, hexafluoroi-
sopropanol (HFIP, pKa =9.3) (Table 1, entry 26).
The ability of Cp*IrIII–NHC complexes to undergo intramolec-
ular aromatic CÀH activation was reported by Peris and co-
workers.[22] They reported that, in most cases, the ortho-metal-
lation occurs under very mild conditions. To investigate the
possible effect of ortho-metallation on the AD reaction, we
studied the reaction of complex 2a with NaOAc (5 equiv) in di-
chloromethane (Scheme 2). We found that 2a was converted
to a new complex 2a’, which could be isolated in 90% yield.
Comparison of the 1H NMR spectra of 2a and 2a’ suggests
that complex 2a’ could arise from intramolecular aromatic CÀ
H activation. The CH2 protons of the benzyl groups display sig-
nals at d=6.03 (d, J=14.4 Hz, 2H) and 5.10 ppm (d, J=
14.4 Hz, 2H) for 2a and at d=5.95 (d, J=14.0 Hz, 1H), 4.99 (d,
J=14.0 Hz, 1H), 4.83 (d, J=14.0 Hz, 1H) and 4.62 ppm (d, J=
14.0 Hz, 1H) for 2a’, which is consistent with the ortho-metalla-
tion results previously reported.[17b,22] The 13C NMR spectrum of
2a’ also supports that ortho-metallation has occurred: the ad-
ditional IrÀCAr signal is observed at d=146.1 ppm.[17b,22] Fur-
thermore, crystals of complex 2a’ suitable for X-ray diffraction
were obtained by diffusion of pentane into a concentrated so-
lution of 2a’ in chloroform, and the proposed structure was
confirmed (Figure 7). As can be seen (Figure 7), ortho-metalla-
tion of the phenyl ring of the imidazol-2-ylidene ligand has oc-
curred, which leads to a chelating coordination of the ligand.
The IrÀCcarbene and IrÀCphenyl bond lengths are 2.05 and 2.04 ꢂ,
respectively, and lie in the expected range.[17b,22]
Figure 6. Molecular structure of 2 f with hydrogen atoms and chloroform
solvent removed for clarity. Selected bond lengths [ꢂ] and angles [8]: Ir(1)À
C(1) 2.023(6), Ir(1)ÀCl(2) 2.3909(16), C(1)ÀN(1) 1.378(7), C(2)ÀN(1) 1.381(8);
C(1)-Ir(1)-Cl(2) 93.21(17), C(1)-N(1)-C(2) 110.5(5).
Table 1. Cp*IrIII–NHC catalysed AD of 3a in TFE.[a]
Entry Cat. [mol% Ir] Solvent Additive [mol %] Time [h] 3a/4a/5 [%][b]
1
2
3
4
5
6
7
8
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (1.0)
2a (0.1)
2a (0.05)
2a (0.05)
2a (0.05)
2a (0.05)
2a (0.1)
2b (0.1)
2c (0.1)
2d (0.1)
2e (0.1)
2 f (0.1)
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
TFE
–
2
2
2
2
2
2
2
2
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
66:20:14
10:10:80
0:13:87
59:41:0
56:44:0
18:82:0
31:69:0
40:60:0
0:100:0
23:77:0
55:45:0
17:83:0
29:71:0
0:100:0
21:79:0
69:31:0
7:93:0
85:15:0
54:46:0
97:3:0
100:0:0
38:62:0
13:87:0
98:2:0
NaBF4 (5.0)
NH4BF4 (5.0)
NaHCO3 (5.0)
Na2CO3 (5.0)
NaOAc (5.0)
KOAc (5.0)
AgOAc (5.0)
NaOAc (5.0)
NaOAc (5.0)
NaOAc (10)
NaOAc (2.5)
NaOAc (1.0)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
NaOAc (2.5)
9
10
11
12
13
14
15
16
17
18
19
20
21
Complex 2a’ was evaluated as a catalyst for the AD reaction
of 3a by using the conditions described in Table 1, entry 14.
[IrCl2Cp*]2 (0.1) TFE
–
TFE
TFE
TFE
22[c] 2a (0.1)
23[d] 2a (0.1)
24
25
26
2a (0.1)
2a (0.1)
2a (0.1)
toluene NaOAc (2.5)
EtOH
HFIP
NaOAc (2.5)
NaOAc (2.5)
100:0:0
11:89:0
[a] Reaction conditions: alcohol (1 mmol), 2 (0.05–1.0 mol%), additive (1-
10 mol%), TFE (1 mL), N2, reflux. [b] Conversion determined from the
1H NMR spectrum. [c] Reaction performed in a closed system. [d] Reaction
vessel was open to air.
versible and is facilitated by the release of H2. In addition, we
detected the evolution of H2 gas during the dehydrogenation
of 3a by GC analysis.[20] However, performing the AD reaction
under an ambient atmosphere also resulted in a lower conver-
Figure 7. Molecular structure of 2a’ with hydrogen atoms removed for clari-
ty. Selected bond lengths [ꢂ] and angles [8]: Ir(1A)ÀC(1A) 2.050(13), Ir(1A)À
C(14A) 2.040(13), Ir(1A)ÀCl(1A) 2.428(5); C(1A)-Ir(1A)-C(14A) 85.1(5), C(1A)-
Ir(1A)-Cl(1A) 92.5(4), C(14A)-Ir(1A)-Cl(1A) 88.0(4).
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Scheme 2. Synthesis of the ortho-metallated Cp*IrIII–NHC complex 2a’.
Good conversion (80%) to 4a was achieved in the presence of
2a’ (0.1 mol%) and NaOAc (2.5 mol%) after 20 h. The results
show that catalyst 2a is more active than the ortho-metallated
analogue 2a’ (cf. Table 1, entry 14), which suggests that ortho-
metallation does not play a significant role, if any, in the AD re-
action.
activity toward the hydrogenation of 7. Indeed, we found that
2a (0.1 mol%) catalysed the quantitative hydrogenation of 7
(1.0 mmol) to 6 under a balloon of H2 (1 atm) in TFE at 408C
after 4 h (Scheme 3). Further evidence of the reversibility of
the AD of 6 is the AD of 7 during the dehydrogenation of 3a
(Scheme 4). When a mixture of 3a (1.0 mmol) and 7 (1.0 mmol)
To explore the scope of the present catalytic AD system, the
reactions of a wide range of secondary alcohols were conduct-
ed under the optimised conditions (Table 2). Various 1-aryletha-
nols (3a–e) bearing electron-donating or electron-withdrawing
substituents at the para-position of the phenyl ring were effec-
tively converted into the corresponding ketones (4a–e) in high
yields by using 2a (0.1 mol%) as the catalyst (Table 2, en-
tries 1–5). Only para-CF3-substituted 1-arylethanol 3e gave
a lower conversion and isolated yield (Table 2, entry 5). More
sterically hindered 1-arylalcohols 3 f–n gave the corresponding
ketones 4 f–n in 83–98% isolated yield (Table 2, entries 6–14).
In addition to the aromatic substrates, aliphatic secondary al-
cohols 3o–s were dehydrogenated to give aliphatic ketones
4o–s (Table 2, entries 15–19). However, some of these sub-
strates required a higher catalyst loading, probably due to
subtle steric effects (for example, Table 2, entries 6–8, 15, 17
and 18).
Scheme 4. Intermolecular hydrogen transfer process.
was subjected to the AD reaction conditions with 2a
(0.25 mol% relative to 3a) 3a was fully converted into 4a,
whereas 7 was reduced to 6 in 96% conversion by “borrow-
ing” the hydrogen atom from 3a (Scheme 4).[23] These results
also show that 2a is an active catalyst for both hydrogenation
and-transfer hydrogenation reactions, and indicate that the
low efficiency of 2a in the AD of 6 is due to easier reduction
of the product.
Next, we examined the AD of primary alcohols. However,
the AD of benzyl alcohol (6) (1.0 mmol) in the presence of 2a
(1.0 mol%) did not proceed well: only 18% conversion to ben-
zaldehyde (7) was observed by NMR spectroscopy (Scheme 3).
Considering that the AD reaction may be reversible and that
aldehydes are easier to reduce than ketones, the low conver-
sion to 7 could be a result of the product being reduced
under the AD conditions. In fact, AD can be seen as a transfer-
hydrogenation reaction, in which the removed hydrogen atom
is not captured by a sacrificial acceptor but released from the
reaction.[10f] To show the possibility of 2a catalysing the reverse
reaction of primary alcohol dehydrogenation, we examined its
The reversibility of the AD of primary alcohols can be ex-
ploited for chemoselective AD reactions. For example, the AD
of 3a (1.0 mmol) and
6 (1.0 mmol) with catalyst 2a
(0.25 mol%) resulted in full conversion of 3a to 4a, but only
3% conversion of 6 to 7 after 20 h (Scheme 5), which demon-
strates the excellent selectivity of the catalyst for secondary
versus primary benzylic alcohols.
Scheme 5. Selective dehydrogenation of secondary alcohols.
Furthermore, we investigated the AD of diols, which can po-
tentially release two equivalents of H2 to form stable lacto-
nes.[11c,12b,15c,24] Thus, in the presence of 2a (1.0 mol%) and
NaOAc (5.0 mol%), 1,2-dibenzenedimethanol (8a) and 1,5-pen-
tanediol (8b) were converted into the corresponding lac-
tones—phthalide (9a) and d-valerolactone (9b)—in 95 and
Scheme 3. Reversible AD of primary alcohol 6.
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92% isolated yield, respectively (Scheme 6). These results show
that it is possible to dehydrogenate molecules that bear pri-
mary alcohol units if there is a functional group available to in-
tercept the newly formed aldehyde.
Table 2. AD of various secondary alcohols catalysed by 2a.[a]
To gain an insight into the rate-limiting step of the present
Cp*IrIII–NHC catalysed AD reaction the kinetic-isotope effect
(KIE) was studied. Figure 8 shows the conversion-time profiles
Entry
1
Product
S/C[b]
1000
Conv. [%][c]
100
Yield [%][d]
96
4a
4b
2
1000
100
97
3
4
4c
1000
1000
100
98
94
91
4d
Scheme 6. AD of diols 8.
5
4e
1000
84
81
6
4 f
4g
4h
4i
500
96
93
88
97
98
83
7
500
93
8
500
100
100
86
9
1000
1000
10
4j
11
12
13
4k
4l
1000
1000
1000
100
100
100
97
95
97
4m
^
*
)
Figure 8. KIE study of the AD reaction. Deuterated ( ) or non-deuterated (
14
4n
1000
92
87
1-phenylethanol (1 mmol), 2a (0.1 mol%), NaOAc (2.5 mol%), TFE (1 mL), N2,
reflux.
15
16
4o
4p
500
88
85
1000
96[e]
95[f]
obtained with 3a and its 1-deuterated analogue. Clearly, deut-
eration has little effect on the kinetics of the AD reaction,
which suggests that CÀH cleavage is not involved in the rate-
limiting step. The linearity of the profiles reveals that the AD
rates do not vary with time (at <50% conversion), and sup-
ports the view that the alcohol is not involved in the rate-limit-
ing step.
17
4q
200
83[e]
82[f]
18
19
4r
4s
1000
200
96[e]
95[e]
96[f]
92[f]
[a] Reaction conditions: alcohol (1 mmol), 2a (0.1–0.5 mol%), NaOAc
(2.5 mol%), TFE (1 mL), N2, reflux, 20 h. [b] Substrate/catalyst ratio.
[c] Conversion was determined from the 1H NMR spectrum. [d] Isolated
yield. [e] Conversion was determined by GC analysis. [f] Yield was deter-
mined by GC, with decane or nonane as an internal standard.
To gain further insight into the reaction mechanism, the AD
of 3a with 2a (20 mol%) in [D3]TFE was monitored by variable
temperature (VT) NMR spectroscopy (Figure 9). No product for-
mation was observed after 30 min at room temperature. In
contrast, a new peak rapidly appeared in the hydridic region
1
of the H NMR spectrum (d=À17.4 ppm) upon warming the
reaction mixture to 508C along with the formation of 4a,
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Figure 9. 1H NMR hydride region of the in situ AD reaction carried out in sealed NMR tube: A solution of 1-phenylethanol (0.5 mmol), 2a (20 mol%) and
1
NaOAc (2.5 mol%) mixed in [D3]TFE at RT and the H NMR spectra was recorded after 30 min. The appearance of a new peak at d=À17.4 ppm was observed
after increasing the temperature to 508C.
which suggested that the catalytically active species was relat-
ed to this iridium hydride. The hydride remained unaltered
during the VT-NMR spectroscopy experiment as conversion of
3a to 4a increased. These observations indicate that the rate-
limiting step in the AD reaction is H2 formation. Rate-limiting
hydrogen formation has been noted for iridicycle-catalysed AD
reactions, and was considered to be facilitated by TFE through
protonation of the intermediate hydride.[18,25]
We also attempted to determine if there was a structure–ac-
tivity relationship in the AD reaction. Thus, non-competitive
dehydrogenation of a series of para-substituted secondary
benzyl alcohols was carried out. By using the initial rate for
each AD reaction, a Hammett plot of log(kX/kH) (krel) against the
substituent constant sp could be constructed. Figure 10 illus-
trates that there is a fairly good correlation between log(kX/kH)
and the sp parameters and a negative slope (1=À1.03, R2 =
0.99). Because the substrate is not involved in the turnover-lim-
iting step, this negative correlation may be indicative of a pre-
equilibrium that involves the necessary substitution of the co-
Figure 10. Hammett plots for catalyst 2a obtained from non-competitive ex-
periments for the AD of para-substituted 1-phenylethanol. Conditions: alco-
ordinated chloride anion of 2a by the alcohol substrate, in
which the substitution is favoured by more nucleophilic alco-
hols.
hol (1 mmol), 2a (0.1 mol%), NaOAc (2.5 mol%), TFE (1 mL), N2, reflux.
On the basis of the observations above, a possible mecha-
nism for the AD reaction is suggested (Scheme 7). Firstly, dis-
placement of a chloride ligand from 2a affords an iridium–alk-
oxide species. This is followed by b-hydrogen elimination to
give an iridium–hydride intermediate,[26] protonation of which
completes the catalytic cycle. The turnover rate is determined
by the protonation step, with the hydride being the catalyst
resting state. All the steps appear to be reversible. An electron-
rich alcohol and the introduction of a catalytic base would
both be expected to shift the equilibrium of the substitution
reaction to the right, which would lead to a higher concentra-
tion of the iridium–alkoxide species and, hence, a faster rate of
the AD reaction.
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Synthesis of Cp*IrIII–NHC complexes
Under an argon atmosphere,
a
mixture of azolium salt
1 (0.5 mmol) and Ag2O (116 mg, 0.5 mmol) was suspended in de-
gassed, dry dichloromethane (5 mL) and stirred at ambient temper-
ature for 1 h shielded from light. [IrCp*Cl2]2 (198 mg, 0.25 mmol)
was added to the suspension and the reaction mixture was stirred
at ambient temperature for an additional 4 h. The resulting sus-
pension was filtered over Celiteꢃ. The remaining solid was washed
with dichloromethane (2ꢄ5 mL) and the filtrate was evaporated.
The residue was purified by column chromatography on silica gel
(9:1 dichloromethane/ethyl acetate) to afford
2 as a yellow
powder.
1
Compound 2a: Yield: 91%, 322 mg. H NMR (400 MHz, CDCl3): d=
7.33 (d, J=8.0 Hz, 4H; ArÀH), 6.87 (d, J=8.0 Hz, 4H; ArÀH), 6.62 (s,
2H; NCH=CHN), 6.03 (d, J=14.4 Hz, 2H; NCH2), 5.10 (d, J=14.4 Hz,
2H; NCH2), 3.80 (s, 6H; OCH3), 1.65 ppm (s, 15H; C5(CH3)5); 13C NMR
(100 MHz, CDCl3): d=159.4 (ArÀC), 156.5 (IrÀC), 130.1 (ArÀC), 128.6
(ArÀC), 121.5 (NCH=CHN), 114.1 (Ar-C), 89.0 (C5(CH3)5), 54.1 (OCH3),
54.1 (NCH2), 9.3 ppm (C5(CH3)5); HRMS (ESI+): m/z calcd for
C29H35ClIrN2O2: 671.2004 [MÀCl]+; found: 671.2005; elemental anal-
ysis calcd (%) for C29H35Cl2IrN2O2: C 49.29, H 4.99, N 3.96; found: C
49.36, H 4.90, N 4.01.
Scheme 7. Suggested mechanism for the AD of secondary alcohols catalysed
by 2a.
Conclusion
1
Compound 2b: Yield: 87%, 281 mg. H NMR (400 MHz, CDCl3): d=
A series of Cp*IrIII–NHC complexes were prepared as catalyst
precursors to promote AD of secondary alcohols in TFE to give
the corresponding carbonyl compounds. Variation of the NHC
ligand framework allowed trends to be established. An elec-
tron-donating para-methoxy group on the N-benzyl group and
imidazole as the NHC skeleton induce higher activity in this
transformation. The most active catalyst 2a was used to con-
vert a variety of primary- and secondary alcohols to aldehydes
and ketones, respectively, accompanied by the release of H2
gas. Mechanistic observations indicated that the rate-limiting
step of the AD reaction is H2 formation. To the best of our
knowledge, the TON achieved is the highest reported for the
acceptorless dehydrogenative oxidation of secondary alcohols
with metal–NHC catalysts. Additionally, the catalyst shows ex-
cellent selectivity for the oxidation of secondary versus primary
benzylic alcohols.
7.36–7.32 (m, 10H; ArÀH), 6.67 (s, 2H; NCH=CHN), 6.01 (d, J=
14.8 Hz, 2H; NCH2), 5.34 (d, J=14.8 Hz, 2H; NCH2), 1.63 ppm (s,
15H; C5(CH3)5); 13C NMR (100 MHz, CDCl3): d=157.5 (IrÀC), 136.8
(ArÀC), 128.8 (ArÀC), 128.4 (ArÀC), 128.0 (ArÀC), 121.9 (NCH=CHN),
89.1 (C5(CH3)5), 54.7 (NCH2), 9.3 ppm (C5(CH3)5); HRMS (ESI+): m/z
calcd for C27H30IrN2: 575.2034 [MÀHÀ2Cl]+; found: 575.2034; ele-
mental analysis calcd (%) for C27H31Cl2IrN2: C 50.15, H 4.83, N 4.33;
found: C 49.99, H 4.87, N 4.31.
1
Compound 2c: Yield: 86%, 335 mg. H NMR (400 MHz, CDCl3): d=
7.62 (d, J=8.0 Hz, 4H; ArÀH), 7.55 (d, J=8.0 Hz, 4H; ArÀH), 6.68 (s,
2H; NCH=CHN), 6.30 (d, J=14.8 Hz, 2H; NCH2), 5.22 (d, J=14.8 Hz,
2H; NCH2), 1.64 ppm (s, 15H; C5(CH3)5); 13C NMR (100 MHz, CDCl3):
d=158.7 (IrÀC), 140.5 (ArÀC), 130.5 (q, J(C,F)=33.0 Hz), 128.9 (ArÀ
C), 125.7 (q, J(C,F)=4.0 Hz), 124.7 (q, J(C,F)=270.0 Hz), 122.2
(NCH=CHN), 89.3 (C5(CH3)5), 54.2 (NCH2), 9.3 ppm (C5(CH3)5);
19F NMR (376 MHz, CDCl3): dÀ62.6 ppm; HRMS (ESI+): m/z calcd for
C29H29ClF6IrN2: 747.1545 [MÀCl]+; found: 747.1535; elemental anal-
ysis calcd (%) for C29H29Cl2F6IrN2: C 44.50, H 3.73, N 3.58; found: C
44.43, H 3.75, N 3.55.
1
Compound 2d: Yield: 90%, 318 mg. H NMR (400 MHz, CDCl3): d=
Experimental Section
7.41 (d, J=8.0 Hz, 4H; ArÀH), 6.85 (d, J=8.0 Hz, 4H; ArÀH), 5.61 (d,
J=14.0 Hz, 2H; NCH2), 4.38 (d, J=14.0 Hz, 2H; NCH2), 3.79 (s, 6H;
OCH3), 3.43 (t, J=9.6 Hz, 2H; NCH2CH2N), 3.28 (t, J=9.6 Hz, 2H;
NCH2CH2N), 1.66 ppm (s, 15H; C5(CH3)5); 13C NMR (100 MHz, CDCl3):
d=186.5 (IrÀC), 159.1 (ArÀC), 129.8 (ArÀC), 128.6 (ArÀC), 113.8 (ArÀ
C), 89.4 (C5(CH3)5), 55.3 (OCH3), 55.1 (NCH2), 48.6 (NCH2CH2N),
9.3 ppm (C5(CH3)5); HRMS (ESI+): m/z calcd for C29H37ClIrN2O2:
673.2166 [MÀCl]+; found: 673.2158; elemental analysis calcd (%)
for C29H37Cl2IrN2O2: C 49.15, H 5.26, N 3.95; found: C 49.13, H 5.31,
N 3.99.
General
Experiments involving air- or moisture-sensitive reagents were per-
formed under an atmosphere of purified N2 by using standard
Schlenck techniques. Unless otherwise specified, all reagents were
obtained commercially and used without further purification. NMR
spectra were recorded with a Bruker 400 MHz NMR spectrometer
and the chemical shifts (d) are reported in parts per million [ppm]
relative to tetramethyl silane (d=0 ppm) or CDCl3 (d=77.0 ppm)
1
1
for the H and 13C NMR spectra, respectively. Melting points were
Compound 2e: Yield: 81%, 305 mg. H NMR (400 MHz, CDCl3): d=
measured with an X-5 Melting Point Apparatus (Beijing Tech Instru-
ment Co.) and are uncorrected. Elemental analysis was carried out
at the Microanalysis Centre, University of Liverpool. Mass spectra
were obtained by Analytical Services at the Chemistry Department,
University of Liverpool and the EPSRC National Mass Spectrometry
Service Centre, College of Medicine, Swansea University. GC analy-
sis was performed with an Agilent 6890N gas chromatograph
equipped with a HP-5 Agilent 19091J-413 column.
7.07 (d, J=8.8 Hz, 4H; ArÀH), 6.99 (t, J=6.0 Hz, 2H; ArÀH), 6.92
(dd, J=6.0, 3.2 Hz, 2H; ArÀH), 6.85 (d, J=8.8 Hz, 4H; ArÀH), 6.18
(d, J=16.4 Hz, 2H; NCH2), 5.90 (d, J=16.4 Hz, 2H; NCH2), 3.79 (s,
6H; OCH3), 1.55 ppm (s, 15H; C5(CH3)5); 13C NMR (100 MHz, CDCl3):
d =172.0 (IrÀC), 158.8 (ArÀC), 135.3 (ArÀC), 128.8 (ArÀC), 127.6
(ArÀC), 122.9 (ArÀC), 114.1 (ArÀC), 112.5 (ArÀC), 90.0 (C5(CH3)5), 55.3
(OCH3), 53.0 (NCH2), 9.2 ppm (C5(CH3)5); HRMS (ESI+): m/z calcd for
C33H35ClIrN2O2: 719.2009 [MÀ2HÀCl]+; found: 719.2005; elemental
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analysis calcd (%) for C33H37Cl2IrN2O2: C 52.37, H 4.93, N 3.70;
found: C 52.29, H 4.88, N 3.72.
sea University. We also thank Dr. R. Sebastian Sprick for per-
forming the H2-evolution experiments and Dr. Konstantin Lu-
zyanin for performing the VT-NMR experiments.
1
Compound 2 f: Yield: 75%, 283 mg. H NMR (400 MHz, CDCl3): d=
6.95 (d, J=8.4 Hz, 4H; ArÀH), 6.91 (d, J=8.4 Hz, 4H; ArÀH), 6.32 (d,
J=16.8 Hz, 2H; NCH2), 5.43 (d, J=16.8 Hz, 2H; NCH2), 3.83 (s, 6H;
OCH3), 1.43 ppm (s, 15H; C5(CH3)5); 13C NMR (100 MHz, CDCl3): d=
161.9 (IrÀC), 158.9 (ArÀC), 128.8 (ArÀC), 126.5 (ArÀC), 118.9 (ArÀC),
114.2 (ArÀC), 89.8 (C5(CH3)5), 55.3 (OCH3), 53.7 (NCH2), 9.1 ppm
(C5(CH3)5); HRMS (ESI+): m/z calcd for C29H35Cl2IrN3O2: 720.1725
[MÀ2HÀ2Cl+NH4]+; found: 720.1716; elemental analysis calcd (%)
for C29H33Cl4IrN2O2: C 44.91, H 4.29, N 3.61; found: C 44.98, H 4.32,
N 3.57.
Keywords: acceptorless dehydrogenation
carbenes · iridium · oxidation
·
alcohols
·
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Synthesis of 2a’
Complex 2a (141 mg, 0.2 mmol) and NaOAc (82 mg, 1.0 mmol)
were suspended in dichloromethane (5 mL) and the mixture was
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ArÀH), 6.87–6.82 (m, 4H; ArÀH and NCH=CHN), 6.59 (d, J=1.6 Hz,
1H; ArÀH), 6.40 (dd, J=8.0, 2.4 Hz, 1H; ArÀH), 5.95 (d, J=14.0 Hz,
1H; NCH2), 4.99 (d, J=14.0 Hz, 1H; NCH2), 4.83 (d, J=14.0 Hz, 1H;
NCH2), 4.62 (d, J=14.0 Hz, 1H; NCH2), 3.80 (s, 3H; OCH3), 3.78 (s,
3H; OCH3), 1.69 ppm (s, 15H; C5(CH3)5); 13C NMR (100 MHz, CDCl3):
d=159.4 (ArÀC), 158.3 (ArÀC), 156.7 (IrÀCcarbene), 146.1 (IrÀCAr), 131.7
(ArÀC), 130.6 (ArÀC), 128.6 (ArÀC), 125.9 (ArÀC), 124.6 (ArÀC), 120.2
(NCH=CHN), 120.2 (NCH=CHN), 114.0 (ArÀC), 108.0 (ArÀC), 90.3
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9.6 ppm (C5(CH3)5); elemental analysis calcd (%) for C29H34ClIrN2O2:
C 51.97, H 5.11, N 4.18; found: C 52.04, H 5.07, N 4.21; HRMS
(ESI+): m/z calcd for C29H33ClIrN2O2: 669.1847 [MÀH]+; found:
669.1827.
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General procedure for the acceptorless dehydrogenative ox-
idation of 3a
Alcohol 3a (1.0 mmol), additive (1.0–10 mol%) and 2a (0.05–
1.0 mol%) were dissolved in TFE (1 mL) in a carousel reaction tube.
The contents of the tube were degassed then reaction mixture
was heated at reflux under N2 for 2–20 h. The reaction mixture was
cooled to RT and the solvent was evaporated under reduced pres-
1
sure. The conversion of 3a was determined by H NMR spectrosco-
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X-ray crystallography
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Crystallographic data and refinement are provided in Tables S1–S7
in the Supporting Information. CCDC 1424271 (<2a), 1470605
(2b),1470606 (2c),1470607 (2d),1470608 (2e),1470609 (2 f) and
1470610 (2a’) 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.
Acknowledgements
The authors thank the Scientific and Technological Research
Council of Turkey (TUBITAK) BIDEB-2219 for postdoctoral fel-
lowships (D.G. and S.G.). Mass spectrometry data were acquired
at the EPSRC UK National Mass Spectrometry Facility at Swan-
Chem. Eur. J. 2016, 22, 1 – 11
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Received: April 8, 2016
Published online on && &&, 0000
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Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
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ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Acceptorless dehydrogenative oxida-
tion of secondary alcohols was achieved
by a Cp*IrIII–N-heterocyclic carbene cata-
lyst with turnover numbers (TONs) up
to 1660 (see figure; Cp*=1,2,3,4,5-pen-
tamethylcyclopentadienyl, PMP=para-
methoxyphenyl).
&
Oxidation Reactions
S. Gꢀlcemal, D. Gꢀlcemal,*
G. F. S. Whitehead, J. Xiao*
&& – &&
Acceptorless Dehydrogenative
Oxidation of Secondary Alcohols
Catalysed by Cp*IrIII–NHC Complexes
Chem. Eur. J. 2016, 22, 1 – 11
www.chemeurj.org
11
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
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These are not the final page numbers! ÞÞ