Iridium phosphine abnormal N-heterocyclic carbene complexes in catalytic hydrogen transfer reactions

Article abstract of DOI:10.1016/j.tetlet.2011.08.058

Several iridium complexes bearing chelating abnormal N-heterocyclic carbenes (NHCs) are shown to be active catalysts for transfer hydrogenation of ketones or enones, dehydrative C-C coupling between primary and secondary alcohols, and dehydrogenation of benzyl alcohol to benzyl benzoate. In the transfer hydrogenation of acetophenone, abnormal NHC complexes give higher activity than a normal analogue. Dehydrative C-C coupling reactions between primary and secondary alcohols result in β-alkylation of the secondary alcohols, using primary alcohols as the apparent alkylating reagents, and such reactions proceed with high yield and selectivity. These catalytic processes are known to involve metal-mediated temporary borrowing of hydrogen from alcohols and subsequent delivery of the hydrogen to CC and /or CO bonds.

Full text of DOI:10.1016/j.tetlet.2011.08.058

Tetrahedron Letters 52 (2011) 5596–5600
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Iridium phosphine abnormal N-heterocyclic carbene complexes
in catalytic hydrogen transfer reactions
Xue Gong a,b, Hong Zhang a, , Xingwei Li b,c,
⇑
⇑
a Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun, Jilin 130024, PR China
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
c Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
Several iridium complexes bearing chelating abnormal N-heterocyclic carbenes (NHCs) are shown to be
active catalysts for transfer hydrogenation of ketones or enones, dehydrative C–C coupling between pri-
mary and secondary alcohols, and dehydrogenation of benzyl alcohol to benzyl benzoate. In the transfer
hydrogenation of acetophenone, abnormal NHC complexes give higher activity than a normal analogue.
Dehydrative C–C coupling reactions between primary and secondary alcohols result in b-alkylation of the
secondary alcohols, using primary alcohols as the apparent alkylating reagents, and such reactions pro-
ceed with high yield and selectivity. These catalytic processes are known to involve metal-mediated tem-
porary borrowing of hydrogen from alcohols and subsequent delivery of the hydrogen to C@C and /or
C@O bonds.
Received 24 May 2011
Revised 8 August 2011
Accepted 9 August 2011
Available online 16 August 2011
Keywords:
Abnormal N-heterocylic carbene
Cyclometalation
Iiridium hydride
Hydrogen-borrowing
Dehydrogenation
Ó 2011 Elsevier Ltd. All rights reserved.
In the past decade neutral carbon-centred ligands, especially N-
heterocylic carbenes (NHCs), have found wide applications in orga-
nometallic chemistry and are actively competing with phosphine
ligands.1–3 As common and representative carbon-centred ligands,
controllable preparation of abnormal carbenes and their com-
plexes should offer new opportunities in catalysis, by allowing ac-
cess to a plethora of new highly active metal complexes.13 Both
experimental and theoretical data suggested that abnormal NHCs
are even stronger electron-donor ligands than the normal ones.14,15
NHCs (typically imidazole-2-ylidenes) hold strong
r-donating but
weak -accepting characters, and they become highly popular in
p
Our recent work demonstrated that besides being strong
r-donors,
catalysis owing to their ability to stabilize transition metals in both
high and low oxidation states and their labilizing effects to facili-
tate the rate-limiting steps.4 NHC complexes have shown advanta-
ges in homogeneous catalysis, such as C–C coupling,5 olefin
metathesis reactions,6 hydrogenation,7 and hydroamination
reactions.8
Imidazole-based ‘abnormal’ NHCs, first discovered by Crabtree
and co-workers in the cycolmetalation of iridium(III) complexes,9
are zwitterionic ligands obtained from the metalation of imidazo-
liums at the C4/5 position (Scheme 1). This relatively less common
binding mode has been further extended to the complexes of other
transition metals in both monodentate and chelation settings.10,11
Furthermore, other non-conventional N-heterocyclic carbenes that
are derived from pyridines, quinolines and pyrazolines have also
been reported and reviewed.10 Significantly, Bertrand and co-
workers have described a strategy of isolating the first free abnor-
mal carbene that is stable at room temperature.12 Thus facile and
abnormal NHCs can be strong -acceptors, and the electronic ef-
p
fects of annulated abnormal NHCs can be even readily tuned by re-
mote modification or by annulation.15 This unique electronic effect
could be partially responsible for the higher catalytic activity of
abnormal NHC complexes of Pd, Rh, and Ru than the corresponding
normal NHC analogues in catalytic transformations involving C–C
coupling16 and hydrogen transfer reactions via metal hydride
intermediates (hydrogen-borrowing processes).17,18
To date most literature reports of abnormal NHC complexes
have focused on their synthetic aspects, structures, and electronic
effects. Catalytic applications of these complexes are still lim-
ited.10,16–18 We now report that iridium abnormal NHC complexes
can catalyse a series of reactions that feature hydrogen transfer,
including transfer hydrogenation of ketones, dehydrative C–C cou-
pling between primary and secondary alcohols, and dehydrogena-
tion of benzyl alcohol.
We19 and others20 recently demonstrated the synthesis of a ser-
ies of iridium(III) abnormal NHC hydride complexes (1 and 2) via
highly selective oxidative addition of the C4/5-H bonds of phos-
phine-tethered imidazolium ions, which can be further converted
to the corresponding Ir(I) complex (3) by base-promoted reductive
⇑
Corresponding authors. Tel.: +86 431 85099372; fax: +86 431 85684009 (H.Z.);
tel.: +86 411 84379089; fax: +86 411 84379737 (X.L.).
E-mail addresses: zhangh@nenu.edu.cn (H. Zhang), xwli@dicp.ac.cn (X. Li).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.058

X. Gong et al. / Tetrahedron Letters 52 (2011) 5596–5600
5597
base
base
N
N
N
N
N
N
N
N
R
R
R
R
R
R
R
R
-HX
-HX
X-
normal NHC
abnormal NHC
Scheme 1. Imidazole-based normal and abnormal NHCs.
elimination (Scheme 2). In comparison, normal NHC complex
8a,19a was obtainable from the reaction of the phosphine-tethered
imidazolium ion and [Ir(COD)( -OtBu)]2 (Scheme 2).
Table 1
Screening of iridium catalystsa
4
l
O
OH
Iridium complexes of normal imidazole21 and pyridine-based
NHCs22 in chelation settings are known to be active catalysts for
transfer hydrogenation reactions. It is important to compare the
catalytic activity of iridium normal and abnormal carbene com-
plexes. The reduction of acetophenone by iPrOH was used to screen
the catalytic activity of complexes 1–4 in 0.1 mol % loading in the
presence of a catalytic amount of KOH (Table 1). In all cases, the
transfer hydrogenation proceeded smoothly and similar activities
were obtained for complexes 1, 2, and 3. The fact that complexes
1 and 3 gave essentially the same result likely suggests that the Ir(-
III) hydride complex enters the catalytic cycle in the form of Ir(I)
abnormal NHC since base-promoted reductive elimination of HCl
is instantaneous and quantitative (Scheme 2). The steric bulk of
the N-alkyl group has no significant effect, while the coordination
mode of the carbene does in that normal carbene complex 4 gives
only 26% yield in 5 h and 28% yield even after 12 h (Table 1, entries
4 and 5). We noted that higher catalytic activity of abnormal NHC
complexes than the normal counterparts has been reported in
some systems.16–18
0.1 mol% [Ir]
5 mol% KOH
iPrOH, reflux
Entry
Catalyst
Time (h)
Yieldb (%)
1
2
3
4
5
1
2
3
4
4
5
5
5
5
92
95(90c)
94
26
28
12
a
Reaction condition: acetophenone (1.0 mmol), KOH (0.05 mmol), and catalyst
(0.001 mmol), iPrOH (3 mL), under nitrogen.
b
NMR yield using 1,3,5-trimethoxybenzene as an internal standard.
Isolated yield.
c
transfer hydrogenation here can follow either stepwise sequence.
For cinnamaldehyde, only 61% 3-phenylpropan-1-ol was isolated
although only one product was observed from crude 1H NMR (Ta-
ble 3, entry 14).
Under the same conditions, other methyl aryl ketones contain-
ing electron-withdrawing or electron-donating groups could also
be reduced to the corresponding alcohols in high isolated yield
(79–95%) using complex 2 as a catalyst (Table 2, entries 1–10).
We further focused on the b-alkylation of secondary alcohols
using primary alcohols (Table 3).24 This catalytic reaction repre-
sents an environmentally friendly process for the synthesis of alco-
hols, and Ru18,25,26 and Ir23a complexes have been reported for this
transformation. This reaction is believed to involve transfer of
hydrogen from both alcohols to the catalyst to give metal hydride
species, a ketone, and an aldehyde. The latter two undergo aldol
condensation, yielding an enone. The metal hydride intermediates
further deliver hydrogen to the C@C and C@O bonds of the enone
intermediate to eventually give the alcohol product.23a Since our
iridium abnormal NHC complexes have been proved to be highly
The scope of the ketone substrate was further extended to
a,b-
unsaturated ketones and aldehydes, where chemoselectivity can
be an issue.23 Both C@C and C@O double bonds of enones are re-
duced with high yield and selectivity (Table 2, entries 13–18). It
is obviously possible that transfer hydrogenation of enones could
occur first to C@C then to C@O group. To further test whether
the other stepwise sequence is possible, we then examined trans-
PhCH@CHC(OH)Me, which can be cleanly hydrogenated under
the same conditions (Table 2, entry 12). These results suggest that
active catalyst for transfer hydrogenation of
a
,b-unsaturated
H
R
N
Ph
P
Ph
P
Cl
2
2
Ph
P
Cs CO
0.5 eq [Ir(COD)Cl]
CH Cl , rt
2
2
3
2
Ir
-
Ir
Cl
Ir
N
R
MeCN, -HCl
N
-
N
2
2
N
PF
6
N
-
PF
6
N
N
-
PF
PF
6
6
R
PPh
R
2
1:
3:
R = Me
R = Me
2: R = iPr
abnormal NHC Ir complex
PPh2
PF6
N
N
Ph2
tBuOK
-
P
0.5 [Ir(COD)OtBu]2
Ir
0.5 [Ir(COD)Cl]2
N
THF, -HOtBu
THF, - KCl
-
N
PF6
4
normal NHC Ir complex
Scheme 2. Synthesis of iridium abnormal and normal N-heterocyclic carbenes.19

5598
X. Gong et al. / Tetrahedron Letters 52 (2011) 5596–5600
Table 2
a
Transfer hydrogenation of acetophenones and enones
O
OH
0.1- 0.5 mol% 2
R
R
5 mol% KOH
iPrOH, reflux
Entry
Substrates
Time
Product
Yieldb
O
OH
Ar
Ar
1
2
3
4
5
6
7
8
9
Ar = Ph
5
10
6
4
4
3
4
3
3
90
78
91
95
88
97
94
92
90
4-MeOC6H4
4-MeC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
3-BrC6H4
2-Napthyl
1-Napthyl
O
10
6
79
O
OH
11
12
13
14
15
6
4
4
6
4
81
87
98
61
97
Ph
Ph
Ph
Ph
Ph
OH
OH
Ph
Ph
Ph
Ph
O
O
OH
Ph
Ph
OH
H
O
O
OH
Ph
Ph
Ph
O
OH
Ph
16
4
88
Ph
MeO
MeO
O
OH
Ph
17
18
2
2
94
93
Ph
Cl
Cl
OH
Ph
Ph
Ph
Ph
a
Reaction conditions: 1 mmol of ketone, KOH (0.05 mmol), 0.1 mol % of catalyst 2 for entries 1–10 and 0.5 mol % of 2 for entries 11–19, iPrOH (3 mL).
Isolated yield.
b
ketone, we reason that they should catalyse this C–C coupling reac-
tion via this ‘hydrogen-borrowing’ mechanism.
acceptor. In this case, appropriate reaction time seems a key factor
in controlling the selectivity. Furthermore, normal NHC Ir complex
4 showed less activity (88% yield) and selectivity (the ratio of 5 and
6 is 91: 9, entry 4) in the transformation. The coupling reactions
between various secondary and primary alcohols are summarized
(Table 3, entries 5–15). The coupling between simple benzyl alco-
hol and different 1-arylethanols bearing either electron-withdraw-
ing or electron-donating groups gave the corresponding products
in high yields. The reaction of 1-phenylethanol and primary ali-
phatic alcohols, such as 1-butanol or 2-phenylethanol, could also
give the b-alkylated products. For most of the reactions, this pro-
cess is very selective in the production of the alkylated alcohols,
The benzylation of 1-phenylethanol and benzyl alcohol was car-
ried out in equimolar ratio. Complex 2 (1 mol %) was used as a cat-
alyst in the presence of KOH (1 equiv) in toluene (110 °C, 2 h),
leading to excellent activity and high selectivity in that the alcohol
product was isolated in 92% yield without any ketone by-product
(Table 3, entry 1). The more dosage of base (2 equiv KOH) has no
significant effect on selectivity (entry 2), while prolonging reaction
time (10 h) led to a small amount of ketone (entry 3), which indi-
cates that the alcohol product may be further dehydrogenated un-
der the catalytic conditions even without any sacrificial hydrogen

X. Gong et al. / Tetrahedron Letters 52 (2011) 5596–5600
5599
Table 3
b-Alkylation of secondary alcohols using primary alcohols catalyzed by 2a
O
OH
OH
[Ir] cat, KOH
toluene, 110 oC
R2
OH
R1
R2
R1
R2
R1
6
5
b
Entry
R1
R2
Time (h)
5:6
5c (%)
1
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
PhCH2
n-Hexyl
2
2
10
2
1.5
2
2
1
1.5
2
2
100:0
100:0
93:7
91:9
87:13
92:8
94:6
96:4
98:2
93:7
81:19
95:5
90:10
88:12
85:15
92
94d
91d
88d
61
73
90
81
90
84
74
84
83
68
61
2e
3
4f
5
6
7
8
4-MeOC6H4
4-MeC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
4-CF3C6H4
3-BrC6H4
2-Naphthyl
1-Naphthyl
Ph
9
10
11
12
13
14
15
1
1
1
2
Ph
a
b
c
d
e
f
Reaction conditions: primary alcohols (2 mmol), secondary alcohols (2 mmol), KOH (2 mmol), catalyst 2 (0.02 mmol, 1 mol %) and toluene (1 mL), under nitrogen, 110 °C.
Determined by 1H NMR based on crude product.
Isolated yield.
NMR yield using 1,3,5-trimethoxybenzene as a standard.
Two equivalents of KOH based on alcohol was used.
Normal NHC complex 4 (1 mol %) as catalyst.
although in some cases a small amount of the corresponding ke-
tones were obtained as by-products.
cesses are known to involve metal-mediated temporary
borrowing of hydrogen from alcohols and subsequent delivery of
the hydrogen to C@C and /or C@O bonds. For the acceptorless
dehydrogenation of benzyl alcohol to benzyl benzoate, moderate
yield and selectivity were obtained. Our results have confirmed
that abnormal NHC complexes can be excellent choices in the de-
sign of active catalysts. Preparation of other non-conventional NHC
complexes is in progress in our laboratory.
The presence of a small amount of ketone byproduct in the reac-
tions given in Table 3 indicates that 2 might be applied as a dehy-
drogenation catalyst. Catalytic acceptorless dehydrogenation of
alcohols has been reported,24b,27,28 and primary alcohols can be
dehydrogenated to esters with the release of two molecular hydro-
gens (Eq. 1).27 We thus applied 2 as a catalyst for the dehydrogena-
tion of benzyl alcohol. In all our experiments, essentially no
benzaldehyde was obtained on the basis of 1H NMR analysis. Benzyl
benzoate was obtained as the major product together with dibenzyl
ether. The formation of the latter is probably due to the Lewis acidic
nature of complex 2. Attempts to inhibit the formation of the diben-
zyl ethyl by addition of water (1 equiv) failed to give any improved
yield of benzyl benzoate. The best results were obtained using
Cs2CO3 as a base and 2 (2 mol %) as a catalyst (toluene, reflux,
48 h), where the conversion is 87%, and the yield of benzyl benzoate
and dibenzyl ether is 52% and 27%, respectively. Further improve-
ment is necessary for this reaction, and neutral Ir(I) or Ir(III) cata-
lysts with low Lewis acidity but high thermal stability are desirable.
Acknowledgements
We gratefully acknowledge financial support by the NSF of Chi-
na (21071027, 20771023), the China High-Tech Development 863
Program (2007AA03Z218) and analysis and testing foundation of
Northeast Normal University. We thank the Dalian Institute of
Chemical Physics, CAS, and Nanyang Technological University for
the financial support. We also thank Dr. Guoyong Song for his gen-
erous help.
Supplementary data
2
2 mol%
Cs2CO3
O
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.08.058.
2H2
Ph
OH
ð1Þ
Ph
OCH2Ph
52%
In summary, several iridium complexes bearing chelating
PhMe, reflux,12h
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