Acceptorless dehydrogenation and dehydrogenative coupling of alcohols catalysed by protic NHC ruthenium complexes

Article abstract of DOI:10.1039/c7ob00542c

A new family of protic NHC Ru complexes ligated with a phosphine-tethered imidazole moiety were prepared, which can act as excellent catalysts for acceptorless dehydrogenation of secondary alcohols and dehydrogenative coupling of primary and secondary alcohols, thus leading to the formation of a variety of carbonyl compounds with release of H₂.

Full text of DOI:10.1039/c7ob00542c

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Acceptorless Dehydrogenation and Dehydrogenative Coupling of
Alcohols Catalysed by Protic NHC Ruthenium Complexes
Received 00th January 20xx,
Accepted 00th January 20xx
a
a,b
a
a
a*
Weihong Chang, Xue Gong, Shuizhong Wang , Ling-Ping Xiao and Guoyong Song
DOI: 10.1039/x0xx00000x
A new family of protic NHC Ru complexes ligated with a phosphine-tethered imidazole moiety were prepared, which can
act as excellent catalysts for acceptorless dehydrogenation of secondary alcohols and dehydrogenative coupling of primary
www.rsc.org/
and secondary alcohols, thus leading to formation of
2
a variety of carbonyl compounds with releasing H .
1
0
catalysis due to their strong donating property. NHC
complexes bearing an NH wingtip (protic NHC) have received
increasing interest, because the bifunctional cooperation of
the metal center and proton-responsive unit in the complex
could facilitate substrate recognition, activation and
Introduction
Catalytic acceptorless alcohol dehydrogenation (AAD) is an
oxidant-free, atom-economical methodology to prepare
1
carbonyl compounds, and this reaction is considered as a key
9
a,10d
transformation.
However, catalytic applications are
step in dehydrogenative coupling reaction of primary and
2
secondary alcohols leading to alkylated ketones or alcohols.
limited of such complexes due to their more difficult synthesis,
since the reaction of an imidazole or pyridine unit with a metal
tends to give an N-coordinated tautomer rather than a protic
The AAD reaction is also relevant to the field of energy
chemistry, as it provides a unique opportunity to produce H
2
1
1
3
from sustainable biomass-derived alcohols. Therefore, various
NHC complex. Bergman and Ellman indicated that the protic
1
NHC Rh species is a key intermediate in coupling reactions.
2
4
transition metal complexes, such as iridium, ruthenium,
5
5
c
6
7
8
Kuwata and Ikariya reported that protic NHC Ru complex
ligated with N-(2-pyridyl-)benzimidazole could catalyse
dehydrative coupling of an N-heterocycle with an allyl
osmium, cobalt, nickel, and iron have been reported to
catalyse the AAD reactions. Among of which, the metal-ligated
systems showed excellent catalytic activity toward AAD-type
reactions, where an acidic group such as OH or NH can act as
an internal Brønsted acid-base catalytic group to promote
reaction without an overall change of the metal’s oxidation
1
3
alcohol. Grotjahn et. al. described that protic NHC Ru and Ir
complexes could serve as efficient catalysts for transfer
1
4a
hydrogenation of acetophenone
and α,β-unsaturated
1
4b
9
ketones. We have reported a series of protic 2-pyridylidene
NHC Ir(I) complexes, which upon anion exchange can undergo
1,3-shift of the NH proton to give Ir(III) hydrides through a
state. Fujita and Yamaguchi reported the Cp*Ir complex
having a 2-hydroxypyridinate (2-hp) ligand could be employed
a,4c
for dehydrogenation of alcohols efficiently,
4
and Rauchfuss
1
5
water-assisted proton-relay mechanism. Recently, Hahn and
co-workers used phosphine-tethered benzimidazole as
later showed that 2-hp ligand is critical for achieving
4
f
turnover. Jones developed a nickel complex ligated with 2-
hydroxyquinoline for acceptorless dehydrogenation of
bidentate ligands to react with [Cp*RuCl]
resulting protic NHC complexes through tautomerization,
and the Ir complex was able to activate molecular hydrogen
4 2 2
and [Cp*IrCl ] ,
1
6
7
a
alcohols, affording ketones, esters and lactones. Additional,
2
g,2h
2i
and Ru complexes having 2-hydroxyl-substituted
the Ir
1
6b
under mild conditions. We envisioned that such protic NHC
complexes might serve as unique catalysts for AAD-type
reactions. Herein, we report the synthesis and reactivity of a
new family of bidentate protic NHC Ru complexes, which can
efficiently catalyse the acceptorless dehydrogenation of
secondary alcohols and dehydrogenative coupling of
secondary and primary alcohols.
pyridine unit were also found to be efficient catalysts for
β
-
alkylation of secondary alcohols with primary alcohols. Given
that the importance of AAD-type reaction, the search new
metal-ligated systems enable the advancement this
transformation would be valuable.
N-heterocylic carbenes (NHCs) are powerful ligands to
stabilize transition metals in organometallic chemistry and in
Results and discussion
i
The P( Pr) -tethered benzimidazole
1 was synthesized from the
2
reaction of 1-(2-chloroethyl)-5,6-dimethyl-benzoimidazole
i
with LiP( Pr) . Reaction of compound
1 with [Cp*RuCl] in THF
4
2
o
at 100 C led to tautomerization of imidazole unit, giving a P,C-
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Scheme 1. Synthesis and reactivity of NH protic NHC Ru complexes
chelating protic NHC complex
2
in 84% yield. The presence of a
1
low field H NMR (C D ) signal (δ = 10.33 ppm, NH), together
6
6
1
with the appearance of a doublet signal in the C NMR
3
spectrum (δ = 209.5 ppm, JP-C = 27.2 Hz, Ru-C1) are consistent
3-16
with typical values reported for protic NHC complexes.
1
X- Figure 1. ORTEP of 2 (upper, CCDC: 1529097) and 3 (lower, CCDC: 1529098) with
thermal ellipsoids at the 30% probability level. Hydrogen atoms have been omitted for
ray crystallographic analysis of 2 revealed that the Ru atom is
clarity. Selected distances [Å] and angles [°] for 2: Ru-C1 1.984(2); Ru-Cl1 2.4637(6); Ru-
P1 2.3016(6); P1-Ru-C1 84.02(6); C1-Ru-Cl1 87.88(6). For 3: Ru-C1 2.030(4); Ru-P1
bonded to one Cp* ring, one phosphine-imidazole moiety
acting as a bidentate ligand and one chloride. The Ru-C1
2
distance (1.984(2)) is within the normal range expected for 90.0(1).
.255(1); Ru-H 1.52(4); Na-H 2.21(4); Na-N2 2.362(5); P1-Ru-C1 84.6(1); H-Ru-C1
1
3,14a
NHC complexes.
Treatment of
0 C gave a red solution, from which an analytically pure NHC
Ru complex bearing N-Na wingtip was isolated (82%) and
2 with excess NaH in THF at
o
essential in present reaction. When protic hydride complex
4
8
was used instead of 2, a similar catalytic effect was observed
3
1
under base-free condition, suggesting that
in the catalytic cycle (entry 7).
4 may be involved
spectroscopically characterized. The H NMR (THF-d ) shows
8
absence of an NH signal and the presence of a hydride signal at
1
3
-
14.70 ppm as a doublet (JP-H = 35.6 Hz). The C NMR
spectrum of exhibits carbene carbon resonance at 201.5
ppm. Single crystals of suitable for X-ray diffraction studies
were obtained by recrystallization from a THF/benzene
2 3
The 2/Cs CO combination was then chosen as a catalyst to
examine acceptorless dehydrogenation reaction of various
alcohols. Some representative results are summarized in Table
3
1
7
3
3
2. Secondary benzylic alcohols having Me, OMe, SMe, CF , Ph
o
and halide substitutes at the meta or para position reacted
solution of
and Na respectively in
Ru-P (2.255(1)) in are slighter longer than those of
3
at -30 C. A bridging hydride is bonded with Ru
. The distances of Ru-C1 (2.030(4)) and
. The
with PrOH gave a protic Ru hydride complex
smoothly to give corresponding acetophenone derivatives in
3
3
i
2
Table 1. Acceptorless dehydrogenation of 1-phenylethanol by ruthenium
a
complexes
reaction of
3
4,
which shows an NH signal at 8.42 ppm as a singlet and a
hydride signal at -13.15 ppm as a doublet with a coupling
1
constant as 35.6 Hz in the H NMR spectrum. In the presence
of NaH,
4 could be transferred into 3 in the THF through an
acid-base reaction.
b
Entry
Cat.
mol %)
2 (5)
2 (1)
2 (5)
2 (5)
2 (5)
2 (5)
4 (1)
Solvent
Base
Time
(h)
24
Yield (%)
To probe the potential of these ruthenium complexes as a
catalyst for AAD reactions, we first examined the
(
(mol%)
1
2
3
4
5
6
7
toluene
xylene
xylene
xylene
xylene
xylene
xylene
Cs
Cs
2
CO
3
3
(5)
(1)
85
dehydrogenation reaction of 1-phenylethanol (5a). In the
o
presence of Cs CO (5 mol%) in toluene at 110 C, complex
c
2
CO
t
NaO Bu(5)
48
93(88 )
2
(5
2
3
48
80
72
79
0
1
8
mol%) afforded acetophenone in 85% yield (Table 1, entry 1).
NaOAc (5)
48
t
Changing the solvent from toluene to xylene, the catalyst
o
amount could be decreased to 1 mol% at 140 C, under which
KO Bu(5)
48
48
acetophenone was isolated in 88% yield in 48 h (entry 2). The
activity of this reaction are influenced by using different bases,
and further screening suggested Cs CO is the best choice
48
91
a
Reaction conditions: 1-phenylethanol (60 mg, 0.49 mmol), catalyst 2 or 4 (1
o
2
3
or 5 mol%), Cs CO (1 or 5 mol%) in toluene (110 C, 1 mL) or in xylene (140
2
3
o
b
c
(entries 3-5). Neither complex
2
nor Cs CO alone afforded
2
C, 1 mL) in a sealed tube. GC yields. isolated yield.
3
acetophenone, which shows that the activation of complex
2
is
2
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The 4/Cs CO also showed an analogous activity toward this
2 3
reaction, while no coupling product 8a was observed in the
absence of Cs CO (see SI),
2
3
Table 3. Dehydrogenative coupling of primary and secondary alcohols catalyzed
a
by complex
2
Entry
Substrate
Time (h)
Yield (%)
Entry
R
1
R
2
Product
8a
8b
8c
Yield (%)
85
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1
2
3
4
5
6
7
R = 4-Me
24
24
24
48
48
48
36
48
24
48
24
24
24
48
48
48
48
48
92
96
93
72
90
85
78
70
94
61
93
90
98
82
84
53
18
20
4-ClC
6
H
4
71
R = 4-OMe
R = 4-SMe
4-OMeC
6
H
4
95
4-MeC
2-MeC
6
6
H
4
4
8d
8e
8f
75
R = 4-CF
3
H
74
R = 4-Ph
R = 4-Cl
4-PhC
3,5-(OCH
2-furyl
6
H
4
58
2
O)C
6
H
4
8g
8h
8i
58
R = 3-Cl
94
b
8
R = 3,4-DiCl
2-thiophene
75
9
R = 3,5-(OCH
2
O)
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
2-naphthyl
8j
86
b
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
R = 2,4-DiMe
4-MeC
4-ClC
4-OMeC
4-SMeC
2,4-DiMe
3,5-(OCH
6
H
4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
8k
8l
76
1-(2-naphthyl)ethanol
1-tetralol
6
H
4
68
6
H
4
8m
8n
8o
8p
8q
8r
90
1-indanol
6
H
4
92
1-phenyl-1-butanol
cycloheptanol
2-octanol
58
b
b
2
O)C
6
H
4
87
2-naphthyl
93
benzyl alcohol
piperonol
4-PhC
6
H
4
Ph
66
b
n
C
b
Ph
7
H
15
8s
20
n
C
b
a
Reaction conditions: 5 (1 mmol), catalyst 2 (1 mol%), Cs
2
CO
3
(1 mol%),
20
6
H
13
Ph
8t
45
o
b
xylene (1 mL), 140 C in a sealed tube, isolated yields. 2 mol% of 2/Cs
2 3
CO
a
2 3
Reaction conditions: 5 (0.5 mmol), 7 (0.5 mmol), complex 2 (2 mol%), Cs CO (2
were used.
o
b
mol%), xylene (1 mL), 150 C in a sealed tube, isolated yields. GC yields.
a
Table 2. Ru-Catalyzed acceptorless dehydrogenation of secondary alcohols
probably because base is necessary for the aldol condensation
process.
2
a
Under the optimized conditions, various
good yields (70-96%, Table 2, entries 1-9). 2-Methyl-
substituted substrate showed a lower activity probably owning
to steric hindrance (entry 10). 1-(2-naphthyl)ethanol, 1-tetralol
and 1-indanol were also dehydrogenated to corresponding
ketone products (entries 11-13). The dehydrogenation
reaction was also observed in the case of 1-phenyl-1-butanol,
dehydrogenative coupling reactions between primary and
secondary alcohols were then examined and the results are
summarized in Table 3. The reactions of 1-phenylethanol with
benzyl alcohols having electron-rich or electron-deficient
substitutes gave the corresponding alkylated carbonyl
compounds 8b-
8g in 58−95% yields in a chemoselective
which gave 1-phenylbutanone in 82% yield (entry 14). In fashion (entries 2-7). 2-furanmethanol, 2-thiophenemethanol
addition to aromatic substrates, the aliphatic secondary and 2-naphthylmethanol are also suitable substrates, giving
alcohols, such as cycloheptanol and 2-octanol were also alkylation products 8h
,
8i and 8j in 75-94% yields (entries 8-
0). The coupling reactions of 4-methyl-, 4-chloro-, 4-
1
applicable in this catalytic conditions, affording corresponding
ketone products (entries 15, 16). When primary alcohols, such
as benzyl alcohol and piperonol were used as substrates, a
significant drop of activity was observed, which gave
corresponding aldehydes in low yields (entries 17, 18).
methoxyl-, 4-methylthio- and 4-phenyl-substituted aromatic
secondary alcohols with benzyl alcohol could also be achieved
analogously in high yields (entries 12-18). In the case of
aliphatic alcohol substrates, an obvious drop in activity was
observed as seen in the case of the reaction of 1-octanol with
The dehydrogenative coupling of primary and secondary
alcohols is a direct and atom-economic method for the
preparation of alkylated ketone derivatives, proceeding via
dehydrogenation of alcohols, aldol condensation and
hydrogenation. On the basis of above results, we reason that
protic NHC ruthenium complex can serve as an efficient
1
-phenylethanol (20 % yield, 48 h, entry 19) and the reaction
of benzyl alcohol with 2-octanol (45% yield, 48 h, entry 20).
/Cs CO (1 mol%) was tested as a catalyst in the coupling
reaction of acetophenone with benzyl alcohol, which resulted
in 93% yield (eq 1). The catalytic hydrogenation of (E)-
2
2
3
2
8
a
chalcone in the presence of H (1 atm) was selective for the
2
catalyst for this transformation. The
2 3
2/Cs CO (2 mol%)
double bond to give 8a in 96% yield in 3 h (eq 2). No saturated
alcohol product was observed after 12 h under current
reaction conditions. The transfer hydrogenation reaction of
(E)-chalcone carried out in PrOH gave a mixture of 1,3-
diphenylpropanol and 8a with a ratio of 1.7:1 (eq 2). The fast
combination was then chosen as a catalyst to examine the
coupling reaction of 1-phenylethanol and benzyl alcohol,
which gave an alkylated product 3-phenylpropiophenone 8a
i
o
selectively in 85% yield in xylene at 150 C (Table 3, entry 1).
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i
selective hydrogenation could be favored by the ability of the the Li Pr P solution was added slowly, and the resulted mixture
2
protic NHC complex to participate in hydrogen bonding with
alkenone as a substrate.
was stirred at room temperature for 24 h. After the reaction
was quenched with methanol (5 mL), all volatile was removed
under vacuum. The residue was then dissolved in
dichloromethane (15 mL) and was filtered to remove inorganic
salts. All the solvent was then removed under vacuum to
O
O
OH
2/Cs2CO3 (1 mol%)
+
(1)
xylene, 150 oC, 24 h
5a
7a
8a, 93%
afford compound
1
as a white solid after washing with hexane.
H NMR (400 MHz, CDCl ): 7.82 (s, 1H), 7.56 (s, 1H), 7.13 (s,
H), 4.27 – 4.20 (m, 2H), 2.40 (s, 3H), 2.37 (s, 3H), 1.89 – 1.84
1
3
O
1
1
3
H2 (1 atm), 3 h
(
m, 2H), 1.82 – 1.75 (m, 2H), 1.12 – 1.03 (m, 12H); C NMR
100 MHz, CDCl ): δ 142.7, 141.8 (2C), 132.0, 130.9, 120.5,
09.7, 44.7 (d, J = 32.9 Hz), 23.3 (d, J = 11 Hz), 22.8 (d, J = 20.9
xylene, 150 o
C
O
(
3
8
a, 96%
2
/Cs2CO3 (5 mol%)
(2)
1
OH
31
Hz), 20.7, 20.3, 19.9 (d, J = 16 Hz), 18.6 (d, J = 9 Hz); P NMR
121 MHz, CDCl ): δ 0.43. Anal. Calcd for C17 P (290.4): C,
0.31; H, 9.37; N, 9.65; Found: C, 69.90; H, 9.84; N, 9.32.
Preparation of complex 2: In a glovebox, compound (290 mg,
.0 mmol), [Cp*RuCl (271 mg, 0.25 mmol) and THF (5 mL)
were charged into a sealed tube and the mixture was stirred at
iPrOH, 100 o
2 h
C
(
3
27 2
H N
+
8a
1
7
63%
37%
1
1
2 4
]
Conclusions
o
1
00 C for 12 h. The solution was then concentrated to ca. 0.5
In summary, we described that ruthenium-mediated
tautomerization of imidazole unit can be achieved, thus giving
mL followed by washed with diethyl ether to afford complex
as an orange solid (471 mg, 84% yield). Single crystals of
2
2
a protic NHC Ru complex
bearing N-Na wingtip was isolated from the reaction of
complex with NaH, which can be transferred into a protic
hydride complex
2. A rare NHC Ru hydride complex 3
suitable for X-ray analysis were obtained by slow diffusion of
o
1
diethyl ether to a solution in CH
MHz, C ): δ 10.33 (br, 1H), 6.65 (s, 1H), 6.25 (s, 1H), 3.97 –
.86 (m, 1H), 3.25 – 3.16 (m, 1H), 2.10 (s, 3H), 2.06 – 1.91 (m,
H), 1.66 (s, 15H), 1.42 (dd, J = 15.2, 7.6 Hz, 3H), 1.25 – 1.23
2 2
Cl at -30 C. H NMR (400
2
6 6
D
i
4
by the reaction with PrOH. These
3
5
complexes represented good catalytic abilities in acceptorless
dehydrogenation of secondary alcohols and dehydrogenative
coupling of primary and secondary alcohols, leading to
formation of variety of carbonyl compounds in good yields.
Explorations of the potential use of protic NHC complexes for
other transformations and detailed mechanism studies are in
progress.
(
m, 2H), 1.06 – 1.03 (m, 3H), 0.90 (dd, J = 14.0, 6.8 Hz, 3H),
13
0
2
1
2
1
6 6
.44 (dd, J = 12.4, 7.2 Hz, 3H); C NMR (100 MHz, C D ): δ
09.5 (d, JP-C = 27.2 Hz), 134.1, 134.0, 129.7, 129.1, 110.0,
08.1, 87.4, 42.8, 30.3 (d, JP-C = 16.2 Hz), 28.4 (d, JP-C = 17.1 Hz),
1.8 (d, JP-C = 23.2 Hz), 20.2, 20.1(d, JP-C = 7 Hz) 19.8, 19.1, 18.3,
31
7.7 (d, JP-C = 3.7 Hz), 10.8; P NMR (125 MHz, C D ): δ 54.7.
6 6
Anal. Calcd for [C27
.98; Found: C, 57.97; H, 7.84; N, 4.72.
Preparation of complex 3: Complex
NaH (24 mg, 1 mmol) and THF (4 mL) were charged into a seal
2
H42ClN PRu] (562.1): C, 57.69; H, 7.53; N,
4
Experimental
2
(100 mg, 0.178 mmol),
General procedure
o
All reactions dealing with air- or moisture-sensitive compounds tube, which was heated at 80 C for 6 h. After reaction, the
were performed by standard Schlenk techniques or in the inorganic salts can be removed by carefully filtration. The pure
nitrogen-filled Vigor glove box. Analytical thin-layer product and single crystals of
3 suitable for X-ray analysis were
chromatography (TLC) was performed on HSGF254 silica gel obtained as a red solid (80 mg, 82% yield) by recrystallization
o
plates. THF, xylene, toluene, diethyl ether and hexane were of the THF/benzene solution at -30 C. H NMR (400 MHz, THF-
1
distilled from sodium and stored over fresh Na chips in a d8): δ 6.74 (s, 1H), 6.58 (s, 1H), 4.25 – 4.13 (m, 1H), 3.82 – 3.72
1 13
glovebox. H and C nuclear magnetic resonance (NMR) (m, 1H), 2.21 (s, 3H), 2.18(s, 3H), 2.03 – 1.98 (m, 2H), 1.82 (s,
spectra of organic products were recorded on a Bruker AVIII 15H), 1.32 – 1.27 (m, 3H), 1.22 – 1.77 (m, 5H), 0.77 (dd, J = 6.4,
400 MHz spectrometer instrument. Data are reported as 14 Hz, 3H), 0.30 (dd, J = 6.8, 12.4Hz, 3H), -14.67 (d, J = 35.6 Hz,
1
follows: chemical shift in ppm (δ), multiplicity (s = singlet, d = 1H). C NMR (100 MHz, THF-d ): δ 201.5 (apparent dd, J =
3
8
doublet, t = triplet, q = quartet, m = multiplet, br = broad 22.1, 7.3 Hz), 146.7, 136.8, 123.5, 121.8, 112.4, 105.3, 86.5 (d,
signal), coupling constant (Hz), and integration.
J = 2.7 Hz), 42.1 (d, JP-C = 5.9 Hz), 29.0 (d, JP-C = 13.4 Hz), 27.1
dd, JP-C =26.0, 4.4 Hz), 22.7 (d, JP-C = 15.3 Hz), 19.5, 19.4, 18.5,
(
Synthesis of compound 1: In a glove box, n-BuLi solution (5.8 18.4, 11.6. P NMR (121 MHz, THF-d ): δ 79.2 (d, J = 23.75 Hz).
3
1
8
mL, 1.6 mol/L in hexane, 93.0 mmol) was added to a solution Anal. Calcd for C H N NaPRu (549.7): C, 59.00; H, 7.70; N,
7 42 2
2
of diisopropylphosphane in hexane (10 g, 10 wt %, 84.5 mmol) 5.10; Found: C, 59.41; H, 7.96; N, 5.19.
at room temperature. The mixture was then stirred at room Preparation of complex 4: To a complex
3 (50 mg, 0.091 mmol)
temperature for 2 days and a white precipitate was observed, solution in THF (5 mL) was added PrOH (0.5 mL), and the
i
to which THF (5 mL) was added to form a clear solution of resulted mixture was stirred at room temperature for 10 min.
.
i
LiP Pr
2
To
a
1-(2-chloroethyl)-5,6-dimethyl-1H- After all volatile was removed under vacuum, the residue was
benzo[d]imidazole (1.678 g, 8.0 mmol) in THF (20 mL) solution, dissolved in benzene (10 mL) and was filtered to remove
4
| J. Name., 2012, 00, 1-3
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Orga Pn l iec a s&e Bd oi o nmo to al ed cj uu sl ta mr Ca rhg ei nms istry
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DOI: 10.1039/C7OB00542C
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ARTICLE
inorganic salts. All the solvent was then removed under
2010, 110, 681; (c) A. J. A. Watson and J. M. J. Williams,
Science, 2010, 329, 635; (d) C. Gunanathan and D. Milstein,
Science, 2013, 341, 249; (e) R. H. Crabtree, Chem. Rev.,
vacuum to afford complex
solid after washing with hexane. H NMR (400 MHz, C
.42 (s, 1H), 6.49 (s, 1H), 6.48 (s, 1H), 3.81 – 3.74 (m, 2H), 2.12
s, 3H), 2.10 (18H, 6CH ), 1.84 – 1.74 (m, 1H), 1.64 – 1.59 (m,
H), 1.21 – 1.12 (m, 6H), 1.08 – 0.91 (m, 2H), 0.69 – 0.56 (m,
4 (46 mg, 97% yield) as a yellow
1
6 6
D ): δ
2
017, DOI: 10.1021/acs.chemrev.6b00556.
8
2
For selected reviews and examples, see: (a) F. Huang, Z.
Liu and Z. Yu, Angew. Chem. Int. Ed., 2016, 55, 862; (b) G.
Chelucci, Coord. Chem. Rev., 2017, 331, 1; (c) G. Guillena,
D. J. Ramón and M. Yus, Chem. Rev., 2010, 110, 1611; (d)
K. Shimizu, R. Sato and A. Satsuma, Angew. Chem. Int.
Ed., 2009, 48, 3982; (e) Y Obora, ACS Catal., 2014, 4,
3972; (f) D. Wang, K. Zhao, C. Xu, H. Miao and Y. Ding,
ACS Catal., 2014, 4, 3910; (g) R. Wang, J. Ma and F. Li, J.
Org. Chem., 2015, 80, 10769; (h) R. Wang, H. Fan, W.
Zhao and F. Li, Org. Lett., 2016, 18, 3558; (i) K.
Chakrabarti, B. Paul, M. Maji, B. C. Roy, S. Shee and S.
Kundu, Org. Biomol. Chem., 2016, 14, 10988.
For reviews, see: (a) M. Trincado, D. Banerjee and H.
Grützmacher, Energy Environ. Sci., 2014, 7, 2464; (b) T. C.
Johnson, D. J. Morris and M. Wills, Chem. Soc. Rev., 2010,
39, 81; (c) Q.-L. Zhu and Q. Xu, Energy Environ. Sci., 2015,
(
3
1
6
2
1
3
H), -13.15 (d, J = 35.6 Hz, 1H); C NMR (100 MHz, C
6 6
D ): δ
05.0 (d, JP-C = 20 Hz), 134.0, 133.8, 108.1, 107.5, 43.1, 28.6 (d,
J
P-C = 23.2 Hz), 27.1 (d, JP-C = 19.6 Hz), 20.0 (d, JP-C = 28.7 Hz),
9.0, 18.7 (d, JP-C = 4.4 Hz), 18.4 (d, JP-C = 5.7 Hz), 17.2, 13.0.
Two aromatic carbon signals were incorporated in the solvent
1
(
3
1
peak). P NMR (125 MHz, C
PRu (527.7): C, 61.45; H, 8.21; N, 5.31; Found: C,
1.81; H, 8.54; N, 5.09.
The observation of conversion complex 4 to 3: In a glove box,
complex (15 mg, 0.028 mmol), NaH (4 mg, 0.17 mmol) and
THF-d (0.5 mL) were charged into a J-Young NMR tube. H
6 6
D ): δ 79.1. Anal. Calcd for
27 43 2
C H N
6
3
4
4
1
8
8
, 478.
For selected examples, see: (a) K. Fujita, N. Tanino and R.
Yamaguchi, Org. Lett., 2007, , 109; (b) M. Nielsen, A.
NMR spectrum indicated that the NH peak was disappeared
after 2 hour at room temperate.
9
General procedure for acceptorless dehydrogenation of alcohols:
In a nitrogen-filled glovebox, 1-phenylethanol (120 mg, 0.98
Kammer, D. Cozzula, H. Junge, S. Gladiali and M. Beller,
Angew. Chem. Int. Ed., 2011, 50, 9593; (c) R. Kawahara,
K. Fujita and R. Yamaguchi, J. Am. Chem. Soc., 2012, 134
643; (d) I. Mena, M. A. Casado, V. Polo, P. García-
Orduña, F. J. Lahoz and L. A. Oro, Angew. Chem. Int. Ed.,
012, 51, 8259; (e) A. H. Ngo, M. J. Adams and L. H. Do,
Organometallics, 2014, 33, 6742; (f) A. M. Royer, T. B.
Rauchfuss, and D. L. Gray, Organometallics, 2010, 29
763 and references cited therein.
,
mmol), complex
2 3
2 (5.5 mg, 1 mol%), Cs CO (3.2 mg, 1 mol%)
3
and xylene (1 mL) were charged in a seal bottle, which was
o
heated at 140 C. The reaction was monitored by GC-MS. After
2
reaction, the solution was allowed to cool to room
temperature, filtered through a silica gel column, and eluted
with petroleum ether. The resulted solution was evaporated
carefully under vacuum to afford acetophenone (100 mg, 85%
,
6
5
For selected examples, see: (a) G. Zeng, S. Sakaki, K.
Fujita, H. Sano and R. Yamaguchi, ACS Catal., 2014, 4,
1
yield). The H NMR and MS spectra of acetophenone were
1
010; (b) K.-N. T. Tseng, J. W. Kampf and N. K. Szymczak,
matched with the previous reports.
Organometallics, 2013, 32, 2046; (c) M. Bertoli, A.
Choualeb, A. J. Lough, B. Moore, D. Spasyuk and D. G.
Gusev, Organometallics, 2011, 30, 3479; (d) E. W. Dahl, T.
Louis-Goffa and N. K. Szymczak, Chem. Commun.,
General procedure for dehydrogenative coupling of secondary
alcohols with primary alcohols: In a nitrogen-filled glovebox, 1-
phenylethanol (60 mg, 0.49 mmol), benzyl alcohol (55 mg, 0.51
2
017,53, 2287; (e) S. Muthaiah and S. H. Hong, Adv.
mmol), complex
2 3
2 (5.5 mg, 2 mol%), Cs CO (3.2 mg, 2 mol%)
Synth. Catal., 2012, 354, 3045 and references cited
therein.
(a) G. Zhang and S. K. Hanson, Org. Lett., 2013, 15, 650;
and xylene (1 mL) were charged in a seal bottle, which was
o
heated at 150 C for 48 h. After reaction, the solution was
6
7
(b) G. Zhang, K. V. Vasudevan, B. L. Scott and S. K.
Hanson, J. Am. Chem. Soc., 2013, 135, 8668.
allowed to cool to room temperature. The pure product 8a
was obtained as a white solid (87 mg, 85% yield) by column
(a) S. Chakraborty, P. E. Piszel, W. W. Brennessel and W.
D. Jones, Organometallics, 2015, 34, 5203; (b) D.
Sengupta, R. Bhattacharjee, R. Pramanick, S. P. Rath, N. S.
Chowdhury, A. Datta and S. Goswami, Inorg. Chem.,
chromatography with petroleum ether/ethyl acetate (100:1) as
1
an eluent. H NMR (400 MHz, CDCl
3
): δ 7.97 (d, J = 7.2 Hz, 2H),
7
.56 (t, J = 7.4 Hz, 1H), 7.46 (t, J = 7.6 Hz, 2H), 7.25 – 7.34 (m,
H), 7.22 (t, J = 7 Hz, 1H), 3.32 (t, J = 7.6 Hz, 2H), 3.09 (t, J = 7.8
2
016, 55, 9602.
4
1
3
Hz, 2H); C NMR (100 MHz, CDCl
8
(a) S. Chakraborty, P. O. Lagaditis, M. Förster, E. A.
Bielinski, N. Hazari, M. C. Holthausen, W. D. Jones and S.
3
): δ 199.2, 141.3, 136.9,
133.1, 128.6, 128.6, 128.5, 128.1, 126.2, 40.5, 30.2.
Schneider, ACS Catal., 2014,
and S. H. Hong, ACS Catal., 2014,
4, 3994; (b) H. Song, B. Kang
4, 2889; (c) P. J.
Bonitatibus Jr, S. Chakraborty, M. D. Doherty, O. Siclovan,
W. D. Jones and G. L. Soloveichik, Proc. Natl. Acad. Sci.
USA. 2015, 112, 1687.
Acknowledgements
This work was supported by the Fundamental Research Fund
for the Central Universities (No. 2015ZCQ-LY-03) and National
Program for Thousand Young Talents of China. We thank Prof.
C.-L. Pan for X-ray analysis.
9
1
For recent reviews on metal-ligand cooperation, see: (a)
S. Kuwata and T. Ikariya, Chem. Commun., 2014, 50,
4290; (b) J. R. Khusnutdinova and D. Milstein, Angew.
1
Chem. Int. Ed., 2015, 54, 12236; (c) R. H. Morris, Acc.
Chem. Res., 2015, 48, 1494.
0 For selected reviews, see: (a) O. Schuster, L. Yang, H. G.
Raubenheimer and M. Albrecht, Chem. Rev., 2009, 109
,
Notes and references
3
2
445; (b) D. J. Nelson and S. P. Nolan, Chem. Soc. Rev.,
013, 42, 6723; (c) K. Riener, S. Haslinger, A. Raba, M. P.
1
For selected reviews, see: (a) M. H. S. A. Hamid, P. A.
Slatford and J. M. J. Williams, Adv. Synth. Catal., 2007, 349
555; (b) G. E. Dobereiner and R. H. Crabtree, Chem. Rev.,
Högerl, M. Cokoja, W. A. Herrmann and F. E. Kühn, Chem.
,
1
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ARTICLE
Rev., 2014, 114, 5215; (d) M. C. Jahnke and F. E. Hahn,
Coord. Chem. Rev., 2015, 293−294, 95.
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1
1
1 D. Kunz, Angew. Chem. Int. Ed., 2007, 46, 3405.
2 S. H. Wiedemann, J. C. Lewis, J. A. Ellman and R. G.
Bergman, J. Am. Chem. Soc., 2006, 128, 2452.
1
1
3 K. Araki, S. Kuwata and T. Ikariya, Organometallics, 2008,
27, 2176.
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Golen, C. E. Moore and A. L. Rheingold, Angew. Chem.
Int. Ed., 2011, 50, 631; (b) J. L. Gomez-Lopez, D. Chávez,
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, 3148.
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Organometallics, 2010, 29, 5283; (b) S. Cepa, C. S. to
Brinke, F. Roelfes and F. E. Hahn, Organometallics, 2015,
1
1
1
1
2
34, 5454.
7 The C NMR signal for carbene carbon (C1) in
13
3 should
be appeared as a double quartet theoretically, however
only apparent double doublet signal was observed with
coupling constants as 22.1 and 7.3 Hz.
8 The dehydrogenation reaction of 1-phenylethanol in
toluene-d was carried out at 110 C in a J-Young NMR
o
8
tube, from which a signal of H
ppm
2
was observed at δ = 4.52
NMR
1
in
the
H
spectrum.
6
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Table of Content
A new family of protic NHC Ru complexes can serve as efficient catalysts for acceptorless dehydrogenative transformations of
alcohols.