Donor functionalized ruthenium N-heterocyclic carbene complexes in alcohol oxidation reactions

Article abstract of DOI:10.1039/c3dt53125b

N-Pyridyl, N′-amido functionalized imidazolium bromides were obtained in high yields as an N-heterocyclic carbene (NHC) precursor and used as bidentate or a pincer ligands to obtain ruthenium complexes via a silver NHC transmetallation route. The incorporation of a phenyl group as an amido-N substituent (R = Ph) results in a bidentate coordination mode through the C _NHC and N_pyridyl donors, whereas in its absence (R = H) a pincer coordination mode was observed through the N_pyridyl^C _NHC^O_amido donors. The ruthenium complex featuring a pincer type NCO coordination mode with a protic NH function adjacent to the coordinating O_amido atom was found to efficiently catalyse the oxidation of activated alcohols effecting quantitative conversions within 30 minutes. However the oxidation of deactivated alcohols required longer reaction times to effect the quantitative transformation. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt53125b

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Donor functionalized ruthenium N-heterocyclic
carbene complexes in alcohol oxidation reactions†
Cite this: Dalton Trans., 2014, 43,
5335
Abbas Raja Naziruddin, Chun-Shiuan Zhuang, Wan-Jung Lin and Wen-Shu Hwang*
N-Pyridyl, N’-amido functionalized imidazolium bromides were obtained in high yields as an N-hetero-
cyclic carbene (NHC) precursor and used as bidentate or a pincer ligands to obtain ruthenium complexes
via a silver NHC transmetallation route. The incorporation of a phenyl group as an amido-N substituent
(R = Ph) results in a bidentate coordination mode through the CNHC and Npyridyl donors, whereas in its
absence (R = H) a pincer coordination mode was observed through the Npyridyl^CNHC^Oamido donors. The
ruthenium complex featuring a pincer type NCO coordination mode with a protic NH function adjacent
to the coordinating Oamido atom was found to efficiently catalyse the oxidation of activated alcohols
effecting quantitative conversions within 30 minutes. However the oxidation of deactivated alcohols
required longer reaction times to effect the quantitative transformation.
Received 5th November 2013,
Accepted 23rd December 2013
DOI: 10.1039/c3dt53125b
www.rsc.org/dalton
A special class of weakly coordinating ligands act as hemi-
labile donors, which upon coordination to catalytically active
Introduction
The excellent catalytic activity and high robustness of metal– metal centers exhibit co-operative effects.16 Mixed chelate com-
N-heterocyclic carbene (NHC) complexes along with their simple plexes feature the coordinated NHC ligand intact whereas a
preparation from readily available azolium precursors as well reversible de-coordination and coordination of the linked
as their ability to catalyse a diverse array of organic reactions hemilabile moiety promote the substrate binding and the
have propelled substantial development in transition metal product dissociation steps of the catalytic cycle to exhibit co-
mediated organic transformations over the past two decades.1 operativity.17 Even though such cooperative catalyses are well
Steric and electronic tuning of NHC ligands can be achieved established in hemilabile groups linked to phosphine donors
through N-wingtip functionalization or backbone modifi- via phenylene bridges in a pincer type coordination mode,18
cation2 to obtain highly selective organometallic catalysts,3 such occurrences involving NHCs donors are relatively rare.19
OLED materials4 or molecules with anticancer activities.5 Silver or gold complexes bearing NHC ligands functionalized
N-donor functionalized NHC wingtips combine the structural with the hemilabile amido group in the N-wingtip are pre-
aspects of NHC ligands along with that of the hetero atom viously known.20 Few ruthenium21 or iridium22 NHC com-
group in these hybrid bidentate3b,c,6 or tridentate7 donor sets. plexes bearing hemilabile O-donors have also been previously
Complexes bearing NHC ligands functionalized with hetero- employed in transfer hydrogenation reactions, whereas palla-
cyclic groups including pyridine,8 pyrazine,9 pyrimidine,8a,f,10 dium complexes with S-functionalized NHCs in a pseudo
quinoline,11 benzimidazole,12 oxazoline13 and phenanthro- pincer fashion were employed in intermolecular hydroamina-
line,14 have been significantly studied over the last few years. tion reactions.17
In addition to N-donor functionalized NHC ligands, few
The oxidation of alcohols to the corresponding aldehydes
reports also describe the direct oxidative insertion of the alkyl or ketones is one of the key steps in conventional organic
wingtip to the ruthenium center with the formation of chelate transformations including natural product synthesis.23
complexes.15
Although a variety of methods have been developed for this
purpose, most of them employ rather harsh reaction con-
ditions and require stoichiometric amounts of oxidant generat-
ing undesirable coproducts, that eventually result in
unselective transformations.24 Aerobic oxidations of alcohols
using metal free,25 homogeneous25,26 or heterogeneous24c,26e,27
transition metal catalysts have been previously developed, but
most of these reactions require the addition of large amounts
of base, heating, or pressured oxygen. Due to the importance
of this transformation in organic synthesis, the development
Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan,
Republic of China. E-mail: hws@mail.ndhu.edu.tw; Fax: +88-6-3-8633570;
Tel: +88-6-3-8633577
†Electronic supplementary information (ESI) available: the crystal data and
refinement parameters are listed in Table S1 and the crystal data, including the
details of data collection, refinement and complete geometric information are
available in CIF format. CCDC number 941843 for 3b and 941842 for 4a. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
C3DT53125B
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of efficient oxidation processes under milder conditions are
highly desirable.26h,28
Several ruthenium complexes have been studied as catalysts
for the oxidation of alcohols.26e,29 In this work, we report three
new ruthenium complexes of potential chelating ligands, fea-
turing an N-donor of the pyridyl wingtip and an O-donor of
the amide wingtips linked to the central NHC precursor. Our
interest in these ligand systems emerge from the fact that
metal complexes with amido functionalized NHC ligands are
rare and to the best of our knowledge, this is the first report of
ruthenium complexes featuring amide/pyridyl functionalized
NHC ligands. In addition we have evaluated the use of these
complexes as precatalysts towards alcohol oxidation reactions.
Results and discussion
The synthesis of the ligand precursors and complexes is out-
lined in Scheme 1. The ligand precursors 1a and 1b were
obtained in high yields by heating equimolar stoichiometric
amounts of 2-imidazol-1-ylmethyl-pyridine and 2-bromo-N,N-
diphenyl-acetamide or 2-bromo-N-phenyl-acetamide in aceto-
1
nitrile under reflux conditions for 6 h. The H NMR spectrum
of 1a exhibited a sharp downfield resonance at δ 9.89 ppm, for
the NCHN imidazolium proton, while the 1H NMR of spectrum
of 1b revealed two such downfield signals at δ 10.65 and
9.71 ppm, corresponding to the NH and NCHN imidazolium
protons respectively. The NMR data of 1a and 1b are in a com-
parable range to those of similar imidazolium NHC precur-
sors.30 The reaction of 1a or 1b with Ag2O in dichloromethane
under the exclusion of light at ambient temperature resulted
in the formation of the Ag–NHC complexes 2a or 2b within
4 h. The ruthenium–NHC complex 3a was obtained in a good
yield (80.7%) via a standard transmetallation reaction of
complex 2a with [Ru(PPh3)3Cl2] in acetonitrile under reflux in
5 h. 3a was further subjected to counter ion exchange by react-
ing with NH4PF6 in an aqueous solution at ambient tempera-
ture to yield 4a. Similar to the synthesis of 3a, the ruthenium
complex 3b was also obtained from the direct transmetallation
reaction of 2b with [Ru(PPh3)3Cl2] in CH3CN at ambient temp-
erature in 5 h. The chloride salt thus obtained was also sub-
jected to an anion exchange reaction using NH4PF6 resulting
in the formation of the pincer complex 3b with PF6 counter
ions.
Scheme 1 The synthesis of N-O functionalized NHC ligand precursors
and their ruthenium complexes via a silver NHC transmetallation route.
All of the ligand precursors and ruthenium complexes are
1
JH–H ∼ 16 Hz due to the diastereotopicity of the methylene
protons arising due to complexation.32 The IR spectra of com-
plexes 2a, 2b, 3a and 4a displayed an amide CvO stretching
band at 1675–1693 cm−1, while 3b exhibited a large lowering
of this stretching frequency to 1621 cm−1, due to the coordi-
nation of the oxygen atom to the ruthenium center. All of the
complexes reported in this work were observed to be air and
moisture stable.
characterized by standard analytical techniques. The H NMR
spectra of all of the complexes are devoid of NCHN imidazo-
lium proton resonances and the 13C NMR spectra exhibited
characteristic carbenoid carbon atom resonance around δ ∼
180 ppm, confirming the coordination of the carbene ligand
in 2a, 2b, 3a, 3b and 4a. The 31P NMR spectra of the complexes
3a, 4a and 3b exhibited the coordinated phosphorus reson-
ances around δ 45–48 ppm.31 The methylene bridges linking
the pyridyl and the amide donor to the central NHC ligand in
3a, 4a and 3b exhibited four sets of doublet resonances in the
range of δ 6.1–5.4 and 4.3–2.9 ppm with a coupling constant of
Single crystals suitable for X-ray diffraction analyses were
obtained by slow diffusion of diethylether into a CH3CN solu-
tion of 4a or by the slow evaporation of a CH2Cl2 solution of
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3b. The crystal data and refinement parameters are listed in
Table S1 (ESI†).
The molecular structure of 3b, as depicted in Fig. 1a, also
confirms a distorted octahedral geometry around the ruthe-
Selected bond lengths and angles are summarized in nium center. The N/O-functionalized mixed NHC donor 1b
Table 1. The molecular structure of 4a, depicted in Fig. 1b, (R = H) acts as a terdentate pincer ligand exhibiting an
shows that the N/O-functionalized NHC ligand 1a (R = Ph) additional weak coordination through the oxygen atom. One
coordinates to the ruthenium center through the carbenoid-C of the weakly bound CH3CN in 4a, is substituted by a CvO
and the pyridyl-N donors with Ru(1)–C(27) and Ru(1)–N(1) donor of an amide functionalized N-wing tip featuring a Ru–O
bond lengths measuring 2.033(3) and 2.148(3) Å, respectively, bond distance of 2.143(4) Å. The ruthenium pincer complex 3b
to form a six-membered chelate ring with a bite angle of exhibit a shorter Ru–CNHC bond distance of 1.978(6) Å, relative
83.96(12). The PPh3 ligand occurs in a trans configuration to to that observed in 4a. Indeed upon the coordination of the
the pyridyl-N atom, featuring a Ru(1)–P(1) bond distance of amide carbonyl (O) donor to the ruthenium center, the CvO
2.352(2) Å. The octahedral coordination sphere of the ruthe- bond distance was elongated to 1.244(7) Å in 3b, from 1.213(4)
nium center is completed by three CH3CN ligands with the Å in 4a. This data is also commensurate with the CvO stretch-
first two Ru–N bond distances measuring 2.028(3) and 2.029(3) ing frequency observed from the IR spectrum. Two CH3CN
Å, whereas the third Ru–N bond that occurs trans to the NHC ligands are observed in a mutually cis configuration with Ru–N
ligand is relatively elongated, measuring 2.102(3) Å. In bond distances measuring 2.113(6) Å (trans to CNHC) and
addition, the bond angles of three trans donor pairs across the 1.994(5) Å (trans to OCvO).
ruthenium center, C(27)–Ru(1)–N(7), N(1)–Ru(1)–N(6), and
The ruthenium complexes reported herein were evaluated
N(5)–Ru(1)–N(6) were observed to be 172.48(12), 174.51(8), and towards the catalytic oxidation of alcohol to the corresponding
175.25(11)°, respectively. The bonding parameters around the aldehyde or ketone. Our optimization experiments confirmed
ruthenium center confirm a slightly distorted octahedral geo- that a combination of KOtBu, toluene and 5 mol% ruthenium
metry and are in a comparable range to those of the closely pre-catalyst at reaction temperature of 100 °C afforded the best
related ruthenium complexes in the literature.33
result for the catalytic oxidation of benzyl alcohol. Furthermore,
we do not observe any over oxidation of the aldehyde to the car-
boxylic acid. The standard oxidation experiment was thus per-
formed on benzyl alcohol (2.0 mmol) using the ruthenium pre-
catalysts (5.0 mol%) and KOtBu (1.0 mmol) in toluene (5 mL) at
100 °C. The product yields were monitored by GC.
Table 1 Selected bond lengths (Å) and angles (°) of 3b and 4a
3b
4a
The catalytic results for the oxidation of benzyl alcohol
using 3a, 4a and 3b are summarized in Fig. 2. The reaction
profiles show that 3a, 4a and 3b demonstrate very good activity
towards the catalytic oxidation of benzyl alcohol resulting in
the almost quantitative formation of benzaldehyde within 100,
50 and 25 min respectively. Complexes 3a and 4a bearing three
CH3CN ligands could facilitate a facile release of the labile
CH3CN ligands to generate a vacant site for the incoming
deprotonated benzyl–alkoxide moiety to initiate the catalytic
reaction and further propel the subsequent β-hydrogen
Ru(1)–C(29)
Ru(1)–N(4)
Ru(1)–P(1)
Ru(1)–O(1)
Ru(1)–N(5)
Ru(1)–N(6)
C(25)–O(1)
C(29)–Ru(1)–N(5)
N(4)–Ru(1)–P(1)
N(6)–Ru(1)–O(1)
C(29)–Ru(1)–N(4)
O(1)–Ru(1)–N(4)
N(5)–Ru(1)–N(6)
1.978(6)
2.147(5)
2.317(2)
2.143(4)
2.113(6)
1.994(5)
1.244(7)
169.90(2)
173.50(14)
176.02(21)
81.69(24)
90.19(17)
85.75(24)
Ru(1)–C(27)
Ru(1)–N(1)
Ru(1)–P(1)
Ru(1)–N(5)
Ru(1)–N(6)
Ru(1) –N(7)
C(29)–O(1)
C(27)–Ru(1)–N(7)
N(1)–Ru(1)–P(1)
N(5)–Ru(1)–N(6)
C(27)–Ru(1)–N(1)
N(6)–Ru(1)–N(7)
2.033(3)
2.148(3)
2.352(2)
2.029(3)
2.028(3)
2.102(3)
1.213(4)
172.48(12)
174.51(8)
175.25(11)
83.96(12)
87.81(11)
Fig. 1 (a) The molecular structure of 3b. (b) The molecular structure of 4a. The hydrogen and PF6 anion are omitted for clarity.
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Table 2 The results of the oxidation of different alcohols catalyzed by
4a and 3b in 30 mina
Entry
1
Alcohol (substrate)
Catal.
Yieldb (%)
3b
4a
99
91
2
3
3b
4a
99
95
3b
4a
65 (97 in 3.0 h)
45 (98 in 4.5 h)
Fig. 2 The reaction profiles for the oxidation of benzyl alcohol
(2.0 mmol) to benzaldehyde with the Ru(II) precatalysts 3a, 4a and 3b
(5.0 mol%) in toluene (5 mL) in the presence of KOtBu (1.0 mmol) at
100 °C. The catalytic conversions of the standard benzyl alcohol sub-
strate were monitored by GC.
4
5
3b
4a
45 (96 in 4.5 h)
29 (99 in 7.0 h)
3b
4a
30 (97 in 7.0 h)
12 (95 in 8.0 h)
elimination reaction to furnish the carbonyl product. A com-
parison of the reaction profiles of 3a vs. 4a also supports the
strong counter ion effect that significantly influences the
results of the catalytic conversion. However, 3b with only two
CH3CN ligands and a labile O-donor via the NHC wingtip,
exhibited even better activity than 3a or 4a; we speculate that
the N-wingtip substituent on the NHC ligand bearing a less
sterically hindered amide-O donor along with the protic NH
function lying adjacent the weakly coordinating oxygen atom
could contribute to the enhanced activities of 3b.
6
3b
4a
98
90
7
8
3b
4a
95
88
3b
4a
93
85
A post-catalytic analysis of 3b and 4a was conducted to
study the stabilizing effect of the chelating N/O functionalized
NHC and phosphine ligands in the catalytic reaction. After the
completion of the catalytic reaction, the reaction mixture was
extracted in an ether–water system. The organic phase was sepa-
rated and the aqueous layer was decanted. The residue was
dissolved in acetonitrile and the solution was filtered. The fil-
trate was further dried in vacuo and the residue was subjected
to NMR and IR analyses. The results confirmed that the N/O
functionalized NHC ligand as well as the phosphine ligand
remains intact at the ruthenium center in 3b and 4a.
To investigate the tolerance of the substituent functionality
on benzyl alcohol and study the generality of the current cata-
lytic system towards activated or unactivated alcohols, a few
selected substrates were screened for catalytic reactions using
3b and 4a under optimized conditions. Pre-catalysts 3b and
4a afforded almost quantitative conversion for activated
aromatic alcohol within 30 minutes, while the unactivated
alcohols required 3–8 h. A relative study of the catalytic
activities of 3b and 4a in 30 minutes is tabulated in Table 2.
Indeed, an expected trend of higher conversions for
activated alcohols using both the pre-catalysts 3b and 4a was
observed. As evident from entries 1 and 6, the conversion of
phenylmethanol or 1-phenylethanol occurred almost quanti-
tatively with the precatalyst 3b (99–98%); whereas the conver-
sion was slightly lowered upon using 4a as the precatalyst
(91–90%). With a p-chloro substituent on the phenyl ring
9
3b
4a
76 (96 in 2.5 h)
59 (97 in 4.0 h)
a Reaction conditions: 2.0 mmol alcohol, 1.0 mmol KO-tBu, and
5.0 mol% precatalyst in 5 mL toluene at 100 °C in 30 min. b Isolated
yields.
(entry 2) higher yields of the corresponding aldehyde (95%)
were observed upon using the precatalyst 4a, while almost
quantitative conversions upon using 3b were retained (99%).
We also tested the effect of the catalytic conversions upon
introducing an electron withdrawing substituent (NO2) at the
p- (entry 3), m- (entry 4) or o- (entry 5) positions of the phenyl-
methanol substrates. In all cases the isolated yields observed
at the end of 30 minutes were from moderate to poor; however
the precatalyst 3b was relatively more efficient for such oxi-
dation of deactivated aromatic alcohols. The pre-catalyst 3b
effected a 65%, 45% and 30% conversion of (p-NO2)-phenyl-
methanol, (m-NO2)-phenylmethanol and (o-NO2)-phenylmetha-
nol, respectively. Especially the poor yields in the oxidation of
(m-NO2)-phenylmethanol, could arise from a mixed factor of
steric and electronic effects as well as the occurrence of intra-
molecular hydrogen bonding between the nitro and hydroxyl
groups that considerably deactivates the substrate. However
such substrates usually require a prolonged reaction time of
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over 20 h to yield quantitative conversions.34 Secondary (0.522 g, 3.28 mmol) and 2-bromo-N,N-diphenyl-acetamide
alcohol substrates like diphenylmethanol and 6-methoxy- (0.960 g, 3.28 mmol) in CH3CN (20 mL) was heated under
1,2,3,4-tetrahydronaphthalen-1-ol (entries 7 and 8) underwent reflux for 6 h. The reaction mixture was cooled and poured
good conversions with the precatalysts 3b (95–93%) and 4a into diethyl ether (100 mL) and the resulting yellow precipitate
(88–85%). Previously an in situ generated Cu–NHC–TEMPO was filtered. The yellow solid was recrystallized three times by
catalyst took 15 h to effect the oxidation of furan-2-ylmethanol dissolving with methanol (10 mL) and titurated with diethyl
with a 71% conversion.34 The precatalyst 3b facilitated a slight ether (100 mL) to obtain white product 1a (1.265 g, 86% yield).
improvement of this catalytic yield (entry 9) (76%), however in 1H NMR (CDCl3): δ 9.89 (s, 1H), 8.36 (dd, JH–H = 4.6, 1.7 Hz,
a remarkably shorter reaction time (30 min).
1H), 7.77 (s, 1H), 7.71 (d, JH–H = 7.8 Hz, 2H), 7.53 (td, JH–H
7.7, 1.8 Hz, 1H), 7.40 (q, JH–H = 7.8 Hz, 3H), 7.31 (t, JH–H = 7.7,
3H), 7.27 (s, 1H), 7.12 (t, JH–H = 7.5 Hz, 3H), 7.04 (d, JH–H
=
=
7.5 Hz, 1H), 5.32 (s, 2H), 5.23 (s, 2H) ppm. 13C NMR (CD3CN):
δ 165.1 (CvO), 153.2 (NCN), 149.7, 142.6, 140.5, 137.9, 137.6,
130.2, 129.5, 129.1, 126.8, 124.3, 123.8, 123.0, 122.1, 53.5,
51.9 ppm. IR (KBr): νmax (cm−1) 1681 (CvO). Mass (MALDI):
m/z 368 for [M+].
Summary
In conclusion, we have prepared a new amide functionalized
NHC ligand precursor, that coordinates to the ruthenium
center in either bidentate or pincer coordination modes,
leading to a distorted octahedral geometry around the metal
center. Amongst the three new ruthenium complexes reported
herein, the complex featuring a pincer coordination mode
renders a hemilabile coordination to the ruthenium center
through the CvO group of the amide function along with a
protic NH moiety adjacent to the coordinating oxygen atom. In
addition the pincer type NHC complex exhibits good catalytic
performances towards the oxidation of alcohols. Activated alco-
hols were almost quantitatively converted into the corres-
ponding aldehydes, whereas the deactivated alcohols were
converted into the aldehydes with a moderate to poor yield. In
this work, we have made a substantial improvement to the
catalytic results by improving the yields within shorter reaction
times and also avoided the usage of halogenated solvents in
the catalytic transformations.
1-Phenylcarbamoylmethyl-3-pyridylmethyl-3H-imidazolium
bromide (1b). A mixture of 2-imidazol-1-ylmethyl-pyridine
(0.340 g, 2.14 mmol) and 2-bromo-N-phenyl-acetamide
(0.456 g, 2.14 mmol) in CH3CN (20 mL) was heated under
reflux for 6 h. Following a procedure similar to that of 1a,
product 1b was obtained as a brown solid (0.717 g, 90% yield).
1H NMR (CDCl3): δ 10.65 (s, 1H), 9.71 (s, 1H), 8.43 (d, JH–H
=
4.5 Hz, 1H), 7.60 (m, 4H), 7.48 (t, JH–H = 8.0 Hz, 2H), 7.14 (m,
3H), 6.95 (t, JH–H = 7.5 Hz, 1H), 5.53 (s, 2H), 5.50 (s, 2H) ppm.
13C NMR (CD3CN): δ 163.0 (CvO), 152.1 (NCN), 149.9, 137.7,
137.6, 137.3, 128.8, 124.5, 124.0, 123.6, 123.5, 122.2, 120.1,
54.1, 52.3 ppm. IR (KBr): νmax (cm−1) 1693 (CvO); 3399 (N–H
stretching). Mass (MALDI): m/z 293 for [M+].
[2a]. Ag2O (0.421 g, 1.81 mmol) was added into a CH2Cl2
(10 mL) solution of 1a (0.540 g, 1.25 mmol) and the resultant
suspension was stirred at ambient temperature for 4 h under
exclusion of light. After this time, the reaction mixture was fil-
tered through Celite and the filtrate was dried in vacuo. The
residue was dissolved in CH2Cl2 (5 mL) and the solution was
added into diether ether (30 mL) to yield a yellow precipitate.
Upon filtration and drying in vacuo, [2a] was obtained as a
Experimental
General synthetic methods, materials and physical
measurements
1
The solvents and reagents were purchased from Sigma Aldrich
or Acros Organics in analytical grade and were used without
further purification. The NMR spectra were recorded on a
Bruker AVANCE DPX-400 spectrometer (1H, 400.13 MHz; 13C,
100.61 MHz) with tetramethylsilane as an internal standard.
Elemental microanalyses were performed at the Taiwan Instru-
mentation Center. The IR spectra were recorded on a Mattson
Genesis Series FT-spectrophotometer. G.C. analyses were per-
yellow solid (0.332 g, 31.4% yield). H NMR (CDCl3): δ 8.54 (d,
JH–H = 5.1 Hz , 2H), 7.66 (t, JH–H = 7.8 Hz, 2H), 7.43 (m, 12H),
7.28 (m, 6H), 7.22–7.15 (m, 10H), 5.35 (s, 4H), 4.87 (s, 4H)
ppm. 13C NMR (CDCl3): δ 182.9 (NCN), 170.1 (CvO), 166.2,
155.8, 155.2, 149.7, 141.7, 140.5, 137.5, 130.7, 129.1, 126.7,
126.0, 123.9, 123.4, 123.2, 123.0, 122.7, 122.2, 121.8, 120.4,
57.0 (CH2), 53.9 (CH2) ppm. IR (KBr): νmax (cm−1) 1687 (CvO).
Mass (MALDI): m/z 846.4 for [M+]. Anal. Calcd for
C46H40N8O2AgBr: C, 59.76; H, 4.36; N, 12.12. Found: C, 59.58;
H, 4.22; N, 11.98.
[2b]. To a solution of 1b (0.232 g, 0.621 mmol) in dichloro-
methane (5 mL) and methanol (0.5 mL) Ag2O (0.220 g,
0.948 mmol) was added and the suspension was stirred at
ambient temperature for 4 h under exclusion of light. The reac-
tion mixture was filtered through Celite followed by drying the
filtrate in vacuo. A solution of the resultant residue in dichloro-
methane (5 mL) was poured into diethylether (30 mL) to yield
formed with
a GC9800 gas chromatography (Shanghai
Kechuang Chromatograph Instrument Co.) equipped with a
flame ionization detector and a 10 m (2.65 μm film thickness)
RESTEK Rtx-2887 fused silica capillary column. 2-Imidazol-
1-ylmethyl-pyridine,33a 2-bromo-N,N-diphenyl-acetamide and
2-bromo-N-phenyl-acetamide35 were prepared according to
literature protocols.
Synthesis
1-(Diphenylcarbamoyl-methyl)-3-pyridylmethyl-imidazolium a yellow precipitate, which was separated and further dried
bromide (1a). A mixture of 2-imidazol-1-ylmethyl-pyridine in vacuo to obtain [2b] as a yellow solid (0.124 g, 26.6% yield).
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1H NMR (CDCl3): δ 8.48 (d, JH–H = 4.8 Hz, 2H), 7.71 (d, JH–H
=
the aqueous reaction mixture and hence the aqueous solution
8.0 Hz, 4H), 7.56 (t, JH–H = 8.1 Hz, 2H), 7.20 (m, 8H), 7.04 (m, 8H), was further treated with NH4PF6 (0.212 g, 1.31 mmol). Other
5.28 (s, 8H) ppm. 13C NMR (CDCl3): δ 182.1 (NCN), 165.4 work-up steps, analogous to that in the preparation of [4a]
(CvO), 155.1, 149.6, 139.0, 138.6, 137.3, 128.6, 123.9, 123.5, were followed to obtain [3b] as a yellow solid (0.177 g, 26.3%
1
123.2, 122.2, 121.2, 120.5, 120.0, 56.8, 54.6 ppm. IR (KBr): νmax yield). H NMR (CD3CN): δ 9.34 (s, 1H), 9.13 (d, JH–H = 3.0 Hz,
(cm−1) 1693 (CvO), 3262 (N–H stretching). Mass (MALDI): m/z 1H), 8.01 (t, JH–H = 7.8 Hz, 1H), 7.73 (d, JH–H = 7.7 Hz, 1H), 7.61
693 [M+]. Anal. Calcd for C34H32N8O2AgBr: C, 52.86; H, 4.18; N, (s, 1H), 7.55–7.32 (m, 20H), 7.21 (t, JH–H = 11.0 Hz, 1H), 6.95 (s,
14.51. Found: C, 52.65; H, 4.06; N, 14.32.
1H), 5.72 (d, JH–H = 16.1 Hz, 1H), 5.49–5.42 (m, 1H), 4.30 (d,
[3a]. [Ru(PPh3)3Cl2] (1.43 g, 1.5 mmol) was added into a JH–H = 16.7 Hz, 1H), 3.74 (d, JH–H = 16.7 Hz, 1H), 1.99 (s, 3H),
solution of [2a] (0.70 g, 0.75 mmol) in CH3CN (5 mL) and the 1.92 (s, 3H) ppm. 13C NMR (CDCl3): δ 188.5 (NCN), 171.5
mixture was heated under reflux for 5 h. Filtration of the reac- (CvO), 157.8, 155.9, 130.1, 136.7, 134.5, 133.3, 132.9, 132.0,
tion mixture yielded a clear solution. The solvent was removed 130.4, 130.0, 129.6, 129.6, 127.6, 127.0, 125.9, 124.6, 124.4,
in vacuo and the residue was suspended in a mixture of 123.5, 121.5, 120.6, 56.2, 52.9, 3.9 ppm. 31P NMR (CD3CN):
CH2Cl2 (50 mL) and H2O (35 mL) and the product was δ 48.21 ppm. IR (KBr): νmax (cm−1) 1621 (CvO), 3396 (N–H
extracted (×3 times) from the aqueous phase. The aqueous stretching). Mass (MALDI): m/z 1027 for [M + 2·PF6]+ and 841
extracts were collected and dried in vacuo to obtain [3a] for [M + PF6 − CH3CN]+. Anal. Calcd for C39H37F12N6O-
(0.536 g, 80.7% yield). 1H NMR (CDCl3): δ 9.58 (d, JH–H = 4.0 P3Ru·CH2Cl2: C, 43.14; H, 3.53; N, 7.55. Found: C, 43.27; H,
Hz, 1H), 7.83 (m, 5H), 7.36 (m, 6H), 7.19 (m, 6H), 7.09 (m, 3H), 3.57; N, 7.51.
7.02 (m, 6H), 6.86 (m, 3H), 6.49 (s, 1H), 6.11 (d, JH–H = 16.0 Hz,
1H), 5.94 (d, JH–H = 16.0 Hz, 1H), 4.30 (d, JH–H = 17.0 Hz, 1H),
A general procedure for the catalytical oxidation of alcohols
2.49 (d, JH–H = 17.4 Hz, 1H), 2.01 (s, 3H), 1.78 (s, 3H), 1.63 (s,
3H) ppm. 13C NMR (CDCl3): δ 180.6 (NCN), 167.0 (CvO),
In a standard catalytic experiment, the alcohol substrate
(2.0 mmol), ruthenium–NHC pre-catalyst (5.0 mol%), KOtBu
156.6, 154.4, 141.3, 139.6, 138.1, 130.6, 129.0, 128.8, 128.7,
128.6, 128.0, 127.9, 127.5, 126.5, 126.2, 125.6, 124.9, 124.7,
124.2, 124.0, 121.3, 55.5, 52.4, 4.7, 4.0 ppm. 31P NMR
(300 MHz, CDCl3): δ 47.25 ppm. IR (KBr): νmax (cm−1) 1675
(CvO). Mass (MALDI): m/z 885 for [M − CH3CN + 2Cl]+ and
767 for [M − 3·CH3CN + Cl]+. Anal. Calcd for C47H44Cl2N7O-
PRu: C, 60.97; H, 4.79; N, 10.59. Found: C, 60.71; H, 4.86; N,
10.50.
(1.0 mmol) and toluene (5 mL) were charged into a screw
capped vial under air and heated at 100 °C in an oil bath. After
the completion of the catalytic reaction, the reaction mixture
was extracted in a diethyl ether–water system (1 : 1, v/v). The
organic phase was separated and ether was removed in vacuo.
The isolated product yields were monitored by NMR
spectroscopy.
[4a]. An aqueous solution (H2O, 5 mL) of [3a] (0.329 g,
0.355 mmol) and NH4PF6 (0.12 g, 0.75 mmol) was stirred at
ambient temperature for 24 h, after which the reaction mixture
was extracted in CH2Cl2 (10 mL × 3) and the solvent was
removed in vacuo. The resultant residue was further washed
with minimal amounts of CHCl3 (1 mL × 3) and dried in vacuo
X-ray crystallography
Single crystals suitable for X-ray diffraction analyses were
obtained by the slow diffusion of diethyl ether into a CH3CN
solution of 4a or by the slow evaporation of a CH2Cl2 solution
of 3b. The data were collected on an APEX II diffractometer,
1
to obtain [4a] as a green solid (0.395 g, 96.7% yield). H NMR
(CD3CN): δ 9.01 (d, JH–H = 5.7 Hz, 1H), 8.09 (t, JH–H = 15.1 Hz,
1H), 7.75 (d, JH–H = 7.7 Hz, 1H), 7.63 (t, JH–H = 6.8 Hz, 2H), 7.56
(m, 4H), 7.36 (m, 12H), 7.22 (d, JH–H = 7.4 Hz, 1H), 7.16 (d, JH–H
= 7.8 Hz, 2H), 7.09 (m, 2H), 7.02 (d, JH–H = 7.6 Hz, 2H), 6.91 (d,
using graphite monochromatic Mo Kα radiation (λ
=
0.71073 Å). The data reduction was performed with SAINT,
which corrects for Lorentz and polarization effects. The
absorption corrections were performed using multiscan
(SADABS).36 All of the H atoms were added in idealized posi-
tions. The structures were solved by the use of direct methods
and the refinement was performed by the least-squares
methods on F2 with the SHELXL-97 package.37 The crystal
data, including the details of the data collection, refinement
and complete geometric information are available in CIF
format.
JH–H = 2.2 Hz, 1H), 5.71 (d, JH–H = 16.6 Hz, 1H), 5.43 (d, JH–H
=
16.6 Hz, 1H), 4.07 (d, JH–H = 16.5 Hz, 1H), 2.94 (d, JH–H = 16.8
Hz, 1H), 2.16 (s, 3H), 1.99 (s, 3H), 1.72 (s, 3H) ppm. 13C NMR
(CDCl3): δ 179.5 (NCN), 167.4 (CvO), 157.0, 155.5, 142.9,
141.2, 140.4, 134.7, 131.6, 131.3, 130.1, 129.8, 129.6, 129.5,
128.5, 127.8, 127.0, 126.8, 126.6, 124.6, 123.6, 122.9, 55.6, 32.1,
4.9, 4.5 ppm. 31P NMR (CD3COCD3): δ 45.0 ppm. IR (KBr): νmax
(cm−1) 1683 (CvO). Mass (MALDI): m/z 920 for [M + PF6
2·CH3CN]+), 733 for [M 3·CH3CN]+. Anal. Calcd for
−
−
C47H44F12N7OP3Ru: C, 49.26; H, 3.87; N, 8.56. Found: C, 49.44;
H, 3.93; N, 8.51.
[3b]. A similar procedure as described for [4a], starting from
Acknowledgements
[Ru(PPh3)3Cl2] (1.280 g, 1.30 mmol) and [2b] (0.505 g, The authors acknowledge the National Science Council
0.653 mmol) was followed to obtain [3b]. An exception being (Taiwan) and National Dong Hwa University for the financial
that the chloride salt of [3b] could not be isolated directly from support.
5340 | Dalton Trans., 2014, 43, 5335–5342
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and G.-X. Jin, Organometallics, 2006, 25, 3565–3569;
(f) Y. Cheng, J. F. Sun, H. L. Yang, H. J. Xu, Y. Z. Li,
X. T. Chen and Z. L. Xue, Organometallics, 2009, 28, 819–
823; (g) X. W. Li, G. F. Wang, F. Chen, Y. Z. Li, X. T. Chen
and Z. L. Xue, Inorg. Chim. Acta, 2011, 378, 280–287;
(h) N. Gurbuz, E. O. Ozcan, I. Ozdemir, B. Cetinkaya,
O. Sahin and O. Buyukgungor, Dalton Trans., 2012, 41,
2330–2339; (i) F. E. Fernández, M. C. Puerta and P. Valerga,
Organometallics, 2011, 30, 5793–5802; ( j) H. Ohara,
W. N. O. Wylie, A. J. Lough and R. H. Morris, Dalton Trans.,
2012, 41, 8797–8808; (k) W. B. Cross, C. G. Daly,
Y. Boutadla and K. Singh, Dalton Trans., 2011, 40, 9722–
9730; (l) F. E. Fernández, M. C. Puerta and P. Valerga, Orga-
nometallics, 2012, 31, 6868–6879.
Notes and references
1 (a) S. Diez-Gonzalez, N. Marion and S. P. Nolan, Chem.
Rev., 2009, 109, 3612–3676; (b) S. Díez-González and
S. P. Nolan, Coord. Chem. Rev., 2007, 251, 874–883;
(c) F. E. Hahn and M. C. Jahnke, Angew. Chem., Int. Ed.,
2008, 47, 3122–3172; (d) R. H. Crabtree, Coord. Chem. Rev.,
2007, 251, 595; (e) H. Jacobsen, A. Correa, A. Poater,
C. Costabile and L. Cavallo, Coord. Chem. Rev., 2009, 253,
687–703; (f) J. C. Lin, R. T. Huang, C. S. Lee,
A. Bhattacharyya, W. S. Hwang and I. J. Lin, Chem. Rev.,
2009, 109, 3561–3598.
2 (a) E. A. B. Kantchev, C. J. O’Brien and M. G. Organ, Angew.
Chem., Int. Ed., 2007, 46, 2768–2813; (b) B. Yiğit, M. Yiğit,
İ. Özdemir and E. Çetinkaya, Transition Met. Chem., 2012,
37, 297–302.
9 C. S. Lee, R. R. Zhuang, J. C. Wang, W. S. Hwang and
I. J. B. Lin, Organometallics, 2012, 31, 4980–4987.
3 (a) F. E. Hahn, ChemCatChem, 2013, 5, 419–430; 10 B. Liu, Q. Xia and W. Chen, Angew. Chem., Int. Ed., 2009,
(b) F. E. Hahn, A. R. Naziruddin, A. Hepp and T. Pape, 48, 5513–5516.
Organometallics, 2010, 29, 5283–5288; (c) A. R. Naziruddin, 11 (a) H. M. Peng, G. Song, Y. Li and X. Li, Inorg. Chem., 2008,
A. Hepp, T. Pape and F. E. Hahn, Organometallics, 2011, 30,
5859–5866; (d) T. Kosterke, T. Pape and F. E. Hahn, J. Am.
47, 8031–8043; (b) H. M. Peng, R. D. Webster and X. W. Li,
Organometallics, 2008, 27, 4484–4493.
Chem. Soc., 2011, 133, 2112–2115; (e) N. Meier, F. E. Hahn, 12 (a) S. B. Fuwei Li and T. S. Andy Hor, Organometallics, 2008,
T. Pape, C. Siering and S. R. Waldvogel, Eur. J. Inorg. Chem.,
2007, 1210–1214.
27, 672–677; (b) F. Li, J. J. Hu, L. L. Koh and T. S. Hor,
Dalton Trans., 2010, 39, 5231–5241.
4 (a) C. S. Lee, S. Sabiah, J. C. Wang, W. S. Hwang and 13 (a) W. A. Herrmann, L. J. Goossen and M. Spiegler, Organo-
I. J. B. Lin, Organometallics, 2010, 29, 286–289; (b) C. S. Lee,
R. R. Zhuang, S. Sabiah, J. C. Wang, W. S. Hwang and
I. J. B. Lin, Organometallics, 2011, 30, 3897–3900; (c) K. Li,
X. Guan, C. W. Ma, W. Lu, Y. Chen and C. M. Che, Chem.
Commun., 2011, 47, 9075–9077; (d) K. Li, G. Cheng,
C. S. Ma, X. G. Guan, W. M. Kwok, Y. Chen, W. Lu and
C. M. Che, Chem. Sci., 2013, 4, 2630–2644.
metallics, 1998, 17, 2162–2168; (b) V. César, S. Bellemin-
Laponnaz and L. H. Gade, Organometallics, 2002, 21, 5204–
5208; (c) M. C. Perry, X. Cui, M. T. Powell, D. R. Hou,
J. H. Reibenspies and K. Burgess, J. Am. Chem. Soc., 2003,
125, 113–123; (d) N. Schneider, V. Cesar, S. Bellemin-
Laponnaz and L. H. Gade, Organometallics, 2005, 24, 4886–
4888; (e) S. Nanchen and A. Pfaltz, Chem.–Eur. J., 2006, 12,
4550–4558; (f) J. Zhao and K. Burgess, Org. Lett., 2009, 11,
2053–2056.
5 (a) L. Kaps, B. Biersack, H. Muller-Bunz, K. Mahal,
J. Munzner, M. Tacke, T. Mueller and R. Schobert, J. Inorg.
Biochem., 2012, 106, 52–58; (b) R. Wai-Yin Sun, A. Lok-Fung 14 S. Gu and W. Chen, Organometallics, 2009, 28, 909–914.
Chow, X.-H. Li, J. J. Yan, S. Sin-Yin Chui and C.-M. Che, 15 (a) S. Burling, B. M. Paine, D. Nama, V. S. Brown,
Chem. Sci., 2011, 2, 728; (c) F. Hackenberg, H. Müller-Bunz,
R. Smith, W. Streciwilk, X. Zhu and M. Tacke, Organometal-
lics, 2013, 32, 5551–5560; (d) T. Zou, C. T. Lum, S. S. Chui
and C. M. Che, Angew. Chem., Int. Ed., 2013, 52, 2930–2933.
6 N. Tsoureas, A. A. Danopoulos, A. A. D. Tulloch and
M. E. Light, Organometallics, 2003, 22, 4750–4758.
M. F. Mahon, T. J. Prior, P. S. Pregosin, M. K. Whittlesey
and J. M. Williams, J. Am. Chem. Soc., 2007, 129, 1987–
1995; (b) S. Burling, M. F. Mahon, R. E. Powell,
M. K. Whittlesey and J. M. J. Williams, J. Am. Chem. Soc.,
2006, 128, 13702–13703; (c) A. E. Ledger, M. F. Mahon,
M. K. Whittlesey and J. M. Williams, Dalton Trans., 2009,
6941–6947.
7 (a) F. E. Hahn, M. C. Jahnke and T. Pape, Organometallics,
2006, 25, 5927–5936; (b) H. Salem, M. Schmitt, U. Herrlich, 16 J. I. van der Vlugt, Eur. J. Inorg. Chem., 2012, 363–375.
E. Kuhnel, M. Brill, P. Nagele, A. L. Bogado, F. Rominger 17 D. Yuan, H. Tang, L. Xiao and H. V. Huynh, Dalton Trans.,
and P. Hofmann, Organometallics, 2013, 32, 29–46;
(c) O. Kaufhold, A. Stasch, P. G. Edwards and F. E. Hahn, 18 P. Elena, G. Mark, J. W. S. Linda, R. Haim, B.-D. Yehoshoa
Chem. Commun., 2007, 1822–1824; (d) E. M. Schuster, and M. David, Chem.–Eur. J., 2004, 10, 4673–4684.
M. Botoshansky and M. Gandelman, Dalton Trans., 2011, 19 (a) M. S. Rosen, C. L. Stern and C. A. Mirkin, Chem. Sci.,
2011, 40, 8788–8795.
40, 8764–8767.
2013, 4, 4193; (b) J. I. van der Vlugt, Eur. J. Inorg. Chem.,
2012, 363–375.
8 (a) K. M. Lee, J. C. C. Chen and I. J. B. Lin, J. Organomet.
Chem., 2001, 617–618, 364–375; (b) Y. Zhou, Z. Xi, W. Chen 20 (a) M. K. Samantaray, V. Katiyar, D. Roy, K. L. Pang,
and D. Wang, Organometallics, 2008, 27, 5911–5920;
(c) V. J. Catalano and A. L. Moore, Inorg. Chem., 2005, 44,
6558–6566; (d) E. Kluser, A. Neels and M. Albrecht, Chem.
Commun., 2006, 4495–4497; (e) X. Wang, S. Liu, L.-H. Weng
H. Nanavati, R. Stephen, R. B. Sunoj and P. Ghosh,
Eur. J. Inorg. Chem., 2006, 2975–2984; (b) M. K. Samantaray,
K. Pang, M. M. Shaikh and P. Ghosh, Inorg. Chem., 2008,
47, 4153–4165.
This journal is © The Royal Society of Chemistry 2014
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View Article Online
Paper
Dalton Transactions
21 C. Gandolfi, M. Heckenroth, A. Neels, G. Laurenczy and
M. Albrecht, Organometallics, 2009, 28, 5112–5121.
22 M. V. Jiménez, J. Fernández-Tornos, J. J. Pérez-Torrente,
and S. Kobayashi, Chem.–Asian J., 2008, 3, 196–214;
(e) N. Mizuno and K. Yamaguchi, Catal. Today, 2008, 132,
18–26.
F. J. Modrego, S. Winterle, C. Cunchillos, F. J. Lahoz and 28 (a) J. Zhang, M. Gandelman, L. J. W. Shimon,
L. A. Oro, Organometallics, 2011, 30, 5493–5508.
23 (a) R. A. Sheldon and I. W. C. E. Arends, in Transition
Metals for Organic Synthesis, Wiley-VCH Verlag GmbH,
H. Rozenberg and D. Milstein, Organometallics, 2004, 23,
4026–4033; (b) Y. Zhu, B. Zhao and Y. Shi, Org. Lett.,
2013, 15, 992–995.
2008, pp. 200–213; (b) M. Pagliaro, S. Campestrini and 29 (a) T. Naota, H. Takaya and S. I. Murahashi, Chem. Rev.,
R. Ciriminna, Chem. Soc. Rev., 2005, 34, 837–845;
(c) S.-I. Murahashi and N. Komiya, in Modern Oxidation
Methods, Wiley-VCH Verlag GmbH & Co. KGaA, 2005,
pp. 165–191.
1998, 98, 2599–2660; (b) A. Dijksman, A. Marino-González,
A. Mairata i Payeras, I. W. C. E. Arends and R. A. Sheldon,
J. Am. Chem. Soc., 2001, 123, 6826–6833.
30 (a) G. Rivera and R. H. Crabtree, J. Mol. Catal. A: Chem.,
2004, 222, 59–73; (b) C. Y. Liao, K. T. Chan, J. Y. Zeng,
C. H. Hu, C. Y. Tu and H. M. Lee, Organometallics, 2007,
26, 1692–1702.
24 (a) R. A. Sheldon and J. K. Kochi, in Metal Catalyzed Oxi-
dations of Organic Compounds, Academic Press, New York,
1981, pp. 350–382; (b) F. A. Luzzio and F. S. Guziec, Jr., Org.
Prep. Proced. Int., 1988, 20, 533; (c) T. Mallat and A. Baiker, 31 P. Marcé, C. Godard, M. Feliz, X. Yáñez, C. Bo and
Chem. Rev., 2004, 104, 3037–3058; (d) R. J. Taylor, M. Reid, S. Castillón, Organometallics, 2009, 28, 2976–2985.
J. Foot and S. A. Raw, Acc. Chem. Res., 2005, 38, 851–869; 32 (a) W. P. Fehlhammer, T. Bliss, U. Kernbach and
(e) T. T. Tidwell, Synthesis, 1990, 857–870; (f) J. Ni, W. J. Yu,
L. He, H. Sun, Y. Cao, H. Y. He and K. N. Fan, Green Chem.,
2009, 11, 756–759; (g) M. Uyanik and K. Ishihara, Chem.
Commun., 2009, 2086–2099; (h) H. Tomioka, K. Takai,
K. Oshima and H. Nozaki, Tetrahedron Lett., 1981, 22,
1605–1608; (i) A. M. Maione and A. Romeo, Synthesis, 1984,
955–957.
I. Brüdgam, J. Organomet. Chem., 1995, 490, 149–153;
(b) W. A. Herrmann, J. Schwarz, M. G. Gardiner and
M. Spiegler, J. Organomet. Chem., 1999, 575, 80–86;
(c) A. A. Danopoulos, A. A. D. Tulloch, S. Winston,
G. Eastham and M. B. Hursthouse, Dalton Trans., 2003,
1009–1015; (d) M. Heckenroth, A. Neels, H. Stoeckli-Evans
and M. Albrecht, Inorg. Chim. Acta, 2006, 359, 1929–1938;
(e) M. C. Jahnke, T. Pape and F. E. Hahn, Z. Naturforsch., B:
Chem. Sci., 2010, 65, 341–346; (f) D. Serra, P. Cao,
J. Cabrera, R. Padilla, F. Rominger and M. Limbach, Orga-
nometallics, 2011, 30, 1885–1895; (g) P. Cao, J. Cabrera,
R. Padilla, D. Serra, F. Rominger and M. Limbach, Organo-
metallics, 2012, 31, 921–929; (h) A. Raba, M. Cokoja,
S. Ewald, K. Riener, E. Herdtweck, A. Pöthig,
W. A. Herrmann and F. E. Kühn, Organometallics, 2012, 31,
2793–2800.
25 M. Shibuya, Y. Osada, Y. Sasano, M. Tomizawa and
Y. Iwabuchi, J. Am. Chem. Soc., 2011, 133, 6497–6500.
26 (a) D. R. Jensen, J. S. Pugsley and M. S. Sigman, J. Am.
Chem. Soc., 2001, 123, 7475–7476; (b) E. M. Ferreira and
B. M. Stoltz, J. Am. Chem. Soc., 2001, 123, 7725–7726;
(c) M. J. Schultz, C. C. Park and M. S. Sigman, Chem.
Commun., 2002, 3034–3035; (d) S. K. Mandal, D. R. Jensen,
J. S. Pugsley and M. S. Sigman, J. Org. Chem., 2003, 68,
4600–4603; (e) B. Z. Zhan and A. Thompson, Tetrahedron,
2004, 60, 2917–2935; (f) S. Mannam, S. K. Alamsetti and 33 (a) J. A. Love, M. S. Sanford, M. W. Day and R. H. Grubbs,
G. Sekar, Adv. Synth. Catal., 2007, 349, 2253–2258;
(g) H. Mizoguchi, T. Uchida, K. Ishida and T. Katsuki, Tetra-
hedron Lett., 2009, 50, 3432–3435; (h) R. Dileep and
B. R. Bhat, Appl. Organomet. Chem., 2010, 24, 663–666;
(i) C. Y. Han, M. Yu, W. J. Sun and X. Q. Yao, Synlett, 2011,
2363–2368; ( j) T. Kunisu, T. Oguma and T. Katsuki, J. Am.
Chem. Soc., 2011, 133, 12937–12939; (k) J. M. Hoover and
S. S. Stahl, J. Am. Chem. Soc., 2011, 133, 16901–16910;
(l) T. Nishii, T. Ouchi, A. Matsuda, Y. Matsubara,
Y. Haraguchi, T. Kawano, H. Kaku, M. Horikawa and
T. Tsunoda, Tetrahedron Lett., 2012, 53, 5880–5882;
J. Am. Chem. Soc., 2003, 125, 10103–10109; (b) W.-Z. Zhang,
R. He and R. Zhang, Eur. J. Inorg. Chem., 2007, 5345–5352;
(c) J. Maurer, M. Linseis, B. Sarkar, B. Schwederski,
M. Niemeyer, W. Kaim, S. Zalis, C. Anson, M. Zabel and
R. F. Winter, J. Am. Chem. Soc., 2008, 130, 259–268;
(d) M. Martín, H. Horváth, E. Sola, A. g. Kathó and F. Joó,
Organometallics, 2009, 28, 561–566; (e) Y. B. Lai, C. S. Lee,
W. J. Lin, A. R. Naziruddin and W. S. Hwang, Polyhedron,
2013, 53, 243–248; (f) A. R. Naziruddin, Z. J. Huang,
W. C. Lai, W. J. Lin and W. S. Hwang, Dalton Trans., 2013,
42, 13161–13171.
(m) S. S. Shen, V. Kartika, Y. S. Tan, R. D. Webster and 34 X. Liu, Q. Xia, Y. Zhang, C. Chen and W. Chen, J. Org.
K. Narasaka, Tetrahedron Lett., 2012, 53, 986–990. Chem., 2013, 78, 8531–8536.
27 (a) R. Sheldon, Green Chem., 2000, 2, G1–G4; 35 E. Galardon, M. Giorgi and I. Artaud, Dalton Trans., 2007,
(b) P. T. Anastas, L. B. Bartlett, M. M. Kirchhoff and 1047–1052.
T. C. Williamson, Catal. Today, 2000, 55, 11–22; 36 APEX, SAINT and SADABS, Bruker AXS Inc., Madison, WI,
(c) R. A. Sheldon and H. van Bekkum, in Fine Chemicals USA, 2004.
through Heterogeneous Catalysis, Wiley-VCH Verlag GmbH, 37 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystal-
2007, pp. 473–551; (d) T. Matsumoto, M. Ueno, N. Wang logr., 2008, 64, 112–122.
5342 | Dalton Trans., 2014, 43, 5335–5342
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