Amide synthesis from alcohols and amines catalyzed by a RuII-N-heterocyclic carbene (NHC)-carbonyl complex

Article abstract of DOI:10.1016/j.jorganchem.2013.12.051

Treatment of [Ru₂(CO)₄(CH₃CN)₆](BF₄)₂ with 3-methyl-1-(pyridin-2-yl)-imidazolium bromide in the presence of tetrabutylammonium bromide at room temperature in dichloromethane affords a Ru^II-N-heterocyclic carbene-carbonyl complex [Ru(py-NHC)(CO)₂Br₂] (1). Catalyst 1 displays diverse substrate scope for phosphine-free acceptorless coupling between alcohols and amines to amides at low catalyst loading. A Ru^II-dihydride/Ru⁰ sequence is proposed in the catalytic cycle.

Full text of DOI:10.1016/j.jorganchem.2013.12.051

Journal of Organometallic Chemistry xxx (2014) 1e7
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Amide synthesis from alcohols and amines catalyzed by a RuIIeN-
heterocyclic carbene (NHC)ecarbonyl complexq
*
Biswajit Saha, Gargi Sengupta, Abir Sarbajna, Indranil Dutta, Jitendra K. Bera
Department of Chemistry and Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208 016, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Treatment of [Ru2(CO)4(CH3CN)6](BF4)2 with 3-methyl-1-(pyridin-2-yl)-imidazolium bromide in the
presence of tetrabutylammonium bromide at room temperature in dichloromethane affords a RuIIeN-
heterocyclic carbeneecarbonyl complex [Ru(py-NHC)(CO)2Br2] (1). Catalyst 1 displays diverse substrate
scope for phosphine-free acceptorless coupling between alcohols and amines to amides at low catalyst
loading. A RuII-dihydride/Ru0 sequence is proposed in the catalytic cycle.
Received 7 December 2013
Received in revised form
26 December 2013
Accepted 27 December 2013
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
Amidation
Catalysis
Dehydrogenative coupling
N-Heterocyclic carbene
Metalemetal bond
1. Introduction
an ensemble of [Ru(COD)Cl2]n, imidazolium chloride as NHC pre-
cursor, phosphine ligand (PCy3) and base KOtBu was employed [6].
Amide bonds are key chemical components in both natural and
synthetic systems. Biopolymer proteins and numerous synthetic
polymers feature amides linkages [1]. The conventional strategy for
amide bond formation involves reactions of activated carboxylic
derivatives with amines [2]. Alternative strategies are also devel-
oped [3] but these reactions lack atom economy generating equi-
molar amounts of chemical waste. We developed a Rh(I) based
bifunctional catalyst for amide synthesis via nitrile hydration [4].
This catalyst is particularly effective for small nitriles such as
acrylonitrile. Transition metal catalyzed amide synthesis strategies
directly from alcohols and amines are receiving considerable at-
tentions in recent years. This reaction does not require the use of
hydrogen acceptors and two molecules of hydrogen are the only
by-products making it a true green process (Scheme 1).
Milstein’s RuePNN pincer complex (Scheme 2, a) was the first
reported metaleligand cooperating catalyst for amide synthesis
directly from alcohols and primary amines without the use of any
base or hydrogen acceptor. The PNN ligand shuttles between
dearomatized and aromatized forms favoring hydrogen abstraction
and liberation in the catalytic cycle [5]. Madsen introduced RueN-
heterocyclic carbene (NHC) system for amidation reaction. Initially,
Williams et al. employed [Ru(p-cymene)Cl2]2, phosphine ligand
bis(diphenylphosphino)butane (dppb) and Cs2CO3 for the synthesis
of secondary amides in presence of excess hydrogen acceptor 3-
methyl-2-butanone [7]. Hong’s catalytic system involved [Ru(p-
cymene)Cl2]2, NHC precursor, pyridine or acetonitrile and strong
base NaH. To unravel the nature of the in situ generated active
catalyst, well defined RueNHC catalysts of the type (p-cymene)
Ru(NHC)X2 (X ¼ Cl, I) (Scheme 2, b) were synthesized and their
catalytic potential were evaluated. Only in the presence of two
equivalents of strong base, these catalysts showed activity that
match with in situ generated RueNHC system [8].
Madsen’s catalytic system needed equimolar amounts of addi-
tive PCy3 for its activity. This was attributed to the labile nature of p-
cymene under catalytic condition. In absence of phosphine, the
catalyst looses molecular integrity and gives poor conversions. The
requirement of expensive, air and moisture sensitive phosphine
compounds possesses a serious disadvantage. We proposed to
replace p-cymene by strongly bound ligands. Accordingly, [Ru(py-
NHC)(CO)2Br2] (1) (py-NHC ¼ 3-methyl-1-(pyridin-2-yl)-imidazol-
2-ylidene) containing two carbonyls and a pyridine functionalized
NHC was synthesized and its catalytic activity for the acceptorless
dehydrogenative coupling of alcohols and amines is examined.
Although the chloro analog of 1 was reported earlier [9], we offer an
oxidative route for high yield synthesis of 1 from metalemetal
bonded diruthenium(I) precursors [Ru2(CO)4(CH3CN)6](BF4)2 and
imidazolium bromide derivative.
q
For the special issue ‘ISOC-2013, Camerino’.
* Corresponding author.
E-mail address: jbera@iitk.ac.in (J.K. Bera).
0022-328X/$ e see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.12.051
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B. Saha et al. / Journal of Organometallic Chemistry xxx (2014) 1e7
Scheme 1. Synthesis of amides from alcohols and amines.
2. Results and discussion
resonate at 202.9 and 195.3 ppm. The ESI-MS exhibits signal at m/z
495, assigned for [1 þ H2O]þ (Fig. S1).
The synthesis of [Ru(py-NHC)(CO)2Cl2] was reported earlier via
transmetallation of the corresponding AgeNHC complex with
[RuCl2(CO)2]n [9]. Several synthetic strategies are developed over
the years for accessing metaleNHC compounds [10]. Albrecht et al.
reported a RhIII bis-carbene complex from Rh2(OAc)4 [11]. We
pursued a methodology that involves oxidative cleavage of the
metalemetal singly-bonded dimetal compound with imidazolium
salt [12]. Thus far only [RuIeRuI] and [RhIIeRhII] precursors are
successfully employed to obtain RuIIeNHC and RhIIIeNHC com-
pounds (Scheme 3). A general pattern has emerged from this work
e 1. Unbridged dimetal complexes appear to give the desired
products; 2. The metal oxidation number increases by one unit; 3.
Only one bidentate NHC is incorporated for Ru irrespective of the
equivalents of imidazolium used whereas two NHC units could be
incorporated for Rh precursor. Initial studies were restricted with
naphthyridine functionalized NHC. Herein we show that the same
protocol is applicable for pyridine derivative (Scheme 4). A tenta-
tive mechanism involves imidazolium CeH oxidative addition
across the RueRu bond followed by electrophilic activation of the
second imidazolium CeH finally resulting in the elimination of a
molecule of dihydrogen. Towards elucidation of the mechanism, we
have shown oxidative heteroaryl CeH/Br addition to [PdIePdI]
room temperature leading to bi- and trinuclear PdII compounds
[13].
The catalytic potential of 1 was evaluated for dehydrogenative
coupling between alcohol and amine. A reaction of 1 mmol benzyl
alcohol and 1.2 mmol of benzylamine in presence of 1 mol% of
catalyst 1 and 5 mol% NaH in toluene for 24 h at 110 ꢀC yielded N-
benzylbenzamide as the single product in 84% yields. Different
bases were screened, the result of which is summarized in Table 1.
No reaction occurred in absence of base whereas strong bases
KOtBu, KOH and NaH were found effective. Use of weak base DABCO
afforded the corresponding imine in high yields.
Substrate scope was examined under optimized conditions
(1 mol% catalyst 1,1:1.2 mol of alcohol:amine, 5 mol% NaH, at 110 ꢀC
in toluene for 24 h). Electron rich p-methoxybenzyl alcohol gave
excellent conversions of the corresponding amides with benzyl-
amine (92%, Table 2, entry 1), p-methylbenzylamine (90%, entry 2),
cyclohexylamine (90%, entry 3), hexylamine (89%, entry 4). The p-
methylbenzyl alcohol and benzyl alcohol afforded good yields with
different amines (69e90%, entries 6e14). Electron deficient p-
nitrobenzyl alcohol and p-fluorobenzyl alcohol gave relatively
lower yields compared to electron rich alcohols (entries 15e17).
Reaction of 2-phenylethanol with benzylamine provided 79%
amide (entry 18). Long chain alcohols octanol and hexanol afforded
lesser yield compared to benzyl alcohol when treated with ben-
zylamine and hexylamine (entries 19e22). With 2-
ethoxymethanol, benzylamine provided 68% amide (entry 23).
Aniline converted to corresponding amides when reacted with p-
methoxybenzyl alcohol (78%), p-methylbenzyl alcohol (72%) and
benzyl alcohol (69%) (entries 5, 9 and 14). This is in contrast to
Madsen’s report which showed poor conversions for aniline even at
higher temperatures. The amidation could also be carried out in an
Treatment of [Ru2(CO)4(CH3CN)6](BF4)2 with 3-methyl-1-(pyr-
idin-2-yl)-imidazolium bromide in the presence of tetrabuty-
lammonium bromide (TBABr) at room temperature in
dichloromethane afforded 1 in 85% yield. The transformation is
marked by the concomitant oxidation of RuI to RuII suggesting
oxidative homolytic cleavage of the RueRu bond. Despite apparent
mechanistic complexity of the reaction, this method affords clean
products without needing base or harsher reaction conditions.
Molecular structure of the complex 1, depicted in Fig. 1, reveals a
central Ru with one chelate bound pyridine carbene (C^N), two cis-
oriented carbonyls and two trans bromides. The Ru1eC2 and Ru1e
intramolecular fashion as seen by the formation of
by using 4-amino-1-butanol (entry 24).
g-butyrolactam
The working proposal is that the catalyst at first dehydrogenates
alcohol to aldehyde. The amine then attacks the metal-coordinated
aldehyde to form hemiaminal. Subsequent metal-catalyzed dehy-
drogenation gives amide [15]. However, the real mechanism does
not appear to be that straightforward. When p-methoxy benzal-
dehyde is directly used with benzylamine, 80% conversion was
observed with 70% of corresponding imine and 30% amide. Addi-
tion of 10% p-methoxy benzyl alcohol in the reaction remarkably
improved the amide conversion to 76% with 24% imine. [8e] A
model amidation reaction in tolueneed8 did not afford deuterated
product which ruled out isotope scrambling from solvent. Reaction
ꢀ
ꢀ
N1 bond distances are 2.087(7) A and 2.146(6) A. A carbonyl resides
at site trans to the carbene carbon which is reflected in longer RueC
distance compared to the other carbonyl (Ru1eC10/C11 ¼1.957(7)/
ꢀ
1.863(8) A). Compound 1 was further characterized from NMR
spectra. 1H chemical shifts are indicative of a single isomer with
trans-Br/cis-CO configuration [14]. A characteristic 13C signal at
d
183.8 ppm is attributed to carbene carbon. Two carbonyl carbons
of benzyl alcohol-a,a-d2 with benzylamine led to hydrogen
scrambling in the product (Scheme 5) [16]. The occurrence of sig-
nificant isotope scrambling is indicative of an oxidative addition-
reductive elimination sequence in the catalytic cycle [17]. Accord-
ingly, a tentative mechanism is proposed that is outlined in Scheme
6. The metal catalyst forms a dihydride [Ru]H2 on reaction with
alcohols producing aldehydes. Reductive elimination of H2 affords a
reactive Ru0 species which dehydrogenates hemiaminal, generated
by the reaction of amine with metal-coordinated aldehyde, to
afford amide and dihydride species [Ru]H2. Alcohols play an
important role in maintaining the [Ru]H2 form of the active catalyst
that is crucial for the overall efficiency of the catalyst.
It is assumed that
b-hydride is the key step during alcohol to
Scheme 2. Two amide producing catalysts.
aldehyde conversion. Hammett studies provide significant insight
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B. Saha et al. / Journal of Organometallic Chemistry xxx (2014) 1e7
3
Scheme 3. Synthesis of MeNHC complexes from metalemetal bonded compounds.
A straight line could be generated only with sD with a negative
slope (
r
¼ ꢁ1.16) (Fig. 2). This suggests generation of positive
charge at alcohol benzylic position supporting the proposition of
hydride elimination for alcohol to aldehyde conversion.
b-
3. Conclusion
Scheme 4. Synthesis of 1.
A RuIIeNHC complex is accessed from metalemetal bonded
RuIeRuI precursor. The title compound catalyzes direct amide
synthesis from alcohols and amines at low catalyst loading. The
activity is similar to Madsen’s RuIIep-cymene based catalysts
without requiring additives like phosphine. Further experiments
suggest the involvement of RuII-dihydride/Ru0 cycle in the catalytic
process. Efforts are on to extend this chemistry with [RhIIeRhII] and
[PdIePdI] to access RhIII and PdII NHC compounds and apply these
for catalytic transformation reactions.
on the reaction pathway. Benzyl alcohol and para-substituted
benzyl alcohols (X ¼ NMe2, OMe, Me, F) were reacted with ben-
zylamine and the products were monitored by GCeMS at regular
time intervals (Scheme 7). Plot of ln(c0/c) for substituted benzyl
alcohols against the same values for that of benzyl alcohol gave a
linear relationship (Table S2). Relative reactivity (kx/kH) could be
calculated from the slope of the straight line (Fig. S2). Four such
values obtained from four different para substituents were plotted
against all possible
s , , s$) available in literature [18].
values (sD sL
4. Experimental section
4.1. General procedures
All reactions with metal complexes were carried out under an
atmosphere of purified nitrogen using standard Schlenk-vessel and
vacuum line techniques. NMR spectra were obtained on JEOL JNM-
LA 500 MHz spectrometer. 1H NMR chemical shifts were referenced
to the residual hydrogen signal of the deuterated solvents. The
chemical shift is given as dimensionless
d values and is frequency
referenced relative to residual solvent for 1H and 13C NMR spec-
troscopy. Elemental analyses were performed on a Thermoquest
EA1110 CHNS/O analyzer. The crystallized compounds were
powdered, washed several times with dry petroleum ether and
dried in vacuum for at least 48 h prior to elemental analyses. ESI-
MS were recorded on a Waters Micromass Quattro Micro triple-
quadrupole mass spectrometer. Infrared spectra were recorded in
the range 4000e400 cmꢁ1 on a Vertex 70 Bruker spectrophotom-
eter on KBr pellets.
4.2. Materials
Solvents were dried by conventional methods, distilled under
nitrogen and deoxygenated prior to use. RuCl3$xH2O was pur-
chased from Arora Matthey, India. All other chemicals were pur-
Fig. 1. Molecular structure (50% probability thermal ellipsoids) of 1 with important
atoms labeled. Hydrogen atoms are omitted for the sake of clarity. Selected bond
chased
from
SigmaeAldrich.
3-Methyl-1-(pyridin-2-yl)-
ꢀ
lengths (A) and angles (deg): Ru1eC2 2.087(7), Ru1eN1 2.146(6), Ru1eBr1 2.533(1),
imidazolium bromide and [Ru2(CO)4(CH3CN)6](BF4)2 were synthe-
sized according to the published procedures [19,20].
Ru1eBr2 2.533(1), Ru1eC10 1.957(7), Ru1eC11 1.863(8), C10eO1 1.095(8), C11eO2
1.105(9). C2eRu1eN1 76.3(2), C11eRu1eC10 87.9(3), C5eN1eC9 118.0(6).
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4
B. Saha et al. / Journal of Organometallic Chemistry xxx (2014) 1e7
Table 1
Base screening for amidation reaction.a
Entry
Base
Conversion (%)b
1
2
3
4
5
KOtBu
NaH
KOH
e
78
84
74
4
DABCO
84c
a
1 mmol benzyl alcohol, 1.2 mmol benzylamine, 5 mol% base, 1 mol% catalyst, 110 ꢀC in toluene for 24 h.
Conversions are determined by GC with dodecane as internal standard.
N-Benzylidene benzylamine is formed.
b
c
Table 2
Amide formation from alcohol and amine by catalyst 1.a
Entry
1
Alcohol
Amine
Amide
Yieldb (%)
92
2
3
90
90
4
5
89
78
6
7
90
87
8
82
9
72
84
10
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B. Saha et al. / Journal of Organometallic Chemistry xxx (2014) 1e7
5
Table 2 (continued )
Entry
Alcohol
Amine
Amide
Yieldb (%)
81
11
12
80
78
13
14
15
16
69
75
66
17
18
78
79
19
77
20
21
72
77
22
23
70
68
45
24
e
a
1 mmol alcohol, 1.2 mmol amine, 5 mol% NaH, 1 mol% catalyst 1 at 110 ꢀC in toluene for 24 h.
Yields are determined by GCeMS with dodecane as internal standard.
b
4.3. Synthesis of 1
6 h. Resulting red solution was concentrated under reduced pres-
sure and diethyl ether was added to induce precipitation. The red
solid was washed with diethyl ether and dried under vacuum.
Crystals suitable for X-ray diffraction were grown by layering pe-
troleum ether over a concentrated dichloromethane solution of the
compound. Yield: 110 mg (85%). Anal Calcd. for RuC11H9N3O2Br2: C,
3-Methyl-1-(pyridin-2-yl)-imidazolium bromide (65 mg,
0.272 mmol) and TBABr (88 mg, 0.272 mmol) were added to a
dichloromethane solution of [Ru2(CO)4(CH3CN)6](BF4)2 (100 mg,
0.136 mmol) and the mixture was stirred at room temperature for
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6
B. Saha et al. / Journal of Organometallic Chemistry xxx (2014) 1e7
Scheme 5. Amidation of benzyl alcohol-a,a-d2 with benzylamine by 1.
Fig. 2. Hammett plot for the amidation of different para-substituted benzyl alcohols.
The frames were indexed, integrated, and scaled using the SMART
and SAINT software packages [21], and the data were corrected for
absorption using the SADABS program [22]. The structure was
solved and refined with the SHELX suite of programs [23].
A single crystal of 1 of dimensions 0.21 ꢂ 0.23 ꢂ 0.31 mm3 was
covered in Paratone oil, mounted on top of a thin glass fiber with
silicone grease and placed in a cold stream of nitrogen. All non-
hydrogen atoms were refined with anisotropic thermal parame-
ters. Hydrogen atoms were included into geometrically calculated
positions in the final stages of the refinement and were refined
according to ‘riding model’. Diamond 3.1e software was used to
produce the diagram (50% probability thermal ellipsoids).
Scheme 6. Proposed mechanism for amide formation.
27.75; H, 1.90; N, 8.83. Found: C, 27.71; H, 1.94; N, 8.86. ESI-MS, m/z:
495, [1 þ H2O]. 1H NMR (500 MHz, DMSO-d6, 298 K):
d 9.21 (d, 1H,
3JHeH ¼ 5.5 Hz, HPy), 8.49 (s, 1H, Im), 8.38 (t,1H, 3JHeH ¼ 8.5 Hz, HPy),
3
3
8.27 (d, 1H, JHeH ¼ 8.5 Hz, HPy), 7.99 (s, 1H, Im), 7.68 (t, 1H, JHe
¼ 6.5 Hz, HPy), 3.97 (s, 3H, CH3). 13C NMR (DMSO-d6, 125 MHz,
H
Acknowledgments
298 K): d 202.9 (CO), 195.3 (CO), 183.8 (NCimN), 156.8 (C, Cim), 153.7
(C, Cim),147.9 (C, Cpy),131.2 (C, Cpy),128.0 (C, Cpy),122.6 (C, Cpy),117.7
This work is financially supported by the Department of Science
and Technology (DST), India. J.K.B. thanks DST for the Swarnajayanti
fellowship. AS thanks UGC, BS and ID thank CSIR and GS thanks IITK
for fellowships.
(C, Cpy), 35.7 (C, CH3). IR (KBr, cmꢁ1): (CO) 2060 and 1996 cmꢁ1
n
.
4.4. General procedure for amide reaction
1 mmol alcohol, 1.2 mmol amine, 5 mol% NaH and 1 mol%
catalyst 1 were introduced successively in Schlenk tube and the
mixture was heated at 110 ꢀC in toluene for 24 h. A small aliquot
was taken from the mixture, dissolved in 5 mL EtOAc, filtered
through a silica column and subjected to GCeMS analysis. Con-
versions were determined by GCeMS (uncorrected GC areas) with
respect to the internal standard dodecane.
Appendix A. Supplementary material
CCDC 974211 contains 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.
Appendix B. Supplementary data
4.5. Details of X-ray data collection and refinements
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.12.051.
Single-crystal X-ray studies were performed on a CCD Bruker
SMART APEX diffractometer equipped with an Oxford Instruments
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Please cite this article in press as: B. Saha, et al., Journal of Organometallic Chemistry (2014), http://dx.doi.org/10.1016/j.jorganchem.2013.12.051