Oxidations and oxidative couplings catalyzed by triazolylidene ruthenium complexes

Article abstract of DOI:10.1021/om101145y

A series of "Ru^II(η⁶-arene)" complexes with 1,2,3-triazolylidene ligands have been prepared and fully characterized. The molecular structure of one of the new complexes has been determined by means of X-ray diffractrometry. The new complexes have been tested in a set of catalytic reactions involving alcohols and amines as substrates, including (i) base-free oxidation of benzylic alcohols to benzaldehydes, (ii) homocoupling of amines to form imines, and (iii) oxidative coupling of amines and alcohols to form amides. The results show the high versatility of the catalysts used and illustrate the application potential of 1,2,3-triazolylidene ligands in the design of effective homogeneous catalysts.

Full text of DOI:10.1021/om101145y

1162 Organometallics 2011, 30, 1162–1167
DOI: 10.1021/om101145y
Oxidations and Oxidative Couplings Catalyzed by Triazolylidene
Ruthenium Complexes
Amparo Prades,†,‡ Eduardo Peris,*,† and Martin Albrecht*,‡
†
´
ꢀ
ꢀ
Dpto. de Quımica Inorganica y Organica, Universitat Jaume I, Avda. Vicente Sos Baynat s/n, 12071
ꢀ
‡
Castellon, Spain, and School of Chemistry & Chemical Biology, University College Dublin, Belfield,
Dublin 4, Ireland
Received December 6, 2010
A series of “RuII(η6-arene)” complexes with 1,2,3-triazolylidene ligands have been prepared and
fully characterized. The molecular structure of one of the new complexes has been determined by
means of X-ray diffractrometry. The new complexes have been tested in a set of catalytic reactions
involving alcohols and amines as substrates, including (i) base-free oxidation of benzylic alcohols to
benzaldehydes, (ii) homocoupling of amines to form imines, and (iii) oxidative coupling of amines
and alcohols to form amides. The results show the high versatility of the catalysts used and illustrate
the application potential of 1,2,3-triazolylidene ligands in the design of effective homogeneous
catalysts.
Introduction
devoted to the development of oxidative cross-coupling
reactions between alcohols and amines to selectively form
imines6,7 or amides,8-13 because these types of car-
bon-nitrogen compounds are key functional groups of
peptides (amides)14 and also because of their versatile applic-
ability in laboratory and industrial synthetic processes
(amides and imines).15 Although the principles governing
the oxidation processes are common to alcohols and imines,
versatile homogeneous catalysts capable of reacting with
both types of substrates are rare, with Shvo’s catalyst being
one of the most representative examples in terms of high
efficiency and versatility.16,17
The formation of carbon-nitrogen bonds is one of the
most important transformations in organic chemistry.1
Homogeneous catalysis has provided effective methodolo-
gies to allow for the alkylation of amines by hydrogen-
borrowing strategies.2-5 In addition, great efforts have been
*To whom correspondence should be addressed: eperis@qio.uji.es
(E.P.); martin.albrecht@ucd.ie (M.A.).
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In the formation of amides by direct coupling between
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r
pubs.acs.org/Organometallics
Published on Web 02/10/2011
2011 American Chemical Society

Article
Organometallics, Vol. 30, No. 5, 2011 1163
a
Scheme 1.
a Reagents and conditions: (i) Ag2O, CH2Cl2, room temperature; (ii) [RuCl2(arene)]2, CH2Cl2, room temperature; (iii) Na2CO3, EtOH, room
temperature.
role.8,11-13,18 Notably, simple well-defined “(η6-arene)Ru-
(NHC)” complexes are among the best catalysts for this
transformation, which can be performed using a wide variety
of amines and alcohols.11 The use of NHC-based ligands
seems to bring a series of advantages to the development of
this catalytic reaction, because NHCs combine a number of
attractive features.19 First, the azolium ligand precursor is
stable toward oxidation.20 Second, the number of possible
coordination modes of the carbene to the metal, paired with
the large library of NHC-type ligands, may allow the elec-
tronic properties of the metal to be fine-tuned, with obvious
implications for catalytic outcomes. Third, the carbene
ligand displays a pronounced mesoionic character,21,22 thus
accommodating positive and negative partial charges in the
same heterocycle. These latter properties may be key for
coordinating metal centers in different oxidation states.23
Low-valent electron-rich centers may be stabilized by an
increased carbene-type neutral resonance contribution,
whereas the mesoionic contribution may prevail on coordi-
nation to an electron-poor metal in a high oxidation state,
resulting from a carbanionic metal binding site that is
charge-compensated intramolecularly by an iminium frag-
ment. Indeed, a number of applications have been disclosed,
predominantly in transfer hydrogenation under basic con-
ditions, which involves the oxidation of a sacrificial hydro-
gen donor.3,24
metal complexes.26 The new ligands were coordinated to a
wide set of metals, such as Ag, Rh, Ir, Ru, and Pd, and their
donor properties were estimated to compare well with the most
basic 2-imidazolylidenes, although being slightly less basic
than other abnormal carbenes.27 In a very recent report,
Bertrand and co-workers were able to isolate and determine
the X-ray molecular structure of one of these 1,2,3-triazoly-
lidenes,22 and these authors signaled its potential application in
nucleophilic organic catalysis. We thought that the new ligand
may have great potential for the development of new catalysts
with unprecedented ligand-induced reactivity patterns, a lar-
gely underdeveloped area.10,28 On the basis of our experience
in employing several “Ru(η6-arene)(NHC)”-derived catalysts
for the coupling of alcohols and amines,5 and because of the
extraordinary industrial interest in these reactions, we investi-
gated the potential of such abnormal carbenes as spectator
ligands for oxidation catalysts using alcohol and amine sub-
strates. Here we report on a first proof of principle of this
approach by using ruthenium(II) complexes comprising a
1,2,3-triazolium-derived carbene ligand as catalyst for the
selective oxidation of benzylic alcohols and amines and for
the oxidative coupling of alcohols and amines to form amides.
Results and Discussion
Synthesis of the Complexes. The new ruthenium(II) com-
plexes 2 comprising a mesoionic triazolylidene ligand were
prepared according to an established protocol from the
corresponding triazolium salt 1 (Scheme 1). This salt is avail-
able from butyl azide and hexyne through Cu-mediated click
chemistry and subsequent alkylation with MeI, according to
a synthetic method previously reported in our group.26 The
ruthenium center was installed via a transmetalation proce-
dure, involving a stepwise reaction of 1 with Ag2O followed
by the addition of [Ru(η6-arene)Cl2]2 (arene = p-cymene,
hexamethylbenzene) to afford complexes 2, as depicted in
Scheme 1.
Aiming to expand the family of abnormal N-heterocyclic
carbene ligands,25 we recently reported the preparation of
1,4-substituted 1,2,3-triazolium salts, which provided versatile
ligand precursors for the synthesis of new 1,2,3-triazolylidene
(18) Ghosh, S. C.; Hong, S. H. Eur. J. Org. Chem. 2010, 4266.
(19) (a) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus,
G. R. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371. (b) Herrmann,
W. A.; Kocher, C. Angew. Chem., Int. Ed. 1997, 36, 2163. (c) Bourissou,
D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39.
(d) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348.
(20) (a) Semeril, D.; Bruneau, C.; Dixneuf, P. H. Adv. Synth. Catal.
2002, 344, 585. (b) Dupont, J.; Spencer, J. Angew. Chem., Int. Ed. 2004,
43, 5296.
(21) (a) Iglesias, M.; Albrecht, M. Dalton Trans. 2010, 39, 5213. (b)
Araki, S.; Yokoi, K.; Sato, R.; Hirashita, T.; Setsune, J. J. Heterocycl.
Chem. 2009, 46, 164.
(22) Guisado-Barrios, G.; Bouffard, J.; Donnadieu, B.; Bertrand, G.
Angew. Chem., Int. Ed. 2010, 49, 4759.
Carbene-type bonding of the triazolylidene ligand was
indicated by the low-field resonance in the 13C NMR spec-
trum (δC 160.9 for 2a, 165.5 for 2b). The chemical shift dif-
ference may reflect the stronger electron-donating power of
hexamethylbenzene to ruthenium as compared to that of
p-cymene. Steric implications of the arene spectator ligand
€
(23) Lalrempuia, R.; McDaniel, N. D.; Muller-Bunz, H.; Bernhard,
1
S.; Albrecht, M. Angew. Chem., Int. Ed. 2010, 49, 9765.
(24) (a) Samec, J. S. M.; Backvall, J. E.; Andersson, P. G.; Brandt, P.
Chem. Soc. Rev. 2006, 35, 237. (b) Albrecht, M.; Miecznikowski, J. R.;
Samuel, A.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21,
3596. (c) Corberan, R.; Mas-Marza, E.; Peris, E. Eur. J. Inorg. Chem.
2009, 1700. (d) Normand, A. T.; Cavell, K. J. Eur. J. Inorg. Chem. 2008,
2781.
(25) (a) Albrecht, M. Chem. Commun. 2008, 3601. (b) Schuster, O.;
Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109,
3445.
are evident when considering the H magnetic resonances
due to the R-CH2 groups of the heterocycle substituents. In
2a, both the NCH2 and the carbon-bound CH2 appear each
(27) (a) Chianese, A. R.; Li, X. W.; Janzen, M. C.; Faller, J. W.;
Crabtree, R. H. Organometallics 2003, 22, 1663. (b) Heckenroth, M.;
Neels, A.; Garnier, M. G.; Aebi, P.; Ehlers, A. W.; Albrecht, M. Chem.
Eur. J. 2009, 15, 9375. (c) Yang, L.; Kruger, A.; Neels, A.; Albrecht, M.
Organometallics 2008, 27, 3161.
(26) Mathew, P.; Neels, A.; Albrecht, M. J. Am. Chem. Soc. 2008,
130, 13534.
(28) Grutzmacher, H. Angew. Chem., Int. Ed. 2008, 47, 1814.

1164 Organometallics, Vol. 30, No. 5, 2011
Prades et al.
Table 1. Catalyst Screening in the Oxidation of Benzyl Alcohola
entry
cat.
t (h)
yield (%)b
1
2
3
4
5
6
7
8
20
16
20
16
16
20
24
24
<2
>95
55
82
85
60
32
95
2a
2b
3a
3b
4
2ac
2ad
Figure 1. ORTEP representation of 2b (50% probability, hy-
drogens omitted for clarity). Selected bond lengths (A) and
angles (deg): Ru(1)-C(1) = 2.093(6), Ru(1)-Cl(1) = 2.4386(19),
Ru(1)-Cl(2) = 2.4377(19), Ru(1)-C(centroid) = 1.718(1); C(1)-
Ru(1)-Cl(1) = 87.11(19), C(1)-Ru(1)-Cl(2) = 90.44(18), Cl-
(1)-Ru(1)-Cl(2) = 86.08(7).
a General conditions: benzyl alcohol (0.2 mmol), catalyst (0.01 mmol)
in toluene (2 mL) at reflux. b Yields determined by 1H NMR (C6Me6 as
internal standard). c At 70 °C. d At 70 °C using 10 mol % of catalyst.
Catalytic Oxidation of Alcohols. Complexes 2 and 3,
together with the known NHC ruthenium complex 4,32 were
evaluated as catalyst precursors in the base- and oxidant-free
oxidation of alcohols. We decided to evaluate the catalytic
properties of compound 4 because of its steric similarity to 2a
(cf. Scheme 1). Hence, catalytic differences can be attributed
predominantly to the distinct electronic properties imparted
by the two different types of carbene ligands. The homo-
geneously catalyzed base-free dehydrogenation of alcohols is
a highly attractive process for the synthesis of reactive
carbonyl species from readily accessible alcohol substrates,
constituting a much more benign methodology than (typi-
cally used) Cr-mediated oxidations.29 Moreover, this process
generates molecular hydrogen as a most useful side pro-
duct.17,33
Oxidation reactions were carried out in refluxing toluene
under base-free conditions, using a catalyst loading of 5 mol
%. An initial test using BnOH as the model substrate
revealed the highest activity for complex 2a (Table 1). Full
conversion to benzaldehyde was observed within 16 h, while
the C6Me6 analogue 2b performed considerably worse. Sub-
stitution of the chlorides in the catalyst precursor for carbo-
nate had diverging effects. While in 3a the activity was
reduced as compared to 2a, a significant increase from
55% to 85% conversion was noted for the hexamethylben-
zene analogue 3b. Lowering the temperature to 70 °C still
provided high conversion, though increased catalyst loading
(10 mol %) and reaction times were required (compare
entries 7 and 8).
as one multiplet, while in 2b, the permethylated arene ring
induces a splitting of these resonances in four distinct multi-
plets. The diastereotopicity of these protons suggests a
sterically rigid conformation of the heterocycle and hindered
rotation about the heterocycle-CH2 and the Ctrz-Ru
bonds.
An X-ray diffraction analysis of single crystals of 2b
confirmed the connectivity pattern and the expected three-
legged piano-stool geometry of the complex (Figure 1). The
Ru(1)-C(1) bond length is 2.093(6) A and hence is slightly
longer in comparison to similar bonds in other “Ru(η6-
arene)(NHC)” complexes.5,29 The long bond may be a con-
sequence of the steric constraints imposed by the hexam-
1
ethylbenzene, as suggested by H NMR spectroscopy. The
chloride ligands and the triazolylidene occupy a regular face
of the ruthenium center, with L-Ru-L0 bond angles be-
tween 86.0 and 90.4°.
Anion metathesis with ethanolic Na2CO3 gave the corre-
sponding carbonate complexes 3 as air- and moisture-stable
yellow solids (Scheme 1). Direct evidence for the presence of
the carbonate group was provided by the new 13C NMR
resonance around 170 ppm and by the strong IR absorption
band at 1610 cm-1, as shown for other previously reported
1
“Ru(η6-arene)(NHC)(CO3)” complexes.30 In the H NMR
spectrum, an upfield shift of the NCH2 resonance (e.g., from
δH 4.61 in 2a to δH 4.34 in 3a) constitutes more evidence for
successful chloride substitution. In addition, the separation
of the multiplets in 3b is considerably less pronounced than
in 2b, which may be attributed to a substantial decrease of the
X-Ru-X bond angle from 86.1° in 2b (X=Cl) to an
estimated 60° in 3b.31 Accordingly, rotation should become
less hindered and the chemical environment of the two
hydrogens in each methylene group is indeed less disparate.
Variation of the substrate revealed the scope and limita-
tions of the best catalyst 2a (Table 2). Both primary and
secondary benzylic alcohols were oxidized readily into ben-
zaldehyde and acetophenone, respectively, with good con-
versions (entries 1 and 2). Aliphatic alcohols such as
2-phenylethanol and 1-octanol were not oxidized (entries 3
and 4). The electronic nature of the aryl group plays an
important role. Electron-withdrawing substituents reduced
the activity of the catalyst, although the conversions to the
final benzylic alcohols were still high (entries 5-7), while
(29) (a) Poyatos, M.; Mas-Marza, E.; Sanau, M.; Peris, E. Inorg.
Chem. 2004, 43, 1793. (b) Cariou, R.; Fischmeister, C.; Toupet, L.;
Dixneuf, P. H. Organometallics 2006, 25, 2126. (c) Ozdemir, I.; Demir,
S.; Cetinkaya, B.; Toupet, L.; Castarlenas, R.; Fischmeister, C.;
Dixneuf, P. H. Eur. J. Inorg. Chem. 2007, 2862. (d) Herrmann, W. A.;
Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R. J. Chem. Eur. J. 1996, 2,
772.
(30) (a) Azua, A.; Sanz, S.; Peris, E. Organometallics 2010, 29, 3661.
(b) Demerseman, B.; Mbaye, M. D.; Semeril, D.; Toupet, L.; Bruneau,
C.; Dixneuf, P. H. Eur. J. Inorg. Chem. 2006, 1174.
(32) Mercs, L.; Neels, A.; Albrecht, M. Dalton Trans. 2008, 5570.
(33) (a) Choi, J. H.; Kim, N.; Shin, Y. J.; Park, J. H.; Park, J.
Tetrahedron Lett. 2004, 45, 4607. (b) Junge, H.; Loges, B.; Beller, M.
Chem. Commun. 2007, 522. (c) Sieffert, N.; Buhl, M. J. Am. Chem. Soc.
2010, 132, 8056. (d) Friedrich, A.; Schneider, S. ChemCatChem 2009, 1,
72.
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2003, 22, 4744.

Article
Organometallics, Vol. 30, No. 5, 2011 1165
Table 2. Oxidation of Different Alcohols using 2aa
Table 4. Amide Formation from Alcohols and Aminesa
entry
R1
R2
t (h)
yield (%)b
entry
cat.
R1
R2
t (h)
yield (%)b
1
2
3
4
5
6
7
8
9
10
C6H5
C6H5
CH2C6H5
H
CH3
H
H
H
H
H
H
H
16
24
20
20
22
22
22
16
16
16
>95
>95
<5
<5
65
68
70
91
>95
90
1
2
3
4
5
2a
2b
4
4
4
CH2C6H5
CH2C6H5
CH2C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C5H11
15
15
15
20
20
68
40
>95
>95
>95
C7H15
4-C6H4-NO2
4-C6H4-Cl
2-C6H4-Cl
4-C6H4-Br
4-C6H4-CH3
4-C6H4-OCH3
C6H5
a General conditions: alcohol (0.2 mmol), amine (0.8 mmol), NaH
(0.04 mmol), catalyst (0.01 mmol) in toluene (2 mL) at reflux. b Yields
determined by 1H NMR (C6Me6 as internal standard).
H
a General conditions: alcohol (0.2 mmol), catalyst (0.01 mmol) in
toluene (2 mL) at reflux. b Yields determined by 1H NMR (C6Me6 as
internal standard).
effectively oxidized, albeit at a lower rate (entries 7 and 8).
The somewhat harsher conditions in amine oxidation as
compared to those for alcohol oxidation may be a conse-
quence of the stronger bonding of imines as opposed to
ketones to the ruthenium(II) center, thus insinuating that
product release from the metal coordination sphere could be
rate-limiting.
Table 3. Oxidative Homocoupling of Amines To Form Iminesa
Catalytic Amide Formation. As a consequence of the
catalytic activity of the complexes in both alcohol and amine
oxidative dehydrogenation, a combination of these two
processes conceivably provides a convenient protocol for
amide synthesis.10-13 Indeed, reaction of benzyl alcohol or
2-phenylethanol with benzylamine or n-hexylamine in the
presence of catalytic amounts of 2 or 4 and NaH gave the
corresponding amide (Table 4, entries 1-3). When complex
4 was used as the catalyst precursor, aliphatic and benzylic
substrates (both as amine or alcohol) were successfully
converted (entries 4 and 5). Interestingly, 2-phenylethanol
behaves as an excellent substrate for this type of coupling,
contrary to what may be suggested by the results shown in
Table 2, where the same substrate is inert toward oxidation
under base-free conditions. The difference in behavior in this
coupling reaction is probably due to the presence of base,
which obviously facilitates the oxidation of the primary
alcohol to the aldehyde in the step preceding the coupling
to the amine. Yields for the different catalyst precursors seem
to correlate with the activity in amine oxidation (cf. Table 3)
and compare well with recent studies using related Ru(NHC)
catalyst precursors.11,13 The absence of any imine in the
product mixture indicates efficient suppression of H2O
elimination from the postulated hemiaminal intermediate.
Presumably, the hemiaminal displays an enhanced stability
when bound to the ruthenium carbene unit, viz. intermediate
A (Figure 2), which is reminiscent of the carbonate com-
plexes 3. Subsequent β-hydrogen elimination to produce the
amide in the metal coordination sphere is apparently more
favorable than ligand dissociation, which would complete
the Schiff base reaction and would result in imine formation
via water elimination. Hydrogen migration from the carbon
in intermediate A to ruthenium is surmised to subsequently
yield a ruthenium dihydride species that has been proposed
as a further intermediate very recently by Hong and co-
workers in the same context.11,12
entry
cat.
R
t (h)
yield (%)b
1
2
3
4
5
6
7
8
9
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C5H11
C3H7
4-C6H4-Cl
20
20
20
20
20
12
22
22
22
<2
>95
>95
<5
<5
>95
70
2a
2b
3a
3b
4
4
4
4
62
90
a General conditions: amine (0.2 mmol), catalyst (0.01 mmol) in
toluene (2 mL) at 150 °C. b Yields determined by H NMR (decane as
internal standard).
1
neutral or electron-donating substituents gave conversions
of 90% or higher (entries 8-10).
Catalytic Oxidation of Primary Amines. In recent years
considerable attention has been devoted to the oxidation of
amines to imines.34,35 Extensive efforts have been made in
particular to develop catalytic systems that use green oxi-
dants for the synthesis of imines from primary amines. Due
to the promising catalytic behavior of complexes 2-4 in the
oxidation of alcohols, we were interested in seeing if an
extension of the catalytic activity to amines under oxidant-
free conditions was accessible, hence contributing to the
development of greener methods for imine production. Our
results are summarized in Table 3. At 150 °C, with a catalyst
loading of 5 mol %, and in the absence of an auxiliary base,
oxidative homocoupling of amines was observed, cleanly
affording the corresponding imines. The normal NHC com-
plex 4 was more active than the triazolylidene-based complex
2 and reached full conversion after 12 h (cf. 20 h for
complexes 2, entries 2-6). In contrast, the carbonate-con-
taining complexes were inactive (entries 4 and 5), perhaps
because the exchange of the carbonate ligand by an amine is
thermodynamically unfavored. Aliphatic amines were also
(34) (a) Chu, G. B.; Li, C. B. Org. Biol. Chem. 2010, 8, 4716. (b)
Landge, S. M.; Atanassova, V.; Thimmaiah, M.; Torok, B. Tetrahedron
Lett. 2007, 48, 5161. (c) Jiang, G.; Chen, J.; Huang, J. S.; Che, C. M. Org.
Lett. 2009, 11, 4568.
(35) Kodama, S.; Yoshida, J.; Nomoto, A.; Ueta, Y.; Yano, S.;
Ueshima, M.; Ogawa, A. Tetrahedron Lett. 2010, 51, 2450.
Conclusions
We have explored the catalytic properties of a series of
“Ru(η6-arene)(NHC)” (NHC = 1,2,3-triazolylidene, imi-
dazolylidene) complexes in various oxidations of alcohols

1166 Organometallics, Vol. 30, No. 5, 2011
Prades et al.
Synthesis of 2a. Silver oxide (107 mg, 0.46 mmol) was added to
a solution of 1 (150 mg, 0.46 mmol) in CH2Cl2 (20 mL). The
suspension was stirred at room temperature for 2 h under the
exclusion of light. The suspension was filtered through Celite,
and [Ru(p-cymene)Cl2]2 (140 mg, 0.23 mmol) was added. The
mixture was stirred at room temperature for 3 h. The suspension
was filtered through Celite, and the solvent was evaporated. The
crude solid was purified by column chromatography. Elution
with a CH2Cl2/acetone (9/1) mixture induced the separation of 2
as an orange band. Addition of cold Et2O induced the precipita-
tion of 2 as an orange solid (164 mg, 71%). 1H NMR (300 MHz,
CDCl3): δ 5.35, 5.03 (2 ꢀ d, 3JHH = 6.0 Hz, 2H, CHp-cym), 4.61
Figure 2. Schematic representation of the ruthenium-bound hemi-
aminal as intermediate in thesynthesis of amides catalyzed by 2.
and amines. The results demonstrate the catalytic potential
of 1,2,3-triazolylidene-based metal complexes and comple-
ment our previous studies on the coordination ability of
these types of abnormal NHC ligands. The catalytic activity
of the triazolylidene-based Ru complexes 2 and 3 has been
compared with that shown by the isostructural imidazolyli-
dene analogue 4. The triazolylidene complexes are more
effective than the imidazolylidene systems in the base-free
oxidation of alcohols, while this trend is inverted for the
oxidative homocoupling of amines and the coupling of
amines and alcohols to form amides. This work thus illus-
trates how subtle changes in the electron-donating properties
of spectator ligands may affect the catalytic activity of
Ru(η6-arene)(NHC) scaffolds. These changes may not have
the same effect in different although related transformations,
as seen here in the catalytic dehydrogneation of alcohols and
amines. Most significantly, the catalytic oxidations take
place in the absence of base and oxidants. Only few catalysts
have shown appreciable activity under such mild conditions
for the oxidation of alcohols, and we are unaware of similar
behavior toward amines.
(m, 2H, NCH2 n-Bu), 3.97 (s, 3H, NCH3), 2.98 (m, 2H, Ctrz
-
CH2 n-Bu), 2.94 (m, 1H, CHiso p-cym), 2.03 (s, 3H, CH3 p-cym), 1.99
(m, 2H, CH2 n-Bu), 1.56 (m, 2H, CH2 n-Bu), 1.48 (m, 4H,
3
CH2 n-Bu), 1.29 (d, JHH = 6.0 Hz, 6H, CH3 iso p-cym), 0.98 (m,
6H, CH3 n-Bu). 13C{1H} NMR (100 MHz, CDCl3): δ 160.9
(Ctrz-Ru), 147.2 (Ctrz-Bu), 106.9, 97.1 (2ꢀCp-cym), 84.9,
82.7 (2 ꢀ CHp-cym), 54.4 (NCH2 n-Bu), 36.3 (NCH3), 33.0
(CHiso
)
32.0, 30.7, 26.0, 23.1 (4 ꢀ CH2 n-Bu), 22.7-
pcym
(CH3 iso p-cym), 20.2 (CH2 n-Bu), 18.7 (CH3 p-cym), 13.9, 13.8
(2ꢀCH3 n-Bu). Electrospray MS (30 V): m/z: 466.1 [M - Cl]þ.
Anal. Calcd for RuCl2C21H35N3: (501.50): C, 50.29; H, 7.03;
N, 8.38. Found: C, 50.08; H, 7.13; N, 8.11.
Synthesis of 2b. Following the same procedure as described
for 2a, reaction of 1 (150 mg, 0.46 mmol) with Ag2O (107 mg,
0.46 mmol) in CH2Cl2 (20 mL) and subsequently with
[Ru(C6Me6)Cl2]2 (140 mg, 0.23 mmol) yielded 2b as an orange
solid (149 mg, 62%). 1H NMR (300 MHz, CDCl3): δ 4.75 (m,
1H, NCH2 n-Bu), 4.06 (m, 1H, NCH2 n-Bu), 3.96 (s, 3H, NCH3),
2.96 (m, 1H, Ctrz-CH2 n-Bu), 2.51 (m, 1H, Ctrz-CH2 n-Bu), 2.05
(m,1H, CH2n-Bu), 1,95 (s, 18H, Car-CH3), 1.87 (m, 1H, CH2n-Bu),
1.47 (m, 4H, CH2n-Bu), 0.96 (m, 6H, CH3n-Bu). 13C{1H} NMR
(100 MHz, CDCl3): δ 165.5 (Ctrz-Ru), 146.7 (Ctrz-Bu),
92.8 (Car-Me), 54.3 (NCH2n-Bu), 36.5 (NCH3), 33.2, 31.5,
26.5, 23.4, 20.6 (5ꢀCH2n-Bu), 15.6 (Car-CH3), 14.1, 14.0
(2 ꢀ CH3n-Bu). Electrospray MS (30 V): m/z: 494.1 [M - Cl]þ.
Anal. Calcd for RuCl2C23H39N3: (529.55): C, 52.17; H, 7.42; N,
7.94. Found: C, 52.15; H, 7.70; N, 7.99.
Synthesis of 3a. Sodium carbonate (42.6 mg, 0.4 mmol) was
added to a solution of 2a (40 mg, 0.08 mmol) in EtOH (15 mL).
The suspension was stirred at room temperature for 3 h. The
yellow solution was filtered through Celite, and the solvent was
removed. The solid was redissolved in CH2Cl2 (10 mL) and
filtered again. Concentration of the solution and subsequent
precipitation with cold Et2O gave 3a as a yellow solid (18 mg,
Experimental Section
General Procedures. The metal complexes [RuCl2(p-cymene)]2,36
[RuCl2(CMe)6]2,37 and 432 and 1,4-dibutyl-1,2,3-triazole26 were
prepared according to literature procedures. All other reagents
were commercially available and used without further purifi-
cation. NMR spectra were recorded on Varian Innova 300 and
400 MHz spectrometers at 298 K. Chemical shifts (δ) were
referenced to the residual protiated solvent signals and are
reported downfield of SiMe4. Electrospray mass spectra (ESI-
MS) were recorded on a Micromass Quatro LC instrument;
nitrogen was employed as the drying and nebulizing gas. Ele-
mental analyses were carried out on a EuroEA3000 Eurovector
analyzer.
Synthesis of 1,4-Dibutyl-3-methyl-1,2,3-triazolium Iodide (1).
To a solution of 1,4-dibutyl-1,2,3-triazole (1 g, 5.5 mmol) in
acetonitrile (7 mL) was added iodomethane (3.5 mL, 55 mmol)
and the mixture was stirred under microwave irradiation for 5 h
at 90 °C. The solvent was removed in vacuo. The oily residue was
washed with diethyl ether several times and dried to afford the
triazolium salt 1. Yield: 1.5 g (85%). 1H NMR (400 MHz,
CDCl3): δ 9.14 (s, 1H, CHtrz), 4.74 (t, 3J(H,H) = 6.00 Hz, 2H,
NCH2 n-Bu), 4.34 (s, 3H, NCH3), 2.98 (t, 3JHH = 6.00 Hz, 2H,
Ctrz-CH2 n-Bu), 2.06 (m, 2H, CH2 n-Bu), 1.80 (m, 2H, CH2 n-Bu),
1.47 (m, 2H, CH2 n-Bu) 0.99 (t, 3JHH = 6.00 Hz, 6H, CH3 n-Bu).
13C{1H} NMR (100 MHz, CDCl3): δ 144.6 (Ctrz-Bu), 129.2
(CHtrz), 54.0 (NCH2 n-Bu), 38.8 (CH3), 31.3, 39.0, 23.7, 22.2, 19.4
(5 ꢀ CH2 n-Bu), 13.6, 13.3 (2 ꢀ CH3 n-Bu). Electrospray MS (30
V): m/z: 196.3 [M - I]þ. Anal. Calcd for C11H22IN3(H2O):
(341.10): C, 38.72; H, 7.09; N, 12.31. Found: C, 38.59; H, 7.17;
N, 12.10.
45%). 1H NMR (400 MHz, CDCl3): δ 5.42, 5.06 (2 ꢀ d, 3JHH
=
8.0 Hz, 2H, CHp-cym), 4.34 (t, 3JHH = 8.0 Hz, 2H, NCH2 n-Bu),
3.96 (s, 3H, NCH3), 2.80 (m, 1H, Ctrz-CH2 n-Bu), 2.74 (m, 1H,
CHiso p-cym), 2.64 (m, 1H, Ctrz-CH2n-Bu), 2.07 (s, 3H, CH3p-cym),
2.03 (m, 1H, CH2n-Bu), 1.91 (m, 2H, CH2n-Bu), 1.73 (m, 1H,
3
CH2n-Bu), 1.46 (m, 4H, CH2n-Bu), 1.32 (d, JHH = 8.0 Hz, 6H,
CH3isop-cym), 1.00 (m, 6H, CH3n-Bu). 13C{1H} NMR (100 MHz,
CDCl3): δ 166.7 (CO3), 165.1 (Ctrz-Ru), 146.5 (Ctrz-Bu), 105.6,
94.3 (2 ꢀ Cp-cym), 82.7, 80.4 (2 ꢀ CHp-cym), 53.8 (NCH2n-Bu), 36.1
(NCH3), 32.2 (CHiso pcym) 31.9, 31.8, 25.4, 23.1 (4 ꢀ CH2n-Bu),
22.9 (CH3isop-cym), 20.2 (CH2n-Bu), 19.1 (CH3p-cym), 13.8, 13.7
(2 ꢀ CH3n-Bu). IR (KBr): ν = 1611 cm-1 (s; CdO). Electrospray
MS (30 V): m/z: 448.2 [M - CO3 þ OH]þ, 492.2 [M þ H]þ. Anal.
Calcd for RuC22H35N3O3 (490.60): C, 53.86; H, 7.19; N, 8.57.
Found: C, 53.61; H, 7.16; N, 8.48.
Synthesis of 3b. According to the procedure described for 3a,
complex 3b was obtained from 2b (53 mg, 0.1 mmol) and
Na2CO3 (42.6 mg, 0.4 mmol) in EtOH (15 mL) as a yellow solid
1
(39 mg, 75%). H NMR (400 MHz, CDCl3): δ 4.25 (m, 1H,
NCH2 n-Bu), 4.15 (m, 1H, NCH2 n-Bu), 3.95 (s, 3H, NCH3), 2.62
(m, 2H, Ctrz-CH2 n-Bu), 2.09 (s, 18H, Car-CH3), 1.84, 1.77 (2 ꢀ
m, 1H, CH2 n-Bu), 1.45 (m, 4H, CH2 n-Bu), 0.97 (m, 6H, CH3 n-Bu).
13C{1H} NMR (100 MHz, CDCl3): δ 169.0 (CO3), 166.8
(Ctrz-Ru), 146.2 (Ctrz-Bu), 91.6 (Car-Me), 53.8 (NCH2 n-Bu),
(36) Bennett, M. A.; Smith, A. K. J. Chem. Soc., Dalton Trans. 1974,
233.
(37) Bennett, M. A.; Huang, T. N.; Matheson, T. W.; Smith, A. K.
Inorg. Synth. 1982, 21, 74.

Article
Organometallics, Vol. 30, No. 5, 2011 1167
36.3 (NCH3), 32.0, 31.2, 25.5, 23.3, 20.4 (5 ꢀ CH2 n-Bu), 16.0
(Car-CH3), 13.9, 13.8 (2 ꢀ CH3 n-Bu). IR (KBr): ν 1602 cm-1
(s; CdO). Electrospray MS (30 V): m/z: 476.2 [M - CO3 þ OH]þ,
520.2 [M þ H]þ. Anal. Calcd for RuC24H39N3O3 (518.6): C, 55.38;
H, 7.58; N, 8.10. Found: C, 55.12; H, 7.70; N, 7.97.
General Procedure for Alcohol Oxidations. A mixture of
alcohol (0.2 mmol) and the ruthenium complex (0.01 mmol)
was refluxed in toluene (2 mL) for the time indicated in Table 1
or 2. The reaction mixture was analyzed by 1H NMR spectros-
copy. Hexamethylbenzene (0.033 mmol) was used in all cases as
internal standard in order to determine conversions and yields.
Products were identified according to commercially available
samples (benzaldehyde, acetophenone, phenylacetaldehyde, oc-
tanal, 4-nitrobenzaldehyde, 4-chlorobenzaldehyde, 2-chloro-
benzaldehyde, 4-bromobenzaldehyde, p-tolualdehyde, and
4-methoxybenzaldehyde).
copy using hexamethylbenzene (0.033 mmol) as internal stan-
dard. Products were identified according to previously reported
spectroscopic data (N-benzylphenylacetamide, N-benzylbenza-
mide, and N-hexylbenzamide).
X-ray Diffraction Studies. Single crystals of 2b were mounted
on a glass fiber in a random orientation. Data collection was
performed at room temperature on a Siemens Smart CCD
diffractometer using graphite-monochromated Mo KR radia-
tion (λ = 0.710 73 A) with a nominal crystal to detector distance
of 4.0 cm. Space group assignment was based on systematic
absences, E statistics, and successful refinement of the structure.
The structure was solved by direct methods with the aid of
successive difference Fourier maps and was refined using the
SHELXTL 6.1 software package.39 All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were assigned to
ideal positions and refined using a riding model. The diffraction
frames were integrated using the SAINT package.40 Further
crystallographic details are compiled in the Supporting Infor-
mation. In addition, CCDC 803583 contains 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.
General Procedure for Amine Oxidations. A mixture of amine
(0.2 mmol) and the ruthenium complex (0.01 mmol) was heated
in toluene (2 mL) to 150 °C in a thick-walled glass tube fitted
1
with a Teflon cap. The reaction mixture was analyzed by H
NMR spectroscopy using decane (0.2 mmol) as internal stan-
dard. Products were identified according to commercially avail-
able samples (N-benzylidenebenzylamine) or previously
reported spectroscopic data (N-hexylidenehexylamine,6 N-buty-
lidenebutylamine,38 and N-(4-chlorobenzylidene)-4-chloroben-
zylamine).35
General Procedure for the Formation of Amides. A mixture of
alcohol (0.2 mmol), amine (0.8 mmol), NaH (0.04 mmol), and
the ruthenium complex (0.01 mmol) was refluxed in toluene
(2 mL). The reaction mixture was analyzed by 1H NMR spectros-
€
Acknowledgment. We thank H. Muller-Bunz for assis-
tance with the crystal structure determination. We ac-
knowledge financial support from the MCINN of Spain
(CTQ2008-04460), Bancaixa (P1.1B2007-04), and the
European Research Council (StG 208561). We also thank
the MICINN for a fellowship (A.P.). We are grateful to
ꢀ
the Serveis Centrals d’Instrumentacio Cientıfica (SCIC)
´
of the Universitat Jaume I for providing us with X-ray
facilities.
(38) Bailey, P. S.; Carter, T. P.; Southwick, L. M. J. Org. Chem. 1972,
37, 2997.
(39) Sheldrick, G. M. SHELXTL, version 6.1; Bruker AXS, Inc.,
Madison, WI, 2000.
(40) SAINT, version 5.0; Bruker Analytical X-ray Systems, Madison,
Supporting Information Available: A CIF file giving crystal-
lographic data for complex 2b. This material is available free of
charge via the Internet at http://pubs.acs.org.
WI, 1998.