Palladium NCN and CNC pincer complexes as exceptionally active catalysts for aerobic oxidation in sustainable media

Article abstract of DOI:10.1039/c1gc15390k

The oxidation of secondary benzyl alcohols is catalyzed by two palladacycles at atmospheric pressure in PEG-400, a sustainable reaction media. Recycling of the active catalytic species is performed up to the 5th run, and catalyst loadings decreased down to 10^-8 mol%, thus achieving unprecedented TON and TOF values. In addition, the same conditions proved to be effective for the aerobic oxidation of benzyl methylene compounds, a scarcely explored process by palladium catalysts.

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PAPER
Palladium NCN and CNC pincer complexes as exceptionally active
catalysts for aerobic oxidation in sustainable media†‡
Garazi Urgoitia, Raul SanMartin,* Maria Teresa Herrero and Esther Dom´ınguez*
Received 11th April 2011, Accepted 9th May 2011
DOI: 10.1039/c1gc15390k
The oxidation of secondary benzyl alcohols is catalyzed by two palladacycles at atmospheric
pressure in PEG-400, a sustainable reaction media. Recycling of the active catalytic species is
performed up to the 5th run, and catalyst loadings decreased down to 10-8 mol%, thus achieving
unprecedented TON and TOF values. In addition, the same conditions proved to be effective for
the aerobic oxidation of benzyl methylene compounds, a scarcely explored process by palladium
catalysts.
relatively high amounts of palladium salts and/or complexes are
Introduction
required to perform the target transformation (0.1–10 mol% of
Since molecular oxygen is a cheap, safe and environmentally
Pd),12g,14 and problems arising from the removal of palladium
friendly oxidant, it has been increasingly used in common
traces from the final products still remain.15
oxidative processes, thus constituting an advantageous alterna-
In recent years our research on palladium pincer complexes
tive to most oxidants, often based on stoichiometric amounts
has dealt with their application to several palladium-catalyzed
of relatively harmful transition metals.1 Aerobic oxidation of
alcohols has been successfully applied to several types of
hydroxy derivatives, including 1,2-diols and polyols, a-hydroxy-
and alkoxy-carboxylic acids, simple aliphatic, allyl and benzyl
alcohols, inter alia.2 Due to the extensive use of aryl ketones
as intermediates for the synthesis of a wide number of phar-
transformations like Suzuki, Sonogashira, Heck, Hiyama and
ketone a-arylation.16 We envisioned that such palladacycles
could be effective catalysts not only for the above cross-
coupling reactions but also for oxidative processes, and to be
precise, for aerobic benzylic oxidation of secondary alcohols.
A major drawback to our proposal was the fact that, to
macological active, natural and other interesting compounds,
the best of our knowledge, no palladium pincer complex has
much effort has been devoted to the aerobic oxidation of benzyl
been used for aerobic oxidation so far. In this paper we
alcohols.3 In fact, the aforementioned safety and environmental
wish to present our most remarkable results in this context,
issues make oxygen an ideal oxidant for such substrates, thus
which also involve an additional application to oxidation of
overcoming other classical procedures employed in this context
methylene groups and the use of sustainable reaction media, as
(Swern,4 PCC, Jones and other Cr(VI) reagents,5 Dess–Martin
well.
periodinane,6 tert-butylhydroperoxide,7 MnO2 and other Mn
reagents,8 silver (I) oxidants,9 oxone,10 and others11). Therefore,
a series of transition metal catalysts (Zr, Mn, Cu, Zn, Ru, Rh,
Result and discussion
Pd, Pt)12,1 have been developed for this aerobic transformation
The pioneering reports by Peterson and Larock14e and other
from arylcarbinols to arylketones. Among all these catalytic
relevant works on this subject12g,14 were taken as a basis
systems, and despite a very recent report on Fe(III)-TEMPO-
to start our work, this time replacing common palladium
catalyzed oxidation,13 palladium catalysts have often shown a
salts (Pd(OAc)2), PdCl2, Pd(OCOCF3)2, etc.), ligands (py, (-)-
sparteine, bathophenanthroline, NEt3, etc.) and additives (MS
high efficiency in terms of lower catalyst loadings.14 Nevertheless,
˚
3–4 A) with our pincer complexes. NCN and CNC-type pincers
1 and 216a,b were elected as the most suitable catalysts for
the target transformation (Fig. 1). Besides, the synthesis of
both complexes was improved, thus overcoming reproducibility
problems.17
In addition, 1-phenylethanol was employed as model sub-
strate to undergo a range of oxidative conditions. Considering
our experience in palladium-catalyzed reactions conducted in
sustainable media,18 preference was given to such solvents rather
Kimika Organikoa II Saila, Zientzia eta Teknologia Fakultatea, Euskal
Herriko Unibertsitatea, PO Box 644, 48080, Bilbao, Spain.
E-mail: raul.sanmartin@ehu.es; Fax: +34 946012748;
Tel: +34946015435
† Dedicated to Professor Carmen Na´jera on the occasion of her 60th
birthday
‡ Electronic supplementary information (ESI) available: The improved
synthesis of palladium pincer complex 1 along with the spectroscopic
data of all aromatic ketones. See DOI: 10.1039/c1gc15390k
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Table 1 Selection of benzylic oxidation assays performed by pincer catalysts 1 and 2
Entry
i
1 (%)a
2 (%)a
1b
DMSO : H2O(1 : 1), 120 ◦C, 24h
—
—
—
—
—
—
—
—
2b
NaHCO3, DMSO : H2O(1 : 1), 120 ◦C, 24h
NaOAc, DMSO : H2O(1 : 1), 120 ◦C, 24h
NaOAc, DMSO : H2O(1 : 1), TBAB, 120 ◦C, 24h
NaOAc, H2O, 120 ◦C, 24h
c
c
3b
c
c
4b
c
c
5f
—
—
c
c
6b
NaOAc, ETG, 120 ◦C, 15h
86
98
93
92
56
95
82
99
37
88
92
—
—
—
—
—
—
—
50
92
80
98
68
7b
NaOAc, glycerol, 120 ◦C, 15h
8b
NaOAc, PEG 400, 120 ◦C, 24h
9b
KOtBu, PEG 400, 120 ◦C, 24h
10d
11d
12d
13e
14f
15f
16f
17b
18b
19d
20g
21h
22i
23b
24b
K2CO3, PEG 400, 120 ◦C, 30h
—
c
NaOAc, TBAB, PEG 400, 120 ◦C, 24h
NaOAc, Pivalic ac., PEG 400, 120 ◦C, 15h
NaOAc, PEG 400 : H2O (1 : 1), 120 ◦C, 48h
NaOAc, PEG 400 : H2O (0.1 : 0.9), 120 ◦C, 48h
NaOAc, PEG 400 : H2O (0.7 : 0.7), 120 ◦C, 48h
NaOAc, PEG 400:H2O(0.9 : 0.1), 120 ◦C , 48h
NaOAc, PEG 400, 80 ◦C, 24h
65
43
98
—
98
97
—
—
—
—
—
—
—
48
c
c
c
NaOAc, PEG 400, TBAB, 80 ◦C, 24h
NaHCO3, PEG 400, TBAB, 80 ◦C, 24h
NaOAc, PEG 400, 120 ◦C, 24h
c
c
c
c
c
c
Trident. ligands, NaOAc, PEG 400, 120 ◦C, 24h
Pd(OAc)2, NaOAc, PEG 400, 120 ◦C, 24h
NaOAc, PEG 400, 120 ◦C, 2 h
c
c
c
c
c
c
NaOAc, PEG 400, 120 ◦C, 13 h
a Determined by 1H NMR spectroscopy. Diethylene glycol dimethyl ether was used as internal standard. b 1.0 eq. of substrate, 0.1 eq. of base, 1 mL
of solvent per mmol of substrate, 1 atm of molecular oxygen, and 1 mol% of the pincer complex were employed. c Only the corresponding alkoxide
was observed. d 1.0 eq. of substrate, 0.1 eq. of base, 0.1 eq. of additive, 1 mL of solvent per mmol of substrate, 1 atm of molecular oxygen, and 1
mol% of the pincer. e 1.0 eq. of substrate, 0.5 eq. of base, 0.1 eq. of additive, 1 mL of solvent per mmol of substrate, 1 atm of molecular oxygen,
and 1 mol% of the pincer. f 1.0 eq. of substrate, 0.1 eq. of base, 1 mL of solvent per mmol of substrate, 1 atm of molecular oxygen, and 0.01 mol%
of the pincer. g 1.0 eq. of substrate, 0.1 eq. of base, 1 mL of solvent per mmol of substrate and 1 atm of molecular oxygen were employed. h Methyl
3,5-bis(pyrazol-1-ylmethyl)benzoate (1 mol%) and 2,6-bis(3-butylimidazolium-1-yl)isonicotinic acid (1 mol%) were added were added instead of
pincers 1 and 2, respectively. 1.0 eq. of substrate, 0.1 eq. of base, 1 mL of solvent per mmol of substrate, 1 atm of molecular oxygen and 0.01 mol%
of the palladium source were employed.
Other assays, including temperature decrease, and the use
of additives, cosolvents and other bases were also performed
in order to improve the reaction rates, but in all cases lower
yields and/or longer times were observed. A blank experiment
under the same conditions as those displayed in entry 8 but
in the absence of pincer complexes was performed, providing
starting material (entry 20). A similar result was obtained when
adding the non-palladated tridentate ligands or precursors of the
catalysts, methyl 3,5-bis(pyrazol-1-ylmethyl)benzoate and 2,6-
Fig. 1 Structure of palladium pincer complexes 1 and 2.
bis(3-butylimidazolium-1-yl)isonicotinic acid (entry 21). Inter-
estingly, the use of commercially available palladium sources like
palladium acetate provided negligible results (entry 22). Finally,
the reaction outcome was monitored at two shorter times (2
h and 13 h, entries 23 and 24 respectively). Although a more
accurate kinetic study is required, there seems to be a relatively
long induction time which could be due to catalyst activation.
Once the best conditions for the proposed transformation
were found, we decided to decrease the catalytic amounts of
complexes 1 and 2 (Table 2). An optimized catalyst loading
of 0.01 mol% was readily established as the most suitable for
further studies on the reaction scope, and as shown in Table 2, a
reduction to exceedingly small amounts of so effective catalysts
than commonly used organic solvents (PhMe, DMSO, CH3CN,
DCE). Table 1 displays a selection of assays performed by our
pincer catalysts 1 and 2. As shown in Table 1, initial experiments
in DMSO/water mixtures and in water provided negligible
results (entries 1–5). We next turned to monomeric polyol
solvents like ethylene glycol (ETG) and glycerol. Surprisingly,
target acetophenone was obtained in good yields (entries 6–
7). Polyethylene glycol is a high-boiling point, environmentally
friendly solvent currently employed in food and pharmacolog-
ical applications.19 To our delight, it turned out to be the most
suitable media, both in terms of yield and sustainability (entry 8).
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Table 2 Minimization of the amounts of catalysts 1 and 2
Table 3 (Contd.)
Entry
6d
Substrate
Product
1 (%)a
2 (%)a
Entry
[Pd] mol (%)
Yield 1 (%)a
Yield 2 (%)a
96
97
1b
2b
3b
4b
5b
6c
7d
8e
9f
1
0.1
0.01
0.001
0.0001
0.00001
0.000001
0.0000001
0.00000001
93
89
99
97
95
97
97
98
98
98
99
97
98
98
93
99
7c
8b
97
98
98
99
g
—
g
—
a Determined by 1H NMR spectroscopy. Diethylene glycol dimethyl
ether was used as internal standard. b 1.0 eq. of substrate, 0.1 eq. of
NaOAc, 1 mL per mmol of substrate and 1 atm of molecular oxygen were
used. Reaction time: 24 h. c Reaction time: 48 h. d Reaction time: 96 h.
e Reaction time: 120 h. f Reaction time: 168 h. g Only the corresponding
alkoxide was observed.
9b
95
90
97
95
10c
was successfully carried out. Both pincer complexes catalyzed
the target oxidation down to 10-6 mol% with excellent yield
(entries 1–7), and only in the last two cases (entries 6–7) longer
reaction times were required. Although complex 2 resulted
inactive in lower amounts, it was possible to further decrease
the loading of palladacycle 1 down to 10-8 mol% still with
excellent yield (entry 9). Even considering the required increase
of reaction times, TON and TOF values were 12 400 000 000 and
73 809 523 respectively, the highest ever achieved for any type of
catalyst in such oxidation processes.
11b
12b
80
70
74
93
Accordingly, the optimized reaction conditions were applied
to a number of commercially or readily20 available secondary
benzyl alcohols in order to extend the scope of the procedure.
Table 3 displays the excellent yields obtained in most cases,
with the exception of 2-methylbenzhydrol (entry 14). To ensure
13b
14b
99
50
97
55
Table 3 Oxidation of secondary benzylic alcohols
15b
16b
17b
96
97
91
94
98
92
Entry
1b
Substrate
Product
1 (%)a
2 (%)a
99
97
2b
3b
4c
5b
97
98
98
99
98
a Determined by 1H NMR spectroscopy. Diethylene glycol dimethyl
ether was used as internal standard. b 1.0 eq. of substrate, 0.1 eq. of
NaOAc, 1 mL of solvent per mmol of substrate, 1 atm of molecular
oxygen and 0.01 mol% of pincer complex were used. Reaction time:
48h. c Reaction time: 72h. d Reaction time: 96h.
94
>99
98
complete conversion rates, reaction time was increased to 48 h,
but in some cases longer times were required for the same
purpose. Fortunately, the reported protocol proved effective for
a variety of alcohols regardless electronic and steric effects.
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been described so far,23a–f and just one report on the oxidation
of methylene compounds by means of a sole palladium catalyst
can be found in the literature.23f The same oxidation procedure
employed for the oxidation of secondarybenzylic alcohols (Table
2, entry 3) were initially applied to ethylbenzene. On the basis of
the excellent result obtained (93–98% yield for acetophenone,
Table 4, entry 1), a number of methylene compounds were
submitted to equivalent reaction conditions.
Table 4 Oxidation of benzylic methylene compounds
Entry
1b
Substrate
Product
1 (%)a
2 (%)a
93
98
The results are displayed in Table 4, showing that not
only diactivated (diarylated) methylene positions (entries 4–7),
but also monoarylated methylene groups (entries 1–3) can be
efficiently oxidized to the corresponding ketones. Ketone, ester
and ether functions were tolerated, and an analogous trend to
that observed in alcohol oxidation regarding the need for longer
reaction times was also noticed in some cases. The extension of
the scope of this advantageous oxidation methodology to other
substrates is currently under investigation.
2e
3c
4b
98
92
76
99
89
77
Conclusions
5d
6c
7b
90
60
98
95
60
94
Two palladium pincer complexes have been for the first time
used as catalysts for aerobic oxidation reactions at atmospheric
pressure. Excellent yields and functional group tolerability
have been observed in two oxidative processes applied to
benzyl substrates, secondary alcohol oxidation and oxidation of
methylene compounds, leading to aromatic ketones. In addition,
both processes have been conducted in an environmentally
friendly, sustainable solvent such as PEG-400, and recycling of
the reaction media containing the active catalytic species has also
been possible up to the fifth run. Moreover, outstanding TON
and TOF values that surpass those of all the palladium catalytic
systems employed in this context by 7 orders of magnitude
have been achieved by these exceptionally active pincer-type
catalysts.
a Determined by 1H NMR spectroscopy. Diethylene glycol dimethyl
ether was used as internal standard. b 1.0 eq. of substrate, 0.1 eq. of
NaOAc, 1 mL of solvent per mmol of substrate, 1 atm of molecular
oxygen and 0.01 mol% of pincer complex were used. Reaction time:
24 h. c Reaction time: 48 h. d Reaction time: 72 h. e Reaction time: 96 h.
Experimental
The next step in the evaluation of catalysts 1 and 2 was their
recycling or reuse. Taking the oxidation of 1-phenylethanol to
acetophenone as a model reaction (Table 3, entry 1), different
work-up procedures were assayed in order to optimize an
efficient reuse that would avoid further treatment of the com-
plexes. Instead of the standard addition of water and extraction
with diethyl ether, a low-temperature extraction (-78 ◦C)21 was
finally used, thus providing an effective catalyst separation from
final ketone products. After this work-up, several runs were
performed with the same PEG media containing the active
catalytic species. Quantitative yields were obtained for both
catalysts up to the fifth run, and it was possible to perform a
last sixth run with palladacycle 2, although with a slightly lower
yield (87%).
Such encouraging results in aerobic oxidation of benzyl
alcohols prompted us to explore an even more interesting trans-
formation, the direct aerobic oxidation of benzylic methylene
compounds. Indeed, although several oxidizing systems have
been reported for this reaction (KMnO4, iodine(V) species,
tBuOOH, m-CPBA, TBHP, inter alia),22 the aerobic version
has been scarcely explored.23 Moreover, to the best of our
knowledge, only three examples using palladium catalysts have
General procedure for the aerobic oxidation of alcohols
In a round bottom flask (25 mL) the alcohol (1 mmol), NaOAc
(0.1 eq.), 0.01 mol% of complex 1 or 2 (0.01 mol% of Pd), and
PEG 400 (1 mL per mmol of the substrate) were added at room
temperature. The system was purged with molecular oxygen,
an oxygen-filled balloon was connected and the reaction was
heated to 120 ◦C under stirring for 24 h. The reaction outcome
1
was monitored by H NMR. Upon completion, the mixture
was cooled to room temperature and water was added (15 mL
aprox.). The resulting solution was extracted Et2O (4 ¥ 6 mL)
and the combined organic layers were washed with brine (1 ¥
10 mL), dried over anhydrous Na2SO4 and evaporated in vacuo
to provide a residue that was analyzed by 1H NMR. Diethylene
glycol dimethyl ether was used as internal standard.
The application of this general procedure to the series of
benzyl alcohols provided the corresponding ketones displayed
in Table 3. See ESI‡ for characterization details
Catalyst loading assays were performed as shown in Table 2.
The same procedure was employed, just reducing the amount of
complex 1 or 2 down to 10-8 mol%.
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General procedure for the recycling
5 (a) M. Hunsen, Tetrahedron Lett., 2005, 46, 1651–1653; (b) C. Wiles,
P. Watts and S. J. Haswell, Tetrahedron Lett., 2006, 47, 5261–5264;
(c) M. Z. Kassaee, S. Z. Sayyed-Alangi and H. Sajjadi-Ghotbabadi,
Molecules, 2004, 9, 825–829.
6 D. B. Dess and J. C. Martin, J. Org. Chem., 1983, 48, 4155–
4156.
In a round bottom flask (25 mL) the alcohol (1 mmol), NaOAc
(0.1 eq.), 0.01 mol% of complex 1 or 2 (0.01 mol% of Pd), and
PEG 400 (1 mL per mmol of the substrate) were added at room
temperature. The system was purged with molecular oxygen,
an oxygen-filled balloon was connected and the reaction was
heated to 120 ◦C under stirring for 24 h. The reaction outcome
7 K. Krohn, I. Vinke and H. Adam, J. Org. Chem., 1996, 61, 1467–
1472.
8 S. Azizian, A. Eftekhari-Bafrooei and H. Bashiri, Kinet. Catal., 2010,
51, 244–249.
9 M. M. Najafpour, M. Halynska, M. Aminim, S. H. Kazemi, T. Lis
and M. Bagherzadeh, Polyhedron, 2010, 29, 2837–2843.
10 M. S. Yusubov, A. A. Zagulyaeva and V. V. Zhadankin, Chem.–Eur.
J., 2009, 15, 11091–11094.
1
was monitored by H NMR. Upon completion, the mixture
was cooled to room temperature and diethyl ether was added
(10 mL). Then the mixture was further cooled to -78 ◦C the
phases were decanted. This extracting protocol was repeated
three times, the combined diethyl ether layers were washed
with brine (1 ¥ 10 mL), dried over anhydrous Na2SO4 and
evaporated in vacuo to provide a residue that was analyzed by
1H NMR. Diethylene glycol dimethyl ether was used as internal
standard.
11 (a) B. Hinzen and S. V. Ley, J. Chem. Soc., Perkin Trans. 1, 1997,
1, 1907–1908; (b) F. Nikaidou, H. Ushiyama, K. Yamaguchi, K.
Yamashita and N. Mizuno, J. Phys. Chem., 2010, 14, 10873–10880.
12 (a) T. Yasu-eda, S. Kitamura, N. Ikenaga, T. Miyake and T. Suzuki, J.
Mol. Catal. A: Chem., 2010, 323, 7–15; (b) A. Cecchtto, F. Fontana,
F. Minisci and F. Recupero, Tetrahedron Lett., 2001, 42, 6651–
6653; (c) I. E. Marko´, A. Gutier, I. Chelle´-Regnaut, P. R. Giles,
M. Tsukazaki, C. J. Urch and S. M. Brown, J. Org. Chem., 1998, 63,
7576–7577; (d) P. Muthupandi and G. Sekar, Tetrahedron Lett., 2011,
52, 692–695; (e) K. Yamaguchi, J. W. Kim, J. He and N. Mizuno, J.
Catal., 2009, 268, 343–349; (f) K. Kaizuka, H. Miyamura and S.
Kabayashi, J. Am. Chem. Soc., 2010, 132, 15096–15098; (g) D. R.
Jensen, J. S. Pugsley and M. S. Sigman, J. Am. Chem. Soc., 2001, 123,
7475–7476.
13 C. X. Miao, J. Q. Wang, B. Yu, W. G. Cheng, J. Sun, S. Chanfreu, L.
N. He and S. J. Zhang, Chem. Commun., 2011, 47, 2697–2699.
14 (a) B. P. Buffin, J. P. Clarkson, N. L. Belitz and A. Kundu, J. Mol.
Catal. A: Chem., 2005, 225, 111–116; (b) E. M. Ferreira and B.
M. Stoltz, J. Am. Chem. Soc., 2001, 123, 7725–7726; (c) I. W. C.
E. Arends, G. J. Brink and R. A. Sheldon, J. Mol. Catal. A: Chem.,
2006, 251, 246–254; (d) T. Yasu-eda, R. Se-ike, N. Ikenaga, T. Miyake
and T. Suzuki, J. Mol. Catal. A: Chem., 2009, 306, 136–142; (e) K. P.
Peterson and R. C. Larock, J. Org. Chem., 1998, 63, 3185–3189; (f) T.
Nishimura, T. Onoue, K. Ohe and S. Uemura, J. Org. Chem., 1999,
64, 6750–6755; (g) D. R. Jensen, M. J. Schultz, J. A. Mueller and
M. S. Sigman, Angew. Chem., Int. Ed., 2003, 42, 3810–3813; (h) M.
J. Schultz, C. C. Park and M. S. Sigman, Chem. Commun., 2002,
3034–3035.
General procedure for the aerobic oxidation of methylene
compounds
In a round bottom flask (25 mL) the methylene compound
(1 mmol), NaOAc (0.1 eq.), 0.01 mol% of complex 1 or 2
(0.01 mol% of Pd), and PEG 400 (1 mL per mmol of the
substrate) were added at room temperature. The system was
purged with molecular oxygen, an oxygen-filled balloon was
connected and the reaction was heated to 120 ◦C under stirring
1
for 24 h. The reaction outcome was monitored by H NMR.
Upon completion, the mixture was cooled to room temperature
and water was added (15 mL aprox.). The resulting solution
was extracted with Et2O (4 ¥ 6 mL) and the combined organic
layers were washed with brine (1 ¥ 10 mL), dried over anhydrous
Na2SO4 and evaporated in vacuo to provide a residue that was
analyzed by 1H NMR. Diethylene glycol dimethyl ether was used
as internal standard.
15 B. A. Steinhoff and S. S. Stahl, J. Am. Chem. Soc., 2006, 128, 4348–
4355.
The application of this general procedure to the series of
benzylic methylene compounds displayed in Table 4 provided
the corresponding ketones with the yields displayed. See ESI‡
for characterization details.
16 (a) F. Churruca, R. SanMartin, B. Ine´s, I. Tellitu and E. Dom´ınguez,
Adv. Synth. Catal., 2006, 348, 1836–1840; (b) F. Churruca, R.
SanMartin, I. Tellitu and E. Dom´ınguez, Synlett, 2005, 20, 3116–
3120; (c) F. Churruca, R. SanMartin, I. Tellitu and E. Dom´ınguez,
Tetrahedron Lett., 2006, 47, 3233–3237; (d) I. Moreno, R. SanMarin,
M. T. Herrero and E. Dom´ınguez, Inorg. Chim. Acta, 2010, 363,
1903–1911.
17 The synthesis of NCN pincer complex 1 was improved by modifying
the amount of NBS (8 equiv.) and the reaction time (31 h) of
the first step of its preparation, the bromination of methyl 3,5-
dimethylbenzoate. In addition, the work-up of the second step was
also modified. For more details see ESI‡.
18 (a) B. Ine´s, R. SanMartin, F. Churruca, E. Dom´ınguez, M. K. Urtiaga
and M. I. Arriortua, Organometallics, 2008, 27, 2833–2839; (b) B.
Ine´s, R. SanMartin, M. J. Moure and E. Dom´ınguez, Adv. Synth.
Catal., 2009, 351, 2124–2132.
19 J. Chen, S. K. Speat, J. G. Huddleston and R. D. Rogers, Green
Chem., 2005, 7, 64–82.
20 Sodium borohydride reduction of deoxybenzoin and benzoin pro-
vided 1,2-diphenylethanol and 1,2-diphenylethane-1,2-diol, respec-
tively. See: M. Nakatsuji, Y. Hata, T. Fujihara, K. Yamamoto,
M. Sasaki, H. Takekuma, M. Yoshihara, T. Minematsu and S.
Takekuma, Tetrahedron, 2004, 60, 5983–6000.
21 Diethyl ether was added to the reaction mixture and then it was
cooled to -78 ◦C, and layers decanted. For more details, see: C.
Chandrasekhar, N. K. Reddy and V. P. Kumar, Tetrahedron Lett.,
2010, 51, 3623–3625.
22 (a) M. Zhou, N. D. Schley and R. H. Crabtree, J. Am. Chem. Soc.,
2010, 132, 12550–12551; (b) M. S. Yusubov, V. N. Nemykin and V.
V. Zhadankin, Tetrahedron, 2010, 66, 5745–5752; (c) M. Moghadam,
V. Mirkhani, S. Tangestaninejad, I. M. Baltork and H. Kargar, J.
Acknowledgements
This research was supported by the University of the Basque
Country/Basque Government (Projects GIC10/52/IT-370-10
and S-PC10UN10) and the Spanish Ministry of Science and
Innovation (CTQ2010-20703). G.U. thanks the University of
the Basque Country (UPV/EHU) for a predoctoral scholarship.
The authors also thank Petronor, S.A. for generous donation
of hexane. Finally, technical and human support provided
by SGIker (UPV/EHU, MICINN, GV/EJ, ESF) is gratefully
acknowledged.
Notes and references
1 L. Liu, B. B. Wayland and X. Fu, Chem. Commun., 2010, 46, 6353–
6355.
2 T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037–
3058.
3 H. F. VanBorcklin, S. M. Hanrahan, J. D. Enas, E. Nandanan and J.
P. O’Neil, Nucl. Med. Biol., 2007, 34, 109–116.
4 K. Omura and D. Swern, Tetrahedron, 1978, 34, 1651–1660.
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 2161–2166 | 2165
©

View Article Online
Mol. Catal. A: Chem., 2008, 288, 116–124; (d) Z. Lounis, A. Riahi,
F. Djafri and J. Muzart, Appl. Catal., A, 2006, 309, 270–272; (e) B.
Karami and M. Montazerozohori, Molecules, 2006, 11, 720–725;
(f) S. J. Singh and R. V. Jayaram, Catal. Commun., 2009, 10, 2004–
2007; (g) R. Miyilmurugan, H. Stoechli-Evans, E. Suresh and M.
Palaniandavar, Dalton Trans., 2009, 5101–5114.
Tada, T. Miura and A. Iron, Tetrahedron Lett., 2010, 51, 4576–4578;
(c) Y. Matsushita, K. Sugamoto, Y. Iwakiri, S. Yoshida, T. Chaen and
T. Matsui, Tetrahedron Lett., 2010, 51, 3931–3934; (d) M. Navarro,
A. Escobar, V. R. Landaeta, G. Visbal, F. Lopez-Linares, M. L. Luis
and A. Fuentes, Appl. Catal., A, 2009, 363, 27–31; (e) M. Miyamoto,
Y. Minami, Y. Ukaji, H. Kinoshita and K. Inomata, Chem. Lett.,
1994, 1149–1152; (f) M. Islam, P. Mondal, A. S. Roy and K. Tuhina,
J. Appl. Polym. Sci., 2010, 118, 52–62.
23 (a) F. Rajabi, R. Luque, J. H. Clark, B. Karimi and D. J. Macquarrie,
Catal. Commun., 2011, 12, 510–513; (b) T. Nobuta, S. Hirashima, N.
2166 | Green Chem., 2011, 13, 2161–2166
This journal is
The Royal Society of Chemistry 2011
©