Gold catalysis: In situ EXAFS study of homogeneous oxidative esterification

Article abstract of DOI:10.1002/chem.200903450

Gold complexes were prepared and investigated as catalysts for the oxidative esterification of aldehydes. Stabilisation by pyridine ligands gave good conversions and the in situ extended X-ray absorption fine structure (EXAFS) study of the reactions indicated that the reaction mixtures contained only mononuclear gold species. Thus, this is the first proof for a homogeneous gold-catalysed oxidation reaction ; the presence of nanoparticles could be excluded experimentally.

Full text of DOI:10.1002/chem.200903450

DOI: 10.1002/chem.200903450
Gold Catalysis: In Situ EXAFS Study of Homogeneous Oxidative
Esterification**
A. Stephen K. Hashmi,*[a] Christian Lothschꢀtz,[a] Martin Ackermann,[a] Renꢁ Doepp,[a]
Sankaran Anantharaman,[b] Benjamin Marchetti,[b] Helmut Bertagnolli,*[b] and
Frank Rominger[a]
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Chem. Eur. J. 2010, 16, 8012 – 8019

FULL PAPER
Abstract: Gold complexes were prepared and investigated as catalysts for the oxi-
dative esterification of aldehydes. Stabilisation by pyridine ligands gave good con-
versions and the in situ extended X-ray absorption fine structure (EXAFS) study
of the reactions indicated that the reaction mixtures contained only mononuclear
gold species. Thus, this is the first proof for a homogeneous gold-catalysed oxida-
tion reaction; the presence of nanoparticles could be excluded experimentally.
Keywords: esters
spectroscopy · gold · oxidation ·
pyridines
·
EXAFS
Introduction
latter could be responsible for the detected oxidation activi-
ty, as known from heterogeneous gold catalysis. Thus, our
work is aimed at the first proof of a truly homogeneous oxi-
dation catalysed by mononuclear gold complexes.
In the last few years, gold-catalysed organic transformations
have become important tools to build up molecular diversi-
ty. Most of the reported reactions deal with cycloisomerisa-
tions or the activation of alkynes and allenes towards nucle-
ophilic attack.[1] In contrast, there are only a few examples
of gold-catalysed homogeneous oxidations. In 2005, Shi
et al. reported the oxidation of alcohols to aldehydes and
ketones with a homogeneous gold catalyst and oxygen as
the oxidant.[2] In another publication they described the
gold-catalysed oxidative cleavage of double bonds.[3] Addi-
tionally, Yuan and Bian published the oxidation of sulfides
to sulfoxides by reductive hydrolysis of AuIII sulfide com-
plexes generated in situ.[4] They derived a mechanism based
upon kinetic studies, as well as upon investigations from the
Hill group.[5,6] Furthermore, there have been publications
that dealt with oxidative-coupling reactions.[7] Inspired by a
number of heterogeneous gold-catalysed oxidations, we
began to investigate homogeneous oxidations catalysed by
gold complexes.[8]
Results and Discussion
We selected to study a reaction of broad interest, namely
oxidative esterification. In an initial screening, we used p-
methoxybenzaldehyde, n-butanol and tert-butyl hydroperox-
ide (TBHP) as a test system with a series of different AuIII
salts (Figure 1). Of the catalyst systems tested, compounds
In our opinion, one of the most interesting reactions with
regard to synthesis is the direct conversion of aldehydes and
alcohols to the respective esters.[9] Hence, our aim was to es-
tablish a homogeneous gold-catalysed version of this reac-
tion. A series of experiments with the “homogeneous” gold
catalysts discussed above, in our hands, all showed a massive
precipitation of elemental gold, even when conducted by
several different co-workers. Thus, we were unconvinced as
to whether these “homogeneous” oxidations are really cata-
lysed by soluble mononuclear gold complexes. Many of the
reactions may form gold nanoparticles in situ and then the
Figure 1. Picture of reaction flasks after the reaction has taken place with
different catalyst systems (% yield). Left: stabilised conditions (pyridines
1–4; I–IV). Right: non-stabilised conditions (V–VII). Reaction condi-
tions: HAuCl4 (4.50 mol%), pyridine compound (9 mol%), aldehyde
(1.00 mmol), n-butanol (3.00 mmol), TBHP (1.50 mmol). For conditions
VII 10 mol% of catalyst was used.
[a] Prof. Dr. A. S. K. Hashmi, Dipl.-Chem. C. Lothschꢁtz,
Dipl.-Chem. M. Ackermann, Dipl.-Chem. R. Doepp, Dr. F. Rominger
Institute of Organic Chemistry, University of Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
Fax : (+49)6221-544205
of AuI and AuIII that contained ligands such as phosphines,
phosphites and mono- and bidentate nitrogen-donor ligands
delivered poor results and were accompanied by the forma-
tion of gold-black precipitate. The use of pyridine additives
as stabilising agents led to a successful outcome and substi-
tution at the 2-position turned out to be the most effective
(Scheme 1).
E-mail: hashmi@hashmi.de
[b] S. Anantharaman, Dipl.-Chem. B. Marchetti, Prof. Dr. H. Bertagnolli
Institut of Physical Chemistry, University of Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Fax : (+49)711-6856-4443
With the use of air or oxygen, auto-oxidation to the acid
was observed. Other oxidants, such as hydrogen peroxide or
cumene hydroperoxide, were inferior to TBHP. Following
these findings, we extended the AuIII-catalysed oxidative
E-mail: h.bertagnolli@ipc.uni-stuttgart.de
[**] EXAFS=extended X-ray absorption fine structure.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200903450.
Chem. Eur. J. 2010, 16, 8012 – 8019
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A. S. K. Hashmi, H. Bertagnolli et al.
InBr3/TBHP. In both cases, the reaction conditions are
[10]
harsh, either the reaction medium contains 70% HClO4
or the reaction is conducted at 1008C.[11] Thus, our method
delivers a similar yield of 5a to the literature, but our reac-
tion conditions are much milder, hence, a higher tolerance
of functional groups is granted. Another comparison can be
made for the synthesis of 5e (Table 1, entry 6). Once again
100% yield has been reported after catalysis with V2O5/
H2O2 (4 mol%).[10] More representative are the values re-
ported for reaction with Oxone in DMF at RT (19%
Scheme 1. Pyridines used as stabilising agents.
esterification to the conversion of either an aldehyde and an
alcohol or two alcohols into the respective esters. Unfortu-
nately, the use of two different alcohols led to inseparable
mixtures of products, therefore, we decided to focus on the
coupling of aldehydes and alcohols (Table 1). The reactions
took place at relatively mild temperatures and could be run
without solvent in most cases. Electron-rich and neutral aro-
matic aldehydes delivered good to excellent yields (Table 1,
entries 1–6). A comparison of our synthesis of 5a (Table 1,
yield),[12] with palladium/phosphinous acid/PhSi
ACTHUNTRGNE(UGN OMe)3/tet-
rabutylammonium fluoride (TBAF) (61% yield)[13] and with
copper perchlorate/TBHP/InBr3 at 1008C (68% yield).[11]
For this compound, our yield is the best reported so far and
our reaction conditions are the mildest, which guarantees a
high functional group tolerance and avoids exotic oxidants,
such as PhSiACTHUNRGTNEUNG(OMe)3. The formation of 5a by our method
entry 1) with the literature[10–13] shows that, in one case,[10]
a
can be directly compared with heterogeneous catalysis by a
TiO2-supported gold catalyst[9a,14] (90 versus 79%). Other
examples of gold-catalysed oxidation delivered aldehydes
and only traces of esters,[9b] or carboxylic acids.[9c] In recent
years, gold catalysts had proven to be superior to other
metals in these heterogeneous oxidations.[1a,15]
yield of 100% was reported for catalysis of the reaction
with V2O5/H2O2 (4 mol%). In a further case,[11] 91% yield
of 5a was obtained with 5 mol% [CuACHTNUTRGNE(UNG ClO4)2]/5 mol%
Table 1. Formation of n-butyl esters 5 from various aldehydes.
The use of 1 equiv of TBHP delivered comparable results
to the use of 1.5 equiv (Table 1, entry 4 versus 5), which in-
dicated that oxidation of the alcohol is much slower than
transformation of the hemiacetal formed in situ to the ester.
The reaction of 2,4,6-trimethylbenzaldehyde (Table 1,
entry 3) shows that even sterically hindered aldehydes can
be transformed into the respective esters in good yield. For
this transformation to 5c we achieved a yield of 78%, one
of the highest reported in literature.[16] When electron-defi-
cient benzaldehydes were used (Table 1, entries 8 and 9),
acetal formation caused a problem.
Entry
1
R
Product
Yield [%][a]
90
5a
2
3
5b
5c
74
78
We also tested the variability of the alcohol component.
Whereas primary alcohols furnished good results (Table 2,
entries 1–3), the increased steric demand of secondary alco-
hols and the weaker nucleophilicity diminished the isolated
4
5
6
5d
5d
5e
72
66[b]
68
Table 2. Formation of esters 6 from 2-naphthaldehyde.
7
8
5 f
5g
32
48
Entry
R
Product
Yield [%][a]
1
2
6a
6b
87[b]
76
3
4
5
6
6c
6d
6e
6 f
78
9
5h
5i
60[c]
45[b]
55
10
47[d]
42
Typical conditions: HAuCl4 (4.50 mol%),
3
(9 mol%), aldehyde
(1.00 mmol), alcohol (3.00 mmol), TBHP (1.50 mmol), 12 h, 55–758C.
[a] Isolated yield. [b] 1.00 mmol of TBHP was used. [c] Product could not
be isolated in pure form. [d] Product is a 10:1 mixture of trans and cis
isomers.
Typical conditions: HAuCl4 (4.50 mol%),
(1.00 mmol), alcohol (3.00 mmol), TBHP (1.50 mmol), 12 h. [a] Isolated
yield. [b] Reaction required CH3CN (1.00 mL) as solvent.
3
(9 mol%), aldehyde
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Homogeneous Oxidative Esterification
FULL PAPER
yields (Table 2, entries 4–6). Comparison with previously re-
ported yields is only possible for 6a (Table 2, entry 1). The
palladium/phosphinous acid/PhSiACTHNUTRGNENUG(OMe)3/TBAF conditions
were reported to deliver 92% yield, but required the use of
expensive silane oxidant and 1 equiv of fluoride.[13]
For combinations of aliphatic aldehydes with previously
tested alcohols (MeOH, nBuOH), the yields were compara-
ble to the examples in which aromatic aldehydes were used.
However, as a result of the low polarity of the esters gener-
ated from MeOH, we could only determine the yields by
GC analysis. To alleviate problems with purification of the
crude products, we used ethylene glycol as the alcohol com-
ponent to generate polar products (Table 3). The lower
Figure 2. Crystal structure of [AuCl4]ꢀ[(1H)]+. With exception of the pyr-
idinium proton all hydrogen atoms are omitted for clarity.
Table 3. Formation of 2-hydroxyethyl esters 7 from aliphatic aldehydes.
drate formation with unlabelled water that emerges from
the catalytic reaction can explain the loss of isotopic purity.
With respect to the nature of the catalyst, we decided to
use X-ray absorption spectroscopy (XAS) methodologies to
prove the absence of nanoparticles as catalytic species. From
X-ray absorption near-edge structure (XANES) and linear-
combination XANES (LC-XANES) analysis information
about the oxidation state of gold at the Au LIII edge and the
possible mixtures of different species with varying oxidation
states can be obtained.[20] LC-XANES analysis on the spec-
tra of HAuCl4 (VIII) and AuCl (IX), as well as the reaction
components X–XIII, was performed (see Tables 4 and 5). In
the XANES spectrum of AuIII (curve IX), a peak that is su-
perimposed on the Au LIII absorption edge was observed
and is attributed to the Au 2p3/2!5d transition.[21] The inten-
Entry
1
R
Product
Yield [%][a]
38
7a
2
7b
42
3
4
7c
7d
59
34
5
7e
0
Typical conditions: HAuCl4 (4.50 mol%),
(1 mmol), alcohol (3.00 mmol), TBHP (1.50 mmol), CH3CN (1.00 mL),
12 h. [a] Isolated yield.
3
(9 mol%), aldehyde
yields might be due, in part, to the application of a diol (see
Table 2, entry 5). We have no evidence for the formation of
the expected diesters derived from a two-fold reaction and
the main by-product was, again, the acetal. Herein, the use
of ethyleneglycol as the alcohol in these oxidative esterifica-
tions is described for the first time.
Table 4. Results from the LC-XANES fit in terms of weighting fraction
of the reference spectra and compositions of the samples investigated.
Weighting of
HAuCl4/CH3CN
(w1)
Weighting of
AuCl (1ꢀw1)
Calculated Au–Cl average
coordination number (N)[a]
VIII
IX
X
1
0
0
1
4 (=NAu3+
)
The proposed mechanism indicates initial Lewis acid acti-
vation of the aldehyde by the gold complex,[17] which leads
to the formation of a hemiacetal by nucleophilic attack of
the alcohol. The next step could be hydride transfer to the
peroxide, which leads to the formation of the ester, tert-bu-
tanol and water. The formation of water could proceed
through protonation of a preformed, unstable AuIII hydrox-
ide by the acidic pyridinium hydrochloride. The presence of
pyridinium hydrochlorides was proven by the isolation of
2 (=NAu +)
3.90
0.951ꢁ0.004
0.049ꢁ0.004
0.066ꢁ0.003
0.118ꢁ0.004
0.856ꢁ0.017
XI
0.934ꢁ0.003
3.87
3.76
2.29
XII 0.882ꢁ0.004
XIII 0.144ꢁ0.017
[a] N=w1N
(Au3+)+
(1ꢀw1)N
(Au+).
ACHTUNGTRENNUNG
Table 5. Reaction conditions in accordance with the conditions outlined
in the Experimental Section.
ACHTUNGTRENNUNG
[AuCl4]ꢀ[PyH]+ salt from a reaction mixture, after the reac-
tion. The salt was characterised by X-ray crystallography
Sample
AuCl
HAuCl4
3
BA[a]
n-butanol
TBHP
t [h]
(Figure 2).[18]
*
VIII
IX
1
1
We could exclude radical-based reaction pathways, as well
as Baeyer–Villiger-type oxidation, by the addition of radical
scavenger 2,6-di-tert-butyl-4-methylphenol (BHT) to the re-
action mixture and by the use of 97% isotopically enriched
18O-benzaldehyde.[19] After the reaction, 38% of the 18O
atoms were still incorporated. An exchange process by hy-
*
*
*
*
*
*
*
*
*
X
XI
XII
XIII
1
1
1
24
*
*
*
*
*
*
*
*
[a] BA=benzaldehyde.
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A. S. K. Hashmi, H. Bertagnolli et al.
sity of this peak is reduced in AuI (curve VIII) relative to
AuIII and is almost absent for Au0. In Figure 3, the Au LIII
edge XANES spectra of the reaction mixture after the addi-
tion of different reactants (curves X–XIII) are compared
with the AuIII and AuI reference spectra (curves VIII and
Figure 3. XANES spectra of Au0 reference (gold metal foil) and of sam-
ples VIII–XIII.
IX). The spectra look similar to the AuIII or AuI references
but not to the Au0 metal spectrum. The LC-XANES fits to
the measured spectra are presented in the Supporting Infor-
mation and the weighting factor of the two reference spectra
necessary to obtain a reasonable fit are given in Table 4.
At any given point in time, the reaction mixture consists
of an assortment of gold species, which coexist in solution.
As a result of the limited analysis options at the cyclotron,
we ensured that the reaction could be run under the pre-
dominant conditions. External parallel reactions with au-
thentic aliquots of the reaction mixture converted whilst the
different XAS spectra were measured. The reaction does
not require absolute conditions and the use of technical-
grade chemicals does not influence the reaction outcome.
Use of the extended X-ray absorption fine structure
(EXAFS) technique provides a structural picture that shows
the average coordination around different Au centres
(Table 4). The Fourier transform of the EXAFS function
does not reveal the contribution of near neighbours either
in gold-metal foil or in nanoparticles, which strongly indi-
cates that no gold(0) participates in the catalytic cycle
(Figure 4).[22] For all the investigated reaction mixtures
EXAFS contribution is only observed from Au–Cl neigh-
bours.
Figure 4. Experimental EXAFS spectra. Top: k3 weighted. Bottom: Four-
ier-transformed EXAFS spectra indicating the Au–Cl and Au–Au near-
neighbour distances and the decrease in the intensity of the Au–Cl peak.
Figure 5. Crystal structure of a AuI compound isolated after the reaction.
Hydrogen atoms are omitted for clarity. Left: top view of the space-filling
presentation. Right: ellipsoid model.
Nevertheless, catalyst deactivation does take place and
sometimes arises from the formation of small gold particles,
which showed no catalytic activity. Moreover, it was possible
to isolate some AuI and AuIII compounds after the reaction
(Figures 2 and 5). Both [AuCl4]ꢀ[(1H)]+ from Figure 1 and
the gold(I) complex from Figure 5 are not active for oxida-
tion. Thus, from these isolated species no evidence for the
involvement or preference of certain oxidation states can be
obtained. This is not surprising as the complexes were isolat-
ed after the reaction was completed, therefore, they origi-
nate from the decomposition of the active species and are
not active themselves.
Figure 5 shows the solid-state structure of a linearly ar-
ranged AuI compound. In the solid state the [AuCl2]ꢀ units
and the [Au(2,6-dimethylpyridine)]+ units alternate. The
distances between the counterions are 333.4 and 332.8 pm in
turn and lie in the typical range for aurophilic interactions.
In this catalysis reaction, 2,6-dimethylpyridine was used as
stabilising agent. In addition to this, we synthesised goldACHTUNGTRENNUNG(III)
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Homogeneous Oxidative Esterification
FULL PAPER
mixture sequentially. The mixture was stirred for 12 h at 758C, the sol-
vent was removed under reduced pressure and the products were purified
by silica-gel column chromatography (petroleum ether (PE)/ethyl acetate
(EA)). Experiments I–VII were carried out in a comparable manner.
Butyl benzoate (5a): Column chromatography (PE/EA 100:0!15:1); col-
ourless oil; yield: 160 mg, 900 mmol, 90%; Rf (PE/EA 15:1)=0.49;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.96 (t, J
ACHTUNGTRENNUNG
3H; CH3), 1.46 (m, J
2H; CH2), 4.31 (t, J(H,H)=6.7 Hz, 2H; CH2), 7.41 (t, J
2H; ArH), 7.52 (t, J
ACHTUGTNRENG(NU H,H)=7.6 Hz, 2H; CH2), 1.73 (m, JACTHUNGTRENNUNG
U
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
8.2 Hz, 1H; ArH); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=13.92,
19.45, 30.96, 64.99, 128.32, 128.45, 130.72, 129.93, 168.83 ppm. All data
are in good agreement with commercially available 5a.
Figure 6. Solid-state structures of [AuCl3·3] (right) and [AuCl3·4] (left).
Hydrogen atoms are omitted for clarity.
Butyl 4-methylbenzoate (5b): Column chromatography (PE/EA 100:0!
15:1); colourless oil; yield: 142 mg, 740 mmol, 74%; Rf (PE/EA 15:1)=
complexes with pyridines 1–4 as ligands. Figure 6 presents
the solid-state structures of [AuCl3·3] and [AuCl3·4]. These
structures show that the stabilising effect might arise from
coordination of the pyridine to the gold centre during the
reaction. The Au–N distances are 204.4 and 203.9 pm for
[AuCl3·3] and [AuCl3·4], respectively. A more detailed study
of these and other AuIII complexes, as well as a study of
their catalytic activity in other reactions, is under examina-
tion.
0.49; 1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.96 (t, J
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
CDCl3, 258C, TMS): d=13.96, 19.50, 21.84, 31.02, 64.85, 128.02, 129.22,
129.76, 143.60, 166.98 ppm. All spectroscopic data are in good agreement
with the previously reported data.[28]
Butyl 2,4,6-trimethylbenzoate (5c): Purification by bulb-to-bulb distilla-
1
tion; colourless oil; yield: 172 mg, 780 mmol; H NMR (300 MHz, CDCl3,
258C, TMS): d=0.95 (t, J
1.72 (m, 2H; CH2), 2.27 (s, 3H; CH3), 2.29 (s, 6H; CH3), 4.31 (t, J-
AHCTUNGTRENNUNG
(H,H)=6.7 Hz, 2H; CH2), 6.64 ppm (s, 2H; ArH); 13C NMR (75 MHz,
ACHTUNGRTENN(NUG H,H)=7.4 Hz, 3H; CH3), 1.44 (m, 2H; CH2),
CDCl3, 258C, TMS): d=13.86, 19.47, 19.89, 21.27, 30.89, 64.89, 128.53,
135.14, 139.29, 170.47 ppm. All spectroscopic data are in good agreement
with the previously reported data.[29]
Conclusion
We have provided the first experimental proof that, in addi-
tion to the known oxidation reactions with gold nanoparti-
cles, mononuclear gold complexes can also be active cata-
lysts in oxidation reactions. The selective oxidative esterifi-
cation of aldehydes and alcohols also opens interesting fur-
ther synthetic perspectives.
Butyl 2-naphthoate (5d): Column chromatography (PE/EA 15:1); colour-
less solid; yield: 164 mg, 720 mmol, 72%; Rf (PE/EA 15:1)=0.33;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.91 (t, J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
1H; ArH), 8.51 ppm (s, 1H; ArH); 13C NMR (75 MHz, CDCl3, 258C,
TMS): d=13.97, 19.50, 31.03, 65.16, 125.43, 126.74, 127.91, 127.96, 128.25,
128.30, 129.45, 131.09, 132.68, 135.65, 166.99 ppm. All spectroscopic data
are in good agreement with the previously reported data.[30]
Experimental Section
Butyl 4-methoxybenzoate (5e): Column chromatography (PE/EA 15:1);
colourless oil; yield: 141 mg, 680 mmol, 68%; Rf (PE/EA 15:1)=0.25;
General methods: All reagents and solvents were obtained from Acros,
ABCR, Alfa Aesar, Sigma–Aldrich or VWR and were used without fur-
ther purification, unless otherwise noted. Deuterated solvents were pur-
chased from Euriso–Top. Solvents were dried by using an MB SPS-800
solvent purification system with the aid of drying columns. Preparation of
air- and moisture-sensitive materials was carried out by using Schlenk
techniques in flame-dried flasks under an atmosphere of nitrogen. Oxida-
tion reactions were performed in dry and degassed solvents. TLC was
performed by using Polygram precoated plastic sheets SIL G/UV254
(SiO2, 0.20 mm thickness) from Macherey–Nagel. Column chromatogra-
phy was performed on silica gel (40.0–63.0 nm particle size) from Ma-
cherey–Nagel. NMR spectra were recorded on Bruker Avance 500,
Bruker Avance 300 and Bruker ARX-250 spectrometers at RT. Chemical
shifts (d, ppm) were referenced to TMS.[23] Signal multiplicity was deter-
mined as s (singlet), d (doublet), t (triplet), q (quartet) or m (multiplet).
13C assignment was achieved with the aid of DEPT90 and DEPT135
spectra. MS spectra were recorded on Vakkum Generators ZAB-2F, Fin-
nigan MAT TSQ 700 or JEOL JMS-700 spectrometers. IR spectra were
recorded on a Bruker Vector 22 FT-IR spectrometer.
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.96 (t, J
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
258C, TMS): d=13.96, 19.50, 21.84, 31.02, 64.85, 128.02, 129.22, 129.76,
143.60, 166.98 ppm. All spectroscopic data are in good agreement with
the previously reported data.[31]
Butyl 2-bromo-4,5-dimethoxybenzoate (5 f): Column chromatography
(PE/EA/CH2Cl2 7:1:1); colourless solid; yield: 141 mg, 320 mmol, 32%; Rf
(PE/EA/CH2Cl2 7:1:1)=0.31; 1H NMR (300 MHz, CDCl3, 258C, TMS):
d=0.94 (t, J
CH2), 2.38 (s, 3H; CH3), 3.86 (s, 3H; CH3), 3.88 (s, 3H; CH3), 4.29 (t, J-
(H,H)=6.7 Hz, 2H; CH2), 7.01 (s, 1H; ArH), 7.37 ppm (s, 1H; ArH);
ACHTUNGTREN(NNUG H,H)=7.4 Hz, 3H; CH3), 1.46 (m, 2H; CH2), 1.73 (m, 2H;
AHCTUNGTRENNUNG
13C NMR (75 MHz, CDCl3, 258C, TMS): d=13.94, 19.52, 28.34, 30.86,
56.30, 56.45, 114.10, 114.22, 117.10, 123.65, 147.99, 152.08, 165.84 ppm;
HRMS (EI+): m/z: calcd for C13H17BrO4: 316.0310 [M]+; found:
316.0303.
Butyl 4-nitrobenzoate (5g): Column chromatography (PE/EA 15:1); col-
ourless solid; yield: 107 mg, 480 mmol, 48%; Rf (PE/EA 15:1)=0.38;
Compounds 1–4: Compounds 1–4 were synthesised according to litera-
ture procedures.[24–27]
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.97 (t, J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
General procedure for the synthesis of esters 5–7: Under an atmosphere
of argon, HAuCl4·xH2O (49% metal content) (17.7 mg, 45.0 mmol) was
mixed with 3 (16.6 mg, 90.0 mmol) and acetonitrile (1.00 mL) to give a
dark-orange solid. Alcohol (3.00 mmol), aldehyde (1.00 mmol) and
TBHP (5.50m in decane, 270 mL, 1.50 mmol) were added to the reaction
6.6 Hz, 2H; CH2), 8.18 (m, 2H; ArH), 8.26 ppm (m, 2H; ArH);
13C NMR (75 MHz, CDCl3, 258C, TMS): d=13.94, 19.45, 30.87, 66.03,
123.72, 136.10, 150.70, 164.95 ppm. All spectroscopic data are in good
agreement with the previously reported data.[32]
Chem. Eur. J. 2010, 16, 8012 – 8019
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8017

A. S. K. Hashmi, H. Bertagnolli et al.
Butyl-3-chlorobenzoate (5h): Column chromatography (PE/EA 15:1);
colourless oil; yield: 127 mg, 600 mmol, 60%; Rf (PE/EA 15:1)=0.49;
spectroscopic data are in good agreement with the previously reported
data.[36]
1H NMR (300 MHz, CDCl3, 258C, TMS): d=1.02 (t, J
ACHTUNGTRENNUNG
Cyclohexyl 2-naphthoate (6 f): Column chromatography (PE/EA 15:1);
colourless solid; yield: 107 mg, 420 mmol, 42%; Rf (PE/EA 15:1)=0.38;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=1.24–1.69 (m, 6H; CH2),
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ArH). Product could not be isolated in pure form; determination of the
yield was accomplished by 1H NMR signal integration. All data is in
good agreement with data for commercially available 5h.
1.78 (m, 2H; CH2), 1.96 (m, 2H; CH2), 5.05 (tt, J
CH), 7.51 (m, 2H; ArH), 7.82 (d, J(H,H)=8.4 Hz, 2H; ArH), 7.91 (m,
1H; ArH), 8.03 (dd, J(H,H)=8.6, 1.7 Hz, 1H; ArH), 8.56 ppm (s, 1H;
ACHTUNGTREN(NUNG H,H)=8.9, 3.8 Hz, 1H;
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
ArH); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=23.96, 25.73, 31.94,
73.45, 125.53, 126.73, 127.93, 128.21, 128.25, 128.49, 129.52, 131.04, 132.72,
135.65, 166.34 ppm; IR (KBr): n˜ =3058, 2939, 2853, 1715, 1631, 1455,
1357, 1320, 1282, 1262, 1234, 1200, 1134, 1106, 1095, 1016, 962, 782,
762 cmꢀ1; HRMS (EI+): m/z: calcd for C17H18O2: 254.1307 [M]+; found:
254.1346.
(E)-Butyl cinnamate (5i): Column chromatography (PE/EA 15:1); col-
ourless oil; yield: 96 mg, 470 mmol, 47%; Rf (PE/EA 15:1)=0.55;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.95 (t, J
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
13C NMR (75 MHz, CDCl3, 258C, TMS): d=13.95, 19.40, 30.98, 64.63,
118.49, 128.23, 129.05, 130.39, 134.66, 144.74, 167.29 ppm. All spectro-
scopic data are in good agreement with the previously reported data.[33]
2-Hydroxyethyl isobutyrate (7a): Column chromatography (PE/EA 2:1);
colourless oil; yield: 50.2 mg, 380 mmol, 38%; Rf (PE/EA 2:1)=0.28;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=1.15 (d, J
ACHTUNGTRENNUNG
6H; CH3), 2.56 (septet, JACHTGNUTRENNUNG
Methyl 2-naphthoate (6a): Column chromatography (PE/EA 15:1); col-
ourless solid; yield: 166 mg, 830 mmol, 83%; Rf (PE/EA 15:1)=0.49;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=3.97 (s, 3H, CH3), 7.56 (m,
4.17 ppm (m, 2H; CH2); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=
19.16, 34.12, 61.49, 66.14, 177.77 ppm. All spectroscopic data are in good
agreement with the previously reported data.[37]
2H; ArH), 7.86 (d, J
ACHTUNGTRENNUNG(H,H)=9.0 Hz, 2H), 7.94 (dd, J=7.8, 1.7 Hz, 1H;
2-Hydroxyethyl pivalate (7b): Column chromatography (PE/EA 2:1); col-
ourless oil; yield: 62.9 mg, 420 mmol, 42%; Rf (PE/EA 2:1)=0.29;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=1.20 (s, 9H; CH3), 2.10 (s,
1H; OH), 3.80 (m, 2H; CH2), 4.18 ppm (m, 2H; CH2); 13C NMR
(75 MHz, CDCl3, 258C, TMS): d=27.39, 39.04, 61.67, 66.31, 179.21 ppm.
All spectroscopic data are in good agreement with the previously report-
ed data.[38]
ArH), 8.05 (dd, J(H,H)=8.6, 1.7 Hz, 1H; ArH), 8.60 ppm (s, 1H; ArH);
ACHTUNGTRENNUNG
13C NMR (75 MHz, CDCl3, 258C, TMS): d=52.45, 125.42, 126.84, 127.58,
127.96, 128.35, 128.43, 129.55, 131.26, 132.68, 135.70, 167.47 ppm. All
spectroscopic data are in good agreement with the previously reported
data.[13]
Ethyl 2-naphthoate (6b): Column chromatography (PE/EA 15:1); colour-
less solid; yield: 128 mg, 640 mmol, 64%; Rf (PE/EA 15:1)=0.43;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=1.44 (t, J
ACHTUNGTRENNUNG
2-Hydroxyethyl butanoate (7c): Column chromatography (PE/EA 2:1);
colourless oil; yield: 60.0 mg, 450 mmol, 45%; Rf (PE/EA 2:1)=0.18;
3H; CH3), 4.44 (q, J
(d, J
ACHTUNGTRENNUNG
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.92 (t, J
ACHTUNGTRENNUNG
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
JACHTUNGTRENNUNG
7.4 Hz, 2H; CH2; overlapping with 1H; OH), 3.79 (m, 2H; CH2),
4.18 ppm (m, 2H; CH2); 13C NMR (75 MHz, CDCl3, 258C, TMS): d=
13.83, 18.60, 36.23, 61.46, 66.06, 174.28 ppm. All spectroscopic data are in
good agreement with the previously reported data.[39]
13C NMR (75 MHz, CDCl3, 258C, TMS): d=14.60, 61.28, 125.45, 126.76,
127.94, 128.26, 128.32, 129.51, 131.22, 132.70, 135.67, 166.95 ppm. All
spectroscopic data are in good agreement with the previously reported
data.[34]
2-Hydroxyethyl hexanoate (7d): Column chromatography (PE/EA 2:1);
colourless oil; yield: 54.5 mg, 340 mmol, 34%; Rf (PE/EA 2:1)=0.16;
Octyl 2-naphthoate (6c): Column chromatography (PE/EA 15:1); colour-
less solid; yield: 208 mg, 730 mmol, 73%; Rf (PE/EA 15:1)=0.71;
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.78 (t, J
ACTHNUGRTENUNG(H,H)=6.9 Hz,
1H NMR (300 MHz, CDCl3, 258C, TMS): d=0.88 (t, J
ACHTUNGTRENNUNG
3H; CH3), 1.28 (m, 4H; CH2), 1.61 (m, 2H; CH2), 2.32 (m, 2H; CH2;
overlapping with 1H; OH), 3.71 (m, 2H, CH2), 4.18 ppm (m, 2H; CH2);
13C NMR (75 MHz, CDCl3, 258C, TMS): d=14.09, 22.50, 24.80, 31.48,
34.36, 61.48, 66.10 ppm. All spectroscopic data are in good agreement
with the previously reported data.[40]
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
1.8 Hz, 1H; ArH), 8.59 ppm (s, 1H; ArH); 13C NMR (75 MHz, CDCl3,
258C, TMS): d=14.31, 22.87, 26.31, 29.01, 29.43, 29.51, 32.03, 65.53,
125.48, 126.78, 127.96, 128.02, 128.29, 128.34, 129.54, 131.13, 132.73,
135.69, 167.04 ppm; IR (KBr): n˜ =2954, 2931, 2855, 1718, 1468, 1390,
1354, 1285, 1228, 1196, 1154, 1131, 1097, 1016, 959, 913, 866, 828, 780,
763 cmꢀ1; HRMS (EI+): m/z: calcd for C19H24O2: 284.1776 [M]+; found:
284.1835.
Acknowledgements
sec-Butyl 2-naphthoate (6d): Column chromatography (PE/EA 15:1); col-
ourless solid; yield: 95.9 mg, 420 mmol, 42%; Rf (PE/EA 15:1)=0.68;
The authors would like to express their special thanks to the Deutsche
Forschungsgemeinschaft (DFG Sonderforschungsbereich SFB 706) as
well as to the Studienstiftung des Dt. Volkes e.V. (fellowship for C.L.).
1H NMR (300 MHz, CDCl3, 258C, TMS): d=1.00 (t, J
ACHTUNGTRENNUNG
3H; CH3), 1.38 (d, JACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[1] For representative reviews on gold catalysis, see: a) A. S. K. Hashmi,
G. J. Hutchings, Angew. Chem. 2006, 118, 8064–8105; Angew. Chem.
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TMS): d=10.02, 19.85, 29.25, 73.23, 125.52, 126.75, 127.94, 128.24, 128.28,
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are in good agreement with the previously reported data.[35]
2-Hydroxyethyl 2-naphthoate (6e): Column chromatography (PE/EA
4:1); colourless solid; yield: 119 mg, 550 mmol, 55%; Rf (PE/EA 4:1)=
0.17; H NMR (300 MHz, CDCl3, 258C, TMS): d=2.43 (s, 1H; OH), 3.99
1
(m, 2H; CH2), 4.52 (m, 2H; CH2), 7.55 (m, 2H; ArH), 7.86 (d, J
8.6 Hz, 2H; ArH), 7.93 (dd, J(H,H)=7.9, 1.6 Hz, 1H; ArH), 8.05 (dd, J-
ACHTUNGTRENNUNG
(H,H)=8.6, 1.6 Hz, 1H; ArH), 8.61 ppm (s, 1H; ArH); 13C NMR
(75 MHz, CDCl3, 258C, TMS): d=61.72, 67.03, 125.40, 126.93, 127.27,
127.99, 128.44, 128.59, 129.57, 131.48, 132.65, 135.83, 167.34 ppm. All
ACHTUNGTRNE(NUNG H,H)=
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Received: December 16, 2009
Revised: April 22, 2010
Published online: July 2, 2010
[18] CCDC-758174
([AuCl2]ꢀ[Au(2,6-dimethylpyridine)]+),
758175
([AuCl3·(4)]), 758176 ([AuCl4]ꢀ
U
Chem. Eur. J. 2010, 16, 8012 – 8019
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8019