Heterogeneous gold-catalysed synthesis of phenols

Article abstract of DOI:10.1002/adsc.200606099

Nanoparticles of gold supported on nanocrystalline CeO₂ catalyse the isomerisation of ω-alkynylfurans to phenols. Initial leaching of gold was observed, which could be minimised by calcining. Subsequent runs showed that once all soluble species

Full text of DOI:10.1002/adsc.200606099

FULL PAPERS
DOI: 10.1002/adsc.200606099
Heterogeneous Gold-Catalysed Synthesis ofPhenols
a
b
a,
Silvio Carrettin, M. Carmen Blanco, Avelino Corma, *
b,
and A. Stephen K. Hashmi *
Instituto de Tecnología Química, UPV-CSIC, Universidad PolitØcnica de Valencia, Avda. de los Naranjos s/n, 46022
a
Valencia, Spain
Fax : (+34)-96-387-7809; e-mail: acorma@itq.upv.es
Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
b
Fax : (+49)-711-685-4321; e-mail: hashmi@hashmi.de
Received: March 13, 2006; Accepted: May 11, 2006
Abstract: Nanoparticles of gold supported on nano- to 391. This is the first time that a heterogeneous
crystalline CeO catalyse the isomerisation of w-al- gold catalyst showed activity in the gold-catalysed
2
kynylfurans to phenols. Initial leaching of gold was phenol synthesis.
observed, which could be minimised by calcining.
Subsequent runs showed that once all soluble species
had leached, the surface-bound, cationic gold species Keywords: alkynes; arenes; furans; gold; homogene-
is still active and can reach turnover numbers of up ous catalysis
Introduction
The gold on cerium oxide catalyst was chosen due
to the high percentage of cationic gold present in this
Since Haruta and co-workers showed the high activity system. As previously described, nanocrystalline
[
1]
of gold catalysts for the selective oxidation of CO,
cerium oxide is able to stabilise cationic species of
[
4,7]
much work has been carried out in the field of gold gold.
Therefore it appeared as a good candidate to
[
2]
catalysis. In spite of the many reactions studied with be employed for the study of CÀC bond formation.
gold catalysts, there are few examples of gold-cata- Previous efforts using a variety of heterogeneous gold
lysed carbon-carbon bond formations, and the vast catalysts like gold on carbon, gold on Fe O , gold on
2
3
majority of these are carried out using homogeneous SiO , gold on TiO and stabilised gold nanoparticles
2
2
[
3]
[8]
catalysts. Only a couple of examples deal with the had failed. In this work we will show a comparison
possible use of heterogeneous gold catalysts for between homogeneous Au(III) complexes and the
AHCTREUNG
[
4,5]
carbon-carbon bond formation.
The isomerisation heterogeneous Au/CeO catalyst, using the cyclisation
2
of w-alkynylfurans to phenols,[ a new reaction first of alkyne 1 to phenol 2 as a test reaction (Scheme 1).
6]
discovered with gold catalysts, is catalysed by AuCl3
and related homogeneous Au(III) complexes. In the
A
H
R
U
G
present work we show for the first time that a hetero- Results and Discussion
geneous gold on cerium oxide catalyst, possessing cat-
ionic gold species, is active and selective for this syn- Initial experiments were carried out using CDCl as
3
thesis of phenols (Scheme 1). The reaction is carried solvent and the reactions were monitored by
1
out under mild conditions leading to almost 100%
conversion and selectivity.
H NMR; the Au/CeO catalyst showed high activity
2
and yield of the wanted product 2 achieving almost
100% conversion after 20 h of reaction. However,
tests carried out to verify that the catalysis was occur-
ring heterogeneously, gave a negative result. Analysis
[
9]
by atomic absorption spectroscopy (AAS) showed
the presence of gold in the reaction mixture and the
filtrate solution possessed high catalytic activity (85%
conversion of 1 after 24 h). The CDCl solvent could
3
be responsible for the gold leaching, since the pres-
Scheme 1. Reaction Scheme of Au/CeO -catalysed phenol ence of gold hydroxide and oxy-hydroxide on the cat-
2
[
4]
synthesis.
alyst surface may determine an interaction with the
Adv. Synth. Catal. 2006, 348, 1283 – 1288
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1283

Silvio Carrettin et al.
FULL PAPERS
halogenated solvent forming soluble gold complexes.
We therefore decided to use a non-halogenated sol-
vent, such as CD CN, to investigate if the leaching
3
problem could be avoided. The use of CD CN result-
3
ed in complete conversion of the substrate 1 to the
desired product 2 after 24 h. Atomic absorption anal-
ysis on the reaction solution revealed the presence of
2
5 ppm of gold in solution (in 3 mL of acetonitrile
this corresponds to 59 mg of gold, which is less than
% of the total amount of gold on the catalyst) and
6
activity tests on the filtrate solution showed a conver-
sion of the 22% of substrate 1 after 24 h reaction
[
3
1
which corresponds to a turnover number (TON) of
27 for the gold present in the filtrate]. A TON of
9.2 (calculated as moles of substrate converted per
Figure 1. 1.8% Au/CeO catalysts for phenol synthesis after
different uses (24 h reaction time, black columns). Reaction
tests of the corresponding liquid phases for catalytic activity
2
mole of gold present on the catalyst) show that the re- (40 h reaction time, grey columns).
action process is catalytic and the results are very sim-
[
6]
ilar to those obtained with AuCl by Hashmi et al.,
3
where the TON reaches 50. Since not all the gold (air) and reducing (H ) atmospheres in order to in-
2
atoms present on the heterogeneous catalysts are cat- crease the interaction of the gold particles with the
alytically active (particle size in the range of 3–4 nm), support and at the same time to discuss the active
it can be stated that the real TON of the active sites gold species (Au3 or Au ) on the heterogeneous cat-
of the supported gold catalyst should be much higher alyst. The sample treated with hydrogen showed poor
than 19, probably even superior to the activity report- catalytic activity leading to only 13% conversion of
ed for the best homogeneous catalysts so far, N,O- substrate 1 even after 6 days of reaction. The reduced
+
0
[10]
complexes of gold
A
H
R
U
G
Further catalyst reuses show amount of cationic gold species, after H treatment,
2
that leaching strongly decreases and has practically could account for the lower activity of this catalyst
disappeared after the third reuse (Figure 1). In this compared to the untreated sample (Table 1).
case the conversion was 35% after 24 hours compared
The catalyst sample treated with air did not differ
to 100% of the original catalyst (first use) at the same in its catalytic activity compared to the untreated one,
reaction time. The leaching test, after the third cata- thus the reaction in CDCl showed significant gold
3
lyst reuse, gave a conversion of 1.3% after 40 hours of leaching and the filtrate solution presented high cata-
reaction for the leached gold in homogeneous phase. lytic activity. As in the case of the untreated catalyst
Despite the fact that the solid catalyst has lost activity sample, the reaction was studied using CD CN as sol-
3
in the second and third reuse, it can still achieve vent, and high catalytic activity was found. Gold
1
00% conversion of the substrate 1 with a longer re- leaching was also investigated in this case by carrying
action time (40 hours for the second and 55 hours for out the reaction using the filtrate solution and by
the third one). In this case, it looks like that an auto- AAS analysis of the reaction mixture. After 24 h the
catalytic behaviour may occur suggesting that for ex- conversion of 1 was 17%, and AAS showed the pres-
ample the decrease of pH by producing the acidic ence of 6.4 ppm of Au in solution (corresponding to a
phenol slightly influences the reaction rate. This very small part, 1.5%, of the gold initially present).
effect is under investigation.
These values are much lower than those obtained
In order to investigate the possibility of reducing or with the uncalcined sample. After one recycle of the
even eliminating gold leaching, the fresh catalyst was calcined sample, leaching is reduced and the homoge-
treated at high temperature (3008C) in both oxidising neous reaction as well, while the heterogeneous reac-
Table 1. Relative amount of the different gold species determined by FT-IR using CO as probe molecule.
3
+
+
0
T
+
3+
0
Entry
Catalyst
Au /Au
Au /AuT
Au /Au
A
H
R
U
G
T
1
2
3
4
5
Au/CeO2
Au/CeO2
Au/CeO2
Au/CeO2
Au/CeO2
0.29
0.27
0.22
0.03
0
0.37
0.34
0.32
0.36
0.37
0.25
0.38
0.32
0.55
0.52
2.67
1.6
1.67
0.71
0.7
Entry 1: untreated; entry 2: air treatment at 3008C; entry 3: H treatment at 1008C; entry 4: sample dried at 1008C for 48 h;
2
0
À1
+
À1
3+
entry 5: H treatment at 3008C. Au : total amount of gold; Au : IR band at 2100 cm ; Au : IR band at 2154 cm ; Au : IR
2
T
À1
band at 2138 cm .
1284
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Adv. Synth. Catal. 2006, 348, 1283 – 1288

Heterogeneous Gold-Catalysed Synthesis of Phenols
FULL PAPERS
tion decreases from 100% to 85%. It appears that, by number of surface exposed gold atoms. Once estimat-
calcinations of the catalyst in an air stream, the inter- ed, the number of 3.5 nm clusters present and the
action of the gold with the support becomes stronger, number of the total gold atoms exposed together with
3
+
tot
further stabilising the active gold species and limiting the ratio Au /Au , determined by FT-IR, we could
3
+
the leaching.
calculate the number of surface exposed Au . From
FT-IR studies, carried out using CO as probe mole- the results presented in Table 2, two conclusions can
cule, showed the presence of different gold species on be drawn. The first one is that the TON values are
the catalyst surface. In particular, untreated catalysts very similar for all catalysts supporting the hypothesis
dried for a short time in an oven and not treated at that the surface Au3 species are the active sites for
high temperature in reducing atmospheres present a the isomerisation of w-alkynylfurans to phenols on
higher fraction of cationic gold and more specifically solid gold catalysts. Secondly, the TON obtained on
+
3
+
Au .
the basis of cationic gold detected by CO adsorption
In the case of the homogeneous catalyst, it was evi- is even superior to the one obtained for homogeneous
[
6]
denced that Au
A
C
H
T
R
E
U
N
G
(III) was the active species. Also in gold catalysts , something which can be explained on
3+
our case when the fraction of cationic gold (Au ), the basis of a surface concentration effect on high sur-
determined by CO adsorption, decreases with respect face area porous catalysts.
[
14]
tot
to the total gold (Au ) the activity decrease (see
Figure 2) and, despite the lack of catalyst samples
Table 2. TON calculated as number of molecules of product
obtained per active site after 24 h reaction.
Entry
Catalysts
Conversion of 1 [%]
TON
1
2
3
4
5
Au/CeO2
Au/CeO2
Au/CeO2
Au/CeO2
Au/CeO2
100
90
88
5
338
326
391
163
0
0
Entry 1: as prepared catalyst; entry 2: air treatment at
008C; entry 3: H treatment at 1008C; entry 4: sample
3
2
dried at 1008C for 48 h; entry 5: H treatment at 3008C.
2
Conclusions
We have shown that nanoparticles of gold supported
on nanocrystalline CeO are able to catalyse the syn-
2
thesis of phenols by isomerisation of w-alkynylfurans.
Cationic gold is the active site for the reaction and
sis. Reaction conditions: 100 mmol of 1, 3 mL of CD CN, these species are very well stabilised on gold support-
Figure 2. Correlation between the first order apparent rate
constant and the amount of cationic gold for phenol synthe-
3
6
08C, Au/CeO (5 mol% of Au with respect to 1).
ed on nanocrystalline CeO2.
2
Caution should be taken about leaching of gold
from the catalyst, because the leached species give
3+
tot
with Au /Au between 0.1 and 0.2, we can see a very high turnover numbers (TON). By calcining the
direct correlation between the fraction of cationic catalyst before use and using acetonitrile as solvent,
gold and the first order apparent rate constant. These leaching was minimised. The cationic gold species
results will support the existence of a parallelism be- measured on the catalyst by IR-CO adsorption give a
tween homogeneous and heterogeneous gold catalysts TON similar to the one reported previously for some
in this case, as has been found for homocoupling and of the homogeneous catalysts.
heterocoupling Suzuki reaction[4,11,12] and in general
for cationic gold as a Lewis acid catalyst.[
13]
In a first approximation, if we consider the cationic
Experimental Section
gold(III) detected by CO adsorption as the active
A
C
H
T
R
E
U
N
G
sites for phenol synthesis; then it is possible to calcu-
late a TON by dividing the number of molecules of
The Au on cerium oxide catalyst was prepared using
HAuCl as precursor and nanocrystalline CeO as support
4
2
[15]
product obtained by the active sites corresponding to following a procedure previously reported
cationic Au. A theoretical estimation of the amount
Different routes to the standard test substrate 1 for this
3
+
of Au active sites was realised considering the total type of chemistry were investigated (Scheme 2). The two
gold present as clusters of 3.5 nm and calculating the commercially available starting materials are 5-methylfufu-
Adv. Synth. Catal. 2006, 348, 1283 – 1288
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
1285

Silvio Carrettin et al.
FULL PAPERS
+
(
80 eV): m/z (%): 147 (100) [M ], 120 (69); anal. calcd. for
C H NO (147.2): C 73.45, H 6.16, N 9.52; found: C 73.34, H
9
9
6
.42, N 9.43.
Synthesis of(5-Methyl fu ran-2-ylmethyl)prop-
-ynylamine (6)
2
Compound 5 (5.00 g, 34.0 mmol) was dissolved in MeOH
30 mL) and sodium borohydride (1.29 g, 34.1 mmol) was
(
added at room temperature in small portions. Most of the
solvent was removed under vacuum and the residue was pu-
rified by column chromatography (hexanes/ethyl acetate,
3
:1). Thus 6 was obtained as a yellow oil; yield: 4.50 g
(
89%); R (hexanes/ethyl acetate, 4:1)=0.13; IR (film): n=
f
3
9
1
2
2
3
293, 2922, 2834, 1569, 1450, 1354, 1330, 1218, 1101, 1020,
À1
1
29, 785 cm ; H NMR (CDCl , 250 MHz): d=1.54 (br s,
3
H), 2.23 (t, J=2.4 Hz, 1H), 2.26 (s, 3H), 3.42 (d, J=
.4 Hz, 2H), 3.81 (s, 2H), 5.86–5.87 (m, 1H), 6.08 (d, J=
13
.9 Hz, 1H); C NMR (CDCl , 64.9 MHz): d=13.4 (q),
3
6.9 (t), 44.5 (t), 71.4 (d), 81.6 (s), 105.8 (d), 108.2 (d), 150.8
+
(
(
1
s), 151.6 (s); MS (80 eV): m/z (%)=149 (18) [M ], 148
27), 120 (48), 95 (100); C H NO (149.2); HR-MS: calcd.
49.0840; found: 149.0839.
9
11
Synthesis of1 fr om 6
Scheme 2. Tested routes to 1.
To
6
(4.00 g, 26.8 mmol) and triethylamine (2.71 g,
2
6.8 mmol) in dichloromethane (30 mL) at room tempera-
ture TsCl (5.11, 26.8 mmol) was added in small portions and
then stirred overnight. After the slow addition of water
ral (3) and 5-methylfurfurylamine (10). Compound 3 is the
cheaper starting material, but here in both possible routes
(
50 mL), separation of the organic layer, two extractions of
the aqueous layer with dichloromethane (2”20 mL), the
(
(
via 5/6 or via 7/8), three steps are needed. Propargylamine
4) is relatively expensive, making the route via 5/6 less fa-
combined organic layers were dried over MgSO and then
4
filtered. Then the solvent was removed under vacuum and
the residue was recrystallised from diethyl ether. Most of 8
crystallised,[ more 8 was isolated from the mother liquor
by column chromatography on silica gel (hexanes/ethyl ace-
tate/dichloromethane, 8:1:1), to provide a combined crop of
vourable. The approach via 10 uses the significantly more
expensive starting material, but only two steps are needed.
16]
Our experiments proved that, if one also takes into account
the yields obtained, the approach via 3/7/8 is economically
the best one while the approach via 10/8 is the fastest one.
8
; yield: 7.88 g, (26.0 mmol, 97%); R (hexanes/ethyl ace-
f
tate, 2:1)=0.44; mp 68–718C; IR (film): n=3286, 2923,
1
9
712, 1598, 1564, 1336, 1349, 1332, 1221, 1162, 1093, 1021,
À1
1
73, 919, 891, 799, 765, 737, 665, 579 cm
;
H NMR
Synthesis of(5-Methyl fu ran-2-ylmethylene)prop-
(CDCl , 250 MHz): d=2.06 (t, J=2.5 Hz, 1H), 2.19 (s, 3H),
3
2-ynylamine (5)
2
.42 (s, 3H), 4.01 (d, J=2.5 Hz, 2H), 4.37 (2, 2H), 5.86 (d,
J=3.1 Hz, 1H), 6.15 (d, J=3.1 Hz, 1H), 7.28 (d, J=8.4 Hz,
To 3 (5.00 g, 45.4 mmol) in dichloromethane (40 mL),
13
2
H), 7.73 (d, J=8.4 Hz, 2H); C NMR (CDCl , 62.9 MHz):
3
MgSO (6.00 g, 49.8 mmol) and 4 (2.50 g, 45.4 mmol) were
4
d=13.35 (q), 21.45 (q), 35.84 (t), 42.70 (t), 73.64 (d), 76.43
s), 106.08 (d), 111.86 (d), 127.62 (d, 2 C), 129.25 (d, 2 C),
35.95 (s), 143.33 (s), 146.31 (s), 152.62 (s); anal. calcd. for
C H NO S (303.4): C 63.34, H 5.65, N 4.61; found: C
added. The reaction mixture was stirred at room tempera-
ture for one day. After filtration, the solvent was removed
under vacuum and the residue was purified by column chro-
matography on silica gel (hexanes/ethyl acetate, 3:1) to
(
1
16
17
3
6
3.09, H 5.51, N 4.66.
afford 5 as a yellow oil; yield: 6.08 g (91%); R (hexanes/
f
ethyl acetate, 3:1)=0.31; IR (film): n=3295, 2925, 2898,
1
9
3
646, 1589, 1532, 1386, 1369, 1353, 1304, 1200, 1023, 996,
À1
1
Synthesis of4-Methyl- N-(5-methylfuran-2-yl-
H), 2.50 (t, J=2.4 Hz, 1H), 4.47–4.49 (m, 2H), 6.05–6.08 methylene)benzenesulfonamide (7)
46, 913 cm
;
H NMR (CDCl , 250 MHz): d=2.33 (s,
3
1
3
(
m, 1H), 6.65 (d, J=3.2 Hz, 1H), 8.30 (m, 1H); C NMR
(CDCl , 64.9 MHz): d=13.7 (q), 46.4 (t), 76.1 (d), 78.4 (s),
This reaction was conducted in analogy to the literature.[17]
Compound (5.00 g, 45.4 mmol), tosylamide (11.6 g,
3
108.0 (d), 116.7 (d), 150.0 (d), 155.7 (s) (1 s is hidden); MS
3
1286
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Heterogeneous Gold-Catalysed Synthesis of Phenols
FULL PAPERS
9
0.8 mmol) and triethylamine (23.0 g, 227 mmol) were dis-
(60 mL), separation of the organic layer, two extractions of
the aqueous layer with dichloromethane (2”20 mL), the
solved in dichloromethane (150 mL) and cooled to 08C.
TiCl4 (4.31 g, 22.7 mmol) in dichloromethane (15 mL) was
added at that temperature, then the ice-bath was removed
and stirring was continued for 30 min (monitoring by TLC).
combined organic layers were dried over MgSO and then
4
filtered. Then the solvent was removed under vacuum and
the residue was recrystallised from diethyl ether. Most of 8
crystallised,[ more 8 was isolated from the mother liquor
by column chromatography on silica gel (hexanes/ethyl ace-
tate 2:1), to afford 8; combined yield: 11.6 g (43.7 mmol,
97%); Rf (hexanes/ethyl acetate, 1:1)=0.41; mp 82–838C;
IR (film): n=3256, 2923, 1598, 1570, 1436, 1350, 1321, 1220,
16]
The mixture was hydrolysed with saturated NaHCO solu-
3
tion, the phases were separated and the aqueous phase was
extracted with dichloromethane (2”70 mL) and the com-
bined organic layers were washed with water (50 mL). After
drying over MgSO4 and filtration, the crude product (93%)
can be used in the next step directly. By recrystallisation of
a portion of the crude material, analytically pure 7 was ob-
À1
1
1186, 1093, 1040, 1018, 930, 815, 790 cm
;
H NMR
(CDCl3, 250 MHz): d=2.11 (s, 3H), 2.41 (s, 3H), 4.09 (d,
J=5.7 Hz, 2H), 4.95 (t, J=5.7 Hz, 1H), 5.76 (m, 1H), 5.95
(d, J=3.1 Hz, 1H), 7.24–7.27 (m, 2H), 7.65–7.73 (m, 2H);
tained in the form of colourless crystals: R (petrol ether/
ethyl acetate, 1:1)=0.37; mp 118–1198C; IR (film): n=3087,
f
À1
13
1
600, 1548, 1507, 1284, 1148, 1088, 1034, 830, 800, 670 cm
;
C NMR (CDCl3, 64.9 MHz): d=13.1 (q), 21.3 (q), 40.1 (t),
1
H NMR (CDCl , 300 MHz): d=2.42 (s, 6H), 6.29 (d, J=
106.0 (d), 108.9 (d), 127.0 (d, 2 C), 129.4 (d, 2 C), 136.8 (s),
3
3.6 Hz, 1H), 7.25 (d, J=3.6 Hz, 1H), 7.28–7.33 (m, 2H),
143.1 (s), 147.4 (s), 152.0 (s); MS (80 eV): m/z (%)=265 (9)
13
+
7.82–7.89 (m, 2H), 8.68 (s, 1H);
C NMR (CDCl3,
[M ], 110 (100), 109 (97), 95 (24); anal. calcd. for
C H NO S (265.3): C 59.04, H 5.86, N 5.09; found: C
7
5.5 MHz): d=14.4 (q), 21.6 (q), 111.0 (d, 2 C), 128.0 (d, 2
13 15
3
5
8.85, H 5.70, N 5.28.
C), 129.7 (d, 2 C), 135.7 (s), 144.3 (s), 147.7 (s), 154.8 (s),
1
1
+
62.1 (d); MS (70 eV): m/z (%)=263 (48) [M ], 172 (21),
55 (22), 108 (34), 91 (100), 65 (17); anal. calcd. for
C H NO S (263.1): C 59.30, H 4.98, N 5.32; found: C
13
13
3
59.24, H 5.00, N 5.22.
Gold Catalysis Reactions
The substrate 1 was taken up in the solvent and the catalyst
was added. The reaction could be monitored by TLC or
NMR, when complete the catalyst was removed by filtration
and column chromatography on silica gel (hexanes/ethyl
Synthesis of4-Methyl- N-(5-methylfuran-2-yl-
methyl)benzenesulfonamide (8) from 7
acetate, 7:1) furnished 2; R (hexanes/ethyl acetate, 5:1)=
f
To 7 (4.00 g, 15.2 mmol) in MeOH (30 mL) NaBH (575 mg,
[6a]
4
0
.27.
15.2 mmol) was added in small portions at room tempera-
ture, then the solution was stirred over night. After hydroly-
sis (30 mL H O), phase separation, drying of the organic
2
phase and the usual work-up by crystallisation and chroma-
trography of the mother liquor 8 was obtained; yield: 73% Acknowledgements
(
68% over two steps).
This work was supported by the European Union (AURI-
CAT EU-RTN, HPRN-CT-2002–00174) and Ministerio de
Educación y Ciencia (Project MAT2003–07945-C02–01 and
02). The authors acknowledge Johnson Matthey plc. (Alfa-
Aesar) for the loan of HAuCl4
Synthesis of1 fr om 8
Compound 8 (4.00 g, 15.1 mmol) was dissolved in acetone
(
30 mL), K CO (4.00 g, 28.9 mmol) and
9
(2.38 g,
2
3
2
0.0 mmol) were added and the mixture was stirred at room
References
temperature for 24 h. Then the solvent was removed under
vacuum, the residue was taken up in water (30 mL) and di-
chloromethane (20 mL) and the product was extracted with
two additional portions of dichloromethane (20 mL). After
removal of the solvent, the residue was recrystallised from
diethyl ether and the mother liquor was purified by column
chromatography on silica gel (hexanes/ethyl acetate/di-
chloromethane, 8:1:1) to afford 1; yield: 4.48 g (14.8 mmol,
[
[
[
1] M. Haruta, N. Yamada, T. Kobayashi, S. Iijima, J.
Catal. 1989, 115, 301–309.
2] G. C. Bond, D. T. Thompson, Catal. Rev. Sci. Eng. 1999,
4
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8%).
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117, 7150–7154; Angew. Chem. Int. Ed. 2005, 44, 6990–
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Synthesis of8 fr om 10
balle, F. Marinelli, Adv. Synth. Catal. 2006, 348, 331–
338.
To 10 (5.00 g, 45.0 mmol) and triethylamine (4.55 g,
4
5.0 mmol) in dichloromethane (60 mL) at room tempera-
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