Ultrasound-assisted synthesis of benzophenones by Stille cross-coupling reactions. Optimization via experimental design

Article abstract of DOI:10.1016/j.jorganchem.2012.10.026

A series of diaryl ketones have been synthesized in moderate to excellent yields through the selective cross-coupling reaction of benzoyl chlorides with arylstannanes using a sonochemical variation of the Stille coupling. Ultrasound significantly enhances this useful organometallic transformation affording the desired products in higher yields and shorter reaction times than conventional reactions. The scope of the protocol has been explored with a selection of arylstannanes and different aroyl chlorides as reaction partners. Remarkably, no by-products resulting from homo-coupling could be detected. The ultrasound-promoted cross-coupling reaction was optimized through experimental design.

Full text of DOI:10.1016/j.jorganchem.2012.10.026

Journal of Organometallic Chemistry 723 (2013) 43e48
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ultrasound-assisted synthesis of benzophenones by Stille cross-coupling
reactions. Optimization via experimental design
,1
2
*
*
Martín Luong, Claudia E. Domini , Gustavo F. Silbestri , Alicia B. Chopa
INQUISUR, Departamento de Química, Universidad Nacional del Sur, Avenida Alem 1253, B8000CPB Bahía Blanca, Argentina
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 July 2012
Received in revised form
A series of diaryl ketones have been synthesized in moderate to excellent yields through the selective
cross-coupling reaction of benzoyl chlorides with arylstannanes using a sonochemical variation of the
Stille coupling. Ultrasound significantly enhances this useful organometallic transformation affording the
desired products in higher yields and shorter reaction times than conventional reactions. The scope of
the protocol has been explored with a selection of arylstannanes and different aroyl chlorides as reaction
partners. Remarkably, no by-products resulting from homo-coupling could be detected. The ultrasound-
promoted cross-coupling reaction was optimized through experimental design.
14 September 2012
Accepted 13 October 2012
Keywords:
Ultrasound
Stille cross-coupling
Aromatic ketones
Arylstannanes
Ó 2012 Elsevier B.V. All rights reserved.
Experimental design
1. Introduction
the generation of the undesired decarbonylation by-products [5].
Recently, we have proposed three alternative and complementary
Aromatic ketones are important building blocks in a large
number of natural products as well as various pharmaceutical
compounds. Electrophilic aromatic substitution, such as Friedele
Crafts acylation [1] and cross-coupling reactions of acyl chlorides
with organometallic reagents [2] are presumed to be preferred for
the synthesis of aromatic ketones. Due to their wide field of
application, investigations related to organometallic chemistry
have increased constantly in the last decades. As a result of their
accessibility and the extraordinary capacity of the tin atom to
transfer organic groups with an excellent chemo-, regio- and
stereo-selective control in: i) cross-coupling reactions, ii) trans-
metallations and iii) electrophilic substitutions, vinyl and, espe-
cially, arylstannanes are considered good substrates for the
effective generation of a carbonecarbon bond [3].
routes for the synthesis of asymmetric sterically hindered benzo-
phenones [6].
It is well known that the sonochemistry effects derive from
a phenomenon known as acoustic cavitation, that is, the formation,
growth and implosion of micro-bubbles inside a liquid [7]. These
effects induce high temperatures (near 5000 K) and very high
pressures (around 1000 atm) inside such cavities, while shock
waves at the interface and bulk liquid are largely responsible for
enhanced mass and energy transfers. By virtue of the effects
mentioned above, the ultrasonic irradiation constitutes a conve-
nient way to accelerate and improve a great number of organic and
organometallic reactions [7a,8].
In these processes, a considerable number of variables (instru-
mental parameters, reagents, times, temperatures, etc.) take part,
so, a large number of experiments must be carried out in order to
define the optimal conditions. The experimental design could be
defined as a methodology based on mathematical tools to advise
and help experimentalists to: i) select the optimal experimental
synthetic strategy through a few number of experiments, ii) eval-
uate the experimental results, guaranteeing maximum reliability in
the conclusions obtained. The multivariable technique has been
already used, for example, in order to improve the optimization
process in chemistry [9]. It has been also applied to different
chemical reactions involving more than one parameter or
response; for example, Guervenou applied a complete factorial
In the last years we have been involved in the application of
arylstannanes as intermediates in organic synthesis. Thus, we have
determined new catalyst-free procedures for the regioselective
synthesis of asymmetric diaryl ketones and triaryl diketones [4], as
well as the efficient synthesis of tertiary alkyl aryl ketones without
*
Corresponding authors. Tel.: þ54 291 4595100; fax: þ54 291 4595187.
E-mail addresses: cdomini@criba.edu.ar (C.E. Domini), gsilbestri@uns.edu.ar
(
G.F. Silbestri).
1
Member of CONICET.
2
Member of CIC.
0
022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.10.026

4
4
M. Luong et al. / Journal of Organometallic Chemistry 723 (2013) 43e48
design in the MichaeliseBecker’ reaction [10] and Bouzidi and
Gozzi applied two types of experimental design to find the best
conditions in the Grignard’ reaction [11].
Based on these preliminary results the study was carried out in
toluene and applying the ultrasound energy inside the reaction
mixture. The system under study was optimized by experimental
design.
Although the Stille reaction applied to the synthesis of aryl
ketones has been known since 1983 [12], few examples are found in
the literature up to now and, generally, the desired products are
2.1. Central composite design (CCD)
ꢀ
obtained in good yields after several hours up to 100 C [13]. Our
purpose in this paper is to study the effect of ultrasonic irradiation
over this reaction. The method was optimized by experimental
design and provides high performance in short reaction times using
a reduced amount of catalyst.
The CCD was proposed by Box and Wilson [14]. This design
consists of the following parts: (1) a full factorial or fractional
factorial design; (2) an additional design or star points; (3) a central
point [15]. Fig. 1 shows a representation of CCD for three-variable
optimizations used in this work.
The levels of the experimental variables and the corresponding
response values of the CCD are shown in Table 2. Statgraphics Plus
2
. Results and discussion
5.1 (Sigma Plus) software was used and a total of 16 experiments,
Our initial exploration started with the cross-coupling reaction
including 8 factorial points, 6 axial points and 2 central points
replicas must be carried out. The predicted values were modeled by
fitting a second-order polynomial. The quadratic polynomial is
given below where yield of 3-methoxybenzophenone (%) is the
measured response and A (power out, %W), B (sonication time,
min) and C (cycles, s.s-1) are the independent variables studied.
of tributyl(3-methoxyphenyl)stannane with benzoyl chloride (1:1.2
ratio) as a model system, using an inexpensive and commercially
available catalyst such as Cl
2
Pd(PPh
3
)
2
(2 mol%) and working at
ꢀ
9
0 C.
At the beginning of our work, we decided to examine the
influence of two variables on the catalytic system. Initially, we
studied the solvent effect carrying out the reactions either in DMF
or toluene; then, we examined the effect of applying ultrasound
inside or outside the reaction mixture. The results are summarized
in Table 1.
Yield of 3-methoxybenzophenone (%) ¼ 71.5954 ꢁ 10.9058A þ
ˇ
6
0
.21877B ꢁ 0.567193C ꢁ 2.50844A 2 þ 7.33AB þ 3.025AC ꢁ
ˇ
ˇ
.807839B 2 ꢁ 7.3025BC þ 4.88793C 2
Reaction times were defined according to the presence of black
Pd (catalyst decomposition) in the reaction mixture. Entries 1 to 3
show that DMF is not an appropriate solvent; thus, the process was
ineffective working either at 480 W or 360 W (80% and 60% output
power, respectively) and 80 cycles (entries 1 and 2) being the
starting substrate almost recovered. Moreover, a decrement of the
cycles from 80 to 20, led to a quantitative yield of anisole as
a consequence of the protodestannylation of the starting substrate
A Pareto chart of standardized effects is shown in Fig. 2. The
Pareto chart was drawn in order to show the significant effects of all
variables (linear, quadratic and interactions between variables). The
vertical line represents 95% of the confidence interval. The effects
crossed by this line are significant values with respect to the
response. As it can be seen from Fig. 2, the three different second-
order interactions can be considered not significant. At the same
time, one of the three main factors, the ultrasound power (Factor
A), has a significant negative effect.
When we applied the optimal reaction conditions obtained by
experiment design (A: power ¼ 44% W (264 W), B: time ¼ 30 min,
and C: cycles ¼ 30) to our model reaction, the desired product was
obtained in a 96% yield and no by-products resulting from homo-
coupling were detected. An experience was carried out in the
absence of Pd and, after 30 min, the arylstannane was almost
recovered supporting that Pd also acts as a catalyst under
sonication.
(
entry 3). On the other hand, the reactions were effective in toluene
as solvent, and different yields of the desired ketone were obtained
working under two different ultrasonic conditions. Experiment 4
shows that the best results were obtained irradiating at 480 W and
8
0 cycles; the ketone was obtained in 39% yield without the pres-
ence of the undesired homo-coupling products. When similar
ultrasonic conditions were applied outside the reaction mixture the
yield of diaryl ketone decreased to 10% (experiment 6).
Table 1
The scope of this sonochemical reaction was evaluated by the
reaction of benzoyl chloride with a series of ortho-, meta- and para-
substituted aryltributylstannanes derivatives (Table 3) under the
optimal conditions previously established.
Preliminary test.a
O
Cl
SnBu3
O
OMe
Cl Pd(PPh )
An analysis of Table 3 shows that meanwhile meta- and para-
substituted arylstannanes gave the desired ketones [16] in good to
excellent yields (66%e98%) (entries 2e6), ortho-substituted
substrates rendered the corresponding ketones in really poor
yields (12%e18%) (entries 1 and 7). Taking into account that in all
experiments we have not noticed the decomposition of the
catalyst (black Pd), and with the main goal of increasing the
yields of sterically hindered unsymmetrical diaryl ketones, we
carried out experiments 1 and 7 but at longer reaction time (2 h).
Unfortunately, no significant increments of the product yields
2
3 2
+
OMe
Entry
Power
Cycles
Time (min)
Solvent
Yieldb
c
1
2
3
4
5
6
80
60
60
80
60
80
80
80
20
80
20
80
10
10
20
40
70
40
DMF
DMF
DMF
Toluene
Toluene
Toluene
0
0
0
39
17
10
c
d
e
f
g
(
15% and 20%, respectively) were produced. These results support,
a
ꢀ
ArSn:PhCOCl, 1:1.2; 2 mol% of Cl
2
Pd(PPh
3
)
2
with respect to arylstannane; 90
C
once more, that cross-coupling reactions are generally limited to
the synthesis of noncrowded ketones [2,17].
In order to define the generality of the protocol, a series of
reactions of tributyl(3-methoxyphenyl)stannane with different
acyl chlorides were carried out.
(
oil bath); under nitrogen atmosphere, ultrasound power is applied inside the
reaction mixture.
b
Quantified by GLC, using the external standard method.
Arylstannane was recovered.
Formation of anisole.
Absence of homo-coupling product.
Together with traces of homo-coupling product.
The ultrasound power is applied outside the reaction mixture.
c
d
e
f
The results summarized in Table 4 show that the reactions take
place with aroyl chlorides supporting either electro-releasing or
g

M. Luong et al. / Journal of Organometallic Chemistry 723 (2013) 43e48
45
Fig. 1. CCD for three-variable optimizations.
electro-attracting groups. Nevertheless, the corresponding ketones
16] were obtained in only moderate to good yields (38%e65%)
observed acceleration would be ascribed to enhance mass and
energy transfers induced by the cavitational collapse. Like in other
organometallic reactions, the existence of coordinative unsaturated
species as intermediates could also be invoked [18].
In order to determine the scope of this protocol for the simul-
taneous diaroylation of aromatic rings, we studied the reaction of
1,4-bis(tributylstannyl)benzene with benzoyl chloride (Eq. (1)).
[
comparing with experiment 2 in Table 3 (96%). It should be
mentioned that in all the experiments carried out under sonication
no undesired homo-coupling by-products were detected (GC/MS).
Next, and in order to reflect the advantage of ultrasound, we
carried out a series of experiments under conventional thermal
conditions which are presented in Table 5.
ꢀ
After 30 min at 90 C, ketones 2, 6 and 9 were obtained in really
poor yields (27%, 20% and 13%, respectively) together with
considerable amounts of homo-coupling by-products (8%e14%).
Longer reaction times (20 hs) produced an increase in the yield of
ketones (47%, 32% and 29%) but also in the yield of homo-
coupling products (15%e25%). Based on these results we are able
to affirm that ultrasound is really effective in the cross-coupling
reaction studied. The corresponding ketones are synthesized in
good to excellent yields in rather short time and no homo-coupling
products are formed, being this key advantage of the ultrasonic
procedure. Probably, these results would be attributed to the
acceleration caused by ultrasound to the catalytic cycle, thus
avoiding formation of side products.
With satisfaction we found that the reaction led to 1,4-
phenylenebis-(phenylmethanone) in almost quantitative yield
after 30 min under irradiation.
3. Conclusions
In conclusion, this work demonstrates the effectiveness of
From a mechanistic viewpoint, further studies will be required
to elucidate the exact role of sonication. At first glance, the
ultrasonically-induced Stille cross-coupling reactions for the
synthesis of benzophenones. The protocol was optimized by
experimental design. Different benzophenones were obtained in
Table 2
ꢀ
moderate to excellent yields within 30 min, at 90 C; moreover, no
Levels of the experimental variables and the corresponding response values of the
CCD.
by-products, resulting from homo-coupling reactions, were detec-
ted and catalytic turnovers of 4800 have been achieved. In contrast,
the conventional thermal method requires much longer reaction
times (20 hs) rendering the ketones in lower yields together with
significant percentages of homo-coupling sub-products. Thus, the
striking advantages of the method are: i. the increment of the rate
of the reaction, reducing energy consumption; ii. the increase in the
Runs
Independent variables
Yield (%)
Factor A
Power (% W)
Factor B
Time (min)
Factor C
Cycles
Coded
levels
Actual
levels
Coded
levels
Actual
levels
Coded
levels
Actual
levels
1
2
3
4
5
6
7
8
9
1
70
40
55
55
70
55
70
55
55
40
40
80
70
55
30
40
ꢁ1
1
15
30
10
23
15
23
30
23
23
15
30
23
30
35
23
15
1
1
0
70
70
55
30
40
55
70
55
80
40
40
55
40
55
55
70
49.87
74.69
74.23
86.64
30.68
76.74
69.13
65.08
92.19
77.66
96.81
52.12
84.76
72.38
84.87
79.14
ꢁ1
0
ꢁ1.68
0
1
0
1
0
0
ꢁ1
ꢁ1
1.68
1
0
ꢁ1.68
ꢁ1
0
1
0
1.68
ꢁ1
ꢁ1
0
ꢁ1
0
1
0
0
10
11
12
13
14
15
16
ꢁ1
1
0
1
1.68
0
ꢁ1
ꢁ1
0
0
ꢁ1.68
0
1
ꢁ1
Fig. 2. Pareto chart of the standardized effects for 3-methoxybenzophenone.

46
M. Luong et al. / Journal of Organometallic Chemistry 723 (2013) 43e48
Table 3
Table 4
Cross-coupling reactions of benzoyl chloride with o-, m- and p-substituted aryl-
Cross-coupling reactions of tributyl(3-methoxyphenyl)stannane with aroyl
chlorides.
tributylstannyl derivatives.a
a
O
Cl
O
Cl
SnBu3
O
SnBu3
O
MeO
Cl Pd(PPh )
2
3 2
Cl Pd(PPh )
Z
2
3 2
+
Z
Z
+
Z
OMe
Entry
1
Z
Product
Ultrasonic method
yield (%)
b,c
Entry
Z
Product
Ultrasonic method
yield (%)
b,c
p-OMe
O
38 (36)
1
o-OMe
12 (10)
O
OMe
MeO
OMe
8
1
O
2
p-Cl
O
59 (56)
OMe
MeO
2
3
m-OMe
96 (94)
Cl
2
3
O
O
9
3
p-NO
2
O
65 (64)
p-OMe
86 (83)
MeO
OMe
NO2
1
0
a
4
m-Me
94 (93)
ArSn:PhCOCl, 1:1.2; Toluene; 2 mol% of Cl
2
Pd(PPh
3
)
2
with respect to aryl-
ꢀ
stannane; power/cycle: 44/33; 90 C (oil bath); 30 min.
b
Quantified by GLC, using the external standard method. Isolated yields between
brackets.
4
c
No homo-coupling products were observed.
O
yield of ketone; iii. the absence of sub-products, simplifying the
isolation of the products (see Experimental section).
5
6
p-Me
98 (97)
66 (63)
Moreover, it should be mentioned that our protocol allows the
simultaneous biaroylation of an aromatic ring rendering the
desired triaryl diketone in high yield. As far as we know, this is the
first example of a Stille reaction applied to the synthesis of triaryl
diketones in a single step.
5
m-Cl
O
Cl
4. Experimental
4.1. General considerations
6
A Cole Parmer 4710 series ultrasonic homogenizer operating at
7
2,4,6-Me
18 (15)
20 kHz (600 W) provided the high intensity ultrasound. This
consists of an ultrasonic generator equipped with a probe that
emits the sound vibration in the solution through a titanium alloy
bar (25 mm diameter) dipped into the top of the liquid in a two-
necked round-bottomed Pyrex flask (volume 25 mL) equipped
with a thermometer and a nitrogen inlet.
O
7
All reactions were carried out under a dry nitrogen atmosphere.
Acid chlorides were commercially available and distilled under
nitrogen before use. Aryltributylstannanes were prepared by
transmetalation of the appropriate Grignard reagents with tribu-
tyltin chloride in anhydrous THF, and their spectroscopic data
a
ArSn:PhCOCl, 1:1.2; Toluene; 2 mol% of Cl
2
Pd(PPh
3
)
2
with respect to aryl-
ꢀ
stannane; power/cycle: 44/33; 90 C (oil bath); 30 min.
b
Quantified by GLC, using the external standard method. Isolated yields between
brackets.
c
No homo-coupling products were observed.

M. Luong et al. / Journal of Organometallic Chemistry 723 (2013) 43e48
47
Table 5
Thermal method vs ultrasonic method.
0
Thermal methoda,b
0 min
Ultrasonic methoda
Entry
ArSnBu
3
Ar COCl
Product
3
20 hs
1
m-OMe
Phe
27 (8)
20 (9)
13 (14)
47 (25)
96
66
59
O
O
OMe
2
2
m-Cl
Phe
32 (15)
Cl
6
3
m-OMe
p-ClPhe
29 (18)
O
OMe
Cl
9
a
Quantified by GLC, using the external standard method.
Yield of homo-coupling product between brackets.
b
agreed with those previously published [19]. NMR spectra were
4.2.2. Synthesis of 3-methoxybenzophenone (2). Ultrasonic
irradiation (B)
1
13
recorded on a Bruker ARX 300 (300.1 MHz for H, 75.5 MHz for C)
using CDCl as solvent. Identity and purity of the products
crude and/or purified) were established using a GC/MS instrument
HP5-MS capillary column, 30 m ꢂ 0.25 mm ꢂ 0.25 m) equipped
3
2 3 2
To a solution of 0.02 mmol of Cl Pd(PPh ) in 10 mL of toluene,
(
(
1.00 mmol of tributyl(3-methoxyphenyl) stannane and 1.20 mmol of
benzoyl chloride were added and the reaction mixture was exposed
m
ꢀ
with a 5972 mass selective detector operating at 70 eV (EI).
to ultrasonic irradiation at 90 C (oil bath) during 30 min. The
ꢀ
ꢀ
ꢀ
Program: 50 C for 2 min increasing 10 C/min up to 280 C.
For gaseliquid chromatography (GLC) an instrument equipped
with a flame-ionization detector and a HP5 capillary column
solution was washed with sodium bicarbonate, water, and filtered
through a celite plug, in order to eliminate the residue of black
palladium. The organic layer was dried over anhydrous magnesium
(
30 m ꢂ 0.25 mm ꢂ 0.25
m
m) was used.
and concentrated under reduced pressure giving 0.199
0.937 mmol, 94%) of 3-methoxybenzophenone (2) as a yellow oil.
In those reactions where lower yields of ketone were obtained,
g
(
4
.2. Synthesis of benzophenones
the purification was simple and in high percentages; it was carried
out by DCVC on silica gel (hexane:EtOAc, 9.5:0.5) in order to
separate the ketone from the starting arylstannane, compounds
with very different retention factors.
All the reactions were carried out following the same procedure.
One experiment is described in detail in order to illustrate the
method used.
4
.2.1. Synthesis of 3-methoxybenzophenone (2). Classical method
4.3. Recovering method for organotin chlorides
(
A)
To a solution of 0.02 mmol of Cl
.00 mmol of tributyl(3-methoxyphenyl)stannane and 1.20 mmol
2 3 2
Pd(PPh ) in 10 mL of toluene,
The recovery of tributyltin chloride was carried out using the
method resumed in our previous work [6].
1
of benzoyl chloride were added and the reaction mixture was
ꢀ
heated on an oil bath at 90 C for 30 min. The solution was washed
Acknowledgments
with sodium bicarbonate, water, and filtered through a celite plug,
in order to eliminate the residue of black palladium. The organic
layer was dried over anhydrous magnesium sulfate. After evapo-
ration of the solvent, the desired product was separated by dry
column vacuum chromatography (DCVC) [20] on silica gel (hex-
This work was partially supported by CONICET, CIC, ANPCYT and
the Universidad Nacional del Sur, Bahía Blanca, Argentina.
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1
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d 8.10e6.96
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9] (a) C.E. Domini, L. Vidal, A. Canals, Ultrason. Sonochem. 16 (2009) 686;
b) C.E. Domini, L. Vidal, G. Cravotto, A. Canals, Ultrason. Sonochem. 16 (2009)
64.
(e) Ketone 11, see ref [4b].
[17] O. Chuzel, A. Roesch, J.P. Genet, S. Darses, J. Org. Chem. 73 (2008) 7800.
[18] J.L. Luche, in: T.J. Mason (Ed.), Advances in Sonochemistry, JAI Press Ltd.,
London, 1993.
[19] Data for arylstannanes: (a) For compounds p-OMe, p-Me, o-Me see:
H. Schumann, I. Schumann (Eds.), Gmelin Handbuch der Anorganischen
Chemie, Zinn-organische Verbindungen, Teil 2, Band 29, Springer Verlag, New
York, 1975;
(
(
[
(
5
[
10] J. Guervenou, P. Giamarchi, C. Coulouam, M. Guerda, C. Le Lez, T. Oboyet,
(b) For compounds m-OMe, m-Me, see: M.B. Faraoni, L.C. Koll, S.D. Mandolesi,
A.E. Zuñiga, J.C. Podestá J. Organomet. Chem. 613 (2000) 236;
(c) For compound m-Cl see: T. Furuya, A.E. Strom, T. Ritter J. Am. Chem. Soc.
131 (2009) 1662;
Chemometrics Intell. Lab. Syst. 63 (2002) 81.
[
[
[
11] N. Bouzidi, C. Gozzi, J. Chem. Educ. 85 (2008) 1544.
12] J.W. Labadie, J.K. Stille, J. Am. Chem. Soc. 105 (1983) 6129.
13] For example: (a) R.J. Linderman, J.M. Siedlecki, J. Org. Chem. 61 (1996) 6492;
(d) For compound 2,4,6-triMe see: A.S.Y. Lee, W.C. Dai Tetrahedron 53 (1997) 859.
[20] D.S. Pedersen, C. Rosenbohn, Synthesis 16 (2001) 2431.
(
b) K.W. Kells, J.M. Chong, J. Am. Chem. Soc. 126 (2004) 15666;