Palladium-catalyzed heck coupling-hydrogenation: Highly efficient one-pot synthesis of dibenzyls and alkyl phenyl esters

Article abstract of DOI:10.1002/adsc.200800460

An efficient method for the synthesis of industrially important dibenzyls and alkyl phenyl esters via sequential Heck coupling and hydrogenation of the alkenyl double bond in one pot with a single recyclable catalyst under mild conditions has been realised. The catalyst was recovered by simple filteration and reused for several cycles with consistent activity.

Full text of DOI:10.1002/adsc.200800460

FULL PAPERS
DOI: 10.1002/adsc.200800460
Palladium-Catalyzed Heck Coupling-Hydrogenation: Highly
Efficient One-Pot Synthesis of Dibenzyls and Alkyl Phenyl Esters
M. Lakshmi Kantam,a,* Rajashree Chakravarti,a Venkat Reddy Chintareddy,a,b
B. Sreedhar,a and Suresh Bhargavac
a
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad – 500007, India
Fax : (+91)-40-2716-0921; phone: (+91)-40-2719-3510; e-mail: mlakshmi@iict.res.in or lkmannepalli@yahoo.com
Present address: 1311 Gilman Hall, Iowa State University, Ames, IA 50011, USA
School of Applied Sciences, RMIT University, Melbourne, Australia
b
c
Received: July 24, 2008; Published online: November 5, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800460.
Abstract: An efficient method for the synthesis of in- filteration and reused for several cycles with consis-
dustrially important dibenzyls and alkyl phenyl tent activity.
esters via sequential Heck coupling and hydrogena-
tion of the alkenyl double bond in one pot with a Keywords: dibenzyls; molecular hydrogen; nanocrys-
single recyclable catalyst under mild conditions has talline magnesium oxide; one-pot synthesis; palladi-
been realised. The catalyst was recovered by simple um(0)
Introduction
and C=C bonds and consecutive Heck reaction fol-
lowed by hydroformylation. The limitation in this
Organic synthesis employing heterogeneous catalysts system is that the activity of the catalyst remains con-
has several beneficial effects on the environment nected to the individual activity of the single support-
since it involves the reuse of precious metal catalysts, ed metal and a number of different metal complexes
thus forming less waste and resulting in lower metal are required to be immobilized. Thus, a single sup-
contamination in the final pharmaceutically important ported metal catalyst, which is applicable to the over-
molecules.[1] In an ideal setting one could make a all reaction, is most useful in this context.
double benefit of the heterogeneous catalysis by using
the same active system for multiple reactions in one- in a variety of transformations in organic synthesis.[1,8]
pot. Therefore, supported palladium catalysts became the
The versatility of palladium catalysts is established
One of the major challenges faced by the fine best choice to be applied to multistep catalytic synthe-
chemicals industry in recent times is the synthesis of ses.
these target molecules by a large number of different
Dibenzyls form important substructures of several
reaction steps involving elaborate work-up and refin- natural and synthetic compounds such as lunularic
ing procedures.[2] The recently introduced one-pot acid, an antifungal agent and bifluranol,[9] an anti-an-
synthetic approach[3] not only helps to improve atom drogen agent. Recently, Djakovitch and co-workers[10]
economy, but also reduces the number of steps in iso- reported the synthesis of dibenzyls in one pot by
lation and purification of the intermediate compounds using Pd on activated carbon. A palladium-catalyzed
in the case of a multistep synthesis.[4]
synthesis of biaryl derivatives via tandem Heck aryla-
A series of homogeneously catalyzed reactions re- tion-hydrogenation was reported by Fagnou and co-
portedly illustrate the applicability of this strategy.[5] workers.[11] Later Corma and co-workers[12] synthe-
However, the negative mutual interactions taking sized important fragrances like alkyl phenyl esters by
place when all the reactions are run in the presence using heterogeneous palladium catalysts.
of the same catalysts poses a problem.[6] To overcome
We have earlier reported Suzuki and Stille cross-
this problem, Arai and co-workers[7] have reported coupling reactions[13] using a ligand-free nanocrystal-
the application of bimetallic Pd and Ru immobilized line magnesium oxide-stabilized palladium(0) catalyst.
on silica for the simultaneous hydrogenation of C=O Later, we have applied the same catalyst for the effi-
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Palladium-Catalyzed Heck Coupling-Hydrogenation
FULL PAPERS
Scheme 1. Palladium-catalyzed synthesis of dibenzyl via Heck coupling and in situ reduction.
cient synthesis of organic amines from the corre-
sponding nitro compounds in the presence of molecu-
lar hydrogen as the reductant under ambient condi-
tions.[14]
Table 1. Screening of different palladium-supported catalysts
for the synthesis of dibenzyls.[a]
Herein, we report a practical and atom economic
synthesis of dibenzyls in good to excellent yields by a
sequential Heck coupling reaction of haloarenes
(chloro, bromo and iodo) and styrenes followed by
hydrogenation of the resulting stilbenes by employing
molecular hydrogen under atmospheric pressure
(Scheme 1) using nanocrystalline magnesium oxide-
stabilized palladium [NAP-Mg-Pd(0)] catalyst.
Results and Discussion
All the catalysts were prepared by counterionic stabi-
lization of PdCl4 as described in the literature.[13,14]
To identify the ideal reaction conditions, a diverse
range of supported palladium catalysts were screened
using bromobenzene and styrene as model substrates
for the sequential reaction and results are summar-
ized in Table 1. A variety of solvents and bases are
also screened (see Supporting Information). After the
completion of the Heck reaction under an N2 atmos-
phere, the reaction mixture was cooled to room tem-
perature and the atmosphere of the reaction vessel
was replaced by hydrogen after flushing out the re-
maining nitrogen.
[a]
Reaction conditions: bromobenzene (1.0 mmol), styrene
(1.2 mmol), NaOAc (1.2 mmol), catalyst (1.43 mol%)
and DMAc (4.0 mL) were stirred at 1008C for an appro-
priate time under an N2 atmosphere. The reaction mix-
ture was then cooled to room temperature and stirred
under an H2 atmosphere.
Time required for the formation of stilbene.
GC yields of stilbene.
Time required for hydrogenation of stilbene to dibenzyl.
GC yields of dibenzyl.
[b]
[c]
[d]
[e]
[f]
0.2 mol% Pd is used, reaction temperature 1208C; yield
is recorded under
a
hydrogen flow of 5 mLminÀ1
Optimum yield of the product (98%) was obtained
by using [NAP-Mg-Pd(0)] (Table 1, entry 1). No prod-
ucts were formed when the reaction was carried out
in the absence of palladium. When the reaction was
(5 bar).[12]
[g]
Reaction temperature 1358C.
carried out in the presence of Pd-SiO2 and added product in good yields but the hydrogenated product
base, the Heck coupling step proceeded very sluggish- was obtained under a hydrogen pressure of 5 bar
ly to form stilbene (5%), but the hydrogenation step (Table 1, entries 6 and 7). Homogeneous PdCl2 re-
did not proceed even at elevated temperatures quired a much higher temperature to yield a trace
(Table 1, entry 2). Pd-C also failed to form any per- amount of the Heck coupled product, but no product
ceptible amount of product in the hydrogenation step was detected in the hydrogenation step (Table 1,
although a small amount of stilbene was formed in entry 8).
the coupling step under our reaction conditions
The enhanced activity of the catalyst [NAP-Mg-
(Table 1, entry 3). Pd-MgLaO showed equal efficiency Pd(0)] over the different supported palladium cata-
in the Heck coupling reaction albeit the hydrogena- lysts screened as well as its homogeneous counter-
tion step proceeded with only moderate yield of prod- parts can be explained by the high basicity and sur-
uct (70%) (Table 1, entry 4). Pd-LDH (Mg-Al layered face area [600 m2 gÀ1] of nanocrystalline magnesium
double hydroxides) also catalyzed the coupling reac- oxide. The presence of edge-corner and other defect
tion with equal ease but was inactive for the hydroge- sites allows the nanostructured MgO materials to pos-
nation step under ambient pressure (Table 1, entry 5), sess a high concentration of reactive surface ions. The
whereas Pd-Al2O3 and Pd-TiO2, yielded the coupling reactive sites on the surface of MgO are as follows: 1)
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Table 2. Palladium-catalyzed one-pot synthesis of dibenzyls from haloarenes and styrenes.[a]
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Palladium-Catalyzed Heck Coupling-Hydrogenation
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Table 2. (Continued)
[a]
Reaction conditions: aryl halide (1.0 mmol), styrene (1.2 mmol), NaOAc (1.2 mmol), NAP-Mg-Pd(0) (1.43
mol%) and DMAc (4 mL) were stirred at 1008C for an appropriate time under an N2 atmosphere. [TBAB
(0.2 mmol) was used in the reactions of chloroarenes]. The reaction mixture was then cooled to room tempera-
ture and stirred under an H2 atmosphere (balloon).
All products are characterized by NMR and mass spectroscopy (see Supporting Information).
Isolated yields after column chromatography.
[b]
[c]
[d]
Yield after sixth cycle.
the Mg2+ site, which is of the Lewis acid type, 2) the oarenes and styrenes were subjected to this sequence
O2À site which is of the Lewis base type, 3) lattice- of reactions to produce the corresponding dibenzyls
bound and 4) isolated hydroxy groups and anionic in good to excellent yields (Table 2).
and cationic vacancies. An edge or even more so, a
Bromobenzene reacted with styrene quantitatively
corner O2À anion is coordinatively unsaturated and is to produce trans-stilbene selectively in the presence
seeking Lewis acids to help stabilize and delocalize its of [NAP-Mg-Pd(0)] (Table 2, entry 6). In all the reac-
negative charge. Conversely, an Mg2+ ion on an edge tions, the ratio of haloarenes to styrenes is 1:1.2 which
or corner is seeking Lewis bases to stabilize and de- gives the optimum yield of the product. Under these
localize its positive charge. Therefore, these coordina- conditions neither the formation of biaryls resulting
tively unsaturated O2À and Mg2+ ions readily accept from competing homocoupling reaction nor branched
incoming reagents with Lewis acid or Lewis base derivatives were detected as by-products. To ensure
character.[15,16]
the active species of palladium, which takes part in
The optimum temperature of the Heck coupling is the reaction, the same reaction (Table 2, entry 6) was
found to be 90–1008C and the hydrogenation of the run using NAP-Mg-PdCl4 as catalyst instead of [NAP-
resulting stilbenes takes place at room temperature Mg-Pd(0)] under the same conditions. It was seen
under ambient pressure. Other conditions were then that only 10% of the desired product is obtained.
optimized using the same model substrates and Leaching of Pd was noticed during the course of the
[NAP-Mg-Pd(0)] as catalyst (see Supporting Informa- reaction as determined by atomic absorption spectros-
tion). The reaction was most productive when dime- copy (AAS). This is in agreement with the observa-
thylacetamide (DMAc) was used as the solvent and tions reported by Djakovitch and Koehler.[18]
sodium acetate (NaOAc) as base. The use of triethyl-
Aryl bromides and iodides containing a variety of
amine or K2CO3 gave the product of Heck coupling substituents underwent this cascade reaction with
but failed to form any hydrogenated product. In the equal ease. Aryl chlorides (Table 2, entries 16–19) al-
absence of any added base, the highly basic NAP- though less reactive, are more desirable substrates
MgO acts as a base in the case of the Heck coupling than their bromide and iodide counterparts in terms
reaction to produce the arylated product in 5% yield of cost and availability.[19] The reactions with aryl
but no product was obtained from the subsequent chlorides also proceeded smoothly albeit with slightly
step. This is in agreement with the currently accepted lower yields compared to their bromide and iodide
mechanism for the Heck reaction which requires stoi- analogues. The nitro group present on the aryl bro-
chiometric neutralization of the hydrohalogenic acid mides and iodides was cleanly reduced to its corre-
by an appropriate base.[17] Other solvents like dioxane sponding amine in addition to the alkenyl double
and NMP were found to be less effective. The use of bond under these reaction conditions (Table 2, en-
less polar solvents such as toluene drastically decreas- tries 2, 7, and 17), whereas ketone and aldehyde func-
es the yield of the coupling product (10%). Under tionality was preserved (Table 2, entries 8, 9, 15, 18
these optimized conditions, a number of different hal- and 19). The reaction was more facile with haloarenes
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M. Lakshmi Kantam et al.
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possessing an electron-withdrawing group in the para- compounds in fragrances and fine chemicals indus-
position (Table 2, entries 2, 7, 8, 9, and 15). However, tries.[12]
presence of an ortho-substituent gave only trace
The coupling reaction proceeded faster with the
amount or no products, this can be attributed to the acrylates, than with their styrene counterparts, albeit
steric factor. In presence of an electron-donating the hydrogenation step was comparatively slower and
group, the reaction was found to be slower (Table 2, proceeded only at elevated temperatures. Aryl bro-
entries 3, 4, 10, 11). This methodology was further ex- mides and chlorides coupled with acrylates with equal
tended to polyaromatic haloarenes (Table 2, entries 5 efficiency in this cascade reaction and results are sum-
and 12) successfully.
marized in Table 3.
Later, we successfully extended the scope of the
The nitro group was cleanly reduced to the corre-
methodology using acrylates as coupling partners sponding amine along with the alkenyl moiety
under the similar reaction conditions (Scheme 2) to (Table 3, entries 2 and 8), while the keto functionally
produce alkyl phenylpropionates, which are useful was greatly tolerated under the present reaction con-
ditions (Table 3, entry 3). In the presence of an elec-
tron-donating group, the reaction proceeds at higher
temperature, and takes a longer time (Table 3,
entry 4). Interestingly, the formation of 4-methoxy-
phenyl propionic acid (20%) was also observed in this
case, which may be attributed to the higher tempera-
Scheme 2. Synthesis of alkyl phenylpropionates.
tures and longer reaction time.
Table 3. Palladium-catalyzed synthesis of alkyl phenylpropionates from haloarenes and
acrylates.[a]
[a]
Reaction conditions: aryl halide (1.0 mmol), acrylate (1.2 mmol), NaOAc (1.2 mmol),
NAP-Mg-Pd(0) (1.43 mol%) and DMAc (4 mL) were stirred at 808C for an appro-
priate time under an N2 atmosphere. The reaction mixture was then cooled to room
temperature and stirred under an H2 atmosphere under ambient pressure at 808C.
[b]
All products are characterized by NMR and mass spectroscopy (see Supporting In-
formation).
Isolated yields.
Reaction temperature: 1208C; Reaction time: 12 h.
[c]
[d]
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Palladium-Catalyzed Heck Coupling-Hydrogenation
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Reusability of the Catalyst
Experimental Section
The catalyst [NAP-Mg-Pd(0)] was prepared according to the
procedure reported in the literature[13,14] (see Supporting In-
formation for details). All other catalysts used were synthe-
sized based on the literature reports.[17,22]
The final product was isolated from the catalyst by
simple filtration after the reaction. The catalyst was
washed first with water and then with diethyl ether to
remove any excess base and organic material. It was
then dried at room temperature and used without any
regeneration for further cycles. It was observed that
the catalyst showed consistent activity until sixth cycle
(Table 1, entry 1). The TEM image of the used cata-
General Procedure for the Synthesis of Dibenzyls and
Alkyl Phenylpropionates
In an oven-dried, 10-mL, two-necked round-bottom flask
lyst did not show any change in the shape and size of was charged the haloarene (1.0 mmol), styrene or acrylate
(1.2 mmol), NaOAc (1.2 mmol), [NAP-Mg-Pd(0)] (0.015 g,
1.43 mol%) and dimethylacetamide (4.0 mL) and the mix-
ture stirred at 1008C (808C for acrylates) under a nitrogen
atmosphere for an appropriate time (4–8 h for styrene sub-
strates and 3–6 h for acrylates) The progress of the reaction
was monitored by thin layer chromatography (TLC). On the
complete formation of the Heck product from the first step,
the reaction mixture was cooled to room temperature and
the nitrogen atmosphere was replaced by flushing with H2
via a hydrogen-filled balloon. The mixture was then again
stirred at room temperature (808C for acrylates) under a hy-
drogen atmosphere for an appropriate time (2–6 h for sty-
rene substrates and 3–12 h for acrylates). After the comple-
tion of the reaction as judged by TLC, the reaction mixture
was diluted with water (20 mL), and then extracted with
EtOAc (3ꢁ10 mL). The combined organic layers were
washed with brine (10 mL) and dried over anhydrous
Na2SO4. The solvent was evaporated under reduced pressure
to yield the crude product, followed by flash chromatogra-
phy over silica gel (60–120 mesh) column using ethyl ace-
tate/hexane (2:100) v/v as an eluent to afford the pure prod-
the support as well as the active species, Pd(0). Thus,
the morphology of the catalyst remains the same even
after several recycles (Figure 1 a and b).
Figure 1. Transmission electron micrographs of (a) fresh cat-
alyst, (b) used catalyst.
A slight decrease in activity was observed in the uct.
subsequent cycles, which may be attributed to the de-
Supporting Information
crease in the surface area due to hydration of the cat-
alyst by water formed in the reaction.[20] No quantifia-
ble amount of leached Pd was detected in the filtrate
by AAS.
The effect of the highly basic nanocrystalline mag-
nesium oxide as demonstrated in the formation of the
dibenzyls and alkyl phenylpropionates, coupled with
the absence of any leached metal in the filtrate sug-
gests that the catalytic process is truly heterogeneous
and the reaction occurs on the solid surface.[8c,13,21]
Complete experimental details for catalyst synthesis, solvent
screening, catalyst optimization studies, 1H and 13C NMR
spectra of the products synthesized are given in the Support-
ing Information.
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