Supported palladium nanoparticle-catalysed Suzuki–Miyaura cross-coupling approach for synthesis of aminoarylbenzosuberene analogues from natural precursor

Article abstract of DOI:10.1002/aoc.3749

A semi-synthetic method has been developed for the synthesis of aminoarylbenzosuberenes (AABs) from naturally occurring himachalenes, an isomeric mixture of sesquiterpenes present in Cedrus deodara oil. Polymer-stabilized Pd(0) nanoparticle-catalysed Suzu

Full text of DOI:10.1002/aoc.3749

Received: 30 September 2016
DOI 10.1002/aoc.3749
Revised: 13 December 2016
Accepted: 4 January 2017
F U L L PA P E R
Supported palladium nanoparticle‐catalysed Suzuki–Miyaura
cross‐coupling approach for synthesis of aminoarylbenzosuberene
analogues from natural precursor
Richa Bharti1,2 | C. Bal Reddy1,2 | Sandeep Kumar1,2 | Pralay Das1,2
1 Natural Product Chemistry and Process
Development Division, CSIR‐Institute of
Himalayan Bioresource Technology,
Palampur 176061HP, India
2 Academy of Scientific and Innovative
Research (AcSIR), New Delhi, India
A semi‐synthetic method has been developed for the synthesis of aminoarylben-
zosuberenes (AABs) from naturally occurring himachalenes, an isomeric mixture
of sesquiterpenes present in Cedrus deodara oil. Polymer‐stabilized Pd(0)
nanoparticle‐catalysed Suzuki–Miyaura cross‐coupling reaction of aminovinyl
bromide‐substituted benzosuberenes has been adopted for AAB synthesis. The
catalyst performed well with different amine substituents, and was recycled up to five
times. The synthesis of such arylated benzosuberene class of compounds from
natural precursors following semi‐synthetic approaches could provide an attractive
alternative method with reduced number of steps.
Correspondence
Pralay Das, Natural Product Chemistry and
Process Development Division, CSIR‐
Institute of Himalayan Bioresource
Technology, Palampur 176061, HP, India.
Email: pdas@ihbt.res.in; pdas_nbu@yahoo.
com
KEYWORDS
benzosuberenes, heterogeneous catalysts, himachalenes, Suzuki–Miyaura cross‐coupling
1 | INTRODUCTION
perbromide as brominating agent, followed by arylation with
anhydrous zinc chloride, phenyllithium and Pd(PPh3)4.[9]
Benzosuberenes are important key structural motifs in vari-
ous natural products such as terpenes,[1,2] alkaloids[3] and
pharmacologically active compounds like nortriptyline which
has antidepressant activity[4] and (−)colchicine which is an
antitumour agent.[5] The role of arylated benzosuberene
derivatives (Figure 1) is established in medicinal chemistry
as anticancer agents because of their structural reminiscence
to colchicine and combretastatin CA4 and CA1.[6]
Various synthetic approaches for the synthesis of arylated
benzosuberene have been developed by Pinney and co‐
workers[7] starting from tetrahydronaphthalene following six
steps with arylation being carried out using trimethoxy-
aryllithium, synthesized from 3,4,5‐trimethoxybromo-
benzene, under harsh reaction conditions with poor overall
yields. The developed method was further modified[8] for
benzosuberene synthesis using arylaldehyde as the starting
molecule following a similar approach. In another report,
Jorden and co‐workers synthesized metabolites of tamoxifen
from benzosuberone using pyridine hydrobromide
All the reported methods followed multistep synthesis
involving lithiation under harsh conditions, selective reduc-
tions and Grignard reagents. In this context, we envisioned
that the aminovinyl bromide‐substituted benzocycloheptene
that was synthesized previously[10] in our laboratory could
be a precursor for the synthesis of arylated benzosuberene.
In addition, another synthetic route for the synthesis of
arylated benzosuberene is the formation of enol triflate from
benzosuberone followed by Suzuki–Miyaura (SM) cross‐cou-
pling with arylboronic acids.[11] Extensive efforts have been
devoted to SM cross‐coupling reactions of vinyl halides
catalysed by copper[12] and palladium.[13] Recently, Li and
co‐workers have reported SM cross‐coupling reaction of
trialkylboranes with aliphatic vinyl halide in Pd(OAc)2/
RuPhos system.[14] Similarly, SM cross‐coupling reactions
of aryl and vinyl halides with aryl and aliphatic boronic acids
have been reported catalysed by trans‐PdBr(N‐Succ)(PPh3)2
(a precatalyst),[15] Pd(P(o‐Tol)3)2,[16] Pd2(dba)3/P(t‐Bu)3
[17]
and Pd(PPh3)4.[18] Although the handling of these
Appl Organometal Chem. 2017;e3749.
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https://doi.org/10.1002/aoc.3749

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BHARTI ET AL.
FIGURE
1
Representative biologically active molecules with
benzosuberene as the central core
FIGURE 3 Powder XRD pattern for Pd@PS catalyst
SCHEME
1
Representative method of aminovinyl bromide‐
substituted benzocycloheptene synthesis from himachalene[10]
air‐sensitive ligands and catalysts is difficult, the methodolo-
gies find their value for the total synthesis of some complex
molecules. The scope of this transformation has been
explored for the synthesis of (+)frondosin A[19] and intramo-
lecular β‐alkyl[20] following SM cross‐coupling for taxane
skeleton.
The development of supported transition metal nanoparti-
cles (NPs) as catalysts for various organic transformations has
attracted much attention in recent years.[21] Recently, our
group has been working in the area of exploration of heteroge-
neous NPs as catalysts in various organic transformations[22]
and synthetic modulation of natural products.[10] Herein, we
report a semi‐synthetic approach to aminoarylbenzosuberenes
(AABs) from aminovinyl bromide‐substituted benzosu-
berenes (synthesized from himachalene, an essential oil of
Cedrus deodara) following SM cross‐coupling reactions
under ligand‐free polymer‐stabilized Pd(0) (Pd@PS)‐
catalysed conditions.
2 | RESULTS AND DISCUSSION
2.1 | Preparation and characterization of
Pd@PS catalyst
The Pd@PS NP catalyst was prepared following a reduction
deposition method[21] of partially borohydride (BH4
)
−
exchanged Amberlite IRA‐900 chloride from resin to influ-
ence faster reduction of Pd(II) salts to Pd(0) and simulta-
neous impregnation into hydrophobic pockets of the
polymeric matrix. The characterization of the Pd@PS cata-
lyst was done by scanning electron microscopy (SEM),
FIGURE 2 (a) TEM image at 5 nm scale. (b) histogram describing
the particle size distribution. (c) high‐resolution TEM image showing
fast‐Fourier transform of marked area. (d) SAED pattern of Pd@PS

BHARTI ET AL.
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TABLE 1 Optimization of reaction conditions for SM cross‐coupling reactiona
Entry
Catalyst (Mol%)
Temp. (°C)
Base
Time (min/h)
Solvent
Yield (%)b
1c
2
Pd@PS (4)
Pd@PS (4)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (3)
Pd@PS (2)
Pd(OAc)2 (3)
Pd(PPh3)4 (3)
Pd/C (3)
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
110
90
K2CO3
K2CO3
DABCO
DBU
15 h
60
60
60
60
60
60
60
60
60
60
60
60
90
60
60
60
60
60
60
45
DMF
DMF
DMF
DMF
DMF
1,4‐Dioxane
EtOH
THF
70
97
3
10
4
Trace
20
5
Et3N
6
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
Cs2CO3
KtOBu
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
40
7
NR
NR
76
8
9
DMA
PEG‐400
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
10
11
12
13
14d
15
16
17
18
19
20
21
95
98
52
54
79
69
80
85
73
54
Pd@PS (3)
Pd@PS (3)
60
110
80
aReaction conditions: 1a (0.27 mmol), 2a (0.32 mmol), Pd@PS (3 mol% of Pd), K2CO3 (0.54 mmol), DMF (1 ml), 110 °C, 60 min.
bIsolated yield of product.
cConventional heating.
d3 equiv. of base.
transmission electron microscopy (TEM), X‐ray diffraction
(XRD) analysis and selected area electron diffraction (SAED)
studies (Figure 2). The dispersion of the Pd NPs in the poly-
mer matrix was analysed using SEM and SEM‐EDS
(supporting information, Figure S1). Further, low‐field
TEM images (Figure 2a) revealed the impregnation of the
Pd NPs with maximum average particle size of between 1
and 3 nm (Figure 2b) as calculated from TEM image
(Figure 2a). The crystal structure of Pd@PS was investigated
using high‐resolution TEM, showing the interplanar distance
between two lattice fringes as 0.22 nm (Figure 2c) corre-
sponding to the (111) plane of Pd. The four Debye–Scherrer
rings observed in the SAED pattern of Pd@PS are assigned
to (111), (200), (220) and (311) planes of Pd NPs (Figure 2
d). The fast‐Fourier transform diffractogram of a selected
region also shows the arrangement of Pd metal in the catalyst
(Figure 2c).
Powder XRD analysis was also performed for the Pd@PS
catalyst, and peaks are clearly visible at ~ 40° and ~ 45°
(Figure 3) corresponding to (111) and (200) planes of Pd
which also correlate with the SAED pattern (Figure 2d).
The Hg(0) poisoning test was also performed to confirm the
heterogeneity of the Pd@PS catalyst.
In our earlier report,[10] we described the synthesis of
aminovinyl bromide‐substituted benzocycloheptene (1) from
himachalene following aromatization, bromination and

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BHARTI ET AL.
amination approaches (Scheme 1). The same strategy was
employed for the synthesis of compounds 1a–1 g, of which
1a–1e were compared with reported data and 1f and 1 g were
characterized using NMR spectral analysis (supporting
information). These synthesized compounds were further
applied for novel AAB molecule synthesis following SM
cross‐coupling reactions (Scheme 1).
Initially we focused on SM cross‐coupling reaction
of aminovinyl bromide‐substituted benzocycloheptene 1a
(1 equiv.) with phenylboronic acid 2a (1.2 equiv.) using
Pd@PS (4 mol%) and K2CO3 (2 equiv.) under conventional
heating at 120 °C for 15 h, which provided the desired
product 3a in 70% yield (Table 1, entry 1). In contrast, the
same reaction under microwave irradiation (100 W) at
110 °C provided 3a in 97% yield with a reaction time of
60 min (Table 1, entry 2). A detailed optimization study
was conducted by screening some organic bases (Table 1,
entries 3–5) and solvents (Table 1, entries 6–10) but no
noticeable change in the product yield was observed. Further
optimization studies were carried out with various inorganic
bases (K2CO3, Na2CO3, Cs2CO3 and KtOBu) (Table 1,
entries 11–15) and changing the catalyst loading (Table 1,
entry 16), from which K2CO3 was found to be a suitable
base. Indeed, the reaction catalysed by homogeneous
Pd(OAc)2, Pd(PPh3)4 and heterogeneous Pd/C catalysts gave
lower yield of desired product (Table 1, entries 17–19). The
product yield dropped on reducing the temperature and
reaction time (Table 1, entries 20 and 21). The optimized
reaction conditions were selected to be: Pd@PS (3 mol%),
K2CO3 (2 equiv.) in DMF solvent at 110 °C under microwave
irradiation (100 W) for 60 min, giving the highest yield of
product 3a of 98% (Table 1, entry 11).
The versatility of the developed protocol was extended
for SM cross‐coupling reactions of various aminovinyl
bromide‐substituted benzocycloheptenes (1) with a variety
of arylboronic acids (Table 2). The coupling reaction of N‐
cyclohexylaminovinyl bromide benzocycloheptene (1a) with
electron‐rich p‐OMe‐, p‐Me‐ and 3,5‐dimethyl‐substituted
phenylboronic acids as nucleophilic partners under
optimized conditions gave the desired AAB products 3b–d
in 75–87% yields (Table 2, entries 1–3). p‐Acetyl‐ and 3,4‐
dichloro‐substituted phenylboronic acids also participated in
TABLE 2 Pd@PS‐catalysed SM cross‐coupling reaction of N‐alkylvinyl bromide benzocycloheptenesa
Entry
1
Reactant (1)
R3
Product (3)
4‐OMe
2
3
1a
1a
4‐Me
3,5‐dimethyl
(Continues)

BHARTI ET AL.
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TABLE 2 (Continued)
4
5
1a
1a
4‐COMe
3,4‐dichloro
6
7
H
1b
1b
1b
4‐OMe
4‐Me
4‐COMe
H
8
9
10
(Continues)

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BHARTI ET AL.
TABLE 2 (Continued)
11
H
aReaction conditions: 1 (1 equiv.), arylboronic acids 2 (1.2 equiv.), Pd@PS (3 mol% of Pd), K2CO3 (2 equiv.), DMF (1 ml), 110 °C, 60 min.
bIsolated yield of product.
cIsolated yield of product in Pd(OAc)2.
TABLE 3 Pd@PS‐catalysed SM cross‐coupling reaction of N‐benzylvinyl bromide benzocycloheptenesa
Entry
12
Reactant (1)
R2
H
Product (3)
13
14
1e
1e
4‐Me
4‐COMe
15
4‐Me
(Continues)

BHARTI ET AL.
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TABLE 3 (Continued)
16
1f
4‐OMe
17
H
aReaction conditions: 1 (1 equiv.), arylboronic acids 2 (1.2 equiv.), Pd@PS (3 mol%), K2CO3 (2 equiv.), DMF (1 ml), 110 °C, 60 min.
bIsolated yield of product.
cIsolated yield of product in Pd(OAc)2.
similar reactions producing desired products 3e and 3f in
considerably good yields of 69 and 80% (Table 2, entries 4
and 5). Gratifyingly, morpholine‐substituted vinyl bromide
benzocycloheptene 1b was found to be an excellent substrate
for the SM cross‐coupling reaction with various electron‐rich
and electron‐deficient arylboronic acids as nucleophilic part-
ners and afforded the desired products 3 g–j in good yields of
75–85% (Table 2, entries 6–9). Interestingly, no significant
steric effects of highly substituted acyclic alkylamine com-
pounds 1c and 1d were observed for SM cross‐coupling reac-
tion to produce the corresponding products 3 k and 3 l in 76
and 72% yields, respectively. N‐benzylaminovinyl bromide
benzocycloheptene 1e (Table 3).was also found to be a good
substrate for this transformation and smoothly coupled with
FIGURE 4 X‐ray crystal structure of 3i with an ellipsoid contour
electron‐rich and electron‐deficient arylboronic acids to give
the desired coupled products 3 m–o (68–74%). Most of the
methoxy‐substituted benzosuberenes (Figure 1) are good bio-
active targets; therefore compounds 3p and 3q were synthe-
sized successfully from 1f following the same approach.
Similarly, methyl substitution at α‐position of benzylamine
in compound 1 g had no effect on the reaction giving the
product 3r in 75% yield (Table 3, entry 17).
probability level of 50% (CCDC 1476591)
used as a substrate under standard reaction conditions. This
reacted smoothly with arylboronic acids to give the corre-
sponding products 5a and 5b in 80–81% yields
(Scheme 2).[23]
The structure of compound 3i obtained as colourless
crystals from DCM–hexane (1:1) mixture was further con-
firmed from X‐ray crystallography studies (Figure 4;
supporting information).
To investigate the applicability of the present method for
other bicyclic vinyl bromides, 2‐bromo‐1H‐indene (4a) was
SCHEME 2 Pd@PS‐catalysed Suzuki cross‐coupling of vinyl
bromides

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BHARTI ET AL.
the Pd@PS catalyst was performed after five reaction
cycles to determine the morphology of the catalyst and dis-
tribution of Pd NPs at 100 nm scale with average particle
size of 4–5 nm (Figure 5).
2.2 | Recyclability experiment
The recyclability of the Pd@PS catalyst was investigated
by following the SM cross‐coupling reaction of 1a and
phenylboronic acid under optimized reaction conditions.
After completion of the reaction the catalyst was recovered
by simple filtration, washed with water and acetone and
dried under reduced pressure. The catalyst was reused for
subsequent reactions and found to be active for up to five
runs without significant loss of activity. TEM analysis of
2.3 | Mercury test
The mercury drop test was conducted to evaluate the catalytic
active species in the reaction solution. The SM cross‐cou-
pling reaction between 1a and phenylboronic acid was cho-
sen as a model reaction using Pd@PS and K2CO3 as a
base. When a drop of mercury was added to the reaction,
no product formation was observed after 60 min due to amal-
gam formation or adsorption of Hg(0) onto the surface of the
heterogeneous catalyst (Pd@PS). This indicated the catalytic
participation of Pd(0) in the reaction, and the catalysis
involved in the reaction is truly heterogeneous.
3 | CONCLUSIONS
In summary, a semi‐synthetic approach was adopted for the
synthesis of AAB classes of compounds from natural precur-
sor with reduced number of steps. Various cyclic, acyclic and
benzylamine‐substituted vinyl bromide benzosuberene ana-
logues were found to be active for SM cross‐coupling reaction
with electron‐rich and electron‐deficient phenylboronic acids.
The catalyst was recycled for five runs without any observable
decrease in efficiency. The present research for novel AAB
compounds from himachalenes could be of interest in acade-
mia and industry for development of biological activity.
4 | EXPERIMENTAL
4.1 | General methods
All solvents and reagents were purchased from reputed
commercial sources. Reactions were monitored by TLC
plates coated with 0.2 mm silica gel 60 F254. TLC plates
were visualized by UV irradiation (254 nm) and iodine spray.
The products were purified by column chromatography
employing silica gel of 60–120 mesh size. HRMS were
conducted with UHR‐QTOF (ultra‐high resolution Q‐time‐
of‐flight). 1H NMR (600 and 300 MHz) and 13C NMR
(150 and 75 MHz) spectra were recorded using CDCl3
and tetramethylsilane as solvent and internal reference,
respectively. The coupling constants (J) are reported in
hertz (Hz) and the following abbreviations are used to
designate signal multiplicity: s = singlet; d = doublet;
t = triplet; m = multiplet; br = broad singlet. CEM
Discover™ focused microwaves (2450 MHz, 300 W) were
used. The temperature on the surface of the reaction flask
was measured with an inbuilt infrared temperature probe
FIGURE 5 (a) TEM image of Pd@PS after five reaction cycles. (b)
histogram showing particle size distribution after five cycles. (c)
recyclability results for Pd@PS catalyst

BHARTI ET AL.
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yl)methyl)cyclohexanamine (1a; 0.26 mmol, 1.0 equiv.),
phenylboronic acid (0.32 mmol, 1.2 equiv.), K2CO3
(0.53 mmol, 2.0 equiv.) and Pd@PS (0.007 mmol, 3 mol%)
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1
the desired product 3a as a semisolid (97 mg, 98%). H
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(d, J = 7.7 Hz, 1H), 3.60 (s, 2H), 2.40 (s, 3H), 2.25–2.22 (m,
2H), 2.10–2.08 (m, 2H), 1.64–1.61 (m, 4H), 1.54 (m, 1H),
1.45 (s, 6H), 1.16–1.09 (s, 4H), 1.00–0.96 (m, 2H). 13C
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126.4, 57.3, 49.1, 48.8, 38.2, 34.3, 33.2, 32.5, 26.2, 24.9,
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ACKNOWLEDGMENTS
[19] E. Z. Oblak, M. D. VanHeyst, J. Li, A. J. Wiemer, D. L. Wright,
The authors are grateful to the Director, CSIR‐IHBT for
providing necessary facilities during the course of the work.
We thank Dr G. Saini, AIRF, JNU‐New Delhi, India for
TEM analysis, Biotechnology Division, CSIR‐IHBT, for
SEM and EDS analysis and AMRC, IIT, Mandi for HRMS
and single‐crystal XRD. R.B. thanks CSIR, New Delhi, for
financial support as part of XII Five Year Plan programme
under the title ORIGIN (CSC‐0108). C.B.R. and S.K. thank
UGC, New Delhi for awarding fellowships.
J. Am. Chem. Soc. 2014, 136, 4309.
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How to cite this article: Bharti, R., Bal Reddy, C.,
Kumar, S., Das, P.. Supported palladium nanoparticle‐
catalysed Suzuki–Miyaura cross‐coupling approach
for synthesis of aminoarylbenzosuberene analogues
from natural precursor. Appl Organometal Chem.
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