Synthesis of 4,5,6-trisubstituted-1,3-dihydroisobenzofurans by virtue of palladium-catalyzed domino carbopalladation of bromoenynes and internal alkynes

Article abstract of DOI:10.1039/c5ra16546f

An efficient hetero-annulation protocol has been developed for the construction of 4,5,6-trisubstituted-1,3-dihydroisobenzofurans via palladium-catalyzed domino carbopalladation of bromoenynes and internal alkynes. The reaction followed domino intramolecular Heck cyclization (5-exo-dig) and termination of the resulting diene with an internal alkyne to give highly substituted isobenzofurans in moderate to good yields.

Full text of DOI:10.1039/c5ra16546f

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Graphical Abstract
Synthesis of 4,5,6-trisubstituted-1,3-dihydroisobenzofurans by virtue of
palladium-catalyzed domino carbopalladation of bromoenynes and internal
alkynes
Munmun Ghosh, Raju Singha, Shubhendu Dhara and Jayanta K. Ray*
An efficient hetero-annulation protocol has been developed for the construction of 4,5,6-
trisubstituted-1,3-dihydroisobenzofurans via palladium-catalyzed domino carbopalladation of
bromoenynes and internal alkynes.
Ar
Br
O
Pd(OAc)2 (5 mol %)
R1
PPh3 (0.25 mmol)
Ar
R1
R2
O
Cs2CO3 (2 mmol)
DMF (5 mL), 90-95 C
°
R2
12 examples
upto 84 % yield
5 h

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Synthesis of 4,5,6ꢀtrisubstitutedꢀ1,3ꢀdihydroisobenzofurans by
virtue of palladiumꢀcatalyzed domino carbopalladation of
bromoenynes and internal alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
Munmun Ghosha, Raju Singhaa , Shubhendu Dharaa and Jayanta K. Ray*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Cylization of 2ꢀbromoꢀ1,n,mꢀenynes and 2ꢀbromoꢀ1,m,nꢀdienynes,
An efficient heteroꢀannulation protocol has been developed
more
particularly,
2ꢀbromoꢀ1,6ꢀenynes
via
sequential
“living”
for
the
construction
of
4,5,6ꢀtrisubstitutedꢀ1,3ꢀ
cabopalladation leading to the formation of
a
dihydroisobenzofurans via palladiumꢀcatalyzed domino
carbopalladation of bromoenynes and internal alkynes. The
reaction followed domino intramolecular Heck cyclization
(5-exo-dig) and termination of the resulting diene with
internal alkyne to give highly substituted isobenzofurans in
moderate to good yields.
alkenylpalladium intermediate and consecutive termination of this
“living” intermediate via further intraꢀ or intermolecular
carbopalladation to other double or triple bonds have gained
considerable attention of a number of chemists.6 Following the same
strategy, herein we report synthesis of previously unexplored class of
highly substituted 1,3ꢀdihydroisobenzofurans from bromoenynes and
internal alkynes via palladiumꢀcatalyzed domino carbopalladation.
Although some reports on other metalꢀcatalyzed synthesis of 4,5,7ꢀ
triphenylꢀ1,3ꢀdihydroisobenzofurans,7aꢀc 4ꢀmethylꢀ5,6ꢀdiphenylꢀ1,3ꢀ
1,3ꢀdihydroisobenzofurans (phthalans) contribute to the family of a
considerable number of natural products demonstrating fascinating
pharmacological activities including antidepressive, antioxidant,
antifungal, antibacterial, antitumor and cardiovascular disease, antiꢀ
inflammatory, cytotoxicity against human cancer cell and so on.1
They are also important in terms of industrial applications2 and as
major building blocks in organic synthesis.3 Figure 1 represents
some selected bioactive phthalans.1aꢀg
dihydroisobenzofurans,7a
dihydroisobenzofurans7a,7d
4,5,6,7ꢀtetraphenylꢀ1,3ꢀ
or
4,7ꢀdimethylꢀ5,6ꢀdiphenylꢀ1,3ꢀ
dihydroisobenzofurans7a,7e are available, no such work has been
reported so far for the palladiumꢀcatalyzed synthesis of 4,5,6ꢀ
triphenylꢀ1,3ꢀdihydroisobenzofurans. Therefore, we targeted to
synthesize 4,5,6ꢀtriphenylꢀ1,3ꢀdihydroisobenzofurans using domino
carbopalladation (Heck reaction followed by termination) strategy.
During the course of the study, we also successfully synthesized 5ꢀ
hexylꢀ4,6ꢀdiphenylꢀ1,3ꢀdihydroisobenzofuran along with the other
regioisomer.
O
HO
NC
Me
O
N
H
O
O
O
O
Br
HO
H
N
OH
Br
Br
OH
HO
OH
HO
OH
O
F
O
7-bromo-1-(2,3-dibromo-4,5-
dihydroxyphenyl)-5,6-dihydroxy-1,3-
dihydroisobenzofuran
Pestacin
(S)-(+)-escitalopram
The Heck precursor [3ꢀ(2ꢀBromoꢀallyloxy)ꢀpropꢀ1ꢀynyl]ꢀbenzenes
(2ꢀbromoꢀ1,6ꢀenynes) (2aꢀ2l) were synthesized from the
corresponding iodo/bromoꢀbenzenes (1mmol) by Sonogashira
coupling with propargyl alcohol (1.2 mmol) using PdCl2(PPh3)2 (5
mol %) and CuI (5 mol %) in refluxing Et3N (6 mL) for 4ꢀ5 h (in
case of iodides) or 8ꢀ10 h ( in case of bromides)8 followed by
allylation of the 3ꢀPhenylꢀ2ꢀpropyn alcohols (1aꢀ1l) using NaH (3
mmol) and 2,3ꢀdibromopropene (1 mmol) in dry THF (5 mL) (0°Cꢀ
25°C) for 1.5ꢀ2 h (Table 1).6c,6g
(+)-egenine
Fig. 1. Phthalan-based pharmacologically active compounds
Owing to such versatile pharmacological, industrial as well as
synthetic applications, development of efficient and economic
methods for the synthesis of phthalans has attracted a number of
chemists over the last few decades. A plethora of transition metal
catalyzed4 as well as metal free5 strategies have been reported for the
construction of substituted 1,3ꢀdihydroisobenzofurans.
Table 1
Synthesis of [3ꢀ(2ꢀBromoꢀallyloxy)ꢀpropꢀ1ꢀynyl]ꢀbenzenesa (2aꢀ2l)
a.Department of Chemistry, Indian Institute of Technology, Kharagpur 721302,
India. E-mail: jkray@chem.iitkgp.ernet.in; Fax: +91 3222282252; Tel: +91
322228332
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Br
(1 mmol)
Br
Br
OH
O
NaH (3 mmol)
R
R
dry THF (5 mL)
0-25 C, 1.5-2h
2a-2l
1a-1l
°
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Br
Br
Br
electron donating and electron withdrawing) 2ꢀbromoꢀ1,6ꢀenynes
2aꢀ2l were reacted with symmetrical as well as unsymmetrical
internal alkynes 3 under the optimized reaction condition. The
results are summarized in Table 3.
O
O
O
Me
Me
Br
Me
2c, Y = 79%
2a, Y = 85%
2b, Y = 86%
Br
O
Br
O
O
MeO
Table 3
MeO
MeO
O
OMe
2f, Y = 79%
2d, Y = 87%
2e, Y = 85%
Br
O
Br
Synthesis of 4,5,6ꢀtrisubstitutedꢀ1,3ꢀdihydroisobenzofuran a,b (4aꢀ4l)
Cl
Ar
2k, Y = 75%
2l, Y = 78%
O
Br
Pd(OAc)2 (5 mol %)
PPh3 (0.25 mmol)
O
R1
O
R2
R1
Ar
a Isolated yields after purification through column chromatography.
Cs2CO3 (2 mmol)
DMF (5 mL), 90-95
5 h
R2
C
°
3
2
4
Table 2
Optimization studiesa
OMe
Me
OMe
Me
Me
O
MeO
O
O
O
O
Br
O
catalyst, base
4d, Y = 80%
4c, Y = 77%
4a,Y = 82%
OMe
O
4b, Y = 84%
4e, Y = 78%
ligand, solvent
90°C, 5h
3a
2a
4a
OMe
Me
Me
O
O
O
Entry Catalyst Ligand Base Solvent Temp (°C) Yield (%)b
O
OMe
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Pd(OAC)2 PPh3 Et3N
Pd(OAC)2 PPh3 Na2CO3 DMF
Pd(OAC)2 PPh3 K2CO3 DMF
DMF
90
90
90
90
70
90
90
90
90
90
48
64
61
82
56
46
55
26
77
72
46
44
33
40
54
4h'
4h
4g, Y = 80%
4f, Y = 77%
Overall yield= 77%; 4h:4h'= 1.15:1
Pd(OAC)2 PPh3 Cs2CO3 DMF
Pd(OAC)2 PPh3 Cs2CO3 DMF
Pd(OAC)2 PPh3 NaOAc DMF
O
O
O
O
Pd(OAC)2 PPh3 KOBut
DMF
Pd(OAC)2 PPh3 HCOONa DMF
Pd(OAC)2 PPh3 Cs2CO3 DMA
Pd(OAC)2 PPh3 Cs2CO3 DMSO
4j'
Overall yield= 78%; 4j:4j'=1.75:1
4j
4i
4i'
Overall yield= 80%; 4i:4i'= 1.04:1
Cl
Me
O
Pd(OAC)2 PPh3 Cs2CO3 1,4ꢀdioxane 90
Pd(OAC)2
ꢀ
Cs2CO3 DMF
90
90
90
90
Pd(OAC)2 PCy3 Cs2CO3 DMF
Pd(OAC)2 BINAP Cs2CO3 DMF
O
O
PdCl2
PPh3 Cs2CO3 DMF
4k, Y = 68%
4l, Y = 75%
16 PdCl2(CH3CN)2
ꢀ
ꢀ
ꢀ
Cs2CO3 DMF
Cs2CO3 DMF
Cs2CO3 DMF
90
90
90
46
50
36
17
18
Pd(PPh3)4
Pd2(dba)3
a
Reagents and conditions: all the reactions were carried out under the
following conditions: substrates 2 (1 mmol), 3 (1.5 mmol), Pd(OAc)2 (5 mol
%), PPh3 (0.25 mmol), Cs2CO3(2 mmol), DMF (5 mL), 90ꢀ95 °C, 5 h
b Isolated yield (after purification through column chromatography) after 5 h.
a Reagents and conditions: 2a (1 mmol), diphenylacetylene 3a (1.5 mmol), Pd
catalyst (5 mol %), base (2 mmol), ligand (0.25 mmol) and solvent (5 mL)
for 5 h.
Yields refer to the isolated yields after purification through column
chromatography.
The reaction was found to be relatively high yielding in case of
electron donating groups present either in 2 or 3 [resulting in 4bꢀ
(4j+4jʹ)] compared to those with electron withdrawing groups
(resulting in 4k and 4l). However, when the reaction was carried out
with diethyl acetylenedicarboxylate (R1 = R2 = CO2Et) and1,1'ꢀ(1,2ꢀ
b
The bromoenyne 2a was then initially reacted with
diphenylacetylene 3a in presence of Pd (OAc)2 catalyst, Et3N base
and PPh3 ligand in DMF at 90 °C for 5 h to give 4,5,6ꢀtriphenylꢀ1,3ꢀ
dihydroisobenzofuran 4a in 48 % yield. After screening through a
number of reaction conditions, the domino carbopalladation reaction
was found to be most effective with Pd(OAc)2 (5 mol %), PPh3 (0.25
mmol), Cs2CO3 (2 mmol) when 1 mmol of 2a was reacted with 1.5
mmol of 3a in DMF (5 mL) at 90 °C for 5 h resulting in the
formation of 82 % of 4,5,6ꢀtriphenylꢀ1,3ꢀdihydroisobenzofuran 4a as
the only isolable product (Table 2, Entry 4).
ethynediyl)bis[4ꢀnitroꢀbenzene] (R1
= R2 = 4ꢀNO2ꢀC6H4), the
starting material and/or the product probably could not persist under
the applied reaction condition leading to no fruitful result.
With unsymmetrical internal alkynes a regioisomeric mixture was
obtained in each case. As indicated by NMR data, 4h and 4hʹ were
obtained in a ratio of 1.15:1(4h:4hʹ≈1.15:1). Similarly, according to
NMR data, 4i:4iʹ≈ 1.04:1 and 4j:4jʹ≈ 1.75:1. However, geometry
optimization was carried out for (4h and 4hʹ), (4i and 4iʹ) and (4j
and 4jʹ) by using the DFT method at the (U)B3LYP level in the
Gaussian 09 program9 and 6ꢀ311G* basis set was used for all the
We next attempted to investigate the general applicability of this
methodology and
a number of variously functionalised (both
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elements. The DFT calculation shows that between 4h and 4hʹ, 4h is
more stable by 0.21 KJ/mol of energy and hence they were obtained
in almost equimolar ratio (4h:4hʹ≈1.15:1 from NMR data). Between
4i and 4iʹ, 4i is more stable only by 0.1 KJ/mol and thus they were
also obtained in equimolar ratio (4i:4iʹ≈1.04:1from NMR data).
Again, between 4j and 4jʹ, 4j is more stable by 6.67 KJ/mol of
energy and hence 4j was obtained as the major product
(4j:4jʹ≈1.75:1from NMR data). Thus the DFT calculation supports
the results obtained experimentally. The results thus obtained also
stand consistent with the regiochemical consequences reported by
Negishi et al. in 1993.6c
Acknowledgement
M.G. thanks CSIR, New Delhi for the fellowship. The DST, India is
also thanked for providing funds for the project and creating 200 and
600 MHz NMR facility under the IRPHA programme.
Notes and references
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To explain mechanistically, it would be rationalized that Pd(II) is
initially reduced by PPh3 to Pd(0) which enters the catalytic cycle by
oxidative addition to the sp2ꢀCꢀBr bond of the bromoenyne 2a
leading to the formation of the alkenylpalladium intermediate A.
This intermediate then undergoes intramolecular carbopalladation to
the triple bond forming a “living” alkenylpalladium intermediate B.
Carbopalladation of diphenylactylene 3a to B furnishes intermediate
C which is then converted to the desired product 4a either via D (6ꢀ
endoꢀtrig carbopalladation) or via E (6πꢀelectrocyclization)followed
by βꢀdehydropalladation sequence.6b,10
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Br
Pd (II)
Pd(0)
O
HBr
PPh3+Cs2CO3
2a
Cs2CO3
H-Pd-Br
O
PdBr
O
4a
A
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H
O
O
PdBr
PdBr
H
E
D
BrPd
O
6
π
-electrocyclization
B
6-endo-trig
carbopalladation
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O
PdBr
C
3a
Scheme 1. Plausible mechanism for the formation of 4,5,6-Triphenyl-1,3-dihydroisobenzofurans
Conclusions
In summary, we have developed an easy access to highly substituted
1,3ꢀdihydroisobenzofurans using domino carbopalladation of
bromoenynes and internal alkynes. Our methodology is
advantageous with respect to good yield and substrate versatility.
Moreover, the starting materials are readily accessible. Hope this
methodology may find successful application in the construction of
phthalanꢀcontaining natural products and also help in materials,
pharmaceutical as well as in industrial research.
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