Rapid access to 1-methyleneindenes via palladium-catalyzed tandem reactions of 1-(2,2-dibromovinyl)-2-alkynylbenzenes with arylboronic acids

Article abstract of DOI:10.1039/b926763h

A novel and efficient route for the synthesis of 1-methyleneindenes via palladium-catalyzed tandem reactions of 1-(2,2-dibromovinyl)-2-alkynylbenzene with arylboronic acids is described. This reaction proceeded under mild conditions with high efficiency and excellent selectivity. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b926763h

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Rapid access to 1-methyleneindenes via palladium-catalyzed tandem
reactions of 1-(2,2-dibromovinyl)-2-alkynylbenzenes with arylboronic
acidswz
Shengqing Ye,a Xiaodi Yangc and Jie Wu*ab
Received (in Cambridge, UK) 18th December 2009, Accepted 13th February 2010
First published as an Advance Article on the web 8th March 2010
DOI: 10.1039/b926763h
A novel and efficient route for the synthesis of 1-methyleneindenes
via palladium-catalyzed tandem reactions of 1-(2,2-dibromovinyl)-
2-alkynylbenzene with arylboronic acids is described. This reaction
proceeded under mild conditions with high efficiency and excellent
selectivity.
Recently, we have witnessed the important progress of using
gem-dihaloolefins as electrophiles in metal-catalyzed reactions.9
These gem-dihaloolefins can be easily accessible from an
aldehyde (or activated ketone) using CBr4/PPh310 or the ylide
CCl2PPh3.11 So far, polysubstituted alkenes,12 enynes,13
polyynes,14 carboxamide,15 and carbo- or heterocycles16,17
could be obtained starting from gem-dihaloolefins. Usually,
good stereoselectivity could be observed for stepwise functional-
ization of gem-dihaloolefin systems12,18 and less attention has
been paid to potentially more efficient tandem reactions.19
Recently, tandem carbon–carbon bond and carbon–heteroatom
bond formation was reported for the reaction of gem-
dihaloolefins.16,17 For instance, Lautens and co-workers
described several efficient routes for indole synthesis via a
transition metal-catalyzed C–N/C–N or C–N/C–C coupling
sequence from gem-dihaloolefins.17 Meanwhile, we have
developed efficient routes for construction of nitrogen-
containing heterocycles starting from 2-alkynylbenzaldoxime
or N0-(2-alkynylbenzylidene)hydrazide.20 Prompted by these
results and the advancement of gem-dihaloolefin chemistry, we
It is well recognized that compounds with an indene core are
found abundantly in many drug candidates with remarkable
biological activities.1 The function of indene-related compounds
has been discovered in the field of materials science as well.2 In
addition, metallocene complexes with an indene core structure
have been utilized in olefin polymerization as catalysts.3 Thus,
a significant effort continues to be given to the development of
new indene-based structures and new approaches for their
construction.4–7 Among the family of indenes, 1-methyleneindenes
have attracted much attention recently since they can be easily
transferred to functionalized indenes. Although several methods
have appeared for the formation of 1-methyleneindene
derivatives,8 development of novel and efficient routes for
the generation of functionalized 1-methyleneindenes under
mild conditions is still of high interest.
conceived that 1-(2,2-dibromovinyl)-2-alkynylbenzenes
1
might be a versatile building block for further transformations
due to the structural similarity. Herein, we would like to
disclose its useful application, namely reaction with arylboronic
acids under mild conditions giving rise to the 1-methyleneindenes
in good to excellent yields. This reaction proceeded with high
efficiency and excellent selectivity.
a Department of Chemistry, Fudan University, 220 Handan Road,
Shanghai 200433, China. E-mail: jie_wu@fudan.edu.cn;
Fax: 86 21 6564 1740; Tel: 86 21 6510 2412
b State Key Laboratory of Organometallic Chemistry,
Shanghai Institute of Organic Chemistry, Shanghai 200032, China
c Laboratory of Advanced Materials, Fudan University,
220 Handan Road, Shanghai 200433, China
1-(2,2-Dibromovinyl)-2-alkynylbenzene could be easily
accessed via condensation of 2-alkynylbenzaldehyde with carbon
tetrabromide. As mentioned above, 1-(2,2-dibromovinyl)-
benzene has been found in several applications for the synthesis
of N-heterocycles.16,17 Since the two vinyl bromides would
undergo cascade coupling reactions, we anticipated that the
presence of a coupling partner such as arylboronic acid in
the reaction of 1-(2,2-dibromovinyl)-2-alkynylbenzene, could
provide access to functionalized 1-methyleneindenes after
subsequent reaction processes (Scheme 1). We reasoned that
after oxidative addition of Pd(0) to 1-(2,2-dibromovinyl)-
2-alkynylbenzenes 1, the subsequent transmetallation of
arylboronic acid and reductive elimination would occur
to generate intermediate B, with the release of Pd(0). The
presence of Pd(0) would react with vinyl bromide B by
oxidative addition, leading to the intermediate C. Intra-
molecular insertion of Pd(II) to the alkynyl group gave rise
to the intermediate D, which then reacted with arylboronic
acid via transmetallation. Subsequently reductive elimination
occurred to give rise to the desired product 3 and Pd(0), which
w Electronic supplementary information (ESI) available: Experimental
section, NMR spectra and crystallography. CCDC 759033. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b926763h
z General procedure for palladium-catalyzed tandem reaction of
1-(2,2-dibromovinyl)-2-alkynylbenzene 1 with arylboronic acid 2:
1-(2,2-Dibromovinyl)-2-alkynylbenzene 1 (0.2 mmol, 1 equiv.) was
added to a solution of Pd(OAc)2 (5 mol%), PPh3 (20 mol%), K3PO4ꢀ
H2O (1.2 mmol, 6 equiv.) and arylboronic acid 2 (0.6 mmol, 3 equiv.)
in THF (2.0 mL). The solution was then stirred at room temperature.
After completion of reaction as indicated by TLC, the reaction was
quenched with water (5.0 mL), extracted with EtOAc (2 ꢁ 10 mL),
dried by anhydrous Na2SO4. Evaporation of the solvent followed by
purification on silica gel provided the product 3. Data of selected
example: Compound 3a. Yield: 86%; red solid, melting point: 177–178
1C; 1H NMR (400 MHz, CDCl3) d 2.13 (s, 3H), 2.18 (s, 3H), 6.35
(d, J = 7.8 Hz, 1H), 6.65 (d, J = 7.8 Hz, 2H), 6.72 (d, J = 7.8 Hz,
2H), 6.76–6.80 (m, 3H), 6.87 (d, J = 7.8 Hz, 2H), 6.89 (s, 1H), 7.09
(t, J = 7.3 Hz, 1H), 7.24 (d, J = 8.3 Hz, 1H), 7.38–7.46 (m, 5H); 13C
NMR (100 MHz, CDCl3) d 20.9, 21.0, 120.4, 123.3, 124.5, 126.9,
127.5, 127.8, 128.5, 128.7, 130.9, 132.3, 133.1, 134.8, 135.2, 136.9,
137.9, 138.2, 138.5, 142.2, 143.0, 144.0, 149.5; HRMS (ESI) calc. for
C30H25 (M + H)+ 385.1956, found 385.1938. Anal. Calc. for C30H24
C, 93.71; H, 6.29; Found: C, 93.65; H, 6.42%.
:
ꢂc
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Scheme
2
Initial studies for palladium-catalyzed reaction of
1-(2,2-dibromovinyl)-2-alkynylbenzene 1a with 4-methylphenylboronic
acid 2a.
the substrate, this palladium-catalyzed 1-methyleneindene
formation was found to be workable with arylboronic acids
2a–2g with electron-withdrawing and -donating substituents
on the aromatic backbone (Table 1, entries 1–7). For example,
reaction of substrate 1a with 4-methoxyphenylboronic acid 2b
afforded the desired product 3b in 75% yield (entry 2). Other
arylboronic acids bearing trifluoromethyl, cyano, carbonyl
and nitro functional groups were all tolerated under the
reaction conditions (entries 4–7). In addition to the aromatic
groups attached to the CRC triple bond, alkyl groups such as
n-butyl were found suitable as well to generate the desired
product 3h cleanly in good yield (62% yield, entry 8). Further-
more, the conditions have proven to be useful for other
substrates. For instance, fluoro- or methoxy-substituted
1-(2,2-dibromovinyl)-2-alkynylbenzene 1c or 1d reacted with
arylboronic acids leading to the expected products in good
yields (entries 9–13).
Scheme
1 Proposed route for the palladium-catalyzed tandem
reactions of 1-(2,2-dibromovinyl)-2-alkynylbenzenes 1 with arylboronic
acids 2.
would re-enter the catalytic cycle. However, there are several
questions associated with the proposed synthetic route, such as
selectivity, compatibility, and relative rates. Thus, to verify the
practicability of the projected route as shown in Scheme 1, we
started to explore the possibility of this palladium-catalyzed
reaction of 1-(2,2-dibromovinyl)-2-alkynylbenzenes 1 with
arylboronic acids 2.
To identify suitable conditions for this proposed trans-
formation, we first evaluated the tandem reaction of
1-(2,2-dibromovinyl)-2-alkynylbenzene 1a with 4-methyl-
phenylboronic acid 2a (Scheme 2). Different conditions
including palladium catalysts, phosphine ligands, bases, and
solvents using a combinatorial approach were screened. At the
outset, the reaction catalyzed by Pd(OAc)2 (5 mol%) was
carried out in the presence of K3PO4ꢀH2O (6.0 equiv.) and
PPh3 (20 mol%) in toluene at room temperature. However,
only a trace amount of product was detected. Similar results
were obtained when the solvent was changed to CH3CN or
DMAc. To our delight, we observed the formation of a
product when 1,4-dioxane was utilized as a replacement, with
65% isolated yield. Further screening of solvents led to the
identification of THF as the most effective condition for the
transformation (86% yield). Structural identification using
1H and 13C NMR, MS and X-ray diffraction analysis (see ESIw)
revealed that the compound obtained was the 1-methylene-
indene 3a. Subsequent reaction optimization of palladium
catalysts revealed that Pd(OAc)2 was the best choice. Decreasing
the amount of palladium catalyst diminished the product yield
(65%). Other phosphine ligands and bases were screened as
well, however, no better results were observed (for details see
ESIw). In the transformation, 3.0 equiv. of arylboronic acid is
essential in order to obtain a respectable yield of product 3a.
Only vinyl bromide B was generated when 1.0 equiv. of
arylboronic acid was employed in the reaction.
In conclusion, we have described a novel and efficient route
for the synthesis of 1-methyleneindenes via palladium-
catalyzed tandem reactions of 1-(2,2-dibromovinyl)-2-alkynyl-
benzenes 1 with arylboronic acids 2. This reaction proceeded
under mild conditions with high efficiency and excellent
selectivity. Further studies by adaptation of the method
described herein for the synthesis of functionalized indenes
Table 1 Palladium-catalyzed tandem reactions of 1-(2,2-dibromovinyl)-
2-alkynylbenzenes 1 with arylboronic acids 2
Entry
R1, R2
R3
Yielda (%)
1
2
3
4
5
6
7
8
H, Ph (1a)
H, Ph (1a)
H, Ph (1a)
H, Ph (1a)
H, Ph (1a)
H, Ph (1a)
H, Ph (1a)
H, n-Bu (1b)
4-F, Ph (1c)
4-F, Ph (1c)
4-F, Ph (1c)
4,5-(OMe)2, Ph (1d)
4,5-(OMe)2, Ph (1d)
4-MeC6H4 (2a)
4-MeOC6H4 (2b)
C6H5 (2c)
4-CF3C6H4 (2d)
4-NCC6H4 (2e)
4-MeO2CC6H4 (2f)
3-NO2C6H4 (2g)
C6H5 (2c)
86 (3a)
75 (3b)
80 (3c)
70 (3d)
72 (3e)
74 (3f)
88 (3g)
62 (3h)
82 (3i)
74 (3j)
75 (3k)
95 (3l)
72 (3m)
Using the palladium-catalyzed conditions (Pd(OAc)2
(5 mol%), PPh3 (20 mol%), K3PO4ꢀH2O (6.0 equiv.), THF, rt),
1-(2,2-dibromovinyl)-2-alkynylbenzenes 1 reacted with various
arylboronic acids 2 to generate a number of 1-methylene-
indenes 3 in good to excellent yields (Table 1). When
1-(2,2-dibromovinyl)-2-alkynylbenzene 1a was employed as
9
C6H5 (2c)
10
11
12
13
4-NCC6H4 (2e)
4-MeO2CC6H4 (2f)
C6H5 (2c)
4-MeO2CC6H4 (2f)
a
Isolated yield based on 1-(2,2-dibromovinyl)-2-alkynylbenzene 1.
ꢂc
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Financial support from the National Natural Science
Foundation of China (No. 20972030) is gratefully acknowledged.
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2952 | Chem. Commun., 2010, 46, 2950–2952