A general method for the synthesis of 3,5-diarylcyclopentenones via Friedel-Crafts acylation of vinyl chlorides

Article abstract of DOI:10.1021/jo900696k

(Chemical Equation Presented) A general approach for the synthesis of 3,5-diarylcyclopentenones was developed. Key aspects of this approach are the intramolecular Friedel-Crafts-type cyclization of vinyl chlorides and subsequent Pd-catalyzed cross-coupling reactions. The requisite vinyl chloride-bearing arylacetic acid precursors are readily available by straightforward alkylation of arylacetic acid esters and undergo cyclization to yield 3-chloro-5-aryl-2- cyclopentenones when treated with AlCl₃. The vinylogous acid chloride functionality present in these immediate products allows for further elaboration via Pd-catalyzed cross-coupling chemistry, leading to a diverse array of products.

Full text of DOI:10.1021/jo900696k

approaches to functionalized organic intermediates, we recently
became interested in the use of alternative nucleophilic com-
ponents to the commonly used arene ring. In particular, we
sought to investigate the use of vinyl chlorides as the nucleo-
philic component in intramolecular F-C acylation reactions.
Inspection of the literature revealed only a few isolated examples
of vinyl chloride-based intramolecular F-C acylations,3 indicat-
ing good potential to expand the scope of this reaction and
develop more generally applicable procedures. In this Note, we
disclose the results from our work in this area, which demon-
strate the utility of intramolecular F-C acylations of vinyl
chlorides for the synthesis of various 3,5-disubstituted cyclo-
pentenone derivatives. These functionalized prochiral intermedi-
ates4 have potential as versatile precursors for various types of
asymmetric catalysis, such as asymmetric conjugate reduction5
and asymmetric conjugate additions to generate quaternary
stereocenters.6
A General Method for the Synthesis of
3,5-Diarylcyclopentenones via Friedel-Crafts
Acylation of Vinyl Chlorides
Yingju Xu,* Mark McLaughlin,* Cheng-yi Chen,
Robert A. Reamer, Peter G. Dormer, and Ian W. Davies
Department of Process Research, Merck Research
Laboratories, Merck & Co., Inc., Rahway, New Jersey 07065
yingju_xu@merck.com; mark_mclaughlin@merck.com
ReceiVed April 2, 2009
Figure 1 depicts our general approach for the preparation of
3,5-diarylcyclopentenones. Given the ready availability of
functionalized arylacetic acids, we envisaged these to be
appropriate starting materials for the construction of a range of
intramolecular F-C cyclization substrates. Carboxylic acid
activation and cyclization under typical Lewis acid conditions
was anticipated to provide 3-chlorocyclopentenone intermediates
that would be amenable to further elaboration via the reactivity
conferred by the vinylogous acid chloride functional group.
Synthesis of the F-C cyclization substrates was accomplished
via alkylation of aryl acetic esters (1a) or, more directly, via
alkylation of the dianion derived from arylacetic acids (1b)7
(Scheme 1). As shown, treatment of a mixture of arylacetic ester
and 2-chloro-3-iodopropene (2) in THF with KHMDS results
A general approach for the synthesis of 3,5-diarylcyclopen-
tenones was developed. Key aspects of this approach are
the intramolecular Friedel-Crafts-type cyclization of vinyl
chlorides and subsequent Pd-catalyzed cross-coupling reac-
tions. The requisite vinyl chloride-bearing arylacetic acid
precursors are readily available by straightforward alkylation
of arylacetic acid esters and undergo cyclization to yield
3-chloro-5-aryl-2-cyclopentenones when treated with AlCl3.
The vinylogous acid chloride functionality present in these
immediate products allows for further elaboration via Pd-
catalyzed cross-coupling chemistry, leading to a diverse array
of products.
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Y.-T.; Meindl, K.; Ru¨hl, S.; Meijere, A. Synlett. 2005, 5, 805. For zirconium-
catalyzed see: (i) Takahashi, T.; Huo, S.; Hara, R.; Noguchi, Y.; Nakajima, K;
Sun, W. J. Am. Chem. Soc. 1999, 121, 1094. (j) Xi, Z.; Fan, H.; Mito, S.;
Takahashi, T. J. Organomet. Chem. 2003, 682, 108. (k) Yuki, T.; Hashimoto,
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see: (l) Tsukada, N.; Sugawara, S.; Okuzawa, T.; Inoue, Y. Synthesis 2006, 18,
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H.; Ito, Y. J. Am. Chem. Soc. 1997, 119, 9307. (n) Matsuda, I.; Ishibashi, H.; Ii,
N. Tetrahedron Lett. 1995, 36, 241. (o) Matsuda, I.; Fukuta, Y.; Tsuchihashi,
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complex see: (p) Stokes, H. L.; Ni, L. M.; Belot, J. A.; Welker, M. E. J.
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Tetraohedron Lett. 1992, 33, 177. For Cobalt-catalyzed syntheses see: (r) Krafft,
M. E. J. Am. Chem. Soc. 1988, 110, 968. (s) Rao, M. L. N.; Periasamy, M.
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The Friedel-Crafts (F-C) acylation reaction represents a
powerful method to introduce new carbon-carbon bonds onto
aromatic compounds and other unsaturated species.1 Acylation
of arenes is a thoroughly established technique for the synthesis
of aromatic ketones. Intramolecular F-C reactions have also
proven highly useful in the formation of various ring systems.2
As part of a continuing effort to develop simple and efficient
(1) Friedel, C.; Crafts, J. M. Compt. Rend. 1877, 84, 1392 and 1450. For
reviews see: (a) Price, C. C. Org. React. 1946, 3, 1. (b) Groves, J. K. Chem.
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Wiley: New York, 1973. (f) Pearson, D. E.; Buehler, C. A. Synthesis 1972, 10,
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10, 2645. (h) Sartori, G.; Maggi, R. Chem. ReV. 2006, 106, 1077. For
enantioselective F-C reactions see:(i) Majer, J.; Kwiatkowski, P.; Jurczak, J.
Org. Lett. 2008, 10, 2955. (j) Poulsen, T. B.; Jørgensen, K. A. Chem. ReV. 2008,
108, 2903. (k) Jørgensen, K. A. Synthesis 2003, 1117. (l) Bandini, M.; Melloni,
A.; Umani-Ronchi, A. Angew. Chem., Int. Ed. 2004, 43, 550.
(2) (a) Franck-Neumann, M.; Miesch-Gross, L.; Nass, O. Tetrahedron Lett.
1996, 37, 8763. (b) Waser, M.; Falk, H. Monatsh. Chem. 2005, 136, 609. For
reviews see: (c) Heaney, H. ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon Press: Oxford, UK, 1991; Vol. 2, p 753. (d) Sethna,
S Friedel-Crafts and Related Reactions, Olah, G. A., Ed.; Interscience: New
York, 1964; Vol. 3, p 911. (e) Gore, P. H. Chem. ReV. 1955, 55, 229–281.
(6) (a) May, T. L.; Brown, M. K.; Hoveyda, A. H. Angew. Chem., Int. Ed.
2008, 47, 7358. (b) Vuagnoux-d’Augustin, M.; Kehrli, S.; Alexakis, A. Synlett
2007, 13, 2057. (c) Zezschwitz, P. Synthesis 2008, 12, 1809.
5100 J. Org. Chem. 2009, 74, 5100–5103
10.1021/jo900696k CCC: $40.75  2009 American Chemical Society
Published on Web 06/01/2009

TABLE 1. Synthesis of 3-Chloro-5-arylcyclopentenones (4)a b
,
FIGURE 1. Access to 3,5-diarylcyclopentenones via intramolecular
F-C acylation of vinyl chlorides and subsequent cross-coupling of the
derived vinylogous acid chloride.
SCHEME 1. Synthesis of Chloroallyl F-C Cyclization
Substratesa
a Conditions A for ester substrates (1a): direct R-alkylation followed by
hydrolysis of the ester. Conditions B for free acid substrates (1b): direct
R-alkylation via dianion chemistry.
in alkylation of the R-position. In most cases some over-reaction
to yield the dialkylated product was observed, although this
could be suppressed (<10%) by slow addition of KHMDS at -
10 °C. Direct basic hydrolysis of the mixture of crude alkylated
products was selective, leaving the sterically congested dialky-
lated esters intact and allowing for straightforward purification
of the desired acid via standard extractive techniques. An
alternative method used for preparation of certain substrates
involved treatment of aryl acetic acids with 2 equiv of n-BuLi
at - 60 °C followed by addition of 2-chloro-3-iodopropene (2)
at -10 °C to obtain the required carboxylic acids directly in a
single step8 (conditions B).
With ready access to the requisite cyclization substrates, study
of the planned F-C reactions was initiated. Following an
established procedure, treatment of the carboxylic acids (3) with
(COCl)2 and a catalytic amount of DMF at room temperature
afforded the corresponding acid chlorides. Next, in situ treatment
with AlCl3 served to promote the desired intramolecular F-C
acylation process. In most cases, both 3-chloro-2-cyclopentenone
(4) and 3,3-dichlorocyclopentanone (5) were observed (by
reversed-phase HPLC) when the reaction mixture was sampled
directly. However, standard aqueous workup involving treatment
with sat. NaHCO3 solution under biphasic conditions led to
facile elimination of HCl from 5 and conversion to 4. As shown
in Table 1 there is reasonable substrate scope and a variety of
vinyl chlorides (3) undergo smooth cyclization to deliver
cyclopentenones (4) in 64-97% yields. The 3-chloro-2-cyclo-
pentenones (4) are stable toward silica gel chromatography and
were generally isolated in good yields (Table 1).
a The reactions were carried out at room temperature with 1.2 equiv
of (COCl)2 in CH2Cl2. Reactions were monitored by LC and upon full
conversion, 1.1 equiv of AlCl3 was added.
A basic workup was
conducted upon complete conversion of the acid chloride intermediates
monitored by LC. b Yields of pure, isolated products (characterized by
1H, 13C NMR, and HR-MS). c Conducted with additional stirring at
room temperature with sat. NaHCO3 added before workup.
Having secured a series of 3-chlorocyclopentenones, we
sought to exploit the vinylogous acid chloride reactivity
displayed by these intermediates in the preparation of 3,5-
disubstituted cyclopentenones. Given the ready commercial
availability of functionalized arylboronic acids, the Pd-catalyzed
Suzuki cross-coupling was an attractive option for increasing
the flexibility of this approach. On the other hand, to our
knowledge there is only one previous report describing a Suzuki
cross-coupling between 3-chlorocyclopentenone and an aryl-
boronic acid.9 Furthermore, an additional complication inherent
to the present substrates was created by the aryl group at the
5-position of the 3-chlorocyclopent-2-enones. The methine
proton at the 5-position is rendered more acidic by virtue of
being both benzylic and adjacent to the carbonyl group.
(7) (a) Chen, C.; Frey, L. F.; Shultz, S.; Wallace, D. J.; Marcantonio, K.;
Payack, J. F.; Vazquez, E.; Springfield, S. A.; Zhou, G.; Liu, P.; Kieczykowski,
G. R.; Chen, A. M.; Phenix, B. D.; Singh, U.; Strine, J.; Izzo, B.; Krska, S. W.
Org. Proc. Res. DeV. 2007, 11, 616. (b) Rosen, J. D.; Nelson, T. D.; Huffman,
M. A.; McNamara, J. M. Tetrahedron Lett. 2003, 44, 365. (c) Chen, C.; Dagneau,
P.; Grabowski, E. J. J.; Oballa, R.; O’Shea, P.; Prasit, P.; Robichaud, J.; Tillyer,
R.; Wang, X. J. Org. Chem. 2003, 68, 2633.
(8) Direct alkylation on free acids with nBuLi (conditions B) gave lower
isolated yield than conditions A.
J. Org. Chem. Vol. 74, No. 14, 2009 5101

TABLE 2. Studies of Base Effect in the Suzuki Couplinga
TABLE 3. 3,5-Diarylcyclopentenones via Suzuki Couplinga b
,
conv at
AY at
entry
base
KF
NaHCO3
K2CO3
K2CO3
NaHCO3
KF
solvent (8 mL/g)
1 h, %
2.5 h, %
1
2
3
4
5
6
dioxane
dioxane
dioxane
PhCH3/H2O (5:3)
PhCH3/H2O (5:3)
PhCH3/H2O (5:3)
>50
,50
∼50
100
.50
100
87
2
49
86
85
87
a The reactions were carried out in either dioxane or toluene/water
mixture. Conversions at 1 h and yields (AY: assay yield) at 2.5 h were
obtained by LC assay.
Deprotonation at this site would presumably generate a cyclo-
pentadiene species that could undergo undesirable side reactions.
Indeed, early attempts to derivatize the 3-chloro-5-arylcyclo-
pentenones as vinylogous esters with K2CO3/MeOH led to dark
reaction mixtures in which extensive degradation had occurred.
With this in mind, it was clear that very mild conditions would
be a prerequisite if the planned Suzuki cross-coupling process
was to be realized. Ligand screening revealed that several of
Buchwald’s biarylphosphines were effective, with the highly
active X-Phos showing particular promise.10 The coupling in a
biphasic solvent system with water proceeded faster than those
in just organic solvents. With X-Phos as ligand, coupling
proceeded very fast in a toluene/water solvent system, when
K2CO3 or KF was used as base (Table 2).
Following some optimization, conditions for the room tem-
perature Suzuki cross-coupling with arylboronic acids (6) were
developed and applied to the preparation of various 3,5-
diarylcyclopentenones (7), as shown in Table 3. Most reactions
went to completion within 1 to 2 h at room temperature. Cross-
coupling with more sterically hindered boronic acids required
increased reaction times to achieve full conversion and the yield
was slightly lower (62% for 7b; 74% yield for 7f, entries 2 and
6, respectively, Table 3). Cross-coupling with electron-rich
boronic acids delivered desired products in excellent yields (95%
yield for 7c, entry 3; 91% for 7h, entry 8, Table 3). An
interesting observation was made when attempting to prepare
3,5-diarylcyclopentenones that were electron deficient. It was
found that the initial products were prone to oxidation at the
5-position, producing a quaternary center bearing a hydroxyl
group. Presumably this is related to increased ease of deproto-
nation at the 5-position and reaction of the resulting enolate
with adventitious oxygen.11 For example, substrate 4d bearing
a The reactions were carried out at room temperature with 2 mol % of
Pd(OAc)2, 5 mol % of X-Phos, 2 equiv of boronic acids (6), and 3
equiv of K2CO3 in a mixture of toluene and water. b Yields of pure,
(9) (a) Murray, A.; Hansen, J. B.; Christensen, B. V. Tetrahedron 2001, 57,
7383. For Pd-mediated coupling of 3-chlorocyclopentenone derivative (Stille
coupling) see: (b) Araki, K.; Saito, K.; Arimoto, H.; Uemura, D. Angew. Chem.,
Int. Ed. 2004, 43, 81. For selective examples of cross-coupling reactions between
3-bromo or 3-iodo cyclopentenones and aryl or vinyl boronic acids see: (c)
Marson, C. M.; Farrand, L. D.; Dunmur, D. A. Tetrahedron 2003, 59, 4377. (d)
Geneˆt, J. P.; Linquist, A.; Blart, E.; Mourie`s, V.; Savignac, M.; Vaultier, M.
Tetrahedron Lett. 1995, 36, 1443. (e) Conchon, E.; Anizon, F.; Aboab, B.;
Prudhomme, M. J. Med. Chem. 2007, 50, 4669. (f) Okamoto, K.; Akiyama, R.;
Kobayashi, S. Org. Lett. 2004, 6, 1987.
(10) X-phos, 2-dicyclohexyldiphenylphosphine, cyclohexyldiphenylphos-
phine, and dppb (5 mol % each) were evaluated with 2.0 equiv of phenylboronic
acid (6a), 3.0 equiv of K2CO3, 2 mol % of Pd(OAc)2, in dioxane or dioxane/
water at 70 °C. X-phos and 2-dicyclohexyldiphenylphosphine gave faster
reactions and higher assay yields, where X-phos gave highest assay yield in
dioxane.
1
13
isolated products (characterized by H, C NMR, and HR-MS).
a nitro group had a relatively lower yield for this coupling (51%
yield for 7d, entry 4, Table 3), with a significant portion of the
mass balance found in the oxidation product. Although genera-
tion of these oxidized products could potentially be of interest,
further investigation of this process was outside the scope of
the current work.
For substrate 4b, which contains a 5-(p-bromophenyl) group,
the Suzuki cross-coupling with phenylboronic acid (6a) under
(11) The solvents used in these experiments were not degassed.
5102 J. Org. Chem. Vol. 74, No. 14, 2009

500 MHz) δ 7.35 (t, J ) 7.4 Hz, 2H), 7.28 (t, J ) 7.4 Hz, 1H),
7.17 (d, J ) 7.3 Hz, 2H), 6.33 (m, 1H), 3.80 (dd, J ) 7.4, 2.6 Hz,
1H), 3.40 (ddd, J ) 18.9, 7.4, 1.5 Hz, 1H), 3.01 (d, J ) 18.7 Hz,
1H); 13C NMR (CDCl3, 126 MHz) δ 204.5, 170.4, 138.3, 130.7,
129.2, 127.8, 127.6, 53.7, 44.1. Exact mass calcd for [C11H9ClO
the developed standard conditions led to unselective reaction
at both the halogen-bearing carbon atoms. In contrast, the aryl
chloride group in substrate 4h was unreactive under the standard
conditions and allowed for clean cross-coupling at the vinylo-
gous acid chloride (95% for 7c, entry 3, Table 3).
+
+ Ag] requires m/z 298.9393, found 298.9388 (ESI+).
In summary, the developed chemistry represents a general
method that employs simple procedures and readily available
starting materials for the synthesis of 3,5-diarylcyclopentenones.
Previously under utilized as a synthetic method, the key
intramolecular F-C acylation of easily accessible vinyl chloride-
bearing compounds was shown to deliver 3-chloro-5-arylcy-
clopentenone intermediates in good yields across a range of
substrates. As a demonstration of the utility of these vinylogous
acid chloride intermediates, Pd-catalyzed Suzuki-type cross-
coupling reactions were applied in the synthesis of a series of
3,5-diarylcyclopentenones. These prochiral compounds are
potential substrates for a variety of asymmetric transformations,
which will be the focus of future investigations in these
laboratories.
Typical Experimental Procedure for Suzuki Cross-Coupling
of 3-Chloro-5-arylcyclopentenone 4a To Deliver 3,5-Diphenyl-
2-cyclopentenone 7a (entry 1, Table 2): To a solution of
3-chlorocyclopentenone derivative 4a (0.200 g, 1.04 mmol) in
toluene/H2O (5:3, 1.6 mL) was added X-Phos (0.025 g, 0.052
mmol), boronic acid 6a (0.253 g, 2.08 mmol), Pd(OAc)2 (0.0047
g, 0.021 mmol), and K2CO3 (0.430 g, 3.11 mmol). The reaction
was stirred at room temperature in a sealed vial for 1 h and LC
indicated complete conversion of 4a. Brine (5 mL) and MTBE (10
mL) were added and the separated aqueous phase was extracted
with a second portion of MTBE (10 mL). The organic layers were
combined, dried over Na2SO4, and concentrated under reduced
pressure to give a crude product. The crude material was further
purified by silica gel flash chromatography eluting with 30% EtOAc/
hexenes to give 3,5-diphenyl-2-cyclopentenone 7a in 85% yield
1
(0.208 g). 7a: H NMR (CDCl3, 500 MHz) δ 7.73 (dd, J ) 7.4,
Experimental Section
1.5 Hz, 2H), 7.50 (d, J ) 7 Hz, 3H), 7.35 (t, J ) 7.3 Hz, 2H), 7.27
(m, 1H), 7.23 (d, J ) 7.6 Hz, 2H), 6.68 (m, 1H), 3.81 (dd, J ) 7.3,
2.6 Hz, 1H), 3.62 (ddd, J ) 18.3, 7.4, 1.6 Hz, 1H), 3.19 (ddd, J )
18.3, 2.5, 1.7 Hz, 1H); 13C NMR (CDCl3, 126 MHz) δ 208.4, 173.0,
140.0, 134.0, 131.7, 129.2, 129.1, 127.8, 127.2, 127.1, 126.6, 52.4,
38.6. Exact mass calcd for [C17H14O + Ag] + requires m/z 341.0096,
found 341.0101 (ESI+).
Typical Experimental Procedure for Friedel-Crafts Cycliza-
tion of Vinyl Chloride 3a To Afford 3-Chloro-5-arylcyclopen-
tenone 4a (entry 1, Table 1): To a solution of alkylated acid
derivative 3a (2.00 g, 9.49 mmol) in CH2Cl2 (35 mL) was added a
few drops of DMF followed by (COCl)2 (0.984 mL, 11.4 mmol).
After 1 h, LC analysis indicated complete conversion of starting
material to the corresponding acid chloride. AlCl3 (1.39 g, 10.4
mmol) was added and the mixture was stirred at room temperature
for 1.5 h until LC analysis indicated the full conversion of the acid
chloride intermediate. The mixture was cooled in an ice bath and
quenched by the careful addition of water. Saturated aqueous
NaHCO3 (20 mL) and MTBE (50 mL) were added and the separated
organic phase was washed with brine (25 mL), dried over Na2SO4,
and concentrated under reduced pressure to afford the crude product.
The crude material was further purified by silica gel flash chro-
matography eluting with 30% EtOAc/hexanes to give the product
4a as a light yellow solid (1.6 g, 87% yield). 4a: 1H NMR (CDCl3,
Acknowledgment. High-resolution mass spectroscopy analy-
sis support from Thomas J. Novak is gratefully acknowledged.
Supporting Information Available: Experimental proce-
dures and analytical data for all the F-C starting materials
(3a-h), products (4a-h) and Suzuki products (7a-h) with
reprints of NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO900696K
J. Org. Chem. Vol. 74, No. 14, 2009 5103