Preparation of trifluoromethylated dihydrocoumarins, indanones, and arylpropanoic acids by tandem superacidic activation of 2-(Trifluoromethyl) acrylic acid with arenes

Article abstract of DOI:10.1021/jo9026275

Chemical Reaction Reprentation Indanones and coumarins are important intermediates for the convenient synthesis of many pharmaceutical and biologically active compounds. Fluoroorganics play a vital role in the design of very effective therapeutics due to significant enhancenment in their lipophilicity, bioavailability, and fast uptake by the presence of fluorine in these molecules. Herein, we report an efficient one-pot synthesis of trifluoromethylated arylpropanoic acids, indanones, and dihydrocoumarins using Friedel-Crafts alkylation or tandem Friedel-Crafts alkylation-cycloacylation of arenes/phenols with 2-(trifluoromethyl)acrylic acid under superacidic conditions using trifluoromethanesulfonic acid. The results have been rationalized by the structure energy calculations of the involved reaction intermediates using ab initio theoretical methods.

Full text of DOI:10.1021/jo9026275

pubs.acs.org/joc
Preparation of Trifluoromethylated Dihydrocoumarins, Indanones,
and Arylpropanoic Acids by Tandem Superacidic Activation of
2-(Trifluoromethyl)acrylic Acid with Arenes
G. K. Surya Prakash,* Farzaneh Paknia, Habiba Vaghoo, Golam Rasul,
Thomas Mathew, and George A. Olah*
Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California,
Los Angeles, California 90089-1661
gprakash@usc.edu
Received December 11, 2009
Indanones and coumarins are important intermediates for the convenient synthesis of many
pharmaceutical and biologically active compounds. Fluoroorganics play a vital role in the design
of very effective therapeutics due to significant enhancenment in their lipophilicity, bioavailability,
and fast uptake by the presence of fluorine in these molecules. Herein, we report an efficient one-pot
synthesis of trifluoromethylated arylpropanoic acids, indanones, and dihydrocoumarins using
Friedel-Crafts alkylation or tandem Friedel-Crafts alkylation-cycloacylation of arenes/phenols
with 2-(trifluoromethyl)acrylic acid under superacidic conditions using trifluoromethanesulfonic
acid. The results have been rationalized by the structure energy calculations of the involved reaction
intermediates using ab initio theoretical methods.
Introduction
been used as intermediates in the synthesis of sulindac 5a,
nonsteroidal anti-inflammatory drug (NSAID),4a-c
a
1-Indanone 1 and dihydrocoumarin (DHC) 6 ring systems
constitute the core structures of many biologically important
compounds. 1-Indanones are present in cytotoxic natural
compound pterosines 2,1 the potent and selective COX-2
inhibitor flosulide 3,2 and the acetylcholinesterase inhib-
itor donepezil hydrochloride 4, used for the treatment of
Alzheimer’s disease3 (Figure 1). 1-Indanones have also
and other novel biologically important compounds.4d,e-g
1-Indanones are also important synthetic precursors for
the synthesis of the substituted indenyl ligand, which is used
in a broad range of metallocene catalysts 5b for olefin
polymerization.5 A number of synthetic approaches have
been reported for the synthesis of the 1-indanone ring system.
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DOI: 10.1021/jo9026275
r
Published on Web 03/10/2010
J. Org. Chem. 2010, 75, 2219–2226 2219
2010 American Chemical Society

Prakash et al.
JOCArticle
process,8 photochemical process,9 and ring-closing metathesis.10
However, little attention has been paid for the synthesis of
trifluoromethylated 1-indanones. Trifluoromethylated 1-inda-
nones have been previously prepared through electrophilic
trifluoromethylation of silyl enol ether11a or enolate anion of
1-indanones.11b
3,4-Dihydro-4-arylcoumarins (neoflavonoids) are present
in natural compounds such as calomelanols.12 Polyphenolic
compound 7 shows biological activity similar to estrogen.13
Diacetoxy dihydrocoumarin 8 is a protein transacetylase
compound.14 Splitomicin 9 and its analogues are known to
be Sir2 inhibitors.15 Natural dihydrocoumarins (DHC) are
of great interest in the flavor industry as well.16 Furthermore,
DHCs have been used extensively as precursors for the
synthesis of important bioactive compounds such as Detrol
LA (tolterodine tartrate) 10, which is a muscarine receptor
antagonist used for the treatment of urinary bladder disorder
(Figure 1).17
The synthesis of the 3,4-dihydrocoumarin ring system
has been accomplished in many ways, such as hydroarylation
of cinnamic acids with phenols in presence of strong acids,18
the catalytic hydrogenation of coumarins,19 reaction of
5-alkylidene Meldrum’s acids with phenol,20 Baeyer-Villiger
oxidation of 1-indanones,21 and palladium-catalyzed cyclo-
carbonylation of 2-vinylphenols,22a etc.22b-d Fluorine sub-
stitution, in particular, the presence of trifluoromethyl group
in organic compounds, often changes their physicochemical
and biological properties significantly.23 Knowing the im-
portance of dihydrocoumarin and indanone ring systems,
one-pot synthesis of trifluoromethylated dihydrocoumarins
FIGURE 1. Biologically active 1-indanones and coumarins and
their use as synthetic precursors.
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TABLE 1. Triflic Acid Catalyzed Preparation of 2-Trifluoromethylindanones and 2-Trifluoromethyl-3-arylpropanoic Acids
*Conversion was 88%.
and indanones using trifluoromethyl-containing building
blocks would be highly desirable.
to the carboxylate moiety has a significant influence on the
biological activity of the compounds, the synthesis of R-tri-
fluoromethylated carboxylic acids are important25 and can be
achieved easily by the developed methodology. The synthetic
results have been augmented with theoritical calculations of the
structure and energy of respective protonated and diprotonated
2-(trifluoromethyl)acrylic and 2-methylacrylic acids.
Herein, we report an efficient direct synthesis of 3-arylpro-
panoic acids, 2-trifluoromethyl-1-indanones, and 3-trifluoro-
methyl-3,4-dihydrocoumarins through superacidic trifluoro-
methanesulfonic acid (triflic acid) catalyzed condensation of
2-(trifluoromethyl)acrylic acid (11) with aromatics. 3-Phenyl-
propanoic acids are important intermediates in the synthesis of
bioactive compounds such as anti-AIDS, nonsteroidal, anti-
inflammatory, and antipsychotic drugs.24 Since the R-position
Results and Discussion
The nature of the products isolated during the superacid
induced reaction of 2-(trifluoromethyl)acrylic acid (11) with
arenes is highly dependent on the reaction conditions. For
€
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(c) O’Hagan, D.; Harper, D. B. J. Fluorine Chem. 1999, 100, 127. (d) Wu,
Y. C.; Chen, Y. J.; Li, H. J.; Zou, X. M.; Hu, F. Z.; Yang, H. Z. J. Fluorine
Chem. 2006, 127, 409. (e) McClinton, M. A.; McClinton, D. A. Tetrahedron
1992, 48, 6555. (f) Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V.
Chem. Soc. Rev. 2008, 37, 320.
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J. Fluorine Chem. 1992, 59, 249. (b) Kitazume, T.; Ohnogi, T.; Ito, K.
J. Am. Chem. Soc. 1990, 112, 6608. (c) Watanabe, S. Jpn. Kokai Tokkyo Koho.
JP 04063598 A, 1992.
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J. Org. Chem. Vol. 75, No. 7, 2010 2221

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SCHEME 1. Triflic Acid Catalyzed Synthesis of 2-Trifluoro-
methyl-3-aryl-1-Propanoic Acids (12) and 2-Trifluoromethyl-1-
indanones (13)
SCHEME 2. Triflic Acid Catalyzed Reaction of
2-(Trifluoromethyl)acrylic Acid with Phenols
in 80% yield after 7 h. Activated phenol such as p-cresol with 11
gave coumarin 14b in a shorter reaction time (4 h) at room
temperature (Table 2, entry 3). However, deactivated phenols
such as 3-nitrophenol gave only the uncyclized vinyl aryl ester 15
as the final product even at 55 °C after 48 h (Table 2, entry 2).
Compared to p-cresol (which possesses “uncongested”
ortho position that can participate in cyclization), condensa-
tion of 3,5-dimethylphenol with 11 was found to be slower
due to the steric effect imparted by the methyl group at the
ortho position to the cyclization center (Table 2, entry 5).
Reaction of 3-chlorophenol gave a mixture of ortho- and
para-cyclized products (14e, 14e0) with high para selectivity
as expected (Table 2, entry 7). Condensation of 2-naphthol
with 11 proceeded smoothly at room temperature with
complete conversion after 5 h yielding a splitomicin analogue
bearing a CF3 group (16) in excellent yield (Table 2, entry 8).
To the best of our knowledge, synthesis of a splitomicin
analogue bearing a CF3 has not been reported. Unlike 2-nap-
hthol, the reaction with 1-naphthol at room temperature was
much slower, and an unidentified mixture of products was
obtained when the reaction was attempted at 70 °C over 24 h.
For a better comparison, Friedel-Crafts reactions of
different arenes with 2-methylacrylic acid (17, the methyl
analogue of 11) in the presence of excess triflic acid have been
studied. Along with the corresponding indanones (18),
2-methyl-1,3-diaryl-1-propanone (19) and vinyl aryl ketone
(20) were also obtained (Scheme 3). No arylpropanoic acids
were isolated. The results are summarized in (Table 3).
The reaction of benzene with 2-methylacrylic acid (17) at
75 °C afforded 2-methyl-1-indanone 18a in 93% yield
(Table 3, entry 1). Lowering the reaction temperature to
45 °C gave rise to same product in 12-42% yield, whereas
2-(trifluoromethyl)acrylic acid (11) gave 2-trifluoromethyl-
3-arylpropanoic acid 13a as the only product under similar
reaction conditions (Table 1). Activated arenes such as
o-xylene and p-xylene undergo successive intermolecular
Friedel-Crafts alkylation and acylation affording the cor-
responding 2-methyl-1,3-diaryl-1-propanone (19, 25-35%)
along with indanones (18, 12-45%) in moderate yields.
With p-xylene, 2-(trifluoromethyl)acrylic acid (11) afforded
indanone 13b as the only product under similar reaction
conditions (Table 1, entry 3), and with o-xylene the co-
rresponding arylpropanoic acid 12c was the only pro-
duct (Table 1, entry 6). With mesitylene, the reaction was
not clean. Reaction of phenol with 2-methylacrylic acid
(17) at room temperature did not lead to coumarin deri-
vatives. Rather, the acylated product 20 was formed exclu-
sively along with a trace of unknown compounds (Table 3,
entry 4). In contrast, condensation of 2-(trifluoromethyl)-
acrylic acid (11) with phenols, as discussed, afforded 3,4-
dihydrocoumarin 14 as the only products in most cases
(Table 2).
example, reaction of 11 with benzene at 45 °C resulted in
2-trifluoromethyl-3-phenyl propanoic acid (12a) as the only
isolated product after 7 h (Table 1, entry 1). When the
temperature was increased to 70 °C, the cyclized product
2-trifluoromethyl-1-indanone (13a) was obtained in 80%
yield along with a small amount (6%) of 2-trifluoromethyl-
3-phenylpropanoic acid (12a) (Table 1, entry 2). This indi-
cates that acid 12 formed by intermolecular Friedel-Crafts
alkylation can undergo Friedel-Crafts intramolecular cy-
cloacylation at higher temperature to form the indanone 13.
Therefore, this reaction can be properly tuned and optimized
for the selective preparation of trifluoromethylated inda-
nones or propanoic acids (Scheme 1, Table 1) under suitable
reaction conditions. However, none of these products were
obtained when the reaction was repeated using weaker acid
such as methanesulfonic acid showing the higher acidity req-
uirement for the reaction.
Reaction of toluene with 2-(trifluoromethyl)acrylic acid in
excess triflic acid led to a mixture of products, which could
not be separated pure. p-Xylene at room temperature af-
forded indanone 13b in 90% yield (Table 1, entry 3). No
arylpropanoic acid was isolated. However, it is interesting to
note that the reaction of m-xylene under similar conditions
gave rise to the corresponding arylpropanoic acid 12b only.
When the temperature was increased to 70 °C, the major
product was indanone 13c (Table 1, entry 5). At room
temperature, reaction of o-xylene also led to arylpropanoic
acid 12c as the only product as observed in the case of
m-xylene. When the temperature was increased to 70 °C, the
trifluoromethylated indanones (13d and 13d0) were obtained
in 50% and 33% yields, respectively. Highly reactive mesity-
lene, at room temperature, provided only arylpropanoic acid
12d due to the absence of free ortho position for further
cyclization (Table 1, entry 8). However, reaction of 1,2,3,4-
tetramethylbenzene afforded the corresponding indanone 13e
at room temperature in high yield (Table 1, entry 9).
Reactions of phenol and its derivatives with 2-(trifluoro-
methyl)acrylic acid (11) in the presence of excess triflic acid
gave rise to 3,4-dihydro-3-trifluoromethyl-2H-1-benzopyr-
an-2-ones (3,4-dihydrocoumarins) 14 in a single step in high
yield and purity (Scheme 2). Condensation of phenol with
2-(trifluoromethyl)acrylic acid (11) at room temperature
gave the expected trifluoromethylated 3,4-dihydrocoumarin 14a
2222 J. Org. Chem. Vol. 75, No. 7, 2010

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TABLE 2. Triflic Acid Catalyzed Preparation of 3,4-Dihydro-3-trifluoromethyl-2H-1-benzopyrane-2-ones
*Conversion was 80%.
03 program.26 The geometry optimizations were performed at
the MP2/6-31G** level. Vibrational frequencies at the MP2/6-
31G**//MP2/6-31G** level were used to characterize station-
ary points as minima [number of imaginary frequency
(NIMAG) = 0] and to evaluate zero-point vibrational energies
(ZPE) which were scaled by a factor of 0.95.27 For MP2/6-
31G** structures further geometry optimizations were carried
out at the MP2/cc-pVTZ level. Final energies were computed at
the MP2/cc-pVTZ //MP2/cc-pVTZ þ ZPE level.
SCHEME 3. Triflic Acid Catalyzed Reaction of 2-Methylacry-
lic Acid with Arenes
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H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.,
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Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
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Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski,
V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
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Gaussian, Inc., Wallingford, CT, 2004.
Calculated Structures and Energies of Protonated
2-(Trifluoromethyl)acrylic Acid (H2CdC(CF3)COOH) and
Their Comparison with Protonated 2-Methylacrylic Acid
(H2CdC(CH3)COOH). To study the mechanistic details
and the nature of intermediates involved in the synthetic
tranformations in superacid medium, computational studies
and calculations on protonated trifluoromethyl as well as
methyl acrylic acids (11, 17) were performed using the Gaussian
(27) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
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JOCArticle
TABLE 3. Products of Triflic Acid Catalyzed Reaction of 2-Methylacrylic Acid with Arenes
*Unidentified products were found for entries 2-4.
TABLE 4. Total Energies (-au), ZPE,a and Relative Energies (kcal/mol)b
MP2/6-31G**
ZPE
MP2/cc-pvtz
rel energy
21
22
23
24
25
26
27
28
602.97091
602.89892
603.02016
603.01807
305.92691
305.88702
306.00374
305.98964
52.0
49.2
56.5
57.3
65.8
63.0
69.1
71.4
603.64129
603.56803
603.68555
603.68490
306.24742
306.20398
306.32034
306.30655
0.0
43.2
0.0
1.2
0.0
24.5
0.0
11.0
aZero-point vibrational energies (ZPE) at MP2/6-31G**//MP2/6-
31G** scaled by a factor of 0.95. Relative energy at MP2/cc-pVTZ//
b
MP2/cc-pVTZ þ ZPE level.
The CdC protonated 22 is characterized as a hydrogen-
bridged structure involving a 2e-3c bond.
Two isomeric diprotonated 2-(trifluoromethyl)acrylic
acid dications, O,O-diprotonated 23 and O- and CdC
diprotonated 24 were located as minima (Figure 2). The
structure 23 is only 1.2 kcal/mol more stable than 24
˚
(Table 4). The central C-C bond length of 1.414 A in 23
FIGURE 2. MP2/cc-pVTZ-calculated structure of 21-24.
is significantly shorter than the corresponding central C-C
bond length of 21 by about 0.05 A. This is due to enhanced
˚
conjugation between the CRdCβ double bond and the
CdO bond in 23. The structure 24 contains a carbonium
ion adjacent to a carboxonium ion center. Thus, the
structure can be considered as a carbonium- carboxonium
dication. Thus, both diprotonated 2-(trifluoromethyl)-
acrylic acid dications 23 and 24 can be considered as
superelectrophiles28 with greatly increased electrophilic
Two structures of protonated 2-(trifluoromethyl)acrylic
acid, oxygen protonated 21 and CdC protonated 22, were
found to be minima on the potential energy surface at the
MP2/6-31G** and MP2/cc-pVTZ levels (Figure 2). The CR
protonated structure does not correspond to a minimum and
converted into 22 upon optimization. Similarly, the Cβ
protonated structure also does not correspond to a minimum
and converted into same 22 upon optimization. The struc-
ture 21 is the global minimum being 43.2 kcal/mol more
stable than 22 (Table 4). The calculated CRdCβ and CR-C-
(28) (a) Olah, G. A. Klumpp, D. A. Superelectrophiles and Their Chemistry;
Wiley-Interscience: Hoboken, NJ, 2008. (b) Olah, G. A.; Klumpp, D. A. Acc.
Chem. Res. 2004, 37, 211. (c) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993,
32, 767.
˚
˚
(CO) distances of 21 are 1.340 A and 1.455 A, respectively.
2224 J. Org. Chem. Vol. 75, No. 7, 2010

Prakash et al.
JOCArticle
SCHEME 4. Suggested Mechanism for the Formation of Indanones and Dihydrocoumarins from 2-(Trifluoromethyl)acrylic Acid and
Arenes/Phenols
reactivity compared to their parent monoprotonated
monocations.
28 were located as minima (Figure 3). The structure 27 is
substantially more stable than the structure 28 by 11.0 kcal/
mol (Table 4). The structure 28 contains a carbenium ion
adjacent to a carboxonium ion center. Thus, the structure
can be considered as a carbenium-carboxonium dication.
The suggested mechanism for the formation of indanones
based on theoretical calculations involves initial protonation
of the carbonyl group, which results in the activation of the
double bond in the acrylic acid. Further protosolvation
would lead to superelectrophilic dications in excess of triflic
acid followed by intermolecular Friedel-Crafts alkylation
of the arenes to give the corresponding arylpropanoic acids.
Further protonation of the resulting arylpropanoic acids
lead to cyclization through intramolecular Friedel-Crafts
acylation. There are two possible mechanistic pathways for
the reaction of phenols. Path (a)18a involves the esterification
of the phenols with 2-trifluoromethyl acrylic acid to the
corresponding vinylic aryl ester, followed by protosolvation
and formation of superelectrophilic dicationic intemediates.
Subsequent ring closure through intramolecular Friedel-
Crafts cyclization (similar to electrocyclization in Nazarov
reaction generally catalyzed by excess strong Lewis or
Brønsted acids) leads to the coumarin ring.6c,29 Path (b)18b,e
involves Friedel-Crafts alkylation at ortho position to hy-
droxyl group to form β-(2-hydroxyphenyl)-R-trifluoro-
methylpropanoic acid, followed by dehydrative cyclization
(Scheme 4).
For comparison, we have also calculated structures of
methyl analog of protonated 2-(trifluoromethyl)acrylic acid,
i.e., 2-methylacrylic acid at the same level. However, in the
latter case, the Cβ-protonated structure 26 in addition to the
O-protonated 25 were found to be minima on the potential
energy surface (Figure 3). The CR carbon protonated struc-
ture does not correspond to a minimum and converted into
25 upon optimization. The structure 25 is 24.5 kcal/mol more
stable than 26 (Table 4). The structure 26 is in fact a carboxyl-
substituted tertiary carbenium ion.
Two isomeric diprotonated 2-methylacrylic acid dica-
tions, O,O-diprotonated 27, and O- and Cβ diprotonated
Conclusion
In summary, we have developed a novel and efficient route
to trifluoromethylated dihydrocoumarins, indanones, and
arylpropanoic acids based on superacid induced Friedel-
Crafts-type reaction of 2-(trifluoromethyl)acrylic acid with
arenes. This manifests the use of 2-(trifluoromethyl)acrylic
(29) Suzuki, T.; Ohwada, T.; Shudo, K. J. Am. Chem. Soc. 1997, 119,
FIGURE 3. MP2/cc-pVTZ-calculated structure of 25-28.
6774.
J. Org. Chem. Vol. 75, No. 7, 2010 2225

Prakash et al.
JOCArticle
acid as a highly useful synthetic precursor for the synthesis of
variety of new biologically important trifluoromethylated
heterocycles and carbocycles. Reactivity profiles of both
2-(trifluoromethyl)acrylic acid and 2-methylacrylic acid to-
ward various arenes in triflic acid medium have also been
studied using computational methods. Computational studies
and calculations suggest the possible involvement of reac-
tive dicationic intermediates formed under superacidic
conditions and support the mechanistic pathways involved
in these reactions.
poured over ice/water (∼25 g) and extracted with CH2Cl2
(3 ꢀ 15 mL). The organic extracts were combined, washed with
water, and dried over anhydrous MgSO4. The solvent was
removed by vacuum evaporation, and crude products were
purified with column chromatography on silica gel (70-230
mesh) using hexane/ethyl acetate as eluent. The products
were characterized by spectral analysis (NMR, GC-MS,
and HRMS).
6-Methyl-3-trifluoromethyl-3,4-dihydrocoumarin (14b). Pre-
pared by the reaction of 11 with p-cresol at room temper-
ature using general procedure B gave 14b in 91% yield. 1H
NMR (400 MHz, CDCl3): δ 2.33 (s, 3H), 3.18 (d, J = 9.15 Hz,
2H), 3.35-3.46 (m, 1H), 6.94 (d, J = 8.24 1H), 7.04 (s, 1H),
7.08-7.11 (m, 1H). 13C NMR (100.5 MHz, CDCl3): δ 20.6, 24.3
Experimental Section
3
2
General Procedure A. Triflic Acid Catalyzed Reactions of
2-(Trifluoromethyl)acrylic Acid with Arenes for the Synthesis of
2-Trifluoromethyl-1-indanones and 2-Trifluoromethyl-3-arylpro-
panoic Acids. 2-(Trifluoromethyl)acrylic acid (0.28 g, 2 mmol)
was mixed with excess of arene (10 mmol) in a pressure tube.
After the mixture was cooled to 0 °C, triflic acid (3 mL, 34 mmol)
was slowly added and the pressure tube was closed. The mixture
was stirred for the required period of time at room temperature
or higher temperatures (Table 1). Progress of the reaction was
monitored by TLC (4:1 hexane/ethyl acetate) and 19F NMR
spectroscopy. After the reaction was complete the mixture was
poured over ice/water (∼25 g) and extracted with CH2Cl2 (3 ꢀ
15 mL). The organic extracts were combined, washed with
water, and dried over anhydrous MgSO4. The solvent was
removed by vacuum evaporation, and crude products were
purified by column chromatography on silica gel (70-230 mesh)
using hexane/ethyl acetate as eluent. The products were char-
acterized by spectral analysis (NMR, GC-MS, and HRMS).
3-Phenyl-2-trifluoromethylpropanoic Acid (12a). Prepared
from the reaction of compound 11 and benzene at 45 °C using
general procedure A in 68% yield. 1H NMR (400 MHz, CDCl3):
δ 3.11-3.21 (m, 2H), 3.38-3.50 (m, 1H), 7.17-7.20 (m, 2H),
7.24-7.32 (m, 3H), 9.12 (br, 1H); 13C NMR (100.5 MHz,
(q, J(C-F) = 2.29 Hz), 43.9 (q, J(C-F) = 28.99 Hz), 116.4,
119.3, 123.8 (q, 1J(C-F) = 278.72 Hz), 128.5, 129.5, 134.9, 148.8,
161.8. 19F NMR (376 MHz, CDCl3): δ -69.25 (d, J(H-F) = 8.10
Hz). GC-MS (EI): m/z 230.6 (Mþ). HRMS (EI): m/z calcd for
C11H9F3O2 230.0555, obsd 230.0546.
General Procedure C. Triflic Acid Catalyzed Reactions of
2-Methylacrylic Acid with Arenes. 2-Methylacrylic acid (0.26 g,
3 mmol) was mixed with an excess of arene (15 mmol) in a
pressure tube. After the mixture was cooled to 0 °C, triflic acid
(4.5 mL, 51 mmol) was slowly added and the pressure tube was
closed. The mixture was stirred for the required period of time
at room temperature or higher temperatures (Table 3). Pro-
gress of the reaction was monitored by TLC (4:1 hexane/ethyl
acetate). After the reaction was complete, the mixture was
poured over ice/water (∼25 g) and extracted with CH2Cl2 (3 ꢀ
15 mL). The organic extracts were combined, washed with
water, and dried over anhydrous MgSO4. The solvent was
removed by vacuum evaporation, and crude products were
purified by column chromatography on silica gel (70-230
mesh) using hexane/ethyl acetate as eluent. The products were
characterized by spectral analysis (NMR, GC-MS, and
HRMS).
2-Methyl-1-indanone (18a). Prepared by the reaction of 17
with benzene at 75 °C using general procedure C in 93% yield.
1H NMR (400 MHz, CDCl3): δ 1.30 (d, J = 7.32 Hz, 3H),
2.64-2.73 (m, 2H), 3.34-3.41 (m, 1H),7.34 (td, J = 7.42, 0.73
Hz, 1H), 7.43 (dt, J = 7.69, 0.82 Hz, 1H), 7.56 (td, J = 7.41, 1.10
Hz, 1H), 7.73 (d, J = 7.69 Hz, 1H). 13C NMR (100.5 MHz,
CDCl3): δ 16.0, 34.7, 41.7, 123.7, 126.3, 127.1, 134.5, 136.1,
153.2, 209.2. GC-MS (EI): m/z 147.6 (Mþ).
3
2
CDCl3): δ 32.0 (q, J(C-F) = 2.29 Hz), 52.2 (q, J(C-F)
=
27.47 Hz), 124.3 (q, 1J(C-F) = 280.76 Hz), 127.3, 128.7, 128.8,
3
135.80, 172.1 (q, J(C-F) = 3.05 Hz). 19F NMR (376 MHz,
CDCl3): δ -68.64 (d, J(H-F) = 7.63 Hz). HRMS (ESI): m/z
calcd for C10H8F3O2 (M - 1) 217.0482, obsd 217.0480.
2-Trifluoromethyl-1-indanone (13a). Prepared from the reac-
tion of compound 11 and benzene at 70 °C using general
procedure A in 80% yield. 1H NMR (400 MHz, CDCl3): δ
3.31-3.50 (m, 3H), 7.44 (t, J = 7.45 Hz, 1H), 7.19 (d, J = 7.90
1,3-Bis(2,5-dimethylphenyl)-2-methyl-1-propanone (19a). Pre-
pared by reaction of 17 with p-xylene at room temperature by
using general procedure C in 35% yield. H NMR (400 MHz,
Hz, 1H), 7.67 (t, J = 7.45 Hz, 1H), 7.82 (d, J = 7.90 Hz, 1H). 13
C
1
NMR (100.5 MHz, CDCl3): δ 27.6 (q, 3J(C-F) = 3.00 Hz), 49.7
(q, 2J(C-F) = 27.43 Hz), 124.70, 124.9 (q, 1J(C-F) = 278.60 Hz),
126.4, 128.2, 135.8, 152.1, 196.8. 19F NMR (376 MHz, CDCl3):
δ -68.23 (d, J(H-F) = 9.15 Hz). GC-MS (EI): m/z 200.9 (Mþ).
HRMS (EI): m/z calcd for C10H7F3O 200.0449, obsd 200.0448.
General Procedure B. Triflic Acid Catalyzed Reactions of
2-(Trifluoromethyl)acrylic Acid with Phenols for the Synthesis
of 3-Trifluoromethyl-3,4-dihydrocoumarins. 2-(Trifluoromethyl)-
acrylic acid (0.28 g, 2 mmol) and phenols (2.2 mmol) were
dissolved in CH2Cl2 (1 mL) in a pressure tube. After the
mixture was cooled to 0 °C, triflic acid (3 mL, 34 mmol) was
slowly added. The pressure tube was closed, and the mix-
ture was stirred for the required period of time at room
temperature or higher temperatures depending on the sub-
strate (Table 2). Progress of the reactions was monitored
by TLC (5:3 hexane/ethyl acetate) and 19F NMR spectro-
scopy. After the reaction was complete the mixture was
CDCl3): δ 1.16 (d, J = 6.96 Hz, 3H), 2.24 (s, 3H), 2.26 (s, 3H),
2.27 (s, 3H), 2.34 (s, 3H), 2.61 (dd, J = 13.73, 7.87 Hz, 1H), 3.08
(dd, J = 13.73, 6.41 Hz, 1H) 3.47-3.56 (m, 1H), 6.88-6.93 (m,
2H), 7.00 (d, J = 7.69 Hz, 1H), 7.06-7.11 (m, 3H). 13C NMR
(100.5 MHz, CDCl3): δ 16.9, 19.0, 20.0, 20.8, 20.8, 36.2, 44.9,
126.9, 128.0, 130.1, 130.6, 131.3, 131.4, 132.8, 134.2, 134.8,
135.1, 137.9, 138.7 208.6. HRMS (EI): m/z calcd for C20H24O
280.1827, observed 280.1823.
Acknowledgment. The support of Loker Hydrocarbon
Research Institute is gratefully acknowledged.
Supporting Information Available: Experimental proce-
dures, 1H, 13C, and 19F NMR, and HRMS of all new com-
pounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
2226 J. Org. Chem. Vol. 75, No. 7, 2010