Cross Redox Coupling of Aryl-Aldehydes and p-Benzoquinone

Article abstract of DOI:10.1021/acs.joc.5b02175

Herein, we report an unprecedented Cross Redox Coupling (CRC) reaction catalyzed by Cu(OAc)₂·H₂O. As a proof-of-concept, direct coupling of aromatic aldehydes (or alcohols) and p-benzoquinone led to an ester in the presence of the Cu(II)-TBHP combination. During the coupling process, the C-H bond of the aldehydes was converted directly to a C-O bond. Mechanistically, we propose that the reaction proceeded via a radical pathway. In addition, atom and electron economies were well-conserved during this CRC reaction.

Full text of DOI:10.1021/acs.joc.5b02175

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Note
Cross Redox Coupling of Aryl-Aldehydes and p-Benzoquinone
Anima Bose, and Prasenjit Mal
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b02175 • Publication Date (Web): 15 Oct 2015
Downloaded from http://pubs.acs.org on October 19, 2015
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Cross Redox Coupling of ArylꢀAldehydes and pꢀBenzoquinone
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Anima Bose and Prasenjit Mal*
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School of Chemical Sciences, National Institute of Science Education and Research (NISER)
Bhubaneswar; Institute of Physics Campus, P.O. Sainik School; Bhubaneswar, Odisha751 005,
India
*Corresponding authors:
Prasenjit Mal, Tel.: +91 9439613856, Eꢀmail: pmal@niser.ac.in, http://www.niser.ac.in/~pmal/
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GRAPHIC FOR TOC
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Cross Redox Coupling (CRC) Reaction
O
OH
O
OH
H or
O
no atom loss
no electron loss
via radical pathway
Cu(II)
H
+
O
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R
O
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R
Condition: Cu(OAc)2.H2O (10 mol%); TBHP (3 equiv); DMSO; 110 oC
ABSTRACT
Herein we report an unprecedented Cross Redox Coupling (CRC) reaction catalyzed by
Cu(OAc)2.H2O. As a proofꢀofꢀconcept, direct coupling of aromatic aldehydes (or alcohols) and
pꢀbenzoquinone led to an ester in presence of Cu(II)ꢀTBHP combination. During the coupling
process, CꢀH bond of the aldehydes were converted directly to CꢀO bond. Mechanistically, we
propose that the reaction proceeded via a radical pathway. In addition, atom and electron
economies were wellꢀconserved during this CRC reaction.
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A redox coupling reaction1 is known in Fischer indolization reaction2,3 in which CꢀC single bond
is oxidatively created4 at the expense of reductive cleavage of NꢀN bond. Overall, indolization
reaction could be rationalized as dehydrogenative αꢀcross coupling of two ketones to a 1,4ꢀ
diketone.5 Similarly, various cross condensation reactions between two carbonyl compounds are
well documented in literature e.g., cross aldol condensation,6 Cannizzaro reaction,7 coupling of
ketones and carboxylate enolates,5 etc. Undesirably, most of these traditional procedures require
strong basic condition, higher temperature and produce unwanted side products.
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Benzoquinone and its derivatives8 are prevalent in organic synthesis. Also, they have versatile
use in many research fields like molecular electronics,9 oxidation chemistry,10 medicinal
chemistry,11,12 radical reactions,13,14 etc. The most common reactions of quinones are:
nucleophilic addition of thiols and amines to derive substituted hydroquinone derivatives via
Michael addition15 and metal16,17 or hypervalent iodine catalyzed18 CꢀH activation.19ꢀ24 However,
examples on acylation of hydroquinone directly from quinone with acylating agents are very few,
if any.25 In synthesis, mono esterification of hydroquinone is challenging because of
unavailability of suitably activated acid or coupling reagents8 and uncontrollable diesterification
reactions.
We report here direct cross coupling reaction for esterification of two carbonyl compounds (pꢀ
benzoquinone and benzaldehyde derivatives) (Fig. 1). To the best of our knowledge, coupling
reaction between two carbonyls via a radical pathway is unprecedented (this work).
Nevertheless, crossꢀcoupling of αꢀalkoxymethylꢀtrifluoroborates with arylꢀ and heteroaryl
bromides has recently been demonstrated using iridium photoredoxꢀcatalyst and Niꢀcatalyst.26
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Generally, esters are synthesized using preꢀactivated acid derivatives (e.g., anhydrides, acyl
halides, activated esters, etc.) and alcohols in presence of stoichiometric amount of bases by
multistep processes.27ꢀ31 Besides, few modified esterification approaches are: oxidative
esterification of aldehydes with βꢀdicarbonyl compounds,32 CꢀO coupling by direct CꢀH bond
activation of formamides for synthesis of enol carbamates,33 Pdꢀcatalyzed oxidative crossꢀ
esterifications,34 Nꢀheterocyclic carbene (NHC)35 catalyzed esterifications of pꢀnapthaquinones
using aldehydes via Breslow intermediate (Fig. 1d),36 etc.
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Figure 1. Cross Redox Coupling (CRC) reaction; newly formed CꢀO bonds are shown as redꢀ
thick line and references as superscript. a) Understanding of CRC reaction. b) This work:
Cu(OAc)2.H2O catalyzed CRC reaction. c) Intramolecular Cannizzaro reaction.7 d) NHC
catalyzed monoacylation of 1,4ꢀnapthaquinopne using aldehyde via Breslow intermediate.36
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Table 1 represents optimization of the reaction condition. Most suitable condition (entry 5) was
identified using 1.0 equiv of 1b (pꢀanisaldehyde), 1.5 equiv of 2 (pꢀbenzoquinone), 3.0 equiv of
TBHP (Caution!!, see Caution paragraph in Experimental Section on page 13) and 10 mol% of
Cu(OAc)2.H2O in DMSO at 100ꢀ110 °C. At high temperature (ca. 110 °C) TBHP is known to
undergo decomposition. Therefore, excess TBHP (3 equiv, 70% aqueous solution) was required
for the reaction. The reaction led to poor yields when temperature was <100 °C and failed in
presence of additives like I2 (entry 8) or KIꢀI2 (entry 9). Sluggish mixture was obtained in
presence of CuCl2 (entry 11), Cu(OTf)2 (entry 13), FeCl3 (entry 12), etc. Addition of K2S2O8
(entry 10) to Cu(OAc)2.H2O did not lead to any improvement of yield. Reaction was also
unsuccessful with solvents like acetonitrile (entry 1) and dimethyl formamide (entry 2).
Conversely, it was poor yielding (ca. 38%) under neat condition (entry 14) and failure under
solvent free ballꢀmilling condition37
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Under optimized condition (Table 1, entry 5), we verified substrates scope for benzoylation of pꢀ
benzoquinone (Fig. 2). Benzaldehyde and its derivative with electron donating substituents
resulted in good yield of the products (3a, 3b, 3c, 3j, 3k and 3l). Halogen substituted monoesters
were also isolated in reasonably fair yields (3e and 3f). Similarly, esters were obtained in good
yields with orthoꢀsubstituted benzaldehydes (3d, 3g, 3j and 3m), heteroꢀaromatic aldehyde (3o)
and polynuclear aromatic aldehydes (3p, 3q and 3r). In presence of TBHP, overꢀoxidation of
benzylic CꢀH was not observed for the substrate 3c, 3d, 3g, 3h, 3m and 3n. One of the major
drawbacks of this CRC reaction in which aldehydes with electron withdrawing group like pꢀnitro
and pꢀcyano failed to produce any esters. Also, under similar reaction condition (Table 1, entry 5)
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naphthoquinone and 2,6ꢀdimethyl benzoquinone led to unidentifiable mixture of products with
benzaldehyde.
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Table 1. Optimization of reaction condition for synthesis of 3b.
Entry TBHPa Additiveb
(equiv) (mol%)
Solventc
Yieldd
(%)
(temp, °C)
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Cu(OAc)2.H2O CH3CN (90 °C) < 5
Cu(OAc)2.H2O DMF (90 °C)
< 5
Cu(OAc)2.H2O DMSO (90 °C) 36
Cu(OAc)2.H2O DMSO
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Cu(OAc)2.H2O DMSO
(10)
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Cu(OAc)2.H2O DMSO
(5)
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30
TBAI (20)
DMSO
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3
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I2 (20)
DMSO
ꢀꢀ e
ꢀꢀ e
50
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KI (50) + I2 (20) DMSO
10
Cu(OAc)2.H2O DMSO
(10) + K2S2O8
(20)
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3
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CuCl2 (20)
DMSO
DMSO
DMSO
ꢀꢀf
FeCl3 (30)
< 5
ꢀꢀ e
38 d
Cu(OTf)2 (20)
Cu(OAc)2.H2O Neat
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17
3g
3
Cu(OAc)2.H2O DMSO
ꢀꢀh
ꢀꢀh
ꢀꢀh
CuI (20)
DMSO
DMSO
3
CuBr (20)
a 70% In water. Cu(OAc)2.H2O used in 20 mol% unless specified. In DMSO, the reactions
b
c
d
e
f
were done at 110 °C, if not shown. Yield based on recovered aldehydes. Not conclusive.
Sluggish reaction mixture. g 5ꢀ6 M TBHP in decane. h No reaction.
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Figure 2. Products from CRC reaction. Compounds identification number, reaction time and
yields are shown. Yields (after column chromatography) were calculated based on recovered
aldehydes.
We have further extended the scope for this CRC methodology to verify multistep synthesis.38
Under optimized condition (Fig. 2), primary alcohols were directly used in presence of 4 equiv
of TBHP. Expectedly, alcohols were in situ oxidized in presence of additional one equivalent of
TBHP. The results of this multistep synthesis are shown in Fig. 3.
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Figure 3. In situ oxidation of alcohols to aldehydes for cross redox coupling with pꢀ
benzoquinones under standard condition. Yields are based on recovered aldehydes.
Control experiments (Fig. 4) in DMSO at 100ꢀ110 °C were performed to understand the
mechanism of CRC reaction (Fig. 5). 2,2,6,6ꢀTetramethylpiperidinꢀ1ꢀylꢀoxy radical (TEMPO)
adduct (5, 78%) of pꢀanisaldehyde (1b) was observed when the reaction was performed in
presence of 2 equiv of TEMPO (Fig. 4a). In absence of either both or any one of the reagents like
TBHP and Cu(OAc)2.H2O, no reaction between pꢀbenzoquinone and benzaldehydes were
observed (Fig. 4b). Formation of 5 indicates that benzoyl radical might be involved as one of the
intermediates (8, Fig. 5).
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Figure 4. a) ꢀ d) Control experiments to understand the mechanism of the reaction.
Furthermore, we could rule out the possibility of complete reduction of pꢀbenzoquinone to
hydroquinone (6) due to inaccessibility of any diester of hydroquinone during reaction. Trace (<
3%) amount of 3b was detected (Fig. 4c) in between reaction of 1b and 6. Likewise, no 3b could
be isolated from reaction of 6 and pꢀanisic acid (7). These facts clearly establish that neither
hydroquinone nor benzoic acid derivatives (anisic acid) was the intermediate in this reaction.
However, when the reaction was continued for more than 48 h, we observed 3b' as a minor
product (< 5%) (Fig. 4d). Interestingly, no direct Cꢀaroylated product before Oꢀaroylation was
observed.
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Figure 5. Plausible mechanism for Cu(II) catalyzed CRC reaction.
Plausible mechanism of Cu(II) catalyzed CRC reaction is proposed (Fig. 5) based on the
outcome of control experiments (Fig. 4). In presence of Cu(II), tꢀbutylperoxy radical (tꢀBuOO•)
was produced from TBHP. After which tꢀBuOO• could led to aroyl radical (8)39 derived from
benzaldehydes (1) with release of Cu(I).40,41 The hydrogen radical (H•) produced from TBHP
might reduce Cu(II) to Cu(I) to generate H+. Further, this aroyl radical possibly combined with pꢀ
benzoquinone via Oꢀaddition42 to result in 4ꢀ(aroyloxy)phenoxide radical (9). Following, Cu(I)
expected to reduce 9 into 4ꢀ(aroyloxy)phenolate anion (10). Finally, 10 yielded the ester 3 in
presence of H3O+ (TBHP used as 70% in water). However, no reaction could be observed using
5ꢀ6 M TBHP in decane (entry 15, Table 1) which confirms the participation of H2O as a source
of proton transfer agent. Again, possibilities of classical disproportionation reactions of carbonyl
compounds like Cannizzaro43 and Tishchenko reactions44 could be easily ruled out due to
observation of aroyl radical intermediate. Thus, the mechanism of CRC reaction is identified
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with the help of Cu(II) as catalyst. Cu(OAc)2.H2O is a wellꢀknown oxidizing agent and probably
caused over oxidation of aldehydes to acids in presence of more than 10 mol%. Hence, poor
yield was observed with 20 mol% of Cu(II) (entry 4, Table 1).
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In summary, we demonstrated Cu(II) catalyzed cross redox coupling (CRC) reaction for
synthesis of esters directly from pꢀbenzoquinone, and aldehydes (or alcohols) using TBHPꢀ
Cu(OAc)2.H2O combination. This newly discovered CRC reaction of aldehydes and pꢀ
benzoquinone is also identified as an example of CꢀH functionalization of the aldehydes.45 In this
reaction none of the carbonyl compounds were preꢀactivated for esterification. Atom or electron
economies were well conserved during the reaction. The proposed CRC methodology requires
easily available precursors and can be performed on several aldehydes. We believe this reaction
will lead to an important contribution in organic synthesis and mechanistic organic chemistry.
EXPERIMENTAL SECTION
General Methods. Column chromatographic purifications of the compounds were performed
using silica gel (mesh 100–200) and hexane – ethyl acetate mixtures as eluent, unless otherwise
specified. NMR spectra were recorded on a 400 MHz instrument at 25 °C. The chemical shift
values are reported in parts per million (ppm) with respect to residual dimethyl sulfoxide (2.50
1
13
13
ppm for H and 40.00 for C). C NMR spectra of all the compounds are recorded as protonꢀ
decoupled carbon spectra (13C{1H}). The peak patterns are designated as follows: s, singlet; d,
doublet; t, triplet; q, quartet; m, multiplet; dd, doublet of doublets; td, triplet of doublets; br s,
broad singlet. The coupling constants (J) are reported in Hertz (Hz). Highꢀresolution mass
spectra (HRꢀMS) were recorded on an ESIꢀTOF (time of flight) mass spectrometer. Infrared
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spectral data are reported in wave number (cmꢀ1). FTꢀIR spectra were recorded after making
pellet of the compounds using anhydrous solid KBr. Melting points of the compounds were
determined using a digital melting point apparatus and are uncorrected.
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Caution. TBHP is a potential shock sensitive chemical.37 Therefore, very high level of safety
precautions should be exercised during reaction with TBHP. The precautions like PPEs (personal
protective equipment) should be used while handling TBHP under neat condition. In this work,
the TBHP was used as 70% in water. Use of blast shields is mandatory at all times during the
reactions.
Yields (after column chromatography) were calculated based on recovered aldehydes. However,
in parenthesis yields are calculated based on aldehydes used for the reaction.
Trapping of acyl radical by TEMPO: TEMPO (228 mg, 1.46 mmol, 2 equiv) was added to an
ovenꢀdried sealed tube charged with a magnetic stirring bar and 1b (100 mg, 0.73 mmol, 1
equiv). Cu(OAc)2.H2O (14.5 mg, 0.073 mmol, 10 mol %), and TBHP (0.3 mL, 2.2 mmol, 3
equiv, 70 % in water ) were added to the mixture in DMSO, and the sealed tube was kept at 100
°C. The reaction was monitored by TLC. After completion of the reaction, the mass was
dissolved in dichloromethane and purified by column chromatography to obtain 2,2,6,6ꢀ
tetramethylpiperidinꢀ1ꢀylꢀ4ꢀmethoxybenzoate (5) . Yield 78% (166 mg); Rf 0.40 (2% diethyl
1
ether/hexane); pale yellow liquid; H NMR (400 MHz, DMSOꢀd6) δ 7.92 (d, J = 8.8 Hz, 2H),
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7.05 (d, J = 8.8 Hz, 2H), 3.82 (s, 3H), 1.54 – 1.34 (m, 6H), 1.17 (s, 6H), 0.96 (s, 6H); C{1H}
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NMR (100 MHz, DMSOꢀd6) δ 165.6, 163.6, 131.5, 121.6, 114.6, 60.1, 55.9, 39.2, 32.0, 20.8,
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6
~
ν
16.9; IR Neat 2974, 2937, 1740, 1601, 1503, 1458, 1364, 1319, 1245, 1172, 1074, 1033, 922,
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849, 771, 686, 608 cmꢀ1; HRMS observed 292.1907 (calculated for C17H26NO3 [M+H]+
292.1913).
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Procedure for preparation of 4ꢀhydroxyphenylꢀ4ꢀmethoxybenzoate (3b): To an ovenꢀdried
sealed tube charged with a magnetic stirring bar and 1b (100 mg, 0.73 mmol, 1 equiv),
Cu(OAc)2.H2O (14.5 mg, 0.073 mmol, 10 mol %), and TBHP (0.3 mL, 2.2 mmol, 3 equiv, 70 %
in water ) in DMSO (2 mL) was added 2 (120 mg, 1.1 mmol, 1.5 equiv). The reaction mixture
was allowed to stir at 110 °C for 16 h. After cooling at room temperature, the reaction mixture
washed with water and followed by extracted with ethyl acetate. Column purification using 15%
ethyl acetateꢀhexane yielded 3b 82% (117 mg, 66%) as pale yellow solid; Rf 0.20 (15% ethyl
acetate/hexane); mp: 162ꢀ164 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.45 (s, 1H), 8.05 (d, J = 8.8
Hz, 1H), 7.10 (d, J = 8.8 Hz, 2H), 7.02 (d, J = 8.8 Hz, 2H), 6.79 (d, J = 8.8 Hz, 2H), 3.86 (s, 3H);
13C{1H} NMR (100 MHz, DMSOꢀd6) δ 165.0, 164.0, 155.5, 143.2, 132.3, 123.0, 121.7, 116.0,
~
ν
114.7, 56.1; IR (KBr) 3438, 2935, 1701, 1597, 1509, 1398, 1321, 1287, 1261, 1168, 1112,
1073, 994, 850, 767, 696 cmꢀ1; HRMS observed 245.0808 (calculated for C14H13O4 [M+H]+
245.0814).
Procedure for in situ oxidation of benzyl alcohols to aldehydes and preparation of 4ꢀ
hydroxyphenylꢀ4ꢀmethoxybenzoate (3b): To an ovenꢀdried sealed tube charged with a
magnetic stirring bar and 4b (100 mg, 0.72 mmol, 1 equiv), Cu(OAc)2.H2O (14.5 mg, 0.072
mmol, 10 mol %), and TBHP (0.4 mL, 2.8 mmol, 4 equiv, 70 % in water ) in DMSO (2 mL) was
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added 2 (117 mg, 1.1 mmol, 1.5 equiv). The reaction mixture was allowed to stir at 110 °C for
20 h. After cooling at room temperature, the reaction mixture washed with water and followed
by extracted with ethyl acetate. Column purification using 15% ethyl acetateꢀhexane yielded 3b
(104 mg, 73%) as pale yellow solid.
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4ꢀHydroxyphenylbenzoate (3a): Rf 0.20 (10% ethyl acetate/hexane); colorless solid; yield 70%
1
(90 mg, 58%) ; mp: 160ꢀ162 °C (lit.46 161ꢀ165 °C); H NMR (400 MHz, DMSOꢀd6) δ 9.48 (s,
1H), 8.10 (d, J = 8.0 Hz, 2H), 7.73 (t, J = 6.8 Hz, 1H), 7.59 (t, J = 7.6 Hz, 2H), 7.06 (d, J = 8.8
13
Hz, 2H), 6.80 (d, J = 8.8 Hz, 2H); C{1H} NMR (100 MHz, DMSOꢀd6) δ 165.4, 155.6, 143.1,
~
ν
134.3, 130.1, 129.6, 129.4, 123.0, 116.1; IR (KBr) 3455, 1714, 1596, 1508, 1398, 1338, 1281,
1209, 1188, 1112, 1066, 1000, 878, 816, 712 cmꢀ1; HRMS observed 215.0703 (calculated for
C13H11O3 [M+H]+ 215.0709).
4ꢀHydroxyphenylꢀ4ꢀmethylbenzoate (3c): Rf 0.20 (10% ethyl acetate/hexane); off white solid;
yield 65%, (93 mg, 49% ); mp: 119ꢀ122 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.48 (s, 1H), 7.99
(d, J = 7.6 Hz, 2H), 7.39 (d, J = 7.6 Hz, 2H), 7.03 (d, J = 8.0 Hz, 2H), 6.79 (d, J = 8.0 Hz, 2H),
13
2.41 (s, 3H); C{1H} NMR (100 MHz, DMSOꢀd6) δ 165.3, 155.5, 144.7, 143.2, 130.1, 129.9,
~
ν
126.8, 123.0, 116.0, 21.7; IR (KBr) 3331, 2919, 1721, 1709, 1601, 1507, 1395, 1286, 1181,
1114, 1080, 822, 747 cmꢀ1; HRMS observed 251.0677 (calculated for C14H12O3Na [M+Na]+
251.0679). .
4ꢀHydroxyphenylꢀ2ꢀmethylbenzoate (3d): Rf 0.20 (10% ethyl acetate/hexane); pale yellow
solid; yield 63% (90 mg, 48%); mp: 114ꢀ116 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.47 (s, 1H),
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8.02 (d, J = 8.0 Hz, 1H), 7.55 (t, J = 7.6 Hz, 1H), 7.38 (t, J = 7.6 Hz, 2H), 7.05 (d, J = 8.8 Hz,
2H), 6.80 (d, J = 8.8 Hz, 2H), 2.57 (s, 3H); 13C{1H} NMR (100 MHz, DMSOꢀd6) δ 166.2, 155.5,
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~
ν
143.1, 140.2, 133.1, 132.2, 131.0, 129.1, 126.6, 123.0, 116.0, 21.6; IR (KBr)
3436, 2962,
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2929, 1719, 1593, 1507, 1446, 1392, 1249, 1209, 1180, 1053, 882, 824, 739, 596 cmꢀ1; HRMS
observed 229.0887 (calculated for C14H13O3 [M+H]+ 229.0865).
4ꢀHydroxyphenylꢀ4ꢀbromobenzoate (3e): Rf 0.20 (12% ethyl acetate/hexane); pale yellow
1
solid; yield 60% (30 mg, 38% ); mp: 126ꢀ127 °C; H NMR (400 MHz, DMSOꢀd6) δ 9.52 (s,
1H), 8.02 (d, J = 8.4 Hz, 2H), 7.81 (d, J = 8.4 Hz, 2H), 7.06 (d, J = 8.8 Hz, 2H), 6.80 (d, J = 8.8
Hz, 2H); 13C{1H} NMR (100 MHz, DMSOꢀd6) δ 164.9, 155.8, 143.2, 132.7, 132.2, 129.0, 128.5,
~
ν
123.1, 116.2; IR (KBr) 3373, 2922, 1730, 1590, 1509, 1458, 1398, 1287, 1207, 1114, 1077,
1009, 867, 848, 814, 749 cmꢀ1; HRMS observed 292.9808 (calculated for C13H10BrO3 [M+H]+
292.9813).
4ꢀHydroxyphenylꢀ4ꢀchlorobenzoate (3f): Rf 0.20 (12% ethyl acetate/hexane); colorless solid;
yield 58% (62 mg, 35%); mp: 117ꢀ119 °C (lit.46 117 °C); 1H NMR (400 MHz, DMSOꢀd6) δ 9.51
(s, 1H), 8.10 (d, J = 8.4 Hz, 2H), 7.66 (d, J = 8.4 Hz, 2H), 7.05 (d, J = 8.8 Hz, 2H), 6.80 (d, J =
13
8.9 Hz, 2H); C{1H} NMR (100 MHz, DMSOꢀd6) δ 164.6, 155.7, 143.0, 139.2, 132.0, 129.6,
~
ν
128.5, 122.9, 116.1; IR (KBr) 3379, 2933, 1730, 1592, 1510, 1486, 1399, 1292, 1208, 1111,
1075, 917, 850, 752 cmꢀ1; HRMS observed 249.0313 (calculated for C13H10ClO3 [M+H]+
249.0319).
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4ꢀHydroxyphenylꢀ2ꢀethylbenzoate (3g): Rf 0.20 (8% ethyl acetate/hexane); off white solid;
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yield 75% (101 mg, 56%); mp: 128ꢀ130 °C; H NMR (400 MHz, DMSOꢀd6) δ 9.48 (s, 1H), 7.97
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(d, J = 7.6 Hz, 1H), 7.58 (t, J = 7.6 Hz, 1H), 7.42 ꢀ 7.36 (m, 2H), 7.05 (d, J = 8.8 Hz, 2H), 6.81
(d, J = 8.8 Hz, 2H), 2.94 (q, J = 7.6 Hz, 2H), 1.19 (t, J = 7.6 Hz, 3H); 13C{1H} NMR (100 MHz,
DMSOꢀd6) δ 166.4 , 155.6, 145.9, 143.1, 133.2, 130.9, 130.8, 129.0, 126.6, 123.0, 116.1, 27.2,
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~
ν
16.43; IR (KBr) 3466, 2960, 1705, 1598, 1520, 1446, 1397, 1254, 1180, 1066, 886, 820, 735,
587 cmꢀ1; HRMS observed 243.1016 (calculated for C15H15O3 [M+H]+ 243.1021).
4ꢀHydroxyphenylꢀ4ꢀethylbenzoate (3h): Rf 0.20 (8% ethyl acetate/hexane); off white solid;
yield 72% (97 mg, 52%); mp: 120ꢀ122 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.49 (s, 1H), 8.01
(d, J = 8.4 Hz, 2H), 7.42 (d, J = 8.4 Hz, 2H), 7.03 (d, J = 8.8 Hz, 2H), 6.80 (d, J = 8.8 Hz, 2H),
2.71 (q, J = 7.6 Hz, 2H), 1.21 (t, J = 7.6 Hz, 3H); 13C{1H} NMR (100 MHz, DMSOꢀd6) δ 165.4,
~
ν
155.6, 150.8, 143.2, 130.3, 128.8, 127.1, 123.0, 116.1, 28.7, 15.7; IR (KBr) 3448, 2971, 2932,
1710, 1599, 1508, 1443, 1397, 1280, 1212, 1175, 1112, 1085, 881, 851, 822 cmꢀ1; HRMS
observed 265.0835 (calculated for C15H14O3Na [M+Na]+ 265.0841).
4ꢀHydroxyphenylꢀ3ꢀbromoꢀ4ꢀmethoxybenzoate (3i): Rf 0.22 (20% ethyl acetate/hexane); pale
yellow solid; yield 75%, (89 mg, 59%); mp: 174ꢀ177 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.50
(s, 1H), 8.22 (d, J = 2.0 Hz, 1H), 8.10 (dd, J = 8.4, 2.0 Hz, 1H), 7.29 (d, J = 8.4 Hz, 1H), 7.04 (d,
13
J = 8.8 Hz, 2H), 6.79 (d, J = 8.8 Hz, 2H), 3.96 (s, 3H); C{1H} NMR (100 MHz, DMSOꢀd6) δ
~
ν
164.0, 160.1, 155.6, 143.1, 134.6, 131.8, 123.1, 123.0, 116.1, 113.1, 111.2, 57.3; IR (KBr)
3406, 2917, 2847, 1729, 1594, 1511, 1400, 1271, 1190, 1115, 1024, 1003, 829, 755 cmꢀ1; HRMS
observed 322.9913 (calculated for C14H12BrO4 [M+H]+ 322.9919 ).
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4ꢀHydroxyphenylꢀ2,4ꢀdimethoxyꢀ6ꢀmethylbenzoate (3j): Rf 0.24 (25% ethyl acetate/hexane);
light orange solid; yield 85% (115 mg, 72%); mp: 112ꢀ114 °C; 1H NMR (400 MHz, DMSOꢀd6) δ
9.46 (s, 1H), 6.98 (d, J = 8.8 Hz, 2H), 6.80 (d, J = 8.8 Hz, 2H), 6.52 (s,1H), 6.49 (s, 1H), 3.82 (s,
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3H), 3.80 (s, 3H), 2.31 (s, 3H); C{1H} NMR (100 MHz, DMSOꢀd6) δ 166.8, 161.8, 158.5,
~
ν
155.5, 143.1, 138.0, 122.8, 116.1, 115.6, 107.5, 96.7, 56.4, 55.8, 19.8; IR (KBr) 3440, 2942,
2837, 1728, 1604, 1503, 1466, 1443, 1335, 1294, 1260, 1204, 1043, 746 cmꢀ1; HRMS observed
289.1071 (calculated for C16H17O5 [M+H]+ 289.1076).
4ꢀHydroxyphenylꢀ3,4ꢀdimethoxybenzoate (3k): Rf 0.30 (25% ethyl acetate/hexane); light
orange solid; yield 78% (103 mg, 63%); mp: 153ꢀ155 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.46
(s, 1H), 7.74 (dd, J = 8.4, 2.0 Hz, 1H), 7.56 (d, J = 2.0 Hz, 1H), 7.13 (d, J = 8.4 Hz, 1H), 7.02 (d,
J = 8.8 Hz, 2H), 6.79 (d, J = 8.8 Hz, 2H), 3.86 (s, 3H), 3.83 (s, 3H); C{1H} NMR (100 MHz,
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DMSOꢀd6) δ 165.1, 155.5, 153.9, 149.0, 143.3, 124.3, 123.0, 121.6, 116.0, 112.5, 111.7, 56.2,
~
ν
56.0; IR (KBr) 3440, 2917, 2847, 1740, 1601, 1510, 1417, 1274, 1191, 1139, 1078, 1012, 906,
755, 526 cmꢀ1; HRMS observed 275.0914 (calculated for C15H15O5 [M+H]+ 275.0919).
4ꢀHydroxyphenylꢀ 3,4,5ꢀtrimethoxybenzoate (3l): Rf 0.30 (30% ethyl acetate/hexane); light
orange solid; yield 80% (98 mg, 64%); mp: 137ꢀ140 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.49
(s, 1H), 7.37 (s, 2H), 7.04 (d, J = 8.8 , 2H), 6.80 (d, J = 8.8, 2H), 3.86 (s, 6H), 3.76 (s, 3H);
13C{1H} NMR (100 MHz, DMSOꢀd6) δ 165.0, 155.6, 153.3, 143.2, 142.7, 124.6, 123.0, 116.1,
~
ν
107.5, 60.7, 56.5; IR (KBr)
3415, 2928, 2852, 1728, 1593, 1509, 1463, 1417, 1398, 1337,
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1215, 1192, 1112, 1025, 996, 763, 702, 618 cmꢀ1; HRMS observed 305.1020 (calculated for
C16H17O6 [M+H]+ 305.1025).
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4ꢀHydroxyphenylꢀ2,4,6ꢀtrimethylbenzoate (3m): Rf 0.20 (12% ethyl acetate/hexane); colorless
powder; yield 85% (116 mg, 67%); mp: 130ꢀ132 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.50 (s,
1H), 7.05 (d, J = 8.8 Hz, 2H), 6.98 (s, 2H), 6.82 (d, J = 8.8 Hz, 2H), 2.3 (s, 6H), 2.27 (s, 3H);
13C{1H} NMR (100 MHz, DMSOꢀd6) δ 168.6, 155.7, 142.8, 139.8, 135.1, 130.5, 128.8, 122.8,
~
ν
116.2, 21.2, 19.8; IR (KBr)
3469, 2928, 1723, 1596, 1508, 1444, 1398, 1264, 1207, 1177,
1112, 1066, 851, 809 cmꢀ1; HRMS observed 257.1192 (calculated for C16H17O3 [M+H]+
257.1178).
4ꢀHydroxyphenylꢀ4ꢀisopropylbenzoate (3n): Rf 0.22 (10% ethyl acetate/hexane); pale yellow
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solid; yield 76% (105 mg, 60%); mp: 115ꢀ119 °C; H NMR (400 MHz, DMSOꢀd6) δ 9.47 (s,
1H), 8.02 (d, J = 8.4Hz, 2H), 7.46 (d, J = 8.4 Hz, 2H), 7.03 (d, J = 8.8 Hz, 2H), 6.80 (d, J = 8.8
13
Hz, 2H), 3.00 (m,1H), 1.24 (d, J = 6.8 Hz, 6H); C{1H} NMR (100 MHz, DMSOꢀd6) δ 165.7,
~
ν
155.7, 155.6, 143.5, 130.6, 127.6, 127.4, 123.3, 116.4, 34.3, 24.2; IR (KBr) 3375, 2962, 2933,
1713, 1593, 1509, 1397, 1276, 1193, 1112, 1075, 768, 705 cmꢀ1; HRMS observed 257.1203
(calculated for C16H17O3 [M+H]+ 257.1178).
4ꢀHydroxyphenylꢀthiopheneꢀ2ꢀcarboxylate (3o): Rf 0.20 (10% ethyl acetate/hexane); brown
solid; yield 68% (86 mg, 44%); mp: 109ꢀ111 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.51 (s, 1H),
8.06 (dd, J = 5.2, 1.2 Hz, 1H), 7.98 (dd, J = 3.6, 1.2 Hz, 1H), 7.29 (dd, J = 4.4, 3.6 Hz, 1H), 7.04
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(d, J = 8.8 Hz, 2H), 6.79 (d, J = 8.8 Hz, 2H); C{1H} NMR (100 MHz, DMSOꢀd6) δ 160.9,
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155.7, 142.8, 135.4, 135.3, 132.6, 129.1, 123.0, 116.1; IR (KBr)
3448, 2923, 2852, 1691,
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1594, 1505, 1408, 1291, 1260, 1205, 1183, 1112, 1067, 994, 917, 855, 807, 728 cmꢀ1; HRMS
observed 221.0267 (calculated for C11H9O3S [M+H]+ 221.0272).
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4ꢀHydroxyphenylꢀ2ꢀnaphthoate (3p): Rf 0.22 (12% ethyl acetate/hexane); pale yellow solid;
yield 74% (98 mg, 58%); mp: 138ꢀ142 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.52 (s, 1H), 8.80
(d, J = 8.4 Hz, 1H), 8.39 (d, J = 7.2 Hz, 1H), 8.27 (d, J = 8.0 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H),
13
7.72 – 7.62 (m, 3H), 7.15 (d, J = 8.8 Hz, 2H), 6.84 (d, J = 8.8 Hz, 2H); C{1H} NMR (100
MHz, DMSOꢀd6) δ 166.2, 155.7, 143.2, 134.5, 133.9, 131.2, 131.1, 129.3, 128.6, 127.0, 126.3,
~
ν
125.5, 125.4, 123.1, 116.1; IR (KBr) 3412, 1696, 1592, 1505, 1433, 1401, 1343, 1282, 1245,
1128, 988, 882, 781 cmꢀ1; HRMS observed 265.0859 (calculated for C17H13O3 [M+H]+
265.0865).
4ꢀHydroxyphenylꢀanthraceneꢀ9ꢀcarboxylate (3q): Rf 0.30 (15% ethyl acetate/hexane); off
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white solid; yield 80% (110 mg, 71%); mp: 230ꢀ234 °C; H NMR (400 MHz, DMSOꢀd6) δ 9.62
(s, 1H), 8.87 (s, 1H), 8.23 (d, J = 8.4 Hz, 2H), 8.17 (d, J = 8.8 Hz, 2H), 7.73ꢀ7.69 (m, 2H), 7.65 –
13
7.61 (m, 2H), 7.36 (d, J = 8.8 Hz, 2H), 6.92 (d, J = 8.8 Hz, 2H); C{1H} NMR (100 MHz,
DMSOꢀd6) δ 168.3, 156.0, 143.0, 130.9, 130.3, 129.3, 128.3, 128.0, 126.9, 126.4, 124.8, 123.1,
~
ν
116.4; IR (KBr) 3500, 2934, 1730, 1597, 1507, 1447, 1401, 1270, 1197, 1171, 1115, 984, 874,
788 cmꢀ1; HRMS observed 337.0835 (calculated for C21H14O3Na [M+Na]+ 337.0841).
4ꢀHydroxyphenylꢀpyreneꢀ1ꢀcarboxylate (3r): Rf 0.30 (15% ethyl acetate/hexane); bright
yellow solid; yield 73% (80 mg, 55%); mp: 210ꢀ213 °C; 1H NMR (400 MHz, DMSOꢀd6) δ 9.54
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(s, 1H), 9.17 (d, J = 9.6 Hz, 1H), 8.84 (d, J = 8.4 Hz, 1H), 8.47ꢀ8.40(m, 5H), 8.32 (d, J = 9.6 Hz,
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1H), 8.20 (t, J = 7.6 Hz, 1H), 7.24 (d, J = 8.8 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H); C{1H} NMR
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(100 MHz, DMSOꢀd6) δ 166.7, 155.8, 143.5, 134.8, 131.07, 131.06, 130.6, 130.4, 130.3, 129.3,
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~
ν
127.7, 127.4, 127.1, 125.1, 124.6, 124.5, 123.8, 123.32 (×2), 122.8, 116.3; IR (KBr) 3432,
2927, 1683, 1586, 1500, 1440, 1324, 1226, 1174, 1129, 1076, 1035, 998, 889, 833, 709 cmꢀ1;
HRMS observed 361.0835 (calculated for C23H14O3Na [M+Na]+ 361.0841).
4ꢀHydroxyꢀ3ꢀ(4ꢀmethoxybenzoyl)phenylꢀ4ꢀmethoxybenzoate (3bʹ): Rf 0.35 (8% ethyl
acetate/hexane); pale yellow solid; yield 12 mg (<5%); mp: 188ꢀ191 °C; 1H NMR (400 MHz,
DMSOꢀd6) δ 10.20 (s, 1H), 8.06 (d, J = 8.8 Hz, 2H), 7.73 (d, J = 8.8 Hz, 2H), 7.28 (dd, J = 8.8,
2.8 Hz, 1H), 7.15 (d, J = 2.8 Hz, 1H), 7.10 (d, J = 8.8 Hz, 2H), 7.06 (d, J = 8.8 Hz, 2H), 7.00 (d,
J = 8.8 Hz, 1H), 3.86 (s, 3H), 3.84 (s, 3H); 13C{1H} NMR (100 MHz, DMSOꢀd6) δ 194.8, 165.0,
164.2, 163.8, 153.9, 143.0, 132.5, 132.4, 130.2, 126.7, 126.1, 122.9, 121.5, 117.6, 114.7, 114.4,
~
ν
56.18 , 56.13; IR (Neat) 3435, 2924, 2852, 1730, 1631, 1604, 1510, 1479, 1420, 1334, 1261,
1169, 1134, 1066, 1028, 970, 845, 785 cmꢀ1; HRMS observed 379.1184 (calculated for
C22H18O6 [M+H]+ 379.1176).
AUTHOR INFORMATION
Corresponding Author: Prasenjit Mal pmal@niser.ac.in.
Notes. The authors declare no competing financial interest.
ACKNOWLEDGEMENT
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We are thankful to DST (New Delhi, India; Grant no. INT/FINLAND/Pꢀ06 and SR/S1/ICꢀ
59/2010) for financial support. AB is thankful to CSIR (India) for fellowship.
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ASSOCIATED CONTENT
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Supporting Information. H, 13C{1H} NMR spectra of all compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
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