Mechanistic and exploratory investigations into the synthesis of 1,3,5-triaroylbenzenes from 1-aryl-2-propyn-1-ones and 1,3,5-triacetylbenzene from 4-methoxy-3-buten-2-one by cyclotrimerization in hot water in the absence of added acid or base

Article abstract of DOI:10.1021/jo301979p

Neat 1-phenyl- and 1-(p-tolyl)-2-propyn-1-ones (1 and 1′, respectively) were heated in water without any additive at 150 °C for 2 h to give 1,3,5-tribenzoyl- and 1,3,5-tri-(p-toluoyl)benzenes (2 and 2′, respectively) in 74 and 52% yields, respectively. The crossed reactions of 1 with the enolate of p-toluoylacetaldehyde (3′) and 1′ with the enolate of benzoylacetaldehyde (3) were carried out to give unsymmetrically substituted 1-toluoyl-3,5-dibenzoylbenzene (Ph₂Tol) and 1,3-ditoluoyl-5-benzoylbenzene (PhTol₂), respectively, corroborating the previously proposed reaction mechanism in which 3 and 3′ that are formed by rate-determining nucleophilic attack of HO^- on 1 and 1′ or its conjugate acids formed by subsequent protonation would serve as a common intermediate for the formation of 2, 2′ and the acetophenone derivatives as byproducts. When 4-methoxy-3-buten-2-one (4) was heated in hot pure water without any additive at 150 °C for 30 min, 1,3,5-triacetylbenzene (5) was obtained in an isolated yield of 77% just by removing water by filtering the crystalline product from the cooled reaction mixture. The reaction did not take place in the absence of water. Slow decompositions of 5 in water set in at the temperature of 300 °C for 30 min.

Full text of DOI:10.1021/jo301979p

Article
pubs.acs.org/joc
Mechanistic and Exploratory Investigations into the Synthesis of
1,3,5-Triaroylbenzenes from 1‑Aryl-2-propyn-1-ones and 1,3,5-
Triacetylbenzene from 4‑Methoxy-3-buten-2-one by
Cyclotrimerization in Hot Water in the Absence of Added Acid or
Base
Tatsuya Iwado,† Keiya Hasegawa,† Toshiyuki Sato,† Masaki Okada,† Kiwamu Sue,‡ Hiizu Iwamura,*,§
and Toshihiko Hiaki*,†
†College of Industrial Technology, Nihon University, 1-2-1 Izumi-cho, Narashino, Chiba 275-8575, Japan
‡Nanosystem Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi
Tsukuba, Ibaraki 305-8565, Japan
§College of Science and Technology, Nihon University, 1-8-14 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8308, Japan
S
* Supporting Information
ABSTRACT: Neat 1-phenyl- and 1-(p-tolyl)-2-propyn-1-ones
(1 and 1′, respectively) were heated in water without any
additive at 150 °C for 2 h to give 1,3,5-tribenzoyl- and 1,3,5-tri-
(p-toluoyl)benzenes (2 and 2′, respectively) in 74 and 52%
yields, respectively. The crossed reactions of 1 with the enolate
of p-toluoylacetaldehyde (3′) and 1′ with the enolate of
benzoylacetaldehyde (3) were carried out to give unsymmetri-
cally substituted 1-toluoyl-3,5-dibenzoylbenzene (Ph2Tol) and
1,3-ditoluoyl-5-benzoylbenzene (PhTol2), respectively, corrob-
orating the previously proposed reaction mechanism in which 3
and 3′ that are formed by rate-determining nucleophilic attack
of HO− on 1 and 1′ or its conjugate acids formed by
subsequent protonation would serve as a common intermediate
for the formation of 2, 2′ and the acetophenone derivatives as
byproducts. When 4-methoxy-3-buten-2-one (4) was heated in hot pure water without any additive at 150 °C for 30 min, 1,3,5-
triacetylbenzene (5) was obtained in an isolated yield of 77% just by removing water by filtering the crystalline product from the
cooled reaction mixture. The reaction did not take place in the absence of water. Slow decompositions of 5 in water set in at the
temperature of 300 °C for 30 min.
contrast to the various nickel-, cobalt- and other transition-
metal-catalyzed cyclotrimerizations of monosubstituted acety-
lenes in which 1,2,4-trisubstituted benzenes are the main
products, with lower amounts of the 1,3,5-isomers also
generally obtained and little, if any, of the 1,2,3-isomers.14−22
Recently, we reported that 1-phenyl-2-propyn-1-one (1)
produces 1,3,5-tribenzoylbenzene (2) in the absence of any
added catalyst in one pot in 65% yields at 200 °C for 7 min or
74% at 150 °C for 60 min in subcritical water (Scheme 1).23
Here again, the strict 1,3,5-trisubstitution regioselectivity was
noted. When the pH of the aqueous media was raised by
addition of sodium hydroxide at the start of the reaction, the
initial rates of the reaction were enhanced. At higher
INTRODUCTION
■
1,3,5-Triaroylbenzenes serve as molecular hosts in solution as
well as in the solid phase.1−4 They are also used as key
intermediates for the synthesis of dendrimers of various
functions, including superparamagnetic polycarbenes5−7 and
superelectrophilic 1,3,3-trisubstituted 2-oxindoles.8 Hyper-
branched polymers with a high degree of branching, such as
poly(aroxycarbonylphenylene)s and ferrocene-containing poly-
(aroylarylene)s, have been prepared.9,10 The physicochemical
properties of the disklike molecules of medium size thus
warrant further synthetic investigations. In most of these cases,
the 1,3,5-triaroylbenzene skeletons are constructed by base-
catalyzed Michael-type cyclotrimerization of the corresponding
1-aryl-2-propyn-1-ones in the presence of organic base such as
diethylamine and pyridine in organic solvents like DMF,
dichloromethane, toluene, etc.5,11−13 Strict 1,3,5-regioselectivity
maintained in these ionic addition reactions is in strong
Special Issue: Howard Zimmerman Memorial Issue
Received: September 11, 2012
© XXXX American Chemical Society
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Scheme 1. X = NH(C2H5)2 in the Previous Work5,11−13 and
OH− Due to Pressurized Hot Water in This Work23
documented in 1947 in Organic Syntheses, a publication of
reliable synthetic procedures for fundamental organic com-
pounds.38 More recently, another procedure treating 4-
methoxy-3-buten-2-one (4) in aqueous ethanol with acetic
acid as a catalyst giving 5 in 61−64% yield at 77 °C for 48 h was
described again in Organic Syntheses.39 Both the procedures use
organic solvents and acetic acid as a Brønsted acid catalyst. We
have examined the latter reaction of 4 in hot water in the
absence of any additive.
RESULTS AND DISCUSSION
■
Scheme 2. Similarity between the Two Reactions
Reactions of the Sodium Salt of the Enol of p-
Toluoylacetaldehyde (3′) with 1-Phenyl-2-propyn-1-one
(1) and the Enol of Benzoylacetaldehyde (3) with 1-(p-
Tolyl)-2-propyn-1-one (1′) in Hot Water in the Absence
of Any Additive. The reaction mechanism proposed in the
previous paper5−7,23 and summarized in Scheme 2 is elaborated
in Scheme 3. According to this mechanism, the enolate of 3
Scheme 3. Reaction Mechanism for the Formation of 1,3,5-
Triaroylbenzenes
temperatures, the yield of acetophenone increased at the
expense of 2 (Scheme 2).
The dielectric constant (εr) of water decreases from 78.4 at
25 °C to ca. 35 (cf. εr = 33 of methanol at room
temperature)24−26 in pressurized hot water at 200 °C. The
ion product of water (pKW = 14) at 25 °C increases almost 3
orders of magnitude to pKW = 11.2 at 200 °C.27,28 These two
factors were concluded to be responsible for the success of the
three consecutive Michael cycloaddition reactions in the
absence of added acid or base.23 The reaction rates increased
with temperature but the yield of 2 decreased at the expense of
the formation of acetophenone as a side product at higher
temperatures. The enol of benzoylacetaldehyde (3) that was
formed by rate-determining nucleophilic attack of HO− on 1
followed by protonation was postulated as an intermediate.23 In
view of these reaction conditions using neither organic solvents
nor added acid or base and therefore consistent with the
concept of green chemistry,29 it seemed worthwhile to study
this mechanism further by preparing the sodium salt of 3 which
would then be allowed to react with 1-phenyl-2-propyn-1-one
(1). In order to determine how many benzoyl groups in 2 are
from 1 and 3 in these test reactions, it seemed appropriate to
label either the benzyol group in 1 or 3 by using the p-methyl
substituent.
Second, 4-methoxy-3-buten-2-one (4) is equivalent to the
methyl ether of the acetyl counterpart of 3. We have been
encouraged to examine whether 4 might be able to produce
1,3,5-triacetylbenzene (5) under similar conditions of pressur-
ized hot water in the absence of any added catalyst (Scheme 2).
In view of the usefulness of 5 for the synthesis of cyanine
dyes,30 functional dendrimers,31,32 phenolic molecular glasses.33
cyclophanes,34 and several ligands for transition metals,35−37 its
synthesis by condensation of the sodium salt of acetoacetalde-
hyde in the presence of acetic acid in 30−38% yield was
should react with 1-phenyl-2-propyn-1-one (1). In order to
determine how many benzoyl groups in 2 are from 1 and 3 in
these test reactions, the aroyl groups in 1 or 3 were labeled by
using the p-methyl substituent. The results are given in Tables
1and 2. See Tables S1 and S2 in the Supporting Information for
the raw data.
Runs 1 and 6 are nothing but the reactions of neat 1-phenyl-
2-propyn-1-one (1) and 1-(p-tolyl)-2-propyn-1-one (1′) giving
1,3,5-tribenzoyl- and tri(p-touoyl)benzenes (2 and 2′),
respectively, serving as references. The two results are almost
comparable. In support of the proposed reaction mechanism,
Ph2Tol was obtained in runs 2−4 in which the highest yield was
run 2. The stoichiometric amount appears not to be enough
and the presence of higher concentration of 1 is favorable for
the nucleophilic addition of 3′ leading to the formation of 1-
toluoyl-3,5-dibenzoylbenzene (Ph2Tol). A similar relation
seems to hold for the formation of 1,3-ditoluoyl-5-benzoylben-
zene (PhTol2) from 1′ and 3 in run 7.
A larger proportion of the enolates leads to the formation of
the acetophenone derivatives. Runs 5 and 10 are such extremes,
showing that the enolate salts of the aroylacetaldehyde cannot
cyclotrimerize to form 1,3,5-triaroylbenzenes.
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Table 1. Production of 1,3,5-Triaroylbenzenes and Acetophenones from the Reaction of 1-Benzoyl-2-propyn-1-one (1) with the
Sodium Salt of p-Toluoylacetaldehyde (3′) at 150 °C for 2 h
a
chemical yield of the products (mol percentage)
molar ratio of the reactants
1,3,5-triaroylbenzenes
acetophenones
run
1:3′
Ph3 (2)
Ph2Tol
PhTol2
Tol3 (2′)
PhCOCH3
TolCOCH3
1
2
3
4
5
1.0:0.0
3.72:1.0
2.05:1.0
0.233:1.0
0.0:1.0
74
27
25
4
0
0
0
0
0
0
0
13
10
15
41
0
0
57.0
26.8
7.3
0
0
11.8
14
0
1.6
0
65
0
69
a
Based on the moles of 3′ in the reactant mixture except for Ph3 and PhCOCH3 for which the yields are based on moles of 1.
Table 2. Production of 1,3,5-Triaroylbenzenes and Acetophenones from the Reaction of 1-(p-Tolyl)-2-propyn-1-one (1′) with
the Sodium Salt of Benzoylacetaldehyde (3) at 150 °C for 2 h
a
chemical yield of the products (mol percentage)
molar ratio of the reactants
1,3,5-triaroylbenzenes
acetophenones
run
1′:3
Ph3 (2)
Ph2Tol
PhTol2
Tol3 (2′)
PhCOCH3
TolCOCH3
6
7
1.0:0
3.5:1.0
2.05:1.0
0.25:1.0
0:1.0
0
0
0
0
0
0
0
52
34
25
9
0
17
8
0
32.5
22.3
8.4
0
6.6
8
0
18
10
26
0
9
4.5
0
34
57
10
0
a
Based on the moles of 3 in the reactant mixture except for Tol3 and TolCOCH3 for which the yields are based on moles of 1′.
Scheme 4. Exchange Reaction of the Aryl Groups
Table 3. Product Distributions of the Reaction of an Equal Molar Mixture of 1-Phenyl-2-propyn-1-one (1) and 1-(p-Tolyl)-2-
propyn-1-one (1′) at 150 °C for 2 h
chemical yield of the products (μmol)
amt of starting materials (μmol)
1,3,5-triaroylbenzenes
acetophenones
run
11
1
1′
Ph3(2)
Ph2Tol
12.4
PhTol2
13.5
Tol3(2′)
PhCOCH3
4.7
TolCOCH3
6.7
106
106
7.05
5.45
The formation of PhTol2 in 1.6% yield in run 4 and Ph2Tol
in 4.5% yield in run 9 are observations that can immediately
have no serious effect on the mechanism of the whole reactions,
but they suggest necessarily the equilibration in Scheme 4.
The predominant structures with respect to the enol, enolate,
and cis- and trans-forms of aroylacetaldehydes are discussed by
means of their NMR spectra in various solvents. Most of them
are in equilibirium with the enthalpy difference of less than a
few kJ/mol and rapidly interconverting.40 According to the
Curtin−Hammett principle, the most populating species does
not necessarily lead to the main product. Therefore, the
presentation in this paper does not necessarily conform to the
most predominant structure of aroylacetaldehydes and their
derivatives.
Crossed Cyclotrimerization of 1-Phenyl-2-propyn-1-
one (1) and 1-(p-Tolyl)-2-propyn-1-one (1′) in Hot Water
in the Absence of Any Additive. When an equal molar
mixture of 1-phenyl-2-propyn-1-one (1) and 1-(p-tolyl)-2-
propyn-1-one (1′) was heated in hot water in the absence of
any additive, the products were obtained as shown in Table 3.
All of the possible combinations were produced, and there
appears to be no selectivity. The molar ratio of the 1,3,5-
triaroylbenzenes are distributed slightly in the direction toward
the binomial coefficients of 1:3:3:1 for statistical combination.
Whereas the reason for the observed trend in Table 3 is not
clear, the results of the cross-reactions obtained in Tables 1 and
2 will open the door for the synthesis of the 1,3,5-
triaroylbenzenes having the unsymmetrical substitution pattern.
Cyclotrimerization of 4-Methoxy-3-buten-2-one (4)
To Give 1,3,5-Triacetylbenzene (5) in Hot Water in the
Absence of Any Additive. The results are listed in Table S3
(Supporting Information), and typical results are shown in
Figure 1. The highest yield obtained by these methods was 88%
in 30 min at 150 °C and 90 min at 110 °C, with values
systematically higher than the isolated yield (see Table S4,
Supporting Information). The variance of the yield data in
these studies were dominated by the uncertainty of the manual
processes and was estimated to be 5%. The reaction product 5
appeared to decompose slightly in water at 150−300 °C. For
example, the recovered yield of 5 at 300 °C in water decreased
from 80.4 to 79.3 and 72.2% for 30, 50, and 100 min,
respectively. In the absence of water at 150 and 200 °C for 30
min, starting material 4 was recovered unchanged with a small
amount of 4,4-dimethoxy-2-butanone detected.
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78.4 at 25 °C to ca. 35 in the pressurized hot water at 200 °C
(cf. εr = 33 of methanol at room temperature).7,8
(2) The ionic dissociation of water is endothermic and
therefore increases with temperature according to Le
Chatelier’s principle. However, as the dielectric constant
decreases with temperature as described in (1), a maximum
value appears at 250−265 °C under the conditions of saturated
vapor pressure .9,10 Its value of pKW = 11.2 is almost 3 orders of
magnitude greater than pKW = 14 at 25 °C.
(3) The solubility of organic compounds in water increases
with temperature. The trend is pronounced for those carrying
hydrophilic substituent such as OH and >CO.
(4) In 1980, Breslow and co-workers reported that the
Diels−Alder reactions of cyclopentadiene with butenone
proceeded 730 times faster in water than in isooctane and
ascribed the phenomenon to the hydrophobic interaction in
water.42,43 The aggregation energy density of organic molecules
in water is due to the hydrogen-bonded structure of water and
decreases with increasing temperature. It disappears at critical
point of water and therefore can be disregarded for the
reactions at higher temperature.
(5) The rate enhancement of organic reactions in pressurized
hot water is interpreted to some extent in terms of the elevated
temperature. According to the Arrhenius equation, a given
reaction with the activation energy of ca. 100 kJ/mol is
accelerated about thousand times from 100 to 200 °C.
Whereas the size of the reaction was limited by the reaction
vessels used (Figure S1, Supporting Information), it could
readily be expanded by employing larger size reactors. The
reaction environment is at subcritical temperature and the
inside pressure of the batch reactor is estimated to be ca. 1.6
MPa at 200 °C. It must not be difficult to find a commercially
available pressure bottle (an autoclave) of the inner volume of
l00−l000 mL. Furthermore, it has been a definite improvement
in flow reactors of recent date using water under sub- and
supercritical conditions in some laboratories.
○
Figure 1. Temporal variation of the yield of product 5 at 110 °C ( )
and 150 °C( ).
●
It is suggested from all of the above results that a series of
cationic aldol condensations are operating. In the enol formed
by protonation at the carbonyl oxygen, the positive charge on
carbon 4 is stabilized by the electron-donating methoxy group
and serve as an electrophile for the second molecule of 4.
Methanol may be eliminated here or later. The older Organic
Syntheses method employs the heating of sodium salt of
acetoacetaldehyde in the presence of acetic acid at 50 °C giving
5 in 30−38% yield.38 Pivaloylacetaldehyde gave 1,3,5-
tripivaloylbenzene in 30% yield when warmed in acetic
acid.41 Therefore, the mechanism depicted in Scheme 5 is
tentatively proposed; the elimination of methanol may take
place at any stage.
Scheme 5. Tentative Mechanism for the Formation of 1,3,5-
Triacetylbenzene (5) from 4-Methoxy-3-buten-2-one (4)
CONCLUSIONS
■
When heated alone in water without any additive at 150 °C for
2 h, 1-phenyl- and 1-(p-tolyl)-2-propyn-1-ones (1 and 1′,
respectively) gave 1,3,5-tribenzoyl- and 1,3,5-tri(p-toluoyl)-
benzenes (2 and 2′, respectively) in 74 and 52% yields,
respectively. The cross-reactions of 1 with the enolate of p-
toluoylacetaldehyde (3′) on the one hand and 1′ with the
enolate of benzoylacetaldehyde (3) on the other gave 1,3,5-
triaroylbenzenes, Ph2Tol and PhTol2, that lack a trigonally
symmetric substitution pattern. These results not only
corroborate the previously proposed reaction mechanism in
which the enolates formed by rate-determining nucleophilic
attack of HO− on 1 and 1′ or its conjugate acids formed by
subsequent protonation would serve as a common intermediate
for the formation of 2, 2′ and the acetophenone derivatives, but
also opened the possibility of new logical method for the
production of unsymmetrically substituted 1,3,5-triaroylben-
zenes. Similarly, 4-methoxy-3-buten-2-one (4) gave 1,3,5-
triacetylbenzene (5) in hot water in the absence of added
acid or base. Whereas the reaction mechanism remains
speculative, a considerably higher isolated yield of 5 in pure
water than that of the procedure described in Organic
Syntheses39 appears simply sufficient to inclusion of our finding
here. All of the reactions described in water without any added
acid or base in this paper are ascribed to the reduced dielectric
constant of water at higher temperatures and the increased ion
In general, we have to take the following factors into account
for the organic reactions in water at high temperature under
high pressure:
(1) The density of water decreases with temperature and
reaches to 0.32 g/cm3 at the critical point of 374.15 °C and
22.12 MPa. The hydrogen bonding among the water molecules
that is responsible for the polarization of water is disrupted, and
as a result, the dielectric constant (εr) of water decreases from
D
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introduced into the reaction vessel, and the contents were purged with
a stream of argon. The capped vessel was immersed in an oil bath at
150 °C. After 30 min, heating was discontinued by removing the
reactor from the heat bath and immersing it promptly in cold water.
The reactor heat-up time was on the order of 30 s, which was generally
short compared with the reaction times employed. After cooling, the
crystalline solid product was flushed out of the reaction vessel with
water and collected on a membrane filter as pale yellow needles
(0.0567 g, mp 164−165 °C).35 The reactions at elevated temperature
in water followed by crystallization during the cooling process may
have had an effect similar to recrystallization from hot water.
Additionally, the effects of reaction temperatures and duration of
the reactions were studied in the ranges 100−400 °C and 10−120 min.
A metal salt bath preheated at 200−400 °C was used when needed. In
these studies, the reaction products were extracted with dichloro-
methane, and an aliquot (1 μL) was injected into a column of a gas
chromatograph−mass spectrometer and developed with He gas with a
flow rate of 0.55 mL/min. The amounts of 5 produced were
determined accurately by reference to added 1-pentanol as an internal
standard and on the basis of calibration curves obtained independently
for the pure samples.
product of water and conform to the 12 criteria of green
chemistry in a number of points.29
EXPERIMENTAL SECTION
■
Starting Materials. Preparation of 1-aryl-2-propyn-1-ones is
described in the previous paper.23 Sodium enolate of benzoylacetalde-
hyde (3) was prepared by the condensation of acetophenone and ethyl
formate by the action of sodium methoxide in dry diethyl ether
according to the literature.44,45 The sodium enolate of p-
toluoylacetaldehyde (3′) was prepared similarly.
Commercially available 4-methoxy-3-buten-2-one (4) of chemical
purity greater than 90% was used without further purification.
Ultrapure water with a specific resistivity greater than 18.2 MΩ and
TOC less than 20 ppb was obtained by passing tap water through an
ultrapure water purification system. Other chemicals and solvents
when necessary were commercially available and used as received.
General Experimental Methods. The reactions were performed
in batch reactors (Figure S1, Supporting Information) fabricated out of
a 1/2-in. tube (diameter of 12.7 mm, wall thickness of 2.1 mm) fitted
with two 1/2-in. caps both of grade 316 stainless steel. The effective
inner volumes of ca. 2.1 and 10 cm3 were used for the preparation of
triaroylbenzenes and 1,3,5-triacetylbenzene, respectively. Prior to its
use in experiments, the reactor was loaded with 2% aqueous hydrogen
peroxide and conditioned for 1 h at 370 °C to remove any lubricants/
oils that might remain from the manufacture of the Swagelok parts and
possible catalytic metal sites on the inner-wall, unless otherwise stated.
The reactor was cleaned with acetone and dried prior to repeated safe
use of ca. 5 times. The internal pressure was subjected to the gas−
liquid equilibrium of water and estimated to be 0.48 MPa at 150 °C.
Preparation of 1,3,5-Triaroylbenzene. The crossed reactions: 1-
phenyl-2-propyn-1-one (1) with sodium enolate of p-toluoylacetalde-
hyde (3′) and 1-(p-tolyl)-2-propyn-1-one (1′) with sodium enolate of
benzoylacetaldehyde (3) were mixed in pure water in molar ratios of
0:1:400−4:1:400. After the mixture was purged with a stream of argon,
the sealed reactor was immersed in an oil bath preheated at 150 °C for
120 min. The reactor was cooled by immersing it in cold water to stop
the reaction. The cap was carefully opened, and the contents of the
reactor were washed with dichloromethane. The organic layer was
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details concerning raw yield data; NMR and MS
spectra of new compounds Ph2Tol and PhTol2. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: iwamura.hiizu@nihon-u.ac.jp, hiaki.toshihiko@nihon-
u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
analyzed by means of GC/MS and H NMR. Quantitative analyses
We thank Prof. Soichiro Watanabe of Toho University for his
permission and advice on the use and separation of the reaction
mixture using preparative gel permeation chromatography. This
research was supported in part by a Grant from the Ministry of
Education, Culture, Sports, Science and Technology to
promote multidisciplinary research projects.
were performed by 1H NMR with 1,3,5-triacetylbenzene as an internal
standard. Separation of the reaction mixtures was performed by
preparative gel permeation chromatography on a couple of columns of
exclusion size limits of MW 1000 and 5000 (in reference to the
molecular weight of the standard polystyrenes), in series using CHCl3
as an elution solvent. Both had a column volume of i.d. 20 mm × 600
mm. After removal of the acetophenone fractions, the triaroylbenzene
fractions were subjected to repeated recycling of ca. 35 times until
effective separation was obtained.
DEDICATION
■
Dedicated to the memory of Professor Howard E. Zimmerman.
1,3-Dibenzoyl-5-(p-toluoyl)benzene (Ph2Tol). NMR (400
MHz, CDCl3): δ 8.38 (t, J = 1.2 Hz, 1H), 8.37 (d, J = 1.2 Hz, 2H),
7.84(d, J = 7.7 Hz, 4H), 7.76 (d, J = 8.0 Hz, 2H), 7.64 (t, J = 7.6 Hz,
2H), 7.52 (t, J = 7.7 Hz, 4H), 7.31 (d, J = 8.0 Hz, 2H), 2.45 (s, 3H)
(see Figure S3a, Supporting Information, for a chart). MS: 404 (parent
peak), 119 (base peak), 105, 91, 77 (see Figure S2, Supporting
Information).
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■
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MHz, CDCl3): δ 8.38 (d, J = 1.2 Hz, 2H), 8.37 (t, J = 1.2 Hz, 1H),
7.84(d, J = 7.7 Hz, 2H), 7.75 (d, J = 8.0 Hz, 4H), 7.64 (t, J = 7.6 Hz,
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