Investigations on the Photochemical Reaction Mechanisms of Selected Dibenzoylmethane Compounds

Article abstract of DOI:10.1021/acs.joc.1c00647

In this work, combined time-resolved spectroscopies of femtosecond transient absorption, nanosecond transient absorption, and DFT calculations were performed to unravel the photocyclization reaction mechanisms of selected dibenzoylmethane (DBM) derivatives, including 2-chloro-1,3-diphenylpropan-1,3-dione (1a), 2-chloro-1-(3,5-dimethoxyphenyl)-3-phenylpropan-1,3-dione (1b), 2-chloro-2-fluoro-1,3-diphenylpropan-1,3-dione (1c), and 2-chloro-2-fluoro-1,3-di(4-methoxyphenyl)propan-1,3-dione (1d). Photocyclization reaction mechanisms for 1a and 1b are similar, where a C-Cl heterolysis occurs yielding an α-ketocation intermediate, followed by cyclization to generate the cation species. On the other hand, 1c and 1d undergo dechlorination primarily producing a radical species, which further experiences cyclization yielding cyclized radical species. The dominant factor leading to the different reaction mechanisms is the involvement of a fluorine atom bonded at α-C. Due to the meta-effect, the p-methoxy substitution on the benzene ring inhibits the photocyclization reaction and reduces the yield of photocyclization.

Full text of DOI:10.1021/acs.joc.1c00647

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Investigations on the Photochemical Reaction Mechanisms of
Selected Dibenzoylmethane Compounds
Junxiao Wang,† Yan Guo,† Jialin Wang, and Jiani Ma*
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ABSTRACT: In this work, combined time-resolved spectros-
copies of femtosecond transient absorption, nanosecond transient
absorption, and DFT calculations were performed to unravel the
photocyclization reaction mechanisms of selected dibenzoyl-
methane (DBM) derivatives, including 2-chloro-1,3-diphenylpro-
pan-1,3-dione (1a), 2-chloro-1-(3,5-dimethoxyphenyl)-3-phenyl-
propan-1,3-dione (1b), 2-chloro-2-fluoro-1,3-diphenylpropan-1,3-
dione (1c), and 2-chloro-2-fluoro-1,3-di(4-methoxyphenyl)propan-
1,3-dione (1d). Photocyclization reaction mechanisms for 1a and
1b are similar, where a C−Cl heterolysis occurs yielding an α-
ketocation intermediate, followed by cyclization to generate the
cation species. On the other hand, 1c and 1d undergo
dechlorination primarily producing a radical species, which further experiences cyclization yielding cyclized radical species. The
dominant factor leading to the different reaction mechanisms is the involvement of a fluorine atom bonded at α-C. Due to the meta-
effect, the p-methoxy substitution on the benzene ring inhibits the photocyclization reaction and reduces the yield of
photocyclization.
α-ketocation intermediate, followed by a cyclization process to
INTRODUCTION
■
generate a cation species (Scheme 2).11
1,3-Dibenzoylmethane (DBM) has attracted considerable
attention for its applications in a variety of fields, especially
in biomedicine and display devices.1,2 As a representative of
dicarbonyl compounds, DBM exists as ketone and enol
tautomers.3,4 Considerable attention has been focused on the
equilibrium populations of each tautomer in various solvents.
The keto−enol tautomer ratio depends on the nature of the
substituents, solvent, temperature, and presence of proton
acceptors and/or donors.5−7
Several investigations have been carried out to understand
the photophysical and photochemical properties of DBM
derivatives. For instance, irradiation of 2,2-dimethyl-1,3-
diphenylpropan-1,3-dione in the presence of triethylamine
led to the formation of anti-2,2-dimethyl-1,3-diphenylcyclo-
propan-1,3-diol as the main product.8 Irradiation of 1-(o-
methylphenyl)-2,2-dimethyl-1,3-diketone in hexane generates
benzocyclobutenol.9 Wirz and co-workers investigated the
primary photochemical reactions of acetate and fluoride
substituted dimethoxybenzoin derivatives using sub-pico-
second laser flash photolysis. Dimethoxybenzoin generates a
preoxetane biradical intermediate through the cyclization
process, followed by a heterolytic dissociation of the leaving
groups with generation of the cation species (Scheme 1).10
Pirrung and co-workers designed dimethoxybenzoin esters as a
photolabile protecting group, which undergoes photoheter-
olytic cleavage reaction from its singlet excited state to form an
̌
Recently, Sket and co-workers synthesized several DBM
derivatives with halogen substitutions and performed steady-
state measurements. These halogen substitutions of DBM
derivatives blocked the keto−enol transformation, allowing
study on the behavior of the pure diketo tautomer.12 It was
demonstrated that the substituted DBM can undergo a new
photocyclization reaction to generate flavono derivatives
(Scheme 3), and the products strongly depend on the type
of halogen substitutions banded at C2 and the position of
electron donor groups in the phenyl rings. From the final
product analysis, the photocyclization reaction via singlet state
was the dominant reaction pathway for 2-chloro-1,3-
diphenylpropan-1,3-dione (1a), 2-chloro-1-(3,5-dimethoxy-
phenyl)-3-phenylpropan-1,3-dione (1b), and 2-chloro-2-fluo-
ro-1,3-diphenyl propan-1,3-dione (1c). On the other hand, the
p-methoxy substituted DBM derivative 2-chloro-2-fluoro-1,3-
di(4-methoxyphenyl)propan-1,3-dione (1d) undergoes the
photocyclization via singlet state, as well as the α-cleavage
Received: March 18, 2021
Published: May 20, 2021
https://doi.org/10.1021/acs.joc.1c00647
© 2021 American Chemical Society
J. Org. Chem. 2021, 86, 7594−7602
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a
Scheme 1. Reaction Pathways of Dimethoxybenzoin (X = OAc, F) in ACN
a
Adapted with permission from ref 10. Copyright 2007 American Chemical Society.
a
Scheme 2. Reaction Pathways of Dimethoxybenzoin Ester (R = SO2CH3) in DMSO
a
Adapted with permission from ref 11. Copyright 2006 Wiley.
a
Scheme 3. Photocyclization Reactions of DBM Derivatives
a
Adapted with permission from ref 12. Copyright 2007 American
Chemical Society.
reaction via triplet state. Although the photochemical products
have been clarified, the reaction mechanisms are still unclear.
First, what is the precursor of the photocyclization reaction?
Second, does the presence of a fluorine atom affect the
photocyclization reaction mechanism? Third, how does the
Figure 1. fs-TA spectra of 1a in ACN.
substitution position of methoxy affect the photochemical
reactions, as the α-cleavage reaction occurs for 1d while it is
not seen for 1b? The cyclization mechanism of DBM
derivatives under irradiation has provided insight for organic
synthesis of cyclic compounds. Systematic research on the
photocyclization reaction mechanism of DBM derivatives can
provide theoretical guidance for the design of DBM derivatives
with higher photocyclization yields and further broaden the
applications of DBM derivatives in the fields of photo-
chemistry, organic synthesis, and medicine.
To unravel the photochemical reaction mechanisms, femto-
second transient absorption (fs-TA) and nanosecond transient
absorption (ns-TA) spectroscopies were performed to detect
the electronic absorption spectra of intermediates and transient
species produced in the photocyclization reactions of 1a, 1b,
1c, and 1d. Density functional theory (DFT) calculations were
utilized to allocate the detected intermediates to further
understand the photocyclization mechanisms.
TA spectra obtained for 1a in neat ACN. Upon irradiation, the
initial change from 340 nm observed within 784 fs was
attributed to an internal conversion (IC) from Sn to S1
(denoted as (1a)1). Later, (1a)1 shifted to around 360 nm
(Figure 1b), accompanied by an isosbestic point at 350 nm. As
the delay time increased from 4.1 to 17.6 ps (Figure 1c), the
absorbance at 360 nm decayed with a simultaneous growth of a
new band with λmax at 440 nm. An isosbestic point was
observed at about 400 nm, suggesting that the former species is
the precursor to the latter. Subsequently, both bands gradually
decreased and a new signal emerged at around 340 nm (Figure
1d). The spectra profile of the newly generated signal
resembled the ns-TA spectra of 1a recorded under parallel
experimental conditions (Figure S9).
Since it has been reported that the yield of the photo-
cyclization reaction of 1a was not remarkably affected by a
radical inhibitor, the pathways involving radical intermediates
such as C−Cl homolysis13 or attack of the carbonyl oxygen on
the phenyl ring10 were ruled out. It is reasonable to propose
that the photocyclization of 1a undergoes C−Cl heterolysis
following the similar reaction pathway of dimethoxybenzoin, as
indicated in Scheme 2.11 That is, (1a)1 directly cleaves the C−
RESULTS AND DISCUSSION
■
Time-Resolved Spectroscopy Studies of 1a and 1b.
To help clarify the photocyclization mechanism of 1a, time-
resolved transient absorption spectroscopy experiments were
examined in selected solvent systems. Figure 1 displays the fs-
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Cl bond with formation of an α-ketocation intermediate
C−Cl from (1a)1, and the time constant of 13 ps was likely
associated with 1a-C+ formed by 1a-C+ through the
(denoted as 1a-C+ ). Then, the cyclic cation (denoted as 1a-
1
2
1
C+ ) formed from 1a-C+ via a cyclization process.
cyclization process. Furthermore, the kinetics of the absorption
band at 440 nm was fit with a growth time constant of 13 ps
and a decay time constant of 66 ps. The rise time constant is
2
1
Accordingly, the electronic absorption spectra of 1a-C+ and
1
1a-C+2 were simulated and exhibited reasonable similarity with
the corresponding experimental spectra showing at 360 and
440 nm in fs-TA spectra, respectively (Figure S10), which
validated the proposed reaction mechanism. Since the cyclic
cation can be quenched by water,10 fs-TA spectra were also
recorded for 1a in ACN−H2O (Figure S11). As expected, the
intensity of 440 nm decreased in ACN−H2O compared to that
in ACN, which further supported the assignment of the signal
associated with the formation of product 1a-C+ . Since 2a is
2
the main photocyclization product of 1a in ACN, the time
constant of 66 ps can be attributed to the decay of 1a-C+
2
resulting in 2a.
Based on the above discussions, the photocyclization
reaction mechanism of 1a in ACN is proposed in Scheme 4.
First, 1a generates an α-keto cation intermediate 1a-C+ with
1
at 440 nm as 1a-C+ . The energy diagram for the photo-
intense absorbance at 360 nm through the C−Cl heterolysis.
2
cyclization pathway of 1a was performed by DFT calculations
(Figure S12) and further verified the rationality of the
photocyclization reaction mechanism of 1a in ACN. The
electronic transition from 1a to (1a)1 was induced upon
Then, the 1a-C+ yields the cation species 1a-C+ though a
1
2
cyclization process. The 1a-C+ with typical signal at 440 nm
2
further undergoes a deprotonation, forming the product 2a.
To investigate the effect of m-methoxy on the photo-
cyclization reaction mechanism of DBM derivatives, fs-TA of
1b in ACN were tested (Figure 3). The species with
irradiation. (1a)1 yields 1a-C+ by an exothermic process
1
(12.41 kcal/mol). Then, 1a-C+ undergoes an exothermic
1
process yielding 1a-C+ (47.33 kcal/mol), followed by an
2
exothermic deprotonation process (32.36 kcal/mol) forming
2a. The simulated absorption spectrum of the triplet state 1a
(denoted as (1a)3) shows a strong oscillator strength at 340
nm (Figure S13). Therefore, the transient species showing at
later delay times in fs-TA at 340 nm was assumed to be (1a)3.
The temporal dependence of the transient absorption
intensity changes for 1a at 365 and 440 nm was monitored
in ACN (Figure 2). The kinetic fitting of the absorption
Figure 3. fs-TA spectra of 1b in ACN.
absorption at 375 and 510 nm immediately observed after
photoexcitation was attributed to the singlet excited state of 1b
(denoted as (1b)1). Having an extra band around 510 nm, the
spectrum of (1b)1 was somewhat different from (1a)1. This
could be due to the methoxy substituents at the meta positions
of 1b, as the singlet excited state of 3′,5′-dimethoxybenzoin
diethyl phosphate (DMBDP) also exhibits a 450 nm bond
besides the 335 nm band in fs-TA spectra.14 Then, a new
species at 400 and 450 nm appeared (Figure 3c).
Subsequently, 400 and 450 nm gradually decreased and
Figure 2. Shown are the kinetics and the fitting plots at 365 and 440
nm for 1a in ACN.
changes at 365 nm by a biexponential function gave a growth
time constant of τ1 = 5.7 ps and a decay time constant of τ2 =
13 ps. The rise time constant τ1 can be assigned to the
formation of 1a-C+ during the heterolytic dissociation of the
1
Scheme 4. Proposed Photocyclization Reaction Mechanism of 1a in ACN
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another new signal emerged at around 360 nm, accompanied
by an isosbestic point at 390 nm.
the decay time constant of 7.6 ps at 364 nm and the growth
time constant of 7.6 ps at 455 nm indicates that the 1b-C+ is
1
The fs-TA spectra of 1b were also obtained in ACN−H2O
(Figure S14), where the species with absorption at 400 and
450 nm as well as 360 nm were absent. As cation species can
be trapped by water, these disappeared signals were supported
as attributed to cation species. The absorption spectra of 1b in
ACN and ACN−H2O were measured at different irradiation
times (Figure S15). In ACN, with increasing irradiation time,
1b absorbing at 254 and 333 nm gradually transformed into a
new species at 260 and 330 nm, which can be assigned to the
photocyclization product 2b based on the reported data for
flavone derivatives.15 While in ACN−H2O, 1b transformed
into another species appearing at 275 and 315 nm upon
irradiation. 1b-C+ is more stable compared to 1b-C+ based
produced from (1b)1. Furthermore, the decay time constant of
417 ps at 364 nm is consistent with the growth time constant
at 455 nm, indicating that the 1b-C+2 was generated through a
cyclization process from 1b-C+ .
1
Based on the above discussions, the photocyclization
reaction mechanism of 1b in ACN is proposed in Scheme 5.
Similar to the photocyclization reaction mechanism of 1a,
(1b)1 undergoes a C−Cl heterolysis to form 1b-C+ with a
1
time constant of 7.6 ps, which is followed by cyclization
yielding the cation species 1b-C+ at 417 ps. Thereafter, a
2
deprotonation process occurs from 1b-C+ to form the final
2
product.
Based on the final product analysis, the photocyclization
yields of 1a and 1b are 45 and 75%, respectively.12 The
photocyclization reaction is affected by the position of the
methoxy substituent. Since the electron donating substituent
enhances the electron density at its ortho and para position, it
would facilitate attack of the electrophilic carbonyl oxygen on
the two points of the methoxy substituent.16 Attack of the
electrophilic carbonyl oxygen is favored for 1b with methoxy
substituted at the meta position,17 which makes the photo-
cyclization yield of 1b higher than that of 1a.
Time-Resolved Spectroscopy Studies of 1c and 1d.
To examine the effect of the fluorine atom bonded at the C2
position, fs-TA measurements for 1c in ACN were performed
(Figure 5). The initial change from 340 and 400 nm observed
within 1.1 ps was attributed to the IC process from Sn to S1
(denoted as (1c)1). Later, (1c)1 shifted to around 360 nm
(Figure 5b), accompanied by an isosbestic point at 350 nm.
Thereafter, a weak signal was found at 400 nm, and then both
of the absorption bands at 360 and 400 nm decayed with
generation of a new species appearing at 335 nm. The spectra
profile of the newly generated species resembled that seen in
ns-TA spectra of 1c recorded under parallel experimental
conditions (Figure S20).
1
2
on the DFT calculations (the difference value is 46.41 kcal/
mol). Therefore, the generated species was assumed to be the
solvolysis product formed by the attack of hydroxyl ion in the
solvent system at the cation intermediate 1b-C+ . Support for
2
this assignment comes from the reasonable agreement between
the experimental spectrum with the calculated electronic
absorption spectrum of the solvolysis product (Figure S16).
Therefore, a similar photocyclization mechanism of 1a is
proposed for 1b. The calculated UV−vis absorption spectra for
1b-C+1 and 1b-C+2 resembled the fs-TA spectra of 1b recorded
at 21.1 ps and 2.5 ns (see Figure S17 and Figure S18),
respectively. The energy diagram for the photocyclization of 1b
in ACN was studied (Figure S19). When populated, (1b)1
underwent C−Cl heterolysis to give rise to 1b-C+ . Thereafter,
1
cyclization of 1b-C+ occurred to form 1b-C+ , and 2b was
1
2
produced after the deprotonation of 1b-C+ . The steps were all
2
exothermic with energy of 13.36, 52.68, and 22.82 kcal/mol,
respectively. This further confirmed that, after excitation of 1b,
a C−Cl bond heterolytic cleavage reaction took place to form
an α-keto cation intermediate 1b-C+ , which subsequently
1
underwent a cyclization reaction to produce the cyclic cation
1b-C+ .
2
The fs-TA spectra of 1c were also collected in ACN−H2O
(Figure S21), showing that the presence of H2O has no
obvious effect on the intensity of the main species seen in the
fs-TA of 1c. As the cation species is unraveled to be quenched
by H2O,10 it is reasonable to assume that the cation species
seen for 1a and 1b is not involved in the photocyclization
reaction of 1c. On the other hand, it is reported that the C-
halogen bond cleavage of organohalides by irradiation is a
general way to produce the carbon radicals.13,18,19 Also, the
stability of the radical is enhanced when connected to the
electron withdrawing groups.20 It is therefore proposed that
(1c)1 generates the corresponding radical species (denoted as
1c-R1) through the C−Cl homolysis. Due to the electron
withdrawing effect of the fluorine atom, the energy of 1c-C+1 is
higher than that of 1c-R1 (the difference value is 180.58 kcal/
mol). Therefore, the C−Cl homolysis yielding the radical
intermediate is preferred for 1c rather than the C−Cl
heterolysis as proposed for 1a and 1b. Then, the cyclization
occurs, forming a cyclized radical species (denoted as 1c-R2).
The radical quenching experiment was conducted for 1c in
ACN. Photolysis of 1c in ACN yielded photocyclization
product 2c with the signal of 292 and 315 nm detected by
UV−vis spectra (Figure S22), while the absorbance of 2c was
gradually quenched by continuous addition of 2,2,6,6-
tetramethylpiperidine-N-oxyl (TEMPO) (Figure S23). This
further supported the assignment of 1c-R1 as the precursor to
Figure 4 shows the best fit at 364 and 455 nm for 1b in
ACN. A biexponential function was used to fit the kinetics at
364 nm, yielding a decay time constant of τ1 = 7.6 ps and a rise
time constant of τ2 = 417 ps. In addition, the fitting for the
kinetics of the absorption changes at 455 nm with a
biexponential function gave a rise time constant of 7.6 ps
and a decay time constant of 417 ps. The coincidence between
Figure 4. Shown are the kinetics and the fitting plots at 364 and 455
nm for 1b in ACN.
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Scheme 5. Proposed Photocyclization Reaction Mechanism of 1b in ACN
changes at 360 nm with a biexponential function gave a rise
time constant of τ1 = 4.5 ps and a decay time constant of τ2 =
45 ps. As 1c-R1 was unraveled to be generated at 360 nm
through C−Cl homolysis from S1, the time constant τ1 was
assigned to the formation time of 1c-R1. Then, 1c-R1 decayed
with a time constant of 45 ps, yielding 1c-R2 through
cyclization. The kinetics at 405 nm was also studied. A growth
time constant of 45 ps and a decay time constant of 146 ps
were obtained, where the rise time constant is associated with
the formation of 1c-R2 and the 146 ps time constant is
attributed to the decay of 1c-R2, producing the final cyclization
product 2c.
Combining the above experimental and calculation results,
the photocyclization reaction mechanism of 1c in ACN was
depicted in Scheme 6. (1c)1 generates the radical 1c-R1
through the C−Cl homolysis with a time constant of 4.5 ps.
Afterward, the radical 1c-R2 was produced via cyclization with
45 ps. Finally, 1c-R2 underwent dehydrogenation, forming 2c
on a 146 ps time scale.
To investigate the effect of p-methoxy on the photo-
cyclization reaction mechanism of DBM derivatives, fs-TA and
ns-TA of 1d in ACN were tested (Figure 6). S1 of 1d (denoted
as (1d)1) appears within 1 ps, showing a pronounced peak at
340 nm (Figure 6a). Then, (1d)1 shifts to absorption bands at
350 and 410 nm. Decay of the bands at 350 and 410 nm
accompanied a simultaneous growth of a new signal with λmax
at 430 nm (Figure 6c). This change is accompanied by an
isosbestic point at 400 nm, suggesting that the former species
is the precursor to the latter. Then, the band at 430 nm
decayed and a new species at 350 nm appeared. A similar
spectral profile was seen in the ns-TA of 1d recorded under
parallel experimental conditions (Figure S28). Based on the
studies of 1a and 1c, the intermediate detected in ns-TA is
tentatively assigned as the triplet state of 1d (denoted as
(1d)3).
Similar to the study of 1c, the presence of H2O has no
obvious effect on the intensity of the bands corresponding to
the main species of 1d (Figure S29). It is therefore reasonable
to propose that the photocyclization reaction of 1d follows a
similar pathway as that of 1c. Similar to 1c, the radical
quenching experiment was also performed for 1d in ACN
(Figure S30 and Figure S31). Obviously, no photocyclization
product 2d was observed when the TEMPO was added, which
validated the involvement of 1d-R1 in the photocyclization.
Figure 5. (a−c) fs-TA spectra of 1c in ACN and (d) the kinetics and
the fitting plots at 360 and 405 nm for 1c in ACN.
the photocyclization. To correlate the experimental observed
species with the possible reactive intermediates proposed, TD-
DFT calculations were performed. Electronic absorption
spectra of 1c-R1 and 1c-R2 were simulated with absorbance
locating at 360 and 405 nm, respectively (Figure S24 and
Figure S25), which validates the above assignment of the
species seen in fs-TA spectra. The energy diagram for the
photocyclization pathway of 1c was performed by DFT
calculation (Figure S26). C−Cl homolysis from (1c)1 to 1c-
R1 was an exothermic process (18.53 kcal/mol); then, 1c-R1
generated the 1c-R2 (52.96 kcal/mol). Finally, 1c-R2 under-
went an exothermic dehydrogenation process (19.23 kcal/
mol), forming 2c. The rationality of the photocyclization
reaction mechanism of 1c in ACN was further verified. The
theoretical prediction absorption spectrum of the triplet state
1c (denoted as (1c)3) spectrum shows reasonable agreement
with the experimental one collected at 120 ns in ns-TA (Figure
S27), suggesting the species that appeared at late delay times in
fs-TA could be assigned to (1c)3.
The dynamics of 1c in ACN was examined at 360 and 405
nm (Figure 5d). The fitting for the kinetics of the absorption
Scheme 6. Proposed Photocyclization Reaction Mechanism of 1c in ACN
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Figure 7. Shown are the kinetics and their fitting plots at 340 and 435
nm for 1d in ACN.
final product analysis, the photocyclization yields of 1c and 1d
are 100 and 59%, respectively.12 The introduction of a
methoxy group at the para position of the benzene ring in 1d
hinders the photocyclization, which makes the conversion to
flavone lower than 1c.
Figure 6. fs-TA spectra of 1d in ACN.
Further support comes from the TD-DFT study showing 1d-
R1 and 1d-R2 have intense oscillator strength at 350 and 410
nm (Figure S32) and 430 nm (Figure S33), respectively. The
energy diagram for the photocyclization pathway of 1d was
performed (Figure S34). C−Cl homolysis, cyclization of 1d-
R1, and the dehydrogenation process were all exothermic
reactions with energy of 19.41, 50.10, and 20.84 kcal/mol,
respectively, and were favorable in thermodynamics. It is noted
that the absorbance of (1d)3 is remarkably intense compared
to that of (1a)3 and (1c)3, which offers more opportunity for
the photochemical reactions from the triplet surface. This is in
CONCLUSIONS
■
In this work, we investigated the photocyclization reaction
mechanisms of substituted DBM derivatives of 1a, 1b, 1c, and
1d using time-resolved transient absorption spectroscopy and
DFT calculations. The cyclization reaction mechanism is
similar for 1a and 1b, which starts from the C−Cl heterolysis
yielding an α-ketocation intermediate; then, the cyclization
process takes place. On the other hand, 1c and 1d undergo
dechlorination first producing a radical species, followed by a
cyclization reaction. The dominant factor leading to the
different reaction mechanisms is the involvement of a fluorine
atom bonded at C2 of 1c and 1d. Due to the meta-effect, the p-
methoxy substitution on the benzene ring inhibits the
photocyclization reaction and reduces the yield of photo-
cyclization. This work unravels the influence of fluorine
substituents and meta-effects on the photocyclization reaction
mechanisms of DBM derivatives and provides useful guidance
for designing DBM derivatives with higher photocyclization
yield.
̌
accordance with the final product analysis by Sket that α-
cleavage reaction occurs from the triplet state of 1d.12 For
benzene rings substituted with electron donating groups,
excitation to the singlet state selectively increased the electron
density at the ortho and para positions.16 Hence, for p-methoxy
substituted 1d, attack of carbonyl oxygen on the meta position
of the methoxy substituent would be inhibited.17 Therefore,
the methoxy substitution on the benzene ring of 1d induced a
barrier for the photocyclization reaction via singlet state and
offers a way for the occurrence of α-cleavage.
Kinetic fitting at 340 and 435 nm was performed (Figure 7).
A triexponential function was used to fit the kinetics at 340 nm,
yielding two decay time constants of τ1 = 2.3 ps and τ2 = 10 ps
and a rise time constant of τ3 = 1868 ps. τ1 can be assigned to
the transformation from (1d)1 to 1d-R1, and τ2 is likely
associated with decay of 1d-R1 generating the cyclized radical
1d-R2. The growth time of 1868 ps correlates with the growth
of (1d)3 via the intersystem crossing. The kinetics at 435 nm
was fit with a biexponential function, generating a growth time
constant of 10 ps and a decay time constant of 415 ps,
respectively. The growth time constant comparable to τ2 could
be due to the transformation from 1d-R1 to 1d-R2, and the 415
ps time constant is likely associated with the formation of the
cyclization product 2d from 1d-R2.
Based on the above discussions, the photocyclization
reaction mechanism of 1d in ACN is proposed in Scheme 7.
Similar to the photocyclization reaction mechanism of 1c,
(1d)1 generates the radical 1d-R1 with a time constant of 2.3
ps. Afterward, the radical 1d-R2 is produced at 10 ps. Finally,
1d-R2 forms 2d with a time constant of 415 ps. Based on the
EXPERIMENTAL SECTION
■
Synthesis of 2-Chloro-1,3-diphenylpropan-1,3-dione (1a). A
mixture of dibenzoylmethane (1 g, 4.5 mmol) and N-chlorosuccini-
mide (NCS) (0.6 g, 4.5 mmol) in CCl4 (10 mL) was heated at reflux
for 2 h. After cooling, the precipitate of succinimide was filtered off
and the solution was concentrated to dryness. The residue was
recrystallized from methanol to give pure 1a as a white crystalline
product (1.05 g, 90% yield).21 1H NMR (CDCl3, 400 MHz): δ −8.00
(d, 4 H, J = 9.0 Hz), 7.61 (m, 2 H), 7.491 (m, 4 H), 6.40 (s, 1 H).
13C{1H} NMR (CDCl3, 100 Hz): δ −189.3, 134.3, 133.8, 129.3,
128.9, 62.9.
Synthesis of 2-Chloro-1-(3,5-dimethoxyphenyl)-3-phenyl-
propan-1,3-dione (1b). To a 100 mL flask were added methyl
iodide (1.8 g, 12.7 mmol), methyl 3,5-dihydroxybenzoate (1.0 g, 5.9
mmol), potassium carbonate (1.7 g, 12.4 mmol), and anhydrous
acetone (15 mL). The mixture was heated at reflux and stirred
vigorously under argon for 48 h. Then, it was allowed to cool and
evaporate to dryness under reduced pressure. The residue was washed
with water. After filtration and recrystallization, methyl 3,5-
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Scheme 7. Proposed Photocyclization Reaction Mechanism of 1d in ACN
dimethoxybenzoate was obtained.22 The methyl 3,5-dimethoxyben-
zoate was used for the next reaction. Then, to a dry flask containing a
solution of acetophenone (0.6 g, 5.1 mmol) and methyl 3,5-
dimethoxybenzoate (1.0 g, 5.1 mmol) in THF (30 mL) was added
quickly 60% sodium hydride (0.2 g, 5.0 mmol). The reaction mixture
was heated under argon at 60 °C for 72 h. The solution was then
acidified with dilute HCl and extracted with CH2Cl2. After removal of
solvent, the pure product 1-(3,5-dimethoxyphenyl)-3-phenylpropan-
1,3-dione was obtained as a yellow solid over a silica gel column.22
Finally, after the 1-(3,5-dimethoxyphenyl)-3-phenylpropan-1,3-dione
(1.136 g, 4.0 mmol) was dissolved in ACN (50 mL), LiClO4 (0.260 g,
2.44 mmol) was added dropwise over a period of 10 min. Then, NCS
(0.59 g, 4.4 mmol) was added and stirring for an additional 3 h. The
solvent was evaporated in a vacuum. The rest was dissolved in CH2Cl2
(50 mL) and washed twice with H2O (25 mL). The organic phase
was separated and dried with anhydrous Na2SO4. After evaporation of
solvent, the crude product was obtained. Crystallization was done
from the mixture of CH2Cl2 and hexane, and pure 1b was obtained as
a yellow crystalline solid (920 mg, 71.9% yield).12 1H NMR (CDCl3,
400 MHz): δ −8.01−7.96 (m, 2H), 7.63−7.56 (m, 1H), 7.50−7.43
(m, 2H), 7.11 (d, 2H, J = 2.3 Hz), 6.67 (t, 1H, J = 2.3 Hz), 6.39 (s,
1H), 3.79 (s, 6H). 13C{1H} NMR (CDCl3, 100 Hz): δ −189.3, 189.1,
161.2, 135.7, 134.5, 133.9, 129.4, 129.1, 107.0, 106.9, 62.9, 55.8.
Synthesis of 2-Chloro-2-fluoro-1,3-diphenylpropan-1,3-
dione (1c). To a stirred solution of dibenzoylmethane (1.13 g, 5
mmol) in ACN (100 mL) at room temperature was added 1-
chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis-
(tetrafluoroborate) (F-TEDA-BF4) (3.40 g, 10 mmol). The mixture
was stirred for 3.5 h. The solvent was evaporated in a vacuum, and to
the rest, CH2Cl2 (50 mL) was added and washed with saturated
NaHCO3 solution and H2O. The organic phase was separated and
dried with anhydrous Na2SO4, the solvent was evaporated in a
vacuum, and the crude product was obtained. Crystallization was
done from the mixture of CH2Cl2 and hexane, and pure 2-fluoro-1,3-
diphenylpropan-1,3-dione was isolated.23 The pure 2-fluoro-1,3-
diphenylpropan-1,3-dione was used for the next reaction. After the
2-fluoro-1,3-diphenylpropan-1,3-dione (968 mg, 4.0 mmol) was
dissolved in ethyl acetate (80 mL), LiClO4 (254.4 mg, 2.4 mmol)
was added at room temperature. After 5 min of stirring at room
temperature, NCS (587.4 mg, 4.4 mmol) was added. The stirring was
continued for an additional 1.5 h. The reaction mixture was washed
with water. The organic phase was separated and dried with
anhydrous Na2SO4. After evaporation of the solvent, the crude
product was obtained. Recrystallization from hexane furnished 1c as a
pale yellow solid (894 mg, 80.5% yield).12 1H NMR (CDCl3, 400
MHz): δ −8.06−7.97 (m, 4H), 7.67−7.58 (m, 2H), 7.52−7.42 (m,
4H). 13C{1H} NMR (CDCl3, 100 Hz): δ −186.7, 186.5, 134.8, 131.5,
131.4, 130.6, 130.5, 129.0.
methoxyphenyl)propan-1,3-dione (533 mg, 1.76 mmol) was dissolved
in ACN (35 mL), LiClO4 (113 mg, 1.06 mmol) was added at room
temperature. After 5 min of stirring at room temperature, NCS (258
mg, 1.93 mmol) was added. The stirring was continued for an
additional 3 h. The solvent was evaporated in a vacuum; the rest was
dissolved in CH2Cl2 (35 mL) and washed with water. The organic
phase was separated and dried with anhydrous Na2SO4. Purification
was done by column chromatography using SiO2 for the stationary
phase and CH2Cl2 for elution, and pure 1d was obtained as a white
crystalline solid (458 mg, 77% yield).12 1H NMR (CDCl3, 400 MHz):
δ −8.02 (d, 4H, J = 8.0 Hz), 6.91 (d, 4H, J = 8.0 Hz), 3.86 (s, 6H).
13C{1H} NMR (CDCl3, 100 Hz): δ −185.2, 185.0, 164.8, 133.2,
133.1, 124.4, 114.3, 55.7.
For characterization of 1a, 1b, 1c, and 1d, please see the 1H NMR
and 13C NMR spectra displayed in Figures S1−S8.
2,2,6,6-Tetramethylpiperidine-N-oxyl (TEMPO) used as the
radical inhibitor was added to the sample solutions to prepare
photolysis solutions. The concentrations of TEMPO were 5, 10, 15,
20, and 25 equiv of samples. Sample solutions (1c (0.1 mM), 1b (0.05
mM), and 1d (0.02 mM)) and sample solutions added with TEMPO
in quartz vessels during irradiation in a Rayonet (RPR 100)
photochemical reactor using 254 nm lamps. The photolysis process
was monitored by measuring the UV−vis absorbance.
The fs-TA experiments use a commercial regenerative amplified
Ti:sapphire laser system and an automated data acquisition system.
The instrument response function was determined to be about 150 fs.
For the present measurements, sample solutions were excited by a 266
nm pump beam (the third harmonic is 800 nm, and the regenerative
amplifier is the fundamental wave) and probed by a white-light
continuum probe beam (330−800 nm) produced from a CaF2 crystal.
A 40 mL sample solution with an optical density of unity at 266 nm
was flowed through a 2 mm path-length cuvette throughout the data
acquisition. The ns-TA experiments were recorded on a LP980 laser
flash photolysis spectrometer (Edinburgh Instruments, UK). The
solutions were purged with Ar for 30 min before measurements. More
detailed descriptions on the fs-TA and ns-TA experiments can be
found in ref 24.
The DFT computations employed the (U)M06-2X/6-311G**
level to find the optimized geometries and electronic absorption
spectra for the transients considered to be possible intermediates for
the reactions. No imaginary frequency mode was observed at the
stationary states of the optimized structures. The effect of solvent
polarity was estimated by utilizing the polarizable continuum model
(PCM) in ACN.25 The DFT26 and TD-DFT27 calculations presented
in this study were carried out using the Gaussian 09 program suite.28
Synthesis of 2-Chloro-2-fluoro-1,3-di(4-methoxyphenyl)-
propan-1,3-dione (1d). To a 50 mL flask were added 1,3-bis(4-
dimethoxyphenyil)propan-1,3-dione (0.71 g, 2.5 mmol), F-TEDA-
BF4 (0.85 g, 2.5 mmol), and ACN (25 mL). The reaction mixture was
stirred at room temperature for 20 h. The solvent was evaporated in a
vacuum, and the CH2Cl2 (50 mL) was added and washed with
saturated NaHCO3 solution and H2O. The organic phase was
separated and dried with anhydrous Na2SO4, the solvent was
evaporated in a vacuum, and the crude product was obtained.
Crystallization was done from the mixture of CH2Cl2 and hexane, and
pure 2-fluoro-1,3-bis(4-methoxyphenyl)propan-1,3-dione was iso-
lated.23 The pure 2-fluoro-1,3-bis(4-methoxyphenyl)propan-1,3-
dione was used for the next reaction. After the 2-fluoro-1,3-bis(4-
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00647.
Synthetic route of compounds with the 1H NMR spectra
discussed in the text, fs-TA and ns-TA spectra for some
of the intermediate species, DFT calculated electronic
absorption spectra, and the Cartesian coordinates and
energies for all calculated structures discussed in the text
(PDF)
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Equilibria of 1,3-Dicarbonyl Compounds Using NMR. J. Chem. Educ.
2007, 84, 1827−1829.
AUTHOR INFORMATION
Corresponding Author
Jiani Ma − Key Laboratory of Synthetic and Natural
Functional Molecule Chemistry of Ministry of Education,
College of Chemistry and Materials Science, Northwest
University, Xi’an 710127, P. R. China; orcid.org/0000-
0003-3325-0762; Email: majiani@nwu.edu.cn
■
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Dicarbonyls Studied by Nuclear Magnetic Resonance Spectroscopy.
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Solvent Dependent Photochemical Reactions of 3-(2-Alkylphenyl)-
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Authors
Junxiao Wang − Key Laboratory of Synthetic and Natural
Functional Molecule Chemistry of Ministry of Education,
College of Chemistry and Materials Science, Northwest
University, Xi’an 710127, P. R. China
Yan Guo − Key Laboratory of Synthetic and Natural
Functional Molecule Chemistry of Ministry of Education,
College of Chemistry and Materials Science, Northwest
University, Xi’an 710127, P. R. China
Jialin Wang − Key Laboratory of Synthetic and Natural
Functional Molecule Chemistry of Ministry of Education,
College of Chemistry and Materials Science, Northwest
University, Xi’an 710127, P. R. China
̌
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Author Contributions
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The research was sponsored by grants from the National
Science Fund of China (21973075), the Project Supported by
Natural Science Basic Research Plan in Shaanxi Province of
China (2021JC-38), and Shaanxi science and technology new
star project (2020KJXX-010) to J.M.
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