Synthetic approach to benzocyclobutenones using visible light and a phosphonate auxiliary

Article abstract of DOI:10.1021/acs.orglett.8b00160

Reported herein is a two-step procedure to synthesize benzocyclobutenones from (o-alkylbenzoyl)phosphonates. It consists of a visible-light-driven cyclization reaction forming phosphonate-substituted benzocyclobutenols and subsequent elimination reaction of the H-phosphonate, which assumes a key role as the recyclable auxiliary. A wide variety of functionalized benzocyclobutenones, which include those difficult to synthesize by conventional methods, are efficiently synthesized.

Full text of DOI:10.1021/acs.orglett.8b00160

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthetic Approach to Benzocyclobutenones Using Visible Light and
a Phosphonate Auxiliary
Takaaki Yano, Tairin Kawasaki, Tatsuya Yuhki, Naoki Ishida, and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
S
* Supporting Information
ABSTRACT: Reported herein is a two-step procedure to
synthesize benzocyclobutenones from (o-alkylbenzoyl)-
phosphonates. It consists of a visible-light-driven cyclization
reaction forming phosphonate-substituted benzocyclobutenols
and subsequent elimination reaction of the H-phosphonate,
which assumes a key role as the recyclable auxiliary. A wide
variety of functionalized benzocyclobutenones, which include
those difficult to synthesize by conventional methods, are efficiently synthesized.
enzocyclobutenone and its derivatives exhibit unique
reactivities to open its four-membered ring in various
heating (e.g., at 630 °C), which diminishes its practicality. The
four-membered ring can be constructed by intramolecular
nucleophilic addition of aryllithium generated from (o-halo-
phenyl)epoxides (method c)8 and (o-halophenyl)acetamides
(method d)9 and by a palladium-catalyzed cyclization reaction
of (o-halophenyl)acetaldehydes (method e).10 UV light
induced cyclization of o-alkylphenyl ketones followed by a
thermally induced retro-aldol reaction (method f)11 merits
special attention because it dispenses with the use of
organolithium reagents. However, the photocyclization suffers
from low yields; the yields of 8 examples out of the 10 listed in
the table11b are less than 50%.
Here, we report a new, two-step procedure to synthesize
benzocyclobutenones through visible light induced cyclization
of (o-alkylbenzoyl)phosphonates that we have recently
developed.12 An H-phosphonate assumes a key role as the
recyclable auxiliary; the H-phosphonate is readily introduced
onto aroyl chlorides, facilitates visible light induced con-
struction of the four-membered ring skeletons, and is removed
for regeneration of a carbonyl group, recovered, and reused. Of
note is the broad functional group tolerance; the present
method allows a diverse array of functional groups including
those incompatible with the conventional methods mentioned
above.
B
modes. The ring-opening reactivities have found a wide range
of applications in organic synthesis,1 thermoset polymers,2 and
pharmaceuticals.3 Recently, exploitation of transition-metal
catalysis has unveiled an unconventional mode of ring
opening,4 which offers a new synthetic route to benzo-fused
cyclic skeletons.5 This has spurred a renewed demand to
develop a more convenient method to synthesize benzocyclo-
butenones with a wide functional group tolerance starting from
easily accessible materials.
The most frequently utilized synthetic method is a [2 + 2]
cycloaddition reaction of arynes with enol derivatives followed
by hydrolysis or oxidation (Scheme 1, method a).6 This
Scheme 1. Conventional Synthesis of Benzocyclobutenones
o-Alkylphenyl ketones undergo an endergonic cyclization
reaction upon irradiation with UV light to give benzocyclobu-
tenols.13 Although the reaction offers a concise way to
construct a four-membered ring, successful and reproducible
examples are significantly limited. We recently reported that the
photoinduced cyclization of (o-alkylbenzoyl)phosphonates 1
forming phosphonate-substituted benzocyclobutenols 4 is far
more efficient and reproducible than that of ordinary ketones
(Scheme 2).12 The origin of the efficiency is mainly ascribed to
the electron-accepting nature of the phosphonate substituent.
method is powerful, particularly for the synthesis of
benzocyclobutenones having an oxy substituent at the 6-
position, which dictates the regioselectivity of the [2 + 2]
cycloaddition.6c−e,g,j In the cases of other substituted
benzocyclobutenones, however, it is often difficult to control
the regioselectivity of the [2 + 2] cycloaddition reaction. Flash
vacuum pyrolysis of aroyl chlorides (method b)7 offers an
alternative access to benzocyclobutenones. It requires rigorous
Received: January 15, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00160
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Photoinduced Cyclization of 1
Table 1. Elimination of H-Phosphonate from 4
The key intermediate to form a four-membered ring by 4π-
electron cyclization is o-quinodimethane 3.14 The electron-
accepting phosphonate substituent lowers the activation energy
of the 4π-electron cyclization to accelerate it.15 The electron-
accepting nature also retards the competing nonproductive
proton-transfer process by lowering the basicity of the diene
moiety. As a result, the photoinduced cyclization of (o-
alkylbenzoyl)phosphonate 1 becomes a highly efficient process
to produce phosphonate-substituted benzocyclobutenols 4 in
excellent yield.
Addition of H-phosphonates onto carbonyl compounds
forming α-hydroxyalkylphosphonates is a reversible process.16
We envisioned that the photocyclized product 4 would undergo
elimination of an H-phosphonate to regenerate a carbonyl
group upon treatment with a base, acting as the precursor of
benzocyclobutenone. However, 4 can follow another pathway
opening the four-membered ring, which is driven by release of
its ring strain.17 Thus, we examined an elimination reaction of
an H-phosphonate from phosphonate-substituted benzocyclo-
butenols 4, which were prepared by photoinduced cyclization
of (o-alkylbenzoyl)phosphonates (Table 1).12 When dimethyl
phosphonate derivative 4a was treated with triethylamine in
toluene at 70 °C for 2 h, a base-induced ring-opening reaction
selectively took place to afford 1a in 13% yield, and 75% of 4a
remained (entry 1). Benzocyclobutenone 5 was not formed at
all. Treatment of 4a with inorganic bases such as NaOH,
K2CO3, and K3PO4 at room temperature afforded a complex
mixture (entries 2−4). On the other hand, diphenyl
phosphonates 4b successfully underwent elimination of the
diphenyl phosphonate moiety upon treatment with Et3N or
K3PO4 to afford 5 in 85% and 89% yield, respectively (entries 5
and 6). Preference for the elimination of diphenyl H-
phosphonate over the opening of the four-membered ring
can be ascribed to the superior acidity of the diphenyl H-
phosphonate (calculated pKa = 9.0 in DMSO),18 which is far
stronger than dimethyl H-phosphonate (calculated pKa = 18.4
in DMSO).18 The higher acidity leads to a better leaving ability.
Comparable results were obtained with cyclic diaryl phospho-
nate 4c to produce benzocyclobutenone 5 in good yields (84−
88%, entries 7−9).
a
Reaction conditions: 4 (0.20 mmol), base (0.20 mmol), toluene, 1
b
c
mL, rt. Isolated yields. Basic Al2O3 (200 mg).
Scheme 3. Preparation of 1b and 1c
the carbonyl moiety of 1c was sterically protected against
hydrolysis and stable enough to be purified by column
chromatography on silica gel. In contrast, 1b was susceptible
to hydrolysis and difficult to isolate by column chromatography
on silica gel. Third, it was high-yielding to recover and easy to
reuse the H-phosphonate 7c after its elimination, whereas
diphenyl phosphonate 7b was difficult to recover after the
elimination reaction due to its lability.
We next performed a sequential procedure consisting of the
photoinduced cyclization and the following elimination of the
H-phosphonate in one pot (Scheme 4). Photoirradiation of 1c
in toluene with a blue LED lamp (peak at 425 nm)21 induced
cyclization and, after 8 h, yielded benzocyclobutenol 4c
quantitatively. Then, basic alumina was directly added to the
toluene solution containing 4c, and the reaction mixture was
stirred at 30 °C for 3 h to complete elimination of the
Thus, the diaryl phosphonates 4b and 4c turned out to be
suitable precursory intermediates, leading to benzocyclobute-
none 5. The sterically bulkier phosphonate 4c was our choice
for further examination because of the following advantages.
First, the acylphosphonate 1c was readily synthesized by a base-
induced addition reaction of H-phosphonate 7c19 to o-toluoyl
chloride 6 (Scheme 3). On the other hand, the addition of
diphenyl H-phosphonate 7b to 6 afforded a mixture of the
acylphosphonate 1b and bis-phosphonate 8, which resulted
from further addition of the diphenyl H-phosphonate to 1b
followed by a phospha-Brook-type rearrangement.20 Second,
B
DOI: 10.1021/acs.orglett.8b00160
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
bromobenzocyclobutenones in 21% combined yield.6f Func-
tional groups that were sensitive toward strong bases such as
ester (18, 21, and 26), secondary carbamate (19), nitro (20),
and cyano (22) groups survived under the present reaction
conditions. Boryl (23), alkynyl (25), and acrylate (26) groups,
which would serve for further derivatization, also remained
intact. Of particular note is the tolerance of the acrylate
functional group. Although the o-quinodimethane intermediate
derived from o-alkylphenyl ketones undergoes a [4 + 2]
cycloaddition reaction with acrylate to produce a six-membered
product,23 the intermediate generated from acylphosphonate
underwent 4π-electrocyclization selectively without touching
the acrylate functional group. The visible light induced
cyclization of the substrates having a longer alkyl chain at the
ortho position (R2 ≠ H) was sluggish under the standard
reaction conditions.24 However, the reaction was accelerated by
adding thioxanthone.25 α-Substituted benzocyclobutenones
27−30 were synthesized in yields ranging from 59% to 79%
by the modified procedure using thioxanthone.26
Scheme 4. Synthesis of Benzocyclobutenone 5
phosphonate moiety. Purification by chromatography on silica
gel gave benzocyclobutenone 5 in 90% isolated yield. H-
Phosphonate 7c was recovered in 84% isolated yield at the
same time. The recovered 7c could be used for preparation of
the acylphosphonate 1c from o-toluoyl chloride 6 without any
problem.
A wide variety of substituted benzocyclobutenones were
successfully synthesized (Figure 1). The synthesis of methy-
Although benzenes fused with two cyclobutenone rings are
intriguing synthetic targets,27 synthesis of 32 has never been
reported in the literature. We applied the present method to
synthesize 32 selectively. The bisphosphonate 31 was prepared
from commercially available 4,6-dimethylisophthalic acid and
7c. Irradiation of 31 with a blue LED lamp successfully induced
double photocyclization. Subsequent treatment with basic
Al2O3 gave 32 in 78% yield (Scheme 5).
Scheme 5. Synthesis of 32
Figure 1. Substituted benzocyclobutenones synthesized. The reaction
conditions for 9−26 were identical to those shown in Scheme 4.
Isolated yields. For 27−30, photoirradiation was performed in the
presence of thioxanthone (0.75 mmol, 0.75 equiv) for 16 h.
In conclusion, we have developed a new, convenient way to
synthesize benzocyclobutenones through a visible light driven
cyclization reaction of (o-alkylbenzoyl)phosphonates and
subsequent elimination of the H-phosphonate. The starting
substrates, (o-alkylbenzoyl)phosphonates, are readily obtained
from aroyl chlorides and H-phosphonates. The H-phosphonate
assumes a key role as the auxiliary that facilitates construction
of the four-membered ring and, afterward, is removed for
regeneration of a carbonyl group, recovered, and reused. The
mild reaction conditions allow a diverse array of functional
groups that conventional methods hardly tolerate. A benzene
fused with two cyclobutenone rings was also synthesized in two
steps from a commercially available dicarboxylic acid.
lated benzocyclobutenones showcases its synthetic advantage.
3-Methylbenzocyclobutenone 9 was selectively produced in
88% isolated yield from (2,3-dimethylbenzoyl)phosphonate.
The methyl group ortho to the carbonyl group selectively
joined the cyclization with the m-methyl group remaining
intact. On the other hand, it has been reported that the
conventional [2 + 2] cycloaddition pathway from 2-
bromotoluene and 1,1-dimethoxyethene furnishes a 3:1 mixture
of 6-methylbenzocyclobutenone (major) and 3-methyl-sub-
stituted one (minor) in 38% combined yield.6c 4-Methyl-
benzocyclobutenone 10 and 5-methylbenzocyclobutenone 11
were also selectively synthesized, both in 94% yields from the
corresponding (dimethylbenzoyl)phosphonates, respectively.22
Another synthetic advantage is shown in the synthesis of halo-
substituted benzocyclobutenones 12−15. 4-Bromobenzocyclo-
butenone 12 was synthesized in 88% isolated yield. In addition,
fluoro-, chloro-, and even iodo-substituted benzocyclobute-
nones 13−15 were all successfully synthesized. The halo-
substituted benzocyclobutenones thus prepared are valuable
synthetic intermediates because a wide variety of derivatizations
would be feasible based on the halogen substituents. On the
other hand, it has been reported that [2 + 2] cycloaddition of
an aryne derived from 1,4-dibromobenzene and sodium amide
with a 1,1-dialkoxyalkene affords a 1:4 mixture of 4- and 5-
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b00160.
Experimental and computational details and NMR
spectra of the products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: murakami@sbchem.kyoto-u.ac.jp.
C
DOI: 10.1021/acs.orglett.8b00160
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
ORCID
Letter
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(f) Mamer, O. A.; Lossing, F. P.; Hedaya, E.; Kent, M. E. Can. J. Chem.
1970, 48, 3606.
Naoki Ishida: 0000-0003-2162-3605
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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This work was supported by JSPS KAKENHI 15H05756,
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D
DOI: 10.1021/acs.orglett.8b00160
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