Synthesis of difluoromethylated diarylmethanesviaFe(OTf)3-catalyzed Friedel-Crafts reaction of 2,2-difluoro-1-arylethyl phosphates

Article abstract of DOI:10.1039/d1cc00765c

The Fe(OTf)₃-catalyzed Friedel-Crafts reaction of 2,2-difluoro-1-arylethyl phosphates with electron-rich (hetero)arenes afforded difluoromethylated diarylmethanes. Control experiments showed that Fe(OTf)₃behaves as the Lewis acid, an

Full text of DOI:10.1039/d1cc00765c

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Synthesis of difluoromethylated diarylmethanes
via Fe(OTf) -catalyzed Friedel–Crafts reaction of
2,2-difluoro-1-arylethyl phosphates†
3
Cite this: Chem. Commun., 2021,
7, 3877
5
Received 9th February 2021,
Accepted 17th March 2021
Yoshihiko Yamamoto, * Tomoya Takase, Eisuke Kuroyanagi and Takeshi Yasui
DOI: 10.1039/d1cc00765c
rsc.li/chemcomm
The Fe(OTf)
3
-catalyzed Friedel–Crafts reaction of 2,2-difluoro-1- successfully transformed into (difluoromethyl)alkenes via an
0
arylethyl phosphates with electron-rich (hetero)arenes afforded S 2 -type substitution reaction using organocuprates. As a
N
difluoromethylated diarylmethanes. Control experiments showed continuation of this work, we investigated the Friedel–Crafts
that Fe(OTf)
3
2
behaves as the Lewis acid, and that the phosphate (F–C) reaction of CF H-substituted benzyl phosphates, which
leaving group and o- or p-alkoxy substituents on the substrates are can be readily prepared from the corresponding benzaldehydes
necessary for the Fe(OTf)
3
-catalyzed reaction to proceed under via the addition of LiCF
Brook rearrangement using K
2
PO(OEt)
2
and subsequent phospha-
as a base in wet DMF
mild conditions.
2
CO
3
(Scheme 1b). Although the synthesis of diarylmethanes via
Diarylmethane is an important structural motif found in a cross-coupling reactions of benzyl phosphates with aryl nucleo-
1
diverse range of biologically active molecules (Fig. 1a). There- philes has been reported by several research groups, the existing
7
fore, numerous synthetic methods have been developed for the methods are limited to the use of primary benzyl phosphates.
2
efficient synthesis of 1,1-diarylmethanes. On the other hand, Moreover, the F–C reaction of benzyl phosphates has rarely been
8
,9
fluoroalkyl groups have attracted much attention in medicinal investigated. Smith and Johnson demonstrated the F–C reac-
chemistry because they can improve the intrinsic potency, tion of phenacyl-substituted benzyl phosphates as challenging
metabolic stability, and bioavailability of the constituent drug substrates bearing an electron-withdrawing group at the benzylic
molecules. There have been several reports on efficient synthetic position, although a stoichiometric amount of the Lewis acid
strategies, including enantioselective methods, for trifluoromethy- and 10 equivalents of arenes were required (Scheme 1c). In
3
9a
4
lated diarylmethanes. In addition, synthetic methods for their addition, Pallikonda and Chakravarty performed catalytic F–C
4a, f,g
difluoromethyl (CF H) analogs are developed (Fig. 1b),
reactions of benzyl phosphates using trifluoromethanesulfonic
2
9b
because the CF
those of the trifluoromethyl group; the CF
lipophilic bioisostere of hydroxy (OH) and mercapto (SH) groups.
2
H group has significant properties distinct from acid (TfOH) as the Brønsted acid catalyst under neat conditions.
2
H group functions as a Recently, Yurino, Ohkuma, and coworkers reported that Al(OTf)
3
5
efficiently catalyzed the F–C reaction of primary benzyl phos-
However, the scope of difluoromethylated diarylmethanes has phates, albeit at a high temperature (100 1C). Thus, we assumed
9c
2
been relatively narrow. This is partly because of the inefficiency that CF H-substituted benzyl phosphates are potential substrates
in the preparation of CF H-substituted benzylic precursors and
2
the requirement of an expensive difluoromethylating agent.
Therefore, a novel strategy for the synthesis of difluoromethylated
diarylmethanes is sought.
We have previously reported the efficient preparation of
allylic phosphates from cycloalkenones via the addition of a
lithiated difluorophosphonate and subsequent phospha-Brook
6
rearrangement (Scheme 1a). The obtained phosphates were
Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical
Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan.
E-mail: yamamoto-yoshi@ps.nagoya-u.ac.jp
†
Electronic supplementary information (ESI) available: Experimental details and
Fig. 1 (a) Biologically active compounds containing diarylmethane motif
NMR charts. See DOI: 10.1039/d1cc00765c
and (b) previous synthesis of difluoromethylated diarylmethanes.
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amounts of a 2 : 1 adduct as a mixture of diastereomers
(
entry 2). When 0.5 equiv. of Et
2
OꢀBF
3
was used, the reaction
did not complete within 5 h and 3aa was obtained in 39% yield
along with 45% recovery of 1a (entry 3). Next, TfOH was tested
as a Brønsted acid promoter.
smoothly in the presence of 1.0 equiv. of TfOH at 0 1C for
9b
The reaction proceeded
2
2
h, affording 3aa in 83% yield along with small amounts of the
: 1 adduct (entry 4). Although, a similar result was obtained
with 0.5 equiv. of TfOH (entry 5), the reaction did not proceed
to completion even in 5 h when the amount of TfOH was
further reduced to 0.2 equiv. (entry 6).
Metal triflates have been used as Lewis acid catalysts, and
they often show Brønsted acid reactivity associated with the
1
2
generation of TfOH under the reaction conditions. Narasaka,
Scheme 1 (a and b) Synthesis of difluoromethylated phosphates via
phospha-Brook rearrangement and subsequent arylation, and (c) F–C Du n˜ ach, and coworkers investigated the effectiveness of metal
1
3
reaction of phenacyl-substituted benzyl phosphate.
triflates as catalysts for the reaction of unsaturated oximes.
They reported that Al(OTf) selectively catalyzed the desired
cyclization, while Fe(OTf) and Cu(OTf) caused the hydrolysis
3
3
2
for the F–C reaction. Nevertheless, the development of an efficient
protocol to catalyze the F–C reaction of benzyl phosphates deac-
of the oxime moiety. With the expectation of the Lewis and
Brønsted acid cooperative effect, several metal triflates were
examined for use as promoters. Interestingly, the use of 20
2
tivated by the electron-withdrawing CF H group under mild
10
conditions is a formidable challenge. Herein, we report our
successful implementation of the synthesis of CF H-substituted
diarylmethanes via the iron-catalyzed F–C reaction of 2,2-difluoro-
mol% Fe(OTf)
afforded 3aa in an improved yield (68%) along with unreacted
a in 19% yield (entry 7). At an elevated temperature (20 1C), the
reaction performed using 20 mol% Fe(OTf) was complete
3
instead of TfOH under the same conditions
2
1
11
1-arylethyl phosphates.
3
At the outset, the reaction of known phosphate 1a with 1,
-dimethoxybenzene (2a) was performed under various condi-
within 2.5 h, affording 3aa in 82% yield (entry 8). Other metal
triflates were also tested; Cu(OTf) gave much lower conversion
3
2
tions to identify the optimal promoter (Table 1). According to
9
a
3
in 5 h than that obtained when using Fe(OTf) , while Al(OTf)3
the report by Smith and Johnson, Et
2
OꢀBF
solution (0.25 M), 1a and
OꢀBF (1.5 equiv.) at 0 1C for
3
was tested as a
led to near completion of the reaction in 5 h (entries 9 and 10).
Therefore, Fe(OTf) was demonstrated to be the most efficient
3
promoter among the metal triflates examined in this study.
Furthermore, when nitromethane was used as the solvent, the
reaction was complete in 1 h even with a reduced amount of
Lewis acid promoter. In a dry CH
2
Cl
2
2a (3 equiv.) were treated with Et
2
3
3
h to afford the desired product 3aa in 86% yield (entry 1). The
reaction was repeated with decreased amounts of 2a (1.5 equiv.)
to afford 3aa in a slightly lower yield (74%) along with small
3
Fe(OTf) (10 mol%), affording 3aa in 87% yield (entry 11). When
reaction was carried out with 5 mol% Fe(OTf) at a 1 mmol
3
scale for 2 h, 3aa was obtained in 85% yield (entry 12). When
FeCl ꢀ6H O was used instead of Fe(OTf) , a longer reaction time
Table 1 F–C reaction of 1a with 2a under various conditions
3
2
3
(
5 h) was required for the reaction completion, demonstrating
the superior efficiency of Fe(OTf)
knowledge, Fe(OTf) has not been used as a catalyst for the F–C
reaction of arenes with benzylic electrophiles.
The scope of the F–C reaction of CF H-substituted benzyl
3
(entry 13). To the best of our
3
1
1
a
Entry
1
Conditions
Yield /%
2
phosphates with (hetero)aromatic compounds was investi-
gated, and the obtained results are summarized in Fig. 2. The
reaction of 1a with various arenes 2b–f bearing alkoxy groups
afforded the corresponding diarylmethanes 3ab–af in good
yields. Phenol derivative 2g and indolinone derivative 2h were
also compatible with this protocol; the desired products 3ag
and 3ah were obtained in 93% and 61% yields, respectively,
although prolonged reaction times were required. In striking
contrast, the less electron-rich p-chloroanisole failed to
undergo the F–C reaction with 1a, and the corresponding
benzyl alcohol 4 (vide infra) was obtained in 54% yield as the
major product. Electron-rich heteroarenes were also examined
as coupling partners. 2-Methylfuran (2i) and benzofuran (2j)
smoothly reacted with 1a to afford the corresponding products
Et
Et
Et
2
2
2
OꢀBF
OꢀBF
OꢀBF
3
3
3
(1.5 equiv.), 0 1C, 3 h
(1.5 equiv.), 0 1C, 3 h
(0.5 equiv.), 0 1C, 5 h
86
74
b
c
2
3
4
5
6
7
8
9
39 (45)
c
TfOH (1.0 equiv.), 0 1C, 1.5 h
TfOH (0.5 equiv.), 0 1C, 1.5 h
TfOH (0.2 equiv.), 0 1C, 5 h
83
85
c
34 (40)
68 (19)
Fe(OTf)
Fe(OTf)
3
(0.2 equiv.), 0 1C, 5 h
(0.2 equiv.), 20 1C, 2.5 h
(0.2 equiv.), 20 1C, 5 h
(0.2 equiv.), 20 1C, 5 h
(0.1 equiv.), 20 1C, 1 h
(0.05 equiv.), 20 1C, 2 h
c
3
82
Cu(OTf)
Al(OTf)
2
46 (44)
82 (trace)
c
10
11
12
13
3
d
c
Fe(OTf)
Fe(OTf)
3
87
de
3
85
81
FeCl
3
2
ꢀ6H O (0.1 equiv.), 20 1C, 5 h
a
b
Isolated yields. Yields of recovered 1a were indicated in parentheses.
c
2
a (1.5 equiv.). Trace amounts of 2 : 1 adduct (o7%) was detected as
d
a mixture of diastereomers. Nitromethane was used as the solvent.
e
1
mmol scale.
3
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conditions, suggesting that the electron-donating ability of the
aromatic substituents on the phosphate substrates plays a decisive
role in their reactivity. These phosphates were subjected to the
reaction with 2a under harsher conditions. In nitromethane, 1l and
2a (3 equiv.) were treated with TfOH (1 equiv.) at 50 1C for 1 h,
affording the expected product 3la in 74% yield. In contrast, 1k was
unreactive under the same conditions. This result could be ascribed
10
to the electron-withdrawing nature of the meta MeO groups. Based
on density functional theory calculations (Fig. S1 and Table S1 in
ESI†), the stability of difluoromethyl-substituted benzyl cations
with an o-, m-, or p-anisyl group decreases in the order para-Int
ꢁ
1
ꢁ1
(
DGrel = 0.0 kcal mol ) 4 ortho-Int (DG = +5.4 kcal mol ) c
rel
ꢁ
1
meta-Int (DGrel = +13.7 kcal mol ). Thus, para-Int and ortho-Int,
which have a quinone methide-type structure, are much more stable
than meta-Int, and these computational data are in good agreement
with the experimental results.
3
To gain insights into the present Fe(OTf) -catalyzed F–C
reaction, additional experiments were conducted, as shown in
Scheme 2. 2,6-Disubstituted pyridines have been used to dis-
criminate between the Lewis- and Brønsted-acid reactivity: they
function as the proton scavengers to selectively inhibit
1
5
Brønsted acid-mediated reaction. The reaction of phosphate
a with 1,3-dimethoxybenzene (2a) was performed in the
presence of 10 mol% Fe(OTf) and 10 mol% 2,6-di(tert-butyl)-
-methylpyridine (5) in nitromethane at 20 1C for 1 h
Scheme 2a). As a result, 3aa was obtained in 84% yield along
1
3
4
(
with the remaining phosphate (1a, 8%), as confirmed by
1
H NMR analysis of the crude reaction mixture. Consequently,
it is suggested that Fe(OTf) promotes the F–C reaction of the
3
2
Fig. 2 The scope of Friedel–Crafts reaction of CF H-substituted benzyl
difluoromethylated benzyl phosphates as a Lewis acid.
Next, benzyl alcohol 4 was subjected to the reaction with 2a
under the standard conditions in order to compare its reactivity
phosphates with (hetero)aromatic compounds.
3ai and 3aj in high yields. The reaction with 7-methoxycoumarin
(
3
2k) occurred at the 3-position to afford natural product analog
ak in 51% yield. In contrast, the reaction with indole did not
14
proceed, and 88% of 1a was recovered intact. This result implies
that the basic indole behaves as a catalyst poison. Thus, the
desired 3-benzylindole derivative 3al was obtained in 56% yield
when the less basic N-acetylindole (2l) was employed instead of
the parent indole.
2
Next, several CF H-substituted benzyl phosphates were
examined as substrates. o-Anisyl derivative 1b was allowed to
react with anisole (2e) under the standard conditions. As a
result, 1b was found to be less reactive than 1a; the reaction was
not complete in 5 h, and the expected product 3be was obtained
in a modest yield (58%) along with the recovered 1b (32%).
Complete conversion was achieved at 50 1C over 2.5 h to afford
3
2
be in 80% yield. The reactions of benzyl phosphates 1c–g with
e delivered the corresponding products 3ce–ge in moderate to
high yields. The reaction of 1h, which has the synthetically
useful Br–Caryl bond, was performed at 50 1C for 1 h to afford
the corresponding product 3he in 93% yield. Diarylmethanes 3ie
and 3je were also obtained in high yields from p-(methylthio)phenyl-
and benzothiophene-3-yl-substituted phosphates 1i and 1j, respec-
tively. In contrast, 3,5-dimethoxyphenyl- and p-tolyl-substituted
Scheme 2 Control experiments and synthesis of triarylmethane 7 and
phosphates 1k and 1l failed to react with 2e under the standard pyrazole 8 (PMP = p-methoxyphenyl).
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with that of phosphate 1a (Scheme 2b). Almost 30% of 4
remained even after the reaction was carried out for 5 h at
2 (a) F. Schmidt, T. T. Stemmler, J. Rudolph and C. Bolm, Chem. Soc.
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2
0 1C, and 3aa was obtained in 62% yield, as suggested by
S. Narayanaperumal and R. S. Schwab, Synthesis, 2020, 1855.
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1
H NMR analysis of the crude reaction mixture. Therefore, this
result clearly demonstrates the superiority of the phosphate
(
(
b) K. M u¨ ller, C. Faeh and F. Diederich, Science, 2007, 317, 1881;
c) S. Purser, P. R. Moore, S. Swallow and V. Gouverneur, Chem. Soc.
leaving group in the Fe(OTf)
theless, benzhydrol 6 could be used as the substrate for
Fe(OTf) -catalyzed F–C reaction. The reaction of 6 was per-
formed in the presence of 20 mol% Fe(OTf) at 50 1C for
.5 h to afford triarylmethane 7 in 90% yield (Scheme 2c).
Recently, Yamazaki et al. reported the base-catalyzed substitu-
tion reaction of CF -substituted benzylic carbonates with 1,
3
-catalyzed F–C reaction. Never-
Rev., 2008, 37, 320; (d) E. P. Gillis, K. J. Eastman, M. D. Hill,
D. J. Donnelly and N. A. Meanwell, J. Med. Chem., 2015, 58,
3
8
315.
3
4
Selected recent examples: (a) G. K. S. Prakash, F. Paknia, T. Mathew,
5
G. Mlosto n´ , J. P. Joschek and G. A. Olah, Org. Lett., 2011, 13, 4128;
(
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Z. Lin, C. Feng and T.-P. Loh, Angew. Chem., Int. Ed., 2017, 56, 9872;
3
16
3
-dicarbonyl compounds. We also performed the reaction of
phosphate 1a with acetylacetone (3 equiv.) in the presence of
0 mol% Fe(OTf) at 50 1C for 5 h to afford the expected product 8
as an inseparable mixture with small amounts of alcohol 4
Scheme 2d). The crude product was treated with hydrazine
(
5
e) M. Brambilla and M. Tredwell, Angew. Chem., Int. Ed., 2017,
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3
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(
hydrate in methanol under reflux to afford pyrazole 8a in 76%
yield over two steps. Similarly, 3,5-heptanedione reacted with 1a
and the corresponding pyrazole 8b was obtained in a higher yield
(82%). Because the reaction using dibenzoylmethane afforded the
(
c) C. D. Sessler, M. Rahm, S. Becker, J. M. Goldberg, F. Wang and
corresponding intermediate as a solid, the subsequent condensa-
tion was performed in EtOH at 80 1C to give the desired product
S. J. Lippard, J. Am. Chem. Soc., 2017, 139, 9325.
6
(a) Y. Yamamoto, Y. Ishida, Y. Takamizu and T. Yasui, Adv. Synth.
Catal., 2019, 361, 3739Also, see: (b) Y. Yamamoto, M. Sakai, Y. Ishida
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8
c, albeit in a diminished yield (60%). Therefore, the nucleophile
is not limited to (hetero)arenes in our Fe(OTf) -catalyzed reaction
of CF H-substituted benzyl phosphates.
3
2
Finally, the present Fe(OTf) -catalyzed F–C reaction protocol
3
could be applied to deactivated benzyl phosphates 9a and 9b,
bearing a phenacyl or cyano group, respectively, to afford the
expected products 10a and 10b in high yields (Scheme 2e).
8
(a) M. Bandini and M. Tragni, Org. Biomol. Chem., 2009, 7, 1501;
b) M. Rueping and B. J. Nachtsheim, Beilstein J. Org. Chem., 2010,
(
In conclusion, we have successfully developed the Fe(OTf)
catalyzed F–C reaction of 2,2-difluoro-1-arylethyl phosphates
with electron-rich (hetero)arenes toward the efficient synthesis
of difluoromethylated diarylmethanes. The Fe(OTf) -catalyzed
3
F–C reaction also proceeded in the presence of a proton
3
-
6, 6, DOI: 10.3762/bjoc.6.6.
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9
(
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Eur. J. Org. Chem., 2020, 2225.
0 The Hammet substituent constant of CF
8
1
1
1
2
H (s
(s
= ꢁ0.27 and s
p
0.32) is comparable
0.36). The Hammett
= 0.12. See:
t
to those of CO Bu (s
p
0.32) and CONH
2
p
scavenger, 2,6-di(tert-butyl)-4-methylpyridine, suggesting that
substituent constants for MeO are s
p
m
Fe(OTf) functions as a Lewis acid catalyst. The p-methoxybenzyl
C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91, 165.
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3
phosphate substrate was found to be superior to the corres-
ponding alcohol. Moreover, Fe(OTf) catalyzed the reaction of
3
p-methoxybenzyl phosphate with 1,3-diketones, and the obtained
products were converted into pyrazole derivatives through the
condensation with hydrazine hydrate.
This research is partially supported by the Platform Project for
Supporting Drug Discovery and Life Science Research (Basis for
Supporting Innovative Drug Discovery and Life Science Research
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Conflicts of interest
1
5 (a) L. Coulombel, M. Rajzmann, J.-M. Pons, S. Olivero and
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There are no conflicts to declare.
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880
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This journal is © The Royal Society of Chemistry 2021