Photoredox-Catalyzed Three-Component Tandem Process: An Assembly of Complex Trifluoromethylated Phthalans and Isoindolines

Article abstract of DOI:10.1021/acs.orglett.6b01257

A novel photoredox-mediated tandem three-component process afforded a wide variety of CF₃-containing phthalans and isoindolines in respectable yields and with moderate to excellent diastereoselectivity.

Full text of DOI:10.1021/acs.orglett.6b01257

Letter
pubs.acs.org/OrgLett
Photoredox-Catalyzed Three-Component Tandem Process: An
Assembly of Complex Trifluoromethylated Phthalans and
Isoindolines
Lucie Jarrige,† Aude Carboni,† Guillaume Dagousset,†,‡ Guillaume Levitre,† Emmanuel Magnier,‡
and Geraldine Masson*,†
́
†Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France
‡Institut Lavoisier de Versailles, UMR 8180, Universite
́
de Versailles-Saint-Quentin, 78035 Versailles Cedex, France
S
* Supporting Information
ABSTRACT: A novel photoredox-mediated tandem three-
component process afforded a wide variety of CF3-containing
phthalans and isoindolines in respectable yields and with
moderate to excellent diastereoselectivity.
rifluoromethylated heterocycles have found extensive use
presence of azidotrimethylsilane (TMSN3, 3a) and Ru-
(bpy)3(PF6)2 photocatalyst 4a under visible light irradiation
(blue LEDs).6−9 The only product formed was not the expected
β-trifluoromethylated azide 5 but, to our surprise, the 1-azido-3-
trifluoroethyl-1,3-phthalan 7a in 49% yield with excellent
diastereoselectivity in favor of the cis-diastereomer (Scheme 1,
eq 2).10 This exciting finding was attributed to the fact that the α-
azido alcohol intermediate 8a, in situ generated by azide addition,
could further undergo a photoredox-mediated intramolecular
oxytrifluoromethylation via a tandem process.9l,n−p To the best
of our knowledge, such a three-component tandem reaction has
never been reported.6−11 This, in conjunction with the general
lack of synthetic methods of CF3-containing phthalans and
isoindolines, prompted us to examine in detail this novel tandem
transformation and to report herein our preliminary results.
We then optimized the reaction conditions of this one-pot
photoredox-catalyzed transformation, and the results are
summarized in Table 1. First, the amount of TMSN3 3a was
examined (entries 1−4). The desired product 7a was reached in
67% yield when 5.0 equiv of 3a were used in CH2Cl2 at rt, and no
improvement was observed with 10 equiv (entry 3). Sub-
sequently, various solvents were screened, revealing that freshly
distilled CH2Cl2 was the best choice for the reaction (entry 4).
The addition of molecular sieves or MgSO4 to remove the water
had adverse effects, possibly due to a poor transmission of light in
this photoredox reaction (entries 5−7). No beneficial effect was
then observed when a Lewis base was used to activate TMSN3
(entries 8−10).12 No product was form with NaN3. Attempts
with other photoredox catalysts, such as Ru(bpy)3Cl2, [Ir-
1,2
Tin medicinal and agricultural chemistry.
Indeed, the
incorporation of a CF3 group into heterocyclic compounds often
exhibits remarkably different chemical, physical, and biological
properties in comparison with their fluorine-free analogues.1,2 As
a result, much effort has been devoted toward the development
of efficient methods for the synthesis of CF3-containing
heterocyclic compounds. However, although phthalans (1,3-
dihydroisobenzofurans) and 1,3-disubstituted isoindolines are
ubiquitous in various pharmaceutical agents,3,4 very few synthetic
methods have been developed for the preparation of
trifluoromethylated analogues.5 This highlights the need for
further synthetic developments in this area.
During our study of photocatalyzed three-component
azidotrifluoromethylation of olefins (Scheme 1, eq 1),6a,b we
had opportunities to attempt the trifluoromethylation of 2-
vinylbenzaldehyde (6a) with Umemoto’s reagent (2) in the
Scheme 1. Use of 2-Vinylbenzaldehyde 6a in the Photoredox-
Catalyzed Azidotrifluoromethylation of Styrenes
Received: April 29, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01257
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Survey of Reaction Conditions
Scheme 2. Scope of the Photoredox-Catalyzed Reaction with
Representative Vinylbenzaldehydes 6
a
b
entry
photocat.
equiv of 3a
additive
yield (%)
1
Ru(bpy)3(PF6)2 (4a)
3
5
49
67
66
2
4a
3
4a
10
5
cd
,
4
4a
79
31
c
c
c
c
c
c
c
c
c
f
5
4a
5
4 Å MS
3 Å MS
MgSO4
6
4a
5
<10
<10
69
7
4a
5
e
8
4a
5
MgO
e
9
4a
5
P(OEt)3
49
e
10
11
12
13
14
4a
5
Ph3PO
30
Rubpy3Cl2 (4b)
[Ir(ppy)2(dtbbpy)]PF6 (4c)
[Ru(bpz)3(PF6)2] (4d)
4a
5
45
5
50
5
<5
0
5
a
General conditions: 6a (0.10 mmol), 2 (0.15 mmol), 4 (0.05 equiv),
b
and TMSN3 in CH2Cl2 (2 mL) irradiated at rt for 2 h. Yields referred
to chromatographically pure product. Reaction performed in dry
CH2Cl2. Maintained at constant room temperature of 20 °C. With
25 mol %. With NaN3.
c
d
e
f
(ppy)2(dtbbpy)]PF6, and [Ru(bpz)3(PF6)2], were not as
successful as Ru(bpy)3(PF6)2 (5 mol %).
The scope of this novel synthesis of 1-azido-3-trifluoroethyl-
1,3-phthalans was next assessed by varying the structures of
vinylbenzaldehyde. As shown in Scheme 2, the ortho-, meta-, and
para-substituted aryl groups were all compatible, leading to the
respective phthalans (7b−7d) in good to excellent yields with
high diatereoselectivities. It was also found that the substrates
bearing a strong electron-withdrawing group such as CF3 (6e) as
well as a strong electon-donating group such as OMe (6f) gave
diminished yields (7e and 7f, respectively). On the other hand, 6-
vinylbenzo[d][1,3]dioxole-5-carbaldehyde (6g) was largely
converted to the azidotrifluoromethylated phthalan 7g (57%)
along with a small amount of two other nontrifluoromethylated
products. The first one was identified as cis-5,7-diazido-6,7-
dihydro-5H-indeno[5,6-d][1,3]dioxole 7g′, and its structure was
confirmed by X-ray diffraction (see Supporting Information (SI)
for details).13 The second product was the corresponding 2,3-
dihydro-1,3-epoxyindene 7g″ in which N3 and CF3 groups were
not incorporated (see the SI for details). Surprisingly, with 3-
vinylfuran-2-carbaldehyde (6h) as the substrate in the photo-
catalyzed tandem reaction, the unexpected diazido-trifluoro-
methylated adduct 7h was the only isolated product (49%). This
probably resulted from an overoxidation of 6h followed by in situ
addition of azide.14 Fortunately, reducing the reaction time from
2 to 1 h resulted in exclusive formation of desired cis-7i, without
detection of diazide product 7h. However, further attempts to
perform the reaction with ketones failed in this tandem reaction,
and only a complex mixture was obtained. Additionally, the
substrate with an internal alkene 6j was also suitable to deliver 7j
in 79% yield as a mixture of two diastereomers (1:1 dr). Then, we
tried to extend this three-component tandem reaction to
trimethylsilyl cyanide (TMSCN).6b Unfortunately, under the
same reaction conditions, the expected 3-trifluoroethyl-1,3-
phthalan 1-carbonitrile (7k) was not observed. Only degraded
products together with some unreacted starting materials were
a
b
Yields referred to chromatographically pure product. General
conditions: 6 (0.10 mmol), 2 (0.15 mmol), TMSN3 3a (0.50
mmol), and 4a (0.05 equiv), in CH2Cl2 (2 mL) irradiated at rt for 2 h.
c
d
e
1
1
>95:5 dr determined by H NMR. For 1 h. Determined by H
f
NMR. General conditions: 6 (0.10 mmol), 2 (0.15 mmol), TMSCN
3b (0.20 mmol), and 4a (0.05 equiv), in CH2Cl2 (2 mL) irradiated at
rt for 2 h.
recovered. However, when 2 equiv of TMSCN were used instead
of 5, we were pleased to see the formation of 7k in 48% yield.
Using these optimal conditions, electron-rich as well as electron-
poor aromatic rings were suitable substrates, affording the
corresponding phthalans 7l and 7m in respectable yields and
with an 80:20 mixture of diastereomers in favor of cis
(determined by X-ray).13 A slightly better yield and excellent
diastereoselectivity were achieved with the ortho-substituted
fluorobenzyl aldehyde 6d. Furthermore, internal alkene 6j was
successfully used in the present system, leading to the
corresponding cyano-trifluoromethylated phthalan 7o as a
mixture of four diastereomers in a ratio of 56:34:7:3. Finally,
we tested the reaction with a ketone-substituted product 6p and
were pleased to isolate the expected 1-cyano-3-trifluoro-
ethylphtalan 7p as the major product (58% yield), together
with the cyanotrifluoromethylated adduct (36% yield).
Next, using the same strategy as for phthalans, the reaction of
4-methoxy-N-(2-vinylbenzylidene)-benzene-sulfonamide (9a)
and TMSN3 (3a) led to the formation of a complex mixture of
products. However, to our delight, this novel tandem reaction
was successful when TMSCN was used, providing the desired
product 10a in excellent. This new multicomponent process
provides a rapid access to isoindoline-1-carboxylic acid, an
important scaffold in the preparation of biologically active
compounds, especially against cancer and for treatment of
B
DOI: 10.1021/acs.orglett.6b01257
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
diabetes, dyslipidemia, atherosclerosis, and obesity.15 As shown
in Scheme 3, a range of vinylbenzaldehydes reacted smoothly
and/or [Ru(bpy)3(PF6)2] 4a were also conducted. The α-azido
alcohol derivative 8h was the sole product formed. Finally, the
reaction was significantly inhibited by radical scavengers such as
TEMPO, benzoquinone (BQ), and 1,4-dinitrobenzene. On the
Scheme 3. Scope of the Photoredox-Catalyzed Reaction with
basis of the above-mentioned results as well as other reports,6−9
a
ab
,
Representative Vinylimines
plausible reaction mechanism is shown in Scheme 4. First, the
Scheme 4. Plausible Reaction Mechanism
nucleophilic addition of azide or cyanide to aldehyde or imine
results in the formation of α-hydroxy or α-amino-azide 8 and 11,
respectively. At the same time, ·CF3 is generated from
Umemoto’s reagent 2 through single electron transfer reduction
by excited-state *Ru(bpy)32+.8 Afterward, regioselective radical
addition to aldehyde 6 or imine 9 would generate a benzyl radical
3+
12 which would then be oxidized by Ru(bpy)3 to afford 13.
a
b
Yields referred to chromatographically pure product. General
conditions: 6 (0.10 mmol), 2 (0.15 mmol), TMSN3 3a (0.50
mmol), and 4a (0.05 equiv), in CH2Cl2 (2 mL) irradiated at rt for 2 h.
Finally, product 7 or 10 is formed by intramolecular trapping of
13 with a hydroxy or amino group, respectively. Based on these
first results, we assume that the kinetically favored oxy- or
aminocyclization and the slow concentration of 13 guarantee the
success of the overall tandem process.
In summary, we have described a novel visible-light photo-
redox promoted three-component tandem process for the
synthesis of CF3-containing phthalans and isoindolines from
readily accessible starting materials. This original tandem-type
process is applicable to a wide range of aldehydes and imines,
including chiral ones. The broad substrate scope makes the
method highly efficient and widely applicable, providing an easy
and entirely novel route toward variously functionalized
trifluoromethylated phthalans and isoindolines. Further explora-
tion of the reaction scope and other types of multicomponent
tandem reactions is currently under investigation.
c
d
e
1
1
>95:5 dr determined by H NMR. For 1 h. Determined by H
f
NMR. General conditions: 6 (0.10 mmol), 2 (0.15 mmol), TMSCN
3b (0.20 mmol), and 4a (0.05 equiv), in CH2Cl2 (2 mL) irradiated at
rt for 2 h.
with TMSCN, affording 10b−d with moderate to excellent
yields. Surprisingly, in the case of 10b and 10c, we observed
complete isomerization of the cis−trans mixture into the cis-
isomer after purification on silica gel. Various sulfonyl groups
were tolerated giving the corresponding cyanotrifluoro-
methylated products 10e−g in good yields.13 It is interesting
to note that, in most cases, the mixtures of diastereomers were
found to be separable by column chromatography. Finally, the
Ellman chiral N-tert-butanesulfinyl imine 9h was successfully
engaged in this tandem multicomponent reaction yielding 10h in
70% yield as a single cis-diastereomer (determined by X-ray).13
To gain more insights into the reaction mechanism, a series of
control experiments was performed. Initially, the photocatalyzed
process of 6h was monitored by 1H NMR spectroscopy (see SI
for details). The formation of α-siloxy-azide 8h was clearly
observed after 20 min. The starting material was completely
consumed 10 min later, and we observed 40% conversion of final
product 7i. This conversion continued for about 1 h until the
starting material 6h was consumed. Moreover, when the azido
alcohol 8h was isolated and subsequently submitted to
photocatalytic trifluoromethylation, only 7i was formed. It is
interesting to note that no azidotrifluoromethylated adduct 5 was
isolated, indicating that 5 is not an intermediate in this reaction.
Additionally, to validate the present photoredox-catalyzed
tandem radical reaction, experiments in the absence of irradiation
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01257.
Detailed experimental procedures and spectral data for all
new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: geraldine.masson@cnrs.fr.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.6b01257
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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(o) Schultz, D. M.; Yoon, T. P. Science 2014, 343, 1239176. (p) Koike,
T.; Akita, M. Inorg. Chem. Front. 2014, 1, 562. (q) Hopkinson, M. N.;
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ACKNOWLEDGMENTS
■
We thank ICSN for financial support and doctoral fellowships to
A.C.; G.D. thanks Labex Charm3at for a postdoctoral fellowship.
(r) Meggers, E. Chem. Commun. 2015, 51, 3290. (s) Karkas, M. D.;
̈
̈
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