Visible Light-Promoted Decarboxylative Di- A nd Trifluoromethylthiolation of Alkyl Carboxylic Acids

Article abstract of DOI:10.1002/chem.201600421

Described herein is a new and straightforward decarboxylative di- A nd trifluoromethylthiolation of alkyl carboxylic acids promoted by visible light. This approach enables the synthesis of biologically relevant alkyl SCF₂H and SCF₃ compounds from cheap and abundant carboxylic acids. The method is operationally simple, using irradiation from household light sources, and its mild reaction conditions make it tolerant of a range of functional groups. The strategy employs electrophilic phthalimide-derived di- A nd trifluoromethylthiolation reagents and exploits the ability of the imidyl radical to carry a radical chain.

Full text of DOI:10.1002/chem.201600421

DOI: 10.1002/chem.201600421
Communication
&
Photocatalysis
Visible Light-Promoted Decarboxylative Di- and
Trifluoromethylthiolation of Alkyl Carboxylic Acids
Lisa Candish+, Lena Pitzer+, Adriµn Gómez-Suµrez, and Frank Glorius*[a]
Abstract: Described herein is a new and straightforward
decarboxylative di- and trifluoromethylthiolation of alkyl
carboxylic acids promoted by visible light. This approach
enables the synthesis of biologically relevant alkyl SCF2H
and SCF3 compounds from cheap and abundant carboxyl-
ic acids. The method is operationally simple, using irradia-
tion from household light sources, and its mild reaction
conditions make it tolerant of a range of functional
groups. The strategy employs electrophilic phthalimide-
derived di- and trifluoromethylthiolation reagents and ex-
ploits the ability of the imidyl radical to carry a radical
Scheme 1. Di- and trifluoromethylthiolation of alkyl carboxylic acids.
SDS=sodium dodecyl sulfate.
chain.
direct formation of C(sp3alkyl)ÀSCF3 bonds. For example, several
methodologies have been reported for the reaction of alkyl
halides with nucleophilic SCF3 transfer reagents,[5] the electro-
philic trifluoromethylthiolation of organometallic nucleo-
philes,[6] or the activation of CÀH bonds adjacent to carbonyl
compounds.[7] Moreover, incorporation of the SCF3 group can
also be achieved through radical pathways, either via the for-
mation of the SCF3 radical[5b,8] or through the single electron
oxidation of CÀH bonds and reaction of the generated alkyl
radical with the highly toxic gas SCF3Cl,[8a,9] or, more recently,
AgSCF3.[10] Controlling the regioselectivity of such reactions is
often challenging, and the product formed is generally dictat-
ed by radical stability.
The incorporation of fluorine atoms into molecules can pro-
foundly influence their physical, chemical, and biological prop-
erties.[1] Accordingly, the systematic derivatization of drug can-
didates through the inclusion of different fluorine-containing
functional groups has become common practice in drug dis-
covery. For example, the incorporation of the trifluorome-
thylthio group is known to increase a molecule’s lipophilicity
and metabolic stability.[2] In addition, the difluoromethylthio
group has recently been found to be a highly lipophilic hydro-
gen bond donor, and is a potential lipophilic OH or NH surro-
gate.[3] Therefore, the development of mild and straightforward
methodologies, capable of tolerating a wide range of function-
al groups, for the incorporation of SCF2X (where X=H or F)
moieties would be highly desirable.
Due to their cheap and abundant nature, alkyl carboxylic
acids are desirable starting materials for the synthesis of valua-
ble compounds. In addition, their decarboxylation, which
yields the traceless by-product CO2, is an excellent method for
accessing alkyl radicals, which can be readily functionalized.
Recently, Shen and co-workers reported the silver(I)-catalyzed
decarboxylative trifluoromethylthiolation of alkyl carboxylic
acids using AgNO3 and K2S2O8 as oxidant (Scheme 1a).[11] While
Shen’s method was effective for the synthesis of 28 and 38
alkyl products, it was found to be inefficient for the prepara-
tion of 18 alkyl-SCF3 species. Since 2013, visible light-promot-
ed[12] decarboxylation has emerged as a very mild approach for
the generation of alkyl radicals from carboxylic acids.[13] As
a result, several decarboxylative processes, such as reducti-
ons,[14a,b] arylations,[14c,d] vinylations,[14e,f] alkylations,[14h,i] oxida-
tive amidation,[14j] fluorinations,[14k–m] and alkynylations[14n,o]
have been reported.
Arguably, the most efficient method for the synthesis of
alkyl di- and trifluoromethylthiolated compounds is through
the direct formation of the CÀSCF2X bond, as this circumvents
the need for initial thiol formation. To date, however, there is
only one reported method, from the group of Shen, for the
direct formation of C(sp3alkyl)ÀSCF2H bonds in b-ketoesters and
2-oxindoles, utilizing the phthalimide electrophilic reagent 1a
(see Scheme 1) developed by the same group.[4] In comparison,
there has been considerably more chemistry developed for the
[a] Dr. L. Candish,+ L. Pitzer,+ Dr. A. Gómez-Suµrez, Prof. Dr. F. Glorius
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster (Germany)
E-mail: glorius@uni-muenster.de
[+] These authors contributed equally to this work.
With the aim of further expanding the field of visible light-
promoted decarboxylative functionalization, improving the
scope of radical trifluoromethylthiolation methodologies, and
Supporting information for this article and the ORCID for one of the au-
thors are available on the WWW under http://dx.doi.org/10.1002/
chem.201600421.
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thus provide a new method for the underdeveloped field of di-
fluoromethylthiolation, herein we disclose a visible light-pro-
moted decarboxylative di- and trifluoromethylthiolation of
alkyl carboxylic acids (Scheme 1b). The method utilizes phthali-
mide derived electrophilic di-[4] and trifluoromethylthiolation[15]
reagents 1a and 1b and is efficient for the transformation of
18, 28, and 38 carboxylic acids, under very mild (reactions run
at room temperature using low-energy visible-light irradiation)
and redox neutral conditions, thus not requiring the use of ad-
ditional oxidants.
Table 1. Scope of the visible light-promoted decarboxylative trifluorome-
thylthiolation.[a]
Investigations commenced with the reaction of tertiary 1-
adamantanecarboxylic acid and electrophilic SCF3-reagent 1b
(2 equiv) in the presence of Cs2CO3 (1.5 equiv) and 2 mol% of
photocatalyst [Ir(dF(CF3)ppy)2(dtbbpy)]PF6 (PCI, dF(CF3)ppy=2-
(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine, dtbbpy=4,4’-
di-tert-butyl-2,2’-bipyridine) in 1,2-dichloroethane (DCE) under
irradiation with blue LEDs (lmax =455 nm). Pleasingly, we ob-
served the formation of the desired trifluoromethylthiolated
adamantyl product 3a in 45% yield (see the optimization table
in Section 3 of the Supporting Information). Changing the sol-
vent to chlorobenzene afforded the product in 66% yield,
whereas using either 1.0 or 0.2 equiv of cesium benzoate
(CsOBz) gave improved, comparable yields of 70% and 71%
respectively. At this point, alternative trifluoromethylthiolation
reagents were explored; however, the phthalimide reagent 1b
provided the product in highest yield (for details, see the Sup-
porting Information). The reaction displayed a high solvent de-
pendency, with fluorobenzene found to give the highest yield
(92%).
Over the course of the optimization, bis-trifluoromethylthio-
lation was observed under a number of conditions, presuma-
bly resulting from hydrogen atom abstraction from the prod-
uct by the intermediate phthalimidyl radical, and subsequent
SCF3-transfer from reagent 1b to the resultant alkyl radical (for
further details, see the Supporting Information). To suppress
this undesired pathway, the addition of a sacrificial hydrogen
atom donor, mesitylene (4) or 3-(methyl) toluate (5), was inves-
tigated. Both additives were found to modestly increase the
yield of the trifluoromethylthiolated adamantyl product 3a
(Table 1, conditions A). However, these additives were found to
have a much more significant effect for other substrates and
thus were used when exploring the reaction scope.
[a] Reaction conditions: alkyl carboxylic acid 2 (0.3 mmol), reagent 1b
(2 equiv), PCI (2 mol%), 4 or 5 (2 equiv) CsOBz (0.2 equiv) fluorobenzene
(6 mL), RT, 4 h, blue LEDs (lmax =455 nm). Percentage values show yields
of isolated products. [b] Conditions B, same conditions as [a] but using
PCII (5 mol%) instead of PCI. [c] Reaction performed with 3 mol% PCI.
With the optimized conditions in hand, the generality of our
light-mediated decarboxylative trifluoromethylthiolation proto-
col was investigated (Table 1). The reaction was found to pro-
ceed well when employing tertiary, secondary, and benzylic
carboxylic acids (3a–h). Gratifyingly, the trifluoromethylthiola-
tion of primary acids was also found to proceed smoothly
when 3 mol% of the photocatalyst was used, providing the
products in moderate to good yields (3i–l). The reaction was
found to be tolerant of protected amines (3 f and 3k), aryl and
alkyl halides (3g and j), terminal alkenes (3h), and ketones
(3l). Following this, the trifluoromethylthiolation of a-amino
and a-oxo acids was also explored, with the products isolated
in acceptable yields of 59% and 71%, respectively (3m and
3n). Given the mild nature of visible light-promoted processes,
they have the potential to become an important tool for the
late-stage functionalization of advanced synthetic intermedi-
ates.[16] Heteroarenes are prevalent in many biologically rele-
vant molecules, though they are also susceptible to nucleophil-
ic radical addition.[17] Therefore, to determine whether our pro-
tocol was tolerant of heteroarenes, carboxylic acids containing
thiophene, pyridine, and indole motifs were subjected to the
reaction conditions. Gratifyingly, they provided the desired
products in moderate to high yields (3o–q) with no by-prod-
ucts resulting from radical addition to the heteroarenes ob-
served.
During optimization of the photocatalyst, we found that
metal-free trifluoromethylthiolation of carboxylic acids could
be achieved employing the strongly oxidizing organic dye 9-
mesityl-10-methylacridinium perchlorate (PCII). Organic dyes
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are inexpensive photosensitizers that have found application
in several visible light-promoted decarboxylative processe-
s.[14a,b,18] Using the standard reaction conditions, but with
5 mol% of PCII instead of PCI, 1-adamantyl carboxylic acid
(2a) was converted to the corresponding SCF3-product 3a in
86% yield after 4 h (cf. 92% with PCI) (Table 1, conditions B).
With these metal-free conditions, the trifluoromethylthiolation
protocol was examined using 18, 28, and 38 carboxylic acids.
Gratifyingly, using PCII, the tertiary SCF3 product 3b was isolat-
ed in 90% yield (cf. 94% with PCI). Secondary and a-oxo acids
also afforded the corresponding products 3e, 3 f, and 3n in
64%, 36%, and 60% isolated yield, respectively (cf. 81%, 76%,
and 71% with PCI). Unfortunately, PCII failed to effect forma-
tion of the primary trifluoromethylthiolated product (3j),
which was isolated in only 7% yield.
Figure 1. Proposed decarboxylative di- and trifluoromethylthiolation mecha-
nism. Phth=phthalimide, counterions omitted for clarity.
that should promote a thermodynamically favorable single-
electron oxidation of the alkyl carboxylate (hexanoate ion
Er1ed = +1.16 V vs. SCE in CH3CN).[19] Decarboxylation of the car-
=
2
boxylate radical would afford the corresponding alkyl radical,
and direct SCF2X (X=H or F) transfer from the phthalimide re-
agents (1a,b) should provide the desired product and the
phthalimidyl radical. Oxidation of the Ir(II) complex through
single-electron transfer (SET) from the phthalimidyl radical
would then regenerate the photocatalyst and furnish the
phthalimide anion. Alternatively, given that imidyl radicals can
act as competent chain carriers,[20] following photoinitiation,
the phthalimidyl radical may participate in a hole-catalyst
chain process,[21] whereby it oxidizes a second alkyl carboxylate
to provide the alkyl radical following decarboxylation. Subse-
quent SCF3 group transfer reforms the phthalimidyl radical and
provides the desired product. A radical chain mechanism has
not yet been proposed for a visible light-promoted decarboxy-
lative process, hence, the reaction’s quantum yield was mea-
sured (quantum yield: F=1.7, see the Supporting Informa-
tion).[22] The quenching fraction (Q) was also measured to de-
termine what fraction of the excited IrIII* complexes participate
in productive electron transfer processes. Using the method
described by Yoon,[22] the quenching fraction was found to be
0.14, suggesting that 86% of excited complexes relax through
radiative decay. The chain length was therefore determined (by
dividing the quantum yield by the quenching fraction (F/Q))
to be 12, suggesting a mechanism involving a smart initiated
hole-catalyst chain.[21]
Having explored the scope of the trifluoromethylthiolation
protocol, we turned our attention to the decarboxylative di-
fluoromethylthiolation (Table 2). We were pleased to find that
Table 2. Scope of the decarboxylative difluoromethylthiolation.[a]
[a] Reaction conditions: alkyl carboxylic acid 2 (0.3 mmol), reagent 1a
(2 equiv), PCI (2 mol%), 4 or 5 (2 equiv) CsOBz (0.2 equiv) fluorobenzene
(6 mL), RT, 14 h, blue LEDs (lmax =455 nm). Percentage values show yields
of isolated products. [b] Reaction performed with 3 mol% PCI.
In conclusion, we have developed a new operationally
simple methodology for the visible light-promoted di- and tri-
fluoromethylthiolation of alkyl carboxylic acids. Mechanistic
analysis suggests a smart photoinitiated hole-catalyst chain is
operational. The method is efficient for tertiary, secondary, and
primary alkyl carboxylic acids, and its mild nature makes it tol-
erant to a range of functionalities, including heteroarenes. In
addition, an alternative metal-free protocol for the efficient tri-
fluoromethylthiolation of secondary and tertiary carboxylic
acids was also developed. Moreover, our decarboxylative di-
fluoromethylthiolation process represents only the second re-
ported method for the direct formation of C(sp3alkyl)ÀSCF2H
bonds and should act as an enabling tool for late-stage difluor-
omethylthiolation reactions.
employing the reaction conditions A, although with longer re-
action times (14 h), the desired products were isolated in simi-
lar yields to the corresponding alkyl-SCF3 analogs. Tertiary di-
fluoromethylthiolated products were isolated in excellent
yields (6a and 6b), whereas secondary (6c), primary (6d), and
a-oxo products (6e) were afforded in good yields. As expect-
ed, the mild reaction conditions were found to be tolerant of
a number of functional groups, highlighting the value of this
process to the underdeveloped field of C(sp3alkyl)ÀSCF2H bond
formation.
Based on the mechanistic research previously reported for
visible light-promoted decarboxylative functionalizations, and
the results of Stern–Volmer luminescence quenching studies
(see the Supporting Information), we propose a mechanism for
the di- and trifluoromethylthiolation reactions as shown in
Figure 1. Photoexcitation of the IrIII photocatalyst produces
a strong oxidant (E1red([IrIII*/IrII])= +1.21 V vs. SCE in CH3CN)
=
2
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Acknowledgements
We thank M. N. Hopkinson, J. B. Ernst, R. Honeker, M. Teders
and E. A. Standley (all WWU Münster) for helpful discussions
and technical assistance. This work was generously supported
by the Deutsche Forschungsgemeinschaft (Leibniz Award) and
the Alexander von Humboldt Foundation (L.C.).
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Keywords: decarboxylation
photocatalysis · trifluoromethylthiolation · visible light
·
difluoromethylthiolation
·
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Published online on February 17, 2016
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