Decarboxylative alkynylation and cyanation of carboxylic acids using photoredox catalysis and hypervalent iodine reagents

Article abstract of DOI:10.2533/chimia.2017.226

Alkynes and nitriles are important functional groups that serve as versatile building blocks in organic synthesis and find applications in material and medicinal sciences. A convenient and straightforward access to both classes of compounds under mild conditions is, therefore, highly desirable. Herein, we disclose the decarb-oxylative alkynylation and cyanation of broadly available carboxylic acids using photoredox catalysis and hyper-valent iodine reagents. Choices of both catalysts and reagents were crucial. Computational and experimental studies revealed two different possible mechanisms that are dictated by the oxidation potential of the reagents: radical for alkynylation, ionic for cyanation.

Full text of DOI:10.2533/chimia.2017.226

226 CHIMIA 2017, 71, No. 4
Laureates: Junior Prizes of the sCs faLL Meeting 2016
doi:10.2533/chimia.2017.226
Chimia 71 (2017) 226–230 © Swiss Chemical Society
Decarboxylative Alkynylation and
Cyanation of Carboxylic Acids using
Photoredox Catalysis and Hypervalent
Iodine Reagents
§
Franck Le Vaillant and Jérôme Waser*
§
SCS-DSM Award for best poster presentation in Organic Chemistry
Abstract: Alkynes and nitriles are important functional groups that serve as versatile building blocks in organic
synthesis and find applications in material and medicinal sciences. A convenient and straightforward access to
both classes of compounds under mild conditions is, therefore, highly desirable. Herein, we disclose the decarb-
oxylative alkynylation and cyanation of broadly available carboxylic acids using photoredox catalysis and hyper-
valent iodine reagents. Choices of both catalysts and reagents were crucial. Computational and experimental
studies revealed two different possible mechanisms that are dictated by the oxidation potential of the reagents:
radical for alkynylation, ionic for cyanation.
Keywords: Decarboxylative alkynylation · Decarboxylative cyanation · Mechanistic studies · Hypervalent iodine
reagent · Photoredox catalysis
Th e chemistry surrounding alkynes and
alkynylations and cyanations would be
to merge hypervalent iodine reagents and
nitriles is extraordinary rich and useful.[
1,2]
In particular, aliphatic alkynes or nitriles visible light induced photoredox catalysis
have a considerable potential as building (Scheme 1). Indeed, photoredox is a topic
blocks in organic synthesis and are also of growing interest, especially for gener-
broadly applicable to many other areas of ating radicals with excellent chemo-selec-
chemistry, ranging from pharmaceutical tivity under mild conditions.[ As trace-
and medicinal sciences to material sci- less activating group and broadly available
10]
[
3]
ences. As a result the synthesis of these substrates, carboxylic acids are a particu-
compounds is extremely valuable. Th e in- larly attractive class of radical precursors.
Franck Le Vaillant was born and troduction of these two moieties usually Furthermore, their decarboxylation under
raised in France. He studied chemistry at takes advantage of the acidity of the sp or- photoredox conditions has already been
[
11]
[9a,12]
the Ecole Nationale Supérieure de Chimie bitals, leading to acetylide and cyanide nu- reported. Introducing an alkyne
or
de Montpellier (ENSCM, France). He then cleophilic substitutions or additions to an a nitrile[ from a carboxylic acid requires
13]
moved to Stanford University (USA) where electrophilic center.[
1,2,4]
Nevertheless, in several steps using established methods.
he joined the Trost group as a visiting stu- some substrates, disconnections using this Th erefore a one-step decarboxylative al-
dent researcher, working on a unique in- innate reactivity are difficult. In this case, kynylation or cyanation under mild condi-
tramolecular palladium-catalyzed alkyne– the polarity of the functional group must tions would provide a faster access. Herein,
alkyne coupling to generate strained mac- be reversed (umpolung), which Seebach we report the visible light photoredox me-
robicyclic enynes. Since November 2014, showed is a useful and efficient concept.[
5]
diated decarboxylative alkynylation and
he started his PhD studies at the Ecole In this regard, hypervalent iodine reagents cyanation of carboxylic acids using EBX
Polytechnique Fédérale de Lausanne are of considerable interest because they and CBX reagents.
(
EPFL, Switzerland) in the group of Prof. allow the umpolung of many functional
In 2015, we began investigating the
Jerome Waser. His current research topic groups, such as alkyne and nitrile. Cyclic photoredox-mediated decarboxylative al-
involves hypervalent iodine reagents and versions of hypervalent iodine reagents, kynylation of aliphatic carboxylic acids
photoredox catalysis.
EthynylBenziodoXolone (EBX) and using EBX reagents.[ N-Cbz protected
14]
CyanoBenziodoXolone (CBX), first made proline 4a was used as model substrate to
[
6]
[7]
by Ochiai and Zhdankin, have been de- optimize the reaction (Scheme 2). Silyl-
[8]
veloped and are now broadly used. Th ese
protected alkynes are synthetically very
reagents, in particular EBX reagents, have useful, due to the easily removable protect-
also demonstrated great potential for func- ing group that gives the most versatile ter-
tionalizing carbon-centered radicals.[
7b,9]
minal alkynes. In that regard, we focused
*
Correspondence: Prof. Dr. J. Waser
Laboratory of Catalysis and Organic Synthesis
Ecole Polytechnique Fédérale de Lausanne
EPFL SB ISIC LCSO, BCH 4306
CH-1015 Lausanne
As many difficult alkynylation and cyana- our efforts on optimizing the transfer of
tion reactions occur at high temperature TI PS-protected alkyne. Using blue LEDs
[4]
or require transition metal catalysis, an as light source, we were delighted to iso-
elegant way of achieving non favorable late 92% yield of the alkynylated product
E-mail: jerome.waser@epfl.ch

Laureates: Junior Prizes of the sCs faLL Meeting 2016
CHIMIA 2017, 71, No. 4 227
were not successful in this transformation.
We then started to explore the scope
(
Fig. 2) of both reactions. Both Cbz and
Boc proline were converted into alkynes
a and 5b in 90% yield at 0.3 mmol
5
scale. Te trahydroquinoline 5c was isolat-
ed in 87% yield as a single regioisomer.
α-Oxy acids were also suitable, as shown
by the conversion of tetrahydropyran-
2
-carboxylic acid and TH F-2-carboxylic
acid to 5d (60% yield) and 5e (quantita-
tive). Hydrocarbon carboxylic acids are
more difficult to functionalize because of
the higher oxidation potential of the cor-
responding carboxylate,[ and we were
17]
pleased to find that adamantane carboxylic
acid was smoothly converted to alkyne 5f
in the presence of 2 mol% of photocata-
lyst. Cyclopentane carboxylic acid yield-
Scheme 1. Our strategy for the decarboxylative alkynylation and cyanation of carboxylic acids.
ed 5g (64%) under the same conditions.
Th e scope of reagents was also broad and
we found that silyl- (5h, 78%), aryl-
5
a after only 5 h of irradiation using 1.5 ated through the combination of water and
equiv. of TI PS-EBX (1), 3 equiv. of cesium
an in situ formed iminium. Cesium ben-
benzoate and 1.0 mol% of photocatalyst zoate was still the base of choice for this
Ir(dF(CF )ppy )dtbbpyPF (3) in a 0.2 M transformation, however a lower amount
(
5i, 88%) and alkyl- (5j, 77%) substi-
tuted alkynes could be transferred. Finally,
3
2
6
natural light can also promote the reac-
tion without any loss of yield (89%) and
the product can be easily converted to tri-
DCE solution. Screening various bases (1.5 equiv.) could be used in comparison
showed the superiority of cesium salts, es- to the previously developed alkynylation
pecially cesium carboxylates. Fluorinated (3.0 equiv.)
azole 8 after TB AF deprotection followed
phenylpyridinato ligands (C-N ligands)
Th e structure of the alkyne and nitrile
by Huisgen 1,3 dipolar cycloaddition with
for the iridium-based photocatalyst were sources was then investigated (Fig. 1). Th e
benzyl azide.
found to be crucial for the reaction, while benziodoxolone core was found to be su-
We then turned our attention to the
the classic Ru(bpy) Cl and fac-Ir(ppy) perior to other structures: 1b gave only a
3
2
3
scope of cyanation (Fig. 3). It was broader
for amino acids (15 examples) than the
alkynylation reaction. Various carbamate
protecting groups could be used (6a–6c,
catalysts were unsuccessful for this trans- 37% yield of 5a, monovalent iodoalkyne
formation.[
15]
1c was, surprisingly, a suitable reagent
Encouraged by this newly developed (82%), whereas no product was obtained
alkynylation, we wondered if another type with either the bromoalkyne 1d or the sul-
8
6–92%), but also an electron-rich benzyl
3
of Csp -Csp coupling could be achieved fone 1e. Finally, acyclic reagent 1f was not
protecting group is suitable for this trans-
under similar photoredox conditions. Th is
suitable for this transformation. A similar
formation (6d, 43%). Free alcohols are tol-
erated, as 6e was isolated in 90% yield. In
addition to proline derivatives, piperidine
and tetrahydroquinoline derivatives are
led to our investigation of decarboxylative trend was observed for cyanation: the es-
[
16]
cyanation (Scheme 2). After optimiza- ter moiety of the reagent is crucial, as 2b
tion (Scheme 2), substrate 4a was suc- did not furnish the expected product, giv-
cessfully converted into the corresponding ing instead TH F-2-carbonitrile. Changing
also well suited for this cyanation (6f, 6g,
nitrile 6a after 4.5 h of blue LED irradia- the stereoelectronics of the CBX core with
7
2%, 65% respectively). N-Cbz-protected
tion using 1.5 equiv. of CBX reagent 2 as a an electron-withdrawing group (2c/2d) or
lysine was converted to the corresponding
cyanide source and 1 mol% of photocata- electron-donating group (2e) improved
nitrile 6h in 83% yield. Primary, secondary
lyst 3. TH F was found to be the best sol-
neither yield nor reaction time. No nitrile
vent. Molecular sieves were important to product was found with acyclic reagent 2f.
avoid the formation of the hemiaminal side Finally, classical cyanide sources, e.g. to-
product 7, which we postulate was gener- syl-cyanide, bromocyanide or iodocyanide
and tertiary radicals are all reactive towards
CBX, leading for example to 6i (66%), 6j
(86%), 6k (49%). Different substituents are
tolerated in the α position of the radical, as
shown with valine derivative 6l (80%) or
methionine derivative 6m (66%). We were
pleased to find out that dipeptides (6n, 6o,
5
6%, 55% respectively) and α-oxy-acids
(6p, 6q, 70%, 66% respectively) were also
suitable substrates for cyanation. However,
thio-acids did not lead to any nitrile prod-
uct, likely because of their ability to be eas-
ily oxidized. Hydrocarbon aliphatic acids
were not transformed well (<20 %), yield-
ing mostly the corresponding anhydrides
(
along with mixed anhydrides formed with
cesium benzoate). Some valuable inter-
mediates in drug syntheses (Vildagliptin,
Idazoxan) can also be synthesized using
our new transformation.[
16,18,19]
Th e cata-
lyst loading can be decreased, as only 0.1
mol% of photocatalyst 3 was required to
Scheme 2. Optimized conditions for the decarboxylative alkynylation and cyanation.

228 CHIMIA 2017, 71, No. 4
Laureates: Junior Prizes of the sCs faLL Meeting 2016
obtain 0.60 mmol of the expected nitrile 6a
1 mmol scale, 60% yield, 48 h), meaning a
(
turnover of 600. Furthermore, this photore-
dox-mediated cyanation can be promoted
by sunlight in only 4 h irradiation (90 %).
Comparing the two reactions reveals
that photocatalyst 3 can be used in both
[
20]
cases and the best structure of the hy-
pervalent iodine reagent is the most simple
benziodoxolone core. However, broader
scope for amino acids is obtained in the
cyanation reaction, along with a lower re-
activity in the case of acids not stabilized
by a heteroatom in the α position. Another
difference is the formation of hemiaminal
side product 7, which was suppressed us-
ing molecular sieves in the cyanation re-
action, whereas this side product was not
observed in the alkynylation. Th ese results
led us to wonder if the two described reac-
tions might follow two divergent mecha-
3
nistic pathways. For these two Csp -Csp
couplings, a similar mechanism based on
the reactivity of the triple bond towards
C-centered radicals (Scheme 3, section a)
could be envisioned. In the seminal work
of Li on silver-catalyzed decarboxylative
alkynylation,[ the proposed mechanism
involved α-addition of the C-centered radi-
cal (intermediate B) to the EBX reagents,
followed by β-elimination leading to the
Fig. 1. Screening of alkynylating and cyanating reagents.
9a]
expected alkyne product. To the best of our
knowledge, nodetailedmechanisticstudies
have been carried out to support this mech-
anism. Th erefore, we started exploring the
mechanism of both transformations, com-
putationally[ and experimentally. It was
21]
found that the decarboxylative alkynyl-
ation and cyanation most likely proceed
through divergent mechanisms. Th e pho-
III
tocatalytic cycle is identical (Ir 3 and 3*,
II red
and Ir 3 as catalytic species), involving
the same cesium carboxylate A as the first
Fig. 2. Selected examples of the scope of the photoredox catalyzed decarboxylative alkynylation.
quencher. Indeed, Stern-Volmer studies
showed that A is an excellent quencher of
3
*, while no quenching was observed with
[
16,22]
CBX.
Because of the high reactivity
com-
of the possible intermediates, DFT
putations were undertaken for the alkynyl-
ation. Th e commonly proposed intermedi-
ate a obtained after α-addition was not
1
located, thus excluding this pathway. As
hypervalent iodine reagents are known to
be oxidants, a single electron transfer be-
tween TI PS-EBX and the α-amino radical
B was also calculated, and appeared fea-
sible (11.3 kcal/mol, Ta ble 1). However the
collapse of radical anion intermediate d1
into the acetylide and carboxyl radical C is
highly disfavored (32.8 kcal/mol). Finally,
alkyne transfer happens via a 3-center tran-
sition state involving the iodine atom. It
could involve the carbon in the α position,
leading to a concerted mechanism with a
transition state b (17.2 kcal/mol) or the
TS
carbon in the β position (transition state
c
16.8 kcal/mol), both pathways could Fig. 3. Scope of the photoredox catalyzed decarboxylative cyanation.
TS

Laureates: Junior Prizes of the sCs faLL Meeting 2016
CHIMIA 2017, 71, No. 4 229
Table 1. Theoretically calculated energy in kcal/mol of transition states in the alkynylation and
cyanation
Acknowledgements
We thank EPFL and European Research
Council for financial support (ERC; Starting
Alkynylation
X = CSiiPr3
Cyanation
X = N
Grant i To ols4MC,
number 334840). Th e
Laboratory for Computational Molecular
Design at EPFL is acknowledged for providing
computational resources.
Tr ansition state b energy
17.2
14.3
T
S
Tr ansition state c energy
16.8
16.3
T
S
Received: January 27, 2017
Tr ansition state d energy
11.3 then 32.8
2.4 then 9.4
T
S
[
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Eds. F. Diederich, P. J. Stang, R. R. Ty kwinski,
On the other hand, CBX bears a more cal B is the less energy demanding pathway
polarized functional group compared to (2.4 kcal/mol for d and 9.4 kcal/mol for
1
TI PS-EBX. Cyclic voltammetry for CBX
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2
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1
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Scheme 3. Tentative mechanism for the photoredox catalyzed decarboxylative alkynylation and cyanation reactions.

230 CHIMIA 2017, 71, No. 4
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