Photocatalytic Aminodecarboxylation of Carboxylic Acids

Article abstract of DOI:10.1002/ejoc.201600620

Aminodecarboxylation of unactivated alkanecarboxylic acids has been accomplished utilizing an organic photocatalyst. This operationally simple reaction utilizes readily available carboxylic acids to chemoselectively generate reactive alkyl intermediates that are not accessible via conventional two-electron pathways. The organic radical intermediates are efficiently trapped with electrophilic diazo compounds to provide aminated alkanes.

Full text of DOI:10.1002/ejoc.201600620

DOI: 10.1002/ejoc.201600620
Communication
Photocatalysis
Photocatalytic Aminodecarboxylation of Carboxylic Acids
Simon B. Lang,[a] Kaitie C. Cartwright,[a][‡] Richard S. Welter,[a][‡] Theresa M. Locascio,[a] and
Jon A. Tunge*[a]
Abstract: Aminodecarboxylation of unactivated alkanecarbox- termediates that are not accessible via conventional two-elec-
ylic acids has been accomplished utilizing an organic photocat- tron pathways. The organic radical intermediates are efficiently
alyst. This operationally simple reaction utilizes readily available trapped with electrophilic diazo compounds to provide amin-
carboxylic acids to chemoselectively generate reactive alkyl in- ated alkanes.
Introduction
Aliphatic amines are ubiquitous across many chemical disci-
plines, especially in synthetic organic chemistry where they
serve as versatile building blocks.[1] For example, N-alkylation
with organic halides is one of the top ten reactions performed
by medicinal chemists.[2] While substitution provides rapid ac-
cess to a wide variety of amines, these reactions could un-
doubtedly be improved if more stable, less expensive and less
toxic alkylating agents could be utilized in lieu of organic hal-
ides. The formation of C–N bonds using abundant carboxylic
acids as alkylating agents would be a first step toward achiev-
ing this goal. Increased functional group diversity, higher atom
economy, and trivial removal of the only stoichiometric byprod-
uct (CO2) are potential additional advantages of replacing alkyl
halides with carboxylic acids.
Classically, aliphatic amines can be prepared directly from
carboxylic acids via the Schmidt reaction or variants of the Cur-
Scheme 1. Strategies for aminodecarboxylation.
tius rearrangement.[3] We postulated that the radical decarboxy-
lation of aliphatic carboxylic acids would be a complementary
Results and Discussion
umpolung approach to regiospecific C–N bond formation that
would allow the formation of amines as well as more oxidized
Optimization studies began with aliphatic carboxylic acids
hydrazine derivatives.[4,5] The trapping of alkyl radicals gener-
which do not undergo anionic decarboxylation. Given that alkyl
ated by decarboxylation of pre-formed activated Barton esters
carboxylates typically have oxidation potentials of +1.1–1.3 V
with electrophilic diazirines and nitroso compounds provided
vs. SCE,[10] the Fukuzumi acridinium family[11,8p] of photocata-
important precedent.[6] Unfortunately, such reactions suffer
lysts with excited state reduction potentials > 2.0 V vs. SCE
from a limited substrate scope, competitive byproduct forma-
seemed promising. Electrophilic dialkyl azodicarboxylates[12] are
tion, and require the preactivation of the carboxylic acid
effective radical traps that can be subsequently transformed to
(Scheme 1). Our group and others have recently utilized photo-
the corresponding hydrazine, amine or protected amide.[4e,4f,14]
redox catalysis[7] to site-specifically generate alkyl radicals di-
Thus, diisopropyl azodicarboxylate (DIAD) was chosen to forge
rectly from carboxylic acids.[8] Thus, we anticipated that photo-
the desired C–N bond upon reaction with the saturated ali-
redox catalysis would obviate the need for stoichiometric pre-
phatic acid 1. Irradiation of an Ar-sparged solution of 1, the
activation and enable the use of carboxylic acids as traceless
non-nucleophilic base 1,8-diazabicylo[5.4.0]undec-7-ene (DBU),
activating groups for selective C–N bond formation.[9]
DIAD, and 1 mol-% of the photocatalyst (Mes-Acr-Ph) with
450 nm LEDs provided the desired product 2a [Equation (1)].
While initial studies were conducted with one equivalent of
DBU, brief optimization revealed that the reaction still reached
complete conversion when a sub-stoichiometric amount of
DBU was added. When DIAD was replaced with di-tert-butyl
azidocarboxylate or bis(2,2,2-trichloroethyl) aziodocarboxylate
[a] Department of Chemistry, The University of Kansas,
2010 Malott Hall, 1251 Wescoe Hall Drive, Lawrence KS 66045, USA
E-mail: tunge@ku.edu
https://tungegroup.ku.edu
[‡] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600620.
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the reaction became sluggish and did not reach completion, could also be performed on a 5 mmol scale but required 24 h
even after extended reaction times. Lastly, no conversion to 2a to reach completion when conducted under more convenient
was detected by GC/MS analysis of the crude reaction mixture concentrated conditions (2c). Aminodecarboxylation also oc-
when light or photocatalyst were omitted from the standard curred at a sterically congested tertiary benzylic position (2d).
reaction conditions.
Additionally, the remote tertiary center of the phenolic ether-
containing hypolipidemic agent, Gemfibrozil, was successfully
aminated (2e). A primary carboxylic acid (2f) required extended
reaction times to reach complete conversion, presumably due
to the slow kinetics of formation of the corresponding primary
radical. An unconjugated cyclic alkene (2g) was also compatible
with the reaction conditions. When 1-cyclohexene acetic acid
was employed, selective amination of the primary over the sec-
ondary allylic position was observed (2h), suggesting that the
reaction is influenced by steric hindrance. para-Amino (2i),
-chloro (2j), and -alkyl (ibuprofen, 2k) substituted phenylacetic
acids afforded the desired aminated products. Additional func-
tional groups including an N-acyl-protected amine (2l) and a
catechol derivative (2m) participated in aminodecarboxylation.
(1)
The scope of the metal-free aminodecarboxylation was then
explored (Scheme 2). Both cyclic (2a) and acyclic (2b, 2c) sec- Heteroaromatics such as the 3-thiophenyl (2n) and N-methyl-
ondary alkyl carboxylic acids were readily aminated in high indol-3-yl derivative 2p were also tolerant of the reaction condi-
yields under the standard reaction conditions. The reaction tions. Indomethacin, an anti-inflammatory 3-indolacetic acid
Scheme 2. Substrate scope [a] isolated yields [b] 5 mmol scale, 2 blue LEDs [0.38M] [c] from 1-cyclohexene-1-acetic acid.
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bearing substituents at the 1,2 and 5 positions was aminated in oxylic acid can quench this charge providing an alkyl carboxyl-
good yield (2o). Finally, enoxolone, which contains a secondary ate to enter the catalytic cycle.
alcohol and an α,ꢀ-unsaturated ketone was well tolerated (2q).
It should be noted that all but one of the carboxylic acids em-
ployed are commercially available. Conversely, only four of the
corresponding alkyl bromides or iodides are available, high-
lighting an advantage of utilizing carboxylic acids for alkylation
chemistry.
Next, experiments were conducted to support the formation
of radical intermediates. First, when cyclopropane acetic acid
was subjected to the standard reaction conditions, the ring-
opened product 2r was isolated in a 35 % yield after 24 h
[Equation (2)]. Second, the addition of two equivalents of
TEMPO as a radical trap afforded 2s in 86 % yield [Equation (3)].
Further analysis of the products in Equations (2)–(4) revealed
that decarboxylation results in the site-specific contra-thermo-
dynamic formation of radicals at the carbon bearing the carb-
oxylate without isomerization to a more stable species prior to
Scheme 3. A plausible mechanism.
amination (e.g. 2a–c, 2f, 2g, 2i, 2l).
Conclusion
A metal-free protocol for the site-specific amination of a wide
variety of alkanes utilizing carboxylic acids as traceless directing
groups was developed. This operationally simple method util-
(2)
izes commercially available reagents and offers a potentially at-
tractive alternative to alkyl halides for site-selective C–N bond
formation.
Acknowledgments
Support for this research was provided by the National Science
Foundation (NSF), USA (grant number CHE-1465172) and the
Kansas Bioscience Authority. Support for the NMR instrumenta-
(3)
tion was provided by NIH Shared Instrumentation grant number
S10RR024664 and NSF Major Research Instrumentation grant
number 0320648.
Although substituted hydrazines are found in a range of
drugs and insecticides, they are also versatile intermediates
used in the synthesis of azapeptides and a variety of hetero-
Keywords: Homogeneous catalysis · Photocatalysis ·
Amination · Carboxylic acids
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An idealized mechanism is presented in Scheme 3. Irradia-
tion of the photocatalyst provides the oxidizing species Mes-
Acr-Ph* which removes an electron from an alkyl carboxylate
(generated with the strong base DBU). This triggers radical de-
carboxylation to provide an alkyl radical and the reduced pho-
tocatalyst. The resulting alkyl radical is trapped by DIAD to pro-
vide the N-centered radical species A which undergoes reduc-
tion by Mes-Acr-Ph·to regenerate the ground state photocata-
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Received: May 14, 2016
Published Online: ■
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Communication
Photocatalysis
Aminodecarboxylation of unactivated
alkanecarboxylic acids has been ac-
complished utilizing an organic photo-
catalyst. This operationally simple reac-
tion utilizes readily available carboxylic
acids to chemoselectively generate re-
active alkyl intermediates that are not
accessible via conventional two-elec-
tron pathways.
S. B. Lang, K. C. Cartwright,
R. S. Welter, T. M. Locascio,
J. A. Tunge* .......................................... 1–5
Photocatalytic
ation of Carboxylic Acids
Aminodecarboxyl-
DOI: 10.1002/ejoc.201600620
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