Cobalt-Catalyzed C(sp2)-H Carbonylation of Amino Acids Using Picolinamide as a Traceless Directing Group

Article abstract of DOI:10.1021/acs.orglett.1c00660

Herein we report an efficient protocol for the C(sp2)-H carbonylation of amino acid derivatives based on an inexpensive cobalt(II) salt catalyst. Carbonylation was accomplished using picolinamide as a traceless directing group, CO (1 atm) as the carbonyl

Full text of DOI:10.1021/acs.orglett.1c00660

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Letter
Cobalt-Catalyzed C(sp2)−H Carbonylation of Amino Acids Using
Picolinamide as a Traceless Directing Group
Lukass Lukasevics, Aleksandrs Cizikovs, and Liene Grigorjeva*
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ABSTRACT: Herein we report an efficient protocol for the C(sp2)−H
carbonylation of amino acid derivatives based on an inexpensive cobalt(II) salt
catalyst. Carbonylation was accomplished using picolinamide as a traceless
directing group, CO (1 atm) as the carbonyl source, and Co(dpm)2 as the
catalyst. A broad range of phenylalanine derivatives bearing diverse functional
groups were tolerated. Moreover, the method can be successfully applied for
the C(sp2)−H carbonylation of short peptides thereby allowing access for
peptide late-stage carbonylation.
catalyzed C−H allylation of a tryptophan moiety in structurally
mino acids and peptides play an essential role in
Amedicinal chemistry, synthetic organic chemistry, and complex peptides (Scheme 1a).8
C−H bond carbonylation using transition metal (TM)
biotechnology; therefore, direct functionalization of amino acid
derivatives and peptides via C−H bond activation has attracted
immense interest during the past decade.1 In addition, the late-
stage modification of peptides has a tremendous potential to
promote the development of innovative peptide drugs.2
Progress on direct C−H bond functionalization of amino
acid derivatives has been achieved mainly using late transition
metal catalysis.1 The functionalization of amino acid C(sp3)−
H bonds is relatively well-developed allowing installation of
various functional groups both in the α-position3 and on the
amino acid side chain.1e However, methods for amino acid side
chain C(sp2)−H bond functionalization are less explored,
particularly for phenylalanine derivatives.4 Therefore, the
development of new synthetic methods for the site selective
modification of phenylalanine derivatives is of great interest.
In the past decade, notable progress has been achieved with
the development of C−H functionalization methodology using
cobalt as an alternative to precious transition metal catalysts.5
The use of cobalt(II) salt catalysts in combination with
bidentate directing groups has been proven to be an effective
strategy for various C−H bond transformations.6 Despite the
progress made so far, substrate diversity that can be used for
C−H functionalization under cobalt catalysis is still under-
developed when compared to noble transition metal catalysis.5
To the best of our knowledge, only two examples of the cobalt-
catalyzed C−H functionalization of amino acids or peptides
have been reported so far. In 2017, Correa and co-workers
demonstrated cobalt-catalyzed selective arylation of glycine
esters and peptides through a radical pathway.7 In 2019, the
Ackermann group reported the first example of cobalt(III)-
catalysis is an efficient tool to introduce a carbonyl group, and
it gives access to various valuable compounds. In recent years,
a number of carbonylation methods have been developed using
cobalt catalysis.9 However, most of them can only be applied
to aliphatic amide or benzamide type substrates. To date, there
are no literature examples for direct carbonylation of
phenylalanine type substrates using first-row TM catalysis. So
far, there have been only two reports for phenylalanine
carbonylation, both exploiting palladium metal. In 2007, the
Vicente group showed a stoichiometric example of ortho-
palladation of a phenylalanine ester and a subsequent reaction
of the formed complex with CO gas (Scheme 1b).10 In 2011,
Garcia, Granell, and co-workers demonstrated palladium-
catalyzed carbonylation of quaternary amino acids (Scheme
1c).11
Herein we report an efficient method for the synthesis of
1,2-dihydroisoquinolinolones via C(sp2)−H carbonylation of
phenylalanine derivatives. Methyl picolinoylphenylalaninate
1aa was chosen as a model substrate for amino acid
C(sp2)−H carbonylation. A picolinamide directing group
(DG) was introduced by Daugulis in 200512 and has been
extensively used for a large diversity of transformations
Received: February 24, 2021
Published: March 16, 2021
https://doi.org/10.1021/acs.orglett.1c00660
© 2021 American Chemical Society
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a
Scheme 1. Selected Examples on C−H Functionalization of
Amino Acids
Table 1. Optimization of Reaction Conditions
b
entry
change from above conditions
yield, %
1
2
3
4
oxidant: Ag2CO3 (3 equiv)
oxidant: Mn(OAc)3·2H2O (3 equiv)
none
37
56
70
0
oxidant: air
5
6
7
8
NaOPiv (2 equiv) instead of PivOH
catalyst: Co(OAc)2 (20 mol %)
catalyst: Co(acac)3 (20 mol %)
catalyst: Co(dpm)2 (30 mol %)
without catalyst
27
24
30
94
0
9
10
11
12
13
solvent: DCE, 100 °C
50
50
32
30
solvent: toluene, 100 °C
“CO” surrogate: DIAD
“CO” surrogate: DEAD
a
Reaction conditions: 1aa (0.1 mmol), CO (1 atm), Co(dpm)2 (0.02
mmol), PivOH (0.2 mmol), oxidant (0.15−0.3 mmol), PhCl (1 mL),
b
120 °C. NMR yields using triphenylmethane as an internal standard.
catalyzed by various TMs.13 In 2017, Cui demonstrated
picolinamide as a traceless DG;14 since then, a number of
reports for picolinamide as a traceless DG have been
reported.15 For optimization studies, conditions similar to
those reported previously for the carbonylation of amino
alcohol derivatives were employed.15b Using Co(dpm)2
catalyst in combination with Ag2CO3 as the oxidant and
PivOH as the additive under 1 atm of CO, we observed the
formation of 1,2-dihydroisoquinolinolone 2aa in 37% yield
(Table 1, entry 1).
Co(dpm)2 = bis(2,2,6,6-tetramethyl-3,5-heptanedionato)cobalt(II)
(CAS 13986-53-3); DEAD = diethyl azodicarboxylate; DIAD =
diisopropyl azodicarboxylate.
(Scheme 2). The reaction of substrate 1aa bearing a methyl
ester functionality with CO gave product 2aa in excellent yield,
a
Scheme 2. Carbonylation of Phenylalanine Derivatives 1
Oxidant screening (entries 1−4) revealed that the yield of
2aa can be increased to 70% using a combination of Ag2CO3
and Mn(OAc)3·2H2O (entry 3). Control experiments
excluding the oxidant or the catalyst showed no formation of
2aa (entries 4, 9). Changing the acid additive to NaOPiv
decreased the yield of 2aa to 27% (entry 5). Other catalysts,
such as Co(OAc)2 (entry 6) or Co(acac)3 (entry 7), formed
2aa in lower yield.16 The final conditions were found when the
product yield additionally was increased to 94% using a higher
catalyst loading, 30 mol % (entry 8). We speculate that the
high catalyst loading is required due to the coordination of 2-
picolinic acid that is released during the reaction to the metal,
which may reduce the catalytic activity of cobalt catalyst.14,17
Indeed, we observed that the product 2aa yield decreased with
an increase in the amount of added 2-picolinic acid.16
Subsequently, other reaction solvents were screened, such as
DCE (entry 10) and toluene (entry 11); however, none of
them gave a better yield. Other CO sources such as DIAD
(entry 12) or DEAD (entry 13) were less effective. It is worth
mentioning that during the optimization we did not observe
formation of either products arising from the radical reaction at
α-center of phenylalanine, nor oxidation or decarboxylation of
1aa. It should be noted that, in comparison with Pd catalysis,
we did not observe the formation of indoline or indole
byproducts resulting from intramolecular C(sp2)−H amina-
tion.18
a
Reaction conditions: 1 (0.5 mmol), CO (1 atm), Co(dpm)2 (0.15
mmol, 30 mol %), PivOH (1.0 mmol, 2 equiv), Ag2CO3 (0.75 mmol,
1.5 equiv), Mn(OAc)3·2H2O (1.0 mmol, 2 equiv), PhCl (5 mL), 120
°C, isolated yields. Reaction in 1.7 mmol scale, 13 h.
b
94%; upscaling the reaction to a 1.7 mmol scale gave product
2aa in 75% yield. At the same time, phenylalanine ethyl, t-
butyl, and benzyl esters 1ba−da gave very good yields (70−
83%). We found that allyl ester 1ea was a competent substrate
as well and gave product 2ea in an acceptable yield, 50%.
We next investigated the scope of the C(sp2)−H carbon-
ylation method with respect to the amino acid derivative 1
(Scheme 3). Gratifyingly, a wide substrate scope with an ortho,
With the optimized conditions in hand, we next examined
the reaction scope with respect to phenylalanine esters
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Scheme 3. Scope for C(sp2)−H Carbonylation of Amino
moderate yield (57%). During the reaction, we observed partial
cleavage of the acetyl group under the reaction conditions. We
were pleased to find that phenylalanine derivative 1ap bearing
the t-butylacetylenyl substituent was a competent substrate and
gave product 2ap in 72% yield. Both naphthyl amino acids 1ar
and 1as reacted smoothly and formed 1,2-dihydroisoquinoli-
none derivatives 2ar and 2as in excellent yield. Moreover, the
thiophene amino acid derivative 1at was compatible with the
reaction conditions and formed the corresponding product 2at
in a good yield. Furthermore, we determined that even
challenging amide derivative 1au, which potentialy could
deactivate the catalyst by coordination, gave carbonylation
product 2au in acceptable yield, 52%.
a
Acid Derivatives 1
Investigating the reaction scope, we found some limitations
(Scheme 4). Substrate 3a was unreactive, suggesting that the
Scheme 4. Unsuccessful Substrates
picolinamide directing group has an essential role for C−H
carbonylation. Quaternary amino acid 3b, homobenzyl amide
3c, and homophenylalanine derivative 3d did not give
carbonylation products, suggesting that both the geometry of
substrate as well as the ester functionality at α-carbon are
crucial for successful transformation.
Encouraged by the result that amide 1au underwent
carbonylation, we decided to apply the method for more
challenging substrates: short peptides. It is known that amide
bonds within peptide can chelate metal ions thereby
deactivating the reaction catalyst. We were pleased to find
that under standard reaction conditions product formation was
observed, albeit a large amount of decomposition byproducts
were detected by NMR that could be related to the fact that
peptides are prone to undergo oxidative cleavage. Excluding
Mn(OAc)3·2H2O and increasing the Ag2CO3 equivalents
improved the product yield. Gratifyingly, dipeptides 4a−c
and tripeptide 4d underwent carbonylation and formed
products 5a−d in acceptable to good yields (Scheme 5).
Unfortunately, under the reaction conditions, partial race-
mization was observed. Product 5a was obtained with 86% ee,
and 5c with 93% ee. Notably, in the carbonylation of
enantiopure 4d, partial racemization most likely occurred,
but purification led to the isolation of the pure major
diasteroisomer in 46% yield.
a
Reaction conditions: 1 (0.5 mmol), CO (1 atm), Co(dpm)2 (0.15
mmol, 30 mol %), PivOH (1.0 mmol, 2 equiv), Ag2CO3 (0.75 mmol,
1.5 equiv), Mn(OAc)3·2H2O (1.0 mmol, 2 equiv), PhCl (5 mL), 120
°C, isolated yields.
meta, and para substitution pattern gave the corresponding
carbonylation products in moderate to excellent yields. Using
meta-substituted phenylalanine derivatives 1ad−af, excellent
regioselectivity was observed, and we selectively obtained only
carbonylation products arising from reaction of less hindered
C−H bond, even using meta-fluoro-substituted phenylalanine
derivative 1ae.19
To understand the reaction mechanism, several additional
experiments were conducted (Scheme 6).
Importantly, diverse functionalities were tolerated under the
reaction conditions, such as alkyl (products 2ad, 2ag), halide
(2ac, 2ae, 2af, 2an, 2ao), trifluoromethyl (2ab, 2ai), phenyl
(2ah), ethyl ester (2aq), methoxy (2aj), methoxymethyl ether
(2ak), and benzyl ether (2al). O-Acetyl tyrosine derivative
1am was less successful and gave a carbonylation product in
We synthesized unsaturated phenylalanine derivative 6 and
found that carbonylation under standard conditions yielded
product 2aa in 91% yield, suggesting that 6 might be an
intermediate for the formation of 2aa. Interestingly, we found
that addition of radical scavenger TEMPO significantly
suppressed the carbonylation reaction of 1aa but only slightly
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a
Co(dpm)2 for the formation of 6. Our attempts to isolate the
C−H activated complex 8 were successful. The structure of
complex 8 was confirmed by X-ray crystallography. We were
pleased to find that under CO (1 atm) at room temperature
complex 8 quantitatively formed product 2av which was
determined by NMR.
Scheme 5. Carbonylation of Peptides 4a−d
On the basis of the control experiments, isolated
intermediates, and previous literature reports,20 we propose
the plausible mechanism (Scheme 7).
Scheme 7. Proposed Mechanism for the Cobalt-Catalyzed
Carbonylation of Phenylalanine Derivatives 1
a
Reaction conditions: 4 (0.2 mmol), CO (1 atm), Co(dpm)2 (0.06
mmol, 30 mol %), PivOH (0.4 mmol, 2 equiv), Ag2CO3 (0.5 mmol,
b
2.5 equiv), PhCl (1 mL), 120 °C, isolated yields. Yield for pure
diastereomer.
Scheme 6. Mechanistic Studies
Initially, the phenylalanine derivative 1 using Co(II) catalyst
under oxidative conditions is oxidized to the corresponding
imine intermediate 6′ which then tautomerizes to the
corresponding enamine 63,7 (both double bond isomers were
detected by TLC and UPLC-MS). Next, Co(II) salt
coordinates with 6 and is oxidized to Co(III) intermediate
A20e followed by the C(sp2)−H bond cobaltation to form
complex B. Complex B may be oxidized to the Co(IV) species
followed by CO coordination and insertion to form Co(IV)
complex C′, which then undergoes a reductive elimination step
to form D (path a). Alternatively, complex B will undergo CO
coordination and insertion to form Co(III) complex C which
gives intermediate D after reductive elimination (path b). Path
c may also be operative according to which intermediate C is
oxidized to C′. Next, the directing group in intermediate D is
hydrolyzed to liberate 2, and cobalt is reoxidized to Co(III) to
form complex 7, which we observed by TLC in all
carbonylation reactions. The catalytic cycle restarts with ligand
exchange in complex 7 to form intermediate A. At this point of
study, it is hard to distinguish between possible pathways, but
according to control experiments, we believe that, in our case,
path b may be more favorable over path a or c, as
stoichiometric complex 8 formed 2av without addition of
oxidant.
a
Standard reaction conditions: 1aa or 6 (0.1 mmol), CO (1 atm),
Co(dpm)2 (0.03 mmol, 30 mol %), PivOH (0.2 mmol, 2 equiv),
Ag2CO3 (0.15 mmol, 1.5 equiv), Mn(OAc)3·2H2O (0.2 mmol, 2
b
equiv), PhCl (1 mL), 120 °C, 16 h. NMR yield using triphenyl-
methane as an internal standard.
affected carbonylation of 6, indicating that the radical reaction
pathway could be involved for the synthesis of unsaturated
intermediate 6. Moreover, under standard conditions by TLC,
we observed the formation of cobalt complex 7 which was
isolated, and its structure was confirmed by NMR. Exploiting
complex 7 as the reaction catalyst for substrate 1aa gave
product 2aa in only 52% yield. Interestingly, substrate 6
carbonylation using catalyst 7 resulted in product 2aa in 96%
yield, suggesting that complex 7 is not as efficient catalyst as
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In summary, we have developed an efficient protocol for the
C(sp2)−H carbonylation of amino acid derivatives based on
the inexpensive Co(dpm)2 catalyst. A broad range of
phenylalanine derivatives bearing diverse functional groups
were tolerated. Carbonylation of the amino acid C(sp2)−H
bond was accomplished using picolinamide as a traceless
directing group and carbon monoxide (1 atm) as the carbonyl
source. In addition, the developed method can be successfully
applied to the C(sp2)−H carbonylation of short peptides
thereby offering opportunity for peptide late-stage carbon-
ylation.
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00660.
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Experimental procedures and characterization data for
all new compounds along with copies of the NMR
spectra and HPLC chromatograms (PDF)
Crystallographic data (PDF)
Accession Codes
CCDC 2054466 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Liene Grigorjeva − Latvian Institute of Organic Synthesis,
Riga LV-1006, Latvia; orcid.org/0000-0003-3497-6776;
Email: liene_grigorjeva@osi.lv
Authors
Lukass Lukasevics − Latvian Institute of Organic Synthesis,
Riga LV-1006, Latvia
Aleksandrs Cizikovs − Latvian Institute of Organic Synthesis,
Riga LV-1006, Latvia
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00660
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mide DG: (a) Nadres, E. T.; Daugulis, O. J. Am. Chem. Soc. 2012,
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Org. Chem. 2020, 85, 4482−4499. (d) Rej, S.; Chatani, N. ACS Catal.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
́
2018, 8, 6699−6706. (e) Martínez, A. M.; Rodríguez, N.; Arrayás, R.
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Notes
The authors declare no competing financial interest.
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(16) Please see the Supporting Information for details.
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ACKNOWLEDGMENTS
■
This research is funded by the Latvian Council of Science,
project [Cobalt catalyzed C−H bond functionalization],
project no. lzp-2019/1-0220. We thank Prof. Dr. A. Jirgensons
(Latvian Institute of Organic Synthesis) for scientific
discussions and Dr. S. Belyakov (Latvian Institute of Organic
Synthesis) for X-ray crystallographic analysis.
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