Carbamoylation of Azomethine Imines via Visible-Light Photoredox Catalysis

Bianca T. Matsuo, Pedro H. R. Oliveira, Jos é Tiago M. Correia, and M á rcio W. Paixao*

Letter

Carbamoylation of Azomethine Imines via Visible-Light Photoredox

Catalysis

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sı Supporting Information

ABSTRACT: A versatile and robust photocatalytic methodology to

install the amide functional group into azomethine imine ions is

described. This protocol is distinguished by its broad scope and mild

reaction conditions, which are well suited for the preparation of

structurally complex compounds in the form of amino acids,

peptides, and small drug-like molecules. Moreover, the generated

pyrazolidinone core could be easily converted into β-alanine

analogues.

ynthetic protocols for the preparation of amide-containing

Last year, the groups of Melchiorre and Jacobi von

molecules are among the most valuable transformations in

Wangelin explored dihydropyridine (DHP) derivatives as

an alternative strategy to install amide bonds on (hetero)aryl

bromides and activated oleﬁns, respectively. By the exploration

of electron donor−acceptor (EDA) complexes with DHPs,

modern organic synthesis, given the abundance of these

motifs in biomolecules (as peptides and proteins), pharma-

ceuticals, and agrochemicals. Although signiﬁcant advances

have been made for the direct synthesis of amides, established

routes are mainly based on C−N bond-forming reactions.

Traditionally, these are performed by the condensation of

carboxylic acids or derivatives with amines (Scheme 1a, left);

however, C−C disconnection to install amide bonds oﬀers new

synthetic strategies to incorporate the amide motif that are

complementary to the well-established routes. In this scenario,

isocyanates and isocyanides have been explored in metal-

Hong disclosed a cross-coupling reaction using N-amidopyr-

idinium compounds, including in the scope examples of C4-

carbamoylated pyridines (Scheme 1b, right). The reactive

radical species generated through these protocols contain the

amide functionality ready to be installed, and in comparison

with metal-catalyzed approaches, it could be considered a

nucleophilic reduced equivalent of isocyanates (Scheme 1c).

Inspired by these initial ﬁndings, we rationalized that an

unprecedented photocatalytic carbamoylation of imines would

represent a mild and easy strategy for constructing biologically

relevant scaﬀolds and would complement those protocols

catalyzed amidations and in multicomponent approaches

Scheme 1a, right).

Considered a contemporary challenge, signiﬁcant eﬀort has

(

involving the functionalization of aryl halides, activated

been focused on the development of new synthetic routes to

selectively forge amide bonds, especially in complex molecules.

The ability to leverage these methodologies for applications in

antibody−drug conjugates within the context of biopharma-

ceutical discovery programs is needed and is essential for their

7b,11,12

oleﬁns, and aza-heterocycles.

The selected coupling partners, azomethine imine ions, are

known as important building blocks that can be easily

transformed into biologically relevant nitrogen-containing

heterocycles. This class of molecules has been exhaustively

explored as 1,3-dipoles; however, only recently has their

reactivity toward photocatalytic radical addition been inves-

success.

Recently, photoredox catalysis have emerged as a valuable

synthetic tool for the synthesis and modiﬁcation of

biomolecules and complex structures. Among the photo-

catalytic amidation approaches reported, direct C−C bond

Received: July 14, 2021

formation strategies rely on the use of photocatalytic generated

carbamoyl radicals as amide synthons. Recent literature has

shown the use of oxamic acids and N-oxyphthalimido

oxamides in oxidative and reductive decarboxylative ap-

proaches, respectively (Scheme 1b, left).

XXXX American Chemical Society

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

Scheme 1. (a) C−C and C−N Strategies for Amide Ligation

Construction, (b) Recent Photocatalytic One-Electron

Generation of Carbamoyl Radicals, (c) Comparison of

Preformed Amide Group in Carbamoyl Radical and

Isocyanate-based Methodologies, and (d) Our Proposal:

Installation of Secondary/Tertiary Amides into Azomethine

Imines

of aryl azomethine imine ions (Scheme 2). As seen in Scheme

2, aryl azomethine imines bearing either electron-withdrawing

Scheme 2. Scope of Azomethine Imines

.0 mmol scale. Reactions were performed using azomethine imine

(

0.15 mmol), carbamoyl radical precursor (1.5 equiv, 0.225 mmol),

and 4CzIPN (1 mol %) in MeCN (3.0 mL). The yields refer to

isolated compounds after puriﬁcation.

substituents (p-CF 4 or p-F 5) or electron-donating groups

(

p-Me 8 or p-OMe 9) underwent carbamoylation with 2a in

moderate to good yield. Importantly, substrates containing aryl

bromides could be successfully tolerated, allowing for further

functionalization by transition-metal-mediated cross-coupling

reactions (compounds 6 and 10). Other substitution patterns

did not show a substantial inﬂuence on the reaction outcome

and, notably, ortho and meta substitutions were well tolerated

(11, 10, and 12, respectively). Likewise, pyridine (13) and

furan (14), privileged pharmacophores, were also competent

toward the carbamoyl radical installation. The azomethine

imine bearing a phenyl substituent on the pyrazolidinone ring

was also compatible with the reaction conditions, furnishing

the product 15.

To demonstrate this method’s applicability in the late-stage

functionalization of dense molecules, we prepared a substrate

derived from etodolac, an anti-inﬂammatory and analgesic

drug. This structure possesses benzylic C−H bonds and acidic

N−H bonds, which are known to interfere with radical

pathways. Gratifyingly, subjecting this substrate to our reaction

conditions, we were able to access the desired product 16 in

excellent yield.

tigated. In this work, we describe the construction of valuable

amide compounds in a C−C bond-forging strategy between

DHP derivatives and azomethine iminium ions in a mild and

robust protocol (Scheme 1d).

Four-substituted 1,4-dihydropyridines (DHPs) are bench-

stable solids. They are easily synthesized from commercially

available starting materials and oﬀer high structural diversiﬁ-

cation. In addition to the exploration of DHPs as alkyl, acyl,

and proton sources, they are also useful precursors for the

generation of the less explored carbamoyl radicals under

photocatalytic conditions. Because of their facile photoredox-

catalyzed oxidation (E ≈ +1.05 V vs SCE), they are prone to

undergo single-electron oxidation under visible-light irradiation

in the presence of a suitable photocatalyst to provide the

corresponding radical after fragmentation. Considering the

advantages of the metal-free organic photocatalysts, we

envisioned 4CzIPN to be a suitable catalyst to promote the

−

proposed carbamoylation reaction (E (PC*/PC ) = +1.35 V

vs SCE).

Recently, the use of nitrones in photoredox protocols has

1/2

been reported in (3 + 2) cycloadditions and in radical

Our study began using the azomethine iminium ion 1a and

the cyclohexylamine-derived dihydropyridine 2a (E _o _x = +1.21

V vs SCE; see cyclic voltammetry in the Supporting

Information (SI)) as model substrates for the evaluation of

the eﬀect of various parameters on the reaction outcome. We

identiﬁed that using only 1 mol % of the photocatalyst, we

were able to obtain the corresponding product N-cyclohexyl-2-

addition reactions. Considering the importance of this

structure to easily access nitrogen-containing compounds and

the presence of N-hydroxylamines in pharmaceuticals, we

were curious if nitrone derivatives could be amenable to our

optimized reaction conditions. To our delight, the carbamoy-

lation proceeded well for all cases, aﬀording the respective

phenylglycine amide derivatives 17−19 in good yields

(Scheme 3).

(

3-oxopyrazolidin-1-yl)-2-phenylacetamide 3 in 70% yield in

acetonitrile under 456 nm blue light-emitting diode (LED)

irradiation. (See the SI for the complete optimization.)

Having established the optimal reaction conditions, we

began to evaluate the scope of this transformation for a range

The growing application of modiﬁed peptides as drug

candidates lead us to evaluate the feasibility of the developed

visible-light-mediated protocol in the direct installation of

proteinogenic amino acids across the azomethine imine

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

Scheme 3. Scope of Nitrones

containing a modiﬁed phenylglycine residue in good to high

yield (compounds 33−39), showcasing possible opportunities

for the exploration of bioconjugation strategies. The dipeptide

scope included sequences having L-Val, L-Met, L-Phe, L-Leu, L-

Pro, L-Ser, L-Ile, and L-Phg residues, which contain useful

handle groups for further modiﬁcation of the peptide side

chain. Notably, oxidation-labile methionine residues were well

tolerated under our protocol.

As a further demonstration of the utility of the radical

addition protocol, we examined the ability to generate hybrid

architectures containing natural products and medicinal agents

(

Scheme 4, bottom, left). As shown, the pendant steroid

lithocholic acid exhibited good reactivity with 1a, giving rise to

the highly functionalized adduct 40 in moderate chemical

yield. Moreover, drug-like small molecules from the anti-

epileptic pregabalin and memantine, used in the treatment of

the Alzheimer’s disease, were successfully installed into 1a in

good to excellent yield (compounds 41 and 42, respectively).

Our method is also amenable to amides derived from other

primary and secondary amines with unmasked functional

handles groups (Scheme 4, bottom, right). Likewise, aromatic

and aliphatic carbamoyl radicals were successfully installed as

Reactions using nitrone (0.15 mmol), carbamoyl radical precursor

1.5 equiv, 0.225 mmol), and 4CzIPN (1 mol %) in MeCN (3.0 mL).

(

The yields refer to isolated compounds after puriﬁcation.

scaﬀold. Such a combination allows the synthesis of non-

natural amino acids and the assembly of modiﬁed peptides in a

single event (Scheme 4, top, left).

Gratifyingly, the reaction tolerated a broad scope of amino

acid derivatives, including those containing a nonpolar side

chain (L-Val, L-Ala, L-Pro, L-Met and L-Ile), polar uncharged (L-

Ser) and aromatic systems (L-Tyr, L-Trp and L-Phe), which

could be smoothly transformed in good chemical yields

the electron-poor 3,5-CF -aromatic carbamoyl radical (47).

Unprotected functional groups including alkyne (45), alkene

(

46), triethyl silicate (48), and phenol (49) did not

dramatically aﬀect the reaction outcome, aﬀording the

respective carmoylated products in moderate to good yields.

To showcase the utility and importance of our carbamoy-

lated products, we next subjected examples of the obtained

analogues to structural modiﬁcation (Scheme 5). The

pyrazolidinone moiety of the obtained structures can be

considered a masked form of the nonproteinogenic amino acid

β-alanine. Notably, the primary amide group is a key structural

motif as a useful building block and is also present in many

pharmaceutical compounds. The carbamoylated product 3

(

compounds 20−30). Nonproteinogenic amino acids from L-

(

+)-α-phenylglycine and 2-methylalanine were nicely incorpo-

rated into the azomethine imine, providing compounds 31 and

2, respectively.

To further demonstrate the versatility of our method, we

next turned our attention to explore the reactivity of dipeptides

with the azomethine imine 1a (Scheme 4, top, right).

Pleasantly, the protocol allowed the synthesis of tripeptides

Scheme 4. Scope of 4-Carbamoyl-1,4-Dihydropyridines

Compounds obtained as a mixture of 1:1 dr. Using the 3-phenyl substituted pyrazolidinone azomethine imine. Using the p-CF C H

azomethine imine. Using the p-BrC H azomethine imine. Reactions using azomethine imine (0.15 mmol), carbamoyl radical precursor (1.5

equiv, 0.225 mmol), and 4CzIPN (1 mol %) in MeCN (3.0 mL). The yields refer to isolated compounds after puriﬁcation.

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

The Supporting Information is available free of charge at

Letter

Scheme 5. Reductive Cleavage of the Pyrazolidinone Moiety

TEMPO adduct I was detected. Control experiments also

disclosed the role of the irradiation source and the photo-

catalyst for the carbamoylation reaction outcome.

In summary, we demonstrate a robust photocatalytic

strategy for the amidation of azomethine imines ions via C−

C bond coupling reaction with readily available DHPs. This

transformation allowed straightforward access to a wide range

of carbamoylated compounds through a simple and versatile

protocol using only 1 mol % of photocatalyst and a small

excess of the radical precursor.

Crucially, the radical addition platform displayed high

functional tolerance and amenability toward biologically

relevant scaﬀolds as amino acids, peptides, drugs and

compounds containing fragile functionalities as tertiary bonds

and reactive groups. Therefore, the protocol represents a new

strategy for the structural diversiﬁcation of the chemical space

of these important classes of compounds. Additionally, the

direct application for obtaining primary amides from the

simple reductive cleavage of the pyrazolidinone nucleus was

demonstrated, leading to β-alanine derivatives containing

multiple reactive sites liable to orthogonal derivatizations.

could be selectively reduced using pretreated Raney Ni under a

H atmosphere at room temperature, delivering the amino

propanamide 53 (Scheme 5a). The product containing the L-

Ala residue 21 was sequentially submitted to the same

reductive conditions, aﬀording the primary amide 54 (Scheme

b).

On the basis of control experiments and the previous

reports, we propose a plausible mechanism for the

carbamoylation reaction in Scheme 6 (See the SI for details).

Scheme 6. Proposed Mechanism for the Carbamoylation of

Azomethine Imines

ASSOCIATED CONTENT

sı Supporting Information

■

https://pubs.acs.org/doi/10.1021/acs.orglett.1c02353.

Details of all experimental, electrochemical, and spectral

data (PDF)

AUTHOR INFORMATION

■

Corresponding Author

Márcio W. Paixa

Sustainable Chemistry (CERSusChem), Department of

Chemistry, Federal University of Sao Carlos − UFSCar, Sao

Carlos, Sao Paulo 13565-905, Brazil; orcid.org/0000-

002-0421-2831 ; Email: mwpaixao@ufscar.br

o − Centre of Excellence for Research in

Authors

Bianca T. Matsuo − Centre of Excellence for Research in

Sustainable Chemistry (CERSusChem), Department of

Chemistry, Federal University of Sao

Carlos, Sao Paulo 13565-905, Brazil

Pedro H. R. Oliveira − Centre of Excellence for Research in

Sustainable Chemistry (CERSusChem), Department of

Carlos − UFSCar, Sao

̃ ̃

Under 456 nm blue LED irradiation, the photocatalyst

CzIPN (λ = 507 nm) is eﬃciently converted to its long-

lived triplet excited state 4CzIPN* (τ = 5.1 μs). The

−

^C^z^I^P^N^*⁽^E¹/2(PC*/PC ) = +1.35 V vs SCE) is oxidizing

Chemistry, Federal University of Sao

Carlos, Sao Paulo 13565-905, Brazil

Carlos − UFSCar, Sao

̃ ̃

enough to promote the SET oxidation of 2a (E 2a = +1.21 V

vs SCE), which after fragmentation, generates the correspond-

ing carbamoyl radical A. The addition of A to the azomethine

iminium 1a gives rise to the amidyl intermediate B after a spin-

center shift event. Furthermore, this reactive intermediate

undergoes SET reduction with the reduced photocatalyst to

provide, after protonation, the desired product. It is important

to emphasize that the SET reduction of 1a should not be

José Tiago M. Correia − Centre of Excellence for Research in

Sustainable Chemistry (CERSusChem), Department of

Chemistry, Federal University of Sao

Carlos, Sao Paulo 13565-905, Brazil

Carlos − UFSCar, Sao

̃ ̃

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.orglett.1c02353

feasible given its highly negative reduction potential (E 1a =

red

Author Contributions

−

1.6 V vs SCE and E₁_/₂(PC /PC*) = −1.04 V vs SCE).

Moreover, the ﬂuorescence quenching study within the

B.T.M. performed most of the experiments in this study and

wrote the original version of this manuscript and the

Supporting Information. P.H.R.O. performed the experiments

for the scope evaluations and contributed to the Supporting

Information preparation. J.T.M.C. prepared starting materials

and, with M.W.P., reviewed, corrected, and provided useful

redox potentials corroborates the identiﬁcation of the reductive

quenching of the photocatalyst as the initial mechanistic step.

Additionally, the reaction in the presence of the radical

scavenger (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO, 3

equiv) did not lead to the formation of the product, and the

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

(10) Cardinale, L.; Konev, M. O.; Jacobi von Wangelin, A.

Letter

contributions to the manuscript. All authors provided input on

the manuscript.

Photoredox-catalyzed addition of carbamoyl radicals to olefins: a

,4-dihydropyridine approach. Chem. - Eur. J. 2020 , 26 , 8239 −8243.

Notes

11) Kim, I.; Park, S.; Hong, S. Functionalization of pyridinium

derivatives with 1,4-dihydropyridines enabled by photoinduced

The authors declare no competing ﬁnancial interest.

charge transfer. Org. Lett. 2020, 22, 8730−8734.

(12) (a) Kim, I.; Kang, G.; Lee, K.; Park, B.; Kang, D.; Jung, H.; He,

ACKNOWLEDGMENTS

■

Y.-T.; Baik, M.-H.; Hong, S. Site-selective functionalization of

pyridinium derivatives via visible-light-driven photocatalysis with

quinolinone. J. Am. Chem. Soc. 2019, 141, 9239−9248. (b) Yi, M.-J.;

Zhang, H.-X.; Xiao, T.-F.; Zhang, J.-H.; Feng, Z.-T.; Wei, L.-P.; Xu,

We are grateful to CNPq (NCT-Catálise/CNPq Grants No

44061/ 2018-5 and 166275/2017-4), FAPESP (14/50249-8,

7/10015-6), and GSK for ﬁnancial support. This study was

G.-Q.; Xu, P.-F. Photoinduced metal-free α -C(sp )−H carbamoyla-

ﬁnanced in part by the Coordenaca

Pessoal de Nível Superior - Brasil (CAPES) - Financial code

01. We also acknowledge Craig Day (The Institute of

̧ o de Aperfeic o̧ amento de

tion of saturated aza-heterocycles via rationally designed organic

photocatalyst. ACS Catal. 2021, 11, 3466−3472.

(13) Nájera, C.; Sansano, J. M.; Yus, M. 1,3-Dipolar cycloadditions

Chemical Research of Catalonia - ICIQ) for the helpful

discussion and comments on the manuscript. We acknowledge

Prof. Dra. Rose Maria Carlos (UFSCar) and the Ph.D.

students Mariana Cali (UFSCar) and Rafael Marchi (UFSCar)

for the electrochemical analysis and fruitful discussions. We

acknowledge Michael Murgu and Waters Technologies of

Brazil for their assistance in obtaining HRMS data.

of azomethine imines. Org. Biomol. Chem. 2015, 13, 8596 −8636.

(14) (a) Matsuo, B. T.; Correia, J. T. M.; Paixao, M. W. Visible-light-

mediated α -amino alkylation of azomethine imines: an approach to N -

J.-A.; Xiang, H. − Y.; Chen, X. − Q.; Yang, H. Photocatalytic

β -aminoalkyl)pyrazolidinones. Org. Lett. 2020, 22, 7891−7896.

(b) Xia, P.-J.; Ye, Z.-P.; Song, D.; Ren, J.-W.; Wu, H. − W.; Xiao,

reductive radical −radical coupling of N,N ′-cyclicazomethine imines

with difluorobromo derivatives. Chem. Commun. 2019, 55, 2712−

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