Letter
Cite This: Org. Lett. 2018, 20, 5861−5865
pubs.acs.org/OrgLett
Cross-Dehydrogenating Coupling of Aldehydes with Amines/ROTBS Ethers by Visible-Light Photoredox Catalysis: Synthesis of
Amides, Esters, and Ureas
Ganesh Pandey,* Suvajit Koley, Ranadeep Talukdar, and Pramod Kumar Sahani
Molecular Synthesis and Drug Discovery Laboratory, Centre of Biomedical Research, Sanjay Gandhi Postgraduate Institute of
Medical Sciences, Lucknow-226014, India
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S Supporting Information
*
ABSTRACT: A straightforward synthesis of amides, ureas, and esters is reported
by visible-light cross-dehydrogenating coupling (CDC) of aldehydes (or amine
carbaldehydes) and amines/R-OTBS ethers by photoredox catalysis. The reaction
is found to be general and high yielding. A plausible mechanistic pathway has been
proposed for these transformations and is supported by appropriate controlled experiments.
C
carrying out a CDC reaction to prepare amides, ureas, and
esters.
Amides are fundamental commodity chemicals found in
natural products, organic materials, agrochemicals, and
pharmaceuticals.9 Classically, this functional group is prepared
by the most common reaction which involves carboxylic acid
derivatives and amines,10 or more recently, by the reaction of
carboxylic acid itself with amines using a large excess of base.11
Dialkylamides are prepared by heating (130 °C) a mixture of
carboxylic acid and N,N-dialkylformamide in the presence of
propylphosphonic anhydride as the promoter.12 Various
amides are also synthesized by employing ruthenium metal
complex catalyzed reactions of alcohols and amines involving
dihydrogen extrusion.13 Apart from this strategy, other
methods, such as oxidative amidation14 of aldehydes and
amines in the presence of chlorite salt,14a NHC catalysts,14b,c
and various other transition metal catalysts,15 are gaining
importance because of the bulk scale availability of reactants.16
Although these methods are scientifically elegant, they suffer
from various limitations such as use of a strong and hazardous
oxidizing agent, low yields, a high catalyst loading of precious
metal catalysts, a tedious workup procedure, and generation of
toxic, irremovable byproducts.10,17
Cho et al. reported the synthesis of amides from aldehydes
and amines by developing a photocatalytic reaction, but
amidation actually occurs by the reaction of an amine and acid
chloride, generated from a corresponding aldehyde during the
course of this reaction.15e
Initially, we set out to evaluate our proposed concept of
CDC of aldehydes and amines by irradiating (LED, blue light)
a mixture of benzaldehyde 1a (1 mmol) and piperidine (2a, 2
mmol) in the presence of BrCCl3 (1.0 mmol) in a roundbottom flask containing Ir[df(CF3)ppy]2(dtbbpy)PF6 (2 mol
%) in degassed acetonitrile as a photocatalyst (Scheme 1; see
Supporting Information (SI), Table S1, entry 1). The progress
ross-dehydrogenative coupling (CDC) has emerged as
one of the most powerful reactions for the coupling of
two chemical entities in recent decades.1 Although, the term
CDC is mostly allied with C−C bond formation2 at the
expense of two C−H protons, currently a C−X (X = N, O)
bond formation is also well recognized by this method.3
Generally, CDC reactions are catalyzed by metal complexes,4
but organo-catalytic CDC reactions are also well-known.5 A
recent trend is also emerging to effect CDC reactions by
involving radicals for the preparation of myriads of
molecules.6,7 In continuation of our interest in developing
CDC reactions for C−X (N and O) formation, using visiblelight photoredox catalytic reactions,8 we envisioned carrying
out a direct coupling of aldehydes/amine carbaldehydes with
amines/R-OTBS ethers to prepare amides, ureas, and esters,
respectively, through a proposed photoredox catalytic cycle as
shown in Figure 1. The concept of this photocycle emerged
Figure 1. Perception of visible light photocatalytic CDC.
from our previous work,8b where intermolecular C−N bond
formation by benzylic C−H bond functionalization was
achieved by the reaction of an amine and alkyl aryls.
Since amides, ureas, and esters are very useful chemicals,
their preparation demands the development of a simple, direct,
and high yielding approach. In this context, we would like to
delineate the success of the concept as shown in Figure 1 by
© 2018 American Chemical Society
Received: August 8, 2018
Published: September 7, 2018
5861
DOI: 10.1021/acs.orglett.8b02537
Org. Lett. 2018, 20, 5861−5865
Letter
Organic Letters
Scheme 1. Photoredox CDC of Aldehydes and Amines
of the reaction was monitored by gas chromatography [GC,
equipped with a split-mode capillary injection system using an
Agilent HP-5 column (30 m × 250 μm × 0.25 μm ID) at 50
°C RAM temperature]. When almost 80% of the aldehyde was
consumed, irradiation was discontinued. GC analyses showed
only the corresponding amide as the major peak. Chromatographic purification of the crude yielded 3aa (GC yield 90%,
isolated yield 75%) (Table S1, entry 1). [Ru(bpy)3Cl2] was
found to be a less effective photoredox catalyst than
Ir[df(CF3)ppy]2(dtbbpy)PF6 (Table S1, entry 2). Use of
other solvents such as DMF and DMSO did not improve yields
any further (Table S1, entries 3 and 4). It was also observed
that use of 2 equiv of amine was optimal for this reaction
(Table S1, entries 5 and 6). The reaction did not take place in
the absence of either light or the photocatalyst (Table S1,
entries 7 and 8) (see Supporting Information).
With this optimized condition, we focused on investigating
the scope and limitations of this reaction with a variety of
aldehydes (1) and piperdine (2a), and the results are
summarized in Figure 2.
Figure 3. Photoredox coupling of benzaldehyde with amines. a GC
yields (based on the consumption of 1). b Isolated yields.
withdrawing and donating, halo groups), heteroaromatic, and
aliphatic (cyclic, acyclic), generated corresponding amides
efficiently. When benzaldehyde was reacted with 2-(benzylamino) ethanol, only N-benzoylated product 3bk was obtained.
Furthermore, amino acid salt L-leucine ethyl ester hydrochloride, in the presence of trimethylamine (1 equiv), reacted
well with 1a yielding corresponding amide 3bv.
After generating a small library of amides, we became
enthusiastic to expand the scope of this protocol for
synthesizing substituted ureas, which are of great importance
in pharmaceuticals, agrochemicals, resin precursors, dyes,
additives to petroleum products, detergents, cellulose fibers,
and polymers.18 Various ureas have also been used as plant
growth regulators, herbicides, pesticides, tranquillizers, and
anticonvulsant medicinal preparations.18
A few unsymmetrically substituted ureas are even known to
act as an HIV-1 protease inhibitor.19 Due to their vast utility,
several methods are known for their preparation but most of
them are restricted to disubstituted ureas rather than
tetrasubstituted ureas, which are more challenging to
prepare.20 The most conventional method for preparing
substituted urea derivatives is the reaction of amines with
phosgene.21 However, due to high toxicity, the use of phosgene
has become limited. Alternatively, substituted ureas are also
prepared from amines and their derivatives by the use of
organic carbonates, CO2, and CO itself as the source of the
carbonyl moiety.22 Some other significant methods include
carbonylation of amines under solvent-free conditions with
carbon monoxide and oxygen using selenium as a catalyst23
using high-density microwave irradiation from primary
amines.24 The obvious disadvantages associated with these
methods led us to extend our photoredox cycle (Figure 1) to
prepare substituted ureas 13 directly by coupling amine-1carbaldehydes25 12 with any amine. Some examples are listed
in Figure 4.
After the success of CDC of an aldehyde (or amine
carbaldehyde) and an amine to form amides and ureas,
respectively, we considered extending the scope of this reaction
for the preparation of esters by coupling an aldehyde and
alcohol directly.
Figure 2. Photoredox coupling of aldehydes with piperidine. a GC
yields (based on the consumption of 1). b Isolated yields.
A variety of aromatic aldehydes having electron-donating or
electron-withdrawing substituents, regardless of their positions,
participated well in the reaction, indicating no obvious
electronic impact. Substituents such as OMe, NO2, F, and Br
groups on the phenyl ring (R1 moiety) were also found to be
compatible under standard photolysis reaction conditions and
did not hamper the reaction processes (Figure 2, 3aa−3aj).
Aldehydes bearing several aliphatic groups such as pentyl, tBu,
and cyclohexyl and most importantly 4-pyridyl as a
heteroaromatic group at the R1 moiety also afforded the
corresponding product in good yields.
Encouraged by these results, we explored the generality of
this reaction with various other amines (Figure 3). It is
noteworthy that not only secondary amines but also primary
amines also reacted well, yielding the corresponding tertiary as
well as secondary amides, respectively (Figure 3). Almost all
types of amines, such as aromatic (containing electron
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DOI: 10.1021/acs.orglett.8b02537
Org. Lett. 2018, 20, 5861−5865
Letter
Organic Letters
Figure 4. Formation of ureas by CDC of amine carbaldehydes and
amines.a GC yields (based on the consumption of 1). b Isolated yields.
Esterification is a fundamental transformation in chemistry
which finds application extensively in fragrances,26 pharmaceuticals, agrochemicals, and materials science27 industries.
Esterification is usually achieved by the reaction of carboxylic
acid derivatives with alcohols in the presence of an acid or base
catalyst.28 However, these approaches require multiple steps to
generate the pre-existing carboxyl and hydroxyl functionalities.
Furthermore, direct oxidative esterification29−34 of aldehydes
with alcohols has also been extensively investigated as a
complementary strategy; however, the key issue here remains
the selectivity between esterification (aldehyde oxidation) and
alcohol oxidation. This selectivity is somewhat handled by the
reaction of aldehydes and alcohols using various metal
catalysts31 in the presence of oxidants, cross-coupling of two
aldehydes,32 and hydrogen transfer using Pd(OAc)2 and
XPhos.33 In addition N-heterocyclic carbene (NHC) catalysts
also have served as effective oxidative catalysts for generating
the ester from aldehydes through a Breslow intermediate.34
Toward realizing direct esterification of an aldehyde with
alcohols (Scheme 2), we first examined the cross-coupling of
Figure 5. Generality of CDC of aldehydes and R-OTBS ethers.a GC
yields (based on the consumption of 1). b Isolated yields.
albeit formation of a new product 15 was noted by GC
(Scheme 4a). All our effort to purify this product by column
Scheme 4. Control Experiments
Scheme 2. Proposed Photoredox CDC of an Aldehyde with
Alcohol
benzaldehyde (1a) and n-butanol by following an identical
reaction protocol as discussed above for amide synthesis.
However, only a trace amount of the corresponding ester was
formed even after 48 h of irradiation. Thereafter, we replaced
the alcohol with its corresponding −OTBS ether; to our
delight, 5aa was formed in high yield (GC yield 85%, isolated
yield 60%) within 24 h (Scheme 3).
chromatography failed, as it decomposed on both silica and
alumina columns. Therefore, this product was identified by
HR-MS (see SI) as a peroxy adduct 15. Similar difficulties are
faced by others35 in attempting to purify other peroxy TEMPO
adducts by column chromatography. The fact that the
formation of 3aa is considerably decreased in the presence of
TEMPO and 15 is formed as a new product supports free
radical pathways (Scheme 1) for this reaction. We also
considered the possibility of another mechanism36 where
•
CCl3 forms an adduct 17 by reacting with 1a, which by
reaction with amine might produce an amide. However, no
evidence of formation of 17 (Scheme 4b) was found. To rule
out the reaction of •CCl3 with 1a, a control experiment was
carried out between 1a (1 mmol) and BrCCl3 (1 mmol) in
identical fashion but without 2a which showed apparently no
reaction. Similarly, formation of TBSCl during the esterification reaction was ruled out by conducting a control reaction
and careful analysis of the reaction mixture.
In conclusion, we have achieved the direct transformation of
aldehydes to amides, ureas, and esters by CDC using visible
light photoredox catalysis. The reaction is found to be versatile
and general. The most prominent feature of this conceptual
protocol is its simplicity, practicality, and environmental
compatibility.
Scheme 3. CDC of Aldehydes and R-OTBS Ethers
The generality and scope of this esterification reaction were
established by taking a range of aldehydes and R-OTBS
(Figure 5) ethers. It may be worth mentioning that iodo or
bromo substitued aromatic aldehydes, aliphatic aldehydes, and
hetaromatic aldehydes all underwent smooth conversion to
give their corresponding esters (5ad, 5ag, 5ai, and 5aj,
respectively).
In order to support our proposed mechanism8b (Figure 1),
we performed a control experiment of the amidation reaction
of 1a (1 mmol) with 2a (2 mmol) in the presence of TEMPO
(14, 4 mmol), under identical reaction conditions as
mentioned above, which gave only a trace amount of 3aa,
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DOI: 10.1021/acs.orglett.8b02537
Org. Lett. 2018, 20, 5861−5865
Letter
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ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02537.
Experimental procedures and characterization data for
all compounds (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: gp.pandey@cbmr.res.in.
ORCID
Ganesh Pandey: 0000-0001-7203-294X
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
G.P. thanks DST, New Delhi, for financial support, and S.K.
thanks DST, SERB, New Delhi. R.T. thanks CBMR, Lucknow,
and P.K.S. thanks CSIR, New Delhi for the award of a research
fellowship.
■
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(36) While rationalizing the formation of 3aa by the radical
mechanism as shown in Figure 1, we had also hypothesized the
possible involvement of acid chloride as an intermediate; however, we
could not come up with any possible mechanism for its formation.
5865
DOI: 10.1021/acs.orglett.8b02537
Org. Lett. 2018, 20, 5861−5865