A one-pot copper catalyzed biomimetic route to n -heterocyclic amides from methyl ketones via oxidative c-c bond cleavage

Article abstract of DOI:10.1021/ol5031266

A direct one-pot Cu-catalyzed biomimetic oxidation of methyl ketones to pharmaceutically important N-heterocyclic amides is reported. The scope of the method is broad, scalable, and mild, and the reaction is tolerant with various acid, base sensitive functionalities with multiple heteroatoms and aryl halides. The extensive mechanistic studies suggest that this reaction follows the Luciferin-Luciferase-like pathway.

Full text of DOI:10.1021/ol5031266

Letter
pubs.acs.org/OrgLett
A One-Pot Copper Catalyzed Biomimetic Route to N‑Heterocyclic
Amides from Methyl Ketones via Oxidative C−C Bond Cleavage
Parthasarathi Subramanian, Satrajit Indu, and Krishna P. Kaliappan*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: A direct one-pot Cu-catalyzed biomimetic oxida-
tion of methyl ketones to pharmaceutically important N-
heterocyclic amides is reported. The scope of the method is
broad, scalable, and mild, and the reaction is tolerant with various
acid, base sensitive functionalities with multiple heteroatoms and
aryl halides. The extensive mechanistic studies suggest that this
reaction follows the Luciferin-Luciferase-like pathway.
hemical conversions/reactions play an important role in
regulating the biomass balances in biological systems.
bioluminescent technique has been widely applied to examine
certain biological processes, such as stages of infection, and
provides other vital sources of information.10
C
Enzymatic decarboxylation1 is a fundamental process among
several biotransformations and is widely applied for the synthesis
of a myriad of compounds.2 Aspiring to imitate the biological
process, synthetic chemists have achieved several decarboxylative
biomimetic methodologies which include Barton,3 Hunsdieck-
er,4 Kolbe,5 Kochi,6 malonic acid half thioester (MAHT) Claisen
condensation,7 and transition metal catalyzed transformations.8
One such biochemical process is the Luciferin-Luciferase
reaction9 [eq 1]. Luciferin belongs to a group of light emitting
Amides are versatile compounds present in biologically active
heterocycles, polymers, proteins, natural products, agrochem-
icals, and synthetic intermediates. The conventional method for
amide preparation often relies on preactivation of carboxylic
acid11 either by acetyl chloride or mix-anhydride formations or
by the use of coupling reagents (DCC, EDC·HCl, HATU, and
etc.). However, these methods possess their own drawbacks
which include requirements of stoichiometric excess of reagents,
sensitivity toward moisture, process of purification, and
expensive coupling reagents. Difficulties in setting up a reliable
protocol of amide synthesis make them attractive targets for
chemists, which has resulted in the development of many name
reactions such as Beckmann,12 Schotten−Baumann,13 Stau-
dinger ligation,14 and Schmidt rearrangements.15 However, these
traditional methods suffer from highly harsh conditions,
hazardous reagents, functional group intolerances, and low
yields. Therefore, the amide synthesis becomes a fascinating field
of research pertaining to developing a mild and scalable protocol.
Over a decade, the immense contribution of transition metals16
and metal-free17 catalytic amide bond formations have resulted
in several reports bearing similar strategies all of which involve a
direct oxidative amidation of aldehyde or alcohol with amine.
Very recently, Jiao and co-workers have reported the utility of
azide rearrangement chemistry for Cu-catalyzed synthesis of
benzamide derivatives from ketones.18
natural products present in numerous bioluminescent organisms
such as bacteria, dinoflagellates, fungi, crustaceans, worms,
insects, and fish. Luciferase is an enzyme which is found to
produce light in the presence of triplet molecular oxygen (3O2)
upon exergonic oxidative decarboxylation reaction with luciferin
(LnH) to generate the singlet-excited state intermediate
oxyluciferin (amide-OLnH*). Thus, this oxyluciferin is respon-
sible for light emission in living bioluminescent organisms. This
Herein we report a scalable, one-pot copper catalyzed
biomimetic synthesis of N-heterocyclic amides from readily
available methyl ketones. Our method involves Cu-catalyzed
cascade sp3 C−H oxidative cyclization of methyl ketone with 2-
Received: October 28, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol5031266 | Org. Lett. XXXX, XXX, XXX−XXX

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Letter
amino heterocycles to the intermediate I [eq 2] analogous to
luciferin LnH [eq 1]. We envisioned that this intermediate I
could complete the biomimetic process of heterocyclic amide
bond formation.
and phenothiazine25 (Scheme 2). It is noteworthy that complete
chemoselectivity was observed; when we intend to functionalize
Scheme 2. Active Pharmacophores
To investigate the proposed biomimetic amide synthesis, we
selected 4′-bromoacetophenone 1 and 2-aminopyridine 2 as
model substrates. After the extensive screening (refer to
Supporting Information (SI)), we could obtain 78% of amide 3
exclusively under 20 mol % CuCl2 in a mixture of NMP and
tBuOH (1:2).
With optimized conditions in hand, the scope of the
biomimetic amide synthesis was examined. A range of methyl
ketones and 2-amino pyridines were converted to the
corresponding amides in good to excellent yields (Scheme 1).
Scheme 1. Transformation of Aryl and Alkyl Methyl Ketones
the C,N-diacetyl phenothiazine, only selective formation of C-
acetyl functionalized product 30 was observed, with N-acetyl
remaining intact. Further we were pleased to find the formation
of bis-amide23,26−28 31 from the sp3 bifunctionalization of
diacetylpyridine in good yield. In addition to 2-amino pyridines
and pyrazine, this method has also worked successfully with 2-
amino benzothiazole to furnish the product 32 in moderate yield.
Many control experiments have been carried out to probe the
mechanism (Scheme 3). To analyze the role of O2, the standard
reaction was carried out under an inert atmosphere, which gave
13% of enamine 33 [eq 3]. When we treated this enamine under
optimized conditions, the reaction affords 57% of amide 3 [eq 4].
To investigate the reactive intermediates further, we carried
out the reaction with 10 mol % of 2-amino pyridine 2 with 1
mmol of p-bromoacetophenone 1, which resulted in the
formation of α-keto aldehyde 34 [eq 5]. This observation is
also supported by precedential reports29 that the possible
generation of α-keto aldehydes is from an oxygenative reaction of
the corresponding enamine via dioxetane formation. To confirm
the formation of imidazopyridinone, the reaction was carried out
with α-keto aldehyde 34 and 2-amino pyridine using standard
reaction conditions without O2, and the amidine analog of
luciferin core30 36 was isolated in 63% yield [eq 7]. The
formation of this amidine moiety might be due to the reactive
imidazopyridinone 35 which might have undergone further
condensation with excess 2-amino pyridine under an inert
atmosphere. Nevertheless, when we treated this amidine 36
under the reaction conditions, the desired amide was isolated in
47% yield [eq 8]. However, this transformation failed to give the
product when the reaction was carried out under inert conditions
or in the absence of a catalyst. Although these reactive
intermediates are in support of the biomimetic Luciferin-
Luciferase pathways, the possibility of direct decarboxylative
amide formation from α-keto aldehyde 34 suggests avoidance of
the biomimetic route.
Particularly compelling is the selective assay of sp3 C−H
oxidation of methyl ketones into desired amides in the presence
of active methyl-sulfone and N-acetyl groups; this provides an
additional opportunity for further functional group manipu-
lations. Despite the presence of halogen substituents, this
biomimetic reaction proceeded smoothly and without any side
reactions. This methodology also proved to work consistently
with electron-rich and -poor methyl ketones to provide the
amides in high yields. Another attractive feature of this method is
its scalability; a couple of substrates have been chosen for this
purpose and tested at the 50 mmol scale. Subsequently, products,
viz. 3 and 8, were obtained in good yields without much variation
in research scale (1 mmol).
However, the reaction with aniline failed to give the product
under the optimized conditions [eq 9]. Hence, this concludes the
biomimetic pathway. To examine the absence of radical
We recognized the potential extension of this method to the
pharmacophore of biologically relevant indole sulphonamides24
B
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Letter
Scheme 3. Control Experiments
Scheme 4. Proposed Mechanism
Further this methodology has also been utilized to synthesize
medicinally valuable drug molecules and their pharmacophore
analogs.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data, and spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: kpk@chem.iitb.ac.in.
Notes
involvements, the reaction was treated with a radical scavenger,
TEMPO; the product was isolated without a reduction in yield.
Further the reaction was performed in dark shield to confirm the
absence of a radical reaction; the desired amide was isolated in
76% yield. After understanding the reactive intermediates, we
sought to examine the oxidation state of the catalyst, which is
responsible for this cascade transformation. Owing to their
various oxidation states and pertaining to the color differences of
the copper species, UV−vis spectroscopy was used to study the
possible conversion of Cu(II) to Cu(I)31 (refer to SI).
Subsequently, we found N-methyl-2-pyrrolidone (used as a
solvent) was responsible for this reduction.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank DAE, BRNS, Mumbai for financial support. P.S. and
S.I. thank UGC and CSIR, New Delhi respectively for
fellowships. The XRD facility of Dept. of Chemistry, IIT
Bombay is gratefully acknowledged. K.P.K. also acknowledges
the generous support from the Astra Zenecca Research
Foundation.
Based on the above investigations, a plausible mechanism for
the biomimetic amide synthesis is outlined here (Scheme 4). The
condensation reaction of methyl ketone and 2-amino pyridine
gives reversible enamine A. Then this enamine might react with
Cu-peroxy radical B to provide dioxetane intermediate31a C.
Opening of the dioxetane ring was followed by oxidation to α-
keto aldehyde and 2-amino pyridine which in turn might
condense to afford the luciferin core structure, imidazopyr-
idinone E. This imidazopyridinone is a required staring material
for the biomimetic Luciferin-Luciferase reactions. Further this
intermediate E could undergo the Cu-catalyzed dioxetane
formation32b to give H, and subsequent decarboxylation of I
delivers the amide.
In summary, we have developed an efficient biomimetic route
for N-heterocyclic amides from readily available substrates. The
reaction is very general, mild, scalable, and tolerant of various
functionalities. The salient features of this methodology include
selective functionalization of C-acetyl in the presence of N-acetyl,
methyl sulfone and bifunctionalization of diacetyl moieties.
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