Sustainable Alkylation of Unactivated Esters and Amides with Alcohols Enabled by Manganese Catalysis

Article abstract of DOI:10.1021/acs.orglett.8b03184

The first example of manganese-catalyzed C-alkylation of the carboxylic acid derivatives is reported. The bench-stable homogeneous manganese complex enables the transformation of the renewable alcohol and carboxylic acid derivative feedstock to higher value esters and amides. The reaction operates via hydrogen autotransfer and ideally produces water as the only side product. Importantly, aliphatic-, benzylic-, and heterocyclic-containing alcohols can be used as alkylating reagents, eliminating the need for mutagenic alkyl halides.

Full text of DOI:10.1021/acs.orglett.8b03184

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Sustainable Alkylation of Unactivated Esters and Amides with
Alcohols Enabled by Manganese Catalysis
†
†
,†,‡
and Osama El-Sepelgy*,†
̈
Yoon Kyung Jang, Tobias Kruckel, Magnus Rueping,*
†Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
‡KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi
Arabia
S
* Supporting Information
ABSTRACT: The first example of manganese-catalyzed C-
alkylation of the carboxylic acid derivatives is reported. The
bench-stable homogeneous manganese complex enables the
transformation of the renewable alcohol and carboxylic acid
derivative feedstock to higher value esters and amides. The
reaction operates via hydrogen autotransfer and ideally
produces water as the only side product. Importantly,
aliphatic-, benzylic-, and heterocyclic-containing alcohols can
be used as alkylating reagents, eliminating the need for
mutagenic alkyl halides.
he catalytic functionalization of the C−H bonds is a
catalyzed alkylation of esters and amides. However, the
1
Tcentral challenge in modern chemistry. Carboxylic acids extensive need for a glovebox may complicate large-scale
application.9
and their derivatives are pervasive in nature. Accordingly, an
For sustainability reasons, the current key challenge is the
development of efficient catalytic systems that rely on the use
of widely abundant, inexpensive metals.10 Manganese, the third
most abundant transition metal, was recently recognized by
several groups including ours as a potential alternative for the
toxic noble metal in the hydrogenation of polar11 and nonpolar
bonds,12 acceptorless dehydrogenation transformations,13 and
C- and N-alkylation with primary alcohols.5e−j,6c,14 Despite
recent advances,15 alkylation of the esters and amides via a
hydrogen autotransfer strategy remains an unsolved challenge.
Herein, we report the first example of manganese-catalyzed α-
alkylation of unactivated esters and amides with alcohols
(Scheme 1). Our catalytic system features a bench-stable
catalyst stabilized by an air-stable PNN ligand.
elegant approach to utilize the readily available esters and
amides is to convert them to more valuable intermediates upon
α-alkylation reaction. Conventional noncatalytic methods for
the α-alkylation of the carbonyl compounds involve the
generation of the corresponding enolate nucleophile using
superbase (such as organolithium or alkali metal amides) at
very low temperature. In addition to the need for toxic alkyl
halide electrophiles, such methods suffer from low atom
economy and the generation of copious amounts of waste.2
Recently, alkylation of the carbonyl compounds with
primary alcohols using a hydrogen autotransfer strategy has
emerged as an environmentally friendly alternative for
conventional alkylation methods.3 The key features of this
hydrogen-neutral process are the use of the alcohols as a green
electrophile, and the only stoichiometric byproduct is water.
Alcohols are renewably available from the lignocellulosic
biomass and a variety of industrial processes.4 In fact, progress
has been made in the α-alkylation of ketones and β-alkylation
of alcohols.5,6 In contrast, the α-C−H bonds of the esters and
amides exhibit comparably low Brønsted acidity owing to the
resonance stabilization effect of the adjacent NR2 group.
Furthermore, esters typically undergo side reactions such as
self-condensation and transesterification. Among the carboxylic
acid derivatives, the functionalization of dimethylacetamide
(DMA) and tert-butyl acetate is of particular interest because
these compounds can be obtained in one step from acetic acid
and are frequently used as solvents in synthetic chemistry.
Only a few reports are known for the α-alkylation of esters and
amides using iridium7 and ruthenium8 catalysis. Very recently,
Kempe and co-workers have reported an elegant cobalt-
The C-alkylation of DMA with benzyl alcohol (1a) was
selected as a model reaction for the optimization of the
reaction conditions (Table 1). Initially, we tested the catalytic
Scheme 1. Unprecedented Manganese-Catalyzed C−H
Functionalization of Esters and Amides
Received: October 5, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03184
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Table 2. Manganese Catalyzed Alkylation of Amides
entry
[Mn]
base
solvent
yield (%)
1
2
3
4
5
6
7
8
Mn-1
Mn-2
Mn-3
Mn-3
Mn-3
Mn-3
Mn-3
Mn-3
Mn-3
Mn-3
Mn-3
Mn-3
t-BuOK
t-BuOK
t-BuOK
t-BuOLi
t-BuONa
Cs2CO3
Na2CO3
NaOH
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
2-Me-THF
diglyme
22
15
91
9
28
0
0
0
0
57
9
9
KOH
10
11
12
t-BuOK
t-BuOK
t-BuOK
toluene
62
a
Reaction conditions: 1a (0.5 mmol), 2a (1.5 mmol), [Mn] (0.015
mmol), base (0.6 mmol), solvent (1 mL), 130 °C, 15 h. Yields
determined by GC analysis using m-xylene as an internal standard.
activity of different manganese complexes (Mn-1−Mn-3)16 in
1,4-dioxane using t-BuOK as a base. While the PNP−Mn
complexes Mn-1 and Mn-2 gave unsatisfactory results, Mn-3
bearing a PNN ligand afforded the desired alkylated amide 3a
in 91% yield (Table 1, entries 1−3). Thus, Mn-3 was selected
for the further optimization of the common reaction
parameters such as base and solvent. It was found that
efficiency of the reaction was reduced upon use of t-BuOLi and
t-BuONa (Table 1, entries 4 and 5). Additionally, relatively
weak bases such as Cs2CO3 and Na2CO3 were inert in this
transformation (Table 1, entries 6 and 7). Similarly, NaOH
and KOH proved unsuitable for this reaction (Table 1, entries
8 and 9). Next, we examined different solvents. The use of
other polar aprotic solvents such as 2-Me-THF and diglyme
resulted in the formation of the desired product in 57% and 9%
yields, respectively (Table 1, entries 10 and 11). The
application of the nonpolar toluene led to lower yield
compared to that of 1,4-dioxane (Table 1, entry 12). In
summary, the reaction works best using 3 mol % of Mn-3 and
1.2 equiv of t-BuOK in 1,4-dioxane at 130 °C.
a
Reaction conditions: 1 (0.5 mmol), 2 (1.5 mmol), Mn-3 (0.015
mmol), t-BuOK (0.6 mmol), 1,4-dioxane (1.0 mL), 15 h, 130 °C.
b
Isolated yields are shown. Mn-3 (0.02 mmol) and t-BuOK (1.0
mmol). 1 mmol scale.
c
With the optimized conditions in hand, we explored the
scope of the α-alkylation of amides with alcohols (Table 2).
First, the scope of the alkylation of N,N-dimethylacetamide
with alcohols was investigated. Benzylic alcohols bearing
electron-donating or electron substituents in the ortho, meta,
and para positions delivered the corresponding alkylated amide
3a−d in good to excellent yield (Table 2, entries 1−4).
Similarly, 1-naphthalenemethanol performed well in this
transformation and gave 3e in 72% yield (Table 2, entry 5).
Importantly, alcohols bearing different heterocyclic moieties;
furan, thiophene, and pyridine, were tolerated, and the high-
value-added amides 3f−h were obtained in good yield (Table
2, entries 6−8). Furthermore, long-chain and α-branched
aliphatic alcohols could be used in this new method to produce
the C-alkylated amide derivatives 3i−k in good yields (Table 2,
entries 9−11). Afterward, we investigated different types of
amides. Pleasantly, when N,N-diethylacetamide and 4-
acetylmorpholine 2b and 2c were subjected to the reaction,
the alkylation products were obtained in moderate yields
(Table 2, entries 12 and 13). Importantly, this catalytic system
was not limited to the alkylation of the tertiary amide. Thus,
the selective alkylation of N-methylacetamide resulted in the
formation of the desired C-alkylated products 3n and 3o in
B
DOI: 10.1021/acs.orglett.8b03184
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
53% and 86% yields (Table 2, entries 14 and 15). It is worth
noting that the conventional alkylation of secondary amides
using alkyl halides often leads to the N-alkylation products.2c
Afterward, we examined our catalytic system for the alkylation
of the 2-oxindole derivative 2p. Indeed, the alkylation reaction
proceeds well followed by C−H hydroxylation to afford the
C3-hydroxy-functionalized 2-oxoindole 3p in 87% yield (Table
1, entry 16).
Encouraged by these results, we decided to further expand
the scope to the alkylation of esters. Thus, the reaction
conditions for the tert-butyl acetate alkylation with benzyl
alcohol were investigated. In summary, the best conditions
involve the use Mn-3 and 2 equiv of t-BuOK in toluene at 100
°C for 4 h. We then focused on the substrate scope (Table 3).
DMA. However, when the catalyst loading was increased to 5
mol %, only 49% yield was observed, which indicates a very
strong kinetic isotope effect. Furthermore, the alkylated amide
3a′ was obtained with >90% deuterium incorporation at C3
and very low deuteration at C2. The deuterium experiment
indicates that the hydrogen autotransfer reaction takes place
via a monohydride mechanism with involvement of both the
metal and the non innocent ligand in the dehydrogenation and
hydrogenation steps.17−19
On the basis of our experimental observations, the proposed
reaction mechanism is presented in Scheme 3. Initially, proton
Scheme 3. Proposed Reaction Mechanism
a
Table 3. Manganese Catalyzed Alkylation of Esters
a
Reaction conditions: 1 (0.5 mmol), 4 (2 mmol), Mn-3 (0.025
transfer from the alcohol to the 16e species A takes place to
generate the manganese alkoxide intermediate B. Subse-
quently, β-hydride elimination leads to the formation of an
aldehyde and the hydrogenated catalyst C, storing the
abstracted hydrogen on the metal ligand catalyst. The base-
catalyzed condensation between the in situ generated aldehyde
and carboxylic acid derivative result in the formation of an
unsaturated amide. The shuttled hydrogen on the hydro-
genated catalyst C is then transferred to the CC bond in two
discrete steps. First, the the CC bond is inserted into the
Mn−H bond to produce the intermediate D. Second, the
desired product is released upon proton transfer and the
manganese species A is regenerated. The proton transfer may
also occur by transferring a proton directly from an alcohol,
producing the desired product and the intermediate B without
the regeneration of the 16e species A.20
mmol) and t-BuOK (1 mmol), toluene (1 mL), 100 °C, 4 h. Isolated
yields are shown.
The application of the benzyl alcohol as a coupling partner
gave the desired product 5a in 58% yield (Table 3, entry 1).
Benzylic alcohols containing p-methyl, p-methoxy, and m-
chloro were compatible with the reaction conditions, and the
C-alkylated esters 5b−d were obtained in moderate yields
(Table 3, entries 2−4). The application of 1-naphthaleneme-
thanol led also to the desired product 5e in moderate yield
(Table 3, entry 5). Noteworthy, thiophene-containing
substrate and products such as 5f are tolerated in this
alkylation reaction (Table 3, entry 6).
In order to gain more insight in the reaction mechanism, we
carried out a deuterium-labeled experiment (Scheme 2). In
more detail, [D2]1a was used an alkylating reagent for the
In conclusion, we have reported the first example of
manganese-catalyzed environmentally benign C-alkylation of
unactivated esters and amides with unactivated alcohols.21 The
reaction tolerates a wide range of functional groups and
heterocyclic moieties, providing highly useful upgraded amides
and esters in excellent efficiency. The presented sustainable
alkylation reaction liberates water as the only side product.
Scheme 2. Deuterium-Labeled Experiment
C
DOI: 10.1021/acs.orglett.8b03184
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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.or-
glett.8b03184.
Experimental details and characterization data (PDF)
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̈
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AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: magnus.rueping@rwth-aachen.de.
*E-mail: osama.elsepelgy@rwth-aachen.de.
ORCID
Magnus Rueping: 0000-0003-4580-5227
Osama El-Sepelgy: 0000-0003-3131-4988
Notes
The authors declare no competing financial interest.
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