Manganese(I)-Catalyzed α-Alkenylation of Ketones Using Primary Alcohols

Article abstract of DOI:10.1021/acs.orglett.9b01327

A simple protocol of manganese catalyzed selective α-alkenylation of ketones using primary alcohols is reported. The reactions proceeded well with a low loading of catalyst (0.3 mol %). The overall transformation operates through O-H bond activation of pr

Full text of DOI:10.1021/acs.orglett.9b01327

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Manganese(I)-Catalyzed α‑Alkenylation of Ketones Using Primary
Alcohols
Suhas Shahaji Gawali, Biplab Keshari Pandia, and Chidambaram Gunanathan*
School of Chemical Sciences, National Institute of Science Education and Research, HBNI, Bhubaneswar-752050, India
S
* Supporting Information
ABSTRACT: A simple protocol of manganese catalyzed selective α-
alkenylation of ketones using primary alcohols is reported. The reactions
proceeded well with a low loading of catalyst (0.3 mol %). The overall
transformation operates through O−H bond activation of primary alcohols
via dearomatization−aromatization metal ligand cooperation in the catalyst
to provide the corresponding aldehydes, which further undergo con-
densation with methylene ketones to deliver α,β-unsaturated ketones. This
selective α-alkenylation proceeds with the release of water and liberation of
molecular hydrogen.
ynthesis of α,β-unsaturated ketones is one of the important
reactions in organic synthesis due to the extensive
Scheme 1. Methods for the α-Alkenylation of Ketones
S
applications of such unsaturated ketones in the synthesis of
pharmaceuticals, biologically active compounds, pesticides,
food additives, solar creams, and other functional materials.1 In
conventional synthesis, aldol condensation between ketones
and aldehydes is employed for the preparation of α,β-
unsaturated ketones using a stoichiometric amount of strong
base. In addition to the high cost, aldehydes are prone to air
oxidation resulting in contamination with carboxylic acids,
which creates a problem in storing these reactive compounds
and requires purification before use.2 These drawbacks urged
the development of alternative methods for the synthesis of
α,β-unsaturated ketones. Rhodium-catalyzed direct α-alkeny-
lation of ketones using internal alkynes was reported recently;
however, this method suffers from the requirement of high
catalyst loading and use of bifunctional ligands in stoichio-
metric amounts (Scheme 1a).3 Synthesis of α,β-unsaturated
ketones directly from the alcohols and ketones can result in
synthetic brevity and circumvent the isolation and purification
of intermediates. Thus, tandem synthetic methods are
developed combining the oxidation of alcohols to aldehydes
and condensation of aldehydes with ketones. Precious
transition metals on alkaline solid supports, such as Pd/
AlO(OH),4a Pd/MgO-Al2O3,4b Au/AlO(OH),4c and Pt/
CeO2,4d containing active sites for oxidation of alcohols and
condensation reactions were developed, which also required
excess alkali additives.5 Moreover, heterogeneous base metal
catalysts such as TiN, NbN, and NbCN were also developed
for the synthesis of α,β-unsaturated ketones, but the
preparation of these catalysts requires tedious and harsh
nitridation of the parent metal oxides.6a,b Recently, CrO3 and
CeO2 catalysts have been reported, while the chromium is
toxic, catalysis by cerium oxide is limited to chalcone synthesis
(Scheme 1b).6c,d
method for the preparation of α,β-unsaturated ketones, which
produce water and H2 as the only byproducts. Over the
decades, homogeneous organometallic complexes have been
developed as catalysts for the acceptorless dehydrogenation of
alcohols followed by condensation of in situ generated
aldehydes, with C−H acidic compounds to afford atom
economical and sustainable chemical transformations.8 Often
hydrogen obtained from dehydrogenation of alcohols is
Alternatively, acceptorless dehydrogenative coupling of
Received: April 16, 2019
alcohols7 and ketones is one of the most prominent synthetic
© XXXX American Chemical Society
A
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Letter
utilized for the hydrogenation of condensation products, thus,
resulting in the term “borrowing hydrogen methods”,9 which
attracted increasing attention for the C−C and C−N bond
formation reactions. The direct use of alcohols in dehydrogen-
ative coupling reactions for the synthesis of α,β-unsaturated
ketones compound is challenging, because CC double
bonds undergo facile hydrogenation to deliver the α-alkylated
ketones or alcohols.10,11 In this direction, Yus and co-workers
reported the ruthenium-catalyzed α-olefination of ketones by
acceptorless dehydrogenative coupling alcohols. However,
poor yields, limited substrate scope, and requirement of a
stoichiometric base necessitate the need for an alternative
protocol.12
Scheme 3. Manganese-Catalyzed Direct α-Alkenylation of 1-
Tetralone Using Primary Alcohols
a
In recent years, in order to attain sustainability in catalysis,
replacement of precious noble-metal catalysts with economical
and environmentally benign earth-abundant base-metal cata-
lysts is in high demand. Thus, atom-economical dehydrogen-
ative coupling of alcohols and ketones to α,β-unsaturated
ketones by base-metal catalysis is a desirable transformation. In
this direction, Mn is one of the most earth-abundant transition
metals, third only to Fe and Ti, and chemists have already
developed excellent catalytic transformations using manganese
complexes.13 Encouraged by our recent results in developing
atom economical and sustainable catalytic methods,14 herein,
we report a dehydrogenative coupling of alcohols and ketones
catalyzed by Kempe’s Mn PNP pincer complexes,15a,b which
selectively produce the α-alkenyl ketones, H2O and H2. To our
knowledge, there is no literature report on α-olefination of
ketones catalyzed by Mn complexes.
a
Using 1-tetralone and benzyl alcohol as model substrates,
extensive optimization studies were carried out to determine
suitable experimental conditions for catalytic α-alkenylation of
ketones catalyzed by complexes 1 and 2 (see Table S1).
Observations revealed that 4-methyl substituted catalyst 2 is
better than catalyst 1 containing a phenyl group in the
backbone. While catalyst 1 provided lower yields and
selectivity, the catalytic reaction using 2 (0.3 mol %) and
Cs2CO3 (5 mol %) with 1-tetralone (0.5 mmol) and benzyl
alcohol (0.6 mmol) provided a 97% conversion of 1-tetralone
and the alkenyl and alkylated products 3a:3a′ were isolated in
80% yield with 97:3 selectivity, respectively (Scheme 2). Use
Reaction conditions: 1-tetralone (0.5 mmol), alcohol (0.6 mmol),
tert-amyl alcohol (1.5 mL), catalyst 2 (0.3 mol %), and Cs2CO3 (5
mol %) were heated at 135 °C under open conditions with an argon
flow. Reported yields correspond to isolated pure compounds.
Conversion of 1-tetralone is given within parentheses and determined
by GC analysis using toluene as an internal standard. α-Alkylated
ketone products were observed in less than 5% yield, in all reactions.
b
c
α-Alkylated ketone product was observed in 10% yield. Reaction
d
time of 20 h. Using 0.5 mol % catalyst 2 and 5 mol % base.
alcohols bearing electron-donating substituents such as methyl,
methoxy afforded the corresponding α-alkenyl ketones in good
yields with excellent conversion. Electron-rich benzyl alcohols
substituted with an ortho and a meta methyl group were
converted into 3b and 3c in excellent (90% and 89%,
respectively) yields, whereas, para-methylbenzyl alcohol
provided 3d in 81% yield. The α-alkenylation of 1-tetralone
with 4-methoxy, 4-isopropyl, and piperonyl benzyl alcohols
afforded the corresponding products 3e−3g in 88%, 77%, and
65% yields, respectively. Notably, 4-methylthiolate substituted
benzyl alcohol provided complete conversion under standard
experimental conditions, and the corresponding product 3h
was obtained in 96% yield. The presence of an electron-
withdrawing substituent such as 3-chloro and 4-bromo on the
aryl ring of benzyl alcohol required a longer reaction time (20
h) to provide good selectivity, and the α-alkenyl ketones 3i and
3j were obtained in 54% and 78% yields, respectively.
Importantly, heteroaryl methanols were tolerated in this
catalytic direct α-alkenylation of ketones. 2-Thiophenemetha-
nol and furfuryl alcohol provided α-alkenylated products 3k
and 3l in good yields with excellent conversion. Similarly, 1-
naphthalenemethanol performed well in this transformation
and delivered 3m in 78% isolated yield. When α-branched
aliphatic alcohol such as 2-ethyl-1-butanol was subjected to
Scheme 2. Mn(I)-Catalyzed α-Alkenylation of 1-Tetralone
Using Benzyl Alcohol
of other bases produced poor results. Control experiments with
base alone (without catalyst) and without both base and
catalyst proved that the combination of catalyst and base is
essential for this transformation.
Using the optimized reaction conditions, a wide range of
primary alcohols were subjected to manganese-catalyzed α-
olefination of 1-tetralone (Scheme 3). In general, benzyl
B
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Letter
catalysis under standard experimental conditions, α-alkenylated
product 3n was isolated in poor yields (37%). Thus, the
catalyst load was increased to 0.5 mol %, which provided the
product 3n in 54% yield. In all these reactions, α-alkylated
ketone products were formed in less than 5% yield.
The scope of various aryl ketones with respect to the
catalytic α-alkenylation reaction using different alcohols was
further investigated (Scheme 4). In general, a variety of
which provided α-alkenylated product 4h in 59% yield. To
expand the substrate scope beyond tetralones, 6,7,8,9-
tetrahydro-5H-benzo[7]annulen-5-one was reacted with ben-
zyl alcohol, which delivered 4i in 61% isolated yield.
Remarkably, acyclic methylene ketone such as propiophenone
was reacted with 2,3-dimethoxybenzyl alcohol and 4-
methoxybenzyl alcohol and the corresponding α-alkenylated
products 4j and 4k were obtained in 71% and 87% yields,
respectively.
A mercury-poisoning experiment was performed to examine
the involvement of any metal nanoparticles formed from
complex 2. The catalytic reaction of 1-tetralone with benzyl
alcohol was carried out in the presence of mercury (5 equiv,
relative to 1-tetralone), which provided the corresponding
alkenylated ketone 3a in 73% isolated yield (against the 80%
yield in the absence of mercury; see Scheme 2), indicating that
the reaction proceeds with involvement of molecular
intermediates (Scheme 5a). To probe whether the PN5P−
Scheme 4. Manganese-Catalyzed Dehydrogenative α-
Alkenylation of Ketones Using Primary Alcohols
a
Scheme 5. Mechanistic Studies for the α-Alkenylation of
Primary Alcohols
a
Reaction conditions: ketone (0.5 mmol), alcohol (0.6 mmol), tert-
amyl alcohol (1.5 mL), catalyst 2 (0.3 mol %), and Cs2CO3 (5 mol
%) were heated at 135 °C under open conditions with an argon flow.
Reported yields correspond to isolated pure compounds. Conversion
of ketone is given within parentheses and determined by GC analysis
b
using toluene as an internal standard. Conversion determined from
1H NMR analysis of reaction mixture using 1,2 dibromoethane as an
c
internal standard. Catalyst 2 (0.6 mol %) and base (10 mol %) were
d
used. α-Alkylated ketone product was observed in 35% yield.
primary alcohols can be employed under these reaction
conditions to deliver α-alkenylated products in moderate to
good yields. Reaction of 4-methoxybenzyl alcohol with 0.3 mol
% of 2 and 5 mol % base resulted in quantitative conversion of
arylketone 5-methoxy-1-tetralone, and α-alkenylated product
4a was isolated in 87% yield with 97% selectivity. The α-
alkenyl ketone structure of 4a is also unequivocally
corroborated using single-crystal X-ray analysis. When 6-
methoxy-1-tetralone and 7-methoxy-1-tetralone were reacted
with 3,4-dimethoxybenzyl alcohol and 4-methylbenzyl alcohol,
the corresponding α-alkenyl ketones 4b and 4c, respectively,
were isolated in good yields. Similarly, 4-(methylthio)benzyl
alcohol and 2-thiophenemethanol showed very good reac-
tivities in the α-olefination of ketones and the products 4d and
4e were obtained in 76% and 89% yields, respectively. When,
4-methyl-1-tetralone was reacted with 4-methylbenzyl alcohol
and furfural alcohol, the corresponding alkenylated products 4f
and 4g were formed in 71% and 86% yields, respectively.
Notably, unactivated aliphatic alcohols are also amenable to
this catalytic α-alkenylation of ketones. 2-Ethyl-1-butanol was
subjected to catalysis under standard experimental conditions,
Mn complex 2 is also involved in the C−C bond formation in
the aldol reaction, reaction of benzaldehyde with 1-tetralone in
the presence and absence of the catalyst were performed,
which resulted in formation of 80% and 70% of 3a, respectively
(Scheme 5b and 5c). This enhanced formation of 3a is an
indication that the catalyst might have moderate involvement
in the key C−C bond formation process. Upon reaction of α-
deuterated benzyl alcohol-d3, product 3a-D was obtained in
which incorporation of 80% deuterium was observed at the
vinyl position (Scheme 5d).
On the basis of these experimental observations and the
previous studies involving the PN5P manganese pincer
catalyst,15 the plausible reaction mechanism for acceptorless
dehydrogenative coupling of alcohols and ketones is proposed
in Scheme 6. Complex 2 reacts with a catalytic amount of base
to generate the dearomatized coordinatively unsaturated
C
DOI: 10.1021/acs.orglett.9b01327
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Scheme 6. Proposed Reaction Mechanism for
Dehydrogenative α-Alkenylation of Ketones Catalyzed by
Manganese Pincer Complex 2
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01327.
■
S
Experimental procedures, spectral data, crystal data of
1
compounds 3g and 4a, and copies of H and 13C NMR
spectra of the products (PDF)
Accession Codes
CCDC 1905606−1905607 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: gunanathan@niser.ac.in.
ORCID
Chidambaram Gunanathan: 0000-0002-9458-5198
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
intermediate I. Further this intermediate I reacts with alcohol
to provide alkoxy-ligated manganese complex II, through O−
H activation of alcohols. Similar manganese alkoxy complexes
have been isolated previously by Milstein,16 Yu,17a and Liu.17b
β-Hydride elimination of manganese-ligated alkoxy ligand with
saturated intermediate II provides the corresponding aldehyde
and Mn-hydride complex III. The liberated aldehyde then
undergoes aldol condensation with ketone to provide the α-
alkenylated ketone and water as the byproduct. Intermediate
III liberates H2 to attain dearomatized intermediate I,
completing one loop in a catalytic cycle. The aromatization
and dearomatization metal−ligand cooperation in manganese
intermediates I−III maintains the same oxidation state (+1) in
all intermediate complexes and plays an important role in this
transformation.
In conclusion, a manganese pincer complex catalyzed
selective synthesis of α-alkenyl ketones by acceptorless
dehydrogenative coupling of primary alcohol with ketones is
demonstrated. Notably, using a minimal base metal−
manganese catalyst (0.3 mol %) and a mild base Cs2CO3 (5
mol %), an assortment of cyclic and acyclic benzylic ketones
can be efficiently and selectively α-alkenylated with primary
alcohols in moderate to excellent yields. Notably, under these
catalytic conditions, heteroaryl, aliphatic, and benzylic primary
alcohols containing methoxy, thiomethyl, chloro, and bromo
functionalities are well tolerated. Mechanistic studies confirm
the involvement of an aldehyde intermediate. The acceptorless
dehydrogenative coupling of alcohols and ketones proceeds via
dearomatization−aromatization metal ligand cooperation.
Overall, 1 equiv of water and molecular hydrogen are liberated
from the reaction as the only byproducts in this environ-
mentally benign transformation.
We thank SERB New Delhi (EMR/2016/002517), DAE, and
NISER for financial support. S.S.G. thanks CSIR for a research
fellowship, and B.K.P. is thankful for a NISER fellowship. We
thank Dr. Sadhika Khullar from Dr. B. R. Ambedkar National
Institute of Technology, Jalandhar for her kind help.
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