Mn(ii)-catalysed alkylation of methylene ketones with alcohols: Direct access to functionalised branched products

Article abstract of DOI:10.1039/c8cc08010k

Herein an operationally simple alkylation of methylene ketones with primary alcohols is reported. Use of an inexpensive and earth abundant Mn/1,10-phenanthroline system enables direct access to a series of functionalised branched ketones including one-pot sequential double alkylation and Alzheimer's drug donepezil. Preliminary mechanistic investigation, determination of the rate and order of reactions and deuterium labeling experiments support the participation of the hydrogen-borrowing strategy for the ketone alkylation.

Full text of DOI:10.1039/c8cc08010k

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Mn(II)-catalysed alkylation of methylene ketones
with alcohols: direct access to functionalised
branched products†
Cite this:DOI: 10.1039/c8cc08010k
Received 7th October 2018,
Accepted 25th October 2018
Lalit Mohan Kabadwal,‡ Jagadish Das‡ and Debasis Banerjee
*
DOI: 10.1039/c8cc08010k
rsc.li/chemcomm
Herein an operationally simple alkylation of methylene ketones
with primary alcohols is reported. Use of an inexpensive and earth
abundant Mn/1,10-phenanthroline system enables direct access to a
series of functionalised branched ketones including one-pot sequential
double alkylation and Alzheimer’s drug donepezil. Preliminary mecha-
nistic investigation, determination of the rate and order of reactions
and deuterium labeling experiments support the participation of the
hydrogen-borrowing strategy for the ketone alkylation.
Branched a,a-di-substituted ketones are ubiquitous in natural
products and pharmaceuticals and broadly used as valuable
intermediates in multi-step organic synthesis.1 In general, pre-
functionalised alkyl halides with toxic bases are used for their
synthesis, relieving stoichiometric waste.2 Therefore, develop-
Scheme 1 (a) Metal-catalysed alkylation of methyl ketones. (b) Precious
metal catalysed alkylation of methylene ketones. (c) Mn-catalysed sustain-
able synthesis of di-substituted branched ketones.
ment of green and sustainable technology for a,a-di-substituted catalysts such as Fe, Mn, Ni and Co would be more attractive
ketones is in demand. In this direction, transition-metal catalysed and sustainable alternatives.5a–c However, to date, applications
alkylation of ketone enolates using primary alcohols is an attractive of homogeneous Fe, Mn, and Co-based catalysts have been only
approach for the synthesis of di-substituted branched products. limited to the monoalkylation of acetophenones with alcohols
Such processes follow the redox neutral or borrowing hydrogen pertaining to the linear ketones and have never been demon-
(BH) approach for the construction of new C–C bonds and generate strated for the branched products.3e, f
water as the sole byproduct (Scheme 1).2
Notably, in this direction, utilisation of manganese for such
Unfortunately, to date, synthesis of branched di-substituted (de)hydrogenative coupling would be a potential sustainable
ketones has scarcely been known and is relatively underdeveloped alternative, as manganese is the third most earth-abundant
(Scheme 1, eqn (a) and (b)).3 For instance, only Ir-4a–e and Ru- metal. Indeed, recently manganese complexes were utilised for
catalysts4f (homogeneous) and Pd-,4g–i Ni-,3a,b or Ag/Mo-catalysts4j,k various key catalytic transformations such as the synthesis of
(heterogeneous) were reported for alkylation of methylene ketones amines with alcohols following the borrowing hydrogen approach;6
with primary alcohols, providing such branched ketones.
N-formylations of amines with methanol;7 synthesis of pyrroles8a
Notably, recently there has been potential drive in catalysis and quinolines;8b,c four-component synthesis of pyrimidines using
research to replace noble metal-catalysts with more highly three different alcohols with amidines,8d imines,9 cyclic amides
abundant base metals due to the potential toxicity, low natural and imides,9 and unsaturated nitriles;10 C-alkylation of ketones,3f
abundance and expensive nature of noble metals.5a,c Therefore, methanol to CO2 and H2,9 and hydrogenation of alcohols to esters,9
key catalytic transformations involving earth abundant metal double bonds and nitriles were also realized.11
It is worth mentioning that most of these manganese-
complexes employ pyridinyl-core PNP ligands, diethylamine-
Department of Chemistry, Laboratory of Catalysis and Organic Synthesis,
core PNP ligands, PN3P ligands and triazinyl-core PN5P ligands
Indian Institute of Technology Roorkee, Roorkee-247667, Uttarakhand, India.
to achieve higher catalytic efficiency.9 Nevertheless, often major
E-mail: dbane.fcy@iitr.ac.in
concerns associated with these phosphine based pincer ligands
are their highly expensive nature in comparison to base-metal
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cc08010k
‡ These authors contributed equally to this work.
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Table 1 Discovery of the reaction conditionsa
Conv. (%)
Deviation from the
Entry
standard conditions
3a
3a0
18
Scheme 2 Manganese catalysed alkylation of methylene ketones.
1
2
3
4
5
6
7
8
—
36 h
Mn(CO)5Br
L2
L3
L4
L5
L6
t-BuONa
K2CO3
K3PO4
Cs2CO3
No base
No catalyst, no ligand
72 (67)b
80 (72)b
45
20
50
58
68
30
42
24
47
33
0
42
27
22
15
57
37
14
4
0
0
o15
catalysts, multi-step synthesis and storage and handling under
standard laboratory environments.9
In our continuous efforts recently we started a research program
to establish sustainable catalytic transformations using renewable
resources in combination with inexpensive metal catalysts using
commercially available bench stable ligands.12 To our delight,
very recently we demonstrated the nickel-catalysed a-alkylation
of methylene ketones with primary alcohols to yield a series of
a,a-di-substituted branched products. Unfortunately, in the case
of sterically hindered 1,2-diphenyl ethanone we observed only a
moderate product yield when a combination of 5 mol% NiBr2
and 6 mol% 1,10-phenanthroline was used.12d Considering this
challenge, we wondered whether a suitable manganese catalyst
with a nitrogen ligand could be beneficial for such gem-di-
substituted ketones.12d
9
10
11
12
13
14
0
0
o25
a
Reaction conditions: Unless otherwise specified, propiophenone 1a
(0.25 mmol), benzyl alcohol 2a (0.3125 mmol), Mn(acac)2 (2.5 mol%), phen
(3 mol%), t-BuOK (0.25 mmol), toluene (2.0 mL), Schlenk tube under a
b
nitrogen atmosphere, 140 1C oil bath, 24 h reaction time. Isolated yield
(average of two runs). L1 = 1,10-phenanthroline. L2 = 2,9-dimethyl-1,10-
phenanthroline. L3 = 4,40-dimethylbipyridine. L4 = 2,20-biquinoline.
L5 = bipyridine. L6 = 1,2-bis(dimethylamino)ethane (TMEDA).
Nevertheless, to the best of our knowledge, to date, no
manganese-catalysed BH protocol (dehydrogenation, conden- Gratifyingly, when 1-naphthylmethanol (2i), heteroaryl substituted
sation and hydrogenation) for coupling of primary alcohols benzyl alcohol (2j) and 2-pyridinemethanol (2k) were employed,
with methylene ketone has been known (Scheme 2).9 Herein, they furnished the di-substituted ketones 3j–3l in 55–76% yields
we report the first example using a manganese catalysed route respectively. Importantly, 2-thiophenemethanol (2l) efficiently
with inexpensive nitrogen ligands for the synthesis of a,a-di- participated under the manganese-catalysed conditions to give
substituted branched products. Notably, in this process only 3m in 42% yield (Table 2). Notably, the catalytic performances of
water is released as the byproduct, rendering them sustainable alkyl alcohols 2m and 2n were sluggish and resulted in 3n and 3o
or environmentally benign.
(30–33%) after 24 h. However, renewable terpenoid intermediate
To further optimise the efficient catalytic protocol, a combi- citronellol (2o) furnished the di-substituted ketone 3p in 50%
nation of 2.5 mol% Mn(acac)2, 3 mol% L1 and t-BuOK in toluene yield. This chemo-selective transformation represents the syn-
was used in the model reaction of propiophenone 1a with benzyl thetic potential of the optimised protocol (Table 2). Moreover,
alcohol 2a, which resulted in branched ketone 3a in 72% isolated benzyl substituted phenyl ketone (1c) resulted in 3q–3r in up to
yield (Table 1, entries 1 and 2). Next, application of different 45% yield. Again, application of n-propyl substituted phenyl
Mn-catalysts, ligands L2–L6 and different bases and solvents ketone (1d) gave a moderate yield of 3s along with 30% reduced
did not display any improvement in the catalytic activity alcohol 3s0 (Table 2). Pleasingly, tetralone derivatives 1e and 1f
(Table 1, entries 3–12, ESI,† Tables S1–S5). However, control gave the desired a-substituted ketones 3u and 3t in up to 76%
experiments revealed the independent role of each component isolated yield. After having witnessed the excellent catalytic effi-
to achieve higher product yields (Table 1, entries 13 and 14). ciency, we explored the optimised protocol for the synthesis of
Notably, in some cases we observed the formation of reduced donepezil, known as the best-selling drug for the treatment of
alcohol 3a0 and 5–10% of the corresponding enone 3a00 during Alzheimer’s disease.13 Pleasingly, using the present manganese
the GC-MS analysis of the crude reaction mixture (Table 1 and catalyzed protocol, donepezil 3v was obtained in moderate yield
Scheme 2). After having the optimal conditions, we utilised (Table 2). Remarkable application of 4-cholesten-3-one (1h) with
1,2-diphenyl ethanone with a variety of functionalised alcohols, benzyl alcohol 2a resulted in an excellent yield of a-benzylated
which resulted in the branched products in good to excellent product 3w (Table 2). Interestingly, fatty acid derived oleyl alcohol
yields (Table 2). Benzyl alcohols substituted with electron rich 2q upon reaction with 1a efficiently transformed into the corres-
methyl, ethyl or iso-propyl groups yielded the desired gem-di- ponding di-substituted ketone 3x without much affecting the
substituted ketones 3b–3e in 69–84% yield, respectively (Table 2). double bond (Table 2).
Pleasingly, methoxy (2e), fluoro (2f), chloro (2g) and trifluoro-
Notably, the catalytic protocol is tolerant to halides (F and
methyl (2h) substituted benzyl alcohols efficiently transformed Cl), alkyl, methoxy, trifluoromethyl and 1,3-dioxolone moieties
into the branched ketones 3f–3i in up to 80% yield (Table 2). including the thiophene and pyridine groups. Application of
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Table 2 Mn-catalysed alkylation of methylene ketonesa
Table 3 Sequential one pot double alkylation using a single catalyst
Reaction conditions: acetophenone 1 (0.25 mmol), benzyl alcohol 2a
(0.25 mmol), Mn(acac)2 (2.5 mol%), phen (3 mol%), t-BuOK (0.25 mmol),
toluene (2.0 mL), Schlenk tube under a nitrogen atmosphere, 140 1C oil
bath. After 2 h alcohol 2 (0.3125 mmol) in toluene (0.5 mL) was added
under a nitrogen atmosphere, 140 1C oil bath, 36 h.
(Scheme 3b and ESI,† Scheme S4). Further, we independently
performed parallel and crossover experiments using a 1 : 1 mixture
of 2a and 2a-d2 under the standard catalytic conditions of Table 2.
Interestingly, the resulting ketones were obtained in 55–77% yield and
displayed variable ratios of the H/D-scrambled product or deuterium
incorporation at the a- and b-positions (Scheme 3b and ESI,† Schemes
S1 and S2).14 However, catalytic experiments using methylene ketones
a
Reaction conditions: Unless otherwise specified, propiophenone 1a
(0.25 mmol), benzyl alcohol 2a (0.3125 mmol), Mn(acac)2 (2.5 mol%),
phen (3 mol%), t-BuOK (0.25 mmol), toluene (2.0 mL), Schlenk tube
under a nitrogen atmosphere, 140 1C oil bath, 36 h reaction time.
b
c
t-BuOK (0.375 mmol) was used. 24 h reaction time.
reducible functionalities such as citronellol and fatty acid alcohols
significantly broadens the scope of the established protocol.
Next, we demonstrated the manganese-catalysed one-pot sequen-
tial bis-alkylation of ketones with two different benzyl alcohols using
a single catalyst (Table 3). We chose acetophenone and 4-methoxy
acetophenone with electronically different benzyl alcohols.
Remarkably, the first step resulted in the selective mono-
benzylation of acetophenone to the linear ketones, then a
sequential addition of second alcohols resulted in hetero-a,a-di
substituted ketones in up to 80% yield (Table 3).
Mechanistically, we envisioned that manganese catalysed
a-alkylation might involve in situ multi-step processes such
as dehydrogenation of an alcohol to the carbonyl compound
followed by base catalysed condensation to intermediate enone
3a00. In the next step in situ hydrogenation of the enone resulted
in the branched ketone 3a (Scheme 2).9 To evidence this, enone
3a00 was independently prepared and was employed with 2a and
2a-d2 (92% D) under standard conditions. The desired branched
ketones 3a and 3a-d2 were obtained in quantitative yields and
exhibited 16% and 26% deuterium incorporation at the a-and
b-positions of 3a-d2 (Scheme 3a and ESI,† Scheme S3). However,
when a-alkylation using methylene ketone 1a with 2a-d1 (98% D)
was performed, we detected only o2% deuterium incorporation
in the branched ketone using 1H-NMR and GC-MS analysis
Scheme 3 Preliminary mechanistic studies. Reaction conditions: a unless
otherwise specified, propiophenone 1a (0.1 mmol), benzyl alcohol 2a
(0.125 mmol), Mn(acac)2 (2.5 mol%), phen (3 mol%), t-BuOK (0.1 mmol),
toluene (1.0 mL), Schlenk tube under a nitrogen atmosphere, 140 1C oil
bath, 36 h reaction time.
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product in 42% yield (Scheme 3b and ESI,† Scheme S5).
It is worth mentioning that all these deuterium labeling
experiments as well as the formation of H/D-scrambled products
provide evidence for the involvement of the borrowing hydrogen
strategy under manganese catalysis.12,14 Further, incorporation of
deuterium at variable amounts in the a- and b-positions in the
branched ketones also strongly supports the micro-reversible
transformation for the alkylation process. Next, we demonstrated
the rate and order of the alkylation process using kinetic studies.
The experimental outcomes displayed first order kinetics with
respect to methylene ketone 1a while considering a steady state
approximation for benzyl alcohol (ESI,† Scheme S6).
In conclusion, we have developed an inexpensive and phos-
phine free manganese-catalysed practical route for the synthesis
of branched di-substituted ketones. Utilisation of renewable
alcohols with a variety of aryl and alkyl methylene ketones yielded
the functionalised branched products in up to 84% yield. As a
highlight, we have demonstrated the sequential one-pot double
alkylation to functionalise hetero bis-alkylated ketones using
2.5 mol% Mn-catalyst. To establish the synthetic utility of the
catalytic protocol; Alzheimer’s drugs, functionalization of steroid
hormones and fatty acid derivatives has been demonstrated.
Preliminary mechanistic investigation, kinetic studies to deter-
mine the rate and order of reaction and a series of deuterium
labeling studies were performed to establish the borrowing
hydrogen approach for ketone alkylation.
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The authors thank SERB, India (Early Career Research Award
to D. B., ECR/2015/000600), and IIT Roorkee (SMILE-32). L. M. K.
and J. D. thank IIT-R for financial support.
Conflicts of interest
There are no conflicts to declare.
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