Manganese-Catalyzed Ring-Opening Coupling Reactions of Cyclopropanols with Enones

Article abstract of DOI:10.1021/acs.orglett.9b01703

A manganese-catalyzed ring-opening coupling reaction of cyclopropanols with enones for the facile and efficient preparation of 1,6-diketones is described. A wide array of synthetically important 1,6-diketones bearing manifold functional groups are obtained with up to 93% yield. These reactions feature broad substrate scopes, environmentally benign conditions, inexpensive catalyst, and operational simplicity.

Full text of DOI:10.1021/acs.orglett.9b01703

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Manganese-Catalyzed Ring-Opening Coupling Reactions of
Cyclopropanols with Enones
Yong-Hui Zhang,†,‡,§ Wen-Wei Zhang,§ Ze-Yu Zhang,† Kai Zhao,*,† and Teck-Peng Loh*,†,‡
†Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center
for Advanced Materials, Nanjing Tech University, Nanjing 211816, China
‡Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
§BGI-Shenzhen, Beishan Industrial Zone, Yantian District, Shenzhen 518083, China
S
* Supporting Information
ABSTRACT: A manganese-catalyzed ring-opening coupling reaction of cyclopropanols with enones for the facile and efficient
preparation of 1,6-diketones is described. A wide array of synthetically important 1,6-diketones bearing manifold functional
groups are obtained with up to 93% yield. These reactions feature broad substrate scopes, environmentally benign conditions,
inexpensive catalyst, and operational simplicity.
1,6-Diketones are privileged and pivotal building blocks in the
construction of biologically and pharmaceutically important
five- and six-membered carbocyclic and heterocyclic com-
pounds.1 In contrast to their structural analogues such as 1,4-
and 1,5-diketones which could be easily and efficiently
prepared through dozens of methods,2 the synthetic strategies
to access 1,6-diketones are considered to be more challenging
and less developed.3 The generally applicable methods leading
to 1,6-diketones include transition-metal-catalyzed hydration
of alkynes,4 Wacker-type oxyfunctionalization of terminal
alkenes,5 oxidative ring-opening cleavage of cyclohexanone,
cyclohexenes and vicinal diols,6 and reductive homocoupling of
enones.7 However, most of the above-mentioned methods
suffer from limitations including relatively harsh reaction
conditions, nonreadily available starting materials, limited
substrate scopes that always gave rise to symmetrical 1,6-
diketones, as well as other limitations in terms of selectivity
and efficiency. Unsymmetrical 1,6-diketones bearing a broad
array of functional groups are recognized as more attractive
synthetic targets in comparison with their symmetrical
analogues in view of their versatile utilities in modern synthetic
chemistry. In the past few decades, the carbon−carbon bond-
forming reaction of β-carbonyl nucleophiles (generated
through copper-, mercury-, or silver-promoted electrophilic
ring opening of siloxycyclopropanes)8 or β-carbonyl radicals
(generated via manganese-promoted ring opening of cyclo-
propanol derivatives)9 with electrophilic enones has emerged
as an efficacious and useful reaction platform for the
preparation of unsymmetrical 1,6-diketones.
Giese8c and Narasaka9 suffered from relative limitations in
terms of the employment of more than a stoichiometric
amount of heavy metal reagents such as mercury acetate
(Scheme 1a) or radical initiators such as tributylhydrido tin
(Scheme 1b), the need of an excess amount of enones, as well
as the selectivity issue attributed to the inevitable formation of
homocoupling products. In this regard, the quest for catalytic
Scheme 1. Ring-Opening Coupling of Cyclopropanol
Derivatives with Enones to Access 1,6-Diketones
Nevertheless, the pioneering examples to furnish unsym-
metrical 1,6-diketones via the reaction of β-carbonyl
nucleophiles/or radicals with enones as demonstrated by
Received: May 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01703
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
protocols for the practical and selective preparation of
unsymmetrical 1,6-diketones under operationally simple and
environmentally benign conditions remains an unmet
challenge for synthetic chemists.
9). It is worth noting that in the presence of InCl3, FeCl3, and
Fe(OTf)3 the reaction proceeded to give the desired product,
albeit in moderate yields (Table 1, entries 10−12).
Subsequently, a number of manganese catalysts were
examined, and it was observed that Mn(OAc)3, Mn(acac)3,
MnBr2, and Mn(OAc)2 were inferior to the reactivity (Table 1,
entries 13−16). To our delight, Mn(acac)2 could efficiently
promote the reaction to forge 3aa in 66% yield. Upon the
elevation of the reaction temperature, a dramatic increase of
the product yields was observed (Table 1, entries 17−20). The
optimal result was obtained when Mn(acac)2 was employed as
a catalyst and propiononitrile bearing a higher boiling point
than acetonitrile was used as a solvent at 120 °C (Table 1,
entry 20).
In the past few years, cyclopropanols prepared through
Kulinkovich reaction10 have been widely employed as
surrogates of homoenolates or β-keto radicals for the
preparation of β-functionalized ketones or aldehydes, as
evidenced by several significant breakthroughs achieved by
the groups of Narasaka,9 Chiba,11 Dai,12 Cha,13 Zhu,14
Walsh,15 Li,16 Kananovich,17 Ma,18 Wang,19 Chen,20 Orella-
na,21 and many others.22 Our group has also demonstrated a
silver-catalyzed oxidative fluorination of cyclopropanols to
yield β-fluorinated ketones.23 Enlightened by those seminal
works, we herein demonstrated a highly selective and efficient
manganese-catalyzed ring-opening coupling reaction of cyclo-
propanols with equimolar amounts of enones, giving rise to a
broad range of synthetically valuable unsymmetrical 1,6-
diketones bearing various functional groups in an atom-
economical and environmentally benign manner without
resorting to substrate prefunctionalization and toxic metal
reagents (Scheme 1c).
With the optimal reaction conditions in hand, we started to
investigate the substrate scope with respect to enones, and the
results were summarized in Scheme 2.
a b
,
Scheme 2. Substrate Scope of Enones
Our investigation commenced by examining the model
reaction of 1-benzylcyclopropan-1-ol (2a) with phenyl vinyl
ketone (1a) to optimize the reaction conditions.
The reaction was performed at 60 °C using acetonitrile as
the solvent. Unsurprisingly, the desired 1,6-diketone (3aa) was
not obtained in the absence of catalyst (Table 1, entry 1).
Meanwhile, a number of transition-metal catalysts such as
CoBr2, Pd(OAc)2, RhCl(PPh3)3, and NiCl2 etc., proved to be
noneffective for the above transformation (Table 1, entries 2−
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
solvent
temp (°C)
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
-
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
C2H5CN
C2H5CN
C2H5CN
C2H5CN
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
80
100
120
140
0
0
0
0
<5%
0
0
0
0
30
53
45
<5%
<5%
0
CoBr2
Co(OAc)2
Fe(acac)3
PdCl2
Pd(OAc)2
NiCl2
[Rh(C5Me5)2]2
RhCl(PPh3)3
InCl3
FeCl3
Fe(OTf)3
MnBr2
Mn(OAc)2
Mn(OAc)3
Mn(acac)3
Mn(acac)2
Mn(acac)2
Mn(acac)2
Mn(acac)2
Mn(acac)2
0
66
70
88
90
88
a
Unless otherwise noted, all reactions were performed with 1a−1z, 4
or 5 (0.5 mmol), 2a (0.5 mmol), Mn(acac)2 (10 mol %), and
a
b
c
Unless otherwise noted, all reactions were performed with 1a (0.5
CH3CH2CN (2 mL), for 8 h. Isolated yields. The reaction was
performed on a 3.4 mmol scale, with 1a (0.45 g, 3.4 mmol), 2a (0.5 g,
3.4 mmol), Mn(acac)2 (0.34 mmol), and CH3CH2CN (35 mL).
mmol), 2a (0.5 mmol), catalyst (10 mol %), and solvent (2 mL), for 8
b
h. Isolated yields.
B
DOI: 10.1021/acs.orglett.9b01703
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In general, enones bearing either electron-donating groups
(1a−1g) or electron-withdrawing substituents (1h−1k) were
viable substrates that could react with 1-benzylcyclopropan-1-
ol (2a) smoothly to give the desired products (3aa−3ja) in
moderate to good yields. It is notable that a diverse range of
synthetically crucial functional groups such as −CF3, −CN,
−CO2Me, −NO2, and alkyne moiety were well tolerated in
standard reaction conditions and remained intact. Notably,
halide-substituted 1,6-diketones 3ma and 3na were also
attained in good yields, rendering them amenable for further
functionalization through cross-coupling reactions. Gratify-
ingly, a diverse array of heterocyclic-substituted enones were
well-adapted to the reaction with 2a to furnish the desired 1,6-
diketones (3pa−3va) in 53−93% yields. Importantly, aliphatic
enones were eligible substrates to give 3wa and 3xa in excellent
yields, respectively. Unsaturated alkene and alkyne substituents
did not result in a deleterious effect on the reaction, thus giving
3ya and 3za in synthetically useful yields. It was notable that α-
methyl-substituted enone 4 could also react with 2a smoothly
to give the coupling product 6, albeit with a slight decrease of
the product yield. In addition, the α,β-unsaturated sulfone 6
proved to be a feasible coupling partner of 2a to forge the
desired product 7 in 27% yield. Remarkably, in all cases,
equimolar amounts of enones were adequate to produce the
1,6-diketones in satisfying yields in an atom-economical
manner. Gratifyingly, a large-scale reaction of 1a of 2a proved
to be practical and efficient, forging 3aa in 72% yield.
good yields. Next, a plethora of phenyl-substituted cyclo-
propanols proved to be viable in this reaction to furnish the
desired products 3ag−3aj in synthetically useful yields. Also
noteworthy was the excellent compatibility of furan-containing
cyclopropanol, affording 3ak in 57% yields. Gratifyingly,
aliphatic-substituted cyclopropanols uneventfully participate
in the transformation to provide 3al−3an with good yields and
selectivities. Notably, the presence of relatively sensitive
functional groups such as an alkene moiety and hydroxyl
group did not attenuate the reaction efficiency, thus giving rise
to synthetically useful products 3ao and 3ap in one step
without resorting to the protection and deprotection steps. It is
notable that the trisubstituted cyclopropanol (2q) also proved
to be a suitable substrate for this transformation, affording 3aq
in 65% yield.
To shed more light on the mechanism of the aforemen-
tioned transformations, a series of control experiments were
conducted. The reaction of 1a with 2a in the presence of
radical scavengers such as 2,2,6,6-tetramethylpiperidine 1-oxyl
(TEMPO) and 2,6-di-tert-butyl-4-methylphenol (BHT) under
optimized conditions was completely suppressed, implicating a
plausible radical pathway (Scheme 4, eq 1). Importantly, the
Scheme 4. Control Experiments
Next, the substrate scope of cyclopropanols was also
investigated, and the results were summarized in Scheme 3.
a b
,
Scheme 3. Substrate Scope of Cyclopropanols
reaction of deuterium-labeled cyclopropanol 2a-[d] (68%
deuterated) with 1a led to the formation of 1,6-diketone
3aa-[d] (57% deuterated), which was probably formed
through the addition of a β-carbonyl radical to enone followed
by an ensuing D-abstraction (Scheme 4, eq 2). The ring-
opening coupling reaction carried out in deuterated acetoni-
trile led to no deuterium-substituted product, thus ruling out
the possibility of solvent as the deuterium source (Scheme 4,
eq 3).
a
Unless otherwise noted, all reactions were performed with 1a (0.5
mmol), 2a−2q (0.5 mmol), catalyst (10 mol %), and solvent (2 mL),
b
for 8 h. Isolated yields.
A plausible radical reaction pathway was proposed to
rationalize the above observations, as depicted in Figure 1.
Initially, the reaction of Mn(acac)2 with cyclopropanol 2a gives
a Mn(I) species and an O-centered radical species (I) through
proton-coupled electron transfer (PCET),24 wherein the
Initially, a broad range of benzyl-substituted cyclopropanols
were examined. To our delight, the substituents on the phenyl
ring and benzylic position did not lead to an obvious negative
effect on the reaction efficiency, giving 3ab−3af in moderate to
C
DOI: 10.1021/acs.orglett.9b01703
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the financial support from Nanjing
Tech University (Start-up Grant Nos. 39837137, 39837101,
and 3827401739), the National Natural Science Foundation of
China (21372210, 21672198, and 21801129), the State Key
Program of National Natural Science Foundation of China
(21432009), and the Natural Science Research Projects in
Jiangsu higher education institutions (18KJB150018).
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01703.
1
Experimental details, analytical data, and H and 13C
NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: ias_kzhao@njtech.edu.cn.
*E-mail: teckpeng@ntu.edu.sg.
ORCID
Teck-Peng Loh: 0000-0002-2936-337X
D
DOI: 10.1021/acs.orglett.9b01703
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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