Mild Ring Contractions of Cyclobutanols to Cyclopropyl Ketones via Hypervalent Iodine Oxidation

Article abstract of DOI:10.1002/adsc.201701237

An iodine-mediated oxidative ring contraction of cyclobutanols has been developed. The reaction allows the synthesis of a wide range of aryl cyclopropyl ketones under mild and eco-friendly conditions. A variety of functional groups including aromatic or alkyl halides, ethers, esters, ketones, alkenes, and even aldehydes are nicely tolerated in the reaction. This is in contrast with traditional synthetic approaches for which poor functional group tolerance is often a problem. The practicality of the method is also highlighted by the tunability of iodine oxidation system. Specifically, combining the iodine(III) reagent with an appropriate base allows the reaction to accommodate a range of challenging electron-rich arene substrates. The facile scalability of this reaction is also exhibited herein. (Figure presented.).

Full text of DOI:10.1002/adsc.201701237

Title: Mild Ring Contractions of Cyclobutanols to Cyclopropyl Ketones
via Hypervalent Iodine Oxidation
Authors: Yan Sun, Xin Huang, Xiaojin Li, Fan Luo, Lei Zhang,
Mengyuan Chen, Shiya Zheng, and Bo Peng
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201701237
Link to VoR: http://dx.doi.org/10.1002/adsc.201701237

10.1002/adsc.201701237
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Mild Ring Contractions of Cyclobutanols to Cyclopropyl
Ketones via Hypervalent Iodine Oxidation
Yan Sun,+a Xin Huang,+a Xiaojin Li,a Fan Luo,a Lei Zhang,a Mengyuan Chen,a Shiya
Zhenga and Bo Penga,b,*
a
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal University, Jinhua
321004, China
Fax: (+86) 579-8228-1739; email: pengbo@zjnu.cn
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
Y.S. and X.H. contributed equally to this work.
b
+
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. An iodine-mediated oxidative ring contraction of
cyclobutanols has been developed. The reaction allows the
synthesis of a wide range of aryl cyclopropyl ketones under
mild and eco-friendly conditions. A variety of functional
groups including aromatic or alkyl halides, ethers, esters,
ketones, alkenes, and even aldehydes are nicely tolerated in
the reaction. This is in contrast with traditional synthetic
approaches for which poor functional group tolerance is
often a problem. The practicality of the method is also
highlighted by the tunability of iodine oxidation system.
Specifically, combining the iodine(III) reagent with an
appropriate base allows the reaction to accommodate a
range of challenging electron-rich arene substrates. The
facile scalability of this reaction is also exhibited herein.
Keywords: ring contraction; hypervalent iodine;
cyclopropyl ketone; cyclobutanol
Scheme 1. Ring contractions of cyclobutanols to
cyclopropyl ketones
Cyclopropyl ketones are core structures in many
natural products and bioactive compounds.[1] They
have also been used in the synthesis of a variety of
invaluable cyclic compounds[2] and are intriguing
structures in mechanistic studies.[3] The synthetic
routes to these compounds include cyclopropanation
of alkenes,[4] ring closure of γ-halogenated ketones,[5]
ring contractions of cyclobutanes,[6-8] and diverse
elaborations of cyclopropyl derivatives.[9] Among
these routes, the contraction of strained cyclobutanols
contracted to even smaller cyclopropyl ketones,
which proceeds via C-C bond cleavage and
reconnection, is mechanistically appealing but rarely
studied.[6,7] Only two protocols for this ring-
contraction process can be found in the literature
(Scheme 1): (1) acid or base promoted ring
contraction of vicinally disubstituted cyclobutanols
(eq 1)".[6] (2) palladium catalyzed oxidative ring
contraction of cyclobutanols (eq 2).[7] Despite
considerable advances made in the synthesis of
cyclopropyl ketones, the development of a mild and
eco-friendly reaction with good functional group
compatibility is highly desirable.
Hypervalent iodine reagents have emerged as
important and unique oxidants in organic synthesis
due to their diverse reactivities, low toxicities,
accessibility and easy handling.[10] Numerous
iodine(III)-mediated transformations have been
developed over the past few decades. An iodine
oxidation protocol has also been applied to the ring
contractions of cyclic ketones and alkenes.[11] Unlike
the reported reactions, we herein disclose an iodine-
mediated ring contraction of cyclobutanols to
cyclopropyl ketones (eq 3).
We commenced the study by using cyclobutanol
1a as a model substrate (Table 1). After thoroughly
surveying
reaction
conditions
(temperature,
concentration, solvent, etc.),[12] PhI(OAc)2/TMSOTf
(1/2) in DCM at -50 ℃ for 6 h were identified as the
optimum conditions for the reaction of 1a. The
1
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10.1002/adsc.201701237
Advanced Synthesis & Catalysis
Table 1. Effect of oxidants and electrophilic promoters[a]
Table 2. Substrate scope[a]
entry oxidant
promoter
TMSOTf
TMSOTf
none
TMSOTf
TMSOTf
TMSOAc
BF3∙Et2O
Tf2O
Tf2O
Tf2O
none
none
yield (%)[b]
85
1
PhI(OAc)2
2
none
0[c]
3
4
5
6
7
8
9
10
11
12
PhI(OAc)2
PhI(CF3CO2)2
PhI(OH)OTs
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OH)OTs
Only PhI=O
PhI(OH)OTs
PhI(CF3CO2)2
0[d]
17[c]
24[c]
0[d]
0[c]
37
43
59
0[c]
0[d]
[a] Reactions were performed on 0.3 mmol scale.
[b] Isolated yield.
[c] Complex mixture was obtained.
[d] 85% of 1a (entry 3), 61% of 1a (entry 6) and 70% of 1a
(entry 12) were recovered.
effects of oxidants and electrophilic promoters were
examined as shown in Table 1. It was found that the
reaction requires both PhI(OAc)2 and TMSOTf
(entries 2 and 3). Compared to other iodine reagents
(PhI(CF3CO2)2 or PhI(OH)OTs), PhI(OAc)2 is the
most suitable for the reaction (entries 4 and 5).
TMSOTf was then found to be the best Lewis acid of
all tested compounds (TMSOAc, BF3∙Et2O) (entries 6
and 7). Notably, the combination of Tf2O with iodine
reagents (PhI(OAc)2, PhI(OH)OTs or PhI=O) could
also promote the transformation in modest yields
(entries 8-10). In contrast, in the absence of
electrophilic
compounds
promoters,
themselves
hypervalent
iodine
or,
[a] Unless otherwise noted, all reactions were performed on
0.3 mmol scale under the optimum conditions.
[b] -50 ℃for 18 h.
(PhI(OH)OTs
PhI(CF3CO2)2) proved ineffective for the reaction
(entries 11 and 12).
[c] Cyclobutanols 1g, 1k and 1l were completely consumed.
[d] HCl was used during work up.[12]
With the best conditions in hand, we next
examined the substrate scope. As shown in Table 2, a
wide range of cyclobutanols were tolerated in the
reaction. Substrates bearing
a
methyl group,
regardless of the position on arene (p-, m-, o-), gave
rise to the desired cyclopropyl ketones 2b-2d in good
to excellent yields (75%, 78% and 91%, respectively).
It seemed that steric hindrance had little impact on
the reaction since the sterically hindered 1d provided
the corresponding product in excellent yield (91%).
Compared to methyl and t-butyl groups (1b-1e),
electron withdrawing groups (1f-1j) have a slight
detrimental effect on the reaction, and the desired
cyclopropyl ketones (2f-2j) were furnished in
relatively low yields (42-74%). This result might be
attributable to the lower nucleophilicity of electron-
deficient substrates. Heteroarene 1k is also well
tolerated in the reaction, although 2k is produced in
modest yield (41%). Remarkably, a wide variety of
functional groups are compatible with the reaction
conditions including ethers (2f and 2m), esters (2n-
2t), a nitrile (2l), a ketone (2q), an alkene (2r), and an
aldehyde (2t); functional group compatibility is a
common challenge faced by conventional methods.[4-9]
The substrates 1g, 1k and 1l were completely
consumed to produce the desired cyclopropyl ketones
in low yields and inseparable complex mixtures. It is
worth noting that the use of aryl and alkyl halides (2i-
2k and 2s) as substrates provides a platform for later
manipulations.
When we switched to cyclobutanol 1u bearing a
phenoxyl group, cyclopropyl ketone 2u was
generated in very low yield even though most of
substrate 1u had been consumed (eq 4). This poor
2
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10.1002/adsc.201701237
Advanced Synthesis & Catalysis
selectivity might be due to the strong background
iodine(III)-mediated arene oxidation.[13] To solve the
region-selectivity issue, we sought to tune the
electrophilicity of PhI(OTf)2 by introducing an
organic base to the oxidation system.[14] To our
delight, through a rapid base screen (for details, see
Supporting Information), we found the combination
of 2,6-
Inspired by the subtle base effect in the reactions
of 1u, we were curious to compare conditions A with
conditions B by evaluating their performance with a
variety of electron-rich arene substrates (Table 3).
Methoxyl-substituted arenes 1v and 1w completely
decomposed under conditions A; However under
conditions B, which involve the assistance of base,
provided 2v and 2w in synthetically useful yields (62%
and 51%, respectively). The use of 2,6-
dichloropyridine also changed the outcome of the
reactions of 1x-1z by significantly improving their
chemical yields. In addition, electron-rich thiophenyl
3a, which was not tolerated under conditions A,
afforded the desired 4a by using extra organic base,
albeit in a relatively low yield (22%). However, even
submitted to the newly developed conditions B, 1l
still provided poor chemical yield of 2l (20%), which
is similar to the reaction under conditions A (Table 2).
Unfortunately, further efforts to expand the substrate
scope to alkyl phenyl cyclobutanol 3b, vinyl
cyclobutanol 3c and alkyl cyclobutanol 3d failed;
reactions with these three substrates in either
conditions A or B merely resulted in complex
mixtures. To date, we have no rational explanation
for these results.
dichloropyridine with iodine(III) enabled an efficient
ring contraction of 1u and provided 2u in very good
yield (85%). The pyridine base likely decreased the
electrophilicity of the iodine(III) reagent, thus
essentially inhibiting the undesired oxidation of the
electron-rich phenyl ring of 1u. We believe that this
discovery is significant for the future development of
hypervalent iodine-mediated transformations.
Table 4. Reactions of 1y, 3e and 3f under conditions A or
B
Table 3. Ring contraction reactions of 1v-1z and 3a-3d
under conditions A or B
[a] 2y was obtained in 14% yield from conditions A and 57%
yield from conditions B, respectively.
Interestingly, the reaction of 2-naphthalene 1y
under conditions A afforded cyclohexanone 5y as the
major product in relatively low yield (Table 4.).
Similar transformations were also observed when
using benzo[b]thiophene 3e and benzofuran 3f as
substrates. Cyclohexanones 5e and 5f were obtained
in modest yields, and none of the corresponding
cyclopropyl ketone products could be identified. The
formation of cyclohexanones might be due to an
[b]
[a] Performed at -50 ℃for 12 h.
2,3-Dihydrophenanthren-4(1H)-one (5y) was obtained in
32% yield.
[c] Cyclobutanol 3a was used as the substrate.
3
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10.1002/adsc.201701237
Advanced Synthesis & Catalysis
intramolecular electrophilic aromatic substitution (see
mechanistic discussion below). Surprisingly, when
we switched to meta-methoxy-substituted phenyl
cyclobutanol 3g (eq 5), in lieu of expected
cyclopropyl ketone 4g, diaryl iodane 6 appeared as
the sole product with quantitative yields under either
conditions A or B. This result can be attributed to the
strong substituent effects of substrate 3g. Similar
transformations for the synthesis of diaryl iodanes
can be found in the literature.[15]
Based on these experimental observations, a
reaction mechanism was postulated (Scheme 2).
According to Wirth’s NMR studies[14b], PhI(OTf)2 is
formed by mixing PhI(OAc)2 with two equiv of
TMSOTf. Cyclobutanol 1 (or 3) interacts with
PhI(OTf)2 affording intermediate A. In the case of 3g,
diaryl iodane 6 is formed through electrophilic
aromatic substitution with iodine(III). Subsequent
internal nucleophilic addition of -OTf to the
cyclobutyl ring of A results in its ring opening while
simultaneously delivering B.[16,17] In the presence of a
strong acid (HOTf), B tautomerizes to its enol form,
C. Through an intramolecular SN2 process, C is
converted to 2 (or 4). Alternatively, C may undergo
an intramolecular electrophilic aromatic substitution
to produce cyclohexanone 5 (blue arrow). Although
the use of extra base has significantly improved the
reactions of some electron-rich arene substrates
(Table 3), a detailed mechanistic interpretation of the
subtle base effect cannot be provided due to lack of
promising evidence.
In summary, an iodine(III)-mediated oxidative ring
contraction of aryl cyclobutanols to aryl cyclopropyl
ketones has been developed. The reaction proceeds
under mild conditions and has a broad substrate scope
and superb functional group compatibility. Notably,
the addition of an organic base (2,6-dichloropyridine)
to the iodine(III) oxidation system allows the
transformation to tolerate more challenging electron
rich-arene substrates. In addition, the gram-scale
reaction
proceeding
with
high
efficiency
demonstrates the practicality of the method. Further
expansion of the substrate scope, detailed mechanistic
studies and application of the technology to some
practical synthesis are currently under way.
Experimental Section
A mixture of PhI(OAc)2 (1.2 equiv) in DCM (3 mL) was
charged in a Schlenk flask under nitrogen atmosphere.
TMSOTf (2.4 equiv) was added dropwise. After stirring
for 10 min at room temperature, the mixture was cooled to
-50 ℃. A solution of cylcobutanol 1 (44.5 mg, 0.3 mmol)
in DCM (1 mL) was added dropwise. After stirring under
the same temperature for 6 h, the mixture was quenched
with NaHCO3 (sat., 1 mL). Then the mixture was diluted
with DCM (15 mL). The organic layer was dried over
anhydrous Na2SO4, filtered, and concentrated under
reduced pressure. After removal of the solvent in vacuo,
the crude material was purified by flash column
chromatography on silica gel to give the cyclopropyl
ketone 2.
Acknowledgements
This work is supported by Zhejiang Normal University, the Na-
tional Natural Science Foundation of China (Grant 21502171),
the Natural Science Foundation of Zhejiang Province
(LY16B020002) and the State Key Laboratory of Fine Chemicals
(KF 1512).
Scheme 2. Proposed mechanism
To examine the practicality of the method, a gram
scale reaction was performed. As shown in eq 6,
model substrate 1a was submitted to the optimum
conditions and efficiently produced 1.4 g of
cyclopropyl ketone 2a (80% yield). Unfortunately,
the current methodology cannot be applied to
cyclopentanol 7. Interestingly, in lieu of 9, tetracyclic
compound 8 was isolated albeit in very low yield (eq
7). Its structure was confirmed by X-ray
crystallography.
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Advanced Synthesis & Catalysis
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6
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10.1002/adsc.201701237
Advanced Synthesis & Catalysis
COMMUNICATION
Mild Ring Contractions of Cyclobutanols to
Cyclopropyl Ketones via Hypervalent Iodine
Oxidation
Adv. Synth. Catal. Year, Volume, Page – Page
Yan Sun, Xin Huang, Xiaojin Li, Fan Luo, Lei
Zhang, Mengyuan Chen, Shiya Zheng and Bo
Peng*
7
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