Strain-Promoted Oxidation of Methylenecyclopropane Derivatives using N-Hydroxyphthalimide and Molecular Oxygen in the Dark

Article abstract of DOI:10.1021/acs.orglett.0c02075

The hydroperoxidation of alkylidenecyclopropanes and other strained alkenes using an N-hydroxylamine and molecular oxygen occurred in the absence of catalyst, initiator, or light. The oxidation reaction proceeds through a radical pathway that is initiated by autoxidation of the alkene substrate. The hydroperoxides were converted to their corresponding alcohols and ketones under mild conditions.

Full text of DOI:10.1021/acs.orglett.0c02075

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Letter
Strain-Promoted Oxidation of Methylenecyclopropane Derivatives
using N‑Hydroxyphthalimide and Molecular Oxygen in the Dark
T. E. Anderson and K. A. Woerpel*
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ABSTRACT: The hydroperoxidation of alkylidenecyclopropanes and
other strained alkenes using an N-hydroxylamine and molecular oxygen
occurred in the absence of catalyst, initiator, or light. The oxidation
reaction proceeds through a radical pathway that is initiated by
autoxidation of the alkene substrate. The hydroperoxides were converted
to their corresponding alcohols and ketones under mild conditions.
he oxidation of organic substrates by molecular oxygen
provides an inexpensive, environmentally benign method
different N-hydroxylamine also underwent the uncatalyzed
transformation. The reaction appears to be initiated by radical
intermediates generated upon autoxidation of the alkene, which
consequently allows an alkylidenecyclopropane to be employed
in catalytic amounts to drive the oxidation of other substrates.
These observations extend the use of ring strain to promote
reactivity in otherwise inert compounds, a strategy that has been
employed in synthetic chemistry,24−29 chemical biology,30,31
and materials chemistry.32−35
T
to prepare synthetically useful products.1−4 Recently, the
application of an N-hydroxylamine such as N-hydroxyphthali-
mide (NHPI, 2) in conjunction with molecular oxygen has
allowed for the addition of two oxygen atoms across an alkene,
affording hydroperoxides as either final products or reactive
intermediates (Scheme 1, 1→3).5−12 NHPI-mediated oxida-
Scheme 1. NHPI-Mediated Oxidation of Alkenes
In the course of our studies on the copper-catalyzed
oxidations of alkenes,7 we attempted to extend the scope of
the reaction beyond styrenes and enynes. We considered that
the reaction could be driven by strain-induced destabilization of
the starting alkene, as opposed to stabilization of a radical
intermediate. Initial studies treating alkene 4a, which can be
prepared in one step from cyclohexanone,36 with NHPI open to
air justified this hypothesis (Table 1, entry 1). The use of an
oxygen atmosphere or blue LED light each resulted in higher
yields, but their combined use did not result in a cumulative
improvement in yield (Table 1, entries 2−4). A further increase
in yield was observed when the reaction mixture was placed in
the dark under an oxygen atmosphere, which proved to be the
optimal conditions for the reaction (Table 1, entry 6). Unlike
under ambient light, in the dark the product yield depended
upon the amount of oxygen (Table 1, entry 5). A slight decrease
in yield was observed when the concentration was higher (Table
1, entry 8).
tions proceed through radical pathways that necessitate
conversion of NHPI to the phthalimide-N-oxyl (PINO) radical
through homolytic cleavage of the hydrogen−oxygen bond,13 a
process that requires radical initiators,9,14,15 transition met-
als,5−8,13,16,17 heat,10,18−20 or light.11,12,21,22 These transforma-
tions generally also require the formation of stabilized alkyl
radical intermediates such as benzylic radicals, so they are largely
limited to oxidations of styrene derivatives.23
The peroxidation reaction was general for a range of
methylenecyclopropane derivatives (Scheme 2). The higher
yields observed in reactions with bi- and tricyclic tetrasub-
Received: June 23, 2020
In this communication, we report the strain-promoted
oxidation of aliphatic alkylidenecyclopropanes with molecular
oxygen and NHPI (2) in the absence of any exogenous activator
or catalyst (Scheme 1, 4 → 5). Other strained alkenes and a
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Minor side products observed following treatment of 4a with
NHPI provided clues as to how the PINO radical could be
generated from NHPI in the absence of light, heat, or catalyst.13
Small amounts of ketones 6 and 7, among a number of other
minor products observed in the unpurified reaction mixture,
indicate that molecular oxygen may add directly to the strained
carbon−carbon double bond (Scheme 3A). Exposure of 4a to
Table 1. Optimization of Reaction Conditions
a
entry
concn (M)
solvent
light
oxidant
yield
a
Scheme 3. Autoxidation of Alkylidenecyclopropane 4a
1
2
3
4
5
6
7
8
0.1
0.1
0.1
0.1
0.1
0.1
0.05
0.5
0.1
0.1
0.1
0.1
0.1
0.1
0.1
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DCE
EtOH
THF
H2O
DMSO
acetone
toluene
ambient
ambient
blue LED
blue LED
dark
dark
dark
dark
dark
dark
dark
dark
dark
dark
dark
air
O2
air
O2
air
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
65%
76%
77%
77%
21%
85%
81%
62%
50%
0%
9%
0%
<1%
79%
8%
9
10
11
12
13
14
15
a
Isolated yields.
Scheme 2. Scope of NHPI-Mediated Oxidation of
Alkylidenecyclopropanes
a
a
Yields of 6 and 7 were determined by NMR spectroscopy of the
unpurified reaction mixture.
oxygen in the absence of NHPI also yielded a trace amount of
autoxidation product 6 (Scheme 3B), confirming that oxygen
can react directly with the strained alkene.
The autoxidation reactions of alkene 4a (Scheme 3A,B)
appear to proceed through a radical mechanism characteristic of
triplet oxygen, as evidenced by the complete inhibition of the
reaction in the presence of butylated hydroxytoluene (BHT,
Scheme 3C).38 Although the addition of molecular oxygen to
alkenes generally involves the excited singlet state of oxygen,39
autoxidation reactions of alkenes and alkynes involving triplet
oxygen have also been observed.40−47 Many of these reactions
involve geometrically strained compounds,48−54 including the
autoxidations of aryl-substituted alkylidenecyclopropanes.55,56
The generation of ketone 6 from the reaction of triplet oxygen
with 4a involves a pathway with several radical intermediates
that could react with NHPI to form the PINO radical (Scheme
4). Addition of triplet oxygen to 4a would initially result in the
53,54
3
formation of the triplet diradical A,
which could undergo
intersystem crossing (ISC) to give the singlet diradical 1A.54,57
Intermediates 3A or 1A can each react with another molecule of
4a to form dimer B as a triplet or singlet species, respectively.
a
Reaction conditions: 4 (0.50 mmol, 1.0 equiv) and 2 (1.2 equiv) in
MeCN (5 mL) for 16 h. Isolated yields.
Scheme 4. Formation of Autoxidation Product 6
stituted alkenes 4a−4d compared to that of monocyclic
tetrasubstituted alkene 4e likely result from reduced steric
hindrance instead of increased ring strain.37 The lower yield
observed in the case of trisubstituted alkylidenecyclopropane 4f
appears to result from both the lower reactivity of the alkene as
well as the instability of the secondary peroxide 5f. While other
substrates were consumed after 16 h, full conversion of 4f was
not observed after 48 h. A longer reaction time did not
substantially improve the yield, however.
B
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Homolysis of the oxygen−oxygen bond58 followed by radical
recombination would result in epoxide C, which can isomerize
to cyclobutanone 6 in the presence of trace amounts of
6C). The similar diastereoselectivities of the catalyzed and
uncatalyzed reactions suggests that they follow a similar
pathway.
acid.59−61 If A or A were sufficiently long-lived to react
intermolecularly with 4a, which the formation of 6 suggests,
reaction of these intermediates with NHPI could also occur to
form the PINO radical.
3
1
The involvement of a radical pathway explains why the use of
blue light and an oxygen atmosphere, while individually
favorable, do not result in a cumulative improvement in yield
(Table 1, entries 1−4). Excessive production of the PINO
radical, as may be expected with the combined use of light and
oxygen, would result in an increase in the second-order decay of
radicals through pathways such as dimerization.66 Accumulation
of alkyl and peroxy radicals D and E, respectively, would also
result in increased termination through dimerization and other
undesirable side reactions.67,68 These termination pathways also
become more prominent at higher concentrations, resulting in
lower yields (Table 1, entry 8).
Following generation of the PINO radical, the oxidation
reaction likely follows a radical addition pathway similar to those
previously reported for aryl-substituted alkenes (Scheme
5).6,8,9,12,62 In this case, however, benzylic or propargylic
Scheme 5. Proposed Reaction Mechanism
The fact that products derived from ring-opening of a
cyclopropylcarbinyl radical (e.g., G) were not observed in any of
the peroxidation reactions is not inconsistent with a radical
mechanism (Scheme 5).69 While cyclopropylcarbinyl radical
ring-opening proceeds at rates approaching the diffusion limit,
this rearrangement can be reversible70,71 and, in some cases, the
ring-closed form is thermodynamically preferred.72 A preference
for the ring-closed form may be expected in this case, as has been
observed in similarly substituted cyclopropylcarbinyl radicals,
because rearrangement results in a primary alkyl radical.73
The oxidation reaction was found to be effective with other N-
hydroxylamines (Scheme 7).7 Treatment of 4a with N-
hydroxybenzotriazole (HOBt, 8) under the optimized con-
ditions gave peroxide 9 in 77% yield.
stabilization of the intermediate alkyl radical is not necessary.
Instead, the PINO radical adds across the strained carbon−
carbon double bond of alkylidenecyclopropane 4a, generating
the more stable alkyl radical D instead of the regioisomeric
cyclopropyl radical.63 Addition of molecular oxygen to D results
in peroxy radical E, and subsequent abstraction of hydrogen
from a second NHPI molecule yields the β-hydroperoxy-N-
alkoxyamine 5a and regenerates the PINO radical.
Scheme 7. Reaction of Alkylidenecyclopropane 4a with
HOBt
Control experiments support the proposed reaction mecha-
nism (Scheme 6). As observed for experiments conducted in the
Scheme 6. Mechanistic Experiments
Other strained alkenes reacted with NHPI under the same
conditions (Scheme 8). Norbornene (10) was oxidized to give
peroxide 11 as a mixture of diastereomers.74 While NHPI
Scheme 8. Reaction of Other Strained Alkenes with NHPI
presence of BHT (Scheme 3C), the addition of the radical trap
(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) suppressed
the reaction (Scheme 6A).64 No addition products were
observed when 4a was treated with NHPI under a nitrogen
atmosphere, indicating that oxygen is necessary for formation of
the PINO radical (Scheme 6B).15,65 Addition of a copper
complex known to generate the PINO radical7 to the reaction
mixture resulted in a more rapid conversion of 4d to the same
product as observed under the uncatalyzed conditions (Scheme
C
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a
appeared to add selectively to the less hindered exo face of the
bicyclic system, molecular oxygen added to both the exo and
endo faces due to its high rate of reaction with alkyl radicals.75
Reaction of trans-dioxasilacyclooctene 12 with NHPI proceeded
selectively to yield 13, with the phthalimido-N-oxyl group in the
pseudoequatorial orientation and oxygen introduced to form the
preferred out,out-bicyclo[5.3.1]undecane ring system.74,76 β-
Pinene (14), which undergoes other strain-promoted reac-
tions,77,78 is not sufficiently strained to react under the present
conditions, even upon irradiation.
Scheme 11. NHPI-Mediated Oxidation of Cumene
The β-hydroperoxy-N-alkoxyamine products formed in these
reactions can be easily functionalized. The hydroperoxide
moiety was rapidly reduced to the corresponding hydroxyl
group upon treatment with tributylphosphine (Scheme 9).
Conversion to the carbonyl group was also possible in the case of
a secondary peroxide (Scheme 10).79
a
Reaction conditions: 17 (1.0 mmol, 1.0 equiv) and NHPI (1.0
equiv), with or without 4e (10 mol %) in MeCN (10 mL), 22 h.
Isolated yields.
Scheme 9. Reduction of Peroxides to Alcohols
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02075.
Experimental procedures, NMR spectra, and analytical
data for all new compounds (PDF)
AUTHOR INFORMATION
Corresponding Author
■
K. A. Woerpel − Department of Chemistry, New York University,
New York, New York 10003, United States; orcid.org/0000-
0002-8515-0301; Email: kwoerpel@nyu.edu
Author
Scheme 10. Elimination of Peroxides to form Ketones
T. E. Anderson − Department of Chemistry, New York University,
New York, New York 10003, United States; orcid.org/0000-
0002-5682-8929
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02075
Notes
The strain-promoted generation of the PINO radical in the
presence of oxygen suggested that other radical reactions could
be performed using an alkylidenecyclopropane as an initiator.
Treatment of cumene (17) with NHPI and a catalytic amount of
heptan-4-ylidenecyclopropane 4e in the dark yielded cumene
peroxide (18) in an amount comparable to that observed when
the same reaction was conducted under blue light without the
strained alkene (Scheme 11). No peroxide formation was
observed when cumene was treated with NHPI in the absence of
4e or light. This NHPI-mediated oxidation of a C(sp3)−H bond
in the dark and under mild conditions suggests promise for the
use of strained alkenes as radical initiators in other reactions.80
In summary, the oxidation of strained alkenes with NHPI and
molecular oxygen expands the scope of alkene oxidation
reactions. Ring strain not only promotes the addition of the
PINO radical to the alkene but also induces formation of this
radical. The generation of the PINO radical by the substrate
avoids the need for any exogenous activator and suggests the
potential use of strained alkenes as cocatalysts in oxidation
reactions of other substrates.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Institute of General
Medical Sciences of the National Institutes of Health
(1R01GM118730). NMR spectra acquired with the Bruker
Avance 400 and 500 NMR spectrometers were supported by the
National Science Foundation (CHE-01162222). The authors
thank Dr. Chin Lin (NYU) for his assistance with NMR
spectroscopy and mass spectrometry.
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