Hydroperoxidations of Alkenes using Cobalt Picolinate Catalysts

Article abstract of DOI:10.1021/acs.orglett.1c01489

Hydroperoxides were synthesized in one step from various alkenes using Co(pic)₂as the catalyst with molecular oxygen and tetramethyldisiloxane (TMDSO). The hydration product could be obtained using a modified catalyst, Co(3-mepic)₂, with molecular oxygen and phenylsilane. Formation of hydroperoxides occurred through a rapid Co-O bond metathesis of a peroxycobalt compound with isopropanol.

Full text of DOI:10.1021/acs.orglett.1c01489

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Letter
Hydroperoxidations of Alkenes using Cobalt Picolinate Catalysts
Zulema Peralta-Neel and K. A. Woerpel*
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ABSTRACT: Hydroperoxides were synthesized in one step from various
alkenes using Co(pic)2 as the catalyst with molecular oxygen and
tetramethyldisiloxane (TMDSO). The hydration product could be
obtained using a modified catalyst, Co(3-mepic)2, with molecular oxygen
and phenylsilane. Formation of hydroperoxides occurred through a rapid
Co−O bond metathesis of a peroxycobalt compound with isopropanol.
characteristics with Co(II) porphyrin complexes, but the
ecause the peroxide functional group is found in many
Bbiologically active natural products and potential drugs, picolinate complexes also permit peroxidation of electron-rich
dienes and alkenes. The reactivity of these catalysts can also be
tuned using substituted picolinic acids.
efforts have been directed toward the development of methods
for the synthesis of peroxides.1−6 The cobalt-catalyzed
silylperoxidation of alkenes7−9 has emerged as a general
method to introduce the peroxide functional group into
unsaturated substrates. These reactions typically use metal
complexes containing 1,3-diketonate ligands, such as 2,2,6,6-
tetramethyl-3,5-heptanedionate (thd or dpm).10−12 The
specific 1,3-diketonate ligand employed does not dramatically
influence the course of reactions, but it can influence their rates
and efficiencies.13 Cobalt porphyrin complexes have also been
used to catalyze the peroxidation of alkenes, but these catalysts
work best with electron-deficient dienes, converting them into
γ-hydroperoxy-α,β-unsaturated carbonyl compounds.14
Initial efforts focused on developing a direct hydro-
peroxidation of alkenes using enone 1 as a model substrate.
The traditional 1,3-diketonate catalysts for these reactions,
such as a cobalt atom complexed to a thd ligand, gave the
expected silyl peroxide 2a under standard reaction conditions
(Table 1, entry 1).21 Attempts to obtain the corresponding
hydroperoxide using other 1,3-diketonate ligands were
unsuccessful (entries 2−4). The use of methanol as a solvent
or co-solvent led to formation of an alcohol, not a peroxide
product (entries 5−6).22
The use of cobalt catalysts with picolinic acid-derived
ligands led to hydroperoxidation instead of silylperoxidation
under similar conditions (Table 2). The use of Ph2SiH2 or
TMDSO were the most effective, particularly with isopropanol
as the solvent (entries 4−5). Although smaller quantities of
Ph2SiH2 could be used (entry 4), the product was
contaminated with alcohol 2b, likely because Ph2SiH2 is a
strong reducing agent.23,24 By contrast, use of TMDSO (entry
5) did not result in formation of alcohol 2b and had the added
benefit of facilitating the separation of the products from
silicon-containing impurities.24
Although the available catalysts address many issues, some
limitations to these reactions remain. The use of Co(thd)2,13
a
particularly efficient catalyst, forms cobalt complexes that can
be difficult to separate from the products,15 which has led to
the development of modified 1,3-diketonate ligands that
facilitate purification.16,17 Catalysis by Mn(III) 1,3-diketonate
complexes can lead to hydroperoxides,18 although the product
is often formed along with the corresponding alcohol.
Porphyrins needed to make catalysts can be costly and,
although they can be prepared by the user, the yields of these
syntheses are often modest.19,20 As a result, a family of readily
prepared and tunable catalysts that give control over which
product is formed (hydroperoxide or alcohol) would be
valuable. Furthermore, considering the biological activity of
cyclic peroxides, it would be desirable to devise catalysts for
peroxidations of alkenes that would enable direct access to
cyclic peroxides.
Experiments designed to optimize the structure of the
catalyst revealed that the reactivities of the cobalt picolinate
complexes were sensitive to substituents on the pyridine ring
(Scheme 1). Complexes possessing an NH2 group at the 3-
position or a chlorine atom at the 5-position of the pyridyl
group (complexes F and G) were unreactive. A complex
bearing a methyl group at the 6-position of the pyridyl group
In this Letter, we report a family of cobalt complexes
constructed with 2-carboxypyridine (picolinate) ligands that
are effective catalysts for alkene hydroperoxidation. These
catalysts are simple to prepare from commercially available
picolinic acids; they can be isolated, handled, and stored
without demanding precautions; and they are effective at
generally low catalyst loadings. These complexes share some
Received: April 30, 2021
Published: June 14, 2021
https://doi.org/10.1021/acs.orglett.1c01489
© 2021 American Chemical Society
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Table 1. 1,3-Diketonato Ligand and Solvent Screen
Scheme 1. Catalyst Screen using Substituted 2-
Carboxypyridine Catalysts
entry
catalyst
solvent
PhCF3
PhCF3
PhCF3
PhCF3
% conv
product
a
50:50 mixture of alcohol 2b and hydroperoxide 3a.
1
2
3
4
5
6
A
B
C
D
A
A
100
0
0
0
100
100
2a
−
−
−
2b
Scheme 2. Substrate Scope
a
PhCF3:MeOH
MeOH
2b
a
90:10 PhCF3:MeOH.
Table 2. Optimization of Silane and Solvent
entry
silane
Et3SiH
Ph3SiH
(i-Pr)3SiH
Ph2SiH2
[(CH3)2SiH]2O
PhSiH3
% conv (MeOH)
% conv (i-PrOH)
1
2
3
4
5
6
10
9
0
37
−
9
10
20
0
ab
,
100
100
36
a
b
Use of 1.2 equiv led to full consumption of alkene. Alcohol 2b was
formed when the reaction was run at 1 mmol scale.
(complex H) reacted much like the parent complex (complex
E). Catalysts bearing picolinate groups with a methyl group at
the 3-position of the pyridine, however, were much more
reactive (complex I), leading to an approximately four-fold
increase in the rate of reaction. The elevated reactivity of 3-
substituted pyridine complexes compared to ones with
substitution at other positions has been noted elsewhere.25,26
This increase in reactivity could be attributed to interactions
between the methyl and carboxylate groups, which cause the
pyridyl group to twist out of the plane,27 instead of being
coplanar with the carboxylate group. Formation of the
hydroperoxide product, however, was accompanied by
considerable quantities of the corresponding alcohol 2b.
The cobalt picolinate-catalyzed hydroperoxidation of alkenes
was general for a number of electron-rich and electron-
deficient alkenes (Scheme 2).28 Both α,β-unsaturated ketones
and esters were effective substrates, although the latter alkenes,
which should be less electron-deficient than enones,29 required
a
% conversion reported
longer reaction times. Addition of t-BuOOH shortened the
induction period of the reactions,30,31 but no compounds
containing OOt-Bu groups were isolated from the reaction
mixture.32,33 Addition of chloroform in some, but not all, cases
increased yields between 5 and 10%.
Reactions involving conjugated dienes showed different
reactivity when using Co(pic)2 compared to reactions using
complexes with 1,3-diketonate ligands (Scheme 3). Subjecting
diene 5a to standard peroxidation conditions21 using Co(thd)2
resulted in a mixture of products. Six C−O−O groups were
identified in the 13C{1H} NMR spectrum of the unpurified
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Scheme 3. Diene Substrate Scope
Scheme 5. Suggested Mechanism for Hydroperoxidation
reaction mixture.31 When the same diene was oxidized with
Co(pic)2, however, only one hydroperoxide product was
observed after 16 h. This observation suggests that the cobalt
picolinate complexes may share characteristics with the cobalt
porphyrin complexes but with the ability to catalyze the
hydroperoxidation of electron-rich and electron-poor al-
kenes.34
The major product of the reaction can be controlled by the
choice of the picolinic acid employed to make the cobalt
complex. Using the methyl-substituted version of the catalyst,
I, and PhSiH3,23 alkenes could be converted to their
corresponding alcohols regioselectively (Scheme 4). This
reaction complements other methods that can be employed
to achieve metal-catalyzed alkene hydration using O2.7
B, would involve exchange of a peroxide ligand on cobalt with
isopropanol to give the hydroperoxide and an isopropoxycobalt
complex, 15, which could react with the silane to regenerate
the cobalt hydride species.39−41 Both of these pathways would
generate an isopropoxysilane, which was observed in the
unpurified reaction mixtures.35
Scheme 4. Alcohol Substrate Scope
Control experiments provided support that the cobalt
picolinate-catalyzed reaction proceeds via pathway B. If the
reaction proceeded through pathway A, the use of isopropanol-
d8 as solvent would result in a cobalt deuteride, so deuterium
atoms would be incorporated into the product. Peroxidation in
isopropanol-d8, however, gave no deuterated hydroperoxide
products after filtration through silica gel, as determined by 1H
and 13C{1H} NMR spectroscopy. This experiment indicates
that the silane is the source of the hydrogen atom on the cobalt
atom. This conclusion was supported by an experiment using
the deuterated silane, Et3SiD (eq 1). Although this silane is not
optimal for the preparative process, deuterated hydroperoxide
16 could be observed in the reaction mixture. Taken together,
these labeling experiments suggest that pathway B is the most
likely pathway.
The direct hydroperoxidation of α,β-unsaturated ketones
allowed for the one-step formation of endoperoxides. Hydro-
peroxidation of enone 17 resulted in the formation of the
corresponding endoperoxide 18 (Scheme 6). When the same
enone substrates were subjected to peroxidation using catalysts
derived from a 1,3-diketone, the uncyclized silyl-protected
peroxides were isolated instead (Scheme 6).
Several mechanistic pathways can be considered for the
formation of hydroperoxides instead of the silyl peroxides
usually obtained with cobalt catalysts. The pathways diverge
after formation of the cobalt-hydroperoxo complex 8, which
would be formed by cobalt-catalyzed hydrometallation in the
presence of O2.31 Complex 8 could lead to silyl-protected
peroxide 2a, which could then be deprotected. This possibility
was discounted by the observation that resubjecting silyl-
peroxide 2a to the reaction conditions led to recovery of
starting material.35 Alternatively, transmetallation of 8 could
occur to form O−H and Si−Co bonds (Scheme 5, pathway
A).36 The resulting silylcobalt complex, 11, could then
undergo reaction with isopropanol to form the product and
regenerate the cobalt hydride.36−38 Another pathway, pathway
The hydroperoxidation/cyclization sequence can be applied
to α,β,γ,δ-unsaturated carbonyl compounds (Scheme 7).
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Experimental procedures, characterization, determina-
tion of regioselectivity, and NMR spectra of new
compounds (PDF)
Scheme 6. One-Step Formation of Endoperoxides
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 7. One-Step Formation of Endoperoxides Using
Dienes
Zulema Peralta-Neel − Department of Chemistry, New York
University, New York, New York 10003, United States;
orcid.org/0000-0002-6491-8326
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01489
Notes
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
(R01GM118730). The authors thank Dr. Chin Lin (NYU) for
his assistance with NMR spectroscopy and mass spectrometry.
The Shared Instrumentation Facility in the Department of
Chemistry was constructed through the support of the
National Center for Research Resources, National Institutes
of Health, under Research Facilities Improvement Award
Number C06 RR-16572-01. NMR spectra acquired with the
Bruker Avance 400 and 500 NMR spectrometers were
supported by the National Science Foundation (CHE-
01162222). The cryogenic probe for the 600 MHz NMR
was acquired through the support of the National Institutes of
Health S10 grant under Award Number OD016343. We thank
Claire Park (Smith College) for experimental support.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge at
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