Cobalt-Catalyzed Aerobic Oxidative Cyclization of 2-Aminophenols with Isonitriles: 2-Aminophenol Enabled O2 Activation by Cobalt(II)

Article abstract of DOI:10.1021/acs.orglett.9b01384

An aerobic cobalt-catalyzed oxidative cyclization of 2-aminophenols and isonitriles is reported. These additive-free conditions furnish a variety of substituted 2-aminobenzoxazoles in moderate to excellent yields. A series of control experiments and spect

Full text of DOI:10.1021/acs.orglett.9b01384

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cobalt-Catalyzed Aerobic Oxidative Cyclization of 2‑Aminophenols
with Isonitriles: 2‑Aminophenol Enabled O2 Activation by Cobalt(II)
Jiaqi Liu and Jessica M. Hoover*
C. Eugene Bennett Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506, United States
S
* Supporting Information
ABSTRACT: An aerobic cobalt-catalyzed oxidative cyclization of
2-aminophenols and isonitriles is reported. These additive-free
conditions furnish a variety of substituted 2-aminobenzoxazoles in
moderate to excellent yields. A series of control experiments and
spectroscopic studies point to the importance of 2-aminophenol
coordination in enabling the aerobic oxidation of cobalt(II).
a
Table 1. Optimization of the Reaction Conditions
omogeneous transition-metal-catalyzed aerobic oxida-
Htion reactions have received considerable attention due
to the benign and atom economic nature of O2 as an oxidant.
Significant advances have been made in the development of
Pd-1 and Cu-2catalyzed aerobic oxidation reactions, whereas
recent efforts seek efficient catalysis utilizing alternative first-
row transition-metal catalysts such as Co. Redox-active
mediators or ligands, such as BQ,3 NHPI,4 and salen ligands,5
are often employed to enable the utilization of O2 as the
terminal oxidant under Co catalysis (BQ = benzoquinone,
NHPI = N-hydroxyphthalimide) (Scheme 1a,b). In addition,
Co/BQ and Co/NHPI cocatalytic systems have been used to
facilitate the use of O2 as the terminal oxidant in Pd-catalyzed
organic oxidation reactions.6
b
entry
[cobalt]
solvent
THF
THF
THF
THF
time (h)
% yield
c
1
2
3
4
5
6
7
8
Co(acac)2
Co(acac)2·H2O
CoCO3
CoCO3·H2O
CoCl2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
Co(OAc)2
24
24
24
24
24
24
24
24
24
24
24
12
6
58
67
51
trace
46
86
89
54
86
78
93(90)
85
79
0
c
THF
THF
1,4-dioxane
2-Me-THF
DMF
DMSO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Despite the widespread utilization of Co-catalyzed aerobic
oxidation reactions exploiting redox-active mediators, there are
fewer examples of Co-catalyzed aerobic oxidation reactions
9
10
11
12
13
14
c
Scheme 1. Redox-Active Cofactors Paired with Cobalt To
Enable Aerobic Oxidation Reactions in Prior Studies and
This Work
24
24
d
15
Co(OAc)2
14
a
Reaction conditions: 1a (0.3 mmol), 2a (0.3 mmol), cobalt(II)
catalyst (10 mol %), solvent (3 mL), at 65 °C with an air balloon for
b
1
24 h. Yield of 3a was determined by H NMR spectroscopy with
c
dimethylsulfone as internal standard (0.03 mmol). Isolated yield.
Reaction was conducted under N2.
d
that operate efficiently in the absence of an external redox-
mediator.7 Achieving such mediator-free oxidation reactions
may be particularly challenging because these cofactors have
recently been shown to assist in the prevention of undesired
oxygenation side products by preventing the accumulation of
Received: April 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01384
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Scope of Substituted 2-Aminophenols
Scheme 5. Oxidative Cyclization of 2-Aminophenol and tert-
Butylisonitrile Mediated by Cobalt under N2
superoxide and peroxide intermediates.3 We hypothesized that
the use of a substrate that can also act as a H-atom donor
might enable the activation of O2 and therefore allow the
selective oxidation of organic substrates (Scheme 1c).
To explore this possibility we selected 2-aminophenol as a
coupling partner due, in part, to its relevance to enzymatic
systems. Cobalt-containing catecholase mimics enable the
oxidation of catechol to ortho-quinone with the concomitant
reduction of O2 to H2O,8 and the related aminophenol-based
scaffolds serve as common redox-active ligands.9 Furthermore,
the coupling of 2-aminophenols with isonitriles generates 2-
aminobenzoxazoles, heterocyclic targets of biological and
a
Isolated yields. Reaction conditions: 1 (0.3 mmol), 2a (0.3 mmol),
and Co(OAc)2 (10 mol %) in 3 mL of CH3CN at 65 °C with an air
10
pharmaceutical importance. Whereas a Pd/O2 catalyst and
b
a Co/tBuOOH11 system have been developed for this and
related transformations (Scheme S1), Co/O2 catalyst systems
have not yet been reported.
balloon for 24 h. 1 mmol scale reaction.
a
Scheme 3. Scope of Isonitriles
We began our studies by evaluating the coupling reaction of
2-aminophenol (1a) and tert-butylisonitrile (2a) under aerobic
conditions in the presence of cobalt salts (Table 1). In early
reactions, air proved to be an efficient oxidant to promote this
oxidative cyclization reaction. Of the CoII salts studied,
Co(OAc)2 gave the highest yield of 3a (entry 6). When
CH3CN was employed as the solvent, 3a was obtained in 93%
yield (entry 11). Shorter reaction times led to reduced yields
(entries 12 and 13). The optimized reaction conditions employ
a simple Co(OAc)2 salt with an air balloon in CH3CN at 65
°C for 24 h. Control reactions conducted in the absence of
Co(OAc)2 (entry 14) or air (entry 15) resulted in 0 and 14%
yield of 3a respectively, highlighting the importance of both
the cobalt catalyst and air in this oxidative transformation.
We next explored the scope of substituted 2-aminophenol
coupling partners (1, Scheme 2). This protocol showed good
tolerance of a variety of functional groups including both
electron-rich (3d, 3e, 3h, and 3i) and electron-deficient
substituents (3f, 3g, and 3k). The influence of a methyl group
in each of the four aromatic positions (3b, 3d, 3i, and 3l)
reveals only a small steric effect, as the yield of 3b bearing the
3-methyl group showed a slightly decreased yield. The 4-, 5-,
and 6-methyl aminophenols all resulted in high yields of the
corresponding 2-aminobenzoxazoles. Similarly, the large
phenyl and tert-butyl groups afforded the desired cyclized
products in good yields (3c and 3h). Finally, the disubstituted
electron-deficient 5-nitro-4-chloro-2-aminophenol 1m afforded
the desired benzoxazole product in 64% yield.
a
Isolated yields. Reaction conditions: 1a (0.3 mmol), 2 (0.3 mmol),
and Co(OAc)2 (10 mol %) in 3 mL of CH3CN at 65 °C with an air
balloon for 24 h. 1 mmol scale reaction.
b
Scheme 4. Oxidative Cyclization Conducted in the Presence
of Radical Trapping Agents
To further evaluate the performance of this cyclization
reaction, we investigated the scope of isonitriles (Scheme 3).
Whereas tert-butylisonitrile is the most commonly employed
isonitrile in related coupling protocols,12 other bulky aliphatic
isonitriles are also excellent coupling partners under our
reaction conditions, furnishing the corresponding products in
high yields (3n−q). Surprisingly, the isopropyl- and cyclo-
hexyl-substituted isonitriles also generated high yields of the
corresponding 2-aminobenzoxazole products (3p and 3q).
These products are expected to contain weak C−H bonds
B
DOI: 10.1021/acs.orglett.9b01384
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 6. (a) Proposed Pathway in Literature for Aerobic Co-Catalyzed Catechol Oxidation and (b) Proposed Pathway for
the Related Oxidation of 2-Aminophenol
4b),17 consistent with the presence of aminophenol
radicals.8b,18 The elevated yield in the presence of TEMPO
could be a result of a TEMPO-mediated H-atom abstraction
pathway to form the aminophenol radical.19
A large series of CoII complexes of aminophenol-, amino-
thiophenol-, and diamine-derived ligands have been character-
ized,9b,20 and Wieghardt and coworkers have shown the
aerobic oxidation of these species to form stabilized
iminobenzosemiquinonato-type species bearing π-radical li-
gands (APSQ).20b In all of these cases, highly substituted
aminophenol ligands are used to avoid ligand dimerization and
enable the isolation of the oxidized complexes. We believe that
a related stabilized ligand radical may be formed under our
reaction conditions from CoII, O2, and 2-aminophenol. When
CNR is present, it is a sufficient trap for the aminophenol
radical,12 whereas in its absence, aminophenol dimerization
occurs.
To determine if O2 is required for the formation of 3 or if it
is only responsible for regenerating CoII after product
Figure 1. Absorption spectrum of Co(acac)2 with 2-aminophenol
under N2 (red trace) exposed to air and monitored over the course of
12 h at rt. The final spectrum (blue trace) is similar to that of
Co(acac)3 treated with 2-aminophenol under N2 after 24 h.
formation, we conducted a series of control experiments in
the absence of O2. When the standard reaction is conducted
with 1 equiv of Co(OAc)2 under N2, only 10% yield of 3a is
obtained, indicating the need for oxygen to efficiently form
product, not simply to regenerate CoII (Scheme 5). Addition-
ally, when the same reaction is conducted with stoichiometric
loadings of the CoIII salt, Co(acac)3, under N2, 3a is formed in
63% yield (Scheme 5). The reduced yield obtained with
Co(acac)3 relative to the standard catalytic conditions with
Co(OAc)2 are attributed to the slow exchange of the acac
ligand. Consistent with this hypothesis, the standard catalytic
reaction conducted with Co(acac)2 leads to 58% yield of 3a,
compared with the 93% yield obtained when Co(OAc)2 is
used (Table 1, entry 11). These combined data suggest that O2
is responsible for the oxidation of CoII to generate an active
CoIII or ligand oxidized semiquinonato CoIIAPSQ intermediate.
Given the importance of oxidation for product formation, we
sought to better understand the aerobic oxidation step. The
oxidation of CoII by O2 is typically proposed to generate a
CoIII−superoxide intermediate. In a recent study by Stahl,
Hammes-Schiffer, and coworkers utilizing salophen-bound
cobalt complexes, the CoIII−superoxide species is responsible
for H-atom abstraction (HAA) from H2Q to generate a CoIII−
hydroperoxide intermediate (Scheme 6a).3 A second HAA step
then generates the quinone and CoII with the release of H2O2.
Under these conditions, H2O2 undergoes disproportionation
to form O2 and H2O. Similarly, the reoxidation of NHPI is
proposed to proceed through an analogous HAA step.4a Thus
we imagined that a CoIII−superoxide species may be
responsible for the HAA of 2-aminophenol under our oxidative
cyclization conditions (Scheme 6b).
(BDE = ∼86 kcal mol−1)13 and are plausible substrates for
further oxidation. The benzylic isonitrile 2r, however, did not
undergo efficient coupling and instead resulted in only a
moderate yield of the coupling product (3r, 32%). Also
observed was the 3-aminobenzoxazine byproduct (16% yield)
that results from the insertion of 2 equiv of isonitrile.14
Surprisingly, no oxygenation byproducts15 were observed in
this reaction, despite the weak C−H bond (∼72 kcal mol−1)13
and the precedent for Co/O2 enabled benzylic oxygenation.4c
Finally, aryl isonitriles also underwent coupling to yield the
corresponding 2-aminobenzoxazole products, with the bulkier
aromatic ring of 1u resulting in the highest product yield
(82%).
The efficient coupling of isonitriles bearing weak C−H
bonds suggested that a free-radical intermediate is unlikely.
Consistent with this hypothesis, efforts to trap free-radical
intermediates with common radical scavengers, such as
TEMPO, 9,10-dihydroanthracene, and 1,1-diphenylethylene,
showed no evidence of trappable radical intermediates. Instead,
when these reagents were included under the standard reaction
conditions, high yields of 3a were still observed (Scheme 4a).16
The absence of trappable free radicals may suggest the
stabilization of a ligand radical through coordination to Co.
When the same reactions were conducted in the absence of
isonitrile, oxidative dimerization of 2-aminophenol occurred to
form 2-aminophenoxazine-3-one in both the presence and the
absence of TEMPO (43 and 10% yield, respectively) (Scheme
C
DOI: 10.1021/acs.orglett.9b01384
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We conducted a series of UV−visible experiments to probe
this oxidation step and, in particular, the ligand environment
that enables aerobic oxidation. First, when Co(acac)2 in
CH3CN is exposed to air for 12 h, no changes in the
absorption spectrum are observed (Figure S3). Similarly, a
mixture of Co(acac)2 and CNtBu in CH3CN showed no
significant changes in the spectroscopic features after exposure
to air (Figure S4). In contrast, when a solution of Co(acac)2
and 2-aminophenol is exposed to air, a color change from light
pink to orange-brown occurs within minutes. This rapid color
change is accompanied by an increase in the absorption around
260 nm (Figure 1). This spectroscopic band is similar to that
observed when Co(acac)3 is combined with 2-aminophenol
under N2 (λmax = 233 nm, Figure S7). In related Co systems
bearing catechol ligands, the semiquinonato species can be
obtained when CoIII starting materials are treated with
catechol,21 and a related redox equilibrium to access
CoII(APSQ) from CoIII may be operative in these aminophenol
systems. Overall, these spectroscopic changes suggest that the
aerobic oxidation of CoII is facilitated by the presence of 2-
aminophenol.
Although our work here has focused on the aerobic
oxidation of CoII, the subsequent coupling of the oxidized
intermediate with isonitrile is also of interest. In related Pd-
catalyzed cyclization reactions of aminophenols with isonitriles,
the C−N bond-forming step is proposed to proceed via the
migratory insertion of the CNR ligand.14 Alternatively, a
radical-based addition to CNR may be operative given the
presence of the proposed semiquinonato radical intermedi-
ates.12
ACKNOWLEDGMENTS
■
We are grateful to West Virginia University and ACS PRF
(56083-DNI3) for financial support of this work. NMR
spectroscopy facilities were partially supported by the NSF
(CHE-1228336).
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ASSOCIATED CONTENT
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* Supporting Information
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Details of reaction development, characterization data
1
for starting materials and products, and H and 13C
NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: Jessica.Hoover@mail.wvu.edu.
ORCID
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DOI: 10.1021/acs.orglett.9b01384
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