A Bioinspired Catalytic Aerobic Functionalization of Phenols: Regioselective Construction of Aromatic C-N and C-O Bonds

Article abstract of DOI:10.1021/acscatal.7b00437

We report a bioinspired approach for the regioselective construction of both aromatic C-O and C-N bonds through the dehydrogenative coupling of phenols and aliphatic amines. Mechanistically, the process hinges on the ortho-oxygenation of phenols to ortho-quinones followed by aromatic C-N bond formation through a condensation/redox isomerization cascade. Overall, the process enables the regioselective formation of aromatic C-O and C-N bonds directly from C-H bonds under mild reaction conditions.

Full text of DOI:10.1021/acscatal.7b00437

Research Article
pubs.acs.org/acscatalysis
A Bioinspired Catalytic Aerobic Functionalization of Phenols:
Regioselective Construction of Aromatic C−N and C−O Bonds
Kenneth Virgel N. Esguerra and Jean-Philip Lumb*
Department of Chemistry, McGill University, Montreal, Quebec H3A 0B8, Canada
S
* Supporting Information
ABSTRACT: We report a bioinspired approach for the regioselective
construction of both aromatic C−O and C−N bonds through the
dehydrogenative coupling of phenols and aliphatic amines. Mechanistically,
the process hinges on the ortho-oxygenation of phenols to ortho-quinones
followed by aromatic C−N bond formation through a condensation/redox
isomerization cascade. Overall, the process enables the regioselective
formation of aromatic C−O and C−N bonds directly from C−H bonds
under mild reaction conditions.
KEYWORDS: aerobic catalysis, ortho-quinone, phenol, amine, C−H functionalization, crossed-dehydrogenative coupling, amino-phenol
incorporation of heteroatoms using simple phenols and amines
he importance of aromatic carbon−heteroatom bonds
Tto the function of materials and biologically active is an unresolved challenge of great interest.
Herein, we describe an approach toward this aim that draws
compounds has motivated considerable efforts to improve the
efficiency of their formation.1 In this context, catalytic aerobic
dehydrogenative coupling reactions, in which aromatic C−O or
C−N bonds are created directly from C−H, O−H, or N−H
bonds, are desirable due to their inherent atom- and step-
efficiency.2 Phenols are an attractive coupling partner for this
class of reaction because of their well-established availability
from petrochemical resources3 and increasing availability from
biomass.4 They are also common structural motifs in natural
products and organic materials.5,6 Nevertheless, their partic-
ipation in dehydrogenative C−O and C−N coupling reactions
is challenged by nonselective oxidation with a broad range of
oxidants.7 Examples of dehydrogenative aryl ether formation
are limited,8 with a noteworthy exception being Hay’s industrial
polymerization of 2,6-dimethyl phenol.9 Likewise, the direct
amination of phenols suffers from challenges that include
limited scope, prefunctionalization of the nitrogen coupling
partner, or the use stoichiometric quantities of a synthetic
oxidant.10 There are only a handful of examples where aromatic
C−N bonds are formed directly from C−H and N−H bonds,11
and there are even fewer that use O2 as the terminal oxidant.11a
More generally, functionalization is typically not regioselective
in the absence of blocking groups and results in derivatization
of the ortho- and/or para-position;7b,f,12 direct functionalization
of the meta-position remains difficult.13 These challenges are
amplified under the oxidative conditions of dehydrogenative
coupling, which often require protection of the hydroxyl
group to avoid nonselective,14 radical-based polymerization.7
Thus, a direct, catalytic aerobic process for the regioselective
inspiration from two very different biosynthetic processes
(Scheme 1). The first is the biosynthesis of melanin pigments,
in which the Type III Cu-enzyme tyrosinase catalyzes the
aerobic ortho-oxygenation of L-tyrosine 1 to L-dopaquinone 2 in
the first and rate-limiting step of pigmentation (Scheme 1A).15
As a natural consequence of dearomatization, the meta-position
of 1 becomes activated for nucleophilic attack by the pendant
amine, leading to L-dopachrome 3 following tautomerization
and reoxidation.16 This constitutes a regioselective means of
oxidizing two aromatic C−H bonds, under conditions that
generate water as the only byproduct.15b The second bio-
synthetic process is the production of collagen fibers, which
capitalizes upon the innate reactivity of ortho-quinones and
amines. In this case, an ortho-imino phenol 5 is produced within
the active site of lysyl oxidase (cf. TTQ) following condensation
and redox-isomerization (Scheme 1B).17 Although the imme-
diate product of this coupling is ultimately hydrolyzed to the
corresponding ortho-amino phenol 6 during enzymatic catalysis,
we envisioned the initial steps leading to 5 as a uniquely efficient
means of forming an aromatic C−N bond. In this manuscript,
we detail our efforts to interface these two, unrelated bio-
synthetic processes into a 1-pot, sequential process (Scheme 2).
This leads to an efficient synthesis of heteroatom-rich aromatic
rings by a formal dehydrogenative coupling of phenols and
Received: February 9, 2017
Revised: March 29, 2017
© XXXX American Chemical Society
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Scheme 1. (A) Tyrosinase-Mediated Dual Functionalization
of L-Tyrosine; (B) Lysyl Oxidase Mediated Deamination of
Lysine during Collagen Biosynthesis
Scheme 3. Classical Synthesis of ortho-Aminophenols
(Scheme 4). For example, benzoxazole 12 or benzoxazinone 13
would result from either oxidative cyclization or redox-neutral
lactonization of ortho-imino-phenols derived from benzylamine
11 or phenylglycine methyl ester 10, respectively. Alternatively,
the use of 2,4-di-nitro-phenyl-hydrazine 14 would provide
valuable “push-pull” azo-phenols (cf. 15), following installation
of the azo-linkage by condensation and tautomerization. Finally,
the use of dihydropyrrole 16 would afford N-arylated pyrrole
17, in a formal dehydrogenative coupling between a phenol and
a pyrrole.
As a means of developing optimized reaction conditions, we
evaluated the dehydrogenative coupling of 4-tert-butyl phenol 7
and benzylamine 11, in a 1-pot sequential process (Table 1).
We have previously reported the oxygenation of 7 using 4 mol %
of Cu(CH3CN)4(PF6) (abbreviated as CuPF6), 5 mol % of
di-tert-butyl-ethylenediamine (DBED) and O2 (2 atm), which
returns a quantitative yield of coupled ortho-quinone 9 by way
of an oxidative coupling between DBED-Cu(II)-semiquinone 8
and 7.23 In the current context, 9 was not isolated, and instead,
one equivalent of 11 was simply added directly to the reaction
mixture. This leads to benzoxazole 12 in 23% isolated yield
following a cascade of condensation/redox isomerization/
cyclization and dehydrogenation (entry 1). Competitive path-
ways contributing to the low yield of 12 included the oxidation
of 11 to benzyl imine 18, and a substitution of the phenoxyl
group with benzyl amine to provide amino-ortho-quinone 19,
consistent with the previous observations of Maumy and
Capdeville (Scheme 5).24 Given that both of these deleterious
pathways might result from a relatively slow dehydrogenation of
dihydrobenzoxazole 12′ to benzoxazole 12 in the final step of
the cascade, we investigated the effects of dessicants because
we have previously observed improvements to the oxidative
capacity of the DBED/CuPF6 system in their presence.23a
Unfortunately, minimal improvements were observed in the
presence of MgSO4 (entry 2), even with 2 equiv of 11 (entries
3−4). Upon increasing the catalyst loading to 8 mol % CuPF6
and 10 mol % DBED, we observed an encouraging increase
in yield to 43% (entry 5). Further adjusting the ratio of DBED
to Cu by raising the ligand loading to 15 mol % provided an
additional increase in yield to 57% (entry 6), which was not
improved by the addition of desiccants (entries 7−11) or
increasing Cu loadings above 8 mol % (entries 10 and 11).
Thus, entry 6 represents our optimized reaction conditions,
which remain efficient for each of the additional nitrogen
coupling partners. This provides benzoxazinone 13, azophenol
15, and N-arylated pyrrole 17 in yields of 85%, 56%, and 83%,
respectively (Scheme 4). Notably, these conditions remain
efficient on gram scale (10 mmol of 7), providing 13 from
7 and 10 in 82% yield. Our regiochemical assignment is based
on a series of X-ray crystal structures (Figure 1) and is
consistent with C−N coupling at the more electrophilic of
the two carbonyls of quinone 9. These can be viewed as two
Scheme 2. Work Described Herein
amines that occurs at room temperature, using an earth-abundant
and commercially available copper catalyst.
An important feature of our proposed methodology is the
versatile reactivity of ortho-imino-phenols, which have been
extensively developed as electrophilic reagents18 and serve as
key intermediates in heterocycle synthesis.19 In spite of their
utility, current syntheses incur atom- and step-inefficiencies
during the preparation of the corresponding ortho-aminophenol
(Scheme 3).20 Representative procedures include the nitration
and reduction of phenols,21 transition-metal catalyzed cross-
coupling of a prefunctionalized aromatic ring,2 or nucleophilic
aromatic substitution.22 In contrast, the tandem ortho-oxygenation/
condensation/redox isomerization approach proposed herein would
offer a direct means for the synthesis of ortho-imino-phenols,
under mild reaction conditions. Once formed, we would anti-
cipate a range of downstream transformations, which could be
guided by careful selection of the nitrogen coupling partner
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a
Scheme 4. Bioinspired Synthesis of Benzoxazoles, Benzoxazinones, Azophenols, and N-Aryl Pyrrole
a
Synthesis of 12: 7 (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 11 (2.0 equiv), O2 (1 atm), 2 h,
23 °C. Synthesis of 13: 7 (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 10 (2.0 equiv) in MeOH
(5 mL), 4 h, 50 °C. Synthesis of 17: 7 (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 16
(2.0 equiv), 2 h, 23 °C. Synthesis of 15: 7 (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 14
(2.0 equiv), MeOH (3 mL), 12 h, 23 °C. Isolated yields are reported for each entry.
Table 1. Initial Studies on the Coupling of Phenol 6 and
Amine 10
Scheme 5. Competitive Pathways in the Aerobic Coupling of
7 and 11
a
CuPF6
DBED
BnNH2
yield of
b
a
entry
(mol %) (mol %) (equiv)
additive
12 (%)
1
2
4
4
5
1.0
1.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
--
23
27
33
32
43
57
55
53
50
37
24
5
MgSO4 (3 equiv)
3
4
5
--
4
4
5
MgSO4 (3 equiv)
5
8
10
15
15
15
15
20
40
--
6
8
--
7
8
MgSO4 (3 equiv)
8
8
M.S. (4 Å, 200 mg)
9
8
Na2SO4 (3 equiv)
10
11
10
20
--
--
a
b
Conducted with 1.0 mmol of 7. Product yield is determined by
1H NMR using hexamethylbenzene as an internal standard.
independent π-systems, consisting of a vinylogous ester and an
α, β-unsaturated ketone, with condensation occurring preferen-
tially at the latter. As a result, the oxygen atom dervied from O2
during the ortho-oxygenation is ultimately replaced by nitrogen
in the subsequent C−N coupling, providing a unique
mechanism for the ortho-amination of phenols.
Figure 1. Single crystal X-ray structure of 13 and selected products in
Table 1 (entries 5 and 7). Hydrogens are omitted for clarity. Blue =
nitrogen, red = oxygen, teal = carbon, yellow = fluorine.
Next, we evaluated the scope of mono- and disubstituted
phenols with each of the amine coupling partners (Table 2).
Monosubstituted phenols bearing para- and meta- substituents
readily couple with various amines to afford benzoxazoles,
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a
Table 2. Scope of Phenols
a
Benzoxazole synthesis: phenol (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 11(2.0 equiv), O2
(1 atm), 2 h, 23 °C; Benzoxazinone synthesis: phenol (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then
10 (2.0 equiv) in MeOH (5 mL), 4 h, 50 °C; N-Aryl pyrrole synthesis: phenol (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2
(0.1 M), 4 h, 23 °C, then 16 (2.0 equiv), 2 h, 23 °C; Azophenol synthesis: phenol (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm),
CH2Cl2 (0.1 M), 4 h, 23 °C, then 14 (2.0 equiv), MeOH (3 mL), 12 h, 23 °C. Isolated yields are reported for each entry.
benzoxazinones, N-aryl pyrroles or azophenols. Notably,
phenols bearing enolizable protons (entries 1−3 and 8), aryl
ethers (entries 4−6) and halogens (entry 6), as well as
2-napththols are all tolerated (entries 9 and 10). The sterically
encumbered 2- and 4-positions of 3,5-diphenyl phenol are also
readily functionalized to afford the corresponding benzoxazi-
nones in good yields (entry 14). Finally, aromatic C−N
coupling remains chemoselective in the presence of a ketone, as
illustrated by the late-stage functionalization of estrone (entries
10 and 11).
The functionalization of phenol (entry 15) and ortho-
substituted phenols (entries 16−18) affords products in which
three C−H bonds have been functionalized in the one-pot
operation, including the installation of two aromatic C−O bonds
and one aromatic C−N bond. Whereas the ortho-oxygenation of
phenol provides a symmetrical ortho-quinone for which there are
no questions of regiochemistry in the amine coupling, the
corresponding oxygenation of ortho-substituted phenols affords
a 3,4,5-trisubstituted ortho-quinone, in which the carbonyls at
C1 and C2 (quinone numbering) are differentiated by sterics
(Table 2, inset). This directs C−N coupling to the more accessible
C1 carbonyl to provide a single regioisomer of the product.
An important feature of the current methodology is the
efficiency with which multiple sites of simple phenols can be
oxidized and activated for additional transformations under
relatively mild reaction conditions. We believe that this sets the
stage for an alternative approach to aromatic-heteroatom coupling
reactions, for which we provide a preliminary proof of principal
in Scheme 6. Standard oxygenative homocoupling of 4-tert-butyl
phenol 7 to ortho-quinone 9 allows downstream conversion
to either ortho-quinone 20 or para-quinone 22, by exposure
to ethanol under basic or acidic conditions, respectively.25
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Scheme 6. Synthesis of N-Aryl Pyrrole from ortho- and para-
Quinones
ACKNOWLEDGMENTS
■
a
Financial support was provided by the Natural Sciences and
Engineering Council of Canada (NSERC, Discovery Grant to
J.-P.L.), the Fonds de Recherche du Queb
́
ecNature et
Technologies (FRQNT, Team Grant to J.-P.L.), the FRQNT
Center for Green Chemistry and Catalysis at McGill University,
the NSERC CREATE Program in Green Chemistry at McGill
University, and the McGill University-Canadian Institutes of
Health Drug Development Trainee Program (doctoral fellow-
ship to K.V.N.E.). We wish to thank Dr. Thierry Maris
(University of Montreal) for help with X-ray crystallography.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.7b00437.
■
S
Experimental procedures and characterization data
including NMR spectra, IR, HRMS, and crystallographic
data (PDF)
X-ray data for compound 13 (CIF)
X-ray data for benzoxazole of Table 2, entry 4 (CIF)
X-ray data for benzoxazole of Table 2, entry 6 (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: jean-philip@mail.mcgill.ca.
■
ORCID
Jean-Philip Lumb: 0000-0002-9283-1199
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acscatal.7b00437
ACS Catal. 2017, 7, 3477−3482