Aerobic Catalyzed Oxidative Cross-Coupling of N, N-Disubstituted Anilines and Aminonaphthalenes with Phenols and Naphthols

Article abstract of DOI:10.1021/acs.orglett.0c00046

The cross-coupling of N,N-dialkyl aniline and aminonaphthalenes with phenols and naphthols using a Cr-salen catalyst under aerobic conditions was developed. Notably, air serves as an effective oxidant affording products in high selectivity. Initial mechanistic studies suggest an outer-sphere oxidation of the aniline/aminonaphthalene partner, followed by nucleophilic attack of the phenol/naphthol. Single products were observed in most cases, whereas mixtures of C-C and C-O coupled products arose from reactions involving aminonapthalene and sterically unencumbered phenols.

Full text of DOI:10.1021/acs.orglett.0c00046

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Letter
Aerobic Catalyzed Oxidative Cross-Coupling of N,N‑Disubstituted
Anilines and Aminonaphthalenes with Phenols and Naphthols
Thomas J. Paniak and Marisa C. Kozlowski*
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ABSTRACT: The cross-coupling of N,N-dialkyl aniline and amino-
naphthalenes with phenols and naphthols using a Cr−salen catalyst
under aerobic conditions was developed. Notably, air serves as an
effective oxidant affording products in high selectivity. Initial
mechanistic studies suggest an outer-sphere oxidation of the aniline/
aminonaphthalene partner, followed by nucleophilic attack of the
phenol/naphthol. Single products were observed in most cases,
whereas mixtures of C−C and C−O coupled products arose from
reactions involving aminonapthalene and sterically unencumbered
phenols.
nsymmetrical biaryls are found in organometallic
Scheme 1. Oxidative Cross-Couplings of 2-
Aminonaphthalenes/Anilines with Naphthols/Phenols
U
chemistry,1 natural product synthesis,2 pharmaceutical
synthesis,3 and materials chemistry.4 The ability to generate
such structures selectively from simple precursors is an
important challenge in organic synthesis. These motifs are
classically synthesized through the metal-catalyzed cross-
coupling of prefunctionalized partners.5 Dehydrogenative
cross-coupling of arenes,6 in particular of phenols,7,8 has
been developed recently to overcome the need for
prefunctionalization.
The 2′-aminobiphenyl-2-ol structural motif is an interesting
unsymmetrical biaryl with examples found in active natural
products9 (Figure 1). Most routes to access these structures
Figure 1. Natural products with 2′-aminobiphenyl-2-ol motif.
elevated temperatures were required. With chiral-auxiliary-
derived aminonaphthalenes, catalytic oxidative conditions give
rise to enantiopure axial chiral versions.12 The coupling of
involve prefunctionalization. Oxidative methods for direct C−
H activation to construct this motif have been developed in the
past decades.10 Seminal work centers on the coupling of 2-
aminonaphthalene with 2-naphthol (Scheme 1a), which was
complicated by the high reactivity of the amino group. More
recently, oxidative couplings of N,N-disubstituted aniline
derivatives with naphthols have been studied using Fe and
Ce catalysts (Scheme 1b).11a,b The scope was confined to 2-
naphthol and t-BuOOH was needed, and in the latter case,
Received: January 5, 2020
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Figure 2. High-throughput experimentation results for the catalyst library screen (internal standard = 4,4′-di-tert-butylbiphenyl).
phenols with similar anilines is much more difficult, with the
first report from Fotie et al. in 2016 (Scheme 1c).13 This
process required a super-stoichiometric Ag oxidant (3 equiv),
with the highest yield of the aniline/phenol coupling being
Table 1. Optimization of Oxidative Cross-Coupling of N,N-
Dimethyl-2-aminonaphthalene and 2-Naphthol
a
70%. Further methods with a hypervalent iodine oxidant,14
a
periodic acid,15 a Pd/Al2O3 catalyst,16 and a heterogeneous Rh
catalyst17 have been reported but with very limited examples
(three to six per report) or the requirement of amino-
naphthalenes and naphthols versus anilines and phenols. For
example, an electrochemical method only uses 2-amino-
naphthalene.18 Herein we describe the development of Cr−
salen-catalyzed cross-coupling of N,N-substituted anilines/2-
aminonaphthalenes with naphthols/phenols. This process
utilizes benign conditions (room temperature (rt) air as
oxidant, Scheme 1d). Interestingly, most reactions result in C−
C coupling products, but some couplings of 2-amino-
naphthalene with phenols lead to the formation of C−O
coupled products as well.
Metal−salen complexes have been shown to be powerful
catalysts in oxidative reactions.8g In particular, our group has
previously reported the mechanism of a Cr−salen-catalyzed
phenol cross-coupling.19 To interrogate the potential of these
metal−salen complexes in aniline/phenol cross-coupling, high-
throughput experimentation screening was implemented with a
library of catalysts. When using 1,1,1,3,3,3-hexafluoro-2-
propanol (HFIP) as the solvent, every catalyst screened
resulted in some level of product formation (Figure 2). To
identify the best catalyst, the top leads were run again on a
larger scale (see the SI) which revealed that the Cr−salen
complex results in the highest yield and minimizes the
formation of undesired side products.
1b:2a cat (mol %)
oxidant
T (°C)
t
yield (%)
1
2
3
4
5
6
7
8
9
10
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:1.5
10
10
10
10
10
20
5
10
10
10
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
t-BuOOH
O2
80
50
rt
0
rt
rt
rt
rt
rt
rt
1 d
1 d
1 d
1 d
6 h
1 d
1 d
1 d
1 d
1 d
33
51
55
23
52
59
52
78
73
74
air
air
a
HFIP: 1,1,1,3,3,3-hexafluoro-2-propanol. Conditions: 1b (0.10
mmol, 0.10 M), t-BuOOH (2.0 equiv), HFIP (1.0 mL). Yields
were obtained by 1H NMR using 4,4′-di-tert-butylbiphenyl as an
internal standard.
even air being suitable for the reaction to proceed (entry 9).
Finally, the reactant ratio was optimized to 1:1.5 aniline to
phenol without a decrease in yield (entry 10).
With these optimized conditions, the scope of the method
was investigated (eq 2). N,N-Dimethyl-2-aminonaphthalene
Optimization studies were performed on the cross-coupling
of N,N-dimethyl-2-aminonaphthalene and 2-naphthol using
this Cr−salen catalyst in HFIP (eq 1, Table 1). Lowering the
temperature to rt provided higher yields (entries 1−3). Lower
yields were observed at 0 °C, potentially due to the decreased
solubility in HFIP (entry 4). Shorter reaction times led to a
slight decrease in the yield, which was found to be more
detrimental with less reactive coupling partners (entry 5).
Different catalyst loadings (5, 10, and 20 mol %) had a small
effect on the yield (entries 3, 6, and 7). Moving toward less
harsh oxidants, O2 was found to be beneficial (entry 8), with
B
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was an effective coupling partner with a wide range of
naphthols and phenols (Figure 3; blue compounds are new
Figure 3. Couplings of N,N-dimethyl-2-aminonaphthalene using the
conditions in eq 2. (Blue compounds are new compounds, not
a
previously reported.) Isolated yield on a 1.0 mmol scale.
compounds, not previously reported). Few byproducts were
observed, and the efficiency was generally good. Bromo- and
methoxynaphthols were well tolerated along with 2-naphthol
(Figure 3, 3ba−bc). Several substituted phenols coupled
effectively as well (3bd−bh) and with very high regioselectivity
(>50:1). On a larger scale, the reaction efficiency was even
higher (3ba, 83%)
Figure 4. Couplings of anilines using the conditions in eq 2 (BRSM =
based on recovered starting material; blue compounds are new
compounds, not previously reported).
The catalyst system was sufficiently reactive that the
couplings of the more difficult aniline derivatives could be
accomplished (Figure 4). In addition to the N,N-dimethyl
congener (3aa−ab), para-toluidines with N,N-diethyl sub-
stitution (3ca−cb) or with the incorporation of the nitrogen
into pyrrolidine (3da−db), piperidine (3ea−eb), or morpho-
line (3fb) rings all coupled to naphthols with good to very
good efficiency. The more electron-rich para-methoxyanilines
were also effectively coupled (3ga−ha). Notably, the coupling
of the aniline analogs with phenols also proceeded in moderate
yield (3dd−de), even for a monosubstituted phenol (3ai) for
which selective couplings are typically very difficult. In all of
these cases, the reactions were fairly clean, giving only one
product along with residual starting material or decomposition
to baseline materials.
Coupling reactions of N,N-dimethyl-2-aminonaphthylene
and phenols led to somewhat unexpected results in certain
instances (Figure 5). Coupling using phenols with multiple
unhindered reactive sites, such as 2-tert-butylphenol, led to a
mixture of ortho- and para-substituted products (3bj, 3bj′).
The product ratio observed (1:2.2) is consistent with the
calculated site nucleophilicities of the ortho and para positions
of the phenolate anion (1.70:2.15)19 showing a preference for
the para-substituted product. When using a phenol with a
sterically unencumbered −OH group, a mixture of C−C (3bi−
bn) and C−O (3bi′−bn′) coupled products was observed. In
para-substituted phenols (3bi−bl), a preference for the C−O
product is observed, with product ratios of 1:1.5−3.1. For each
of these cases, the site nucleophilicities (see the SI) of the
phenolates predict that the oxygen is more reactive in accord
with the observed trends. The greatest preference for C−O
products is seen with an electron-donating methoxy group
(3bl, 3bl′). In contrast, increasing the steric bulk around the
OH leads to more C−C coupled product (3bm). Furthermore,
Figure 5. Couplings of N,N-dimethyl-2-aminonaphthalene with two
outcomes (C−Cortho vs C−Cpara or C−C vs C−O; blue compounds
are new compounds, not previously reported).
increased steric bulk around the ortho positions (by 3,5-
substitution) leads to a greater preponderance of C−O
product (3bn′).
This method required certain structural and electronic
parameters in order for coupling to occur. Specifically, para
substitution of the aniline derivative is required. N,N-
Dimethylaniline as well as ortho-substituted N,N-dimethylani-
C
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lines did not undergo coupling with this method. Furthermore,
electron-withdrawing substituents on the aniline or phenol
partner were not tolerated. The incorporation of the nitrogen
into a ring (e.g., N-methylpyrrole or N-methylindole) did not
afford cross-coupled product using this method.
To gain greater insight into the reaction, the active catalyst
of the system was determined. The addition of 100 mol % oxo-
Cr(V) was found to result in 100% conversion of starting
material. The addition of sterically hindered base (2,6-di-tert-
butyl-4-methylpyridine) increased the rate of loss of oxo-Cr(V)
(1 min vs 20 s). This finding is consistent with the reported
work on the cross-couplings of phenol with the same Cr−salen
catalyst.19
Cyclic voltammetry and calculated nucleophilicities were
used to probe which substrate likely initiates the reaction. The
onset oxidation potential of N,N-dimethyl-2-aminonaphtha-
lene (0.33 eV, relative to Fe/Fe+) was found to be significantly
lower than that of the most oxidizable phenolic partner, 2,6-
dimethoxyphenol (0.89 eV),19 suggesting that the amino-
naphthalene was the more oxidizable species in the reaction.
Support for this oxidation order is the rapid quantitative
formation of a homodimer when N,N-dimethyl-2-amino-
naphthalene alone is subjected to the catalyst.
oxidation occurring in the reaction. Such an oxidation by the
oxo-Cr(V) species II would yield a radical cation and Cr(IV)
species III. A computational study of the site nucleophilic-
ities19 of the coupling partners revealed that the deprotonated
phenol/naphthol (1.67−2.88; see the SI and previous work19)
partner is considerably more nucleophilic at the ortho-carbon
than N,N-dimethyl-2-aminonaphthalene (0.95). Thus, after
deprotonation of the phenol by III, the attack of the more
nucleophilic phenolate onto the radical cation accompanied by
a one-electron oxidation would induce the selective cross
coupling and yield IV. The addition of base suppresses the
formation of the aminonaphthalene homodimer by ∼6% in the
cross-coupling of N,N-dimethyl-2-aminonaphthalene and 4-
chlorophenol, which further supports the role of the phenolate
anion. No enantioselectivity was observed in the couplings of
3ba, 3bb, and 3bc (see the SI), which is consistent with a
mechanism where the coordination of the phenol does not
occur. Ultimately, tautomerization and water release lead to
the product and regenerate the Cr(III)−salen catalyst I. The
complementary nature of the coupling partners (oxidizability
vs nucleophilicity) is similar to that invoked in some phenol
cross-couplings.8d
For some cases (i.e., unhindered phenols), phenol oxidation
may involve coordination to the Cr(IV) species III and inner-
sphere electron transfer; however, the dissociation of the
resulting phenoxyl equivalent needs to be invoked to explain
the C−O coupling outcomes (Figure 5). For more hindered
phenols (e.g., 2,6-di-tert-butylphenol), outer-sphere electron
transfer appears more likely.
Free-radical inhibitor butylated hydroxytoluene (BHT, 1.06
eV) was found to alter the reaction outcome dramatically.
When N,N-dimethyl-2-aminonaphthalene (eq 3, no phenol
In conclusion, we have developed an effective catalytic
oxidative cross-coupling of N,N-disubstituted aniline deriva-
tives with naphthols and phenols. The method proceeds under
benign conditions, using O2 in air as the oxidant at room
temperature. Mechanism experiments suggest that oxidation of
the aniline portion occurs first, which then engages in a
coupling with the more nucleophilic species (naphthol/
phenol) at the more nucleophilic site via a Cr(V) to Cr(III)
redox couple.
present except BHT) was subjected to the Cr−salen catalyst
under air with BHT, the aminonaphthalene homocoupling that
was otherwise observed (see above) was completely sup-
pressed, and compound 4 was formed instead.
On the basis of the above data, a catalytic cycle is proposed
(Figure 6). Binding of the sterically hindered N,N-dimethyl-2-
aminonaphthalene to the Cr−salen catalyst would be
disfavored,20 which suggests the possibility of an outer-sphere
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00046.
Experimental and spectroscopic data (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Marisa C. Kozlowski − Department of Chemistry, University of
Pennsylvania, Philadelphia, Pennsylvania 19104, United
States; orcid.org/0000-0002-4225-7125; Email: marisa@
sas.upenn.edu
Author
Thomas J. Paniak − Department of Chemistry, University of
Pennsylvania, Philadelphia, Pennsylvania 19104, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00046
Notes
Figure 6. Proposed catalytic cycle for oxidative cross-coupling.
The authors declare no competing financial interest.
D
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ACKNOWLEDGMENTS
■
We are grateful to the NSF (CHE1764298) and the NIH (R35
GM131902, RO1 GM112684) for financial support of this
research. Partial instrumentation support was provided by the
NIH and NSF (1S10RR023444, 1S10RR022442, CHE-
0840438, CHE-0848460, 1S10OD011980, CHE-1827457).
Dr. Charles W. Ross, III (UPenn) is acknowledged for
obtaining accurate mass data. This work was supported by the
Vagelos Institute for Energy Science Technology at the
University of Pennsylvania. We thank Dr. Sergei Tcyrulnikov
and Lucille Wells (UPenn) for the calculation of nucleophil-
icity parameters.
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F
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