Controllable synthesis of 2- And 3-aryl-benzomorpholines from 2-aminophenols and 4-vinylphenols

Article abstract of DOI:10.1039/d0cc02662j

We present herein a method for the controllable synthesis of 3-aryl-benzomorpholine and 2-aryl-benzomorpholine cycloadducts via cross-coupling/annulation between electron-rich 2-aminophenols and 4-vinylphenols. Molecular oxygen was successfully used in the reaction as the terminal oxidant and the complete inversion of chemoselectivity was achieved by the adjustment of the solvents and bases at room temperature.

Full text of DOI:10.1039/d0cc02662j

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DOI: 10.1039/D0CC02662J
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Controllable Synthesis of 2- and 3-Aryl-Benzomorpholines from 2-
Aminophenols and 4-Vinylphenols
Kui Dong,a Xiao-Ling Jin,a Shihao Chen,a Li-Zhu Wu b and Qiang Liu*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
We present herein a method for the controllable synthesis of 3-aryl- for assembling aryl-benzomorpholines directly from 2-
benzomorpholine and 2-aryl-benzomorpholine cycloadducts via aminophenols and aryl alkenes remains a formidable challenge.
the cross-coupling/annulation between electron-rich 2-
aminophenols and 4-vinylphenols. Molecular oxygen was
successfully used in the reaction as the terminal oxidant and the
complete inversion of chemoselectivity was achieved by the
adjustment of solvents and bases at room temperature.
Benzomorpholines are an important class of benzoheterocycles,
which are common in drugs and bioactive molecules.1
Particularly, the skeletons bearing aryl at the C3 or C2 position
have attracted much attention due to their unique bioactivities
(Figure 1).2 Over the past few decades, several synthetic
methods for 2- and 3-aryl-benzomorpholines have been
reported such as metal-catalyzed intramolecular O-arylation
and N-arylation,3 hydrogenation of the C=N double bond in 2H-
1,4-benzoxazines,4 arylation of 2-hydroxy-1,4-benzoxazines5,2d
Figure 1. Representative biologically active 2- and 3-aryl
benzomorpholines scaffolds.
and others among which 2-aminophenols were always used as
the starting materials to access aryl-benzomorpholine
derivatives with partners such as activated halides and
epoxides.6 However, these methods suffered from additional
functionalization of subcomponents, multistep processes as
well as the lack of atom economy. In this regard, oxidative
coupling between 2-aminophenols and aryl alkenes is an ideal
approach to aryl-benzomorpholines synthesis due to its high
atom economy and readily available chemicals. To date the only
successful reported method for this purpose is tandem
oxidation–Diels–Alder reaction of 2-aminophenol derivatives
and 3,3-diphenyl enamines via electrochemical or MnO2-
involved oxidative process, yet suffers from a narrow substrate
scope.7 Thus, the development of an aerobic oxidation strategy
4-Vinylphenol motifs such as caffeic acid derivatives,
isoeugenol, and resveratrol etc. are widely prevalent in nature
and can be obtained from many natural phenolics.8 They
represent an ideal and sustainable class of starting materials for
diverse modes of bond construction in the constitution of more
complicated molecules.9 A few years ago, our group disclosed
visible light-catalyzed aerobic oxidative self-coupling of
resveratrol and its analogues.10a In continuation of our previous
work on phenols coupling and visible light-driven aerobic
oxidation,10 we sought to explore an aerobic oxidative
annulation of 4-vinylphenols with 2-aminophenols for the direct
synthesis of 2-AMs (2-aryl-benzomorpholines) and 3-AMs (3-
aryl-benzomorpholines). To the best of our knowledge,
controllable coupling of phenols remains challenging.11 Herein,
a.State Key Laboratory of Applied Organic Chemistry, Lanzhou University, 222 South
Tianshui Road, Lanzhou 730000, P. R. China.
we
present
base-
and
solvent-controlled
cross
E-mail: liuqiang@lzu.edu.cn
coupling/annulation between 2-aminophenols and 4-
vinylphenols for the synthesis of 2-AMs and 3-AMs at room
temperature in a highly regioselective manner.
b.Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, the Chinese Academy of Sciences,
Beijing 100190, P. R. China
† Electronic supplementary information (ESI) available. See
DOI: 10.1039/x0xx00000x
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Due to both the reactivity and the reaction pathway of substituted 2-aminophenol substrates with electVrioewn-AdrtoiclneaOtnilninge
phenols are sensitive to environmental pH,10, 12-13 we started our substituent groups, such as methyl, t-buDtyOlI,: a10n.d103m9e/Dth0CoCxy0l2,6t6h2eJ
investigation by evaluating the aerobic oxidative annulation of reaction proceeded well, giving 3-AMs in high to excellent yields
1a and 2a with various bases in different solvents. The reaction
was first carried out in acetonitrile solution containing 1 equiv successfully participated in this reaction in moderate yield (3ha).
of KHCO3 as a base and 1 mol % of Rose Bengal (RB) as the The reaction tolerated halide substituents such as chloride (3da
(3aa-3ca). Moreover, 3-methyl-substituted 2-aminophenol also
)
photocatalyst under blue LED irradiation at room temperature, and bromide (3ea), which are important from a synthetic
afforded 3-AM (3-aryl-benzomorpholine) 3aa as the product in perspective since these substituents provide useful handles for
96% isolated yield as a 6:1 mixture of diastereomers (Table 1, further structural elaboration through metal-catalyzed coupling
entry 1). The use of different bases had a dramatic impact on reactions. The molecular structure of 3da was also verified by
the reaction rate and the annulation regioselectivity (Table 1, single-crystal X-ray analysis (CCDC 1990318). However, 2-
entries 1-8). When the base was changed to K2CO3, the reaction amino-4-fluorophenol failed to give the product 3fa. Then, we
proceeded with 84% yield and longer reaction time (Table 1, turned our attention to the scope of 4-vinylphenols. As
entry 2). Intriguingly, the replacement of KHCO3 with KOH led to expected, the electronically enriched aromatic rings of 4-
the production of both 3-AM 3aa (35%) and 2-AM (2-aryl- vinylphenols were well tolerated, giving rise to the desired 3-
benzomorpholine) 4aa (33%) (Table 1, entry 3). Without the AMs (3ad, 3af, 3ag, 3aj) smoothly. Also delightful was the
addition of the base, only 3-AM 3aa was obtained but with comparably excellent yields for 4-vinylphenols with
longer reaction time and lower yield (Table 1, entry 4). To our electronically enriched alkenes moiety (3ab, 3ac, 3ae, 3ai).
satisfaction, the complete inversion of regioselectivity of the Moreover, 2-aminophenols protected by methyl, phenyl and
annulation was achieved via the adjustment of solvent and base benzyl gave the corresponding 3-AMs in good to high yields
(Table 1, entries 5-7), and the product 2-AM was obtained in 88% (3ia
yield with DMSO and KOH (Table 1, entry 7). Analogously, the
base was highly essential to the formation of 2-AM in DMSO
(Table 1, entry 8). Moreover, control experiments showed that
air is necessary for the desired reactions (Table 1, entries 9-10).
Table 1. Optimization of the reaction conditionsa,b
-
3ka).
Table 2. Scope of substrates in method A.
3aa
(%)(dr)
4aa
(%)(dr)
entry
solvent
base
yieldc (%)
1
2
CH3CN
CH3CN
CH3CN
CH3CN
DMSO
DMSO
DMSO
DMSO
CH3CN
DMSO
KHCO3
K2CO3
KOH
-
96 (3h)
84 (7h)
68 (7h)
81 (15h)
85 (20h)
82 (12h)
88 (9h)
48 (12h)
96 (6:1)
84(5:1)
35(3:1)
81(4:1)
73(4:1)
26(4:1)
ND
ND
ND
a1 (0.2 mmol),
2 (0.22 mmol), RB (1 mol %), KHCO3 (1 equiv) in
CH3CN (3 mL) under air with 3 W blue LEDs irradiation at rt. The
diastereomeric ratios were determined by flash column
chromatography and then 1H NMR spectroscopic analysis.
3
33(5:1)
ND
4
5
KHCO3
K2CO3
KOH
-
12
6
56(6:1)
88 (8:1)
ND
The scope of the annulation was also investigated in
KOH/DMSO system (method B) as shown in Table 3. Different
substituents (R = Me, t-Bu, Cl, Br) on 2-aminophenols part were
tolerated to give 2-AMs with moderate to excellent yields (4aa-
4ha). The structure of 2-AM 4da was further confirmed by
single-crystal X-ray analysis (CCDC 1990329). For 5-substituted
2-aminophenols, the yields of 2-AMs decreased as compared
7
8
48(4:1)
trace
9d
KHCO3 trace (22h)
ND
10d
KOH trace (22h)
ND
trace
aReaction conditions: 1a (0.2 mmol), 2a (0.22 mmol), RB (1 mol %), base
(1 equiv) were added in 3 mL solvent under air with 3 W blue LEDs
irradiation at rt. bND: not detected. Isolated yields. The reaction was
carried under Ar. The diastereomeric ratios were determined by flash
column chromatography and then 1H NMR spectroscopic analysis.
c
d
with corresponding 4-substituted substrates
(4la, 4ma).
Unfortunately, 2-amino-4-fluorophenol did not deliver the
annulation product 4fa under the present reaction condition.
Next, we examined the compatibility of the reaction with 4-
vinylphenols. Electron-rich 4-vinylphenols bearing alkoxy or t-
Bu groups were the reactive cycloaddition partners (4aj, 4ag).
The potentially sensitive functional group such as alcohol could
We then tested the generality of the reaction against a wide
array of 2-aminophenols and 4-vinylphenols in the optimized
conditions (method A: Table 1, entry 1; method B: Table 1, entry
7). Firstly, the preparation of 3-AMs using method A were
investigated and the results are presented in Table 2. For 4-
be present on the alkene substrate (4ah). The reaction was also
2 | J. Name., 2012, 00, 1-3
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tolerant of steric bulk at the
COMMUNICATION

position of the styrene (4ae). In
There are two possible ways in which the O-C boVniedwfAortricmleaOtniloinne
DOI: 10.1039/D0CC02662J
addition, the scale-up experiment of method B proceeded of 4-vinylphenols with phenoxyl radicals could be accounted for,
smoothly to deliver 4aa in 72% yield, highlighting the scalability one is addition of phenoxyl radicals to double bond of 4-
of the present visible light-promoted aerobic annulations (see vinylphenols, the other is cross-coupling of phenoxyl radicals
ESI†).
Table 3. Scope of substrates in method B.
with quinone methide radicals.10a If the former pathway is
possible, 2-AM product should be produced when 2-
aminophenol 1a and O-methyl-protected 4-vinylphenol
4 were
subjected to the reaction with method A. However, no 2-AM
was detected (Figure 2), indicating the phenolic hydroxyl group
of 4-vinylphenols is crucial for the reaction. Because 4-
vinylphenols are easily oxidized to quinone methide radicals
under visible light-driven aerobic conditions,10a the pathway via
radical cross-coupling is reasonable. Under the optimized
conditions, the self-coupling of 4-vinylphenols can be inhibited
and the cross-coupling products can be achieved smoothly.
According to these preliminary results and the previous
literatures about oxidative radical cross-coupling of phenols,8b,
14
a free-radical coupling mechanism has been suggested in
Figure 3. Initial excitation of the photocatalyst RB under visible
light irradiation leads to the formation of the excited species RB*,
which is quenched by 1a affording 2-aminophenol radical la•
through single-electron oxidation and extrusion of proton. The
addition of a weak base such as KHCO3 can neutralize the
protons produced in the system. Subsequently, electron
transfer from RB•- to molecular oxygen produces a superoxide
radical anion (O2•-) and regenerates RB. 4-Vinylphenol 2a
undergoes similar oxidation/deprotonation process to generate
the reactive quinone methide radical species 2a•. The coupling
of radical la• and another radical 2a•, followed by intramolecular
nucleophilic attack to the methylene of the intermediate
semiquinone, gave 3-AM 3aa (Figure 3).
a1 (0.2 mmol),
2 (0.22 mmol), RB (1 mol %), KOH (1 equiv) in
DMSO (3 mL) under air with 3 W blue LEDs irradiation at rt. The
diastereomeric ratios were determined by flash column
chromatography and then 1H NMR spectroscopic analysis.
To gain insight into the possible reaction mechanism, a series
of experiments and DFT calculations were performed (see ESI†).
The radical process was supported by (i) the EPR experiments
and (ii) radical trap experiments using 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO). The lifetimes of
phenoxyl radicals generated from 2-aminophenols with bulky
groups would probably be long enough for them to be detected
directly by EPR at room temperature. 15 We indeed observed an
intense EPR signal (g = 2.0039) when the solution of 4,6-di-tert-
buty-2-aminophenol (1n), KHCO3 (1 equiv) in an aerobic CH3CN
was stirred at room temperature (see Figure S3). In addition,
addition of 2 equivalents of TEMPO to the reaction mixture
lowered the product yields from 96% to 10% (3aa) (Figure 2).
Figure 3. Plausible mechanism
(
the spin density at O and N for
the radical 1a• and radical anion 1a•- are showed).
Figure 2. The calculated pKa values and control experiments.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Am. Chem. Soc., 2012, 134, 2442; (d) S. M. Wei, X. Q. Feng and
We further focused on understanding the origin of the
inversion of regioselectivity in KOH/ DMSO system (method B).
The pKa of the substrates, the bases and the 2-aminophenol
radical la• in CH3CN and DMSO were calculated and shown in
Figure 2. The controllable regioselectivity observed in the
present work might be attributed to the event of releasing
proton from -NH2 of phenoxyl radical la• in KOH/DMSO
system.12 The pKa value of KHCO3 in CH3CN is much smaller than
that of KOH in DMSO, indicating DMSO containing KOH is more
favorable for deprotonation. Moreover, the acidicity of la• in
DMSO (pKa, 24.2) is stronger than that of which in CH3CN (pKa,
35.4). As a result, deprotonation of phenoxyl radical la•
proceeds spontaneously in KOH/DMSO system and leads to the
formation of radical anion 1a•-, which shows more electron spin
density on nitrogen according to DFT calculation data (Figure 3).
Finally, the phenyl amino radical anion 1a•- couples with
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quinone methide radical 2a• to furnish the 2-AM 4aa
.
In summary, we have demonstrated that readily accessible 2-
aminophenols and 4-vinylphenols can undergo cross-
coupling/annulation to furnish 2-AMs and 3-AMs in high
regioselectivities. The visible light-promoted reactions feature
controllable regioselectivity, high atom-economy and aerobic
conditions.
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Conflicts of interest
There are no conflicts to declare.
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