Manganese/cobalt-catalyzed oxidative C(sp3)-H/C(sp3)-H coupling: A route to α-tertiary β-arylethylamines

Article abstract of DOI:10.1039/c7cc08512e

Reported herein is an oxidative coupling reaction of an α-C(sp³)-H bond of amine with a benzylic C(sp³)-H bond through Mn or Co catalysis to provide diverse collections of α-tertiary β-arylethylamines. This protocol features an easily installed and removable coordinating activation group, a wide scope of substrates, low-cost metal catalysts, easily available starting materials and synthetic simplicity.

Full text of DOI:10.1039/c7cc08512e

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Manganese/cobalt-catalyzed oxidative C(sp3)−H /C(sp3)−H
coupling: a route to α-tertiary β-arylethylamines
Received 00th January 20xx,
Accepted 00th January 20xx
Meiling Tan, Kaizhi Li, Jiangliang Yin and Jingsong You*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Reported herein is an oxidative coupling reaction of an α-C(sp3)−H
bond of amine with a benzylic C(sp3)−H bond through Mn or Co
catalysis to provide diverse collections of α-tertiary β-
arylethylamines. This protocol features an easily installed and
H
H
oxidative C(sp3)−H /C(sp3)−H
R2HN
Ar
FG
R1
H
H
R2HN
R1
FG
cross-coupling
+
Ar
Scheme 1. Shortcut to α-tertiary β-arylethylamine derivatives.
removable coordinating activation group,
a wide scope of
substrates, low-cost metal catalyst, easily available starting
materials and synthetic simplicity.
Previous works:
(a) Palladium-catalyzed C(sp3)−H arylation (DG: directing group)
R1
α-Tertiary β-arylethylamines extensively exist in natural
products, biologically active molecules, and pharmaceuticals,
and are also important intermediates in synthetic organic
chemistry.1,2 Although a number of methodologies have been
developed to construct the α-tertiary β-arylethylamine
derivatives, traditional strategies generally require tedious
multiple synthesis with highly functionalized starting materials,
and encounter rigorous reaction conditions such as very strong
bases (e.g., LDA), strong reductive reagents (e.g., LiAlH4),
organometallic reagents (e.g., Grignard reagents, and
organozinc reagents),3 which may be incompatible with
sensitive functional groups. Thus, it is highly demanded to
develop general, economical, and eco-friendly routes to α-
tertiary β-arylethylamines. Over the past decade, transition
metal-catalyzed oxidative C−H/C−H cross-coupling reactions
have attracted great attentions in carbon−carbon bond
forming reactions.4,5 In the view of simplicity, efficiency and
atom-economy, oxidative cross-coupling between an α-
C(sp3)−H of amine and a benzylic C(sp3)−H would be a distinct,
R1
FG
DGHN
R2
X
[Pd]
FG
R2
DGHN
H
+
Ar
C−H/C−X coupling
Ar
X = I, Br, Bpin, Si(OEt)3
(b) Synthesis of β-aromatic α-amino acids through the coordinating activation strategy
(PAHN: picolinamido group)
O
R1
H
R1
H
O
PAHN
OR2
Ar
Ni2+, [O]
oxidative C−H/C−H coupling
OR2
R3
+
N
Ar
R3
H
N
O
coordinating activation group
This work:
(c) Synthesis of α-tertiary β-arylethylamines through the coordinating activation strategy
H
H
O
H
FG
PAHN
R1
H
[Mn] or [Co], [O]
R1
N
FG
+
Ar
oxidative C−H/C−H coupling
H
N
Ar
Scheme 2. Strategies to access β-arylethylamines through the
functionalization of C(sp3)−H bond.
arylethylamines (Scheme 2a).6 However, this type of coupling
straightforward gateway to access diverse collections of β- reaction typically requires noble metal palladium as the
catalyst and stoichiometric silver as the additive and is
sensitive to steric hindrance. The aza-Henry-type or Mannich-
type cross-coupling reactions of α-C(sp3)–H of amines with
nucleophilic C(sp3)–H have been thought as a promising
strategy for the synthesis of amine derivatives,5a,5c,5e,7,8 but are
generally quite sterically sensitive and exhibit a narrow
substrate scope. The nucleophiles are often limited to α-
C(sp3)–H adjacent to electron-withdrawing groups, C(sp2)–H of
electron-rich (hetero)arenes and organometallic compounds.
Recently, we proposed the coordinating activation strategy to
accomplish the radical oxidative cross-coupling between a
C(sp3)–H bond of α-amino acid and an arylmethyl free radical
through nickel-catalysis, which could afford β-aromatic α-
arylethylamine derivatives with an α-tertiary carbon center
(Scheme 1).
In recent years, chelation-directed transition metal-
catalyzed arylation of β-C(sp3)−H of amines with aryl halides/
surrogates has been developed to prepare diverse α-tertiary β-
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amino acids (Scheme 2b).9 The coordinating activation strategy
enables to activate the α-C(sp3)−H bond neighboring to the
amino group by installing the 2-pyridine carbonyl group at the
nitrogen atom for the coordination toward a metal center.9,10
Inspired by this work, we herein attempt to advance the
coordinating activation strategy to construct a large library of
α-tertiary β-arylethylamines from diverse frequently occurring
amines and benzylic derivatives (Scheme 2c).
(0.5 mL), Mn(OAc)2·4H2O (20 mol %) and DTBP (4.0 equiv) at
o
150 C under N2 for 18 h. Isolated yield. MnCl2·4H2O (20 mol
b
c
d
o
e
%). Ni(acac)2 (20 mol %) at 140 C for 6 h. The dr value was
calculated by 1H NMR. fCo(OAc)2.4H2O (20 mol %) at 140 oC for
4 h. PAHN = picolinamido group.
With the optimized reaction conditions in hand, we
examined the scope of benzylic substrates as shown in Scheme
3. To our delight, a wide range of benzylic substrates could
couple with 1a. Mono or multi methyl-substituted benzene
Considering that β-aryl-α-(pyridin-2-yl)ethan-1-amines are
important structural scaffolds in pharmaceuticals, bioactive
compounds and are also synthetically important precursors,11
the arylmethylation of pyridin-2-ylmethanamine was first
investigated through the installation of the coordinating 2-
pyridinecarbonyl group. Thus, the oxidative cross-coupling of
N-(pyridin-2-ylmethyl)picolinamide 1a with toluene 2a was
performed as a model reaction to explore the reaction
conditions (Table S1). The previously reported catalytic
systems involving FeCl3·6H2O and Ni(acac)2 did not almost
derivatives could proceed (Scheme 3, 3a-3e). When the
benzene ring of toluene was substituted by the electron-
withdrawing halogen, electron-donating methoxyl or aryl ether,
the desired products could also be delivered in synthetically
useful yields (Scheme 3, 3f-3i). 1-Methylnaphthalene and 2-
methylnaphthalene afforded the desired product 3j and 3k in
47% and 45% yields, respectively. In addition to arylmethanes,
arylmethylene groups could also smoothly gave β,β-
deliver the desired product 3a 9,10a
When Ni(acac)2 was used,
.
disubstituted-α-(pyridin-2-yl)ethan-1-amines (Scheme 3, 3l
–
only the dibenzylated product 3ab was obtained (Table S1,
entry 3). Fortunately, simple manganese salts enabled to
afford the target product 3a. Among all manganese salts
investigated, Mn(OAc)2·4H2O worked well and gave rise to 3a
3p).
Ethylbenzene,
butylbenzene
and
(methoxymethyl)benzene gave the coupled products 3l
,
3m
and 3n in 67%, 65% and 52% yields, respectively. Notably,
when the benzylic group of substrate was substituted by ester,
α-aromatic β-amino acid derivative 3o could be obtained in 75%
yield. In addition, polycyclic aromatic hydrocarbon involving an
arylmethylene unit could also couple with 1a and the coupled
product 3q was given in 57% yield. Finally, cyclohexylbenzene
and cumene containing an arylmethine group reacted with 1a
to afford the coupled products 3r and 3s in 58% and 77% yields,
respectively, which demonstrated a good compatibility of
benzylic substrates.
o
in 62% yield using 4.0 equiv of DTBP as the oxidant at 150 C
for 18 h (Table S1, entry 26). Among a set of oxidants (e.g.,
DTBP, TBHP, TBPB, DCP, K2S2O8 and H2O2) screened, only
peroxide ethers such as DTBP and DCP were capable of
promoting this reaction (Table S1, entries 12-17). When the
amount of toluene decreased from 1.0 mL to 0.5 mL, the yield
dropped to 56%.
Scheme 4. Coupling reactions of amines with benzylic substrates.
aReaction condition: 1 (0.25 mmol), 2 (0.5 mL), Co(OAc)2·4H2O (20
Scheme
ylmethyl)picolinamide with benzylic substrates. aReaction
condition 1a (0.25 mmol),
(1.0 mL), Mn(OAc)2·4H2O (20 yield. c140 oC. d6 h. e2 (1.0 mL). fMn(OAc)2·4H2O (20 mol %).
mol %), and DTBP (4.0 equiv) at 150 oC under N2 for 18 h.
Reaction condition 1a (0.25 mmol), (10.0 equiv), benzene
3
.
Coupling
reactions
of
N-(pyridin-2-
o
b
mol %), and DTBP (4.0 equiv) at 150 C under N2 for 24 h. Isolated
A
:
2
gTrifluorotoluene as solvent. h18 h. i5.0 equiv of DTBP. j32 h.
kMnCl2·4H2O (20 mol %).
B
:
2
2 | J. Name., 2012, 00, 1-3
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Subsequently, we evaluated the scope of amine derivatives, di-tert-butyl-4-methylphenol (BHT), was added to the reaction
and found that a range of amines could smoothly react with system involving Co(OAc)2·4H2O or Mn(OAc)2·4H2O as the
benzylic substrates to form α-tertiary β-arylethylamines in the catalyst, the oxidative reaction was suppressed dramatically
presence of Co(OAc)2·4H2O12,13 or Mn(OAc)2·4H2O (Scheme 4). (Scheme 7a and 7b). These observations demonstrate that this
α-Aminoketones, an important type of amines, could work type of reaction is likely to undergo a process involving free
well with toluene or heterocyclic aromatics containing an radical. Next, the use of substrate that contains an NMe group
arylmethylene group in moderate to good yields (Scheme 4, instead of an NH group did not deliver any desired product,
4a
-4f). Glycine derivatives could be used as the substrate to indicating a crucial role of a free NH group (Scheme 7c).
provide α-tertiary β-aromatic α-amino acid derivatives 4g and Considering
that the coupling reaction of N-
4h in 52% and 53% yields by using Co(OAc)2·4H2O as the benzhydrylpicolinamide with toluene could afford 4s in 25%
catalyst, respectively. Phenyl methanamines with various yield (Scheme 4), we subsequently prepared the imine 1s'. The
substituents such as bromine, cyano, trifluoromethyl and reaction of 1s' with toluene gave 4s in 37% yield (Scheme 7d).
methoxyl could be tolerated and gave rise to the desired These results imply that the cross-coupling reaction could
products in moderate yields (Scheme 4, 4i-4n). There is no involve the formation of imine intermediate. Finally, the
obvious difference in yield when the substituent groups on the primary kinetic isotopic effect (KIE) experiments were
benzene ring were changed from electron-withdrawing groups conducted with regard to the C(sp3)–H/D bonds of toluene
to electron-donating groups. 4o and 4p could be obtained by under two catalytic systems. The KIE values (kH/kD = 6.14 and
the
reactions
of
2-aminomethylfuran
and
2- 6.69) illustrate that the cleavage of benzylic C–H bond may be
aminomethylnaphthalene with cumene, respectively. To our the rate-limiting step in this reaction (Scheme 7e and 7f).
delight, the lowly reactive alkyl amines such as butylamine
could couple with toluene to produce 1-phenylpentan-2-amine
4q. In addition, the reaction condition was compatible with
aminoacetonitrile, offering 4r in a moderate yield.
Scheme 5. Scale-up experiment.
Scheme 6. Removal of coordinating activation group and
transformation of α-tertiary β-arylethylamine.
With the generality of the cross-coupling reaction
investigated, we attempted to perform the scale-up
experiment for the synthesis of α-(pyridin-2-yl) β-
arylethylamine and the removal of the coordinating activation
group as shown in Scheme 5 and 6a. These experiment results
illustrate the practical value of this strategy. It is known that β-
aryl-α-(pyridin-2-yl)ethan-1-amines can produce the 3-
substituted isoquinoline derivatives through the Pictet‒
Spengler reaction.14 Thus, the sequential removal of the
picolinamido group, the condensation with formaldehyde and
oxidative aromatization gave rise to 3-(pyridin-2-
yl)isoquinoline 5b, a type of structural scaffold in drugs or
biologically active molecules (Scheme 6b).15
Scheme 7. Control experiments and isotope effect experiments.
Based on the experimental data and the precedent
reports,8a,9,10a,16 the possible reaction mechanism is proposed
(Scheme 8). At the beginning, low-valent Co(II) or Mn (II) is oxidized
to high-valent species by DTBP with the concomitant formation of
tert-butoxyl radical (tBuO.).17 The high-valent metal catalyst
coordinates with the picolinamide to produce the intermediate
IM1. Then, tert-butoxyl radical (tBuO·) abstracts a hydrogen atom in
alpha from amine to form the radical IM2. The resulting radical
undergoes an intramolecular single-electron transfer (SET) to afford
an imine intermediate IM3 with the dissociation of proton,18
followed by an oxidation to form a high-valent metal species IM4.
This electrophilic imine could then undergo the attack of the
To further understand the reaction process, a set of control
experiments were performed. First, when a radical trapping
reagent, 2,2,6,6-tetramethylpiperidine oxide (TEMPO) or 2,6-
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tBuO- + tBuO
Ar
R
DTBP
O
O
H
4
5
M3+
M2+
N
N
H
R
H
N
N
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Ar
O
H
O
H
N
R
N
R
N
IM1
M3+
tBuO
N
M2+
M = Mn or Co
IM5
DTBP
Ar
H
6
tBuOH
O
tBuO
SET
H
O
H
H
N
N
R
R
Ar
N
M3+
N
O
M3+
IM2
IM4
N
R
SET
H+
tBuO- + tBuO
N
M2+
7
8
DTBP
IM3
Scheme 8. Proposed mechanism.
In summary, a simple and efficient method to prepare α-
tertiary β-arylethylamine skeletons has been developed by the
oxidative cross-coupling reaction between an α-C(sp3)−H of
amine and a benzylic C(sp3)−H of arylmethane, arylmethylene
and arylmethine. In combination with an easily installed and
removable coordinating activation group, inexpensive metal
catalyst, a wide scope of substrates, operation simplicity, and
easily available starting materials make the protocol
practicable.
9
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This work was supported by grant from the National NSF of
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Graphic Abstract
Manganese/cobalt-catalyzed oxidative C(sp3)−H /C(sp3)−H coupling: a route to α-tertiary
β-arylethylamines
Meiling Tan, Kaizhi Li, Jiangliang Yin and Jingsong You*
An oxidative coupling reaction of α-C(sp3)−H bond of amine with benzylic C(sp3)−H bond provides
diverse collections of α-tertiary β-arylethylamines.