A practical copper-catalyzed approach to β-lactams: via radical carboamination of alkenyl carbonyl compounds

Article abstract of DOI:10.1039/c9cc05668h

Functionalized β-lactams are highly important motifs in synthetic chemistry. We report an efficient and novel approach to the synthesis of β-lactams via a copper(i)-catalyzed cascade process involving C(benzyl)-H radical abstraction, intermolecular alkene addition, and intramolecular amination reaction. Variously substituted alkenes were synthesized from vinylacetic acid, leading to the corresponding β-lactams in moderate to good yields. Preliminary studies indicate that the reaction undergoes a free radical mechanism via a Cu(i)/Cu(ii)/Cu(iii) catalytic cycle.

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A Practical Copper-Catalyzed Approach to β -
Lactams via Radical Carboamination of Alkenyl
Carbonyl Compounds †
Cite this: DOI: 10.1039/x0xx00000x
Peng Shi, Jie Wang, Zixu Gan, Jingyu Zhang, RunSheng Zeng* and
Yingsheng Zhao*
Received 00th January 2018,
Accepted 00th January 2018
DOI: 10.1039/x0xx00000x
www.rsc.org/
moderate to good yields. Preliminary studies indicated that
the reaction undergoes a free radical mechanism via a Cu (I)/
Cu (II)/Cu (III) catalytic cycle .
Functionalized β-lactams are highly important motifs in
synthetic chemistry. We report an efficient and novel approach
to the synthesis of β-lactams via a copper (I)-catalyzed cascade
process involving
C
(benzyl)–H radical abstraction,
intermolecular alkene addition, and intramolecular amination
reaction. Variously substituted alkenes were synthesized from
vinylacetic acid, leading to the corresponding β-lactams in
moderate to good yields. Preliminary studies indicate that the
reaction undergoes a free radical mechanism via a Cu (I)/Cu
(II)/Cu (III) catalytic cycle.
O
Directing group
O
R2 assisted activation
inert olefin with
R
R2
Copper(III)- promoted
Methylene
O
N
R
N
R
C(sp3)-N formation
N
copper catalysis
I
Challenging
Reductive elimination
or -H elimination
R1
R1
R1
II
R1H
O
H
R2-X
HO
+
Activated by
Functionalized β-lactams are a class of highly important
scaffolds in pharmaceutical chemistry, since they are
ubiquitous as a substructure in various bioactive compounds,
copper catalysis
O
From simple
Vinylacetic Acid
O
R2
R2
RHN
HO
[1]
[2]
[3]
[4]
such as Ezetimibe ,
Various approaches to synthesize β-lactams have been
developed including the well-known Beckmann
rearrangement, Kinugasa alkyne-nitrone cycloaddition, and
penicillin,
azthreonam,
etc.
Scheme 1. Retrosynthetic analysis for the synthesis of β-
lactams.
[5]
Schmidt reactions.
Recently, the C–H functionalization
The groups of Wolfe, Michael, Chemler, and Molander
have successfully developed a series of Palladium-, Copper,
and Nickel-catalyzed intramolecular alkene carboamination
of γ-alkenyl amides to form the γ-lactams. [10] However, the
strategy for the formation of challenging β-lactam by
intramolecular alkene carboamination reaction has not been
reported. Recently, Engle and coworkers reported several
examples of transition-metal-catalyzed 1,2-difunctionalization
of 8-aminoquinoline-coupled vinylacetic acid with various
strategy has provided a straightforward pathway to synthesize
β-lactams. For example, Shi et al. reported an efficient
palladium-catalyzed
sequential
C
(sp3)–H
monoarylation/amination to prepare β-lactam backones. [6a,b,c]
Later the groups of Wu, Ge, Kanai, You and Chantani
developed a series of palladium-,[6d,e] cobalt-,[7] copper-[8] and
nickel-[9]catalyzed intramolecular aminations to construct β-
lactams with aliphatic or amino acids as substrates. Although,
there has been significant progress, the difficulty to prepare
functionalized aliphatic acids limits the molecular diversity of
the β-lactams synthesized through the intramolecular C–H
amination reaction. Herein, our group reports the discovery of
[11]
nucleophiles to build important synthetic architectures,
which showed the synthetic utility of vinylacetic acid.
Through retrosynthetic analysis, a highly efficient approach to
build numerously substituted lactams from vinylacetic acid
was envisioned (Scheme 1). The unactivated double bond in
vinylacetic acid can be activated by the combination of a
transition metal with a directing group. Additionally, a free
alkyl radical could selectively react with the substrates at the
less sterically hindered position, which would result in β-
a
copper-catalyzed cascade of
C
(benzyl)–H radical
addition, and
abstraction,
intermolecular
alkene
intramolecular amination of 8-aminoquinoline coupled-
vinylacetic acid derivatives to synthesize β-lactams.Variously
functionalized β-lactams were obtained via cascade of C
(benzyl)–H radical abstraction and alkene carboamination in
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lactam formation instead of pyrrolidinone formation. Since
copper catalysts have different oxidation states (Cu (I), Cu
(II), and Cu (III)), it was postulated that copper would be
thesuitable catalyst to enable this highly regioselective
cascade C–H difunctionalization reaction.
Table 1. Functional group tolerance on vinylacetic acid
derivatives [a]
R
O
Me
O
R1
Q
Cu(CH3CN)4PF6 (10 mol%)
N
Bn
N
Q
H
DTBP (3 equiv)
130 oC, 8 h
R
1
R1
With these considerations in mind, we treated the directing
group-protected vinylacetic acid with a benzyl free radical in
the presence of copper (II) to explore whether the cascade
benzyl radical addition/ intramolecular amination could be
performed to realize β-lactam formation in a single step.
Initially, the 8-aminoquinoline-coupled vinylacetic acid was
reacted with copper acetate (10 mol %) and DTBP (3 equiv)
3
2a
O
O
O
O
Me
Et
i-Pr
N
N
N
N
Bn
Bn
Bn
Bn
Bn
Q
Q
Q
Q
3a
3b
3c
3d
, 70%
, 85%
, 80%
, 77%
O
O
O
Bn
O
Bn
Bn
N
N
N
N
Bn
Bn
Q
Q
Q
o
Q
in toluene at 130 C for 8 h. The cyclized product 3a was
3e
3f
3g
3k
3h
, 73%
, 74%
, 79%
, 75%
obtained in 26 % yield, and some of the starting material was
recovered (shown in SI). Encouraged by this result, several
Me
Me
O
O
O
EtO2CH2C
O
MeO
Bn
protecting
groups,
such
as
p-toluenesulfonamide,
N
N
N
N
Bn
Bn
Bn
Q
Q
Q
Q
pentafluoroaniline, 2-methylsulfanylaniline, 2-picolylamine
and 1-naphthylamine were tested but were all found to be
ineffective and resulted in recovery of starting material.
Several copper catalysts were further screened to improve the
efficiency of the cascade benzyl radical addition/
intramolecular amination. Both copper (II) bromide and
copper (II) trifluoromethanesulfonate provided yields of 3a
greater than 50 %. Copper (I) bromide (shown in SI) and
copper (I) thiocyanate only provided yields of less than 35 %
for product 3a. Interestingly, (Cu(CH3CN)4PF6) gave the best
yield of 3a at 85 % yield. The control experiment clearly
showed that (Cu(CH3CN)4PF6) and di-tert-butyl peroxide
were indispensable for this reaction, indicating that a copper-
promoted free radical addition pathway was involved (shown
in SI).
3i
3j
3l
, 55%
, 72%
, 65%
, 70%
O
O
O
O
N
N
N
N
Me
Et
Bn
n-Hex
Q
Q
Q
Q
Bn
3m
Bn
Bn
Bn
3p
3n
3o
, 20%
, 40%
, 25%
, trace
[a]Reaction conditions: 1 (0.2 mmol), DTBP (0.6 mmol), Cu(CH3CN)4PF6
o
(0.02 mmol) in 1 mL toluene at 130 C for 8 h; Isolated yield; ‘dr’ decided
by NMR is ＞20:1 if not stated otherwise.
The functional group compatibility on the aromatic ring
oftoluene was next explored under the standard reaction
conditions (Table 2). A variety of functional Me, F, Cl, Br,
CF3, etc and ester all performed well, leading to the
corresponding products in good yields. The substituents on
the ortho-, meta-, or para-position were all compatible,
providing the products in 45 %-81 % yields. It is worth noting
that the iodide functional group was well tolerated, leading to
the corresponding products in good yields (5o-5q).
Mesitylene and cumene also provided the corresponding
products (5r and 5s, respectively) in good yields.
Interestingly, the p-cymene selectively generated the product
5t, which was probably due to the stability of the free radical.
Diphenylmethane and 1-methylnaphthalene led to the
corresponding β-lactams (5u and 5v, respectively) in good
yields. A further study revealed that 2-methylthiophene, 3-
methylthiophene and 2-methylfuran were shown to be good
substrates for this reaction.
With the optimized reaction conditions in hand, we explored
the scope of 8-aminoquinoline coupled vinylacetic acid
derivatives as presented in Table 1. We found that various α-
substituted terminal alkenes were well tolerated and led to the
corresponding β-lactams in good to excellent yields.
Monosubstitution at the α-position has little effect in these
ascade C(benzyl)–H functionalization alkene carbomination
reactions. A variety of functional Me, F, Cl, Br, CF3 etc
generate β-lactams in good yields. The cyclopropane
methylene and cyclobutane methylene were also compatible,
and reacted to form the corresponding products in good yields
without any ring opening side products. The ester and
methoxy functional groups also performed well. Interestingly,
when the diene substrate 3e was subjected to the standard
reaction conditions, the desired β-lactam product was formed
rather than the five-membered pyrrolidinone products,
indicating the copper catalyst prefers to proceed via a five-
member metalacycle intermediate rather than the six-
membered alternative mechanism. The α,α-dimethyl
substituted alkene only reacted to form the corresponding β-
lactam in 55 % yield, along with starting material recovery.
When the γ-substituted alkene was employed as the substrate,
a dramatically decreased yield of lactams was obtained
(Table1, 3m-3p), along with starting material recovered,
respectively. It might be that the steric hindrance effectgreatly
reduces the reactivity of the substrates.
To further demonstrate the synthetic utility of this reaction,
a gram-scale reaction was performed, which led to the
product 3a in 68 % (1.02 g) yield, along with the recovery of
starting material (Scheme 2a). The product 3a could be easily
transformed into azetidine[12] or β-amino[13] acid derivatives in
good yields (6a and 6b). 5-Methoxyquinolin-8-amine (MQ)
is a removable directing group for the synthesis of
pyrrolidinones via intramolecular amination, which was
introduced by the Chen group. The substrate 7a, which was
prepared from vinylacetic acid, could be coupled with MQ to
form the compound 7b.
A
cascade C(benzyl)–H
functionalization / alkene carboamination with copper as
catalyst followed, leading to the product 7c in 62 % yield
2 | J. Name., 2012, 00, 1-3
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Table 2. Functional group tolerance on the aromatic ring of
used
instead
of
tetrakis(acetonitrile)copper
(I)
toluene.[a]
hexafluorophosphate under standard conditions, products 3a
and 8b were not formed and the starting material 1a was
recovered (Scheme 3b). This result indicated that the alkene
cannot trap the benzyl radical without coordination with the
copper (II) complex. The substrate, 8-aminoquinoline, was
coupled with allylacetic acid and directly subjected to the
reaction conditions, where N-benzylated product 8c was
isolated at 67 % yield, while the expected product,
pyrrolidinone, was not formed (Scheme 3c). This result
further indicates that complex I is essential to activate the
inert double bond in substrate 1a. Due to the stability,
complex II may not be generated under these reaction
condition, leading to the product 8c formation. When tert-
butylbenzene was used as the solvent instead of toluene, the
starting material 1a was completely recovered, without
formation of product 3a . This result could rule out an
alternative mechanism that Heck-type coupling of the benzyl
O
DTBP (3 equiv)
O
Q
N
Cu(CH3CN)4PF6 (10 mol %)
130 oC, 8 h
N
R2
H
Q
R2
5
1a
2
5f, R2=o-CH2COOEt, 80%
5k, R2=m-CF3, 62%
2
5a
, R =o-Me, 81%
2
2
5g
5h
, R =m-OMe, 72%
2
5b
5c
, R =o-F, 78%
5l
, R =p-Me, 58%
2
2
, R =o-Cl, 77%
, R =m-Me, 75%
2
5m
5n
5o
, R =p-Cl, 53%
2
2
5d
5e
2
, R =o-Br, 75%
5i
5j
, R =F, 60%
, R =p-CF3, 45%
2
2
2
, R =o-CF3, 71%
, R =m-Cl, 72%
, R =o-I, 80%
O
O
O
O
O
Q
Q
Q
Q
N
Q
N
N
N
N
Me
I
Me
Me
Me
I
Me
Me
Me
5p
5q
5s
5t
, 50%
Q
, 62%
Q
, 84%
, 66%
O
5r
, 45%
Q
O
O
O
O
Q
Q
N
N
N
N
N
S
S
O
Ph
5u, 80%
radical to generate a more substituted alkene intermediate[16]
,
5v
5w
5x
5y
, 78%
, 65%
, 80%
, 55%
followed by intramolecular radical-based hydroamination
from the Q amide.
[a]Reaction conditions: 1a (0.2 mmol), DTBP (0.6 mmol), Cu(CH3CN)4PF6
(0.02 mmol) in 1 mL solvent at 130 oC for 8 h;
a) Trapping of intermediate with TEMPO
Bn
O
N
Cu(CH3CN)4PF6
(10 mol%)
O
O
a) Gram scale reaction and further application
Me
TEMPO
N
Q
Bn
HN
Q
DTBP (3 equiv)
toluene, 130 o
C
O
Q
N
Cu(CH3CN)4PF6 (10 mol%)
DTBP (4 equiv), 130 oC, 8 h
1a, 0.2mmol
Q
3a, 0%
8a
, 28%
N
O
b) Control experiment
H
3a
, 1.02g, 68%
ferrocene
(10 mol%)
1a,
5mmol
Me
O
O
O
Q
N
Q
HN
Q
NH
O
DTBP (3 equiv)
Bn
CH3ONa (1.5 equiv)
CH3OH, reflux, 2 h
LiAlH4
0 0C, 4 h
HN
toluene, 130 o
C
N
Q
Ph
Bn
Bn
Q
OCH3
8b, N.D.
1a
3a
, 0%
6a
, 85%
6b
, 75%
c) Parallel experiment
b) The synthetic application
O
Cu(CH3CN)4PF6
(10 mol%)
Me
Me
O
Q
Cu(CH3CN)4PF6
(10 mol%)
O
Me
Me
COOH
H
N
N
OCH3
Bn
i-PrI
HATU
DTBP (0.6 mmol)
N
Q
HN
Q
DTBP, (4 equiv)
Bn
toluene, 130 o
C
LDA
toluene, 130 0
Me
C
Pyridine
N.D.
COOH
O
N
8c, 67%
7a
7b
, 65%
, 78%
O
O
Me
Me
Me
O
Me
O
N
Me
N
Me
Ph
Boc2O
O
ref.14
Me
Ph
VS
O
CAN
N
II
N
N
Boc
N
CuII
L
NH
CuII
L
NBoc
Ph
I
Ph
N
OCH3
Stable 5 ~ 6-membered
metalic cycle
Unfavorable 6 ~ 7 membered
metalic cycle
7c
7d,
60%
7e, 68%
7f
, 62%
Scheme 3. Control experiments.
Scheme 2. Late-stage functionalization of vinylacetic acid
derivatives and further transformations
Ⅰ
(Scheme 2b). The directing group was removed according to
Chen’s method to provide 7d in 60 % yield. [14] The product
7d could be easily protected by Boc (7e) and further
transformed into γ-lactam7f, which is an important synthon
for the preparation of biologically valuable derivatives. [15]
Cu
3a
DTBP
O
t-BuO-
Ⅱ
t-BuO
ArCH3
Cu
N
Cu
Bn
Ⅲ
N
Ⅱ
To further understand the reaction pathway of this copper-
catalyzed cascade C(benzyl)–H functionalization / alkene
carboamination, several experiments were performed. When
the standard reaction was performed with 2 equivalents of
TEMPO, the product 8a was isolated in 28 % yield based on
the amount of TEMPO used, suggesting the benzyl radical
was formed in the reaction (Scheme 3a). When ferrocene,
which can promote benzylradical generation with DTBP, was
ArCH2
t-BuOH
1a
O
N
N
CuII
I
Scheme 4. Plausible mechanism.
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Based on these experimental results and previous reports[17]
,
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a plausible catalytic cycle is proposed in Scheme 4. The
copper (I) catalyst reacted with di-tert-butyl peroxide, leading
to the copper (II) catalyst and a benzyl radical. [18] The copper
(II) catalyst coordinated with the substrate and generated
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[19]
to complex II,
followed by reductive elimination and
yielding the β-lactam product and copper (I) catalyst.
In conclusion, we have developed an efficient and novel
copper (I)-catalyzed cascade C–H benzylation/intramolecular
amination reaction to form various β-lactam backbones in
moderate to good ields under the assistance of a bidentate
directing group. Various substituted alkenes were synthesized
from vinylacetic acid leading to the corresponding β-lactams
in moderate to good yields. Preliminary studies indicated that
the reaction undergoes a free radical mechanism via a Cu
(I)/Cu (II)/Cu (III) catalytic cycle.
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