Photoredox-Catalyzed α-Aminomethyl Carboxylation of Styrenes with Sodium Glycinates: Synthesis of γ-Amino Acids and γ-Lactams

Article abstract of DOI:10.1021/acs.orglett.1c00536

A visible-light photoredox-catalyzed reductive α-aminomethyl carboxylation of styrenes with sodium glycinates and CO2 has been developed to synthesize a series of α,α-disubstituted γ-amino acids and γ-lactams with high efficiency and regioselectivity. Notably, CO2 released from the decarboxylation step can be reused for the subsequent carboxylation. Distinct from the previous reactions with the same type of substrates leading to simple decarboxylation and olefin hydroalkylation, this process involves additional CO2 sequestration, thus leading to olefin α-aminomethyl carboxylation. These findings not only provide new access to α,α-disubstituted γ-amino acids and γ-lactams but also serve as a proof of concept for CO2 reutilization in decarboxylation reactions.

Full text of DOI:10.1021/acs.orglett.1c00536

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Letter
Photoredox-Catalyzed α‑Aminomethyl Carboxylation of Styrenes
with Sodium Glycinates: Synthesis of γ‑Amino Acids and γ‑Lactams
Cong Zhou, Miao Li, Jianwei Sun, Jiang Cheng,* and Song Sun*
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ABSTRACT: A visible-light photoredox-catalyzed reductive α-
aminomethyl carboxylation of styrenes with sodium glycinates and
CO2 has been developed to synthesize a series of α,α-disubstituted
γ-amino acids and γ-lactams with high efficiency and regioselectiv-
ity. Notably, CO2 released from the decarboxylation step can be
reused for the subsequent carboxylation. Distinct from the previous
reactions with the same type of substrates leading to simple
decarboxylation and olefin hydroalkylation, this process involves
additional CO2 sequestration, thus leading to olefin α-aminomethyl
carboxylation. These findings not only provide new access to α,α-disubstituted γ-amino acids and γ-lactams but also serve as a proof
of concept for CO2 reutilization in decarboxylation reactions.
Scheme 1. Introduction to Decarboxylation of α-Amino
Acids for Olefin Addition
ecarboxylative cross-coupling reaction of carboxylic acids
Dprovides versatile access to diverse useful functional
molecules. This is partly owing to the fact that carboxylic acids
are readily available, inexpensive, and stable chemical feed-
stock.1 While significant progress has been made in transition-
metal-catalyzed decarboxylative coupling reactions, there
remain certain drawbacks in the known systems.2 For example,
harsh conditions were required for many occasions. Moreover,
the carboxylic acid substrates are restricted to those aryl and
alkenyl (sp2-hybridized) ones.3 Similar processes for aliphatic
carboxylic acids remain as a challenge. Recently, visible-light-
mediated radical decarboxylative coupling reactions of aliphatic
carboxylic acids have emerged as an attractive complement to
address the above limitation.4 Along this line, further
development of these useful decarboxylative coupling reactions
to milder and operationally simpler protocols remains in high
demand.
It is worth noting that in almost all the decarboxylation-
involving reactions of carboxylic acids, CO2 is typically
generated as a byproduct, and efforts to reutilize this important
C1 building block for further transformation has met with
limited success. On the other hand, α-amino acids have been
established as useful decarboxylation substrates toward useful
nitrogen-containing molecules. For example, decarboxylative
arylation of α-amino acids with cyanoaromatics can generate
highly functionalized primary and secondary benzylic amines.5
Moreover, diverse functionalized amines or amino esters can
be synthesized from α-amino acids via visible-light-mediated
decarboxylative Giese addition of alkyl radicals to electron-
deficient olefins.6 However, in all these examples, CO2 was
uniformly produced as a byproduct, without further utilization
for cascade bond formation (Scheme 1a).7
Received: February 14, 2021
Published: March 30, 2021
https://doi.org/10.1021/acs.orglett.1c00536
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2895−2899
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a
Achieving atom-economic reactions is an important pursuit
of sustainable development.8 If the CO2 released from
decarboxylation could be sequestered as a C1 building
block9 for subsequent bond formation, the whole process
could be carbon-neutral and thus contribute to atom-economy.
In continuation of our effort in CO2 conversion11 and inspired
by the above-mentioned developments in decarboxylation,
herein we report our progress toward the above goal.
Specifically, we were curious about whether CO2 released
from decarboxylation could serve as an electrophile to react
with the carbanion prior to protonation. If successful, α-
aminomethyl carboxylation of styrenes would occur instead of
simple hydroalkylation (Scheme 1c). It is important to note
that such a process could provide straightforward access to
those useful α,α-disubstituted γ-amino acids12 and their
derivatives, such as γ-lactams (Scheme 1d). During the
preparation of our manuscript, Yu and co-workers reported a
work based on the similar concept, in which CO2 released from
decarboxylation of α-amino acids and peptides could be reused
for carboxylation (Scheme 1b).10
Table 1. Evaluation of Conditions
b
entry
PC
Solvent
3ab yield (%)
1
2
3
4
Ir[(dF(CF3)ppy)]2(dtbbpy)PF6
4CzIPN
Ru(bpy)3Cl2·6H2O
Ru(bpy)3·2PF6
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
CH3CN
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
36
<5
22
40
49
16
NR
74
68
0
57
68
64
70
27
c
5
Ru(bpy)3·2PF6
6
7
Ru(bpy)3·2PF6
Ru(bpy)3·2PF6
Ru(bpy)3·2PF6
Ru(bpy)3·2PF6
d
8
9
de
,
df
,
10
11
12
13
14
15
Ru(bpy)3·2PF6
d
d
d
d
d
Ru(bpy)3Cl2·6H2O
Ir[(dF(CF3)ppy)]2(dtbbpy)·PF6
Ir[(2.4-2Fppy)]2(dtbbpy)·PF6
Ir[(2.4-2Fppy)]2(bpy)·PF6
4CzIPN
A proposed mechanism of this design is depicted in Scheme
2. With photocatalysis, decarboxylation takes places to
a
Reaction conditions: ethene-1,1-diyldibenzene 1a (0.2 mmol),
sodium 2-(methyl(phenyl)amino)acetate 2b (0.3 mmol), catalyst (2
Scheme 2. Design Plan
mol %), under N2, solvent (2 mL), 48 h, in a sealed Schlenk tube,
b
irradiated by a 2 × 3 W blue LEDs, unless otherwise noted. Isolated
c
d
yield. Sodium N-methyl-N-phenylglycinate 2b (0.6 mmol), CO2 (1
e
f
atm) was used in place of N2, Catalyst (1 mol %). Without catalyst
or blue LEDs. NR = no reaction.
Ru(bpy)3·2PF6 proved to be the best catalyst (3ab, 40%, entry
4). When the amount of 2b was increased to 3 equiv, the yield
of 3ab could be improved to 49% (entry 5). A survey of
solvents indicated that DMSO worked best (entries 4, 6, and
7). In this reaction, it is clear that the CO2 released from the
decarboxylation step was used as the carbon source of the
newly formed carboxylic acid group. Subsequent considerable
efforts were devoted to further optimization. Unfortunately,
higher efficiency was not achieved. Nevertheless, it serves as an
important proof of concept for CO2 reutilization. The low
yield might be attributed to the low concentration of CO2
released from 2b. Therefore, in order to promote the reaction
to complete carboxylation, external CO2 (1 atm) was used to
replace N2, and the yield of 3ab increased to 74% (entry 8).
Lowering the catalyst loading to 1 mol % resulted in slightly
diminished efficiency (68%, entry 9). Furthermore, control
experiments confirmed that both photocatalyst and blue light
are essential for this reaction (entry 10). Finally, other catalysts
were compared again with the best conditions, but they were
all inferior (entries 11−15).
With the optimized conditions (entry 8, Table 1), the
substrate scope of styrenes was investigated. As shown in
Scheme 3, these reactions proceeded smoothly under the
standard conditions regardless of the electronic nature of the
substituents in the para-position of the phenyl ring (3ab−3ib).
Furthermore, steric hindrance has no effect on the reaction
efficiency. Those substrates bearing an m- (1i) or o-methyl
(1j) group could afford both of the corresponding products
3jb and 3kb in 70% and 67% yields, respectively. Multi-
substituted styrenes 1k and 1l were also good substrates,
generating the desired products in good yields (3 lb, 75%;
3mb, 67%). To our delight, this reaction also proceeded
smoothly with styrenes bearing 2-naphthyl (1n), 2-furyl (1o),
and 2-thienyl (1p) substituents, forming the corresponding
generate aminomethyl radical B, which then adds to the olefin
double bond to form carbon radical D. Single-electron transfer
from the reduced form of photocatalyst to this radical
generates carbanion E. Next, sequestration of the released
CO2 with this anion followed by protonation leads to γ-amino
acid 3. Additionally, when the amino moiety containing a free
N−H bond (R2 = H), intramolecular lactamization of
intermediate 3 could generate diverse γ-lactams 5.
At the outset, ethene-1,1-diyldibenzene 1a and N-methyl-N-
phenylglycine 2a were employed to test our hypothesis (see
Table S1 for details). Unfortunately, initial examination of the
reaction under N2 led to trace desired product 3a. Instead,
hydroalkylation to form 6 as the main byproduct, even with
various photocatalysts (Table S1, entries 1−4). When 2 equiv
of K2CO3 was added, the yield of 3ab was dramatically
increased to 42%; other inorganic bases gave comparable
results, while organic bases such as TBD and DBU only
afforded 6 as the main product (Table S1, entries 5−9).
When sodium 2-(methyl(phenyl)amino)acetate 2b was used
instead of 2a, the desired product 3ab was isolated in 36%
yield in the presence of 2 mol % of Ir[(dF(CF3)-
ppy)]2(dtbbpy)PF6 as the catalyst (Table 1, entry 1). To
further improve the reaction efficiency, other catalysts were
screened. 4CzIPN showed low catalytic activity (entry 2).
Ru(bpy)3Cl2·6H2O resulted in only 22% yield (entry 3), but
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and 3ai in 78% and 76% yields, respectively. A range of N-
heterocyclic glycinates were also reactive toward the desired
products with good efficiency. For example, those with indole
(2j), tetrahydroquinoline (2k), and pyrrolidine (2l) rings gave
the corresponding products 3aj−3al in 69%, 77% and 69%
yields, respectively. Delightedly, common carboxylic acids 1m
and 1n were also good substrates in this reaction, affording the
desired products 3am and 3an in 56% and 54% yields,
respectively.
Scheme 3. Synthesis of γ-Amino Acids*
Considering that the γ-lactams and their derivatives
represent a privileged and valuable structural motif present in
many therapeutic agents possessing diverse biological
activitie,13 the construction of these structures is still in high
demand.14 Notably, simple modification of the reaction
conditions allowed these substrates to form diverse γ-lactams
in moderate to good yields (see the SI for details). The lactam
products were simply a result of lactamization of the initial γ-
amino acid products with assistance by acetic anhydride. As
shown in Scheme 4, all the examples led to moderate to good
a
Scheme 4. Synthesis of γ-Lactams
*
Reaction conditions: alkene 1 (0.2 mmol), 2 (0.3 mmol), Ru(bpy)3·
2PF6 (0.004 mmol, 2 mol %), CO2 (1 atm), DMSO (2.0 mL),
a
irradiated by a 2 × 3 W blue LEDs, 48 h, rt. Reaction conditions:1a
(1.0 mmol), 2a (1.5 mmol), Ru(bpy)3·2PF6 (0.02 mmol, 2 mol %),
DMSO (10.0 mL), irradiated by a 16 W blue LED strips, 48 h, CO2
b
balloon, rt. Ir[(dF(CF3)ppy)]2(dtbbpy)·PF6 (0.004 mmol, 2 mol %)
was used in place of Ru(bpy)3·2PF6. Pivalic acid (0.3 mmol) and CsF
c
(0.6 mmol) were used instead of sodium glycinate, 12 h. After that,
MeI (0.6 mmol) and K2CO3 (0.4 mmol) were added into the reaction
mixture, which was then stirred at 70 °C for about 1 h. 2-
Tetrahydrofuroic acid (0.3 mmol) and CsF (0.6 mmol) were used
instead of sodium glycinate, 12 h.
a
Reaction conditions: alkene 1 (0.2 mmol), 4 (0.3 mmol),
d
Ir[(dF(CF3)ppy)]2(dtbbpy)·PF6 (0.004 mmol, 2 mol %), CO2 (1
atm), DMSO (2.0 mL), irradiated by a 2 × 3 W blue LEDs for 48 h,
rt, then acetic anhydride (1.5 equiv, 30 μL) was added into the
reaction mixture, and the reaction mixture was stirred at 80 °C for
about 1 h.
products 3nb−3pb in moderate to good yields (34%−64%). In
particular, styrene 1q within an internal double bond was also
reactive in this reaction, albeit with low efficiency (3qb, 31%).
When monoaryl alkene 1r was employed, the desired 3rb
could be also isolated in 46% yield. However, prop-1-en-2-
ylbenzene 1t could not undergo this reaction, and the desired
product 3tb could not be obtained.
Next, substituted sodium glycinates were examined using
styrene 1a as the reaction partner. As expected, they also
worked well to deliver the desired products 3ac−3al in good
yields (69%−80%). Notably, a range of functional groups, such
as chloride (3ae) and bromide (3af), were compatible with the
mild conditions, which provided a versatile handle for further
functionalization. Moreover, sodium glycinates with N-(2-
naphthyl) and N-benzyl groups also worked well to afford 3ah
efficiency. The electronic property of the substitutions in the
phenyl ring of 4 had no obvious influence to the reactivity.
Substrates bearing electron-donating or electron-withdrawing
groups all afforded the desired products successfully. In the
case of o-methyl substitution, 5ag was isolated in 76% yield.
Furthermore, 2-naphthyl-subsituted analogue (4h) and sodium
2-(phenylamino)propanoate 4i were also good reaction
partners to generate 5ah and 5ai in 60% and 57% yields,
respectively. Additionally, a variety of substituted styrenes
could also react smoothly with 4a under standard conditions,
forming the products 5ca−5pa in good yields (53%−81%).
Unfortunately, the internal styrene 1p resulted in the low yield
(5qa, 32%), which might be attributed to steric hindrance.
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Typically, electron-deficient alkenes, such as acrylamide 1s,
also reacted smoothly under the standard conditions to afford
the desired 5sa in 48% yield.
To demonstrate the product utility of the γ-amino acid
products, a further transformation of 3a was demonstrated.
Reduction of the carboxylic acid group with LAH resulted in
the formation of δ-amino alcohol 7 in 45% yield (Scheme 5a).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00536.
■
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Experimental procedures and spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Scheme 5. Derivatization and Control Experiments
Jiang Cheng − School of Petrochemical Engineering, and
Jiangsu Key Laboratory of Advanced Catalytic Materials &
Technology, Changzhou University, Changzhou 213164, P.R.
China; Email: jiangcheng@cczu.edu.cn
Song Sun − School of Petrochemical Engineering, and Jiangsu
Key Laboratory of Advanced Catalytic Materials &
Technology, Changzhou University, Changzhou 213164, P.R.
China; orcid.org/0000-0002-9974-6456;
Email: sunsong@cczu.edu.cn
Authors
Cong Zhou − School of Petrochemical Engineering, and
Jiangsu Key Laboratory of Advanced Catalytic Materials &
Technology, Changzhou University, Changzhou 213164, P.R.
China
Miao Li − School of Petrochemical Engineering, and Jiangsu
Key Laboratory of Advanced Catalytic Materials &
Technology, Changzhou University, Changzhou 213164, P.R.
China
Jianwei Sun − School of Petrochemical Engineering, and
Jiangsu Key Laboratory of Advanced Catalytic Materials &
Technology, Changzhou University, Changzhou 213164, P.R.
China; Department of Chemistry, the Hong Kong University
of Science and Technology, Kowloon, Hong Kong SAR,
China; orcid.org/0000-0002-2470-1077
To gain insights into the reaction mechanism, we also
carried out some control experiments. First, luminescence
studies showed the photoactivated Ru(bpy)3·2PF6 was
reductively quenched by sodium glycinate 2b in the initial
step (see Figures S4 and S5). When TEMPO (3 equiv) was
used as additive, the standard reaction of 1a and 2a was
completely inhibited, which suggested that a radical pathway
might be involved (Scheme 5b). Second, with D2O (10 equiv)
as additive, the standard reaction of 1a and 2a formed amine 8
in 34% yield, together with 42% of recovered 1a (Scheme 5c).
This result implied that γ-amino benzylic carbanion is likely an
intermediate. Finally, when the hydroalkylation product 6 was
subjected to the standard conditions in the presence of CO2,
no carboxylation product 3a was detected, thus ruling out the
benzylic C−H carboxylation pathway in the standard protocol
(Scheme 5d).
In conclusion, we have developed a new visible-light-
induced photoredox-catalyzed α-aminomethyl carboxylation
of styrenes, leading to diverse α,α-disubstituted γ-amino acids
and γ-lactams with high efficiency and regioselectivity. Distinct
from the previous processes of similar substrates that led to
simple decarboxylation and hydroalkylation, our reaction
involves an addition carboxylation of the resulting carbanion
intermediate, thus representing α-aminomethyl carboxylation
of olefins that involves CO2 sequestration. While reutilization
of the CO2 generated from the decarboxylation step was not
achieved with complete success, this work provided a proof of
concept and represents a new step forward in the development
of atom-economic decarboxylative reactions. Finally, these new
findings not only provided efficient access toward α,α-
disubstituted γ-amino acids and γ-lactams, but also enriched
the chemistry of sodium glycinates for olefin difunctionaliza-
tion.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00536
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(21602019, 21971025), Jiangsu Key Laboratory of Advanced
Catalytic Materials & Technology (BM2012110), and
Advanced Catalysis and Green Manufacturing Collaborative
Innovation Center for financial support. C.Z. thanks the
Postgraduate Research & Practice Innovation Program of
Jiangsu Province (SJCX20_0947).
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