Stereoselective amination: Via vinyl-silver intermediates derived from silver-catalyzed carboxylative cyclization of propargylamine

Article abstract of DOI:10.1039/d0cc02273j

The stereoselective synthesis of aminovinyloxazolidinones based on the electrophilic amination of a vinyl-silver intermediate, generated by silver-catalyzed carbon dioxide incorporation on a propargyl amine, was achieved. The geometry of the aminated product was determined by a single crystal X-ray analysis and NOE measurement and it was elucidated that the geometry was proved to be opposite to the geometry predicted from the previous silver-catalyzed carbon dioxide fixation on a propargyl amine derivative. This unexpected stereoselectivity could be successfully explained by a radical mechanism. This journal is

Full text of DOI:10.1039/d0cc02273j

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Stereoselective amination via vinyl-silver
intermediates derived from silver-catalyzed
Cite this:DOI: 10.1039/d0cc02273j
carboxylative cyclization of propargylamine†
Received 28th March 2020,
Accepted 7th July 2020
Yuta Sadamitsu, Kodai Saito and Tohru Yamada
*
DOI: 10.1039/d0cc02273j
rsc.li/chemcomm
The stereoselective synthesis of aminovinyloxazolidinones based on
the electrophilic amination of a vinyl-silver intermediate, generated
by silver-catalyzed carbon dioxide incorporation on a propargyl
amine, was achieved. The geometry of the aminated product was
determined by a single crystal X-ray analysis and NOE measurement
and it was elucidated that the geometry was proved to be opposite
to the geometry predicted from the previous silver-catalyzed carbon
dioxide fixation on a propargyl amine derivative. This unexpected stereo-
selectivity could be successfully explained by a radical mechanism.
Carbon dioxide is one of the attractive carbon sources, having
a low toxicity and availability at low cost as well as easy
handling.1 Several reactions were evaluated for carbon dioxide
fixation, however, it sometimes leads to synthetic difficulty due
to the instability of the carboxylate generated from the addition
of a nucleophile and carbon dioxide.2 Therefore, a sequence to
Scheme 1 Silver-catalyzed CO2 incorporation into a propargylamine.
stabilize the carboxylate is required in many cases to incorporate bromination,5c to give functionalized vinyloxazolidinones with-
carbon dioxide into a substrate.3
out the loss of the geometry (Scheme 1a).
The successive intramolecular cyclization based on the transition
The formation of a vinyl-silver complex has been often proposed
metal-catalyzed activation of an alkyne is one of the most utilized as an intermediate in the silver-catalyzed reactions,6 although, due
and reliable methods to afford a variety of heterocycles. We have to their instability, few successful reports were found about the
reported silver-catalyzed carbon dioxide incorporation reactions on isolation of vinyl-silver complexes.7,8 Therefore, the reactivity of a
propargyl alcohols4 and propargyl amines.5a In these reactions, the vinyl-silver complex is not completely understood. Perfluoroalkenyl-
corresponding cyclic carbonates and carbamates bearing Z exo- silvers are known as isolable vinyl-silver complexes, and their
olefin were obtained through the anti-addition of a hemi- applications for halogenation, methylation, silylation, and
carbonate or -carbamate to the internal alkyne activated by a transmetalation have been studied.7 The direct transformation
silver catalyst, followed by the protonation of the vinyl-silver of the in situ generated vinyl-silver intermediate was also
intermediate. The theoretical calculation also supported this examined in the silver-catalyzed reaction, but was limited to
reaction mechanism.4 It was elucidated that the vinyl-silver halogenation8a–e and stannylation.8f Thus, it is considered that
intermediate generated from the carbon dioxide fixation on the the catalytic reaction based on the vinyl-silver intermediates is
propargyl amine through the anti-addition could also be still being developed. In this paper, we report the amination of
applied for further transformation, such as iodination5b and the vinyl-silver intermediate generated from the silver-catalyzed
carbon dioxide incorporation reaction on the propargyl amine
(Scheme 1b). Based on this study, an interesting reactivity of the
vinyl-silver intermediate and the unexpected stereoselectivity
Department of Chemistry, Keio University, Hiyoshi, Kohoku-ku,
Yokohama 223-8522, Japan. E-mail: yamada@chem.keio.ac.jp
were discovered.
† Electronic supplementary information (ESI) available. CCDC 1993300 and
As an electrophilic aminating agent, an azodicarboxylate9
1993299. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/d0cc02273j
was selected for this investigation. When the propargyl amine
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Table 1 Examination of reaction conditions
Entry
X/MPa
t/1C
Yielda
2a/%
3aA/%
2a : 3aA
1
2
3
4
5
6
7
1.0
1.0
1.0
1.0
30
50
15
À15
À30
À30
À40
À45
À40
70
96
65
55
31
21
10
11
9
30
2
36
45
58
67
89
70 : 30
495 : 5
64 : 36
55 : 45
35 : 65
24 : 76
10 : 90
11 : 89
9 : 91
1.0
Balloon
Balloon
Balloon
Balloon
8
87
9b,c
87 (80)d
a
b
c
Determined by 1H NMR. DBAD (1.5 equiv.) was used. AgOAc
d
(5 mol%) was used. Isolated yield.
1a was treated with 1 MPa pressure of carbon dioxide,
1.0 equivalent of DBU (= 1,8-diazabicyclo[5.4.0]undec-7-ene),
3.0 equivalents of DBAD (= di-tert-butyl azodicarboxylate), in the
Scheme 2 Scope of substrates.
presence of a catalytic amount of silver acetate, and molecular were next evaluated. When the reaction of the substrate having
sieves 3A in acetonitrile at 30 1C, the desired aminovinyloxazo- p-methoxyphenyl group (1b) was carried out, the desired products
lidinone 3aA was obtained in 30% NMR yield along with the 3bA and 3bB were obtained in 94% and 499% yields, respectively.
corresponding vinyloxazolidinone 2a in 70% NMR yield The propargyl amine bearing p-dimethylaminophenyl group (1c)
(Table 1, entry 1). The product 3aA was clearly identified as a also converted to 3cA and 3cB in 98% and 499% yields,
single product without the formation of the other stereoisomer. respectively. The substrate bearing an electron withdrawing
Next, the reaction of 1a was carried out under higher tempera- group on R1 was also examined, however, the reactions using
ture conditions (50 1C) but resulted in the formation of 2a in the propargyl amine having p-chlorophenyl group (1d) resulted
96% yield (entry 2). Due to the DBAD decomposition at higher in moderate yields of 3dA and 3dB (47% and 68%, respectively)
temperature,10 the protonation of the vinyl-silver intermediate due to the lower nucleophilicity of the vinyl-silver intermediate.
would be preferred rather than the amination. Therefore, a lower In the examination of substrate bearing a 2-thienyl group on R1
temperature was next subjected to this reaction to improve the (1e), when DBAD was used, the lower yield (25%) of 3eA was
selectivity of 3aA. When the reaction was performed at 15 1C, observed, whereas the use of DEAD resulted in a high yield of
À15 1C, and À30 1C, the yields of 3aA were increased to 36%, 3eB (82%). The difference in yields between 3eA and 3eB might
45%, and 58%, respectively (entries 3–5). It was discovered that reflect the reactivity of the aminating reagents.12 Further studies
the balloon pressure of carbon dioxide was effective to improve focused on the substituent effect of R1 on substrates having an
the selectivity at À30 1C, and 3aA was obtained in 67% yield o-substituted phenyl group (1f and 1g) were carried out. When
(entry 6). When the reaction was performed at À40 1C, the the substrate bearing o-tolyl group (1f) was employed, the amination
selective amination reaction proceeded to afford 3aA in 89% proceeded to afford 3fA and 3fB in 89% and 99% yields, respectively.
yield (entry 7). Although a further improvement of the yield of 3aA The reaction using 1-naphthyl group-substituted propargyl amine
was not observed at a lower temperature (entry 8), it was found that (1g) resulted in the formation of the corresponding aminated
1.5 equivalents of DBAD was sufficient to obtain the aminated products 3gA and 3gB in 66% and 96% yields, respectively. The
product 3aA in 87% yield (80% isolated yield) (entry 9).
effect of the substituent R2 was next examined. The reaction using
The optimized reaction conditions were successfully applied the propargyl amine having benzyl group on R2 (1h) successfully
to various propargyl amines (Scheme 2). In this examination, proceeded to give 3hA and 3hB in 80% and 499% yields, respec-
t
DBAD (A: R4 = Bu) or DEAD (B: R4 = Et) was employed as the tively. The substrate bearing p-methoxybenzyl group on R2 (1i) also
aminating reagent. The reaction using propargyl amine 1a converted to 3iA and 3iB in 61% and 99% yields, respectively. For
resulted in the corresponding aminovinyloxazolidinones 3aA examination of the substituent of R3, the diethyl substituted
and 3aB in 80% and 499% yields, respectively.11 The propargyl propargyl amine (1j) was employed for this reaction, and 3jA and
amines bearing electron donating groups on R1 (1b and 1c) 3jB were obtained in 72% and 98% yields, respectively.
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Fig. 1 X-ray crystallographic data and NOE experiment.
Scheme 3 Control experiments.
The geometry of the exo-olefin was next confirmed. The According to our previous study,5 the silver-catalyzed carbon
single crystals of aminovinyloxazolidinone 3bA and 3hA were dioxide incorporation on a propargyl amine can proceed at
obtained from the reaction using DBAD as an aminating ambient temperature without a base, thus it was indicated that
reagent for the substrates bearing p-methoxyphenyl group on DBU might increase the nucleophilicity of the nitrogen atom to
R1 (1b) and benzyl group on R2 (1h) and an X-ray analysis was attack the carbon dioxide under the low temperature conditions.
performed using these single crystals. When NOE experiment Next, the mechanism of the amination step was investigated. It was
of 3bA was performed, the correlation of the protons between reported that the amination using an azodicarboxylate ester
the methyl and PMP groups (1.4%) was observed to also proceeded by a radical mechanism.14 When TEMPO (= 2,2,6,6-
support the geometry resulting from the X-ray analysis. As a tetramethylpiperidine 1-oxyl) was added as a radical scavenger
result, the exo-olefin geometry of 3bA and 3hA were determined in this reaction, the protonated product 2a was only obtained in
to be Z13 and surprisingly proved to be opposite to the geometry 79% yield (Scheme 3c). Therefore, it is considered that the
predicted from our past findings (Fig. 1).4,5 In the reactions amination could involve a radical mechanism. According to the
using DEAD as an aminating reagent, no single crystal was literature, the combination of DEAD with triphenylphosphine
obtained, thus the exo-olefin geometry was determined by NOE generated the corresponding N-radical species.15 Also, in this
experiment. The aminated product 3bB synthesized from 1b reaction, the similar N-radical species can be formed by a single
using DEAD was measured, and the proton interaction between electron transfer to DEAD from DBU. Thus, DBU might play
the PMP group and the methyl groups was observed as 3bA. double roles as a base to enhance the nucleophilicity of the
Based on these analyses, it was confirmed again that the exo-olefin substrate at a lower temperature and as a single electron
geometries of the aminated products in this reaction were opposite reductant against DEAD to generate the N-radical species. The
to the already reported silver-catalyzed vinyloxazolidinone synthesis.5 reaction of 1b was then performed using DPPH (= 1,1-diphenyl-
This result suggested that the mechanism of this reaction could 2-picrylhydrazyl free radical) as the N-radical species instead of
be different from the previously reported transformations of the DBAD or DEAD, and the corresponding N-substituted vinylox-
vinyl-silver intermediates.4,5
azolidinone 4b was certainly obtained in 8% yield (Scheme 3d).
The control experiments were performed to investigate the This result suggested that the amination of this reaction proceeded
reaction mechanism (Scheme 3). When the reaction was carried through a radical process.
out without the silver salt, the protonation and amination hardly
The plausible reaction mechanism is summarized in Fig. 2.
proceeded with recovery of the starting material 1a (Scheme 3a). The vinyl-silver intermediate I is first generated via the silver-
This result suggested that a silver salt was necessary for the catalyzed carbon dioxide incorporation on the propargyl amine 1
cyclization of the carboxylate into the alkyne the same as the using DBU as a base. When the vinyl-silver intermediate I reacts
standard silver-catalyzed carbon dioxide incorporation. The reaction with a proton source, such as the protonated DBU (H-DBU+), the
without DBU was also examined and resulted in no reaction. corresponding vinyloxazolidinone 2 is obtained (Fig. 2, path A).
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Fig. 2 Plausible reaction mechanism.
On the other hand, the reaction of I and N-radical species
generated from an azodicarboxylate and DBU is assumed to
produce the vinyl radical II via the homolysis of the vinyl-silver
bond (Fig. 2, path B). It is known that vinyl radical bearing an aryl
group forms a p-radical having a linear structure.16 Therefore,
another N-radical species would access and react with II while
avoiding the sterically demanding substituent R3 to afford the
corresponding aminated product 3.14e,17 This might be the
reason why the exo-olefin geometry of the aminated product 3
was opposite that of the vinyl-silver intermediate I.
In conclusion, we have developed the stereoselective synthesis
of aminovinyloxazolidinones via the silver-catalyzed carbon dioxide
incorporation. In this reaction, the nucleophilic addition of a
propargyl amine to carbon dioxide and sequential silver-catalyzed
cyclization proceeded to generate the vinyl-silver intermediate, and
the azodicarboxylate reacted with this intermediate to afford the
corresponding aminated product in high yield. This reaction is
the first example of the amination of a vinyl-silver bond, and it
was suggested that this amination would proceed by a radical
mechanism.
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ESI†.
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13 The geometries of the aminated product 3 and the protonated
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priority. Thus, it indicated that the electrophilic amination and
protonation of the vinyl-silver intermediate for the production of 3
and 2 occurred through the inversion and retention of the geometry
of the vinyl-silver intermediate, respectively.
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Conflicts of interest
The authors declare no conflict of interest.
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