Bronsted acid catalyzed amination of 1,3-dicarbonyl compounds by iminoiodanes

Article abstract of DOI:10.1055/s-0033-1340108

A synthetic method to aminate 1,3-dicarbonyl compounds with PhI=NTs using Bronsted acid catalysis is described herein. The method was shown to be applicable to β-keto esters and phosphonates as well as 1,3-diones, providing the corresponding α,α-acyl amino acid derivatives in moderate to excellent yields. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1340108

LETTER
▌201
letter
Brønsted Acid Catalyzed Amination of 1,3-Dicarbonyl Compounds by Imino-
iodanes
Brønsted Acid Catalyzed Amination of 1,3-Dicarbonyl Compounds
Ciputra Tejo, Hui Quan Yeo, Philip Wai Hong Chan*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
Fax +6567911961; E-mail: waihong@ntu.edu.sg
Received: 24.09.2013; Accepted after revision: 08.10.2013
With ethyl benzoylacetate (1a) as the model substrate, we
Abstract: A synthetic method to aminate 1,3-dicarbonyl com-
began to establish a variety of reactions conditions (Table
pounds with PhI=NTs using Brønsted acid catalysis is described
1). This initial study showed that treating the β-keto ester
herein. The method was shown to be applicable to β-keto esters and
with two equivalents of PhI=NTs and 20 mol% of TFA in
the presence of 240 mg of 4 Å molecular sieves (MS) in
dichloromethane at room temperature for 15 minutes gave
the corresponding α-aminated product 2a in 85% yield
(Table 1, entry 1). Subsequent investigations revealed that
slightly lower yields of 75–79% could be obtained on ei-
ther decreasing the catalyst loading from 20 mol% to 10
mol% or the amount of iminoiodane from two equivalents
to 1.5 or 1.2 equivalents (Table 1, entries 2–5). Lower
product yields (61–72%) were also afforded when the re-
action was repeated with 5 mol% of TFA and/or at 0 °C
(Table 1, entries 6, 7, and 9). On the other hand, the anal-
ogous reactions with 10 mol% of the Brønsted acid at 0 °C
for 90 minutes gave 2a in 86% yield (Table 1 entry 8). In
our hands, amination of 1a by PhI=NTs under these latter
reaction conditions in other solvents was found to be less
effective (Table 1, entries 10–12). Lower product yields
of 11–68% were furnished when THF, MeCN, or toluene
in place of dichloromethane was used as the reaction me-
dium. Likewise, reactions with HCl, H2SO4, H3PO4, and
AcOH as the catalyst gave lower yields of 51–78% even
on prolonging reaction times to 18 hours at room temper-
ature (Table 1, entries 13–16). On the basis of these re-
sults, amination of 1a by PhI=NTs (1.2 equiv) in the
presence of 10 mol% of TFA and 4 Å MS (240 mg) in di-
chloromethane at 0 °C for 90 minutes was deemed to pro-
vide the optimal reaction conditions.7,8
phosphonates as well as 1,3-diones, providing the corresponding
α,α-acyl amino acid derivatives in moderate to excellent yields.
Key words: α,α-acyl amino acid derivatives, iminoiodanes, amina-
tion, Brønsted acid catalysis, 1,3-dicarbonyl compounds
Compounds containing unnatural α-amino acids, those
outside the set of 20 used by Nature, represent important
targets in medicinal chemistry because of their potential to
exhibit new modes of bioactivity.1,2 Not surprisingly, the
field has witnessed a number of elegant synthetic methods
to prepare derivatives of the biomolecule in an efficient
and convenient manner. Recently, we reported one of
such approach to ketone-substituted α-amino acid deriva-
tives that relied on Cu(OTf)2-catalyzed amination of a
1,3-dicarbonyl compound by PhI=NSO2Ar (Ar = 4-
MeC6H4, 4-O2NC6H4).2c,3–5 At about the same time,
Zhang and co-workers showed that the analogous reac-
tions of β-keto amides, esters, phosphonates, and 1,3-dio-
nes with TsNH2 and PhI=O mediated by Zn(ClO)4·6H2O
could be achieved in good to excellent yields.2f In view of
these works, we queried whether a Brønsted acid cata-
lyzed version of this functional-group transformation
could be realized.6 In doing so, we disclose herein the de-
tails of this study involving TFA-mediated amination of
β-keto esters and phosphonates and 1,3-diones by
PhI=NTs (Scheme 1). This process provides a convenient
and operationally straightforward route to the correspond-
ing α,α-acyl amino acid derivatives in moderate to excel-
lent yields.
To assess the generality of the present procedure, we next
turned our attention to the amination of a variety of 1,3-
dicarbonyl compounds, and the results are summarized in
Scheme 2. These experiments revealed reactions of β-keto
esters containing an electron-donating (1b–e) or electron-
withdrawing (1f–i) phenyl group with PhI=NTs proceed
well to afford the corresponding α,α-acyl amino acid de-
rivatives 2b–i in 53–86% yield. Likewise, β-keto esters in
which the phenyl group was replaced with a furan moiety
(1j) or alkyl substituent (1l,m) were well tolerated, giving
2j,l,m in 47–73% yield. The only exception was the reac-
tion of 1k with PhI=NTs providing 2k in a lower yield of
27%. Changing the β-keto ester to its phosphonate ana-
logue 1n was also found to give the corresponding α-am-
inated product 2n in 53% yield on treatment with 10
mol% of TFA and 1.2 equivalents of PhI=NTs under stan-
dard reaction conditions. Similarly, the reaction condi-
O
O
O
O
TFA (10 mol%)
PhI=NTs
+
R1
R2
R1
R2
CH2Cl2
4 Å MS, 0 °C
1
NHTs
2
Scheme 1 Brønsted acid catalyzed amination of 1,3-dicarbonyl
compounds by PhI=NTs
SYNLETT 2014, 25, 0201–0204
Advanced online publication: 19.11.2013
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9
3
6
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5
2
1
4
1
4
3
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0
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DOI: 10.1055/s-0033-1340108; Art ID: ST-2013-D0908-L
© Georg Thieme Verlag Stuttgart · New York

202
C. Tejo et al.
LETTER
tions could be extended to the 1,3-diones 1q–s to give the acid catalyst in facilitating the enolization of the 1,3-di-
corresponding 2-amino-1,3-dione products in 57–63% carbonyl compound would be consistent with the recov-
yield. However, replacing the alkyl substituents in the 1,3- ery of the substrate in reactions with less acidic dialkyl
dione substrate with presumably more bulky tert-butyl malonates.9
groups, as in 1t, was found to result in recovery of the sub-
strate in near quantitative yield. Additionally, the analo-
gous reactions with the alkyl malonates 1o and 1p were
The formation of iodonium ylide 3u could be due to pref-
erential α-H elimination in intermediate A (path b in
Scheme 4). This could be due to the more stable nature of
found, in both instances, to lead to the recovery of the 1,3-
the species provided by secondary bonding interactions
dicarbonyl compound in near quantitative yield based on
between the carbonyl oxygen atoms and the iodine(III)
TLC analysis and 1H NMR measurements of the respec-
center on generating the cyclic form of the hypervalent io-
tive crude mixtures. As shown in Scheme 3, the amination
dine product.10 The side product additionally argues in fa-
of 1,3-cyclohexadione (1u) was also investigated but was
vor of species A being the actual intermediate that
found to afford the iodonium ylide 3u in 83% yield.
undergoes reductive elimination to afford the product.
A tentative mechanism for the present Brønsted acid cat- Added to this, competition experiments on the amination
alyzed amination reaction is illustrated in Scheme 4. Un- of β-keto esters 1d,e,g–i revealed log(Kx/KH) values of
der the acidic conditions, this could involve enolization of 0.02 (1d), 0.09 (1e), –0.09 (1g), –0.07 (1h), and –0.06
1 to its enolate 1′ that subsequently adds to PhI=NTs to (1i). This suggests that 1,3-dicarbonyl compounds substi-
give the putative iodine(III) species A (path a in Scheme tuted with an electron-donating group are more reactive
4).2f Reductive elimination of the hypervalent iodine(III) than 1a, whereas those with an electron-withdrawing sub-
moiety in this key intermediate would result in the release stituent retard the reaction. Fitting (by least-squares meth-
of PhI and deliver the product 2. The role of the Brønsted od) the log(KX/KH) data to the σp scale gave rise to
Table 1 Optimization of the Reaction Conditionsa
O
O
O
O
catalyst
PhI=NTs
+
Ph
OEt
solvent
4 Å MS, temp
Ph
OEt
NHTs
1a
2a
Entry
Catalyst
Catalyst loading
(mol%)
PhI=NTs loading Solvent
(equiv)
Temp (°C)
Time (min)
Yield (%)b
1
2
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
TFA
HCl
20
20
10
20
10
5
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
PhMe
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
0
15
10
85
77
79
79
75
61
62
86
72
68
51c
11c
52
78
66
51
1.5
1.5
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
2
3
10
4
15
5
10
6
15
7
20
10
5
90
8
0
90
9
0
90
10
11
12
13
14
15
16
10
10
10
20
20
20
20
0
90
0
90
THF
0
90
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
r.t.
r.t.
r.t.
r.t.
1080
1080
1080
1080
H2SO4
H3PO4
AcOH
2
2
2
a Unless otherwise stated, all reactions were performed in the presence of the given catalyst and 4 Å MS (240 mg) in given solvent (2 mL) at
given temperature and time with 0.5 mmol of 1a.
b Isolated yield.
c Product yield was determined by 1H NMR analysis of the crude mixture with CH2Br2 as the internal standard.
Synlett 2014, 25, 201–204
© Georg Thieme Verlag Stuttgart · New York

LETTER
Brønsted Acid Catalyzed Amination of 1,3-Dicarbonyl Compounds
203
O
O
O
O
H+
TFA (10 mol%)
R1
R2
R1
R2
PhI=NTs
+
R1
R2
R1
R2
CH2Cl2
4 Å MS, 0 °C
1
O
O
HO
O
NHTs
1
1'
2
O
O
O
O
PhI=NTs
OEt
OEt
path b
R
NHTs
NHTs
2j (41%)
R1
R2
O
= (CH2)4
(1u)
O
O
2b R = 2-Me (65%)
2c R = 3-Me (84%)
2d R = 4-Me (84%)
path a
– PhI
R1
R2
O
O
O
O
O
O
H
– NH2Ts
I
I
NHTs
2e R = 4-OMe (63%)
2f R = 4-I (53%)
R1
OR2
Ph
Ph
NHTs
3u
2
A
NHTs
2g R = 4-Cl (65%)
2h R = 4-Br (60%)
2i R = 4-F (58%)
2k R1 = Me, R2 = Me (27%)
2l R1 = Me, R2 = Et (73%)
2m R1 = Me, R2 = t-Bu (47%)
Scheme 4 Plausible mechanism for amination of 1,3-dicarbonyl
compounds with PhI=NTs mediated by TFA
O
O
P
OEt
O
O
OEt
NHTs
RO
OR
2n, (53%)
NHTs
2o R = Me, (–)b
2p R = Et, (–)b
O
O
R1
R2
NHTs
2q R1 = R2 = Ph (63%)
2r R1 = Ph, R2 = Me (57%)
2s R1 = R2 = Me (62%)
2t R1 = R2 = t-Bu (–)b
Scheme 2 TFA-mediated amination of 1b–t by PhI=NTs. Refer to
the Supporting Information for the reaction times. Values in parenthe-
ses denote isolated product yields. For compounds 2o,p,t: no reaction
based on TLC and 1H NMR analysis of the crude mixture.
Figure 1 Linear free-energy correlation of log(KX/KH) against σp on
the Brønsted acid catalyzed amination of 1,3-dicarbonyl compounds
1d,e,g–i with PhI=NTs
O
O
TFA (10 mol%)
I
PhI=NTs
+
Ph
CH2Cl2
4 Å MS, 0 °C
O
O
Acknowledgment
1u
3u (83%)
This work was supported by a College of Science Start-Up Grant
from Nanyang Technological University and an A*STAR-MSHE
Joint Grant (122 070 3062) from A*STAR, Singapore.
Scheme 3 Formation of iodonium ylide 3u
moderate linearity (R2 = 0.947) with a ρ value of –0.306
(Figure 1).11 The small and negative ρ value indicates
minimal positive-charge development on the aromatic
ring of the intermediate A and implies that its formation
could be the rate-limiting step.12
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
References and Notes
In summary, we have developed a Brønsted acid cata-
lyzed method to access unnatural α,α-acyl amino acid de-
rivatives from 1,3-dicarbonyl compounds and PhI=NTs.
Our studies show the reaction to be applicable to β-keto
esters and phosphonates as well as 1,3-diones and compli-
ment earlier works that make use of Cu(II)2c and Zn(II)2f
catalysis. Further exploration on the synthetic utility of
the present transformation is currently underway and will
be reported in due course.
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 201–204

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LETTER
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To a degassed mixture of PhI=NTs (0.6 mmol, 224 mg) and
powdered 4 Å MS (240 mg) was added CH2Cl2 (1 mL). The
reaction was cooled to 0 °C, and a solution of TFA (0.05
mmol, 3.83 μL) in CH2Cl2 (1 mL) was added. The 1,3-
dicarbonyl compound was added, and the reaction was
monitored by TLC analysis. Upon completion, the reaction
mixture was filtered, washed with EtOAc (40 mL),
concentrated under reduced pressure, and purified by flash
chromatography [n-hexane–EtOAc (4:1) as eluent] to
furnish the title compound.
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Ethyl 2-(4-Methylphenylsulfonamido)-3-oxo-3-
phenylpropanoate (2a)
Reaction time = 1.5 h; yield 86%; 0.162 g; white solid. 1H
NMR (400 MHz, CDCl3): δ = 7.98 (d, J = 7.4 Hz, 2 H), 7.73
(d, J = 8.3 Hz, 2 H), 7.62 (t, J = 7.4 Hz, 1 H), 7.47 (t, J = 7.7
Hz, 2 H), 7.24 (d, J = 8.1 Hz, 2 H), 6.00 (d, J = 8.9 Hz, 1 H),
5.58 (d, J = 8.9 Hz, 1 H), 3.97 (q, J = 7.1 Hz, 2 H), 2.38 (s,
3 H), 1.04 (t, J = 7.1 Hz, 3 H). 13C NMR (100 MHz, CDCl3):
δ = 190.2, 165.9, 144.1, 144.0, 136.6, 134.7, 129.8, 129.5,
128.9, 127.4, 62.6, 60.9, 21.6, 13.8.
Ethyl 3-(4-Bromophenyl)-2-(4-methylphenyl-
sulfonamido)-3-oxopropanoate (2h)
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Reaction time = 1.5 h; yield 60%; 0.114 g; white solid. 1H
NMR (400 MHz, CDCl3): δ = 7.86 (d, J = 7.2 Hz, 2 H), 7.72
(d, J = 8.3 Hz, 2 H), 7.62 (d, J = 8.7 Hz, 2 H), 7.25 (d, J = 8.2
Hz, 2 H), 5.96 (d, J = 8.7 Hz, 1 H), 5.52 (d, J = 8.7 Hz, 1 H),
4.02–3.93 (m, 2 H), 2.39 (s, 3 H), 1.05 (t, J = 7.2 Hz, 3 H).
13C NMR (100 MHz, CDCl3): δ = 189.6, 165.9, 144.2, 136.5,
132.3, 130.9, 130.2, 129.8, 127.4, 62.9, 60.9, 21.6, 13.8.
N-(2,4-Dioxopentan-3-yl)-4-methylbenzenesulfonamide
(2s)
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7.71 (d, J = 8.1 Hz, 2 H), 7.32 (d, J = 8.1 Hz, 2 H), 6.16 (s,
1 H), 5.16 (s, 1 H), 2.44 (s, 3 H), 1.87 (s, 6 H). 13C NMR (75
MHz, CDCl3): δ = 194.1, 144.3, 136.6, 130.1, 127.4, 110.1,
22.1, 21.6.
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