Iridium(I)-catalyzed regio- and enantioselective allylic amidation

Article abstract of DOI:10.1016/j.tetlet.2007.08.009

Ir(I)-catalyzed intermolecular allylic amidation of ethyl allyl carbonates with soft nitrogen nucleophiles under completely 'salt-free' conditions is described. A combination of [Ir(COD)Cl]₂, a chiral phosphoramidite ligand L^*, and DBU as a base in THF effects the reaction. The reaction appears to be quite general, accommodating a wide variety of R-groups and soft nitrogen nucleophiles, and proceeds with excellent regio- and enantioselectivities to afford the branched N-protected allylic amines. The developed reaction was conveniently utilized in the asymmetric synthesis of N-Boc protected α- and β-amino acids as well as (-)-cytoxazone.

Full text of DOI:10.1016/j.tetlet.2007.08.009

Tetrahedron Letters 48 (2007) 7094–7098
Iridium(I)-catalyzed regio- and enantioselective allylic amidation
Om V. Singh and Hyunsoo Han*
Department of Chemistry, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA
Received 5 July 2007; revised 27 July 2007; accepted 1 August 2007
Available online 6 August 2007
Abstract—Ir(I)-catalyzed intermolecular allylic amidation of ethyl allyl carbonates with soft nitrogen nucleophiles under completely
‘salt-free’ conditions is described. A combination of [Ir(COD)Cl]2, a chiral phosphoramidite ligand L*, and DBU as a base in
THF effects the reaction. The reaction appears to be quite general, accommodating a wide variety of R-groups and soft nitrogen
nucleophiles, and proceeds with excellent regio- and enantioselectivities to afford the branched N-protected allylic amines. The
developed reaction was conveniently utilized in the asymmetric synthesis of N-Boc protected a- and b-amino acids as well as
(À)-cytoxazone.
Ó 2007 Elsevier Ltd. All rights reserved.
Ir(I)-catalyzed stereoselective allylic amination reactions
have quickly become a method of choice for the prepa-
ration of optically enriched allylic amines due to their
mild reaction conditions, as well as high regio- and
enantioselectivity.1–5 As an alternative and complemen-
tary approach, we recently reported that a catalytic sys-
tem involving [Ir(COD)Cl]2, a chiral phosphoramidite
ligand L*, DBU, and proton sponge (PS) in THF en-
abled highly regio- and enantioselective decarboxylative
allylic amidation of substituted allyl benzyl imidodicarb-
oxylates 1 to the branched N-Cbz protected allylic
amines 2, and the catalytic reaction was likely to pro-
ceed through the Ir-p-allyl complex (Eq. 1 in Fig. 1).6,7
Three possible pathways are conceivable for the car-
bon–nitrogen bond formation step in the reaction (Eq.
2 in Fig. 1). Oxidative addition of 1 to the Ir(I) catalyst
gives rise to intermediate I, which after proton shift
from the nitrogen to the oxygen attacks the Ir-p-allyl
complex to form the carbon–nitrogen bond. Alterna-
tively, intermediate I can undergo decarboxylation to
generate intermediate II, where the anion of benzyl
carbamate acts as a nucleophile toward the Ir-p-allyl
complex. The third possibility is an intermolecular reac-
tion between the Ir-p-allyl complex and the anion of 1
generated in the reaction.
OMe
O
Cbz
O
O
[Ir(COD) Cl]2, L*
Cbz
R
HN
N
H
O
P
N
+
(1)
(2)
DBU, PS, THF
R
2
1
We were interested in developing the intermolecular ver-
sion of the above decarboxylative allylic amidation reac-
tion, and envisioned that pathways I and III could be
mimicked by reactions between ethyl allyl carbonates 3
and di-benzyl imidodicarboxylate (or other soft nitrogen
nucleophiles) in the presence of a base and the iridium
catalyst, whereas the reaction between 3 and benzyl carb-
amate under similar conditions would follow pathway II
(Fig. 2).
OMe
L*
+
+
L*'
L*'
(COD)Ir
L*'
(COD)Ir
(COD)Ir
vs
vs
R
-
R
R
+
-
-
HN
HO
N
Cbz
R'O
N
Cbz
Cbz
O
III
I
II
O
Figure 1. Ir(I)-catalyzed decarboxylative allylic amidation reaction of
allyl benzyl imidodicarbonates and three possible reaction pathways.
O
Ir(I), L*, base
Cbz
R
Cbz
Cbz
HN(Cbz)2
N
HN
EtO
O
or
or
Keywords: Iridium; Phosphoramidite ligand; Asymmetric amidation;
a-Amino acid; b-Amino acid; Cytoxazone.
Corresponding author. Tel.: +1 210 458 4958; fax: +1 210 458
7428; e-mail: hyunsoo.han@utsa.edu
R
R
H2NCbz
2
4
3
*
Figure 2. Ir(I)-catalyzed intermolecular allylic amidation.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.08.009

O. V. Singh, H. Han / Tetrahedron Letters 48 (2007) 7094–7098
7095
Herein, we disclose that the Ir(I)-catalyzed intermole-
cular enantioselective allylic amidation reaction occurs
between ethyl allyl carbonates 3 and di-benzyl imidodi-
carboxylate (or other soft nitrogen nucleophiles) in the
presence of DBU to give the corresponding branched
N-bis-Cbz protected allylic amine 4 (or N-bis-Boc/N-
Ac,N-Boc protected allylic amines) in good to excellent
yield and stereoselectivity (Fig. 2 and Table 1). So far, to
the best of our knowledge, there has been only one liter-
ature report on the Ir(I)-catalyzed enantioselective
allylic amidation reaction, which can give N-Ac, N-
Cbz, and N-Boc (arguably three most popular N-protec-
tion groups) protected chiral allylic amines.8 While we
were preparing the present paper, Helmchen’s group
reported a similar work on the enantioselective allylic
amidation. Based on all data reported to date, our cata-
lytic systems involving Ir(I), L*, and DBU appear to be
more general than the Helmchen’s, accommodating a
variety of alkyl/aryl groups and soft nitrogen nucleo-
philes such as (Ac)(Boc)NH, (Boc)2NH, (Cbz)(Boc)NH,
and (Cbz)2NH, especially under ‘salt-free’ conditions.8
yield to 95% and reduced the reaction time to 30 min
without affecting regio- and enantioselectivities. Subse-
quently, the amount of DBU was adjusted to
20 mol % without any detrimental effects on reaction
yield as well as regio- and enantioselectivities (entry 1
of Table 1). The determined optimal conditions were ap-
plied to ethyl cinnamyl carbonate (3b) (the correspond-
ing cinnamyl benzyl imidodicarboxylate was a slower
and more difficult substrate in the decarboxylative allylic
amidation reaction under similar conditions), and the
allylic amidation occurred equally well to give the
branched product 5b (entry 2).
Other soft nitrogen nucleophiles such as di-tert-butyl
imidodicarboxylate (7b), benzyl tert-butyl imidodicarb-
oxylate (7c),11 and di-benzyl imidodicarboxylate (7d)11
were examined with both ethyl trans-2-hexenyl carbon-
ate and ethyl cinnamyl carbonate. All these reactions
proceeded well to give the corresponding branched allyl-
ation products in high yields and excellent regio- and
enantioselectivities (entries 3–6). In the case using 3a
and 7d, a lower reaction temperature (room tempera-
ture) was necessary to suppress the transesterification
reaction between the branched allylation product 5g
and ethoxide anion generated in the reaction (entry 7).
The reaction of 3b with 7d was sluggish at room tempera-
ture, and gave a mixture of the transesterification pro-
duct and the usual allylation product (entry 8).
An allylic amidation reaction was initially performed on
ethyl trans-hex-2-enyl carbonate (3a) using [Ir(COD)Cl]2
(2 mol %),
a
chiral phosphoramidite ligand L*
(4 mol %), the anion of benzyl carbamate (generated
in situ from benzyl carbamate and n-BuLi at 0 °C) as
a nucleophile (1.2 equiv), and DBU as a base (1 equiv)
in THF (0.5 mL). Not even a trace amount of the de-
sired product was observed, and the starting materials
were recovered. All efforts using other counterions for
the anion of benzyl carbamate, other solvents, and
halide anions proved to be fruitless.9
Selective/mono deprotections of the different nitrogen
protecting groups of 5a–g were studied (Scheme 1).
Selective deprotection of the Boc group of 5a-5b was
effected by TFA in CH2Cl2, while selective deprotection
of the acetyl group was achieved by K2CO3 in MeOH.
Mono deprotection of the Boc groups of 5c–d was
accomplished by Zn in refluxing MeOH.12 Similarly,
treatment of either K2CO3 in MeOH or TFA in CH2Cl2
could be used for selective/mono deprotection of the
Cbz/Boc groups of 5e–g.
However, when N-acetyl N-tert-butyl carbamate (7a)10
was used as a nucleophile, the corresponding branched
allylation product 5a formed in 60% yield after 5 h with
excellent regio- and enantioselectivities. Increasing the
reaction temperature to 55 °C improved the reaction
Table 1. Allylic amidation of ethyl allyl carbonates with different imidodicarboxylatesa
O
Ir(I), L*, DBU
R1
R
R2
THF, 50 º C
N
R2
EtO
O
+
R
N
R1
R2
R1
R
N
H
6a-6h
5a-5h
3a: R=
n-Pr
7a-7d
3b: R= Ph
Entry
Carbonate
Nucleophile
Product
Time (h)
Yieldb (%)
5/6c
% ee of 5d
1
2
3
4
5
6
7
8
3a
3b
3a
3b
3a
3b
3a
3b
7a: R1 = Ac- R2 = Boc
7b: R1 = Boc- R2 = Boc
7c: R1 = Cbz- R2 = Boc
7d: R1 = Cbz- R2 = Cbz-
5a
5b
5c
5d
5e
5f
0.5
1.0
0.5
1.0
0.5
1.0
95
94
93
90
94
92
97:03
>99:01
98:02
>99:01
98:02
>99:01
99:01
f
96
>99
96
>99
96
>99
96
f
5g
5h
3.0e
90
f
f
a All reactions were performed at 0.2 mmol scale.
b Isolated yields.
c Ratios of regioisomers were determined by 1H NMR of crude reaction mixtures.
d Enantiomeric excesses (ee’s) were determined by converting 5 to their mono N-Cbz/Boc derivatives and using chiral HPLC (Chiracel OD-H).
e The reaction was conducted at room temperature.
f See the text part.

7096
O. V. Singh, H. Han / Tetrahedron Letters 48 (2007) 7094–7098
Boc
The generality of the one-pot allylic amidation and
HN
K2CO3, MeOH
deprotection sequence was probed with 7a as a nitrogen
nucleophile and a variety of different ethyl allyl carbon-
ates, and the results were summarized in Table 2.
According to the data, the Ir(I)-catalyzed allylic amida-
tion and selective deprotection sequence appears to be
quite general, tolerating a wide variety of substituents
such as linear and branched alkyl (entries 1 and 2), con-
jugated alkenyl (entry 3), heteroatom functionalized
alkyl (entry 4), aryl (entries 5 and 6), and heteroaryl
groups (entries 7 and 8). In all cases, the N-Boc pro-
tected branched allylic amines 5 were obtained in high
yields and excellent regio- and enantioselectivities.
R
30 min, 94%
Ac
R
Boc
8a/8b
Ac
N
HN
TFA, CH2Cl2
1 h, 98%
5a/5b
R
9a/9b
8a/8b
Boc
R
Boc
N
Zn-MeOH, reflux
16 h, 90%
5c/5d
K2CO3-MeOH
10 h, 90%
8a/8b
Cbz
Boc
N
Cbz
HN
TFA, CH2Cl2
1 h, 96%
R
5e/5f
R
To explore a synthetic utility of N-Boc protected chiral
allylic amines and to ascertain their stereochemistry,
compound 11 (from entry 5 of Table 2) was converted
to the known a- and b-amino acids 12 and 14 as
depicted in Scheme 2. The oxidative cleavage of 11 by
2a/2b
2a
Cbz
Cbz
N
K2CO3-MeOH
12 h, 90%
Pr
5g
13
Scheme 1. Selective/mono deprotections of the different nitrogen
protection groups.
RuCl3 and NaIO4 produced N-Boc protected (R)-
(À)-phenylglycine (12).14 On the other hand, the hydro-
boration–oxidation15 of 11 followed by oxidation of
the resulting alcohol 13 by RuCl3 and NaIO4 gave
N-Boc protected b-amino acid 14.16
Having established the reaction conditions for the allylic
amidation and deprotection reactions, we pursued an
one-pot operation of two reactions. The allylation reac-
tion between 7a and 3a in THF at 55 °C followed by the
addition of K2CO3 and MeOH at room temperature fur-
nished the corresponding N-Boc protected allylic amine
8a in reaction yield and stereoselectivities comparable to
those from the corresponding two step reactions.
As shown in Scheme 3, combined with the Sharpless
dihydroxylation reaction,17 the developed allylic amida-
tion reaction could also provide a convenient route for
the asymmetric synthesis of (À)-cytoxazone,18 a novel
cytokinin modulator isolated from Streptomyces sp.19
The dihydroxylation reaction of the N-Boc protected
Table 2. One-pot allylic amidation and deprotection using methyl tert-butyl imidodicoxylate as a nitrogen nucleophilea
O
i. Ir((I), L*, DBU,
Boc
7a, THF, 55 o
C
HN
EtO
O
Boc
+
R
N
H
R
ii. K2CO3, MeOH
rt, 30 min
R
10
8
3
Entry
1
R=
Time (h)
0.5
Yieldb (%)
95
8/10c
97:03
ee of 8 (%)
97d
2
1.5
85
99:01
>99e
3
4
0.5
0.5
92
94
98:02
99:01
96e
99e
3
TBDPSO
MeO
5
1.0
94
>99:01
>99d
6
7
1.0
1.0
92
85
>99:01
>99:01
>99d
98.5d
O
S
8
1.0
80
97:03
ndf
a All reactions were performed at 0.2 mmol scale.
b Isolated yields.
c Rations of regioisomers were determined by 1H NMR of crude reaction mixtures.
d Enantiomeric excesses were determined using chiral HPLC.
e Enantiomeric excesses were determined after converting to the respective Cbz derivatives.
f The racemic compound could not be separated on HPLC using chiral columns OD-H, OJ-H, and OB-H in repeated attempts.

O. V. Singh, H. Han / Tetrahedron Letters 48 (2007) 7094–7098
7097
selective allylic aminations developed by other research
groups2–5 and should be of particular value in the
asymmetric synthesis of nitrogen-containing chiral
compounds.
Boc
Boc
HN
HN
RuCl3, NaIO4
CO2H
MeCN:CCl4:H2O
(1:1:1.5), 85%
12
11
9-BBN, THF, 0 º C,
then H2O2, NaOH,
0 oC to rt, 92%
Acknowledgments
Financial support from National Institute of Health
(GM 08194) and The Welch Foundation (AX-1534) is
gratefully acknowledged. We are also grateful to Dr.
K. Ditrich, BASF, Germany, for a generous gift of a
chiral amine for the synthesis of ligand L*.
Boc
HN
Boc
HN
CO2H
RuCl3, NaIO4
OH
MeCN:CCl4:H2O
(1:1:1.5), 90%
13
14
Scheme 2. Synthesis of N-Boc protected a- and b-amino acids.
References and notes
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OH
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HN
HN
17
OTBDPS
O
O
OH
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O
O
HN
Boc
1. NaH, THF
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OH
20; (-)-Cytoxazone
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OH
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(DHQ)2–PHAL ligand in the dihydroxylation reaction
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with NaH in THF, 19 cyclized to form the O-TBDPS
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