Enantioselective transformation of allyl carbonates into branched allyl carbamates by using amines and recycling CO2 under iridium catalysis

Article abstract of DOI:10.1002/chem.201402388

Enantioselective transformation of allyl carbonates into branched allyl carbamates by using amines and recycling CO₂ in the presence of an Ir complex and K₃PO₄ was accomplished. This provided branched allyl carbamates in fair to excellent yields with up to 98:2 regioselectivity and 93% ee. The role of CO₂ in this transformation is discussed as well.

Full text of DOI:10.1002/chem.201402388

DOI: 10.1002/chem.201402388
Communication
&
Synthetic Methods
Enantioselective Transformation of Allyl Carbonates into Branched
Allyl Carbamates by Using Amines and Recycling CO2 under
Iridium Catalysis
Sheng-Cai Zheng, Min Zhang, and Xiao-Ming Zhao*[a]
Chem. Eur. J. 2014, 20, 1 – 7
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Communication
knowledge, such a reaction has not been explored despite its
potential in both atom-economic synthesis and our mechanis-
tic understanding of p-allyl–metal chemistry. Allyl carbamates
are an important class of compounds; there is a broad interest
in them with regards to both synthetic intermediates[9] and
biologically active molecules.[10] Allyl carbamates are generally
synthesized from the reaction of allyl alcohols with isocya-
nates.[11] Therefore, a new synthetic method for the preparation
of chiral allyl carbamates is highly desirable. Furthermore, we
are confronted with an additional issue; the branched allyl car-
bamate is a good substrate for Ir-catalyzed allylic aminations
as well.[12] Herein, we report the first enantioselective transfor-
mation of allyl carbonates into branched allyl carbamates in
the presence of amines.
Abstract: Enantioselective transformation of allyl carbo-
nates into branched allyl carbamates by using amines and
recycling CO2 in the presence of an Ir complex and K3PO4
was accomplished. This provided branched allyl carba-
mates in fair to excellent yields with up to 98:2 regioselec-
tivity and 93% ee. The role of CO2 in this transformation is
discussed as well.
Transition-metal-mediated allylation has emerged as a powerful
method for the regio-, diastereo-, and enantioselective forma-
tion of carbon–carbon[1] or carbon–heteroatom[2] bonds. How-
ever, the selectivities depends on various factors such as the
nature of the metal complex, the substitution pattern of the
substrate, the nucleophile, the leaving group, the solvent, and
the temperature.[3] When allylic carbonates are employed as
substrates, linear products are preferentially obtained with Pd
catalysts[4] (Scheme 1, reaction 1); however, branched products
In an initial test of our hypothesis, we explored a model re-
action of (E)-cinnamyl methyl carbonate 2a with n-propyl-
amine in the presence of different types of bases and a well-
known iridacycle,[5b] which is generated from 2 mol% of [{Ir-
(cod)Cl}2] and 4 mol% of ligand 1a,[13–14] under various reaction
conditions. We found that employing propyl amine 3a in this
reaction in N,N-dimethylformamide (DMF)[15] at 358C led to 6a
as the sole amination product in 32% yield (Table 1, entry 1).
Surprisingly, in the presence of CsF, a trace amount of the
branched carbamate 4a[16] was obtained (Table 1, entry 2). Sig-
nificant improvement in efficiency, regioselectivity, and enan-
tioselectivity (55% yield, 4a/5a 81:19, 88% ee) was achieved
when Cs2CO3 was employed; 15% of 6a and a minor allylic al-
cohol were obtained as well (Table 1, entry 3). These results
strongly suggest that the domino reaction occurred as specu-
lated and that the nature of the base exerts a significant effect
on this reaction. As a result, a range of bases were investigat-
ed. Among these bases, K3PO4 gave 4a in fair yield with 4a/5a
90:10 and 85% ee (Table 1, entry 5), whereas the remaining
bases gave rise to poor results (Table 1, entries 4, 6–8). Exami-
nation of a range of solvents revealed that DMSO is the opti-
mum solvent (Table 1, entries 5, 9–11).
Scheme 1. Transition-metal-catalyzed allylation of allyl carbonates with
amines.
are favored in the presence of other transition metals, such as
iridium[5] and ruthenium.[6] Detailed mechanistic studies on the
Ir-catalyzed allylic substitution have been carried out by several
research groups.[7] The essential step is the formation of a p-
allyl–iridium complex through decarboxylation, an intermedi-
ate that then undergoes various transformations. In these reac-
tions, CO2 is produced as a co-product in the decarboxylation
step (Scheme 1, reaction 2).[7a] The conversion of CO2 into
useful chemicals has gained great attention from the view-
point of carbon resources and environmental issues.[8] We
speculated that CO2 may be recycled by an attack of a nucleo-
phile, such as an amine, that is, an amidation reaction, fol-
lowed by an Ir-catalyzed allylation reaction to give a branched
allylic carbamate (Scheme 1, reaction 3). To the best of our
In terms of transition-metal-catalyzed allylic substitution, ele-
vated reaction temperatures promotes the reductive elimina-
tion process.[17] Indeed, a change of the reaction temperature
has a dramatic influence on efficiency and enantioselectivity
(Table 1, entries 11–13). The reaction at room temperature[18]
gave 4a in 43% yield with 4a/5a 94:6 with 93% ee; 13% of
amination product 6a was obtained as well (Table 1, entry 12).
Upon raising the temperature to 358C, the yield of 4a was im-
proved to 80%; however, the ee value of 4a was reduced to
86% while the regioselectivity was maintained at 94:6 (Table 1,
entry 11). Upon further elevating the reaction temperature to
508C, both regio- and enantioselectivity were reduced; 5% of
amination product 6a was obtained (Table 1, entry 13).
A set of chiral ligands, including Feringa’s ligand, 1a, 1b,[14]
1c,[13] 1d,[14] 1e,[19] and PHOX ligand 1 f[20] (Figure 1), was evalu-
ated. The reaction with 1a at 258C gave superior results; 7%
yield of 6a was obtained as well (Table 1, entry 11). Ligand 1d,
which bears a simple biphenyl backbone, afforded the product
in good yield (69%) and regioselectivity (86:14), albeit with
slightly lower enantioselectivity (80% ee, Table 1, entry 16). The
use of ligands 1b and 1c, which have bulky groups on the
[a] S.-C. Zheng, M. Zhang, Prof. Dr. X.-M. Zhao
Department of Chemistry
State Key Laboratory of Pollution Control and Resource Reuse
Tongji University
1239 Siping Road, Shanghai 200092 (P. R. China)
Fax: (+86)21-65981376
E-mail: xmzhao08@mail.tongji.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402388.
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Communication
nation products 6a–e were ob-
tained in 8–15% yields (Table 2,
entries 1–5). Lower reaction tem-
peratures led to improved levels
of enantioselectivity (Table 2,
entry 1 and entry 4, 93% and
90% ee, respectively, at 258C). A
range of amines, including n-bu-
tylamine, isopropylamine, and al-
lylamine, were explored by using
(E)-methyl 5-phenylpent-2-enyl
carbonate (2 f) as the substrate
and the corresponding products,
4 f[21] and 4g–i, were obtained in
good to excellent yields (70–
94%) with excellent levels of re-
gioselectivity (95:5–98:2) and
76–80% ee; a trace amount of
6 f–i, respectively, was found in
these cases (Table 2, entries 6–9).
Alkyl-substituted allylic carbo-
nates, such as 2g and 2h, were
well tolerated, providing the
branched carbamates 4j and 4k
in 88 and 78% yield, with the
same excellent regioselectivity
(96:4) and 88 and 78% ee, re-
spectively (Table 2, entries 10
and 11).
Table 1. Optimization of the reaction conditions.[a]
Entry
L*
Base
Solvent
4a/5a[c]
Yield 6a [%][c]
Yield 4 [%][b]
ee[d]
[e]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1 f
–
CsF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
--
32
–
–
–
n.d.[f]
85:15
n.d.
Trace
55
25
46
28
39
29
–
–
80
43
73
23
18
69
Trace
Trace
n.d.
88
n.d.
85
n.d.
28
78
–
Cs2CO3
K2CO3
K3PO4
KOAc
DABCO
DBU
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
15
n.d.
12
trace
trace
25
38
21
7
13
5
11
15
11
–
90:10
90:10
83:17
55:45
--
9
THF
10
11
12[g]
13[h]
14
15
16
17
18
CH2Cl2
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
--
–
94:6
94:6
89:11
85:15
57:43
86:14
n.d.
86
93
80
90
79
80
n.d.
n.d.
n.d.
–
[a] All the reactions were carried out on a 0.2 mmol reaction scale by using 2 mol% of [{Ir(cod)Cl}2], 4 mol% of
1a–f, 120 mol% of base, 120 mol% of 2a, and 100 mol% of 3a (0.1m). [b] Yields of isolated product. [c] Deter-
mined by 1H NMR analysis of the crude reaction mixture. [d] Determined by HPLC analysis by using a chiral
column (Chiralcel OD-H column). [e] Not detected. [f] n.d.=not determined. [g] At room temperature. [h] At
508C.
Table 2. Scope of allyl carbonates 2 and amines 3.[a]
Entry
2
R
R1
4/5[c]
Yield 6
[%][c]
Yield 4 ee
[%][b]
[%][d]
1[e]
2
2a
Ph
nPr
nPr
nPr
94:6
94:6
96:4
93:7
89:11
95:5
98:2
97:3
6a, 13
6b, 9
6c, 8
6d, 15
6e, 11
6 f, trace
6g, trace 4g, 80
6h, trace 4h, 70
4a, 43
4b, 77
4c, 80
4d, 54
4e, 52
4 f, 94
93
90
87
90
88
77
76
76
80
88
78
94
Figure 1. Chiral ligands 1a–f.
2b 4-ClC6H4
2c 4-BrC6H4
2d 3-MeOC6H4 nPr
3
4[e]
5
phenyl ring of the amine moiety, resulted in poor yields (18–
23%, Table 1, entries 14 and 15). Notably, the reaction com-
pletely failed when both 1e and 1 f were employed (Table 1,
entries 17 and 18).
2e
2 f
2 f
2 f
2 f
2g
2h
2c
4-MeC6H4
PhCH2CH2
PhCH2CH2
PhCH2CH2
PhCH2CH2
nPr
nPr
nPr
iPr
nBu
allyl 97:3
nPr
nPr
iPr
6
7
8
9
10
11
12
6i, trace
6j, trace
6k, trace
4i, 75
4j, 88
4k, 78
4l, 58
We further examined the scope and generality of this reac-
tion. Aryl-substituted substrates, such as (E)-cinnamyl methyl
carbonate 2a and allylic carbonates (2b–e), with either elec-
tron-withdrawing groups (4-Cl and 4-Br) or electron-donating
groups (3-OCH3 and 4-CH3) gave the respective products in
moderate to high yields (52–80%) with high levels of regiose-
lectivity (89:11—96:4) and enantioselectivity (88–93% ee); ami-
96:4
96:4
85:15 6l, 7
Et
4-BrC6H4
[a] Reaction conditions as in Table 1, entry 12 at either 258C or 358C.
[b] Yield of isolated products. [c] Determined by 1H NMR analysis of the
crude reaction mixture. [d] Determined by HPLC analysis by using a chiral
column or GC. [e] The reaction was performed at room temperature.
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We present three piece of ex-
perimental evidence for the
above mechanistic hypothesis:
1) a domino reaction of (E)-(3-
chloroprop-1-enyl)benzene,[24]
CO2, and nPrNH2 occurred in the
presence of p-allyl–Ir complex
(1aa, 0.20 mol%, prepared ac-
cording to Helmchen’s meth-
od)[7d] and K3PO4 in DMSO at
258C, providing the same
S
product 4a (Scheme 4) as that
for entry 1 of Table 2; 2) under
the optimized conditions, using
4a/2a (1:2) as the reactants, iso-
merization of 4a into thermody-
namically stable 5a (4a/5a 94:6)
Figure 2. X-ray structure of (S)-4l.
X-ray analysis of 4l[22] revealed that its absolute configura-
tion is S (Figure 2).
This methodology was extended to bis(allylcarbonates) and
the synthesis of (3R,7R)-nona-1,8-diene-3,7-diyl bis(propylcarba-
mate) (4m) was demonstrated (Scheme 2). The domino reac-
tion of the bis(allylcarbonate) 2j under the optimal reaction
conditions occurred twice to give the corresponding product
4m in 80% yield with 99% ee and 1:3.7 d.r.
Scheme 4. Domino reaction of (E)-(3-chloroprop-1-enyl)benzene, propyl-
amine, and CO2.
Scheme 2. Domino amidation—allylation reaction of 2j.
1
was confirmed through H NMR analysis of the crude products;
the branched allylic alcohol,[25] but not amine 6a, was ob-
For the mechanism, we propose that it begins by insertion served as well; 3) using 5a as the reactant, no reaction oc-
of iridium into the allyl–oxygen bond to liberate an ionized p- curred (see the Supporting Information).
allyl–Ir complex (Int A), methoxide, and CO2. Subsequently, an
In summary, we have developed a domino reaction of allyl
amidation reaction between CO2 and an amine occurs to form carbonate, amine, and recycling CO2, a reaction that occurs in
a carbamate ion[23] (ÀOCONHR), which in turn attacks Int A to the presence of K3PO4 and DMSO with the assistance of an iri-
yield an allyl carbamate 4 and regenerating the catalyst. In the dium complex and provides the branched allyl carbamates 4 in
presence of K3PO4, Int A is directly attacked by the amine to good yields with excellent levels of regioselectivity and good
give an extrusion product, allylamine 6 (Scheme 3).
to excellent levels of enantioselectivity. Essential for the suc-
cess of the reaction was the role of CO2, which is generated in
the course of this reaction and then recycled.
Experimental Section
General procedure
The yellow iridacycle[5b] made from [{Ir(cod)Cl}2] (0.004 mmol,
2 mol%) and phosphoramidite ligand 1a (0.008 mmol, 4 mol%)
was placed in a dry Schlenk tube filled with argon. Then, allylic car-
bonate 2 (0.24 mmol, 120 mol%), K3PO4 (0.24 mmol, 120 mol%),
amine 3 (0.20 mmol, 100 mol%), and DMSO (2.0 mL) were added.
The reaction mixture was stirring at 358C. After the completion of
the reaction, the crude reaction mixture was diluted with water
and extracted with ethyl acetate. The crude residue was purified
Scheme 3. Proposed mechanism.
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´
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Keywords: allylcarbamates
reactions · enantioselectivity · iridium
·
carbon dioxide
·
domino
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Chem. Eur. J. 2014, 20, 1 – 7
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Communication
c) Pd-catalyzed reaction of an allylic carbonate in the presence of an
amine gave allylic carbamates as the by-products, see: J. E. Bꢃckvall,
K. L. Granberg, A. Heumann, Isr. J. Chem. 1991, 31, 17.
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[24] (E)-(3-chloroprop-1-enyl)benzene was successfully used as the allylating
agent for the preparation of 1aa; see ref. [7a].
Received: February 27, 2014
Published online on && &&, 0000
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Communication
COMMUNICATION
&
Synthetic Methods
S.-C. Zheng, M. Zhang, X.-M. Zhao*
&& – &&
Enantioselective Transformation of
Allyl Carbonates into Branched Allyl
Carbamates by Using Amines and
Recycling CO2 under Iridium Catalysis
Branching out: Allyl carbonates can be
enantioselectively transformed into
branched allyl carbamates by using
amines and recycling CO2 in the pres-
ence of an Ir complex and K3PO4.
Branched allylcarbamates are obtained
in fair to excellent yields, up to 98:2 re-
gioselectivity, and up to 93% ee (see
scheme; cod=1,4-cyclooctadiene).
Iridium Catalysis
In their Communication on page && ff., X.-M. Zhao et al.
show that allyl carbonates can be enantioselectively
transformed into branched allyl carbamates in the presence
of an iridium catalyst and an amine. CO2, which is initially
expelled upon oxidative addition, is recycled by being
trapped by the amine, the resulting carboxylate adduct
undergoing nucleophilic addition with the allyl–iridium
intermediate.
Chem. Eur. J. 2014, 20, 1 – 7
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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