Enantioselective domino reaction of CO2, amines and allyl chlorides under iridium catalysis: Formation of allyl carbamates

Article abstract of DOI:10.1039/c4cc00413b

The enantioselective domino reaction between CO₂ (1 atm), amines and linear allyl chlorides in the presence of an iridium complex, DABCO and toluene at 15 °C was realized, which gave branched allyl carbamates in acceptable to high yields with u

Full text of DOI:10.1039/c4cc00413b

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Enantioselective domino reaction of CO2, amines
and allyl chlorides under iridium catalysis:
formation of allyl carbamates†
Cite this: Chem. Commun., 2014,
50, 4455
Received 17th January 2014,
Accepted 4th March 2014
Min Zhang, Xiaoming Zhao* and Shengcai Zheng
DOI: 10.1039/c4cc00413b
www.rsc.org/chemcomm
The enantioselective domino reaction between CO2 (1 atm), amines and been exploited.5 Especially, an asymmetric version of the domino
linear allyl chlorides in the presence of an iridium complex, DABCO and reaction involving transition-metal mediated allylation represents a
toluene at 15 8C was realized, which gave branched allyl carbamates in big issue to be overcome in the area of CO2 chemistry. Allyl alcohol
acceptable to high yields with up to excellent regioselectivity (99/1) and derivatives (e.g., –OH, –OCO2R, –OAc, –OPO(OR)2 as leaving groups)
94% ee. This is the first example of the synthesis of chiral allyl have been widely used as the allyl substrates.4 However, allyl
carbamates through catalytic domino reactions using CO2.
chlorides have been less employed in iridium-catalyzed allylation
reactions.6 We hypothesized that a domino reaction between a linear
Carbon dioxide (CO2) is an abundant, low cost, and nontoxic allyl chloride, CO2 and a nucleophile such as an amine7 with the
renewable carbon resource; therefore, CO2 fixation for the synthesis assistance of an Ir-complex may occur and it will produce branched
of high value-added compounds has gained increasing attention allyl carbamates8 (Scheme 1, eqn (2)). The traditional synthetic
over the last decade.1 It represents a big challenge in exploring new method of allyl carbamates relies on the reaction of allyl alcohols
concepts1a and opportunities for the academic and industrial devel- with isocyanates.9 Herein, we firstly report a three-component
opment of catalytic processes.2 CO2 has been applied as a carb- domino reaction using CO2 in the presence of an iridium complex.
oxylating agent in the palladium-catalyzed ring-opening cyclization
In an initial exploration of our hypothesis, we carried out a
reaction of vinylaziridines, which are involved in an allylation process domino reaction between CO2 (1, 1 atm), propylamine (3a) and
(Scheme 1, eqn (1)).3 In addition, iridium-catalyzed allylation reac- (E)-(3-chloroprop-1-enyl)benzene (2a) in the presence of an iridium
tion has become a powerful tool for the synthesis of useful synthetic complex (Table 1). We found that the reaction with the species10
intermediates in the last decade.4 Notably, a three component generated from 4 mol% of [Ir(COD)Cl]2 and 8 mol% of Feringa’s
domino reaction involving an iridium-catalyzed allylation has hardly ligand L111,12 in toluene at 15 1C gave the branched allyl carbamate 4a
in both a fair yield and ee value albeit with an excellent regioselectivity
(entry 1). We also encountered a problem that an allylamine 6a was
obtained with 33% yield, which may stem from either the direct allyl
amination of 3a or decarboxylation of 4a (entry 1).13 Moreover, the
formation of 4a and 5a strongly suggested that this domino reaction
occurred under the iridium complex catalysis. The nature of bases
has a crucial impact on the reaction outcome in the presence of the
O-nucleophile.14 As a result, a range of bases including K2CO3, CsF,
DBU, and DABCO was tested (entries 2–5). DABCO gave superior
results, a 51% yield with a high level of both regio- and enantioselec-
tivity (entry 5), whereas the others gave fair outcomes (entries 2–4).
Variation of solvents and reaction temperatures has a dramatic effect
on the efficiency and enantioselectivities of the domino reaction
(entries 6–10). A decrease of the iridacycle10 resulted in lowered
enantioselectivity (entry 11 vs. entry 5). Interestingly, we found that
the enantioselectivity strongly depends upon the ratio of 2a/3a with a
narrow scope. Therefore, the ratio of 2a/3a ranging from 1/1.0 to 1/1.6
was screened (entries 5, 12–14) and among them, 2a/3a 1/1.2 gave the
best results (entry 5).
Scheme 1 Domino reactions involving Pd or Ir-catalyzed allylation reaction.
Department of Chemistry, State Key Laboratory of Pollution Control and Resource
Reuse, Tongji University, 1239 Siping Road, 200092 Shanghai, P.R. China.
E-mail: xmzhao08@mail.tongji.edu.cn
† Electronic supplementary information (ESI) available: Experimental procedures
and analysis data for new compounds, the CIF file of (R)-4i (CCDC 970275). For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc00413b
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Table 1 Screening of the reaction conditionsa
Temp.
(1C)
Time
(h)
Yield of 4ab
(%)
Yield of 6ab
(%)
ee of 4ad
(%)
Entry
L
Additive
Solvent
2a/3a
4a/5ac
1
2
3
4
5
6
7
8
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L1a
—
K2CO3
CsF
Toluene
Toluene
Toluene
Toluene
Toluene
DCM
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.0/1
1.4/1
1.6/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
1.2/1
15
15
15
15
15
15
15
15
5
25
15
15
15
15
15
15
15
15
15
15
12
12
12
12
1
12
2
12
4
1
1
1
1
1
1
1
1
1
1
1
24
36
17
55
33
16
19
14
13
—
7
—
12
18
17
10
14
17
7
97/3
91/9
98/2
92/8
90/10
—
52
53
58
30
94
—
83
—
90
98
73
78
76
72
13
DBU
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
51
Trace
32
THF
85/15
—
CH3CN
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
N.R.f
43
9
78/22
95/5
90/10
90/10
96/4
93/7
80/20
70/30
86/14
—
10
11e
12
13
14
15
16
17
18
19g
20h
24
54
37
52
55
14
13
15
12
22
—
7
ꢀ47
63
N.R.
11
52
—
91/9
90/10
ꢀ57
13
94
a
Reaction conditions: [Ir(COD)Cl]2 (4 mol%), L1 (8 mol%), 2a (0.24–0.32 mmol), 3a (0.20 mmol), base in solvent (2 mL) under 1 atm CO2 at 5–25 1C.
b
c
d
e
Isolated yield. Determined by 1H NMR of the crude product. Determined by a chiral HPLC analysis. The iridacycle made from 2 mol% of
f
g
h
[Ir(COD)Cl]2 and 4 mol% of L1 was used. N.R. = No reaction. [Ir(COD)Cl]2/L6 (1/1) was used in this case. (p-allyl)-Ir-complex L1a was used.
The presence of 2a15 in the system suppressed the decarboxylation
of 4a to 6a, for example, under the identical conditions, 4a (1.0 eq.)
together with 2a (0.2 eq.) gave 4a/6a in a ratio of 15/1; in the absence
of 2a it led to 4a/6a 5/1, respectively. These outcomes indicated that
allylamine 6a is mainly derived from the direct allylic amidation of 3a.
A range of ligands L1–L6 was studied in order to figure out how
structural variation of ligand L has an influence on efficiency and
enantioselectivities. Ligands L211 and L316 both with a bulky group in
either the biphenyl or amine moiety resulted in 4a with poor to
acceptable enantioselectivities (entries 15 and 16). Ligand L412 with a
Fig. 1 Ligands L1–L6 were used in the domino reaction.
simple biphenyl fragment gave rise to 4a with a moderate enantio-
selectivity (entry 17). The PHOX ligand L517 was ineffective in this
domino reaction (entry 18). Ligand L618 with three binaphthol positive effect on both yield and enantioselectivity. (2) Aryl-
moieties gave a fair enantioselectivity (entry 19). Furthermore, substituted allyl chlorides 2e and 2l with the electron-withdrawing
(p-allyl)-Ir-complex L1a (0.8 mol%) generated according to Hartwig’s groups (e.g., 4-Br and 4-Cl) on the phenyl ring led to the branched
method19 was used as a catalyst and it gave rise to similar results as 4e, 4i and 4l in acceptable to good yields and excellent regioselec-
entry 5 (entry 20 vs. entry 5). Ligands L2–L4, and L6 led to a tivities but with lower enantioselectivities (Table 2). (3) Substrate 2j
considerable amount of 6a, which was revealed by the corresponding bearing an aliphatic group was tested and it provided the corre-
ratio of 4a/6a; the formation of 6a was considerably suppressed with sponding allyl carbamate 4j with a lower ee value (Table 2). (4)
ligand L1 (entries 15–17, 19, and 5) (Fig. 1).
Propylamine (3a), butylamine (3b), isopropylamine (3c), but-3-en-1-
The scope of the domino reactions was demonstrated by reacting amine (3d), phenylmethanamine (3e) and aniline (3f) were explored.
allyl chlorides 2a–i and j with a range of amines 3a–d in the Among them, 3a, 3b, 3d, and 3e led to similar results; the bulky 3c
presence of CO2 under the optimized conditions presented in entry gave somewhat lower yields; 3f failed to give the corresponding allyl
5 of Table 1. The results revealed the following facts: (1) 2a and aryl- carbamate 4o (Table 2). (5) Notably, the formation of 6a–j was
substituted allyl chlorides 2b–d, 2f–h with the electron-donating diminished in all cases; trace to 17% yields of 6a–j were obtained,
groups (e.g., 2-Me, 3-Me, 3-MeO and 4-Me) on the phenyl ring gave respectively (see ESI†).
the branched allyl carbamates 4a–d, 4f–h, and 4m–n in fair to good
yields with a high level of both regio- and enantioselectivities determined as R through X-ray crystallography (Fig. 2).21
(Table 2). Amazingly, the process was not hampered by the ortho This finding was further extended to substrates containing double
substituent;20 the presence of –Me in the ortho position had a allyl chloride moieties such as 1,2-bis((E)-3-chloroprop-1-enyl)benzene
The absolute configuration of 4i (94% ee, from hexane) was
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Table 2 Scope of allyl chlorides and aminesa,b,c,d,e
a SN20 reaction of (R)-4k with EtMgBr under Cu/(R)-L1 catalysis was
carried out according to Alexakis’s method22 to form (R,-)-7k along
with an inseparable linear product 8k in 51% yield with 1/3.1 dr, b/l
84/16, and 98% ee (Scheme 2).
We developed a domino reaction of CO2 (1 atm), amines and
linear allyl chlorides in the presence of an iridium complex and
DABCO, which provides the branched allyl carbamates with the
high level of both enantio- and regioselectivities. The method
allows the use of CO2 under mild conditions, shows excellent
ortho substituent tolerance of aryl substrates, and provides a
direct way for the synthesis of chiral allyl carbamates.
This work was supported by the Chinese NSF (21272175).
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