I2-Mediated oxidative bicyclization of 4-pentenamines to prolinol carbamates with CO2 incorporating oxyamination of the C=C bond

Article abstract of DOI:10.1039/c7gc01992k

A metal-free oxyamination reaction of alkenes with ambient CO₂ is reported. In the presence of I₂ and DBU, CO₂ is applied in situ as a protecting group to regulate the nucleophilicity of the amino group and facilitate the bicyclization of 4-pentenamines with high chemoselectivity. Moreover, this reaction provided a feasible approach to prepare prolinol carbamates with good tolerance of functional groups and high efficiency under mild conditions.

Full text of DOI:10.1039/c7gc01992k

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I2-Mediated Oxidative Bicyclization of 4-Pentenamines to Prolinol
Carbamates with CO2 Incorporating Oxyamination of C=C Bond
Sheng Wang,a Xiaowei Zhang, a Chengyao Cao, a Chao Chen,* a and Chanjuan Xi* a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A metal-free oxyamination reaction of alkenes with ambient CO2 reported for the construction of prolinol derivatives via
is reported. In the presence of I2 and DBU, CO2 is applied in situ as sequentially intramolecular amination of alkene and oxygen-
protecting group to regulate nucleophilicity of amino group and based nucleophiles. For example, Cu,5a,5d Os,5b Pd,5c,5f Au,5e and
facilitate the bicyclization of 4-pentenamines with high Mn5g were applied as catalyst to realize synthesis of prolinol
chemoselectivity. Moreover, this reaction provided a feasible derivatives in the presence of oxidant (Figure 2, a), which
approach to prepare prolinol carbamates with good tolerance of provided a straightforward method for the construction of the
functional groups and high efficiency under mild conditions.
prolinol derivatives. Metal-free oxyaminations to construct this
motif were also disclosed.6 Although all works mentioned
above for preparation of the prolinol derivatives are efficient,
preprotected amine was required, which increase synthetic
steps and decrease atom-economy. In addition, in some cases,
reaction proceeded in both 5-exo and 6-endo cyclization
modes to afford a mixture of products. Therefore, the
development of a general and sustainable method for the
synthesis of prolinol derivatives from easily available starting
materials with good chemoselectivity and regioselectivity is
highly desired.
Prolinol and derivatives have been widely utilized in many
asymmetric organic synthesis as ligands, catalysts or chiral
auxiliaries.1 Furthermore, the prolinol and derivatives are also
significant scaffold frequently existing in the natural products2
and pharmaceutical3, for instance, (+)-allopumiliotoxin2a and
stemonine2f as natural products contain prolinol skeletons.
Iminoalditol served as reversible inhibitors of D-glycosidase3b
and A-84543 displayed affinity for central neuronal nicotinic
acetylcholine receptors3a (Figure 1). Consequently, many
processes have been developed for efficient synthesis of
prolinol derivatives. Traditionally, prolinol derivatives are often
obtained from transformation of prolines. This method has a
limitation for the synthesis of functionalized prolinols due to
the weakness of getting functionalized prolines on the
pyrrolidine.
Carbon dioxide (CO2) is an abundant, low-toxic,
environment-friendly and sustainable one-carbon synthons
possessing great potential in preparation of value-added
products. It has been utilized in the synthesis of carbamates7
and various important heterocycles such as oxazolidones and
cyclic carbonate.8 However, relatively few chemical processes
incorporating CO2 were reported, which may be attributed to
low reactivity of CO2. Recently, combination of allylamines
with CO2 and some electrophiles to afford functionalized 2-
oxazolidinones has been reported.9 These works have well
applied oxygen-based nucleophilic center generated from
reaction of amine with CO2 and other electrophilic reagent
such as t-BuOI,9a Togni’s reagents,9b and perfluoroalkyl
iodides9c to add to alkenes. We envisioned that 4-pentenamine
to react with CO2 may realize simultaneous oxyamination of
C=C bond to form prolinol carbamate via tandem cyclization,
which would provide a strategy to construct a variety of
prolinol derivatives. Although oxyamination of 4-pentenamine
using excess sodium bicarbonate in several days to form
prolinol carbamate has been reported,10 it is still a challenge
for direct incorporation of CO2 as synthon to form the prolinol
carbamate via oxyamination of C=C. Inspired by the above-
Me
n-C3H7
HO
NH2
H
N
H
Me
H
N
H
Me
O
N
H
O
N
H
O
O
OH
HO
OH
O
N
H
OH
A-84543
stemonine
D-glycosidase inhibitors
(+)-allopumiliotoxin 267A
Fig.1 Selected biologically active prolinol derivatives
Due to the significance of alkenes for the past decades,4
transition-metal-catalyzed oxyamination of alkenes has been
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mentioned successes and our ongoing project on CO2 with five-membered
Journal Name
(
1h
)
and six-membered
(1i)
chemistry,11 herein, we reported a oxidative bicyclizcation of cycloalkeneamines could also generate the corresponding
diverse 4-pentenamines with ambient CO2 in the presence of I2 spiro-bicyclic carbamates 2h and 2i in 81% and 70% yield,
and DBU at room temperature to produce prolinol carbamate, respectively. Substrate containing different substituents of R1
which are easily transferred into useful prolinol derivatives and R2 (1j
, 1k) were also tested. Monosubstituted substrate 1j
(Figure 2, b).
afforded a product with high diastereoselectivity. The NOE
spectrum showed the major one
Table 1 Optimization of reaction conditionsa
Fig.2 Oxyamination of alkenes
Initially, 2,2-diphenyl-4-pentenamine 1a was selected as
starting material. In principle, the oxidant is definitely needed
to take away two redundant electrons for this reaction. We
first examined the reaction of 1a with CO2 (1 atm) in the
presence of PhI(OAc)2 as an oxidant and DBU as the base in
acetonitrile at room temperature (entry 1). Although trace
desired product 2a was observed by GC-MS, the oxidation of
amine to the corresponding nitrile was obtained as major
product in this case,12 which means PhI(OAc)2 was too reactive
for this reaction to obtain the desired product. Other
hypervalent iodine(III) reagents such as PhI(OTf)2 and
PhI(OH)OTs were employed and the reaction failed to give
desired product. We thought that iodine as a milder oxidant
might lead to desired product. To our delight, when iodine was
added and the desired product 1a was obtained in 94 % NMR
yield (entry 4). Base might enhance the nucleophilicity of an
amine or activate CO2. Several bases such as tBuOK, TBD,
Cs2CO3 and Et3N were explored (entries 4-8), we were
delighted to identify DBU as the best choice (entry 4). When
DMF, THF, EtOH, and EtOAc were used as solvent, the yield of
2a decreased (entries 9-12), nevertheless, EtOH and EtOAc
demonstrate potential to replace MeCN as greener solvents.
Furthermore, when H2O was used as solvent, the yield of 2a
was obtained in 9% (entry 13). When the temperature was
increased, 2a diminished (entry 14). When amount of I2 or DBU
was reduced, yield of 2a decreased (entries 15-16), which
suggested that sufficient amount of I2 and base were
necessary to facilitate the bicyclization. The structure of 2a
was confirmed by X-ray crystallography (see supporting
information).13
aReaction conditions: 1a (0.5 mmol), 1 atm CO2, oxidant (0.5 mmol, 1.0 equiv.), base
(1.0 mmol, 2.0 equiv.), solvent (5 mL), room temperature (rt), 16 h. bNMR yields were
detected with dibromomethane as an internal and isolated yield was demonstrated in
parenthesis. c60 oC. dI2 (0.25 mmol, 0.5 equiv.). eBase (0.5 mmol, 1.0 equiv.). f8 h. DBU =
1,8-diazabicyclo[5.4.0]undec-7-ene, TBD = 1,5,7-triazabicyclo[4.4.0]dec-5-ene , DMF =
N,N-dimethylformamide, DCM = dichloromethane, THF = tetrahydrofuran.
is large group (Ph versus H) in the cis product. When
disubstituted substrate 1k was employed and the product 2k
was isolated in 69% yield with diastereoselectivity in 5:4 ratio.
Electronic effect of R1, R2 was obvious in this reaction.
Substrate with electron-donating group such as methyl (1h) in
benzene ring was more reactive to produce the product 2d in
88% isolated yield while electron-withdrawing group such as
fluorophenyl (1e) and ester group (1g) gave the products 2e
and 2g in 65% and 47% yield, respectively. Except for
substrates with different functional groups of R1 and R2,
cyclosubstrates 1l and 1m also underwent tandem cyclization
smoothly and afforded tricyclic carbamates 2l and 2m in good
yields, but moderate diasteroselectivity were obtained.
Notably, the two isomers could be easily seperated by flash
chromatography on silica gel. We further explored the
substrates with more steric hindrance in C=C bond, which is
more challenging in difunctionalization reaction. When 1,1-
With optimal reaction condition (Table 1, entry 4) in hand,
we explored the substrate scopes and the representative
results are summarized in Scheme 1. Substituent effect of R1
and R2 was first investigated. Starting material bearing phenyl
(
1a), benzyl (1b), allyl (1c), p-tolyl (1d), p-flurophenyl (1e), 2-
thiophene (1f), ester (1g) of R1 and R2 proceeded smoothly to
afford the desired products 2a-2g in good yields. Substrates
2 | J. Name., 2012, 00, 1-3
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disubstituted alkeneamine 1n was treated with CO2 under the 2,2-diphenyl-4-pentenamine 1a with one equivalent of DBU
conditions and expected carbamate 2n was formed in 40% under ambient CO2 in CD3CN at room temperature, the
yield. When 1,2-disubstituted alkenylamine 1o was employed, reaction was monitored by NMR. The NMR data showed that
the reaction failed to give the desired product 2o. Notably, substrate 1a was totally transformed to carbamic salt
A. ESI
substrate with longer carbochain such as 5-hexenamine 1p data also supported formation of compound A (see supporting
was used and tetrahydro-1H-oxazolo[3,4-a]pyridin-3(5H)-one information). Subsequently, addition of another one
2p was obtained in 24% yield even at 60 oC for 36 h. When equivalent of DBU and one equivalent of I2 to the reaction
non-substituted 4-pentenamine 1q was employed, the mixture containing carbamic salt
A and desired product 2a was
reaction failed to afford the corresponding product 2q, which obtained in 90 % yield (Scheme 2, a). Secondly, when the
may be contributed to flexibility of chain without substituents. reaction of 1a was proceeded under the standard conditions in
Furthermore, 2b, 2c, 2f, and 2h could also be generated in the absence of CO2, any well-defined products were not
good yields when the reaction was conducted in EtOH or observed,14 unfortunately. Whereafter, CO2 was injected to
EtOAc.
the above reaction mixture and no any product was obtained
(Scheme 2, b). These experiments demonstrated that CO2 is
necessary to tune the reactivity of amino group in this tandem
oxidative cyclization. Finally, radical scavengers such as TEMPO
were added in the reaction under the standard conditions, the
desired product 2a was also observed in 85% yield (Scheme 2,
c), which rules out the possibility of radical pathway in this
reaction.
R3
R3
O
1.0 equiv. I2
2.0 equiv. DBU
R2
R1
NH2
R4
R1
R2
N
n
+
CO2
O
CH3CN, rt, 16h
(1 atm)
n
R5
R4
2
R5
1
O
O
Bn
Bn
N
N
Ph
Ph
O
O
2b, 65% (80%)a
(78%)b, (71%)c
O
2a, 84%
X-ray of 2a
O
O
N
O
p-F-Ph
N
Tol
Tol
N
O
O
p-F-Ph
2c, 83% (91%)a
(87%)b, (84%)c
O
2e, 65%
2d, 88%
O
S
S
O
N
O
N
O
MeO2C
MeO2C
N
O
2h, 81% (88%)a
(80%)b
2f, 68% (78%)a
(75%)b, (61%)c
O
2g, 47%
O
O
Ph
H
N
O
N
Ph
Scheme 2 Control experiments
O
N
O
Me
2j, 46% (d.r.> 20:1) 2k, 69% (d.r.= 5:4)
In light of all results, we proposed a possible mechanism for
this reaction (Scheme 3). First, in the presence of DBU, amino
2i, 70%
O
O
O
Ph
Ph
N
group is trapped by CO2 to generate the carbamic salt
has been identified by NMR and ESI. Then, C=C bond may be
activated by I2 to generate intermediate , and meanwhile
reversible proton exchange occurs to generate active
intermediate C, which leads to iodonium intermediate and
releases [DBUH]+I-. The nitrogen atom attacks the C=C bond via
5-exo-trig to form compound However, possible
A, which
O
N
N
O
O
Me
B
2l, 75% (d.r.= 2:1)
2m, 64% (d.r.= 3:2)
2n, 40%
O
O
O
D
Ph
N
O
N
Ph
Ph
Ph
N
O
O
E
.
intermediate with N-I bond generated from nucleophilic
substitution of N atom and I2 directly could not be excluded,
2p, 24%d
2q, 0%
Me
2o, 0%
Scheme 1 Scope of 4-pentenamines.a aThe standard reaction conditions, yields
are those of the isolated products, the NMR yields are shown in parentheses.
bThe reaction proceed in the EtOH. cThe reaction proceed in the EtOAc. d60 oC,
36 h.
which is capable of yielding compound
Then, another molecular DBU captures the proton from
carbamic acid The iodine of
to form intermediate F 15
intermediate is intramolecularly substituted by the oxygen of
the carbamate to give oxyamination product
E via iodocyclization.
E
.
F
Additional experiments were performed to gain more
insight of this reaction. According to previous work, it is
suspected that
F
2.
carbamic salt
A generated from amine with CO2 might be the
possible intermediate in this reaction. As a result, we first
mixed
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Scheme 3 Proposed reaction mechanism
Considering significance of prolinol and derivatives in organic
synthesis, gram-scale experiment and transformation starting
from prolinol carbamate are worthy of investigation. We
conducted the reaction in gram-scale and obtained prolinol
carbamate in comparable yield (Scheme 4). The reaction of 2a
9
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methylprolinol in 89% yield.
3 in
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Conclusions
In summary, metal-free oxidative oxyamination of 4-
pentenamines with ambient CO2 was realized efficiently to
generate privileged prolinol carbamates via bicyclization under
a mild condition. CO2 plays a vital role in tuning the reactivity
of amine to achieve oxyamination of alkene with excellent
regioselectivity. This reaction is scalable and compatible with
various functional groups. Furthermore, prolinol carbamates
could be easily transformed into prolinol derivatives.
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Acknowledgements
This work was supported by National Natural Science Foun-
6
7
dation of China (21472106, 91645120
, 21372138 and
21672120), the Fok Ying Tong Education Foundation of China
(Grant No. 151014) and the National Key Basic Research
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