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FULL PAPER
Stereodivergent Carbamate Synthesis by Selective In Situ
Trapping of Organic Carbonate Intermediates
Wusheng Guo,[a][b] Victor Laserna,[a][b] Eddy Martin,[a] Eduardo C. Escudero-Adán[a] and Arjan W.
Kleij*[a][c]

Abstract: Trans-carbamate structures can be prepared in a
diastereoselective approach by a judicious one-pot combination of in
situ prepared organic carbonates and suitable amine reagents under
appropriate reaction conditions. This unprecedented approach allows
for stereo-divergence from a single oxirane substrate with easy
access to both cis and trans carbamate isomers with high stereoselectivity (dr > 19:1). Key to control the diastereo-selective nature of
the conversions leading to the trans carbamates is the in situ
formation of trans-configured oligo/polycarbonates through Alcatalysis providing the targeted products after aminolysis. The present
results demonstrate the valorization of a renewable carbon-based
reagent (CO2) into new valuable scaffolds and an unusual stereocontrol exerted through carbonate intermediates. A series of control
experiments support the proposed mechanistic rationale towards the
trans-carbamate products which is based on the trapping of an in situ
formed trans-configured oligo/polycarbonate.

conditions when using simple alkylamines and mono-substituted
COC reagents.[7] The efficient aminolysis of di- and tri-substituted
COCs has been seldom reported and should proceed with
retention of configuration (Scheme 1). Cis-carbamates are
produced straightforwardly from their respective cis-COCs
derived from cyclic oxiranes and CO2 for which the synthetic
methodology has advanced significantly over the years. [8]
However, the trans-carbamates cannot be easily accessed as
there exists no general method for the formation of trans-COCs
from cyclic oxirane precursors and CO2.[9]

Introduction
Carbamates are ubiquitous scaffolds found in a range of synthetic
products of general use in agrochemical and pharmaceutical
applications,[1] and in polymer science.[2] Their seemingly
privileged incorporation into a wide range of chemical structures
has been enabled by methodologies that are conventionally
based on the use of toxic reagents including phosgene and its
derivatives.[3] Driven by the need for more sustainable
approaches towards carbamates, both linear[4] and cyclic organic
carbonates[5] have become common reaction partners for amines
to afford these widely targeted molecules. The latter category of
carbonate structures can be easily obtained through epoxide/CO 2
coupling reactions[6] thus providing a way towards the valorization
of a waste combustion product being safe, cheap and readily
accessible.
The aminolysis reaction of cyclic organic carbonates (COCs)
is well-documented and typically proceeds under mild reaction

[a]

[b]
[c]

Dr. W. Guo, V. Laserna, Dr. E. Martin, E. C. Escudero-Adán, Prof.
Dr. A. W. Kleij, Institute of Chemical Research of Catalonia (ICIQ),
The Barcelona Institute of Science and Technology, Av. Països
Catalans 16, 43007 Tarragona, Spain
E-mail: akleij@iciq.es
These authors contributed equally.
Prof. Dr. A. W. Kleij
Catalan Institute of Research and Advanced Studies (ICREA), Pg.
Lluís Companys 23, 08010 Barcelona, Spain
Supporting information for this article is given via a link at the end of
the document

Scheme 1. Divergent approach towards carbamate synthesis using carbonate
intermediates via a sequential (12; cis) or one-pot (1+2; trans) approach. Cn
stands for a cycloalkyl skeleton with n representing the number of C-atoms.

In order to chase this synthetic challenge, we envisioned that
in situ formation of a trans oligo/polycarbonate from cyclic
oxiranes and CO2 could trigger the formation of a trans-carbamate
by aminolysis. The use of appropriate catalytic copolymerization
conditions would selectively lead to a growing trans-configured
oligo-carbonate chain with subsequent aminolysis of the metalbound oligomer giving rise to trans-carbamates in a one-pot
approach (Scheme 1). Here we report on a general, stereodivergent method for a wide range of functionalized carbamate
structures derived from organic carbonate intermediates. Both the
cis and trans diastereoisomers of these carbamate scaffolds can
be formed with excellent stereo-control, with the family of unusual
trans-configured products being generated through a
conceptually new approach.

FULL PAPER
Results and Discussion
As part of a screening study we first considered the one-pot
conversion of cyclopentene oxides (cyclopentene oxide, CPO,
and 3,4-epoxyfuran), three different primary amines R´-NH2 (R´ =
Cy, Bu, Oc) and CO2 using Al-catalysis (Table 1).[10] Suitable
nucleophilic additives, including NBu4Br (Br) and PPNCl (Cl; PPN
= bis(triphenylphosphine)iminium) were used as these were
previously shown to have the best potential in the formation of
polymeric carbonates using aminotriphenolate complexes. [11] It
should be noted that cyclohexene oxide (CHO) generally has
excellent copolymerization potential, whereas cyclopentene
oxides (Y = C or O; Scheme to Table 1) are known to be more
challenging substrates. Thus, we envisioned that successful in
situ formation of oligomeric carbonates derived from
cyclopentene oxides would serve better to validate a more
general approach towards trans carbamates.
First, 3,4-epoxyfuran was investigated using both Al-catalysts
A and B varying the co-catalyst nature and amount, and the
amount of solvent (MEK; methylethyl ketone), see entries 111 of
Table 1. In the absence of Al-catalyst and nucleophile no
conversion was noted (entry 1), while the use of complex A or
nucleophile (TBAB) alone gave poor carbamate yields of 15% and
9%, respectively (entries 2 and 3). Combination of both catalyst
components gave more productive catalysis and afforded the
desired carbamate product in much higher yields of up to 76%
with excellent stereo-control (dr > 99:1, entry 4). Additional
variation of the amount of solvent, changing TBAB for PPNCl or
using Al-complex B (entries 511) did not further improve these
results, and at higher tem-peratures (90ºC, entry 10 versus 4) loss
of stereo-selectivity was noted.
In the case of cyclopentene oxide as substrate (entries 1221),
similar observations were done although in these reactions Alcomplex B combined with PPNCl provided the best compromise
in terms of yield and diastereo-selectivity: the results reported in
Table 1 show that the use of typical copolymerization conditions
(i.e., using Cl as nucleophile, Al-catalyst B, 60ºC)[10,11] leads to the
preferred formation of the targeted trans-carbamate product with
high dr´s of up to 96:4 (entry 14) when n-octylamine is used, and
89:11 in the case of n-butylamine (entry 19). Again, in the
absence of nucleophilic additive (as noted with 3,4-epoxyfuran),
much lower yields of carbamate are noted (entries 16, 17 and 20)
likely to be the result of a relatively slow background reaction
based on a carbamate nucleophile derived from the amine and
CO2 (Figure 1).[12] Longer reaction times do not seem to
significantly affect the yield of carbamate (cf. entries 16 versus 17)
under low-pressure conditions (10 bar).
We therefore tried the carbamate synthesis also under more
severe pressure conditions (40 bar, entry 20 Table 1; cf. entry 21)
and found only a small increase in the yield of the carbamate
derived from CPO. This further confirms that for efficient formation
of the trans-carbamate product under these relatively mild
reaction conditions the presence of an external nucleophile is
required, and offers the major pathway leading to transcarbamate product through in situ trans-carbonate formation.
Table 1. Screening of suitable catalytic conditions to form the trans-carbamate
products from cyclopentene oxides (Y = C, O), Amines R´-NH2 and CO2.[a] Nu

Stands for nucleophile additive, Cy for cyclohexyl, Oc for n-octyl and Bu for nbutyl.

Entry

Y/R1

Cat
[mol%]

Nu
[mol%]

T/t
[ºC]/[h]

Yield
[%][b]

trans/
cis[c]

1[d]

O/Cy





70, 70

<1

-

2[d]

O/Cy

A, 0.5



70, 70

15

>99:1

3[d]

O/Cy



Br, 2.0

70, 70

9

36:64

4

O/Cy

A, 0.5

Br, 2.0

70, 70

76[e]

>99:1

5[d]

O/Cy

A, 0.5

Br, 2.0

70, 70

81

76:24

6

O/Cy

A, 0.5

Cl, 2.0

70, 70

57

79:21

7[f]

O/Cy

A, 0.5

Cl, 2.0

70, 70

55

63:37

8[g]

O/Cy

B, 1.0

Cl, 1.0

70, 70

29

91:9

9[d]

O/Cy

B, 2.0

Cl, 2.0

70, 70

35

57:43

10[f]

O/Cy

A, 0.5

Br, 2.0

70, 70

46

70:30

11[f]

O/Cy

A, 2.0

Br, 2.0

70, 70

26

27:73

12

C/Oc

A, 0.5

Br, 2.0

70, 70

45

72:28

13

C/Oc

B, 0.5

Cl, 1.0

60, 24

7

85:15

14

C/Oc

B, 2.0

Cl, 2.0

60, 60

45[e]

96:4

15

C/Oc

B, 2.0

Cl, 2.0

70, 60

49

83:17

16

C/Oc

B, 2.0



60, 24

5

>99:1

17

C/Oc

B, 2.0



60, 60

9

>99:1

18

C/Bu

A, 0.5

Br, 2.0

70, 70

73[e]

57:43

19

C/Bu

B, 2.0

Cl, 2.0

60, 24

51[e]

89:11

20

C/Bu

B, 2.0



60, 24

4

>99:1

21[h]

C/Bu

B, 2.0



60, 24

11

>99:1

[a]

Conditions: epoxide (4 mmol), R1-NH2 (1.2 equiv; R = Oc, Cy: 4 equiv.; R =
Bu) using 0.5 mL MEK (methylethyl ketone), p(CO2)º = 10 bar. [b] NMR yields
using mesitylene as internal standard. [c] Determined by 1H NMR. [d] Using 1.0
mL of MEK. [e] Isolated yields. [f] Using 2.0 mL of MEK. [g] Neat conditions. [h]
Reaction carried out at 40 bar.

FULL PAPER
Similar behavior in carbamate formation was also noted for
one other cyclic and one acyclic epoxide when they were probed
under similar temperature/pressure conditions (see Tables S1
and S2). In these latter cases good to excellent selectivity for the
trans-carbamates (dr >99:1) was noted validating further the
current approach using an external nucleophilic additive.

Figure 1. Potential background reaction to form a trans carbamate through the
formation of a carbamate nucleophile from an amine and CO2.

Having established a set of suitable reaction conditions
leading selectively to trans-configured carbamates, we then
investigated the scope in more detail (115, see Figure 2). As
reference materials for the assignment of the trans-products
1b15b, the cis-carbamates 1a15a were separately prepared by
prior formation of their cis cyclic carbonates derived from the
respective epoxides under appropriate conditions. [6d] The
aminolysis reaction was then carried out by addition of the
appropriate amine and details are reported in the Supporting
Information. In general, the stereo-control towards the transcarbamates 1b15b is excellent providing the compounds with
dr´s of ≥93:7 (apart from 4b and 13b; dr = 89:11 and 77:23,
respectively). The approach is applicable towards various fiveand six-membered cyclic and also acyclic epoxides (cis-2-butene
oxide, cf. formation of 12b and 13b) without compromising
significantly the diastereo-selectivity of the reaction. Various
functionalized amines are tolerated bearing allyl (2b and 15b),
cyclopropyl (8b), thiophenelyl (9b), and pyridyl (11b) groups
whereas the use of more functional epoxides allows for the
incorporation of potentially useful furan (3b, 5b, 7b and 8b) or
cyclohexenyl (2b and 14b) fragments.
The molecular structures for carbamates 115 (both
diastereoisomers) were fully supported by 1D/2D NMR and IR
spectroscopic techniques, and HRMS; for 5a and 5b the X-ray
molecular structures (see Figure 3 and Supporting Information) [13]
were determined and further supported their cis and trans
assignment in line with the spectroscopic data. The isolated yields
for the cis-carbamates 1a15a are generally higher than those
obtained for the trans diastereo-iosmers 1b15; this suggest that
the one-pot approach with the amine present at the early stage of
the reaction may, as expected, slow down overall kinetics as the
amine may compete with epoxide coordination to the Al-center in
complexes A or B.

Figure 2. Scope for the formation of the trans-carbamates 1b15b. All reactions
were carried out on a 4 mmol scale using 0.5 mL MEK, p(CO2)º = 10 bar, cat/cocat amounts are in mol%, and AlCl = complex A; AlMe = complex B. All dr values
were determined by 1H NMR.

Figure 3. X-ray molecular structures determined for cis-5a (top) and trans-5b
(below) together with part of the numbering scheme.

FULL PAPER

Scheme 2. Proposed mechanism for the formation of trans-carbamates through (1) formation of a trans-configured cyclic carbonate intermediate following
aminolysis, or (2) a stepwise aminolysis of a metal-bound oligo/polycarbonate; pathway (3) shows the control experiment done with an isolated polycarbonate
sample. The cation of the initial nucleophile is not shown here.

The isolated yields for the cis-carbamates 1a15a are
generally higher than those obtained for the trans diastereoiosmers 1b15b; this suggest that the one-pot approach with the
amine present at the early stage of the reaction may, as expected,
slow down overall kinetics as the amine may compete with
epoxide coordination to the Al-center in complexes A or B. In
order to investigate the mechanism of trans-carbamate formation,
several additional experiments were conducted (see Scheme 2).
First, an isolated sample of a copolymer based on CHO/CO2
(cf., path 3, Scheme 2) was subjected to the general reaction
conditions of the one-pot carbamate formation (i.e., in the
presence of [Al] complex A, TBAB, MEK, 70ºC, 18 h) but the
presence of the n-butyl amine did not provoke any observable
aminolysis between 25-80ºC as evidenced by 1H NMR
comparative studies (Supporting Information). Then we decided
to freshly prepare the CHO/CO2 copolymer in the absence of the
amine using [AlCl] catalyst A (0.5 mol%) and TBAB (2.5 mol%) at
70ºC for 24 h; at this stage 1H NMR analysis showed virtually full
conversion into a fully alternating poly(cyclohexene)carbonate
(PCHC, 99% polymer selectivity) with trace amount (1%) of transcyclohexene carbonate (trans-CHC). This copolymer (without
applying the usual work up and with the metal likely still bound to
the polymer)[14] was treated with 3 equiv of n-butyl amine for 10 h
at 70ºC and then again analysed by 1H NMR. Fortunately,
conversion (65%) to the carbamate 1b (dr > 99:1) was noted with
the remaining 35% consisting of a mixture of 85% PCHC and 15%
of trans-CHC (i.e., a clear increase in the cyclic
carbonate/polymer ratio is noted). The relative increase in transCHC can be explained by a depolymerisation of the metal-free
copolymer[15e] when adding the amine which may compete for
coordination to the Al-center (Scheme 2, pathway 1). The

decoordinated polymer then leads to trans-CHC by a relatively
fast alkoxide back-biting process towards the trans-carbonate and
in situ aminolysis to give trans-configured 1b as product.
A similar polymer degradation/aminolysis sequence (Scheme
2, pathway 1) seems plausible for the acyclic cis-2-butene oxide
case. Control experiments were performed in the absence of
amine reagent following the reaction conditions reported for 12btrans and 13b-trans (Figure 2). The 1H NMR analysis showed the
mixture to contain only the cyclic carbonate product with a trans
selectivity of up to 83% which is in reasonable agreement with the
reported dr values for the two carbamate products derived from
cis-2-butene oxide. Copolymers derived from cis-2-butene oxide
have been reported recently by Darensbourg et al.[9c] showing that
polymer-selective bifunctional M(salen) catalysts (M = Co, Cr)
give virtually only trans-carbonate upon alkoxide backbiting; this
suggests that the trans-carbamates derived from cis-2-butene
oxide also originate from pre-polymer formation.
However, the diastereoselective formation of transcarbamates using substrates such as CPO and 3,4-epoxyfuran
cannot be explained via pathway 1 (Scheme 2). The group of
Darensbourg has demonstrated that such a pathway is
thermodynamically not feasible due to a high ring strain present
in this trans cyclic carbonate, and therefore preferably results in
degradation towards the starting epoxide and CO2. Under high
pressure conditions, carbonate rather than alkoxide backbiting is
facilitated resulting in exclusive cis-carbonate formation,[15]
excluding trans-carbamate formation through a trans cyclic
carbonate intermediate.
We
have
recently
shown
by
a
combined
experimental/computational approach that the coupling between
limonene oxide (LO; Scheme 3) and CO2 gives exclusive

FULL PAPER
formation of a fully alternating copolymer with a relative high
barrier for (trans) cyclic carbonate formation.[10] In this Almediated copolymerization process the CO2 insertion step was
shown to be rate-limiting, suggesting that possible polymer
degradation would have to proceed through alkoxide back-biting;
both metal-bound and metal-free oligo/polycarbonates based on
LO, however, are kinetically incompetent to give a trans cyclic
carbonate via depolymerisation. This behaviour is reminiscent of
copolymers derived from CPO which also cannot produce transCOC through alkoxide back-biting. Therefore, another
degradation process (i.e., pathway 2 in Scheme 2) leading to
formal trans-carbamate formation was considered.

control is a new tool in the synthesis of CO2-derived fine
chemicals showing potential in other types of synthetic
transformations focusing on this renewable C1 building block.

Experimental Section
Typical procedure for the formation of the cis-carbamates
The cyclic carbonates were first synthesized from carbon dioxide
according to reported literature procedures[6d,17] using Al(III) aminotriphenolate complex [AlCl] as catalyst. After that, the respective carbonate
(1 mmol, 1 eq.) and the corresponding amine (1.2 eq. for non-volatile
amines; 4 eq. for volatile amines) were charged into a 5 mL round bottom
flask and the reaction mixture was stirred at 50-70 ºC for an appropriate
time frame. After the reaction, the analytically pure carbamate product was
isolated by flash chromatography. The Supporting Information contains all
details regarding these cis-carbamates.
Typical procedure for the formation of the trans-carbamates

Scheme 3. Carbamate formation reactions using tri-substituted epoxides as
reagents. Conditions: 4 mmol scale, 0.5 mL MEK, p(CO2)º = 10 bar, cat/co-cat
amounts in mol%. All dr values were determined by 1H NMR.

In order to investigate this in more detail, we envisioned that
carbamate formation from tri-substituted cyclic oxiranes such as
LO (Scheme 3) could serve to mimic the reactivity of CPO and
related cyclic oxiranes. When we subjected both 16 (methylcyclohexene oxide) and 17 (cis-LO) to reaction conditions
typically used for the formation of carbamates 1b15b, we only
observed the formation of trans-carbamates 1820. The origin of
the trans-selectivity is ascribed to an attack of the amine on the
oligo/polycarbonate itself being either metal-bound (pathway 2a)
or metal-devoid (pathway 2b) through aminolysis at a carbonate
linkage retaining thus the trans configuration in the final
carbamate product.[16]

Conclusions
In summary, we here present an unprecedented catalytic
approach towards trans-configured carbamates by selectively
trapping of trans-carbonate intermediates with amine reagents.
Supporting experiments have revealed that either in situ
aminolysis of trans cyclic carbonates obtained through
polycarbonate backbiting or aminolysis of in situ prepared transconfigured (metal-bound) oligo/polycarbonates is a new approach
to exert a high degree of diastereo-selective control. Such a

The respective epoxide (4 mmol, 1 eq.), Al-complex (AlR , 0.52.0%; R =
Cl or Me), amine (1.2 eq. for non-volatile amines; 3 eq. for volatile amines),
TBAB/PPNCl and MEK (1 mL) were charged into a 30 mL stainless steel
autoclave. The autoclave was then subjected to three cycles of
pressurization and depressurization with carbon dioxide (0.5 MPa, 5 bar),
before final stabilization to a pressure of 1 MPa (10 bar). The autoclave
was sealed and heated to 6090 ºC and left stirring for an appropriate time
frame. Then, the autoclave was cooled to rt and carefully depressurized.
The analytically pure carbamate product was then isolated by flash
chromatography. The Supporting Information contains all details regarding
these trans-carbamates.

Acknowledgements
We thank ICIQ, ICREA, and the Spanish Ministerio de Economía
y Competitividad (MINECO) through projects CTQ-2014–60419R, and the Severo Ochoa Excellence Accreditation 2014–2018
through project SEV-2013–0319. Dr. Noemí Cabello, Sofía Arnal,
and Vanessa Martínez are acknowledged for the mass analyses.
WG thanks the Cellex foundation for further financial support and
VL acknowledges the Generalitat de Catalunya for a FI
predoctoral fellowship.
Keywords: aluminium • carbamates • carbon dioxide •
diastereoselective synthesis • organic carbonates
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[17]

Commun. 2011, 47, 212-214; b) D. J. Darensbourg, P. Bottarelli, J. R.
Andreatta, Macromolecules 2007, 40, 7727-7729; c) D. J. Darensbourg,
W.-C. Chung, Macromolecules 2014, 47, 4943-4948; The selective
formation of a trans-carbonate from cis-2-butene oxide was also reported
recently: d) C. J. Whiteoak, E. Martin, E. C. Escudero-Adán, A. W. Kleij,
Adv. Synth. Catal. 2013, 355, 2233-2239.
Al(aminotriphenolate)
complexes
are
effective
epoxide/CO2
copolymerization catalysts, see: L. Peña Carrodeguas, J. GonzálezFabra, F. Castro-Gómez, C. Bo, A. W. Kleij, Chem. Eur. J. 2015, 21,
6115-6122.
For Fe(III) based aminotriphenolates a selective switch in chemoselectivity between the cyclic and polycarbonate was observed in
cyclohexene oxide/CO2 couplings, see: M. Taherimehr, S. M. Al-Amsyar,
C. J. Whiteoak, A. W. Kleij, P. P. Pescarmona, Green Chem. 2013, 15,
3083-3090.
Under the reaction conditions, the amine may react with CO2 to form a
carbamate species which can ring-open the epoxide to form the transcarbamate albeit in very low yield. Thus, this is a much slower reaction
than the ones studied here with added nucleophiles. This background
reaction proceeds somewhat better under much harsher pressure
conditions (i.e., 50 bar) with concomitant formation of substantial
aminoalcohol side-product, see: F. Kojima, T. Aida, S. Inoue, J. Am.
Chem. Soc. 1986, 108, 391-395. Apparently, under the mild conditions
reported here the formation of a carbamate nucleophile derived from the
amine and CO2 is either catalytically incompetent or only formed in very
low concentration. The use of secondary amine reagents leads only to
amino alcohol formation, i.e. aminolysis of the oxirane substrate occurs.
For more details: CCDC-1039414 and CCDC-1423217.
For living copolymerisation see: J. G. Kim, C. D. Cowman, A. M. LaPointe,
U. Wiesner, G. W. Coates, Macromolecules 2011, 44, 1110-1113.
a) D J. Darensbourg, A. D. Yeung, Macromolecules 2013, 46, 83-95; b)
D. J. Darensbourg, A. D. Yeung, S.-H. Wei, Green Chem. 2013, 15,
1578-1583; c) D. J. Darensbourg, S.-H. Wei, A. D. Yeung, W. Chadwick
Ellis, Macromolecules 2013, 46, 5850-5855. See also: d) D. J.
Darensbourg, A. D. Yeung, Polymer Chem. 2014, 5, 3949-3962; e) D. J.
Darensbourg, A. D. Yeung, Polymer Chem. 2015, 6, 1103-1117.
Note that the regio-selectivity for the formation of 18b is different from
the ones observed for trans 19 and 20. This is likely a function of the
amine reagent as the formation of other carbamates from methylcyclohexene oxides using p-methoxy-benzylamine under similar reaction
conditions was shown to give a regio-isomeric mixture (combined yield
32%; regio-selectivity 58:42). See for full details the Supporting
Information.
C. J. Whiteoak, N. Kielland, V. Laserna, F. Castro-Gómez, E. Martin, E.
C. Escudero-Adán, C. Bo, A. W. Kleij, Chem. Eur. J. 2014, 20, 22642275.

FULL PAPER
Entry for the Table of Contents:

FULL PAPER
Cuts both ways: Stereodivergence in carbamate synthesis
has been achieved through a
selective trapping of carbonate
intermediates under Alcatalysis using cyclic oxiranes,
amines and CO2 as reaction
partners. Both trans and cis
carbamates were produced
with dr values >19:1.
Mechanistic investigations support the view of a key transconfigured oligo/polycarbonate
intermediate controlling the
stereo-selectivity in this
conceptually new catalytic CO2
conversion approach.

Wusheng Guo, Victor Laserna,
Eddy Martin, Eduardo C.
Escudero-Adán and Arjan W.
Kleij*

Page No. – Page No.

Stereodivergent Carbamate
Synthesis by Selective In Situ
Trapping of Organic Carbonate
Intermediates