A Metal-Free Synthesis of N-Aryl Carbamates under Ambient Conditions

Article abstract of DOI:10.1002/anie.201504956

The first chemo- and site-selective process for the formation of N-aryl-carbamates from cyclic organic carbonates and aromatic amines is reported. The reactions proceed smoothly under extremely mild reaction conditions using TBD (triazabicyclodecene) as an effective and cheap organocatalyst, thus providing a sustainable and new methodology for the formation of a wide variety of useful N-aryl carbamate synthons in good to excellent yields. Computational investigations have been performed and show the underlying reason for the observed unique reactivity as related to an effective proton-relay mechanism mediated by the bicyclic guanidine base. By relay: The previously unknown site-selective attack of arylamines on cyclic carbonates to deliver N-aryl carbamates as the principal product is reported. The organocatalyst TBD guides an effective proton-relay process, thus mediating a chemoselective formation of the carbamate target under extremely mild reaction conditions. The new methodology represents a sustainable, cheap, and attractive process towards these important N-aryl carbamate synthons.

Full text of DOI:10.1002/anie.201504956

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201504956
German Edition: DOI: 10.1002/ange.201504956
Organocatalysis
A Metal-Free Synthesis of N-Aryl Carbamates under Ambient
Conditions
Wusheng Guo, Joan Gónzalez-Fabra, Nuno A. G. Bandeira, Carles Bo, and Arjan W. Kleij*
Abstract: The first chemo- and site-selective process for the
formation of N-aryl-carbamates from cyclic organic carbo-
nates and aromatic amines is reported. The reactions proceed
smoothly under extremely mild reaction conditions using TBD
(triazabicyclodecene) as an effective and cheap organocatalyst,
thus providing a sustainable and new methodology for the
formation of a wide variety of useful N-aryl carbamate
synthons in good to excellent yields. Computational investiga-
tions have been performed and show the underlying reason for
the observed unique reactivity as related to an effective proton-
relay mechanism mediated by the bicyclic guanidine base.
Scheme 1. Reaction manifold of cyclic organic carbonates with amine
nucleophiles and new reactivity towards NARC formation under mild
reaction conditions.
T
he conversion of carbon dioxide into value-added organic
compounds continues to be a vivid area of research in
academic and industrial settings.[1] The valorization of CO2 is
important to create value from a waste material, and currently
efforts have already shown great potential towards the use of
CO2 to store energy,[2] and as a synthon for the creation of new
polymers[3] and fine-chemicals.[4] Another area of widespread
interest and importance concerns the preparation of organic
carbonates. More recently, focus has been shifted towards the
use of these carbonates as intermediates in organic syn-
thesis.[5] An attractive route towards the conversion of cyclic
carbonates into useful products concerns their aminolysis by
aliphatic amines, thus affording N-alkyl carbamate structures
(Scheme 1).[6] However, the corresponding site-selective
aminolysis induced by aromatic amines to yield N-aryl
carbamates (NARCs) is surprisingly unknown.
on the a-carbon atom (Scheme 1) of the cyclic carbonate,
hence yielding a plethora of decarboxylated side-products
including N-alkylated amines and their derivatives.[7] Rele-
vant studies[8] concerning the reaction of noncyclic dialkyl-
carbonates with aromatic amines were reported, albeit with
a limited scope at temperatures greater than 808C. Thus, it
still remains highly challenging and attractive to selectively
prepare NARCs through a site-specific aminolysis reaction
using aromatic amines (Scheme 1, below) from cyclic carbo-
nates under mild reaction conditions. Such a new and
sustainable process would represent a valuable alternative
to reported (metal-based) processes[9] which require either
harsh reaction conditions or more expensive reagents/metal
precursors, and conventional routes to NARCs based on
isocyanates.[10]
Furthermore, the NARC compounds may offer syntheti-
cally attractive scaffolds as they partially mimic fragments of
pharmaceutical compounds such as Efavirenz and Retigabine
(Scheme 1).[11] Also, thermolysis of NARCs offers a useful,
phosgene-free route towards aryl isocyanates which are key
reagents in the synthesis of polyurethane polymers.[12]
Inspired by this unresolved challenge, we set out to explore
a new preparative method towards NARCs and envisioned
that the use of hydrogen-bond activation of cyclic carbonates
could offer a viable substrate conversion strategy as recently
demonstrated for aminolysis reactions involving alkylami-
nes.[6a] Herein we report on the unprecedented chemoselec-
tive formation of (functionalized) NARCs from cyclic carbo-
nates under extremely mild reaction conditions using aryl-
amines as reagents, thus providing a highly sustainable
method for these important scaffolds.
The low nucleophilic character of aromatic versus ali-
phatic amines poses a huge challenge to prepare NARCs.
Recent work[7] concerning the catalytic reaction between
cyclic carbonates and aromatic amines has revealed that high
reaction temperatures (> 1408C) are needed to achieve
appreciable conversion rates. However, these temperature
requirements significantly compromise the chemoselectivity
of the process with no observable formation of the NARC. At
high reaction temperatures aromatic amines prefer the attack
[*] Dr. W. Guo, J. Gónzalez-Fabra, Dr. N. A. G. Bandeira, Prof. Dr. C. Bo,
Prof. Dr. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ)
Av. Països Catalans 16, 43007 Tarragona (Spain)
E-mail: akleij@iciq.es
Prof. Dr. C. Bo
Departament de Química Física i Inorgànica, Universitat Rovira
i Virgili, Marcel lí Domingo s/n, 43007 Tarragona (Spain)
Prof. Dr. A. W. Kleij
Catalan Institute of Research and Advanced Studies (ICREA)
Pg. Lluís Companys 23, 08010 Barcelona (Spain)
Our initial screening phase (Table 1 and Table S1 in the
Supporting Information) focused on the use of aniline (A)
and propylene carbonate (PC; B) as substrates, and various
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504956.
11686
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 11686 –11690

Angewandte
Chemie
Table 1: Selected entries from the optimization of reaction conditions for
conversion level as noted at 1008C (60%), the chemo-
the organocatalyzed N-aryl carbamate formation from aniline and PC.[a]
selectivity for C was markedly improved.[13]
An increase in TBD loading (Table 1, entries 5 and 6)
showed higher conversion levels but still with substantial diol
formation, therefore we decided to first screen other nitrogen
bases as potential catalysts (entries 7–13) and a previously
reported hydrogen-bond activator (PG).[14] In all cases
studied the conversion levels were (much) lower than those
observed for TBD under similar reaction conditions (see
entry 6). The addition of a solvent gave poorer results and
generally we performed the substrate scope reactions under
neat conditions. However, in some cases a very small amount
of CH3CN (typically 20 mL) was necessary to maintain a liquid
phase (see the Supporting Information).
Finally, the reaction temperature and amount of aniline
were further optimized (Table 1, entries 14–18), and satisfying
results were finally achieved using 30 mol% of TBD at 208C
and 1.5 equivalents of aniline, thus providing 98% PC
conversion and good selectivity for C (yield of isolated
product: 74%). When the reaction was performed at 1408C
with TBD as the catalyst (entry 19), a highly complex mixture
of components was formed with only trace amounts of diol
present; C could not be detected in the crude reaction
mixture. This result strongly suggests that for high chemo-
selectivity towards C and site-selective attack of the aromatic
amine onto PC (Scheme 1), a low temperature combined with
a sufficiently high loading of TBD are required.
Entry
1
A
Cat.
T
B
C
D
(equiv) (mol%)
[8C] Conv. [%][b] Yield [%][b] Yield [%][b]
1.5
–
100
0
0
25
14
40
48
38
10
26
0
0
31
47
19
34
30
15
19
0
2[c] 1.2
TBD (5)
TBD (10)
TBD (5)
TBD (10)
TBD (10)
MTBD (10)
DBU (10)
PG (10)
100 60
100 69
70 60
70 82
70 75
70 26
70 46
3[d] 1.2
4
5
6
7
3
3
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
8
9
70
0
10
11
12
13
14
DMAP (10)
TMG (10)
DABCO (10 70 <2
70 12
70 35
4
19
0
7
16
<2
0
22
25
21
20
20
trace
14
HMTA (10)
TBD (10)
TBD (10)
TBD (30)
TBD (30)
TBD (40)
TBD (30)
TBD (30)
70
0
0
55 72
45 65
20 98
20 99
20 98
140 94
20 95
48
40
76[e]
70
75
0
15[d] 1.5
16
17
18
1.5
3
1.5
Encouraged by the results from the screening phase, the
substrate scope was then investigated using various anilines
and monosubstituted, functional cyclic carbonates[15] as reac-
tion partners (Scheme 2) under mild reaction conditions. In
general, good to excellent yields (up to 93%) of the isolated
NARCs 1–19 were obtained. A variety of functional groups
are tolerated using this procedure, including electron-donat-
ing and, remarkably, even electron-withdrawing groups (F, I,
19[f] 1.5
20[g] 1.5
77
[a] Reaction conditions: 2 mmol of B, the indicated number of equiv-
alents of A, and the catalyst were combined and reacted for 20 h (no
solvent was used); see Table S1 for more entries for the optimization
process. [b] Conversion determined by 1H NMR spectroscopy. Yield
determined by relative integration of the signals of the methyl groups.
[c] 16 h. [d] 40 h. [e] Yield of isolated product is 74%. [f] 16 h, complex
mixture noted by 1H NMR spectroscopy. [g] Performed under anhydrous
conditions. See the Supporting Information for details. DABCO=1,4-
diazabicyclo[2.2.2]octane, DBU=1,8-diazabicyclo[5.4.0]-undec-7-ene,
DMAP=4-dimethylaminopyridine, HMTA=hexamethylenetetramine,
MTBD=1-methyl-1,5,7-triazabicyclo[4.4.0]-dec-5-ene, PG=pyrogallol,
TBD=triazabicyclodecene, TMG=1,1,3,3-tetramethylguanidine.
N-heterocyclic structures as potential organocatalytic activa-
tors. It is important to emphasize that the reaction run at
1008C for 20 hours in the absence of a catalyst (entry 1) did
not show any observable conversion of the substrates and was
in line with the challenging nature of this conversion. We were
pleased to note that at 1008C (entry 2) the use of TBD
(5 mol%) gave an appreciable PC conversion of 60% with
a 25% yield of the targeted NARC product C. However,
under these reaction conditions, substantial formation of the
diol D was also observed. Performing the reaction for a longer
period of time gave only a slightly higher conversion at higher
TBD loading (entry 3, 69%) with significantly higher
amounts of undesired diol being formed. The reaction at
708C (entry 4) showed a promising result when using a higher
amount of aniline: while maintaining virtually the same
Scheme 2. TBD-catalyzed formation of NARCs from monosubstituted
cyclic carbonates (only the major isomer is shown).
Angew. Chem. Int. Ed. 2015, 54, 11686 –11690
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11687

.
Angewandte
Communications
using aniline and butylamine as representative amine nucle-
ophiles, and PC as a carbonate substrate. This data demon-
strates that the reaction of aniline with PC is not feasible
because of an activation barrier of 40.9 kcalmolÀ1 (solid red
line in Figure S1 in the Supporting Information). Moreover,
the calculations also point out that a direct butylamine attack
(dashed red line) is not kinetically favored at room temper-
ature, with a free energy barrier of 33.3 kcalmolÀ1. Therefore,
the presence of water acting as a proton-relay catalyst was
considered (blue traces) as the reactions in general are not
performed under anhydrous conditions. This new mechanism
indeed decreases significantly the barrier for the aniline
pathway from 40.9 to 33.9 kcalmolÀ1, though this value still
remains considerably high for the reaction to occur under
ambient conditions. For the butylamine case, the barrier was
also effectively lowered to 24.2 kcalmolÀ1, thus confirming
the experimental observation that the reaction proceeds
smoothly at room temperature.[6a] To reinforce the accuracy of
the computational method, the obtained structures of the
butylamine pathway underwent single-point CCSD(T) calcu-
lations (Figure S1). The obtained absolute barrier was deter-
mined at 41.5 kcalmolÀ1 (orange line), thereby unequivocally
demonstrating that the presence of water is crucial for the
process to occur and also revealing that the B97-D3 func-
tional, at most, only slightly underestimates the barriers. The
calculated energy barrier for the butylamine attack under
water catalysis using CCSD(T) is 33.2 kcalmolÀ1 and thus
9.8 kcalmolÀ1 higher than observed from B97-D3/6-311G**
(see the Supporting Information).
Scheme 3. TBD-catalyzed formation of the NARCs 20–25 from five-
and six-membered disubstituted cyclic carbonates.
CN, and Ph). Substituents on the various positions on the
aniline scaffold (ortho, meta, or para) were also tolerated. Of
further note is that the synthesis of 2 could be easily scaled up
(60 mmol, 9.2 g) with a slightly improved yield of 79% upon
isolation. The use of carbonates other than PC as a reagent
allows the introduction of various groups (e.g., 12–19)
including useful bromoaryl (13), morpholine (14), alkene
(16–17), and alkyne (19) groups.[16] The molecular structure of
13a was also further supported by X-ray analysis (Supporting
Information).[17] Apart from monosubstituted carbonates,
disubstituted five/six-membered carbonates[5d,15a–b] also
showed good potential as substrates in the formation of
NARCs (Scheme 3; 20–25). The formation of all products
(except 24) proceeded with high levels of stereoretention. The
proposed atom connectivity in 23 (only one regioisomer was
isolated) was fully supported by two-dimensional NMR
analysis. Secondary aromatic amines exhibited much lower
reactivity under the present reaction conditions (see Table S2
in the Supporting Information).
The results for the TBD-mediated reactions (Figure 1)
show that the mechanism for the transformation of butyl-
amine and aniline involves several steps. Both reactions have
remarkably similar barriers, that is, 18.1 and 17.5 kcalmolÀ1
for aniline and butylamine, respectively. These values are
consistent with the reactions taking place at room temper-
ature (see Table 1 and Table S1). The mechanistic pathways
are slightly different for each substrate. The first intermediate
(INT-0) could only be optimized for the butylamine pathway.
Given the screening studies[13] we hypothesized that
formation of nonsymmetrical ureas would be feasible by
treatment of pure NARCs with aniline derivatives under
appropriate reaction temperatures. Indeed, such ureas
(Scheme 4; 26 and 27) are the major products noted (other
Scheme 4. TBD-catalyzed formation of the nonsymmetrical ureas 26
and 27 from NARC 2 (concomitant formation of diol byproduct
observed). Reaction conditions: 1008C, 20 h, TBD (30 mol%).
than the formation of D) when 2 is treated with 3 equivalents
of the respective aniline. These results reinforce the idea that
NARCs are indeed intermediates towards urea formation
under high-temperature conditions, and this corroborates the
observation of rather chemoselective formation of the NARC
product under ambient conditions.
Figure 1. Gibbs free energy profile for the TBD-catalyzed reaction
mechanism of PC with aniline (solid line) and butylamine (dashed
line).
To get better insight into the operative mechanism of the
NARC formation, quantum chemical studies were performed
11688 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 11686 –11690

Angewandte
Chemie
How to cite: Angew. Chem. Int. Ed. 2015, 54, 11686–11690
In this step, the amine approaches the carbonate group, with
the carbon center adopting a tetrahedral geometry. Next, TS-
1 is a mutual step and rate-determining for both pathways.
This transition state constitutes an ion pair comprising
a protonated TBDH+ and an alkoxide (INT-1). Formation
Angew. Chem. 2015, 127, 11852–11856
[1] a) M. Aresta, A. Dibenedetto, A. Angelini, Chem. Rev. 2014,
114, 1709 – 1742; b) N. Kielland, C. J. Whiteoak, A. W. Kleij,
Adv. Synth. Catal. 2013, 355, 2115 – 2138; c) Q. Liu, L. Wu, R.
Jackstell, M. Beller, Nat. Commun. 2015, 6, 5933; d) M. Cokoja,
C. Bruckmeier, B. Rieger, W. A. Herrmann, F. E. Kühn, Angew.
Chem. Int. Ed. 2011, 50, 8510 – 8537; Angew. Chem. 2011, 123,
8662 – 8690; e) C. Martín, G. Fiorani, A. W. Kleij, ACS Catal.
2015, 5, 1353 – 1370; f) Y. Tsuji, T. Fujihara, Chem. Commun.
2012, 48, 9956 – 9964; g) B. Yu, L.-N. He, ChemSusChem 2015, 8,
52 – 62.
[2] a) W. Leitner, Angew. Chem. Int. Ed. Engl. 1995, 34, 2207 – 2221;
Angew. Chem. 1995, 107, 2391 – 2405; b) W. Wang, S. Wang, X.
Ma, J. Gong, Chem. Soc. Rev. 2011, 40, 3703 – 3727; c) G. Centi,
E. A. Quadrelli, S. Perathoner, Energy Environ. Sci. 2013, 6,
1711 – 1731; d) M. Mikkelsen, M. Jorgensen, F. C. Krebs, Energy
Environ. Sci. 2010, 3, 43 – 81.
À
of INT-2 is characterized by an elongated C O bond (1.66 Š)
in the cyclic species and is stabilized by two hydrogen bonds
with the TBDH+. In the subsequent step, INT-3 is produced
and the substrate is now linear, thus retaining still an alkoxide
character. The final step is the proton transfer from TBDH+
to the substrate through the transition-state TS-2. Compared
with the reaction assisted by water, the TBD is a much more
effective proton-relay catalyst,[18] thus providing significantly
lower kinetic barriers. Also, when MTBD (methylated TBD)
is used, much poorer catalysis behavior was noted (Table 1,
entries 6 and 7), thus clearly indicating that hydrogen-bond-
ing stabilization is also crucial in this reaction.
In summary, we have presented a new and highly
attractive route towards the challenging formation of N-aryl
carbamates derived from readily available cyclic carbonates
and aromatic amines under virtually solvent-free and metal-
free conditions. TBD is shown to be an effective organo-
catalyst for the site-selective and chemoselective formation of
the N-aryl carbamate products and DFT studies have
revealed an interesting proton-relay mechanism. The present
methodology is operationally simple, easily scalable, and has
great potential in synthetic chemistry.
[3] a) M. R. Kember, A. Buchard, C. K. Williams, Chem. Commun.
2011, 47, 141 – 163; b) X.-B. Lu, D. J. Darensbourg, Chem. Soc.
Rev. 2012, 41, 1462 – 1484; c) G. W. Coates, D. R. Moore, Angew.
Chem. Int. Ed. 2004, 43, 6618 – 6639; Angew. Chem. 2004, 116,
6784 – 6806; d) L. PeÇa Carrodeguas, J. Gónzalez-Fabra, F.
Castro-Gómez, C. Bo, A. W. Kleij, Chem. Eur. J. 2015, 21,
6115 – 6122.
[4] a) E. Blondiaux, J. Pouessel, T. Cantat, Angew. Chem. Int. Ed.
2014, 53, 12186 – 12190; Angew. Chem. 2014, 126, 12382 – 12386;
b) S. Wesselbaum, U. Hintermair, W. Leitner, Angew. Chem. Int.
Ed. 2012, 51, 8585 – 8588; Angew. Chem. 2012, 124, 8713 – 8716;
c) K. Beydoun, T. vom Stein, J. Klankermayer, W. Leitner,
Angew. Chem. Int. Ed. 2013, 52, 9554 – 9557; Angew. Chem. 2013,
125, 9733 – 9736; d) Y. Li, X. Fang, K. Junge, M. Beller, Angew.
Chem. Int. Ed. 2013, 52, 9568 – 9571; Angew. Chem. 2013, 125,
9747 – 9750; e) Y. Li, T. Yan, K. Junge, M. Beller, Angew. Chem.
Int. Ed. 2014, 53, 10476 – 10480; Angew. Chem. 2014, 126, 10644 –
10648.
[5] a) A. Khan, R. Zheng, Y. Kan, J. Ye, J. Xiang, Y. J. Zhang,
Angew. Chem. Int. Ed. 2014, 53, 6439 – 6442; Angew. Chem.
2014, 126, 6557 – 6560; b) A. Khan, L. Yang, J. Xu, L. Y. Jin, Y. J.
Zhang, Angew. Chem. Int. Ed. 2014, 53, 11257 – 11260; Angew.
Chem. 2014, 126, 11439 – 11442; c) Y. J. Zhang, J. H. Yang, S. H.
Kim, M. J. Krische, J. Am. Chem. Soc. 2010, 132, 4562 – 4563;
d) V. Laserna, G. Fiorani, C. J. Whiteoak, E. Martin, E. C.
Escudero-Adµn, A. W. Kleij, Angew. Chem. Int. Ed. 2014, 53,
10416 – 10419; Angew. Chem. 2014, 126, 10584 – 10587.
[6] For recent rational approaches towards N-alkyl (poly)carba-
mates: a) M. Blain, L. Jean-GØrard, R. Auvergne, D. Benazet, S.
Caillol, B. Andrioletti, Green Chem. 2014, 16, 4286 – 4291; b) L.
Annunziata, A. K. Diallo, S. Fouquay, G. Michaud, F. Simon, J.-
M. Brusson, J.-F. Carpentier, S. M. Guillaume, Green Chem.
2014, 16, 1947 – 1956; c) X. Sheng, G. Ren, Y. Qin, X. Chen, X.
Wang, F. Wang, Green Chem. 2015, 17, 373 – 379.
Experimental Section
Typical NARC formation: the respective carbonate (2 mmol,
1 equiv), amine (1.5 equiv) and TBD (30 mol%) were charged into
a 5 mL round-bottom flask and the reaction mixture was stirred at RT
for the required time. The analytically pure N-aryl carbamate product
was then isolated by flash chromatography. The NARCs were fully
1
characterized by H/13C NMR and two-dimensional NMR spectros-
copy (COSY, HSQC, HMBC and DEPTQ135 when necessary), as
well as IR and HRMS. Full details are provided in the Supporting
Information.
Computational results are available in full format under https://
iochem-bd.iciq.es:8443/browse/handle/100/137.
Acknowledgements
We thank ICIQ, ICREA, and the Spanish Ministerio de
Economía y Competitividad (MINECO) through projects
CTQ-2014-60419-R and CTQ2014-52824-R, and the Severo
Ochoa Excellence Accreditation 2014–2018 through project
SEV-2013-0319. We also thank AGAUR for support through
2014-SGR-409. Dr. Noemí Cabello, Sofía Arnal and Vanessa
Martínez are acknowledged for the mass analyses, and
Eduardo C. Escudero-Adµn and Dr. Marta Martínez-Bel-
monte for the X-ray analyses. N.A.G.B. gratefully acknowl-
edges the COFUND grant 291787-ICIQ-IPMP and W.G.
thanks the Cellex foundation for further financial support.
[7] M. Selva, M. Fabris, V. Lucchini, A. Perosa, M. No›, Org.
Biomol. Chem. 2010, 8, 5187 – 5198.
[8] a) J. Gao, H. Li, Y. Zhang, Y. Zhang, Green Chem. 2007, 9, 572 –
576; b) C. Han, J. A. Porco, Jr., Org. Lett. 2007, 9, 1517 – 1520.
[9] Illustrative examples include: a) E. V. Vinogradova, N. H. Park,
B. P. Fors, S. L. Buchwald, Org. Lett. 2013, 15, 1394 – 1397; b) S.-
Y. Moon, U. B. Kim, D.-B. Sung, W.-S. Kim, J. Org. Chem. 2015,
80, 1856 – 1865; c) L. Ren, N. Jiao, Chem. Commun. 2014, 50,
3706 – 3709; d) M. Feroci, M. Orsini, L. Rossi, G. Sotgiu, A.
Inesi, J. Org. Chem. 2007, 72, 200 – 203.
[10] D. Chaturvedi, Tetrahedron 2012, 68, 15 – 45.
Keywords: anilines · cyclic carbonates · hydrogen bonds ·
N-aryl carbamates · organocatalysis
[11] a) E. S. Carl, G. Seden, K. Peter, Nat. Rev. Drug Discovery 2011,
10, 729 – 730; b) Y. R. Natella, Expert Opin. Drug Metab.
Toxicol. 2010, 6, 95 – 103.
Angew. Chem. Int. Ed. 2015, 54, 11686 –11690
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11689

.
Angewandte
Communications
[12] a) V. L. K. Valli, H. Alper, J. Org. Chem. 1995, 60, 257 – 258;
b) R. Juµrez, A. Corma, H. García, Top. Catal. 2009, 52, 1688 –
1695.
[16] The different regioisomers of the NARC products 12, 13, and 16
can be easily separated by flash chromatography; see the
Supporting Information for details.
[13] In some cases we were also able to detect some diphenylurea as
a by-product between 70–1008C; quantification was troubled by
overlapping signals and only partial solubility of the urea by-
product.
[17] CCDC 1402414 (13a) and 1402415 (27) contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre.
[14] C. J. Whiteoak, A. Nova, F. Maseras, A. W. Kleij, ChemSusChem
2012, 5, 2032 – 2038.
[18] Similar proton-relay mechanisms involving TBD have been
reported, see for instance: A. Chuma, H. W. Horn, W. C. Swope,
R. C. Pratt, L. Zhang, B. G. G. Lohmeijer, C. G. Wade, R. M.
Waymouth, J. L. Hedrick, J. E. Rice, J. Am. Chem. Soc. 2008,
130, 6749 – 6754.
[15] Such functional cyclic carbonates are easily accessible. See:
a) C. J. Whiteoak, N. Kielland, V. Laserna, E. C. Escudero-
Adµn, E. Martin, A. W. Kleij, J. Am. Chem. Soc. 2013, 135,
1228 – 1231; b) C. J. Whiteoak, E. Martin, E. Escudero-Adµn,
A. W. Kleij, Adv. Synth. Catal. 2013, 355, 2233 – 2239; c) C. J.
Whiteoak, N. Kielland, V. Laserna, E. Martin, E. C. Escudero-
Adµn, A. W. Kleij, Chem. Eur. J. 2014, 20, 2264 – 2275.
Received: June 1, 2015
Published online: August 12, 2015
11690 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 11686 –11690