Iron-Catalyzed Synthesis of Five-Membered Cyclic Carbonates from Vicinal Diols: Urea as Sustainable Carbonylation Agent

Article abstract of DOI:10.1002/ejoc.201600476

A new iron-catalyzed synthesis of cyclic carbonates from the corresponding vicinal diols and urea is described. This straightforward transformation allows for the preparation of a variety of five-membered carbonates by employing an inexpensive and environmentally benign iron salt as the catalyst. The use of readily available feedstocks such as urea and polyols makes this a sustainable process. As ammonia is formed as the only stoichiometric byproduct, this process can also be characterized by its high atom economy.

Full text of DOI:10.1002/ejoc.201600476

DOI: 10.1002/ejoc.201600476
Full Paper
Iron Catalysis
Iron-Catalyzed Synthesis of Five-Membered Cyclic Carbonates
from Vicinal Diols: Urea as Sustainable Carbonylation Agent
Miguel Peña-López,[a] Helfried Neumann,[a] and Matthias Beller*[a]
Abstract: A new iron-catalyzed synthesis of cyclic carbonates use of readily available feedstocks such as urea and polyols
from the corresponding vicinal diols and urea is described. This makes this a sustainable process. As ammonia is formed as the
straightforward transformation allows for the preparation of a only stoichiometric byproduct, this process can also be charac-
variety of five-membered carbonates by employing an inexpen- terized by its high atom economy.
sive and environmentally benign iron salt as the catalyst. The
Introduction
allows for high atom efficiency as ammonia is the only stoichio-
metric waste that is formed. The possibility of the regeneration
of the starting urea by treating such a byproduct with carbon
dioxide would improve the sustainability of the process,[12]
whereby urea becomes a potential feedstock for the indirect
use of CO2 (Scheme 1, b).
Cyclic carbonates are valuable synthetic targets because of their
relevant properties and numerous applications.[1] They have
been used as high boiling aprotic polar solvents, and because
of their low toxicity and good biodegradability, they are capa-
ble of replacing traditional hazardous solvents.[2] In addition,
they have served as components of electrolytes in Li-ion re-
chargeable batteries[3] and protecting groups in carbohydrate
chemistry,[4] and they are found in several natural products and
potential pharmaceuticals.[5] Of particular relevance is the use
of such heterocyclic compounds as intermediates in the manu-
facturing of fine chemicals,[6] more specifically as monomers,
for the preparation of polycarbonates, polyurethanes, and other
polymeric material with applications in the field of plastics engi-
neering.[7]
Within this class of compounds, five-membered derivatives
are most significant according to the literature. Consequently,
several catalytic methods have been published to date. Cur-
rently, the main route for their synthesis involves the reaction
of epoxides with carbon dioxide, for which a large number of
catalytic systems have been successfully developed (Scheme 1,
a).[8] However, the synthesis of these starting epoxides is typi-
cally achieved by using olefins along with powerful oxidants
and procedures that generate waste. Furthermore, epoxides can
be toxic and difficult to handle. An alternative approach for the
synthesis of cyclic carbonates has involved the use of 1,2-diols,
which are generally bioavailable from renewable resources.[9] In
this case, with the exception of CO2,[10] carbonylation agents
such as phosgene and carbon monoxide in combination with
different oxidants have been used (Scheme 1, a).[11] Still, these
latter reagents have significant drawbacks. Alternatively, a safe
and easily accessible carbonate precursor is urea. This reagent
Scheme 1. Catalytic processes for the synthesis of cyclic carbonates from
epoxides and diols. Urea is used as CO2 equivalent.
These advantages led us to focus our research on the cata-
lytic coupling of diols with urea. So far, there have been reports
of different protocols that regularly use catalysts with the ap-
propriate acidic and basic properties such as metal oxides, zinc,
and magnesium chloride as well as other sophisticated hetero-
geneous systems.[13] However, many of these methods have dis-
advantages such as the need of a vacuum system to remove
the generated ammonia to accelerate the process. In addition,
these protocols often have a limited substrate scope. For these
reasons, we became interested in a general study of the synthe-
sis of cyclic organic carbonates and the development of new
benign catalysts.
[a] Leibniz-Institut für Katalyse e.V. an der Universität Rostock,
Albert-Einstein-Straße 29a, 18059 Rostock, Germany
E-mail: Matthias.Beller@catalysis.de
Because of the increasing interest in the implementation of
green chemistry principles, iron-based catalysts have been
widely employed in organic synthesis in recent years.[14] As iron
is one of the most abundant metals, a variety of inexpensive
and nontoxic derivatives are commercially available. Compared
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available on the WWW under http://dx.doi.org/10.1002/ejoc.201600476.
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with noble metal complexes, their use in various transforma- approach and establishing iron(II) bromide to be the most effec-
tions generally provides economic and ecological benefits. Ad- tive catalyst, we decided to explore other general factors to
ditionally, Lewis acid catalyzed reactions with iron have been determine the optimal reaction conditions. Performing the
well-known for a long time.[15] For example, the corresponding same reaction in other solvents such as tert-amyl alcohol, tolu-
electrophile of a nucleophilic substitution reaction is activated ene, and acetonitrile decreased the reactivity (37–70 % yield,
by metal coordination, which makes it more susceptible to the Table 1, Entries 1–3), whereas studying different ratios of 1 and
attack of an incoming nucleophile.[16] Applying this principle, 2a led to an excellent 91 % yield by using 1.5 equiv. of the
we herein describe a new sustainable synthesis for cyclic carb- corresponding diol (Table 1, Entries 4 and 5). We then examined
onates from ureas and vicinal diols in the presence of an eco- the effect of changing the catalyst loading and temperature. In
friendly iron catalyst.
the presence of 0.01 mmol FeBr2, the reaction afforded the de-
sired product in a lower 71 % yield, whereas the use of higher
amounts of iron did not significantly improve upon the earlier
reactivity (92 % yield, Table 1, Entries 6 and 7). The temperature
serves an important role in this transformation as 3a was ob-
tained in a moderate 61 % yield at 140 °C, and surprisingly no
reactivity was observed at a lower temperature (Table 1, En-
tries 9 and 10). Next, we studied the potential effect of addi-
tives, but the use of an acid or base in substoichiometric
amounts (5 mol-%) afforded the corresponding cyclic carbonate
in lower yields (Table 1, Entries 11 and 12). Decreasing the reac-
tion time led to a drop in the conversion (Table 1, Entry 13),
and the experiment provided the cyclic carbonate in only 24 %
yield in the absence of the catalyst (Table 1, Entry 14).
Results and Discussion
Our research began by selecting an effective catalyst for the
model reaction of urea (1) with 1-phenylethane-1,2-diol (2a,
Scheme 2). Initially, we compared the reported reactivities of
catalysts that have been used in this transformation under a
given set of reaction conditions (0.5 mmol of 1, 0.5 mmol of
2a, 1,4-dioxane, 150 °C, 18 h).[13m–13o] Employing the standard
ZnO (0.02 mmol) as a catalyst provided the desired 4-phenyl-
1,3-dioxolan-2-one (3a) in 69 % yield. The use of other zinc and
magnesium salts, which are also known for carbonate synthesis,
provided the same product but in lower yields (45 and 27 %,
respectively).
Table 1. Screening of reaction conditions for the iron-catalyzed synthesis of
4-phenyl-1,3-dioxolan-2-one (3a).[a]
Entry
Catalyst Additive
Solvent
T [°C]
Yield [%][b]
1
2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
FeBr2
–
–
–
–
–
–
–
–
–
tert-amyl alcohol
toluene
MeCN
150
150
150
150
150
150
150
160
140
130
150
150
150
150
55
70
37
58
91
71
92
89
61
–
3
4[c]
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
5[d]
6[d,e]
7[d,f]
8[d]
9[d]
–
–
10[d]
11[d,g]
12[d,g]
13[d,h]
14[d]
K2CO3
pTsOH
–
–
21
65
53
24
[a] Unless otherwise specified, all reactions were carried out with catalyst
(0.02 mmol), urea (1, 0.5 mmol), and 1-phenylethane-1,2-diol (2a, 0.5 mmol)
in a solvent (1 mL) at the indicated temperature for 18 h. [b] Yields deter-
mined by GC analysis using hexadecane as the internal standard. [c] Urea
(0.75 mmol) was used. [d] Diol (0.75 mmol) was used. [e] Catalyst (0.01 mmol)
was used. [f] Catalyst (0.03 mmol) was used. [g] An additive (0.025 mmol)
was used (pTsOH = para-toluenesulfonic acid). [h] Reaction time was 5 h.
Scheme 2. Variation of catalysts for the synthesis of 4-phenyl-1,3-dioxolan-2-
one (3a) from urea (1) and 1-phenylethane-1,2-diol (2a). Reagents and condi-
tions: catalyst (0.02 mmol), 1 (0.5 mmol), and 2a (0.5 mmol) in 1,4-dioxane
at 150 °C for 18 h (OTf = trifluoromethanesulfonate, acac = acetylacetone).
Yields were determined by GC analysis using hexadecane as the internal stan-
dard.
Next, we studied this transformation by using various iron
compounds. Here, we observed that classical inexpensive
Remarkably, this process takes place selectively to avoid the
iron(II) salts provided the corresponding five-membered cyclic formation of related side products, such as polycarbonates or
carbonate 3a in good yields and obtained the best 75 % yield urethanes, a common occurrence in such transformations. In
by using simple FeBr2 (Scheme 2). Additionally, analogous addition, the atom economy of this process is another signifi-
iron(III) species and triiron(0) dodecarbonyl also showed some cant feature, as the substrates are converted into the product
reactivity, although lower conversions were observed in all along with only two equivalents of ammonia as a byproduct.
cases (25–68 % yield, Scheme 2). After investigating this first The ease of availability of the coupling partners as well as the
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safety and low price of the iron salt provides added value to
this synthetic method.
Table 2. Iron-catalyzed synthesis of substituted cyclic carbonates 3a–3p from
urea (1) and vicinal diols 2a–2p.[a]
After the model reaction was studied, our next step involved
investigating the reactivity of different vicinal diols to make this
method more general. First, we applied the previously opti-
mized conditions to the reaction of urea (1) with terminal diols.
Such an approach would allow access to different 4-substituted
cyclic carbonates. As shown in Table 2, we found that styrene
diol derivatives that contain electron-withdrawing groups on
the phenyl ring such as 1-(4-chlorophenyl)- and 1-(4-trifluoro-
methylphenyl)ethane-1,2-diol (2b and 2c) provided the corre-
sponding carbonates 3b and 3c in excellent yields (98 and
85 %, respectively, Table 2, Entries 2 and 3). Nevertheless, the
same reaction with the electron-rich substrate 2d afforded the
product in only a low 23 % yield (Table 2, Entry 4), probably a
result of undesired reactions caused by the electronic proper-
ties of the starting diol. However, benzyl derivative 2e was ef-
fectively converted into the desired product 3e in 92 % yield
(Table 2, Entry 5). In contrast, this transformation also takes
place with aliphatic diols. Thus, ethane-1,2-diol (2f) gave rise to
ethylene carbonate (3f) in a moderate 65 % yield (Table 2, En-
try 6), whereas the reaction of urea with hexane- and 7-octene-
1,2-diol (2g and 2h) under the previously determined condi-
tions led to the desired heterocycles 3g and 3h in almost quan-
titative yields (92 and 97 %, respectively, Table 2, Entries 7 and
8). Notably, demanding substrates that contain heteroatoms
such as derivatives 2i and 2j gave the desired carbonates, albeit
in much lower yields (15 and 21 %, respectively, Table 2, En-
tries 9 and 10). The reactivity of 2i may be related to the result
from 2d. Chlorohydrin derivative 2j is susceptible to undergo
multiple undesired reactions, which explains the low yield of its
reaction .
To expand the scope of our protocol, we tested internal diols,
which would afford the corresponding 4,5-disubstituted carb-
onates. Applying the optimal conditions (0.5 mmol of 1,
0.75 mmol of 2k, and 0.02 mmol of FeBr2 in 1,4-dioxane at
150 °C), we obtained the 4,5-dimethyl-1,3-dioxolan-2-one (3k)
in a good 86 % isolated yield (70:30 dr, Table 2, Entry 11). The
reactions between urea and cis-1,2-cyclopentane- and cis-1,2-
cyclohexanediol (2l and 2m) provided carbonates 3l and 3m in
96 and 75 % yield, respectively (Table 2, Entries 12 and 13).
Pinacol (2n), a vicinal diol with two tertiary alcohol moieties,
even gave rise to the desired heterocycle 3n in good yield
(Table 2, Entry 14). However, the reaction with catechol (2o)
showed no conversion, and only the unaltered starting material
was recovered (Table 2, Entry 15). In this case, the formation of
the carbonate did not occur because of the low nucleophilicity
of aromatic alcohols, which are not capable of a nucleophilic
attack on the iron-activated urea. Finally, (R,R)-hydrobenzoin
(2p) afforded the five-membered heterocycle 3p in 95 % yield,
thus providing access to compounds substituted with two aro-
matic groups.
[a] Unless otherwise specified, all reactions were carried out with FeBr2
(0.02 mmol), urea (1, 0.5 mmol), and diol 2a–2p (0.75 mmol) in 1,4-dioxane
(1 mL) at 150 °C for 18 h. [b] Isolated yields are provided. [c] NMR yield with
durene as the internal standard. [d] Carbonate 3k was obtained as a mixture
of diastereoisomers (70:30). [e] Only the remaining starting diol was ob-
served.
Encouraged by these results, we were interested in using
diols from renewable feedstock for the synthesis of cyclic
carbonates (Scheme 3). To this end, glycerol, a side product of
the biodiesel industry, is especially interesting.[17] Because of
the bulk production of this alcohol, its use as a platform chemi-
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cal provides valuable opportunities for new products.[18] For ex- ureas 6, the reaction of which would generate both ammonia
ample, the resulting cyclic carbonate is a biodegradable high and RNH2 as byproducts. Thus, the iron-catalyzed reaction of
boiling polar solvent with low toxicity and multiple applica- 6a–6d with vicinal diol 2a in 1,4-dioxane at 150 °C provided
tions.[19] Despite the apparent simplicity of carbonate produc- the five-membered cyclic carbonate 3a in all cases in low to
tion from glycerol and urea, this reaction could lead to the for- moderate yields (26–53 % yield, Table 3, Entries 1–4). It seems
mation of side products such as urethanes, oxazolidinones, and that the generated primary amine in these cases may partici-
cyclic carbonates with larger rings. Gratifyingly, the reaction of pate in undesired reactions, thereby decreasing the yield of the
urea (1, 0.5 mmol) with glycerol (4a, 0.75 mmol) in the presence transformation. In fact, the least effective reaction involved de-
of iron(II) bromide as the catalyst selectively afforded glycerol rivative 6d with highly nucleophilic benzylamine formed as a
carbonate (5a) in an excellent 95 % yield (Scheme 3). Addition- side product. To enhance the versatility of the system, we chose
ally, this reaction was also tested with other valuable glycerol a urea analogue that contained a better leaving group such as
derivatives such as the corresponding 1-monoethers.[20] Hence, the corresponding carbamates 7 (Table 3, Entries 5–8). Again,
related experiments that employed 3-allyloxy- and 3-(4-meth- low yields were observed for the reaction of 2a with methyl,
oxyphenoxy)-1,2-propanediol (4b and 4c) as substrates gave tert-butyl, and benzyl carbamates. However, the synthesis of the
rise to the formation of carbonates 5b and 5c in very good desired product 3a from the phenyl carbamate (7c) was
isolated yields (89 and 91 %, respectively, Scheme 3). Next, we achieved in a good 89 % yield because of the increased leaving
applied this method to other bio-based diols such as the ethyl group ability and low nucleophilicity of phenols (Table 3, En-
ester of (L
)-(+)-tartaric acid.[21] Here, the ester moieties induce a try 7). Finally, another well-known alternative for the synthesis
lower nucleophilicity of the hydroxyl groups to provide the de- of the desired products includes the transesterification reaction
sired cyclic carbonate 5d in only 16 % yield (Scheme 3). In con- of acyclic carbonates with 1,2-diols,[24] which is one of the sim-
trast, we studied the reactivity of a long-chain internal diol that plest methods to be broadly applied in industrial processes for
was derived from oleic acid, a monounsaturated omega-9 fatty the manufacturing of biodiesel. The use of our previously opti-
acid that is present as triglycerides in biological systems.[22] In- mized conditions for the reaction of 2a and carbonates 8a–8d
terestingly, the reaction of urea with such an (R,R)-diol under led to low yields in most cases (Table 3, Entries 9–12). As for
iron catalysis afforded the five-membered heterocycle 5e in the case of the carbamates, the formation of the primary alco-
88 % yield (Scheme 3). Finally, we expanded the scope of the hol and the possibility of its involvement in side reactions de-
reaction to an α-pinene-derived diol.[23] This substrate satisfac- creases the effectiveness of the transformation, with the excep-
torily participated in the process to give tricyclic compound 5f tion of diphenyl carbonate (8c), which afforded 3a quantita-
in excellent yield (93 %, Scheme 3). In general, the use of diols tively.
from renewable resources enables the selective synthesis of
Table 3. Iron-catalyzed synthesis of 4-phenyl-1,3-dioxolan-2-one (3a) from the
reactions of ureas, carbamates, and carbonates (i.e., 6–8) with 1-phenyl-
ethane-1,2-diol (2a).[a]
bio-based carbonates in high yields, which benefits this method
by making it sustainable.
[a] Unless otherwise specified, all reactions were carried out with FeBr2
(0.02 mmol), either a urea, carbamate, or carbonate derivative (i.e., 6–8,
0.5 mmol), and diol 2a (0.75 mmol) in 1,4-dioxane (1 mL) at 150 °C for 18 h.
[b] Isolated yields are provided.
Scheme 3. Iron-catalyzed synthesis of cyclic carbonates 5 by using diols from
renewable resources (i.e., 4a–4f). Reagents and conditions: 1 (0.5 mmol), 4a–
4f (0.75 mmol), and FeBr2 (0.02 mmol) in 1,4-dioxane at 150 °C for 18 h.
Isolated yields are provided.
Once the scope and limitations of this protocol were ana-
lyzed, we proposed a reaction mechanism, as shown in
After studying the general applicability of this transforma- Scheme 4.[25] In the first step, the carbonyl group of urea is
tion, we compared the activity of urea with other convenient activated by coordination to the metal through the oxygen
CO sources in the reaction with 1-phenylethane-1,2-diol (2a, atom. Here, the iron(II) bromide behaves as a Lewis acid to form
Table 3). First, we extended this protocol to N-monosubstituted intermediate I. Then, one hydroxy group of diol 2, which is
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probably activated by hydrogen bonding, attacks the electro- formation confirms the ability of the iron salt to activate both
philic carbonyl carbon of urea to generate intermediate II. Mi- coupling partners.
gration of a proton from the oxygen to the nitrogen atom pro-
vides species III, which undergoes an intramolecular substitu-
tion reaction with the oxygen atom of the other OH group to
form five-membered ring IV and ammonia. Finally, the release
of the catalyst gives carbonate 3 and the second molecule of
NH3 to complete the catalytic cycle. The high rate of the intra-
molecular cyclization of intermediate III is the reason of the
high selectivity that is observed in this transformation. The for-
mation of the heterocyclic structure appears to be more favora-
ble than a nucleophilic substitution with a second diol mol-
ecule, a reaction that would lead to polycarbonates.
Scheme 5. Additional experiments: (a) Synthesis of isotopically labeled 4-
phenyl-1,3-dioxolan-2-one from 13C-urea; (b) Isolation of intermediate carb-
amate from trans-1,2-cyclopentanediol; (c) Reaction of urea with styrene ox-
ide. Isolated yields are provided.
Conclusions
In summary, we have developed a general synthesis for five-
membered cyclic carbonates from readily available urea and
vicinal diols from renewable feedstocks. This versatile transfor-
mation allowed us to prepare a broad scope of carbonates in
good yields by using a safe, inexpensive, ligand-free, and envi-
Scheme 4. Proposed reaction mechanism for the iron-catalyzed synthesis of
cyclic carbonates from urea and vicinal diols.
ronmentally benign iron salt as the catalyst. The use of bio-
available diols allows for the valuable use of renewable resour-
ces in a straightforward manner.
On the basis of these results, additional experiments were
performed. The use of 13C-labeled urea (1′) confirms the origin
of the carbonyl group in the cyclic carbonate (Scheme 5, a).
Under the previously established reaction conditions, isotopi-
Experimental Section
General Methods: Unless otherwise is stated, all reactions were
conducted under argon with exclusion of moisture from reagents
and glassware by using standard techniques for the manipulation
of air-sensitive compounds. Reaction temperatures refer to external
bath temperatures. TLC analysis was performed on silica gel 60 F254
(layer thickness: 0.2 mm), and components on the developed plates
were visualized by observation under UV light and/or treating the
plates with either phosphomolybdic acid solution or p-anisaldehyde
reagent followed by heating. Column chromatography was per-
formed on silica gel (230–400 mesh) with 30 % ethyl acetate/hept-
cally labeled 3a′ was obtained in 90 % yield, an identical result
to that observed in the experiment with standard urea (Table 2,
Entry 1). To isolate an intermediate carbamate, we carried out
the iron(II) bromide catalyzed reaction of urea (1) with trans-
cyclopentane-1,2-diol (9). In agreement with our mechanistic
proposal, we obtained compound 10 in 78 % isolated yield
(Scheme 5, b). Clearly, the trans configuration of the hydroxy
groups prevents an internal nucleophilic substitution reaction
from taking place, and less favorable intermolecular reactions
do not take place to give any oligomeric or polymeric species. ane as the eluent. The NMR spectroscopic data were recorded with
a Bruker Avance 400 spectrometer. The residual solvent signal (for
CHCl3, δ = 7.26 ppm) was used as the internal standard for 1H NMR,
and the solvent signal (for CDCl3, δ = 77 ppm) was used as the
internal standard for 13C NMR. All measurements were carried out
at room temperature, unless otherwise stated, and analysis by DEPT
was used to assign the types of carbon atoms. Mass spectrometry
was, in general, recorded on a MAT 95XP or an HP 5973N mass
selective detector. Gas chromatography was performed on an HP
6890N chromatograph with an HP5 column. Unless otherwise
stated, commercial reagents were used as received without further
Lastly, this reaction was also studied with other coupling part-
ners such as styrene oxide (11, Scheme 5, c), and its reaction
with urea under the same conditions afforded oxazolidin-2-one
12 in a moderate 57 % yield as the only regioisomer. Besides
urea, iron(II) bromide can also activate the epoxide to make it
more electrophilic. Then, the regioselective ring-opening by at-
tack of one amino group of urea to the more substituted
carbon of 11 gives the intermediate N-(2-hydroxy-1-phenyl-
ethyl)urea, which subsequently undergoes an intramolecular
substitution to provide heterocyclic compound 12. This trans- purification.
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General Procedure for the Iron-Catalyzed Synthesis of Cyclic
Carbonates: In a glass pressure tube (25 mL) under argon, FeBr2
(4.3 mg, 0.02 mmol), urea (30.1 mg, 0.5 mmol), and the diol
(0.75 mmol) were dissolved in 1,4-dioxane (1 mL). The pressure tube
was then closed, and the resulting mixture was stirred in an oil bath
at 150 °C for 18 h. After cooling to room temperature, the crude
mixture was directly purified by flash chromatography on silica gel,
and after the combined fractions were concentrated and dried un-
der high vacuum, the corresponding cyclic carbonates were af-
forded in the reported yields.
[9]
[10]
Acknowledgments
The authors thank the Federate State of Mecklenburg-Vorpom-
mern, Germany and the Bundesministerium für Bildung und
Forschung (BMBF), Germany for financial support. M. P.-L.
thanks the European Union (EU), European Commission for a
Marie Curie Intra-European Fellowship within the 7th Framework
Programme (PIEF-GA-2012-328500). The Analytical Department
of the Leibniz Institut für Katalyse (LIKAT) is thanked for techni-
cal support.
[11]
[12]
[13]
Keywords: Green chemistry · Carbonylation · Cyclization ·
Oxygen heterocycles · Iron · Diols
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Received: April 15, 2016
Published Online: ■
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Full Paper
Iron Catalysis
M. Peña-López, H. Neumann,
M. Beller* .............................................. 1–8
Iron-Catalyzed Synthesis of Five-
Membered Cyclic Carbonates from
Vicinal Diols: Urea as Sustainable
Carbonylation Agent
A new catalytic approach for the syn- ing high atom economy make this an
thesis of five-membered cyclic carbon- operationally simple and sustainable
ates from the reaction of urea with process.
vicinal diols has been developed. The
use of an inexpensive nontoxic iron
catalyst and readily available sub-
strates in combination with the result-
DOI: 10.1002/ejoc.201600476
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8
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