Selective Catalytic Synthesis of 1,2- and 8,9-Cyclic Limonene Carbonates as Versatile Building Blocks for Novel Hydroxyurethanes

Article abstract of DOI:10.1002/chem.201905561

The selective catalytic synthesis of limonene-derived monofunctional cyclic carbonates and their subsequent functionalisation via thiol–ene addition and amine ring-opening is reported. A phosphotungstate polyoxometalate catalyst used for limonene epoxidation in the 1,2-position is shown to also be active in cyclic carbonate synthesis, allowing a two-step, one-pot synthesis without intermittent epoxide isolation. When used in conjunction with a classical halide catalyst, the polyoxometalate increased the rate of carbonation in a synergistic double-activation of both substrates. The cis isomer is shown to be responsible for incomplete conversion and by-product formation in commercial mixtures of 1,2-limomene oxide. Carbonation of 8,9-limonene epoxide furnished the 8,9-limonene carbonate for the first time. Both cyclic carbonates underwent thiol–ene addition reactions to yield linked di-monocarbonates, which can be used in linear non-isocyanate polyurethanes synthesis, as shown by their facile ring-opening with N-hexylamine. Thus, the selective catalytic route to monofunctional limonene carbonates gives straightforward access to monomers for novel bio-based polymers.

Full text of DOI:10.1002/chem.201905561

A Journal of
Accepted Article
Title: Selective catalytic synthesis of 1,2- and 8,9-cyclic
limonene carbonates as versatile building blocks for novel
hydroxyurethanes
Authors: Katarzyna A. Maltby, Marc Hutchby, Pawel Plucinski, Matthew
G. Davidson, and Ulrich Hintermair
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To be cited as: Chem. Eur. J. 10.1002/chem.201905561
Link to VoR: http://dx.doi.org/10.1002/chem.201905561
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Chemistry - A European Journal
10.1002/chem.201905561
Selective catalytic synthesis of 1,2- and 8,9-
cyclic limonene carbonates as versatile
building blocks for novel hydroxyurethanes
Katarzyna A. Maltby,a Marc Hutchby,a Pawel Plucinski,a Matthew G. Davidson,a Ulrich
Hintermair*a
a
Centre for Sustainable and Circular Technologies, University of Bath, Claverton Down, BA2 7AY Bath, UK
u.hintermair@bath.ac.uk
Abstract
The selective catalytic synthesis of limonene-derived monofunctional cyclic carbonates and their
subsequent functionalization via thiol-ene addition and amine ring opening is reported.
A
phosphotungstate polyoxometalate catalyst used for limonene epoxidation in 1,2 position is shown to
also be active in cyclic carbonate synthesis, allowing a two-step-one-pot synthesis without intermittent
epoxide isolation. When used in conjunction with a classical halide catalyst, the polyoxometalate
increased the rate of carbonation in a synergistic double-activation of both substrates. The cis isomer is
shown to be responsible for incomplete conversion and by-product formation in commercial mixtures of
1
,2-limomene oxide. Carbonation of 8,9-limonene epoxide furnished the 8,9-limonene carbonate for the
first time. Both cyclic carbonates underwent thiol-ene addition reactions to yield linked bis-carbonates
which can be used in linear non-isocyanate polyurethanes synthesis as shown by their facile ring-
1
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Chemistry - A European Journal
10.1002/chem.201905561
opening with N-hexylamine. Thus, the selective catalytic route to monofunctional limonene carbonates
demonstrated gives straightforward access to monomers for novel bio-based polymers.
Introduction
1
Terpenes are a large and diverse class of hydrocarbons naturally occurring in the oils of plants and
2
fruits that have been used for flavour and fragrance manufacturing since antiquity. Due to their structural
diversity and low oxygen content, investigations into their use as renewable feedstocks for chemical
building blocks3-5 have recently received increasing attention. The most prevalent and widely available
6
terpenes are limonene, α- and
-pinene. Pinene is typically extracted from crude turpentine, a by-product
of the paper industry generated at a scale of 300,000 tonnes per annum, containing 40-85 % α-pinene
and 0.5-28 %

-pinene.7,8 Limonene is a by-product of the citrus industry with an estimated global
9
production of 70,000 tonnes per annum. The range and volume of available terpenes may be even
higher through industrial biotechnology to provide geographically flexible supplies of terpenes via
fermentation of plant sugars and cellulose waste.10,11 In contrast to other renewable feedstocks such as
carbohydrates, terpenes are partially unsaturated hydrocarbons which are chemically similar to
petrochemical feedstocks long used in industry. Their potential to be used as drop-in replacements for
renewable commodity manufacturing through well-established olefin chemistry makes terpenes
particularly attractive biogenic feedstocks.12 However, in order to develop truly sustainable processes
based on terpene feedstocks we will need sustainable and scalable methodologies tailored to their
upgrading.13
Limonene (1
) and other terpenes have been used in the synthesis of a variety of building blocks,14-20
including cyclic carbonates21-23 which have subsequently been used for the synthesis of non-isocyanate
polyurethanes (NIPU). NIPUs have similar properties to commercially available polyurethanes, but avoid
the use of toxic isocyanates in the synthesis.24 Thus, the development of terpene-based cyclic
carbonates can lead to new bio-based polymer materials with unexplored and potentially tuneable
properties. This assumption is supported by recent work of Mülhaupt et al. on 1,2,8,9-limonene
carbonate and related NIPU thermosets and thermoplastics (Figure 1).22,23 The initially synthesised
limonene-derived NIPUs yielded rigid and brittle pre-polymers,22 but subsequent work reported a
modification that allowed for an improved purification of 1,2,8,9-limonene carbonate which significantly
reduced the colourization and improved the mechanical and thermal properties of the resulting limonene-
based NIPUs.23
2
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Figure 1. Limonene-based NIPU synthesis via 1,2,8,9-limonene cyclic carbonate according to Mülhaupt et al.22,23
Thus, clean, efficient and scalable routes to these cyclic carbonates are key to sustainable
manufacturing of terpene-based NIPUs. The quaternary ammonium salt used by Mülhaupt et al. is a
2
cheap and relatively efficient catalyst for the insertion of CO into the epoxide moieties of 1,2,8,9-
limonene bis-oxide. A wide range of catalysts for the synthesis of cyclic carbonates from epoxides has
been described in literature, including metal complexes, metal organic frameworks, quaternary
ammonium salts, imidazolium halide salts and supported ionic liquid phase catalysts.25-32 After formation
of cyclic carbonates such as 1,2,8,9-limonene carbonate, the ability to further modify the monomer or
carry out post-polymerization functionalization is an important factor. With their multiple sites of
unsaturation, terpenes can be ideally exploited in this way, however, examples such as 1,2,8,9-limonene
carbonate can only be tuned through the linking co-monomer. Selectively functionalising only one of the
double bonds available would provide further opportunities to introduce additional modifications.
Selective formation of 1,2-limonene cyclic carbonate from commercially available 1,2-limonene oxide (
has previously been investigated in the literature (Table 1). All of the catalysts used showed more
favourable formation of trans-1,2-limonene cyclic carbonate (trans- ) over the cis isomer (cis- ).
Although some catalyst mediate the carbonation reaction at milder temperatures of 75 – 100 C and
2)
3
3
°
lower pressures in the range of 10 – 50 bar, those reactions required long reaction times of 16 – 66 h.
Moreover, some systems required the use of organic solvents (Table 1 Entry 1 and 2) and co-catalytic
additives (Entry 1 – 5). All of those studies were performed using commercially available
2, and no
system has thus far been investigated for performing an epoxidation - carbonation tandem reaction.
3
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Table 1 Literature reports of the carbonation of commercial 2.
Entry
1
catalyst
Co-catalyst
10 mol %
solvent
MeCN
T
t
p
Yield 3 cis-3:trans-3
Ref.
33
[
°C]
[h]
48
[bar]
50
[%]
10 mol %
75
80
14:86
CaI
2
3
Ph P
10 mol %
crown ether
derivative
1 mol % Al
amino
2
3
3 mol %
[PPN]Cl
MEK
neat
85
66
16
10
10
57
43
1:99
8:92
21
34
trisphenolate
1 mol %
4 mol %
100
Lanthanum
Heteroscorpi
onate
4
[Bu N]Cl
4
0.1 mol %
trimetallic
cobalt
5 mol %
[Bu N]Cl
neat
80
24
20
33
Not reported
35
4
complex
5
6
1 mol % Al
complex
3 mol %
[Bu N]Cl
None
neat
neat
80
90
66
18
10
12
48
2
8:92
36
37
4
1 mol %
Not reported
formazanate
ferrate (II)
Recently, a solvent-free protocol for limonene epoxidation was reported where a catalyst consisting of an
3-
ammonium phase transfer catalyst (PTC) and the [PW
4
O
24] polyoxometalate anion ([PTC]
3 4
[PW O24] =
A) yielded 94 % 1,2-limonene oxide after 1 h reaction time at room temperature using H
2
O
2
as the
oxidant.38 Although
A is highly efficient for limonene epoxidation under these conditions, it was shown to
be difficult to recover and lost activity upon recycling. Thus, instead of recycling the catalyst for repeat
epoxidation runs, we set out to test whether it could be used in the following carbonation reaction without
intermittent epoxide isolation (Figure 2).
4
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Figure 2 General scheme of epoxidation - carbonation sequence catalysed by A.
Here, we report the development of such an effective catalytic system that gives access to trans-1,2-
limonene cyclic carbonate ( ) and 8,9-limonene cyclic carbonate ( ) using benign oxidants and solvents
Figure 3). We found that for 1,2-limonene oxide ( ) a single polyoxometalate catalyst ( ) can catalyse
both the epoxidation and the CO insertion step. Furthermore, we demonstrate effective CO insertion to
,9-limonene oxide ( ) for the first time. Both chiral cyclic carbonates are shown to undergo radical
3
5
(
2
A
2
2
8
4
coupling with 1,3-propanedithiol and react with N-hexylamine to result in novel hydroxyurethanes as a
demonstration of their utility in the synthesis of bio-based NIPUs.
Figure 3. Limonene cyclic carbonates and corresponding hydroxyurethanes investigated in this work.
5
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Results and discussion
3 4
Polyoxometalates such as those originally reported by Ishii and Venturello, [PTC] [PW O24] (PTC =
methyl tricapryl ammonium) (
A) are excellent catalytic systems for the epoxidation of a variety of
alkenes, including terpenes.39-42 Limonene epoxidation using
A
may take place under solvent-less
2 2
conditions and mild temperatures using H O as an oxidant. Moreover, the reaction can be set up in
batch or in flow reactor, reaction times are short (10 min residence time in a flow setup), and the
selectivity toward epoxides is as high as 90%.38 We therefore investigated the possibility of telescoping
the benign epoxidation using
polyoxometalates, such as [(n-C
A directly into the synthesis of cyclic carbonates. Silicotungstate
7
H
15
)
4
N]
6
[α-SiW11
O
39Co] and [(n-C
7
H
15
)
4
N]
6
[α-SiW11O39Mn], have been
reported as an efficient nonhalogen anionic catalyst for coupling CO
2
and epoxides.43 Moreover, a
combination of spectroscopic techniques and computational calculations showed that transition metal
.44,45 Leitner et al. reported synergistic effects of [Bu
N]Br and [(n-
4
polyoxometalates can coordinate CO
2
7 15 4 5
C H ) N] [CrSiW11O39] in carbonation reactions of unsaturated carboxylic acids to give increased
reaction rates and improved stereoselectivity.46 The prominent non-silicotungstate epoxidation catalyst
has not yet been investigated for its ability to catalyse epoxide carbonation.
A
Carbonation of cyclohexene oxide
We first investigated catalytic CO
solvent-free conditions. At 120 °C [Bu
cyclic carbonate 11 after 4 hours (Table 2, entry 3). The epoxidation catalyst
active under the same conditions, however, with lower selectivity, giving about the same yield of 11
Table 2, entry 2). To our delight we found that a combination of [Bu N]Br and gave high activity whilst
2
insertion into cyclohexene oxide (10) as a model compound under
N]Br showed good activity, providing 53 % of the corresponding
was found to be more
4
A
(
4
A
maintaining high selectivity to give 11 in >80 % yield after 4 hours at 120 °C (Table 2, entry 1) through a
synergistic double-activation of both substrates as in a related system reported by Leitner et al.46
Although
A could not be recycled from these reactions either, the opportunity to use the same catalyst
for two consecutive transformations represents a significant advantage over its single use in epoxidation
only.
6
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4
Table 2. Cyclohexene cyclic carbonate synthesis from cyclohexene oxide catalysed by A and [Bu N]Br.
a
b
b
Entry Catalyst
T [°C]
120
p [bar]
75
t [h]
4
Conv. [%]
Chemoselectivity 11 [%]
1
2
3
4
2 mol % A+ 3 mol % [Bu
4
N]Br
97
78
60
0
84
69
88
-
2 mol % A
3 mol % [Bu
none
120
75
4
4
N]Br
120
75
4
120
75
4
Reaction conditions: 6.3 mmol 10; 10 mol % biphenyl as an internal standard
1
a) pressure of CO
2
at 40 °C. b) derived from quantitative H NMR spectroscopy.
Carbonation of commercially available 1,2-limonene oxide
Following these encouraging results, the same methodology was applied to commercially available
2
in
order to evaluate the difference between CO
system consisting of [Bu N]Br and more closely (Table 3). Initially we applied 3 mol % [Bu
bar CO which gave high conversion but with poor selectivity (20 %), with higher pressures of 75 bar
improving selectivity to 41 % (Table 3, entry 2). The addition of 2 mol% to 3 mol% [Bu N]Br improved
both conversion and selectivity further to give in about 40 % yield after 24 hours (Table 3, entry 3). An
increased loading of [Bu N]Br alone gave even higher conversion but chemoselectivity to
below 60 % (Table 3, entry 4).
2
insertion catalysed by [Bu
4
N]Br and the combined catalytic
4
A
4
N]Br at 30
2
A
4
3
4
3
was still
7
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Table 3. 1,2-Limonene carbonate synthesis from commercially available 1,2-limonene oxide.
Entry Catalyst
T
p
Conv.
Chemoselectivity trans : cis 3
a
b
b
[
°C] [bar]
[%]
3 [%]
20
1
2
3
4
3 mol % [Bu
4
N]Br
N]Br
140 30
140 75
140 75
140 75
79
61
73
95
96:4
94:6
92:8
95:5
3 mol % [Bu
4
41
2 mol % A + 3 mol % [Bu
5 mol % [Bu N]Br
4
N]Br
53
4
52
Reaction conditions: 2.3 mmol 2; 10 mol % biphenyl as an internal standard, 24 h, solvent-less.
1
a) pressure of CO
2
at 40 °C b) determined by quantitative H NMR spectroscopy.
Interestingly, all catalysts and conditions tested predominantly produced the trans isomer of
3
with >90 %
stereoselectivity. Starting with a 6:4 mixture of trans/cis epoxide isomers in commercial
1H NMR spectra of the crude reaction mixture showed predominantly cis-
2, analysis of the
2
to remain after the reaction.
This finding suggested cis-
2
2 and trans-2 to differ in their reactivity towards CO insertion, an observation
that has also been reported by Kleij and co-workers recently.21 Indeed, considering conformational and
2
steric effects of the double inversion pathway of halide-catalysed CO insertion (Figure 4), it is clear that
-
nucleophilic activation of cis-
accessible trans-
isomers cis- and trans-
between cis- and trans-
2
by halide (X ) is less favourable than in the case of the more easily
2
.21,47 Although we have not computed their thermodynamics, we expect the carbonate
to be very close in energy, and thus propose the observed discrimination
in the CO insertion step to be purely kinetic in nature.
3
3
2
2
2
8
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Figure 4. Different reactivity of cis/trans isomers of 1,2-limonene oxide in the nucleophilic halide activation for CO
2
insertion21,46
Reactivity of cis- and trans-1,2-limonene oxides in carbonation
To investigate this assumption further, we separated the two isomers of
2 according to previously
reported methods.48,49 In line with our catalytic results using the commercial 6:4 mixture, the cis isomer of
2
alone did get converted but with very poor chemoselectivity to
of hydrolysis and rearrangement products were obtained under the conditions applied (Figure 5). The
pure trans isomer of however showed good reactivity and excellent chemoselectivity to trans- under
the same conditions (Table 4, entry 5 and 6, and Figure 6). Thus, not only is trans- clearly more
reactive towards carbonation than cis- , but the latter gives rise to unwanted side products instead.
3 (Table 4, entry 1). Instead, a number
2
3
2
2
2
Table 4. Reactivity of different mixtures of cis-2 and trans-2 towards CO insertion.
a
Entry
trans:cis 2
t [h]
Conv. [%]
trans:cis 2
after reaction)
Chemoselectivity 3 [%] trans: cis 3
(
1
2
3
4
5
6
0:100
13:87
57:43
97:3
24
24
24
24
4.5
4.5
60
80
83
91
60
61
0:100
13:87
33:67
91:9
12
26
64
85
95
89
0:100
55:45
99:1
99:1
97:3
96:3
98:2
100:0
100:0
100:0
4
Reaction conditions: 4.6 mmol 2, 3 mol % [Bu N]Br, 2 mol % A, 10 mol % biphenyl as an internal standard, 140 °C, 75 bar CO
2
at 40 °C, solventless.
a) determined from quantitative H NMR spectroscopy
1
2
Figure 5. Product distribution from the attempted CO insertion into cis-2 as determined by GC-MS and NMR analysis (% selectivity).
9
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2
Figure 6. Product distribution from CO insertion into trans-2 as determined by GC-MS and NMR analysis (% selectivity).
Reaction profiles a commercial sample of
2 (57:43 trans:cis) showed global conversion of the mixture to
be lower than for the pure trans isomer due to a lower concentration of the reactive isomer (Figure 7).
1
0
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2 4
Figure 7 Top: Reaction profiles at 140 °C and 75 bar CO , 2.83 mmol 2, 2 mol % A, 3 mol % [Bu N]Br, 10 mol % biphenyl as an internal
standard. Commercial 2 was sourced from Sigma Aldrich, trans-2 has been synthesised in the procedure described in SI, it is optically 100 %
1
trans-2 which contains 10 % impurities (See SI IVc). Bottom: H NMR spectra showing reaction progress. The presented spectra correspond to
data points collected for commercial 2 (57:43 trans:cis).
1
1
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Temperature influence on carbonation of trans-1,2-limonene oxide
Varying the reaction temperature from 40-140
depended on temperature (Table 5). Applying 120
reaction maintained high carbonate selectivity. Lowering the temperature further to 80
significantly diminished conversion and selectivity, and at 40 C no reactivity was observed at all. These
⁰
C with pure trans-
2
showed that conversion strongly
C, but the
C showed
⁰
C resulted in lower conversion than 140
⁰
⁰
⁰
findings are consistent with data previously reported for 1,2,8,9-limonene carbonate, in which the
C.22 Reaction
conversion was high for reactions between 120-140 ⁰C but significantly lower at 100 ⁰
temperatures above 100
⁰C are typical for cyclic carbonate synthesis catalysed by quaternary
ammonium salts, however, the optimum reaction temperature often depends strongly on the epoxide
used.43,46,50,51 For trans-
2, conversion increased with higher temperature and the high selectivity of
reaction was not affected up to 140 ⁰C.
Table 5 The influence of temperature on trans-5 synthesis
a
Entry
Catalyst
T [°C]
t [h]
Conv. [%]
Selectivity trans-3 [%]
Trans : cis 3
1
2
3
4
A+[Bu
A+[Bu
A+[Bu
A+[Bu
4
N]Br
4
N]Br
4
N]Br
4
N]Br
140
120
80
4.5
4.5
4.5
4.5
65
42
10
5
80
78
45
0
1:0
1:0
1:0
-
40
4
Reaction conditions: 1.73 mmol trans-2 (see ESI IVc), 2 mol% A, 3 mol % [Bu N]Br, 10 mol % biphenyl as an internal standard,
stirrer speed 300 rpm, 75 bar CO
2
at 40 °C
1
a) determined from quantitative H NMR spectroscopy.
Pressure influence on carbonation of trans-1,2-limonene oxide
Varying the CO
g/mL) showed that selectivity was strongly affected by the amount of CO
densities between 0.15 and 0.24 g/mL (60 – 70 bar) conversion and selectivity remained high. However,
reactions carried out at CO densities of 0.06 g/mL (30 bar) or below showed significantly lower
2
pressure in the range of 20 - 75 bar (corresponding to CO
2
densities of 0.037 – 0.238
2
added (Table 6). At CO
2
2
selectivity whilst retaining the same level of substrate conversion. A reaction carried out under
atmospheric pressure of air yielded mostly hydrolysis and rearrangement products 20 and 21 from non-
productive epoxide activation by the halide (Table 6, entry 5).
1
2
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Table 6. The influence of pressure on trans-3 synthesis.
b
Entry
Catalyst
T [°C]
p
t [h]
Conv, [%]
Selectivity trans-3 [%]
Trans : cis 3
a
[
bar]
1
2
3
4
5
A+[Bu
A+[Bu
A+[Bu
A+[Bu
A+[Bu
4
N]Br
4
N]Br
4
N]Br
4
N]Br
4
N]Br
140
140
140
140
140
75
4.5
4.5
4.5
4.5
4.5
68
62
65
45
16
76
79
55
35
1:0
1:0
1:0
1:0
1:0
60
30
20
c
d
1
25
4
Reaction conditions: 2.16 mmol trans-2 (see ESI IVc), 2 mol% A , 3 mol % [Bu N]Br, 10 mol % biphenyl as an internal
standard, stirrer speed 300 rpm
1
a) pressure of CO
2
at 40 °C. b) determined from quantitative H NMR spectroscopy. c) atmospheric pressure of air. d) low
concentration of product; main side products identified as 20 and 21 by GC-MS.
Catalyst variation
When reducing the amount of
by 12% with no significant changes in selectivity. It was also found possible to use 5 mol %
any addition of [Bu N]Br (Table 7, entry 4), but at a rather unfavourable mass ratio (5 mol %
equal to 80 weight% of the reaction).
A
from 2 mol % to 1 mol % (Table 7, entry 1-3) the conversion decreased
without
being
A
4
A
Bromide is typically reported as the most efficient halide for cyclic carbonate synthesis due to its
balanced nucleophilicity and leaving group character.22,27,46,52,53 However, for trans-
3
synthesis we found
chloride to be the most active and selective halide co-catalyst, both on its own and in combination with
A
(
Table 7). Activities decreased when moving to the less nucleophilic bromide and iodide. Due to its
4
corrosive and toxic nature, combined with reports of [Bu N]F being ineffective in cyclic carbonate
synthesis,46 fluoride was not tested. Yasuda et al. reported that quaternary ammonium salts with longer
alkyl chains were more active in cyclic carbonate synthesis,43 however we found that substituting
[
Bu
4
N]Cl for Aliquat 336 (a commercial mixture of C8 and C10 quaternary ammonium salts) did not result
in any significant change in activity (Table 7, entry 5 and 8).
1
3
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Table 7. The synthesis of trans-6 catalysed by various catalysts.
a
Entry
Catalyst
t [h]
Conv. [%]
Selectivity trans-6 [%]
Trans : cis 6
1
2
3
4
5
6
7
8
9
2 mol % A+ 3 mol % [Bu
4
N]Br
4.5
4.5
24
68
56
91
83
35
45
15
34
83
55
93
96
98
76
77
64
72
82
59
52
69
78
39
74
55
67
-
1:0
1:0
1:0
1:0
1:0
1:0
1:0
1:0
1:0
1:0
1:0
1:0
1:0
-
1 mol % A+3 mol % [Bu
4
N]Br
1 mol % A+ 3 mol % [Bu
4
N]Br
5 mol % A
4.5
4.5
4.5
4.5
4.5
4.5
4.5
24
3 mol % [Bu
3 mol % [Bu4N]Br
3 mol % [Bu N]I
3 mol % Aliquat 336
4
N]Cl
4
b
2 mol % A+ 3 mol % [Bu
2 mol % A+ 3 mol % [Bu
2 mol % A+ 3 mol % [Bu
2 mol % A+ 3 mol % [Bu
2 mol % A+ 3 mol % [Bu
none
4
4
4
4
4
N]Cl
10
11
12
13
14
N]I
N]Cl
N]Br
N]I
24
24
c
4.5
-
2
Reaction conditions: 2.0-2.6 mmol trans-2 (see ESI IVc), 140 °C, 75 bar CO at 40 °C, 10 mol % biphenyl as an internal
standard, stirrer speed 300 rpm
1
a) determined from quantitative H NMR spectroscopy. b) Aliquat 336 = commercial mixture of C
8
and C10 quaternary
ammonium salts with predominately N-methyl-N,N,N-trioctylammonium chloride. c) 5 % mass loss.
Integrated epoxidation and carbonation of 1,2-limonene cyclic carbonate
With an efficient catalyst system in hand, we explored the possibility of using
A
in a direct epoxidation –
carboxylation sequence starting from terpenes. Limonene
2 2
1 was epoxidized with H O using 1 mol% A to
yield a 59:41 mixture of cis/trans-
2
as previously reported.38 The resulting bi-phasic mixture was
separated, and the organic phase used directly in CO
catalyst (Figure 8). Pleasingly, we found that after 24 h 85 % of
chemoselectivity and 99% stereoselectivity to trans- , which could be isolated in pure form by
2
insertion after addition of 3 mol % [Bu
4
N]Br co-
2
had reacted to with 42 %
3
3
chromatography (crystallization has been reported as an alternative purification method21,23).
Figure 8 Two-step synthesis of cyclic carbonates from limonene without intermittent epoxide isolation (res. time = residence time).
1
4
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Chemistry - A European Journal
10.1002/chem.201905561
The overall yield of trans-3 across both steps starting from limonene of 39 % was mainly limited by the low
epoxidation selectivity to trans-2. Nevertheless, this is the first example of using the same catalyst for epoxidation
and carbonation without intermittent epoxide isolation. In order to assess the effectiveness of our approach we
compared it to literature methods for preparing 3, including stereoselective stoichiometric epoxidations54 and
some low-pressure carbonation methods,21 in terms of process mass intensity (PMI) factors across both steps
(Table 8). As can be seen from entries 1-5, although stereoselective oxidation provides almost exclusively trans-2
and consequently higher yield of trans-3, the required intermediate workup of the epoxide leads to a 35 % higher
PMI than our telescoped epoxidation/carbonation sequence catalysed by A despite lower yields. Conventional
routes to trans-3 involving classical mCPBA oxidation (Entries 4 and 5) have the highest PMI due to the wasteful
workup and non-selective epoxide production.
Table 8 Calculated Process Mass Intensities (PMI) for various limonene epoxidation/carbonation systems.
a
Entry Reaction scheme
1
PMI
ref
b
thisd
2
2
2
5
0
work
2
3
8
7
0
54, 21
54, 47
c
4
21
c
5
thise
work
132
All calculations performed for 5 mmol limonene, for details see the supporting information.
a) carbonate isolation was not included in PMI calculations. b) calculations do not include waste generated during start-up,
before the flow reactor achieves steady state. c) no data is available on commercial preparation of limonene oxide, mCPBA
oxidation was chosen as an example: 5 mmol limonene, 5.5 mmol mCPBA, 5 mL DCM, assumed 90 % yield. d) see Figure 8.
e) see Table 3 Entry 2.
1
5
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Chemistry - A European Journal
10.1002/chem.201905561
Synthesis of 8,9-limonene cyclic carbonate
Cyclic carbonate formation at the 1,2 position of limonene is known to be facile due to the ease of
selectively epoxidizing the more electron-rich trisubstituted double bond. However, access to the
corresponding 8,9 epoxide and carbonate would provide an underexplored and potentially useful building
block. 8,9 selectivity over 1,2 functionalization would most easily be achieved on the basis of steric
differentiation. Indeed, Mizuno has reported [Bu
for the selective epoxidation of limonene to 8,9-limonene epoxide
proceeded with high regioselectivity to
found CO insertion with to be more facile than with the 1,2 epoxides
40 °C high conversions were achieved after only 2.5 hours (Table 9 Entry 1). Lowering the temperature
to 120 °C still gave good conversion with excellent selectivity of 99 % (Entry 3). In contrast to are
less sterically hindered so that the nucleophilic attack is equally favourable on both isomers, and no
significant difference in the isomer ratio was observed. While was found to also catalyse the
formation of , the promoting effect of adding to [Bu N]Br was much lower than for (Entries 3-5). The
attempted use of Mizuno’s catalyst for CO insertion yielded no cyclic carbonate but gave the 8,9-
limonene aldehyde 41 as the major product irrespective of the amount of CO added (Entry 7 and 8).
Thus, unlike the 1,2-selective epoxidation catalyst , the 8,9-selective epoxidation catalyst does not
catalyse carbonate formation and produces antagonistic effects in combination with halide co-catalysts.
4
N]
4
[
HPV
2
W
10
O
40] (
B
) as a bulky POM-based catalyst
4
.55 The epoxidation of limonene using
B
4
as a 6:4 mixture of two diastereoisomers. Pleasingly, we
. Using 3 mol% [Bu N]Br at
2
4
2
4
1
2, 4
4
A
5
A
4
3
B
2
2
A
B
1
6
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Chemistry - A European Journal
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2
Table 9. Reaction of 4 with CO .
Entry Catalyst
T [°C]
p
t [h]
R,S/R,R 5
Conv. Chemoselectivity R,S /
a
b
c
[
bar]
after reaction
[%]
13 [%]
R,R 13
1
2
3
4
5
6
7
8
9
3 mol % [Bu
3 mol % [Bu
3 mol % [Bu
3 mol % [Bu
2 mol % A
4
4
4
4
N]Br
N]Br
N]Br
140
120
120
120
120
120
120
120
120
75
75
75
75
75
75
75
-
2.5
4.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
54:46
58:42
59:41
64:36
64:36
61:39
62:38
56:44
63:37
80
76
52
68
46
57
96
91
99
98
86
76
70:30
65:35
67:33
73:27
83:17
63:37
-
N]Br + 2 mol% A
3 mol % [Bu
4
N]Br + 2 mol % B
d
e
2 mol % B
63
0
f
g
2 mol % B
75
0
-
h
No catalyst
75
-
-
-
Reaction conditions: 2.3 mmol 4 (initial ratio of 63:37 R,S / R,R), 10 mol % biphenyl as an internal standard, stirrer speed 300
rpm
1
1
2
a) pressure of CO at 40 °C. b) determined from quantitative H NMR spectroscopy. c) from H NMR using doublet at 4.25 ppm,
overlapping doublets were deconvoluted using MNova. d) up to 63% depending on the batch of B
e) reaction yields 41 with 50% selectivity. f) up to 75 % depending on the batch of B. g) reaction yields 41 with 47% selectivity.
h) 6.5 % mass loss.
Novel hydroxyurethanes from monofunctional cyclic carbonates
In contrast to 1,2,8,9-limonene carbonate, the use of monofunctional terpenoid cyclic carbonates
3 and 5
has remained unexplored so far, although they represent potentially useful building blocks for novel bio-
renewable NIPUs. Here we demonstrate their utility by linkage through a thiol-ene reaction on their
remaining double bond to show that it is possible to create bi-functional carbonate dimers (
which may be further ring opened to hydroxyurethanes using n-hexylamine as a model substrate (Figure
). Note that the sulfide linker is not required for actual NIPU synthesis and that the presented ring
6 and 7)
9
opening reaction is not optimised but serves as proof-of-concept; more efficient procedures have
recently been published by Kleij and co-workers.57
1
7
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Chemistry - A European Journal
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Figure 9. Synthesis of dithiol-linked bis-carbonates from 3 and 5 via radical addition, and carbonate ring-opening to hydroxyurethanes 8 and 9.
Radical addition of di-thiol on
3
to afford the linked bis-monocarbonate
6
could be carried out thermally
without any exogeneous initiator.14 The endocyclic double bond in
5
however required addition of AIBN
and longer reaction times to furnish the corresponding bis-monocarbonate
was confirmed by NMR spectroscopy and mass spectrometry (Figure 10).
7. The formation of
6
and
7
1
8
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Chemistry - A European Journal
10.1002/chem.201905561
Figure 10. MS and 1H NMR analysis of 6 and 7 (further analytical details can be found in the SI).
Both bi-functional carbonate dimers
6 and 7 readily reacted with an excess of n-hexylamine resulting in
ring opening of their cyclic carbonate functionalities after heating to 100 °C for 24 hours. NMR analysis of
the reaction mixtures was challenging due to the presence of a number of regio- and stereoisomers
present, but IR spectra showed complete consumption of cyclic carbonates in both cases through a
characteristic shift of their carbonyl band at 1799 cm-1 and 1792 cm-1 for
6
and
7 respectively to 1689
cm-1 and 1698 cm-1 in the corresponding hydroxyurethanes
8
and 9 (Figure 11). Mass spectrometry
+
+
further confirmed formation of
8 and 9 by detecting characteristic signals of [M+H] and [M+Na] adducts
(
additional data and details can be found in the SI).
1
9
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Chemistry - A European Journal
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Figure 11. ATR-IR spectra of 6 and 7 before (yellow) and after (blue) reaction with n-hexylamine, showing the formation of novel
hydroxyurethanes 8 and 9, respectively. Carbonate C=O resonances are highlighted in grey, and amide C=O resonances in green.
The successful synthesis of hydroxyurethanes
terpene-derived cyclic carbonates for renewable polymer synthesis: while the reaction of
diamines would furnish linear polyurethanes, the reaction of and with polyfunctional amines would
8
and
9
demonstrates the potential of monofunctionalised
6
and with
7
3
5
give access to pre-polymers that may be further functionalised on the remaining double bond.57-59
Conclusions
The growing interest in bio-derived NIPUs and the commitment of the coating industry to improve their
sustainability60 makes terpene-derived cyclic carbonates desired compounds. 1,2,8,9-limonene
biscarbonate has previously been used in NIPU synthesis to show promising results.22,23 Here, we have
synthesised limonene-derived monocarbonates and their corresponding hydroxyurethanes using benign
and scalable catalytic methods. The synergistic interactions between the widely used Ishii-Venturello
epoxidation catalyst
carbonate directly from limonene. A different epoxidation catalyst (Mizuno’s polyoxometalate
selectively gave access to 8,9-limonene oxide that could easily be transformed into its corresponding
cyclic carbonate which was fully characterised for the first time. Both cyclic carbonates and have
been shown to undergo thiol-ene reactions with 1,3-propanedithiol to yield di-monocarbonates and
2
A and CO allowed for a telescoped two-step synthesis of 1,2-limonene cyclic
B)
5
3
5
6
7
as versatile building blocks for polymer synthesis by nucleophilic ring-opening with amines. Using
combinations of various dithiols and polyamines with these limonene carbonates will give access to a
2
0
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Chemistry - A European Journal
10.1002/chem.201905561
range of novel NIPUs with tuneable properties and consisting of >50 wt% renewables. Application to
other terpenes with less stereochemical restraints on the carbonation than limonene will give access to a
wide range of bio-based building blocks in even higher yields.
Acknowledgements
This work was supported by the Royal Society (UF160458; University Research Fellowship to UH), the
EPSRC Centre for Doctoral Training in Sustainable Chemical Technologies (EP/L016354/1; PhD
studentship to KS), and the EPSRC grant Terpene-based Manufacturing for Sustainable Chemical
Feedstocks (EP/K014889/1). The authors would like to thank Dr Catherine Lyall of the Dynamic Reaction
Monitoring Facility at the University of Bath, and the National Mass Spectrometry Facility at Swansea
University for support and assistance with this project.
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