Stereochemical divergence in the formation of organic carbonates derived from internal epoxides

Article abstract of DOI:10.1002/adsc.201201070

Catalysis of the challenging cycloaddition of carbon dioxide to internal epoxides has been studied using iron (III) amine triphenolate complexes and particular focus has been given to the stereochemical regulation of this process. When pure cis- or trans-2,3-epoxybutane is used as substrate, the stereochemistry of the product can be controlled yielding selectively cis- or trans-cyclic carbonates for both epoxidic substrates. This stereochemical divergence relates to two accessible catalytic pathways leading to either the cis or trans product via two distinct ring-closure steps. The involved mechanism and stereocontrol is a function of the catalyst/co-catalyst loading, and is further influenced by the medium, temperature and catalyst/co-catalyst structure. Other trans-internal epoxides could also be successfully converted into the pure trans-cyclic carbonate products without any loss of stereochemical information.

Full text of DOI:10.1002/adsc.201201070

FULL PAPERS
DOI: 10.1002/adsc.201201070
Stereochemical Divergence in the Formation of Organic
Carbonates Derived from Internal Epoxides
Christopher J. Whiteoak,a Eddy Martin,a Eduardo Escudero-Adꢀn,a
and Arjan W. Kleija,b,*
a
Institute of Chemical Research of Catalonia (ICIQ), Av. Paꢁsos Catalans 16, 43007 Tarragona, Spain
Fax : (+34)-977-920-828; phone: (+34)-977-920-247; e-mail: akleij@iciq.es
Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluꢂs Companys 23, 08010 Barcelona, Spain
b
Received: December 6, 2012; Revised: January 16, 2013; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201070.
Abstract: Catalysis of the challenging cycloaddition steps. The involved mechanism and stereocontrol is
of carbon dioxide to internal epoxides has been stud- a function of the catalyst/co-catalyst loading, and is
ied using ironACHTUNGTRENNUNG(III) amine triphenolate complexes and further influenced by the medium, temperature and
particular focus has been given to the stereochemical catalyst/co-catalyst structure. Other trans-internal ep-
regulation of this process. When pure cis- or trans- oxides could also be successfully converted into the
2,3-epoxybutane is used as substrate, the stereochem- pure trans-cyclic carbonate products without any loss
istry of the product can be controlled yielding selec- of stereochemical information.
tively cis- or trans-cyclic carbonates for both epoxidic
substrates. This stereochemical divergence relates to
two accessible catalytic pathways leading to either Keywords: carbon dioxide fixation; cycloaddition;
the cis or trans product via two distinct ring-closure homogeneous catalysis; iron; stereoselectivity
Introduction
cyclic carbonates can be mediated by many catalyst
systems, there remain important synthetic challenges
The field of carbon dioxide catalysis is currently to be solved. Amongst these challenges are the effi-
a thriving area of research providing improved and cient conversion of internal epoxides,[11,16,21–25] the
new carbon fixation methodologies.[1–3] These methods asymmetric synthesis of carbonates[10,26–30] and the
allow for the conversion of a waste material into vari- preparation of enantioenriched polycarbonates.[31–35]
ous value-added organics and CO2 can be considered Stereocontrol in organic carbonate synthesis is a topic
as an alternative and renewable carbon feedstock for that has recently become an important objective as
a number of fossil fuel-based chemicals.[4,5] One suc- the properties of both cyclic as well as polycarbonates
cessful example of CO2 utilization is the atom-effi- depend on the relative configuration of pending sub-
cient synthesis of cyclic organic carbonates through stituents and the degree of stereoregularity.
addition of CO2 to terminal epoxides which has been
For instance, cis-2,3-epoxybutane is a liquid where-
extensively investigated,[6–8] whilst the use of the more as trans-2,3-epoxybutane is a solid under ambient con-
challenging internal epoxides as substrates has histori- ditions. Furthermore, Darensbourg and co-workers
cally received much less attention. Cyclic carbonates have recently shown that polycarbonates derived
offer potential building blocks during the synthesis of from (chiral) epichlorohydrin[36] and styrene oxide[37]
pharmaceutical and fine chemical intermediates, may suffer from some loss of stereochemical informa-
amongst other useful applications.[9]
tion (up to 20%) due to ring-opening at both the me-
The most popular and successful type of catalyst thine as well as methylene carbon centres (Figure 1)
system for the cycloaddition of CO2 to epoxides con- of the epoxidic substrate, resulting in formal inversion
sists of a Lewis acid site or metal complex and a nucle- and retention of configuration, respectively. This ster-
ophilic co-catalyst (i.e., a binary system),[10–17] and the eodivergence in polycarbonate formation remains
latter may also be incorporated into the catalyst struc- a crucial aspect to control as it affects important poly-
ture resulting in a bifunctional mediator.[18–20] Where- mer properties such as the glass transition tempera-
as the easy conversion of terminal epoxides into their ture, stereoregularity and thermal stability.
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Figure 1. Ring-opening mediated by a nucleophile at both
the methylene carbon (Ca) and methine carbon (Cb) poten-
tially leading to loss of stereo-information. M here denotes
a metal catalyst.
As stated, most of the catalyst systems described to
date for the cycloaddition of CO2 to epoxides display
ample scope for the conversion of terminal epoxides
as internal epoxides are significantly more difficult to
convert. The simplest internal epoxide is 2,3-epoxy-
Figure 2. IronACTHUNTGRNEUNG(III) amino triphenolate complexes 1–3 used
in this study.
ACHTUNGTRENNUNGbutane, for which there are limited examples of Co-,
Fe-, Cr-, Sn- and V-based catalyst systems capable of relative configuration of the methyl groups in the
converting it into the cyclic carbonate product, albeit cyclic carbonate product appears to have a strong de-
with different degrees of efficiencies.[11,16,21–25] Interest- pendence on the TBAB loading. In addition, and as
ingly, examples of the conversion of this substrate typ- would be expected, it can be seen that upon reducing
ically start from either pure cis- or trans-2,3-epoxybut- the TBAB loading, also the isolated yield decreases.
A
Complex 1 is able to selectively catalyze the con-
products show some loss of stereochemistry from the version of cis-2,3-epoxybutane to the corresponding
original configuration of the starting epoxide. This cis-cyclic carbonate (i.e., retention of configuration)
slight loss of configuration cannot be explained by the at higher TBAB loadings (Table 1, entry 1), at a low
generally involved combination of two SN2 reactions TBAB loading of 0.125 mol% (0.25 equiv. of TBAB
taking place at the ring-opening and ring-closure per Fe centre) this catalyst system is able to form the
stage of the process, as this would lead to an overall
retention of configuration.
Table 1. Conversion of cis-2,3-epoxybutane into cis- and
trans-cyclic carbonate products using complexes 1 and 3.[a]
Inspired by these latter results and their relation-
ship with the challenging and important stereoregular
formation of polycarbonates, we here communicate
the conversion of internal epoxides with a series of
ironACHTUNGTRENNUNG(III) amine triphenolate catalysts into their re-
spective carbonates. This work has focused particular-
ly on the stereochemical manipulation in these reac-
tions in order to gain a deeper understanding on how
to increase the stereoregulation.
Entry Catalyst TBAB
[mol%]
cis[b]
[%]
trans[b]
[%]
Yield[c]
[%]
1
2
3
4
5
6
7
8
1
1
1
1
1
1
3
3
3
3
3
3
4.0
2.5
>99
95
69
26
18
11
94
84
22
18
5
<1
5
68
47
32
30
26
19
72
61
56
52
48
41
1.25
0.625
0.313
0.125
4.0
21
74
82
89
6
16
78
82
95
>99
Results and Discussion
The ironACHTUNGTRENNUNG(III) amine triphenolate complexes 1 and 2
used in this study (Figure 2) have been previously re-
ported[21,22] and complex 3 was synthesized in a similar
manner from the corresponding amine triphenol
ligand.
2.5
9
1.25
0.625
0.313
0.125
10
11
12
Initially, the stereochemistry obtained from the
conversion of 2,3-epoxybutane was investigated as
previous work showed that under certain conditions
some loss of configuration from the starting epoxide
was noted.[21] In order to investigate this observation
in more detail, the effect of the tetrabutylammonium
bromide (TBAB)[38] loading on the ratio of cis- and
trans-cyclic carbonate products from the conversion
of cis-2,3-epoxybutane was studied, using complexes
1 and 3 as catalysts (see Table 1). Interestingly, the
<1
[a]
Conditions: cis-2,3-epoxybutane (1.0 g), catalyst=
1 (0.25 mol%), or 3 (0.5 mol%), TBAB (loading indicat-
ed), 808C, 18 h, 10 bar initial CO2 pressure in a 30 mL
autoclave.
Determined by H NMR (CDCl3) from the isolated prod-
uct.
[b]
[c]
1
Isolated yield, and selectivity for the cyclic carbonate
products in all cases was >99%.
2
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Stereochemical Divergence in the Formation of Organic Carbonates Derived from Internal Epoxides
trans-cyclic carbonate (Table 1, entry 6) with 89% se- served (Table 2, entry 6), which is different to the be-
lectivity.
haviour observed using complexes 1 and 3 at this
Complex 3 is able to exclusively form the trans- TBAB loading; in these latter cases significant
cyclic carbonate from cis-2,3-epoxybutane at a TBAB amounts of the cis-cyclic carbonate are obtained
loading of 0.125 mol% (Table 1, entry 12). The occur- (Table 2, entries 4 and 9, 45% and 36%). At a very
rence of two different products and the strong effect low TBAB loading of 0.0625 mol% and a 0.5 mol%
of the TBAB loading indicate that there are likely to loading of complex 1 (0.0625 equiv. of TBAB per Fe
be two different ring-closure mechanisms operating centre) using an extended reaction time of 66 h, 55%
during the cycloaddition reaction, one resulting in of the cis-cyclic carbonate could be obtained (Table 2,
formal retention of configuration and the other one entry 5). The conversion of trans-2,3-epoxybutane
leading to formal inversion of configuration.
into the cis-cyclic carbonate product is extremely
In order to further investigate the effect of TBAB challenging as a result of the trans-cyclic carbonate
loading on the configuration of the cyclic carbonate being the thermodynamically favoured product. If the
products and gain further insight into the proposal of temperature of the reaction is decreased from 80 to
two ring-closure mechanisms, trans-2,3-epoxybutane 508C, a successive increase in the cis-cyclic carbonate
was also studied as substrate for this reaction product is observed, with up to a remarkable 85% for-
(Table 2).[39] It can be seen that both complexes 1 and mation of the cis-cyclic carbonate, although at the ex-
3 selectively convert trans-2,3-epoxybutane into the pense of the overall yield (Table 2, entries 10–12).
trans-cyclic carbonate products at TBAB loadings of
The initial screening experiments were performed
2.5 and 1.25 mol%, respectively (Table 2, entries 1 without the presence of solvent, therefore the sub-
and 7). In addition, when complex 2 was used as cata- strate and cyclic carbonate products play this role and
lyst, even at a low catalyst loading of 0.313 mol%, se- therefore in order to investigate the effects of chang-
lectivity towards the trans-cyclic carbonate is ob- ing the medium in which the reaction occurs, different
solvents were tested (Table 3). Under the conditions
used, all reactions carried out using a co-solvent re-
sulted in lower overall yields due to the dilution of
the reaction mixture and hence lower conversion
Table 2. Conversion of trans-2,3-epoxybutane into cis- and
trans-cyclic carbonate products using complexes 1–3.[a]
levels. The use of methyl ethyl ketone (MEK) or ace-
tonitrile (MeCN) results in a reduced amount of cis-
cyclic carbonate product (Table 3, entries 2 and 3)
compared to the reaction without co-solvent. This is
likely a result of the coordinating potential of these
Entry Catalyst TBAB
[mol%]
cis[b]
[%]
trans[b]
[%]
Yield[c]
[%]
solvents and their polarity leading to bulk stabiliza-
1
2
3
1
1
1
1
1
2
3
3
3
1
1
1
2.5
1.25
<1
6
>99
94
70
55
45
>99
>99
91
64
34
67
58
36
23
28
23
71
59
32
20
12
6
Table 3. Variation of solvent during the conversion of trans-
2,3-epoxybutane into trans- and cis-cyclic carbonate prod-
ucts using complex 1.[a]
0.625
0.313
0.0625
0.313
1.25
0.625
0.313
0.0625
0.0625
0.0625
30
45
55
<1
<1
9
36
66
76
85
4
5[d]
6
7
8
9
10[e]
11[f]
12[g]
Entry
Solvent
cis[b] [%]
trans[b] [%]
Yield[c] [%]
24
15
1
2
3
4
5
none
MEK
MeCN
CHCl3
toluene
54
22
30
44
50
46
78
70
56
50
32
18
12
18
15
[a]
Conditions: trans-2,3-epoxybutane (1.0 g), catalyst; 1 or
2=0.25 mol%, 3=0.5 mol%, TBAB (loading indicated),
808C, 18 h, 10 bar initial CO2 pressure in a 30 mL auto-
clave.
[b]
[c]
1
[a]
Determined by H NMR (CDCl3) from the isolated prod-
Conditions: trans-2,3-epoxybutane (0.50 g), catalyst=
1 (0.5 mol%), TBAB (0.0625 mol%), solvent (1.0 mL),
808C, 66 h, 10 bar initial CO2 pressure in a 30 mL auto-
clave.
Determined by H NMR (CDCl3) from the isolated prod-
uct.
uct.
Isolated yield, and selectivity for the cyclic carbonate
products in all cases was >99%.
0.5 mol% of 1, 66 h.
0.5 mol% of 1, 708C, 66 h.
0.5 mol% of 1, 608C, 66 h.
0.5 mol% of 1, 508C, 90 h.
[d]
[e]
[f]
[b]
[c]
1
Isolated yield, and selectivity for the cyclic carbonate
[g]
products in all cases was >99%.
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Table 4. Variation of co-catalyst structure during the conver-
sion of trans-2,3-epoxybutane into trans- and cis-cyclic car-
bonate products using complex 1.[a]
use of chloroform (CHCl3) and toluene result in little
difference in the amount of cis-cyclic carbonate ob-
tained (Table 3, entries 4 and 5) compared with the
case where no co-solvent is used.
The effect of halide nucleophile and variation of
the counter ion has also been studied (Table 4). The
results reveal that the tetrabutylammonium based co-
catalysts result in higher yields and selectivities for
the cis-cyclic carbonate products compared with the
bis(triphenylphosphoranylidene)ammonium-based co-
catalysts. These results also indicate that the highest
yield and selectivity is obtained when using TBAB.
The combined results obtained in this study allow
us to propose that there exist two distinct ring-closure
mechanisms during the cyclic carbonate product for-
mation. Both of these proposed mechanisms are
shown in Scheme 1 and are in line with the catalytic
findings reported in Table 1, Table 2, Table 3 and
Table 4. The first step in the mechanism is coordina-
tion of the epoxide substrate (in this case trans-2,3-ep-
oxybutane, intermediate I), then subsequent ring-
opening of the epoxide by the bromide anion (inter-
mediate II), which results in an inversion of configu-
ration at this carbon atom. After insertion of CO2
there are two possible ring-closure steps. In pathway
Entry Co-catalyst cis[b] [%] trans[b] [%] Yield[c] [%]
1
2
3
4
5
TBAC
TBAB
TBAI
PPN-Br
PPN-Cl
46
48
33
28
26
54
52
67
72
74
47
77
64
63
46
[a]
Conditions: trans-2,3-epoxybutane (0.50 g), catalyst=
(0.5 mol%), co-catalyst (0.313 mol%), 808C, 66 h,
10 bar initial CO2 pressure in 30 mL autoclave.
1
a
TBAC=tetrabutylammonium chloride; TBAI=tetrabu-
tylammonium iodide; PPN=bis(triphenylphosphoranyli-
dene)ammonium.
Determined by H NMR (CDCl3) from the isolated prod-
uct.
[b]
[c]
1
Isolated yield, and selectivity for the cyclic carbonate
products in all cases was >99%.
tion of intermediates in the ring-closure event leading A (i.e., at a relatively low concentration of TBAB),
to formal retention of configuration. In contrast, the the ring-closure is controlled by the metal catalyst
Scheme 1. Proposed ring-closure mechanisms (substrate=trans-2,3-epoxybutane) occurring in the coordination sphere of the
Fe complex (pathway A) resulting in formal inversion of configuration, and outer-sphere ring-closure (pathway B) resulting
in formal retention of configuration.
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Stereochemical Divergence in the Formation of Organic Carbonates Derived from Internal Epoxides
Table 5. Cycloaddition of carbon dioxide to internal epox-
with the linear carbonate anion coordinated. Amino
triphenolate Fe complexes with small ortho substitu-
ents (cf., 1 and 2) have been shown to be able to form
hexacoordinated complexes[22] with an available cis-
coordination potential unlike in the case of, for in-
stance, metallosalens.[8] The bromide in intermediate
III is proposed to dissociate from the coordinated
linear carbonate towards the metal complex using the
vacant cis-coordination site on the metal centre; then,
the resultant sp2-hybridized carbon centre undergoes
a pseudo-SN1 type ring-closure and hence an overall
inversion of the configuration occurs.
ides catalyzed by 1.[a]
Entry Substrate
1
Product
Yield[b,c] [%]
>99
The other pathway B (Scheme 1) is associated with
a relatively high concentration of bromide anions in
the reaction mixture. As a consequence, there is com-
petition between coordination of the linear carbonate
anion and the excess bromide anions resulting in
a higher probability for an outer-sphere ring-closure
step. This seems to be supported by the observation
that polar solvents favour the formation of the trans-
carbonate product (Table 3) as these solvents are
more effective in stabilizing the anion in intermediate
V. Thus, in this case the ring-closure follows a typical
SN2 type elimination of the bromide and hence
a second inversion occurs at this carbon atom, result-
ing in an overall retention of configuration (i.e., for-
mation of the trans product). The absence of any
polymeric products in the 1H NMR spectra of the
crude products leads us to believe that the formation
of the inversion product is not via a depolymerisation
route as suggested by Kruper and co-workers for
2
3
>99
85
4
87
[a]
Conditions:
Substrate
(1.52 mmol),
catalyst=
1 (2.0 mol%), co-catalyst (5.0 mol%), 808C, 66 h, 10 bar
initial CO2 pressure in a 30 mL autoclave.
[b]
[c]
1
Determined by H NMR (CDCl3) from the isolated prod-
uct.
Isolated yield, and selectivity for the cyclic carbonate
products in all cases was >99% with >99% selectivity
for the trans isomer.
their Cr
In order to further exemplify the potential for re-
ACHTUNGTRENNUNG
(III) porphyrinate catalyst system.[23]
tention of configuration in the epoxide substrate, we terpretation of the data[40] is complicated as the cyclic
have converted four other internal trans-epoxides into carbonate can also arise from the main product
the corresponding trans-cyclic carbonates with >99% formed under these conditions (polycyclohexene car-
retention of configuration in high isolated yields bonate, see the Supporting Information) by several
(Table 5). The ability of this complex to convert these different types of back-biting mechanisms.[41] These
epoxides into the cyclic carbonate products also ex- latter results may hold relevance to enantioselective
emplifies the power of this catalyst system as there CO2/cyclohexene oxide couplings to afford chiral car-
are very few examples of catalyst systems able to con- bonate-based polymers.[42]
vert this type of challenging substrates. We chose to
use MEK as solvent due to the solid nature of the
products, allowing optimal yields to be obtained. The Conclusions
trans nature of the products was also confirmed by
1
the absence of cross-peaks in the H NOESY NMR In summary, we have found an interesting stereo-
spectra.
chemical control in the formation of cyclic carbonates
Preliminary investigations with cyclohexene oxide derived from internal epoxides. The effect of co-cata-
(a cyclic oxirane) as substrate using Fe catalyst 1 and lyst loading and other factors on these conversions
a high and low TBAB loading indicate that also in was studied in detail and two distinct ring-closure
these cases stereochemical divergence can be ob- mechanisms are proposed leading to a formal reten-
served for the cyclic carbonate product with the cis- tion or inversion of the relative configuration of both
product (with 97% selectivity) almost exclusively substituents in the starting epoxide. The formal inver-
formed at a high loading of TBAB (2.5 mol%) while sion process is proposed to be controlled in the coor-
isolating the trans carbonate as the most prominent dination sphere of the metal stemming from a pseudo
species (with 72% selectivity) at a low TBAB loading SN1 process, whereas the observation of retention is
(0.157 mol%). However, in this particular case the in- due to two consecutive SN2 processes at the same
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carbon centre occurring in the ring-opening and ring- Typical Catalytic Experiment
closure steps of the mechanism.
To a 30-mL stainless steel reactor equipped with a stirrer
The catalyst system 1 has also been shown to be
able to convert other trans-internal epoxides whereby
selectively trans-cyclic carbonates are formed. The
knowledge obtained through these studies is highly
important in the context of asymmetric synthesis of
cyclic carbonate derivatives, and also provides further
insight for the preparation of chiral polycarbonates of
which the properties depend on the stereoregularity
of the polymer backbone.
bar and containing 2,3-epoxybutane (1.0 g) was added cata-
lyst 1 (32.5 mg, 0.25 mol%) and TBAB (89 mg, 2.5 mol%).
Three cycles of pressurization and depressurization of the
reactor (with carbon dioxide at 5 bar) were carried out
before finally stabilizing the pressure at 10 bar. The reactor
was then heated at 808C with stirring for 18 h. After this
time, the reactor was cooled to room temperature and the
unreacted epoxide removed under reduced pressure. The
residue was passed through a silica pad using dichlorome-
thane as the eluent and the solvent removed under reduced
pressure to yield analytically pure cyclic carbonate product.
Experimental Section
Product Characterization
All purified cyclic carbonate products obtained were charac-
General Methods
1
terized by H, 13C{H} NMR and IR spectrometry. HR-MS
The synthesis of the ironACTHUNRGTNE(NUG III) amine triphenolate complex,
3, was carried out using standard vacuum line, Schlenk or
analyses were obtained for all compounds that have not
been previously reported. All original spectra and tabular
data can be found in the Supporting Information.
cannular techniques and once synthesized the compound
was stored in a vial kept in air. H and 13C NMR spectra
1
were recorded on a Bruker AV-400 or AV-500 spectrometer
and referenced to the residual deuterated solvent signals. IR
spectra were obtained using a Bruker Alpha FT-IR spec-
trometer. Mass spectrometric analysis and X-ray diffraction
studies were performed by the Research Support Group at
the ICIQ.
Acknowledgements
We thank ICIQ, ICREA, the Spanish Ministerio de Econo-
mꢀa y Competitividad (MINECO) through project CTQ2011-
27385. We thank Giovanni Salassa for help with the DFT cal-
culations.
Reagents
All substrates are commercially available and were used as
received. Methyl ethyl ketone (MEK), all other reagents
and carbon dioxide (purchased from PRAXAIR) were used
as received without further purification or drying prior to
their use. The ligands used during the synthesis of com-
plexes 1–3 were synthesized as described previously.[21,22,43]
References
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Preparation of Complex 3
A tetrahydrofuran solution (30 mL) of the requisite amino
triphenol ligand (1.0 g, 1.49 mmol) was slowly added to a sus-
pension of sodium hydride (107 mg, 4.46 mmol) in tetrahy-
drofuran (10 mL). The suspension was stirred overnight and
then added to anhydrous ironACTHNUGTRNEUNG(III) chloride (241 mg,
1.49 mmol), whereby the suspension immediately turned
dark brown. The mixture was stirred for a further 5 h before
being filtered through a pad of Celiteꢄ and the solvent re-
moved under reduced pressure to afford a brown powder;
yield: 998 mg (84%). Crystals suitable for X-ray crystallo-
graphic analysis were grown by slow evaporation of a tetra-
hydrofuran solution of the complex and the structure and
refinement parameters can be found in the Supporting In-
formation.[44] HR-MS (MALDI+, dctb): m/z=724.4487,
calcd. for [MÀ THF]+: 724.4392; IR (neat): n=2953 (m),
2902 (w), 2867 (w), 1682 (vw), 1604 (vw), 1466 (m), 1438
(m), 1413 (w), 1389 (vw), 1361 (m), 1302 (w), 1260 (s), 1239
(s), 1204 (w), 1169 (m), 1131 (w), 1067 (w), 1028 (w), 979
(vw), 914 (w), 875 (m), 837 (s), 810 (w), 773 (w), 749 (m),
693 (w), 646 (w), 605 (m), 554 (s), 483 (s), 450 (vw),
432 (vw), 387 cmÀ1 (m); UV-Vis (toluene):
l
(e in
LmolÀ1 cmÀ1)=424 nm (4570), 332 nm (5460).
6
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7
ÞÞ
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FULL PAPERS
8 Stereochemical Divergence in the Formation of Organic
Carbonates Derived from Internal Epoxides
Adv. Synth. Catal. 2013, 355, 1 – 8
Christopher J. Whiteoak, Eddy Martin,
Eduardo Escudero-Adꢀn, Arjan W. Kleij*
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Adv. Synth. Catal. 0000, 000, 0 – 0
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These are not the final page numbers!