Carbon Dioxide as a Protecting Group: Highly Efficient and Selective Catalytic Access to Cyclic cis-Diol Scaffolds

Article abstract of DOI:10.1002/anie.201406645

The efficient and highly selective formation of a wide range of (hetero)cyclic cis-diol scaffolds using aminotriphenolate-based metal catalysts is reported. The key intermediates are cyclic carbonates, which are obtained in high yield and with high levels of diastereo- and chemoselectivity from the parent oxirane precursors and carbon dioxide. Deprotection of the carbonate structures affords synthetically useful cis-diol scaffolds with different ring sizes that incorporate various functional groups. This atom-efficient method allows the simple construction of diol synthons using inexpensive and accessible precursors and green metal catalysts and showcases the use of CO₂ as a temporary protecting group. Protective Carbon: Aminotriphenolate complexes of Fe^III and Al^III are highly efficient and selective catalysts for the conversion of functional (multi)cyclic oxiranes into the corresponding cis carbonates. Basic hydrolysis of the latter provides a series of useful cyclic cis-diol scaffolds in high yield. In this process, CO₂ acts as both a temporary protecting group and an oxygen donor.

Full text of DOI:10.1002/anie.201406645

Angewandte
Chemie
DOI: 10.1002/anie.201406645
Temporary Protecting Groups
Hot Paper
Carbon Dioxide as a Protecting Group: Highly Efficient and Selective
Catalytic Access to Cyclic cis-Diol Scaffolds**
Victor Laserna, Giulia Fiorani, Christopher J. Whiteoak,* Eddy Martin, Eduardo Escudero-
Adꢀn, and Arjan W. Kleij*
Abstract: The efficient and highly selective formation of a wide
range of (hetero)cyclic cis-diol scaffolds using aminotriphe-
nolate-based metal catalysts is reported. The key intermediates
are cyclic carbonates, which are obtained in high yield and with
high levels of diastereo- and chemoselectivity from the parent
oxirane precursors and carbon dioxide. Deprotection of the
carbonate structures affords synthetically useful cis-diol scaf-
folds with different ring sizes that incorporate various func-
tional groups. This atom-efficient method allows the simple
construction of diol synthons using inexpensive and accessible
precursors and green metal catalysts and showcases the use of
CO2 as a temporary protecting group.
centers,[8] whereas polycarbonate formation followed by
a depolymerization/hydrolysis pathway results in the opposite
stereochemistry (Scheme 1).[4b] We recently reported power-
C
arbon dioxide (CO2) is an interesting carbon feedstock for
organic synthesis with a number of attractive features as
a renewable, inexpensive, and readily available resource.[1]
Recent progress in the area of catalytic transformations with
[2]
CO2 has led to a spectacular increase in the application of
this C1 synthon providing access to important and functional
chemical intermediates[3] and precursors for (bio)renewable
plastics.[4] One of these intensively studied areas is the
preparation of organic carbonates from oxirane/oxetane
precursors giving rise to five- and six-membered cyclic
structures.[5] Although this area has been developed to
a sophisticated level,[6] important challenges remain to be
solved, including the efficient conversion of internal cyclic
epoxides[7] and their selective conversion into the correspond-
ing cyclic carbonate rather than the polycarbonate. The latter
selectivity is especially vital to control the stereochemical
outcome of the reaction as the formation of the cyclic
carbonate through a double inversion pathway leads to
retention of the relative configuration at both carbon
Scheme 1. Natural compounds with cyclic cis-diol scaffolds and syn-
thetic approach towards cis diols through intermediate carbonate
formation using metal catalysts 1 and 2.
ful catalyst systems that are based on aminotriphenolate
ligands (M = Fe, Al; Scheme 1, compounds 1–2), which
showed great versatility in the formation of both cyclic[7d,9]
as well as polycarbonates.[10] Moreover, these systems cata-
lyzed the conversion of acyclic internal epoxides with ample
functionality and high turnover numbers.
Cyclic cis-diol scaffolds are key structural elements of
various naturally occurring compounds, such as (À)-trihy-
droxyheliotridine and Mupirocin (Scheme 1)[11] and are thus
interesting synthetic targets. One of the most well-known
methods for cis-diol formation is based on the Sharpless
dihydroxylation of alkene precursors using osmium cataly-
sis.[12] We envisaged that the catalytic formation of organic
carbonates from cyclic oxiranes and CO2 using earth-abun-
dant and inexpensive metal catalysts followed by deprotec-
tion under basic conditions could offer a useful and alter-
native sustainable approach towards cyclic cis diols.[13] In this
method, CO2 has a dual role as it acts both as a temporary
protecting group (TPG; Scheme 1) that helps to steer the
process stereoselectivity, and as an oxygen source. Although
various methods for diol formation from organic carbonates
have been reported,[14] to date, the substrate scope has
[*] V. Laserna, Dr. G. Fiorani, Dr. C. J. Whiteoak, Dr. E. Martin,
E. Escudero-Adꢀn, Prof. Dr. A. W. Kleij
Institute of Chemical Research of Catalonia (ICIQ)
Av. Paꢁsos Catalans 16, 43007 Tarragona (Spain)
E-mail: chriswhiteoak@googlemail.com
akleij@iciq.es
Homepage: http://www.iciq.es
Prof. Dr. A. W. Kleij
Catalan Institute for Research and Advanced Studies (ICREA)
Pg. Lluꢂs Companys 23, 08010 Barcelona (Spain)
[**] G.F. kindly acknowledges financial support from the European
Community through the FP7-PEOPLE-2013-IEF project RENOVA-
CARB (622587). ICIQ, ICREA, and the Spanish MINECO (CTQ2011-
27385) are thanked for financial support. V.L. thanks the Generalitat
de Catalunya for an FI fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406645.
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remained limited and has been primarily based on terminal
epoxides, which are relatively facile to convert, whereas cyclic
cis-diol formation using CO2 as a key reagent has rarely been
studied.[15] Herein, we present a general approach towards
functional cyclic cis-diol synthons using a versatile catalytic
strategy that allows for the efficient conversion of (internal)
(hetero)cyclic oxirane substrates, suppressing undesired poly-
carbonate formation and thus controlling the stereoselectiv-
ity. Furthermore, this CO2-based method could be applied to
a broad substrate scope and exhibits good functional group
tolerance, providing a wide range of functional cis-diol
scaffolds in good to excellent yields.
Our first efforts focused on the chemoselective conversion
of some benchmark substrates, including cyclopentene oxide
and cyclohexene oxide (using 1 as the catalyst; see the
Supporting Information, Table S1), which have previously
been shown to easily form polycarbonates.[10,16] This screening
provided useful information about the required reaction
conditions and the nature of the (co)catalysts and their
loadings to achieve high and selective conversion into the
cyclic carbonate.[17] Whereas for the conversion of cyclo-
pentene oxide (Table S1, entries 1–9) both high selectivity for
the cyclic carbonate and appreciable conversion levels were
obtained at 708C (entries 4–6), for cyclohexene oxide under
similar reaction conditions (entries 10–14, 18–21), high selec-
tivity towards formation of the (undesired) polycarbonate
was observed. Two important requisites for selective con-
version into the cyclic organic carbonates were revealed upon
further variation of the reaction medium and the relative ratio
between complex 1 or 2 and the nucleophile (NBu4X,
bromide was generally found to be the best nucleophile;
entries 15–17). The use of a co-solvent (2-butanone) and an
excess of the nucleophile suppressed multiple, alternate CO2
and/or epoxide insertions, which would lead to undesired
polycarbonate formation with the opposite stereochemistry in
the carbonate unit (Scheme 1 and Table S1). Once the
reaction conditions had been optimized,[18] a series of cyclic
oxiranes were screened as substrates to form the correspond-
ing bi-, tri-, and tetracyclic organic carbonates 3a–3q
(Figure 1).
We were pleased to find that the optimized reaction
conditions gave access to a wide range of organic carbonates
in high yields and selectivities using catalysts 1 or 2 and
NBu4Br as the nucleophile additive. It was eventually
observed that catalyst 2 was slightly preferred.[18] In most
cases, rather mild temperatures (708C) could be used whereas
for the synthesis of bis(carbonate) 3h (66% yield), a some-
what elevated reaction temperature was required to achieve
high conversion of both oxirane fragments into their respec-
tive carbonates. Five-, six-, and seven-membered oxiranes
were all smoothly converted into the targeted products (3a–
3p), whereas the eight-membered-ring carbonate 3p could
only be isolated in 15% yield. The latter result suggests that
activation of larger cycloalkenyl-based epoxides through
coordination to the Lewis acid (such as 2) is more difficult,
and the ring-opening step[19] with the nucleophile is made
more complicated by the rather unreactive conformation of
the oxirane substrate. This hypothesis was further supported
by the fact that the reaction with cyclooctadiene oxide as the
Figure 1. Substrate scope for the formation of cyclic carbonates 3a–3r.
Reaction conditions, unless otherwise noted: 1 ([Fe]) or 2 ([Al]) as the
catalyst, NBu4Br ([Br]) as a co-catalyst in 2-butanone (MEK; 0.5–
2.0 mL), 18 h, 708C, p(CO2)=10 bar. Yields of isolated products are
given. Unless otherwise indicated, the carbonate configuration was
>98% cis. Reported d.r. values were determined by 1H and/or
13C NMR spectroscopy and relate to the relative configuration of the
carbonate unit and the C6 ring substituent. Fg=functional group.
substrate successfully furnished carbonate 3q (63%): The
presence of a double bond in the backbone is sufficient to
result in significantly higher conversion and yield of the cyclic
carbonate, which is likely to be the result of a more rigid
nature of the substrate backbone, which appears to play an
important role. However, it also (slightly) influenced the cis/
trans ratio of the carbonate unit (87:13) as testified by the
characteristic NMR patterns of the cis and trans isomers.
When the bis(oxirane) derived from cyclooctadiene was used
as the substrate, the oxirane-derived trans carbonate 3r,
which, as 3d, bears a synthetically useful epoxide group, was
isolated in 52% yield. Surprisingly, the presence of the
epoxide ring in 3r (sp3- versus sp2-hybridized carbon centers
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when compared to 3q) thus directed the stereoselectivity
towards the trans carbonate.
Interestingly, the catalyst systems based on 1 or 2 tolerate
a number of useful functional groups, including ether (3b),
oxirane (3d, 3r), endo- and exo-cyclic double bonds (3 f, 3j,
and 3q), trimethoxysilyl (3l), and ester (3m) moieties.
Remarkably, the tetracyclic carbonate product 3d was
obtained from the corresponding bis(oxirane) derivative
without affecting the other epoxide unit and thus provides
clean access to a bifunctional intermediate.[20] The prepara-
tion of the substituted bicyclic carbonates 3h–3m is note-
worthy; for these products, analysis by NMR spectroscopy
was more challenging owing to the presence of three
stereocenters in these molecules; however, for a representa-
tive example, we were able to separate the set of diastereo-
isomers (3m) by column chromatography (Figure S1) and
analyzed one of these (the minor isomer 3ma; ester group
equatorial) by X-ray analysis.[21] The other, major diastereo-
isomer 3mb (ester group axial) was isolated as a viscous liquid
(for details, see the Supporting Information). The combined
data was used to assign the configurations of both isomers
that are present in the isolated product and also confirmed the
cis nature of the carbonate unit.
The next step involved the formation of the cis-diol
products from the carbonate precursors 3a–3r by treatment
with a suitable base. Most diol targets were readily formed in
good to excellent yields (Figure 2). The deprotection of
carbonate 3l was complicated as the resulting product was
difficult to analyze because it was formed as a mixture of
rather insoluble components. It is likely that the silyloxy
fragment gave rise to insoluble gels through cross-hydrolysis.
However, basic deprotection of 3m could be carried out with
a weaker base (K2CO3), selectively giving cis diol 4m in high
yield (80%). The yield of tetraol 4h (40%) was lower than
generally observed for the other diol products as its isolation
was more difficult. The oxirane group in 4d (89%) surpris-
ingly remained unaffected under basic conditions showing
again its high stability. The relative cis configuration in these
diols was further supported by X-ray analysis of diol
derivatives 4c and 4q (Figure 2, insets).[21] These results
clearly show the generality of this approach towards poten-
tially useful cyclic cis-diol scaffolds.
Figure 2. Formation of the cis-diol products 4a–4r from precursors
3a–3r. Reaction conditions: NaOH (5 equiv) in H2O, 3 h, RT. For 4m,
K2CO3 (3 equiv in MeOH) was used. Yields of isolated products are
given. Unless otherwise specified, the diol configuration was >98%
cis. Reported d.r. values were determined by 1H and/or 13C NMR
spectroscopy and relate to the relative configuration of the carbonate
unit and the C6 ring substituent, except for in the case of 4r. Further
details for the analysis of 4r are provided in the Supporting Informa-
tion. Fg=functional group.
as a temporary protecting group. These attractive features
combined with the simple operational characteristics of this
catalytic method (no special precautions required) may give
a valuable starting point for the synthesis of tri- and even
tetra-substituted cis-diol synthons and may stimulate the
advancement of alternative asymmetric preparations of this
important class of organic compounds.
In summary, we have presented an efficient and practical
method towards the formation of cyclic cis diols with ample
scope and functional group tolerance. These synthons were
afforded in high yield and (diastereo)selectivity with the
preparation of their cis-configured organic carbonate precur-
sors being the key to success. The selective formation of cyclic
carbonate versus poly(carbonate) (see Scheme 1) from the
cyclic epoxides suggests that two consecutive SN2 reactions
take place, thus preserving the initial relative configuration of
the two carbon centers in the oxirane unit and leading to cis-
diol formation after basic treatment. This double-inversion
mechanistic pathway has been independently studied and
proposed by various groups.[22]
Experimental Section
Typical procedure for organic carbonate formation: A 30 mL stainless
steel autoclave was charged with the epoxide precursor (0.5 g) along
with the correct loadings of catalyst, co-catalyst, and MEK (0.5 mL).
The autoclave was subjected to three cycles of pressurization and
depressurization with CO2 (0.5 MPa, 5 bar) before final pressure
stabilization (1.0 MPa, 10 bar). The autoclave was sealed and heated
to the required temperature for 18–66 hours. Afterwards, an aliquot
of the crude reaction mixture was collected and analyzed by 1H NMR
spectroscopy to determine the reaction conversion (solvent: CDCl3).
Then, the product was isolated/purified by filtration through a silica
pad or by column chromatography on silica gel.
The current approach is characterized by the use of easily
accessible, air-stable, and powerful catalysts that are derived
from aminotriphenolate complexes and incorporate inexpen-
sive and earth-abundant metals, and the use of carbon dioxide
Typical deprotection procedure: The respective carbonate
(1 mmol) was dissolved in NaOH (5 mL, 1m). The mixture was left
stirring for 2–3 hours until complete dissolution of the starting
material and formation of a homogeneous, pale yellow solution was
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achieved. Hereafter, the diol was extracted with EtOAc and isolated
by removal of the solvent in vacuo.
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Received: June 27, 2014
Revised: July 15, 2014
Published online: && &&, &&&&
Keywords: aluminum · carbonates · epoxides ·
.
homogeneous catalysis · syn diols
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[17] As previously reported for the FeIII complex 1, acyclic internal
oxiranes can also be smoothly converted into the corresponding
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we have used this method to access the cyclic carbonate product
starting from cis-2,3-epoxybutane (68% yield, cis/trans ratio of
the carbonate: 99:1) and converted it into the corresponding
acyclic cis diol (87%). For details, see the Supporting Informa-
tion.
[18] Although iron complex 1 was initially used, we found that
complex 2 consistently resulted in slightly higher yields and
selectivities, and thus the latter was used throughout the rest of
the scope studies; see also Table S1, entry 16 versus entry 15.
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[20] Separate control experiments carried out with an oxirane based
on this subunit (i.e., 3-oxatricyclo[3.2.1.02,4]octane or norbor-
nene oxide) gave the crude carbonate product in only 6% yield
after prolonged reaction times (> 3 days) and at elevated
temperatures (1008C), showing the unreactive nature of this
oxirane unit. For more details, see the Supporting Information.
[21] CCDC 1005696 (3ma), 1005695 (4c), and 1009477 (4r) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[22] See, for instance, Ref. [7a], [7d], and [19].
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Temporary Protecting Groups
V. Laserna, G. Fiorani, C. J. Whiteoak,*
E. Martin, E. Escudero-Adꢁn,
A. W. Kleij*
&&&& — &&&&
Carbon Dioxide as a Protecting Group:
Highly Efficient and Selective Catalytic
Access to Cyclic cis-Diol Scaffolds
Protective Carbon: Aminotriphenolate
complexes of FeIII and AlIII are highly
efficient and selective catalysts for the
conversion of functional (multi)cyclic
oxiranes into the corresponding cis car-
bonates. Basic hydrolysis of the latter
provides a series of useful cyclic cis-diol
scaffolds in high yield. In this process,
CO2 acts as both a temporary protecting
group and an oxygen donor.
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