Communications
Two control experiments were also carried out. Complex 3c
and tetrabutylammonium bromide were used as catalysts for
the reaction of substrates 6 at room temperature and one bar
pressure, and 100% retention of the epoxide stereochemistry
was observed.[7] Cis-6 was also treated with one equivalent of
carbonato complex 5 at 508C under 1 bar nitrogen in the ab-
sence of carbon dioxide. Exclusive formation of trans-7 was ob-
served, indicating that this reaction proceeded with complete
inversion of epoxide stereochemistry.
The stereochemical consequences at the more hindered
carbon atom of the epoxide was investigated using (R)-styrene
oxide [(R)-1c] and (S)-glycidyl phenyl ether [(S)-1d] at 508C
and 10 bar carbon dioxide pressure. In the presence of tetrabu-
tylammonium bromide, cyclic carbonate synthesis from either
enantiomerically pure epoxide proceeded with 100% retention
of the epoxide stereochemistry as determined by chiral
HPLC.[7] However, in the absence of tetrabutylammonium bro-
mide, (R)-1c gave a 97:3 ratio of (R)- and (S)-2c and (S)-1d
gave a 99.5:0.5 ratio of (S)- and (R)-2d indicating that some
cleavage of the more substituted carbon-oxygen bond of the
epoxide occurred.
Figure 3. 13C NMR spectrum of complex 3c in CDCl3 in the presence of 13CO2
(top) in comparison to parent 3c in CDCl3 (bottom).
3c at À788C, confirming that the insertion is feasible and
facile, as predicted by DFT calculations. The resulting 13C NMR
spectrum (Figure 3) showed a resonance at 165.60 ppm, char-
acteristic of a carbonato bridged complex,[11] and a shift in the
other resonances relative to 3c. 13C NMR exchange spectrosco-
py (EXSY) experiments showed that complex 5 was in equilibri-
um with 3c and magnetisation transfer was observed between
the carbonato group of complex 5 (165.60 ppm) and dissolved
13CO2 (124.83 ppm).[7]
A catalytic cycle for cyclic carbonate synthesis catalysed by
complexes 3 in the absence of tetrabutylammonium bromide
consistent with the above results is shown in Scheme 4.
Carbon dioxide first inserts into the aluminium-oxygen bond
of complex 3 to give carbonato complex 5. Subsequent com-
plexation of the epoxide gives adduct 8 which undergoes in-
tramolecular epoxide ring-opening at the less hindered carbon
atom of the epoxide by the carbonate with inversion of config-
uration to give intermediate 9. From complex 9 two pathways
lead to the cyclic carbonate. In path 1, the alkoxide of complex
9 cyclises onto the carbonyl of the carbonato group forming
the cyclic carbonate and regenerating complex 3. This path-
way involves a single inversion of configuration at the epoxide
and requires no additional carbon dioxide beyond formation
of 5. Hence it accounts for the inversion of epoxide configura-
tion observed when complex 5 is used to induce the reaction
in the absence of carbon dioxide.
To investigate the relevance of carbonato complex 5 to the
catalytic cycle, it was dissolved together with tetrabutylammo-
nium bromide in epoxide 1b under nitrogen at room tempera-
1
ture. H NMR and GC analysis[7] confirmed the formation of car-
bonate 2b, showing that complex 5 is catalytically active and
that the carbonato bridge acts as a source of carbon dioxide.
The reaction mechanism in the absence of tetrabutylammo-
nium bromide was then studied using monodeuterated cis-
and trans-decylene oxide[12] 6 as substrates at 508C and 10 bar
carbon dioxide pressure to investigate the stereochemistry of
the reaction (Scheme 3). The results indicate that the reaction
proceeds mainly with retention of epoxide stereochemistry,
giving a 3:1 ratio of retention to inversion products. Formation
of both cis- and trans-cyclic carbonate 7 in the absence of tet-
rabutylammonium bromide indicates that two different reac-
tion pathways are operative.
Pathway 2 involves a second carbon dioxide insertion into
the aluminium alkoxide bond[13] of complex 9 to form bis-
carbonato complex 10. The carbonato groups in complex 10
will be good nucleophiles and good leaving groups, allowing
formation of the cyclic carbonate to occur by a second intra-
molecular substitution reaction with a second inver-
sion of stereochemistry, giving overall retention of
epoxide stereochemistry. Formation of complex 10
also provides an explanation of the partial racemiza-
tion by SN1 type cleavage of the carbonato group at-
tached to the more substituted carbon atom.
Additional DFT calculations were carried out to
support the routes shown in Scheme 4.[7] Thus, carbo-
nato bridged complexes were found to coordinate
epoxides as required for the formation of species 8.
The six-coordinate aluminium in complex 8 was
found to exist in a cis-b configuration,[14] which ena-
bles the intramolecular rearrangement to form com-
plex 9.
Scheme 3. Cyclic carbonate synthesis from deuterated epoxides 6.
ChemSusChem 2016, 9, 791 – 794
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