Titanium(salen)-catalysed synthesis of Di- and trithiocarbonates from epoxides and carbon disulfide

Article abstract of DOI:10.1002/cctc.201400005

The combination of a bimetallic titanium(salen) complex [Ti(salen)O] ₂ and either tetrabutylammonium bromide or tributylamine forms a highly active catalyst system for the reaction between epoxides and carbon disulfide to lead to di- and/or trithiocarbonates. Reactions can be performed at 90 °C by using just 0.5-1.0 mol % of the catalysts. The reactions proceed with the inversion of the epoxide configuration and on the basis of kinetic and spectroscopic evidence, a mechanism to account for the results is proposed. Salen away on a kinetic sea: The combination of [Ti(salen)O]₂ and tetrabutylammonium bromide or tributylamine catalyzes the addition of carbon disulfide to epoxides to form predominantly dithiocarbonates. Ten examples are reported that give the dithiocarbonates in 33-99 % isolated yield. A mechanistic study based on reaction kinetics, stereochemistry, and NMR spectra of reaction mixtures allow a catalytic cycle to be proposed.

Full text of DOI:10.1002/cctc.201400005

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DOI: 10.1002/cctc.201400005
Titanium(salen)-Catalysed Synthesis of Di- and
Trithiocarbonates from Epoxides and Carbon Disulfide
Christopher Beattie[b] and Michael North*[a, b]
The combination of a bimetallic titanium(salen) complex [Ti-
(salen)O]2 and either tetrabutylammonium bromide or tributyl-
amine forms a highly active catalyst system for the reaction
between epoxides and carbon disulfide to lead to di- and/or
trithiocarbonates. Reactions can be performed at 908C by
using just 0.5–1.0 mol% of the catalysts. The reactions proceed
with the inversion of the epoxide configuration and on the
basis of kinetic and spectroscopic evidence, a mechanism to
account for the results is proposed.
Introduction
The reaction between epoxides and heterocumulenes is
known to lead to a wide range of five-membered ring hetero-
cycles, which include cyclic carbonates 1,[1] cyclic mono-, di-
and trithiocarbamates 2–4,[2] oxazolidinones 5,[3] 2-oxathiolani-
Scheme 1 often requires harsh reaction conditions (high tem-
peratures and pressures), even in the presence of a suitable
catalyst.[1–6]
In recent papers, we have reported that the combination of
bimetallic aluminium(salen) complex 9 and tetrabutylammoni-
um bromide catalyses the formation of cyclic carbonates
1 from epoxides 10 and carbon dioxide[15] or the formation of
di- and trithiocarbonates 3 and 4 from epoxides and carbon
disulfide.[16] Subsequently, mononuclear aluminium(salen) com-
plexes were shown to catalyse the reaction between epoxides
and carbon disulfide under harsher reaction conditions.[17]
Complex 9 (in the absence of a cocatalyst) would also catalyse
the synthesis of oxazolidinones 5 from epoxides and isocya-
nates[18] (Scheme 2).
mines 6,[4] 2-oxazolidinimines[5]
7 8
and cyclic sulfites[6]
(Scheme 1). These heterocycles have a wide range of applica-
Scheme 1. Synthesis of heterocycles from epoxides and heterocumulenes.
tions. For example, cyclic carbonates 1 are used as electrolytes
in lithium ion batteries[7] and as polar aprotic solvents.[8] Oxazo-
lidinones 5 have a range of biomedicinal applications[9] and are
precursors of b-aminoalcohols.[10] Dithiocarbonates 3 and tri-
thiocarbonates 4 have been shown to possess radioprotective
activity.[11] Dithiocarbonates have also been used in polymer
syntheses,[12,13] and trithiocarbonates have been found to pos-
sess insecticidal activity.[14] However, the chemistry shown in
[a] Prof. M. North
Green Chemistry Centre of Excellence
Department of Chemistry
The University of York
Heslington, York, YO10 5DD (UK)
Fax: (+44)01904-322-705
E-mail: Michael.north@york.ac.uk
Scheme 2. Synthesis of heterocycles by using catalyst 9.
[b] C. Beattie, Prof. M. North
School of Chemistry
We have also reported the use of salen complexes of alumi-
nium,[19] titanium[20,21] and vanadium(V)(salen)[12,20] complexes 9,
11 and 12a–g, respectively, as catalysts for the asymmetric ad-
dition of various cyanide sources to aldehydes and ketones
(Scheme 3). In this paper we show that although complexes 11
Newcastle University
Newcastle upon Tyne, NE1 7RU (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201400005.
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conversion of styrene oxide to trithiocarbonate 4a (Table 1,
entry 2). The reaction conditions for the use of complex 11
were then optimised by using 1,2-epoxyhexane (10b) as sub-
strate as styrene oxide was known to be exceptional in provid-
ing only the trithiocarbonate product.[15] The results of this
study are shown in Table 2. Complex 11 displayed some cata-
Table 2. The use of complex 11 to catalyse the reaction between 1,2-ep-
oxyhexane and carbon disulfide.[a]
Entry
Catalyst
[mol%]
Bu4NBr
[mol%]
t
[h]
Conv.
[%]
3b/4b
Scheme 3. Asymmetric cyanohydrin synthesis by using catalysts 9, 11 and
12a–g.
1
2
3
4
5
6
7
8
11 (5)
11 (5)
11 (5)
11 (5)
0
11 (1)
11 (0.5)
9 (5)
0
0
5
5
5
1
0.5
5
3
24
3
24
24
24
24
16
20
62
62
100
67
100
97
45:55
100:0
100:0
64:36
60:40
75:25
75:25
43:57
and 12a–g have only very low catalytic activity for the synthe-
sis of cyclic carbonates from epoxides and carbon dioxide,
complex 11 is a more active catalyst than aluminium complex
9 for the reaction between epoxides and carbon disulfide.
87
[a] Reaction conditions: 1.8 equiv. CS2, 908C. Conversions and 3b/4b
1
ratio determined by H NMR analysis of the reaction mixture.
Results and Discussion
Initially, the optimal conditions for the synthesis of styrene car-
bonate (1a) from styrene oxide (10a) and carbon dioxide, pre-
viously determined when using complex 9 as catalyst,[22] were
applied to complexes 11 and 12a–g (with seven different L
groups). All the complexes, however, displayed negligible cata-
lytic activity compared to that of complex 9 under identical
conditions (Table 1). Similarly disappointing results were ob-
tained for the synthesis of trithiocarbonate 4a from styrene
oxide (10a) and carbon disulfide using complexes 12a–g
under the conditions previously optimised for this reaction
with aluminium complex 9.[16] Interestingly, the vanadium iso-
thiocyanate complex (12g) was much more catalytically active
than any other vanadium(salen) complex, although it still gave
less than half the conversion obtained using aluminium
complex 9.
lytic activity even in the absence of the tetrabutylammonium
bromide cocatalyst (Table 2, entries 1 and 2). However, the ad-
dition of tetrabutylammonium bromide had a synergistic effect
on the catalytic activity (compare Table 2, entries 1 and 3 or 2
and 4) just as observed previously for reactions catalysed by
complex 9.[16] At a 5 mol% catalyst loading, tetrabutylammoni-
um bromide also had significant catalytic activity if used in the
absence of complex 11 (Table 2, entry 5). The combination of
complex 11 and tetrabutylammonium bromide was a more
active catalyst system than complex 9 and tetrabutylammoni-
um bromide and this allowed the catalyst loading to be re-
duced to 1 or even 0.5 mol% of each catalyst (Table 2, entries 6
and 7), which still gave a higher conversion than that obtained
by using 5 mol% of complex 9 and tetrabutylammonium bro-
mide (Table 2, entry 8).
Titanium complex 11 was found to be a much more active
catalyst for di- and trithiocarbonate synthesis to give complete
Another contrast between the reactions catalysed by com-
plexes 9 and 11 is the ratio of di- to trithiocarbonate formed.
At 908C, complex 9 gave trithiocarbonate 4b as the major
product, whereas complex 11 gave predominantly or exclu-
sively dithiocarbonate 3b. The dithiocarbonate is the initial
product of the reaction,[16] and these results suggest that its
conversion to trithiocarbonate 4b is also catalysed by com-
plexes 9 and 11, for which 9 is the more effective catalyst for
the rearrangement.
Table 1. The use of complexes 11 and 12a–g as catalysts for styrene
carbonate 1a or styrene trithiocarbonate 4a synthesis.
Entry
Complex (L)
Conv. to
1a [%][a]
Conv. to
4a [%][b]
1
2
3
4
5
6
7
8
9
9
11
12a (Cl)
12b (F)
12c (Br)
12d (EtOSO3)
12e (NO3)
12 f (CF3SO3)
12g (NCS)
98[22]
13
12
9
3
2
2
3
2
91[16]
100
0
0
19
16
23
16
44
To explore the generality of this process, the conditions of
entry 7 in Table 2 were taken initially as standard and applied
to other terminal epoxides to give the results detailed in
Table 3. For the unfunctionalised substrates propylene oxide
(10c) and 1,2-epoxydecane (10d), the use of 0.5 mol% of each
catalyst component continued to give high conversions to the
dithiocarbonates 3c and d (Table 3, entries 1 and 2). However,
the chloro-functionalised substrate epichlorohydrin (10e) gave
a much lower conversion under these conditions (Table 3,
entry 3). Therefore, the catalyst loadings were increased to
[a] Reaction conditions: 2.5 mol% catalyst, 2.5 mol% Bu4NBr, solvent free,
1
RT, 1 bar CO2 pressure for 24 h. Conversions determined by H NMR analy-
sis of the reaction mixture. [b] Reaction conditions: 1.8 equiv. CS2, 5 mol%
catalyst, 5 mol% Bu4NBr, solvent free, 908C, 24 h. Conversions determined
by 1H NMR analysis of the reaction mixture.
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Table 3. The use of complex 11 and Bu4NBr to catalyse the reaction be-
tween terminal epoxides 10a–h and carbon disulfide.[a]
Table 4. The use of complex 11 to catalyse the reaction between disub-
stituted epoxides 13a–d and carbon disulfide.[a]
Entry
11/Bu4NBr
[mol%]
10 (R1)
Conv. or yield
[%]
3/4
Entry
Epoxide
Conv.
[%]
Yield
[%]
14/15
1
2
3
4
5
6
7
8
0.5:0.5
0.5:0.5
0.5:0.5
1:1
1:1
1:1
1:1
1:1
1:1
1:1
10c (CH3)
79 (conv.)
83 (conv.)
36 (conv.)
94 (yield)
99 (yield)
58 (yield)
98 (yield)
96 (yield)
76 (yield)
98 (yield)
86 (yield)
66:34
80:20
44:56
60:40
64:36
34:66
81:19
75:25
75:25
81:19
0:100
1
2
3
4
5[b]
13a
13b
13c
13d
13d
52
0
0
12
42
50
–
–
–
33
63:37
–
–
83:17
83:17
10d (nOct)
10e (CH2Cl)
10c (CH3)
10d (nOct)
10e (CH2Cl)
10 f (nDec)
10b (nBu)
10g (Et)
[a] Reaction conditions: 1.8 equiv. CS2, 908C for 24 h. Conversions and 14/
15 ratio determined by H NMR analysis of the reaction mixture. [b] Reac-
tion time 72 h.
1
9
10
11
10h (CH2OPh)
10a (Ph)
1:1
1
son of the H and 13C NMR spectra with those previously re-
[a] Reaction conditions: 1.8 equiv. CS2, 908C. Conversions and 3/4 ratio
determined by H NMR spectroscopy of the reaction mixture.
1
ported[16] confirmed that both 14a and 15a had trans-fused
rings as found previously for the product of the reaction cata-
lysed by complex 9 and tetrabutylammonium bromide. How-
ever, attempts to extend this chemistry to other cyclic epox-
ides were unsuccessful as neither cyclopentene oxide (13b)
nor cyclooctene oxide (13c) underwent any reaction (Table 4,
entries 2 and 3). 1,1-Disubstituted epoxide 13d gave a low
conversion predominantly to dithiocarbonate 14d under the
standard conditions (Table 4, entry 4), which could be in-
creased to 42% conversion with 33% isolated yield by increas-
ing the reaction time (Table 4, entry 5).
1 mol% (cf. Table 2, entry 6) and used to catalyse the reaction
between eight terminal epoxides 10a–h and carbon disulfide
(Table 3, entries 4–11). These reactions were all worked up, and
thiocarbonates 3 and 4a–h were purified by chromatography
as reported previously[16] to give isolated yields of the thiocar-
bonates. Unfunctionalised aliphatic epoxides 10b–d, f and g
gave high yields predominantly of dithiocarbonate 3b–d, f
and g under these conditions (Table 3, entries 4, 5, 7–9). The
yield of thiocarbonate obtained from epichlorohydrin 10e im-
proved compared to the use of 0.5 mol% catalyst, and this
substrate gave predominantly trithiocarbonate 4e (Table 3, en-
tries 3 and 6). 3-Phenoxypropylene oxide (10h) was an excel-
lent substrate that gave predominantly dithiocarbonate 3h
product (Table 3, entry 10). Styrene oxide 10a gave only tri-
thiocarbonate 4a, as reported previously for reactions cata-
lysed by complex 9 and tetrabutylammonium bromide.
A kinetic study was performed to investigate the reaction
mechanism. This required the use of a solvent, and preliminary
studies showed that the reaction between 3-phenoxypropy-
lene oxide 10h and carbon disulfide catalysed by complex 11
(5 mol%) and tetrabutylammonium bromide (5 mol%) pro-
ceeded to over 85% conversion at 508C in CDCl3 over 12 h.
This allowed kinetic experiments to be performed in CDCl3 and
monitored by 1H NMR spectroscopy. A typical set of data is
shown in Figure 1, which illustrates that under these condi-
tions the reaction has an appreciable induction period (approx-
imately 2.5 h). However, once the reaction starts, it shows
a good fit to first-order kinetics exactly as found earlier for re-
actions catalysed by complex 9.[16] Reactions performed at vari-
ous initial concentrations of epoxide 10h and carbon disulfide
confirmed that, after the induction period, the reaction was
first order in epoxide concentration and zero order in carbon
disulfide concentration (rate=k[epoxide]). The reactions show
no change in the rate at 50% epoxide conversion, which indi-
cates that this system will not accomplish the kinetic resolu-
tion of epoxides even though an enantiomerically pure catalyst
is used with a racemic substrate.
In view of the high catalytic activity of the complex 11/tetra-
butylammonium bromide system and to further extend the
scope of the chemistry, the use of disubstituted epoxides 13a–
d was investigated (Scheme 4). Disubstituted epoxides are
Scheme 4. Di- and trithiocarbonate synthesis from disubstituted epoxides.
known to be much more difficult substrates for thiocarbonate
synthesis, and for reactions catalysed by complex 9 and tetra-
butylammonium bromide, only cyclohexene oxide (13a) was
found to be a substrate, which gave trans-di- and trithiocar-
bonates 14a and 15a.[16] Reactions with disubstituted epoxides
were performed by using 1 mol% of complex 11 and tetrabu-
tylammonium bromide, and the results are presented in
Table 4.
Complex 11 is known to exist in CDCl3 in a concentration-de-
pendent equilibrium with the corresponding monometallic
species 16 (Scheme 5).[23] At the catalyst concentrations used
in this work (2–7 mm), the bimetallic species 11 is known to be
the major species present in solution, but either 11 or 16
could be the catalytically active species. To investigate this, the
reaction order with respect to catalyst concentration was de-
termined by performing kinetic experiments at four different
catalyst concentrations. Each kinetic run was performed in du-
plicate, and Figure 2 shows the resulting plots of log[11]
Cyclohexene oxide 13a was found to be a substrate for the
complex 11-catalysed reaction (Table 4, entry 1) and compari-
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Figure 2. Plots of log[11] versus log(k1avg) (top) and [11] versus k1avg
(bottom). Reactions were performed in duplicate at 508C in CDCl3 with
[10h]0 =1.4m, [CS2]0 =2.6m, [Bu4NBr]=71 mm and [11]=28–71 mm.
Figure 1. Kinetic plots for a reaction performed at 508C in CDCl3 with
[10h]0 =1.4m, [CS2]0 =2.6m, [11]=71 mm and [Bu4NBr]=71 mm. Top: con-
version versus time plot that shows the induction period. Bottom: first-order
kinetics plot with the time adjusted to ignore the first 12600 s of data.
Scheme 5. Equilibrium between bimetallic and monometallic complexes 11
and 16.
versus log(k1avg) and [11] versus k1avg, which both indicate that
the reaction is first order with respect to the catalyst concen-
tration. We have shown previously[23] that a first-order depend-
ence of a reaction rate on the concentration of 11 indicates
that at least one species within the catalytic cycle contains the
same number of metal ions as the major species (11 or 16)
present in solution, which implies a bimetallic species in the
catalytic cycle in this case.
The NMR spectroscopic data acquired from the kinetic ex-
periments also provided important information on the species
present during the reaction. A typical stacked plot of spectra
are shown in Figure 3. The peaks that correspond to com-
plexes 11 and 16 are labelled, and the spectra confirm that,
even at 508C, complex 11 is the predominant species present
in solution at the start of the reaction. In addition to the imine
peaks that correspond to complexes 11 and 16, the NMR spec-
tra presented in Figure 3 show three additional peaks (labelle-
d A) in the region d=8.25–8.55 ppm, in which the imine
groups of monometallic titanium(salen) complex 16 appear.
Control experiments (Figure 4) showed that these peaks were
not present in the spectra of the pure catalyst, if the catalyst
and carbon disulfide (40 equivalents) were mixed or if the cata-
1
Figure 3. The imine region of H NMR spectra recorded every 30 min (from
top to bottom) during the conversion of 3-phenoxypropylene oxide 10h
into dithiocarbonate 3h at 508C in CDCl3.
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1
Figure 4. The imine region of the H NMR spectra of complex 11 (top),
Scheme 7. Proposed reaction mechanism in the presence of Bu4NBr.
11+40 equivalents of carbon disulfide (next to top), 11+40 equivalents of
10h (middle), 11+40 equivalents of 3h (next to bottom) and 11+1 equiva-
lent of Bu4NBr (bottom).
monium bromide acts as an in situ source of tributylamine.
There is literature precedent for this reverse Menschutkin reac-
tion.[16,22] The tributylamine can then react with carbon disul-
fide to form a dithiocarbamate 18.[2a,g,h,24] Complex 11 is in
equilibrium with monometallic species 16. One molecule of 16
can act as a Lewis acid to form adduct 19 with the epoxide,
and the other can react with dithiocarbamate 18 to form com-
plex 20. Complexes 19 and 20 can re-assemble to form the
key bimetallic complex 21. Complex 21 is the key species in
the catalytic cycle as it contains both an activated epoxide and
a dithiocarbamate-based nucleophile that can ring-open the
epoxide intramolecularly to form monometallic complex 22
and regenerate one molecule of 16. The catalytic cycle is com-
pleted by the collapse of 22 to form the dithiocarbonate prod-
uct and regenerate the other molecule of 16.
lyst and tetrabutylammonium bromide (1 equivalent) were
mixed. However, a mixture of catalyst and 40 equivalents of 3-
phenoxypropylene oxide 10h did give rise to three new peaks
in this region, consistent with the formation of complex 17 by
coordination of the epoxide to the free coordination site of
complex 16 (Scheme 6).
This mechanism involves a single substitution reaction at
the epoxide and hence accounts for the inversion of stereo-
chemistry observed if cyclohexene oxide 13a is used as the
substrate. It is also consistent with the first-order kinetics ob-
served with respect to catalyst 11 as the catalytic cycle in-
volves bimetallic complex 21. The greater catalytic activity of
complex 11 compared to aluminium complex 9 can be ex-
plained by the higher Lewis acidity of 11.[25]
Catalyst 11 also shows some catalytic activity in the absence
of tetrabutylammonium bromide (Table 2, entries 1 and 2),
which indicates that an alternative mechanism that does not
involve tetrabutylammonium bromide or tributylamine must
also be possible. A catalytic cycle for this situation is shown in
Scheme 8. In this case, complex 16 rather than tributylamine
reacts with carbon disulfide to form 23. Complexes 19 and 23
can again re-assemble to form 24, which again contains an ac-
tivated epoxide and a good sulfur-based nucleophile. Intramo-
lecular ring-opening of the epoxide in 24 leads to neutral com-
plex 25, which possesses a nine-membered ring. Complex 25
can collapse in a single step to form the dithiocarbonate prod-
Scheme 6. Formation of complex 17.
Towards the end of the reaction, three new peaks (label-
led B) appear in the region d=7.80–8.25 ppm in which the
imine groups of bimetallic titanium(salen) complex 11 occur.
The 1H NMR spectrum of a mixture of complex 11 and 40
equivalents of 3/4h showed three new peaks in this region of
the spectrum (Figure 4), so these peaks are assigned tentative-
ly to a catalyst–product complex. The other feature of the
NMR spectroscopic data shown in Figure 3 is that all the peaks
change position over time, which indicates that the titanium-
(salen) species present are all in equilibrium with one another.
This equilibrium changes as the concentration of epoxide de-
creases and the concentration of dithiocarbonate increases
during the reaction.
A reaction mechanism consistent with the kinetic and NMR
data is shown in Scheme 7. In this mechanism, tetrabutylam-
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Figure 5. Conversion versus time plot for a reaction performed at 508C in
CDCl3 with [10h]0 =1.4m, [CS2]0 =2.6m, [11]0 =71 mm and [Bu3N]0 =71 mm.
form a minimum concentration of a species later in the catalyt-
ic cycle. The most likely explanation is, therefore, the need to
form bimetallic complex 21, which requires a reaction between
two monometallic species, both of which will be present at rel-
atively low concentrations.
Scheme 8. Proposed reaction mechanism in the absence of Bu4NBr.
uct and re-form 11. The preference for the catalytic cycle
shown in Scheme 7 in the presence of tetrabutylammonium
bromide may be because tributylamine is a better nucleophile
to react with carbon disulfide and/or because neutral complex
25 is relatively stable, which slows the catalytic cycle shown in
Scheme 8.
Conclusions
The combination of bimetallic titanium(salen) complex 11 and
tetrabutylammonium bromide or tributylamine forms a highly
active catalyst system for the synthesis of di- or trithiocarbon-
ates from epoxides and carbon disulfide. Compared to the bi-
metallic aluminium(salen) catalysed reactions reported previ-
ously, the catalyst loading can be reduced by 5–10-fold and
the reactions exhibit a greater propensity to form the di-
rather than trithiocarbonate. A combination of kinetic and
spectroscopic studies allowed a catalytic cycle to be proposed,
which is compatible with all the experimental observations.
As Scheme 7 suggests that the only role of tetrabutylammo-
nium bromide is to act as an in situ source of tributylamine, it
should be possible to use tributylamine instead of tetrabuty-
lammonium bromide as the cocatalyst. This has been demon-
strated previously for reactions catalysed by aluminium com-
plex 9[16] and was also found to be the case for reactions cata-
lysed by titanium complex 11 (Table 5). A comparison of the
Table 5. The use of complex 11 and Bu3N to catalyse the reaction be-
tween terminal epoxides and carbon disulfide.[a]
Entry
10a–h (R1)
Conv.
[%]
Yield
[%]
3/4
Experimental Section
Catalysts 11 and 12a–g were prepared as described previous-
ly.[12,20,21] All other compounds were commercially available and
used as supplied. 1H NMR and 13C NMR spectra were recorded in
CDCl3 as the solvent at 25–508C by using a Bruker Avance300
spectrometer operating at 300 or 75 MHz, respectively, or
a JEOL400 spectrometer operating at 400 or 100 MHz, respectively.
Chemical shifts are quoted in ppm relative to tetramethylsilane. IR
spectra were recorded at RT by using a Varian 800 FTIR Scimitar
series spectrometer, and we measured specific absorbance intensi-
ties as: broad (br), strong (s), medium (m) or weak (w). Melting
points were determined by using a Stuart SMP3 system.
1
2
3
4
5
6
7
10a (Ph)
95
94
100
100
100
87
83
89
94
97
94
82
96
0:100
10b (nBu)
10c (CH3)
10d (nOct)
10 f (nDec)
10g (Et)
75:25
63:37
83:17
80:20
72:28
74:26
10h (CH2OPh)
100
[a] Reaction conditions: 1.8 equiv. CS2, 1 mol% of both complex 11 and
Bu3N, reaction time 24 h at 908C. Conversions and 3/4 ratio determined
by 1H NMR analysis of the reaction mixture.
data in Tables 3 and 5 shows that essentially identical yields
and 3/4 ratios were obtained by using either tetrabutylammo-
nium bromide or tributylamine as cocatalyst, consistent with
the role of tetrabutylammonium bromide being to generate
tributylamine in situ.
General procedure for reactions catalysed by complex 11
and Bu4NBr
CS2 (0.09 mL, 1.49 mmol), an epoxide (0.835 mmol), 11 (1 mol%,
10 mg, 0.00835 mmol) and Bu4NBr (1 mol%, 2.8 mg, 0.00835 mmol)
were placed in a sealed Young flask and stirred at 908C for 24 h.
The solution was evaporated and, if necessary, the residue was pu-
rified by column chromatography (CHCl3/hexane 1:1) to give an
unseparated mixture of the di- and trithiocarbonate, except for 4a,
which was obtained as the pure trithiocarbonate, and 3b/4b, 3h/
4h and 14a/15a, for which the di- and trithiocarbonates were sep-
arable by chromatography. The NMR spectroscopic data for the di-
and trithiocarbonate mixtures matched those in the literature.[16]
The reaction between 10h and carbon disulfide catalysed by
1
11 and tributylamine was monitored by H NMR spectroscopy
at 508C in CDCl3. The induction period observed for reactions
with tetrabutylammonium bromide as the cocatalyst is still
present if tributylamine is used as the catalyst (Figure 5). This
suggests that the induction period is not because of the need
to build up a concentration of tributylamine but the need to
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Data for 4a: M.p. 83–848C (lit.[16] 85–868C); IR (ATR): n˜max =3050
Acknowledgements
1
(m), 2918 (w), 1487 (s), 1451 cmÀ1 (s); H NMR: d=4.05 (1H, dd, J=
12.0, 6.0 Hz), 4.20 (1H, dd, J=12.0, 9.0 Hz), 5.67 (1H, dd, J=12.0,
6.0 Hz), 7.2–7.6 ppm (5H, m); 13C NMR: d=49.6, 64.0, 127.4, 129.0,
129.1, 135.0, 227.2 ppm.
The authors thank the EPSRC for a studentship to C.B.
Keywords: aluminium · N,O ligands · reaction mechanisms ·
titanium · vanadium
Data for 3b: IR (ATR): n˜max =2932 (m), 2861 (w), 1647 cmÀ1 (w);
1H NMR: d=0.87 (3H, t, J=6.7 Hz), 1.2–1.4 (3H, m), 1.4–1.5 (1H,
m), 1.7–1.8 (1H, m), 1.9–2.0 (1H, m), 3.35 (1H, dd, J=9.5, 6.2 Hz),
3.52 (1H, dd, J=11.0, 6.5 Hz), 5.0–5.1 ppm (1H, m); 13C NMR: d=
13.8, 22.3, 27.4, 33.4, 39.3, 91.8, 212.1 ppm.
[1] For recent reviews see: a) M. North, R. Pasquale, C. Young, Green Chem.
2010, 12, 1514–1539; b) A. Decortes, A. M. Castilla, A. W. Kleij, Angew.
Chem. Int. Ed. 2010, 49, 9822–9837; Angew. Chem. 2010, 122, 10016–
10032; c) P. P. Pescarmona, M. Taherimehr, Catal. Sci. Technol. 2012, 2,
2169–2187; d) X.-B. Lu, D. J. Darensbourg, Chem. Soc. Rev. 2012, 41,
1462–1484; e) N. Kielland, C. J. Whiteoak, A. W. Kleij, Adv. Synth. Catal.
2013, 355, 2115–2138; f) M. North in New and Future Developments in
Catalysis: Activation of CO2 (Ed.: S. L. Suib), Elsevier, London, 2013,
chap. 13.
Data for 4b: IR (ATR): n˜max =2932 (m), 2861 (w), 1350 cmÀ1 (m);
1H NMR: d=0.86 (3H, t, J=6.7 Hz), 1.2–1.4 (4H, m), 1.7–2.0 (2H,
m), 3.64 (1H, dd, J=11.9, 8.0 Hz), 3.90 (1H, dd, J=11.9, 5.4 Hz),
4.2–4.4 ppm (1H, m); 13C NMR: d=13.8, 22.3, 30.3, 33.2, 48.2, 60.9,
227.9 ppm.
Data for 3h: IR n˜max =2925 (w), 1597 (m), 1493 (m), 1184 cmÀ1
;
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1H NMR: d=3.67 (1H, dd, J=11.2, 7.2 Hz), 3.74 (1H, dd, J=11.2,
7.8 Hz), 4.22 (1H, dd, J=10.3, 5.6 Hz), 4.26 (1H, dd, J=10.3, 5.6 Hz),
5.3–5.5 (1H, m), 6.8–6.9 (2H, m), 6.9–7.0 (1H, m), 7.2–7.3 ppm (2H,
m); 13C NMR: d=36.3, 66.2, 87.7, 114.5, 121.9, 129.7, 157.7,
211.3 ppm.
Data for 4h: IR (ATR): n˜max =2936 (w), 1598 (m), 1488 (m), 1461 (m),
1
1033 cmÀ1 (s); H NMR: d=4.00 (1H, dd, J=12.2, 3.8 Hz), 4.11 (1H,
dd, J=9.0, 5.4 Hz), 4.15 (1H, dd, J=11.7, 5.7 Hz), 4.29 (1H, t, J=
9.8 Hz), 4.5–4.7 (1H, m), 6.8–6.9 (2H, m), 6.9–7.0 (1H, m), 7.2–
7.3 ppm (2H, m); 13C NMR: d=44.9, 57.2, 66.5, 114.6, 121.8, 129.7,
157.7, 226.4 ppm.
Data for 14a: M.p. 60–618C (lit.[16] 63–648C); IR (ATR): n˜max =2940
1
(m), 2860 (m), 1447 cmÀ1 (m); H NMR: d=1.3–1.5 (2H, m), 1.5–1.7
(1H, m), 1.7–1.8 (1H, m), 1.8–2.0 (2H, m), 2.1–2.3 (1H, m), 2.4–2.5
(1H, m), 3.71 (1H, td J 11.9, 3.6 Hz), 4.33 ppm (1H, td J 11.8,
3.9 Hz); 13C NMR: d=23.7, 25.1, 28.3, 29.8, 56.4, 94.7, 212.5 ppm.
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Data for 15a: M.p. 165–1668C (lit.[16] 163–1648C); IR (ATR): n˜max
=
2940 (m), 2859 (m), 1643 cmÀ1 (w); 1H NMR: d=1.4–1.5 (2H, m),
1.6–1.8 (2H, m), 1.9–2.0 (2H, m), 2.1–2.3 (2H, m), 4.0–4.2 ppm (2H,
m); 13C NMR: d=24.9, 29.0, 64.4, 227.1 ppm.
General procedure for reactions catalysed by complex 11
and Bu3N
CS2 (0.09 mL, 1.49 mmol), an epoxide (0.835 mmol), 11 (1 mol%,
10 mg, 0.00835 mmol) and Bu3N (1 mol%, 1.5 mg, 0.00835 mmol)
were placed in a sealed Young flask and stirred at 908C for 24 h.
The solution was evaporated and, if necessary, the residue purified
by column chromatography (CHCl3/hexane 1:1) to give an unsepa-
rated mixture of the di- and trithiocarbonate, except for 4a, which
was obtained as the pure trithiocarbonate.
General procedure for kinetic measurements
The appropriate amounts of catalyst 11, Bu4NBr and epoxide 10h
were dissolved in CDCl3 (0.37 mL) to give the concentrations re-
quired for a particular experiment. The solution was added to an
NMR tube, and the required amount of CS2 was added. The NMR
tube was kept in the 400 MHz NMR spectrometer with the probe
heated to 508C. A 1H NMR spectrum was collected every 30 min
for 12.5 h, and the relative intensities of the signals for unreacted
epoxide, dithiocarbonate and trithiocarbonate were used to deter-
mine the extent of reaction.
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Published online on && &&, 0000
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C. Beattie, M. North*
&& – &&
Titanium(salen)-Catalysed Synthesis of
Di- and Trithiocarbonates from
Epoxides and Carbon Disulfide
Salen away on a kinetic sea: The com-
bination of [Ti(salen)O]2 and tetrabuty-
lammonium bromide or tributylamine
catalyzes the addition of carbon disul-
fide to epoxides to form predominantly
dithiocarbonates. Ten examples are
reported that give the dithiocarbonates
in 33–99% isolated yield. A mechanistic
study based on reaction kinetics, stereo-
chemistry, and NMR spectra of reaction
mixtures allow a catalytic cycle to be
proposed.
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9
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