formation of tributylphosphine oxide and bromoform was
with ethylene glycol, cyclization took place to produce ethylene
confirmed. Moreover, the peak due to phosphonium species in
carbonate in 78.1% yield.
31
the P NMR spectrum was detected when the reaction of CO2
with benzyl alcohol was followed by NMR spectroscopy. On
the basis of the above observations, the following mechanism is
proposed to explain the present reaction using the tributyl-
phosphine–carbon tetrabromide–CyTMG system (Scheme 3);
Experimental
Materials and methods
Carbon dioxide from a gas cylinder was dried through a silica
gel tube. Alcohols and DMF were purified by distillation.
1
2
CyTMG was prepared according to the literature. Other
O
CyTMG
1
reagents were used without further purification. H NMR spec-
ROH
+
CO2
RO
C
O
H CyTMG
tra were recorded on a Varian Mercury 200 spectrometer.
J-Values are given in Hz. IR spectra were recorded on a Horiba
FT-200 spectrometer.
1
n-Bu3P
+
CBr4
n-Bu3P CBr3
The identities and purities of dialkyl carbonates (RO) CO
2
Br
were confirmed by TLC analysis on silica gel 60 with
n-pentane–diethyl ether = 5:1 (R = Me and Et) or cyclohexane–
ethyl acetate = 5:1 (other carbonates) solvent system as follows
2
(
R/R ): Me/0.63; Et/0.81; n-Pr/0.34; i-Pr/0.30; n-Bu/0.34;
f
O
C
O
C
s-Bu/0.34; n-hexyl/0.55; cyclohexyl/0.71; allyl/0.53; benzyl/0.69;
CH CH –/0.24; column chromatography was performed on
1
+
2
RO
O
P-n-Bu3
RO
O
P-n-Bu3
–
2
2
CBr3
(Wakogel C-200) silica gel.
CBr3
3a
3b
Reaction of CO with benzyl alcohol
2
To a stirred solution of benzyl alcohol (0.216 g, 2.00 mmol),
+
H CyTMG
Br
tributylphosphine (0.303 g, 1.50 mmol) and CyTMG (0.394 g,
3
2
.00 mmol) in DMF (2.00 cm ) was added CO2 at room
temperature. After 15 min, carbon tetrabromide (0.663 g, 2.00
mmol) was added to the mixture, and the reaction system
was sealed and stirred for 2 h. Then, the reaction mixture was
O
ROH
+
3b
RO
C
OR
+
n-Bu3P
O
+
CHBr3
Scheme 3
diluted with ethyl acetate, washed successively with 0.5 mol
Ϫ3
dm aqueous HCl and saturated aqueous NaHCO , and dried
3
over Na SO . Diphenylmethanol as an internal standard
2
4
a similar mechanism was suggested to be involved in the form-
was added to the organic solution and the yield of dibenzyl
carbonate was determined by the integrated ratio between a
methylene peak of the carbonate and a methine peak of di-
ation of cyclic urethanes from amino alcohols and CO using
2
the triphenylphosphine–carbontetrachloride–triethylamine sys-
10
tem. The first step is the formation of carbonate anion 1 by
1
phenylmethanol in the H NMR spectrum of the ethyl acetate
the reaction of alcohol with CO in the presence of CyTMG.
2
solution. Dibenzyl carbonate, prepared by a larger scale re-
On the other hand, the reaction of tributylphosphine with car-
bon tetrabromide takes place to form the (tribromomethyl)-
phosphonium bromide 2. Then an ion-exchange reaction
between these two species 1 and 2 occurs, giving rise to phos-
phonium intermediate 3a, which is probably in equilibrium
with 3b. A phosphoniumoxy group in 3b is a good leaving
group; the nature of anion 1 can be changed from nucleophilic
to electrophilic by the conversion into 3b having this leaving
group. Therefore, nucleophilic attack of the alcohol onto the
carbonyl carbon of 3b can occur, followed by Arbuzov-type
reaction to lead to a carbonate, tributylphosphine oxide, and
bromoform.
action of CO with benzyl alcohol (1.08 g, 10.0 mmol), was
isolated by column chromatography (cyclohexane–ethyl
2
acetate = 50:1) in 67.3% yield (0.816 g), δ (200 MHz) 5.17
H
Ϫ1
(
(
4H, s, CH ), 7.30–7.43 (10H, m, C H ); νmax(NaCl)/cm 1747
2
6
5
C᎐O), 1260 (C–O).
᎐
Reaction of CO with methanol, ethanol, propan-1-ol, propan-
2
2
-ol allyl alcohol and ethylene glycol
The reaction was started according to the experimental pro-
cedure used in synthesis of dibenzyl carbonate, and the reaction
system was sealed and stirred for 2 h. The yields of the lower
boiling and/or water-soluble carbonates from those alcohols
were directly determined by the integrated ratio between the
following characteristic peaks due to the methylene or methine
Although the present condensing agent has traditionally
8
been used as a halogenating agent of alcohols, the formation
and participation of alkyl halides in the present reaction sys-
tems were excluded since benzyl bromide was not detected in
protons neighboring the OC᎐O group and the methine peak of
᎐
1
1
the H NMR spectrum of the reaction mixture from benzyl
diphenylmethanol as an internal standard of the H NMR
alcohol and CO . Furthermore, benzyl bromide was totally
spectra of the reaction mixtures; δH 3.79 (dimethyl carbonate),
4.18 (diethyl carbonate), 4.09 (dipropyl carbonate), 4.87 (diiso-
propyl carbonate), 4.64 (diallyl carbonate), and 4.59 (ethylene
carbonate).
2
recovered from the reaction mixture, and the yield of
dibenzyl carbonate was the same as that of entry 1 in Table
4
, when the reaction of benzyl alcohol with CO was carried
2
out in the presence of an equimolar amount of benzyl
bromide.
Reaction of CO with hexan-1-ol
2
Procedure was as described in synthesis of dibenzyl carbonate.
The yield of dihexyl carbonate was determined by the inte-
grated ratio of the peak at δ 4.11 due to methylene protons
neighboring OC᎐O and the methine peak of diphenylmethanol
Conclusions
Direct condensation of CO with alcohols was achieved using
the tributylphosphine–carbon tetrabromide–CyTMG system as
2
1
as an internal standard of the H NMR spectrum of the reac-
a condensing agent. The reaction of CO with primary alcohols
tion mixture. Dihexyl carbonate, prepared by the larger-scale
2
gave the corresponding carbonates in high or relatively high
reaction of CO with hexan-1-ol (20.4 g, 20.0 mmol), was iso-
2
yields. From CO and secondary alcohols, however, the carbon-
lated by column chromatography (cyclohexane–ethyl acet-
2
ates were produced in only low yields. In the reaction of CO2
ate = 50:1) in 24.2% yield (0.557 g), δ (200 MHz) 0.89 (6H, t,
H
J. Chem. Soc., Perkin Trans. 1, 1999, 2205–2208
2207