Chemistry Reference
In-Depth Information
The reaction of acetals is especially attractive because the starting material is much
less expensive compared with ortho esters, and the co-produced carbonyl com-
pounds 979 can be recycled. Hence, the reaction can be regarded as a synthesis of
DMC from CO
2
and methanol. In these reactions, carbonate complexes generated
by CO
2
insertion into the tin-oxygen bond of R
2
Sn(OMe)
2
are proposed as key in-
termediates [704, 706].
Another route to the synthesis of dialkyl carbonates from CO
2
is the reaction
with alcohols and tertiary amines mediated by acetylene. As alcohols, ethanol, iso-
propanol, and allyl alcohol have been employed. The resulting dialkyl carbonates
are obtained in yields of 12-30% [707].
A direct condensation reaction of CO
2
with alcohols, using a trisubstituted
phosphine/tetrabromomethane/base system to prepare dialkyl carbonates, has
been developed. Optimal conditions require CyTMG as a strong, hindered, non-
nucleophilic, and highly polarizable base, which is more effective than other bases
such as DBU. The solvent of choice is DMF [708]. Yields of dialkyl carbonates 981
derived from various primary alcohols 980 are 54-91%; from secondary alcohols
980 they are 14-22%.
+
+
980
R-OH
CO
2
n-Bu
3
P
CBr
4
cyTMG
O
+
+
_
R
_
n-Bu
3
P-CBr
3
Br
O
H-cyTMG
O
_
CBr
3
O
+
+
_
+
R
P-n-Bu
3
H-CyTMG
Br
O
O
R-OH
O
+
+
981
R
R
n-Bu
3
P=O
CHBr
3
O
O
Typical procedure. Dibenzyl carbonate 981 [708]: To a stirred solution of benzyl al-
cohol (0.216 g, 2.00 mol), tributylphosphine (0.303 g, 1.50 mmol), and CyTMG
(0.394 g, 2.00 mmol) in DMF (2.00 mL), CO
2
was added at room temperature.
After 15 min, tetrabromomethane (0.663 g, 2.00 mmol) was added, the reaction
vessel was sealed, and the contents were stirred for 2 h. Thereafter, the reaction
mixture was diluted with ethyl acetate, washed successively with 0.5 m aqueous
HCl and saturated aq. NaHCO
3
solution, and dried over Na
2
SO
4
. Diphenylmetha-
