Phosgenation Reactions - Phosgenations - page 260

Here, the use of cesium bases is crucial due to the inherently enhanced nucleo-
philicities of the corresponding cesium alkoxides generated in situ from various
aliphatic alcohols. Primary and secondary alcohols are easily incorporated into
CO 2 , and then the products, in turn, react with diverse halides, including second-
ary bromides, which are usually resistant to alkylations due to eliminations. The
stereochemical sense is lost to a negligible extent, if at all. Using chiral templates,
carbonates are formed e ciently without any elimination or hydrolysis, and little
or no racemization is observed [711].

Typical procedure. O-4-Phenylbutyl-O 0 -butyl carbonate 989 [711]: To 4-phenyl-1-buta-
nol 985 (100 mg, 0.67 mmol) in anhydrous DMF (1.6 mL, 0.4 m) were added ce-
sium carbonate (625 mg, 2.10 mmol, 3 equiv.) and tetrabutylammonium iodide
(208 mg, 0.67 mmol, 1 equiv.). CO 2 gas (flow rate 25-30 mL min 1 ) was bubbled
into the reaction mixture for 2-3 min, and then 1-bromobutane 988 (274 mg,
0.22 mL, 2.0 mmol) was added to the suspension. The reaction proceeded at am-
bient temperature with CO 2 gas bubbling for 3.5 h, after which time the starting
material (4-phenyl-1-butanol) had been consumed. The reaction mixture was then
poured into water (30 mL) and extracted with hexanes/EtOAc (3:1 v/v, 60 mL). The
organic layer was washed with water (2

30 mL) and brine (30 mL), and dried
over anhydrous sodium sulfate. Evaporation of the solvent followed by flash col-
umn chromatography (hexanes/EtOAc, 9:1 v/v) afforded O-4-phenylbutyl-O 0 -butyl
carbonate 989 (157 mg, 94%) as a colorless oil.
Cyclic carbonates can be obtained by reactions of alkali metal carbonates with
epi-halohydrins. 1-Chloro-2,3-epoxypropane 990 reacts with potassium hydrogen
carbonate to form 4-hydroxymethyl-1,3-dioxolan-2-one 991 in 41% yield [712].

41 %

H O

1. KHCO 3

Cl

O O

O 18-crown-6

80°C, 36 h

990 O

2. H 2 O

991

Typical procedure. 4-Hydroxymethyl 1,3-dioxolan-2-one 991 [712]: A stirred mixture of
potassium hydrogen carbonate (10.0 g, 0.1 mol), 18-crown-6 (0.2 g, 0.76 mmol),
and 1-chloro-2,3-epoxypropane 990 (27.6 g, 0.3 mol) was heated at 80 C for 36 h.
After cooling and removal of the potassium salt by filtration, the organic layer
was washed with water and 991 was distilled at 152-160 C/0.6-0.8 mmHg; yield
4.83 g (41%); IR: n max 3500, 1800 cm 1 .

The stereoselective conversion of 1,2-diols into alkane-1,2-diyl carbonates at
room temperature by adding them to acetonitrile solutions containing CO 3 2 or
HCO 3 anions in the presence of tetraalkylammonium cations has been described.
These solutions can be prepared by electrochemical or chemical routes [713]. The

¼