Phosgenation Reactions - Phosgenations - page 258

nol was added to the organic solution as an internal standard and the yield of di-
benzyl carbonate was determined from the relative integrals of a methylene peak of
the carbonate and a methine peak of diphenylmethanol in the 1 H NMR spectrum
of the ethyl acetate solution. Dibenzyl carbonate, prepared by a larger scale reac-
tion of CO 2 with benzyl alcohol (1.08 g, 10.0 mmol), was isolated by column chro-
matography (cyclohexane/ethyl acetate, 50:1) in 67.3% yield (0.816 g); IR (NaCl):
n max 1747 cm 1 .

A series of methods has been published for the preparation of dialkyl carbonates
from CO 2 and carbonate salts using alkyl halides. Carbonic esters can be prepared
in a phase-transfer catalytic reaction from primary alkyl halides and a mixture of
dry potassium hydrogen carbonate and dry potassium carbonate in non-polar sol-
vents. Yields of dialkyl carbonates obtained are 67-83%. The conversion is inef-
fective in the absence of the hydrogen carbonate and or a phase-transfer catalyst
(PTC) [709].

¼

General procedure. Dialkyl carbonates [709]: Dry potassium hydrogen carbonate
(10.0 g, 0.01 mol) is mixed with dry potassium carbonate (14.0 g, 0.1 mol). To this
mixture is added a solution of aliquat 336 (400 mg, 1 mmol) and alkyl halide
(0.1 mol) in toluene or petroleum ether (10 mL). The reaction mixture is stirred at
100 C for 8-15 h, then filtered and concentrated, and the residue is fractionated.
Dialkyl carbonates are purified by distillation in a kugelrohr apparatus or by re-
crystallization.
Dialkyl carbonates can also be prepared in a three-component coupling system
of aliphatic alcohol/CO 2 /alkyl halide under a pressure of CO 2 (160 psig) and in the
presence of a peralkylated guanidine. In this way, di-n-butyl carbonate 984 is ob-
tained in 73% yield (by GC) [710].

CO 2
160 psig

CO 2

O

160 psig

O O

OH

CyTMG
MeCN

85°C, 16 h

982 984 73 %

80 psig CO 2

40°C MeCN

1 h

Cl

983

Typical procedure. Di-n-butyl carbonate 984 [710]: A 160 mL Parr autoclave was
charged with butanol 982 (2.22 g, 0.03 mol), N-cyclohexyl-N 0 ,N 0 ,N 00 ,N 00 -tetrame-
thylguanidine (CyTMG; 6.9 g, 0.035 mol), and acetonitrile (30 mL). The autoclave
was attached to a pressure head, and 160 psig CO 2 was introduced with stirring at
room temperature. An exothermic reaction ensued, leading to an increase in tem-
perature to ca. 40 C. In a Fischer-Porter bottle was placed a solution of 1-chloro-
butane 983 (8.33 g, 0.09 mol) in acetonitrile (10 mL). This bottle was attached to