Phosgenation Reactions - Phosgenations - page 342

Primary, secondary, and benzylic alcohols are converted into the corresponding
chlorides when treated with tetrachlorosilane in the presence of potassium carbo-
nate at room temperature. For example, benzyl alcohol, 1-octanol, 2-hexanol, cy-
clohexanol, cinnamyl alcohol, borneol, and tert-butyl alcohol were treated with
K 2 CO 3 and SiCl 4 in dichloromethane at room temperature for 50-70 min to give
benzyl chloride, 1-chlorooctane, 2-chlorohexane, chlorocyclohexane, cinnamyl chlo-
ride, bornyl chloride, and tert-butyl chloride, respectively, in 94-97% yield [978].
Potassium carbonate reacts with tetrachlorosilane to form trichlorosilyloxy-
carbonyl chloride (Cl 3 SiOCOCl), which subsequently reacts with another mole of
tetrachlorosilane leading eventually to phosgene in chlorinated solvents. Tri-
chlorosilyloxycarbonyl chloride or phosgene generated in situ in this way have
proved to be very effective for the chlorination of a wide variety of alcohols to give
the corresponding chlorides [978].
Activated alcohols can be converted to alkyl chlorides by reaction with phosgene
in the presence of N,N-dimethylformamide [971].
Because of the thermal instability of certain chloroformates, the production of
halides can often occur in the absence of a catalyst [1, 969, 970, 972]. Most chloro-
formates are amenable to facile decomposition in the presence of appropriate cat-
alysts [973].
Decarboxylation of alkyl chloroformates 1299 in the presence of onium salts as
catalysts has been extensively studied [974, 979-981].

onium salts
catalysts

O

l + CO 2 R

R H O

R Cl

1298 1299 1300

The advantages of the partial chlorination of alcohols (60-95% conversion) with
HCl and completion of the chlorination by a catalytic phosgenation and subse-
quent decarboxylation of the resulting chloroformates have been combined in a
two-stage process [974, 982]. Only small amounts of dialkyl ethers, alkenes, iso-
meric chloroalkanes, or dialkyl carbonates are claimed to be formed as side prod-
ucts.
The availability of triphosgene as a stable solid alternative to phosgene together
with a quantification of the intermediate chloroformates gave the reaction a more
preparative character.
A clean conversion to cephem chloride 4 has been observed by chlorinating the
cephem nucleus 2 at the C-3 position using triphosgene [13]. The reaction is
thought to proceed through the unstable cephem chloroformate 3 (reaction and
experimental procedure described in Section 4.2.1).
The reaction proceeds primarily according to an S N 2 mechanism, with some con-
tribution from S N 1 and/or S N i mechanisms, as discussed in Section 4.2.1.
Recently, SNPE reported that the inversion of configuration that occurs in bi-
molecular nucleophilic substitutions is being used to prepare chiral 2-chloro-
propionates 1302.