Chemistry Reference
In-Depth Information
O
Cl
C
AlCl
3
or other Lewis acid
AlCl
4
+
Cl
Cl
O
H
COCl
AlCl
4
COCl
+ AlCl
3
+ HCl
H
On analyzing the reactions of phosgene and phosgene equivalents, the stepwise
nature of phosgenation processes becomes apparent. It is observed that the initial
step of most of these processes involves COCl (chlorocarbonyl) transfer to the
appropriate reaction center of the molecule, with formation of the corresponding
derivative, i.e. chloroformate, carbamoyl chloride, etc. This COCl transfer is usually
followed by a nucleophilic attack to give tricoordinated derivatives, or by CO
2
and/
or HCl elimination to give dicoordinated derivatives of carbonic acid. The scheme
below illustrates the reaction of amides with the phosgene equivalent triphosgene.
The dehydration of primary amides starts with chloroformylation of the substrate.
Further elimination of CO
2
and HCl assisted by an appropriate amine affords the
nitrile.
Cl
R
2
O l
Cl
O
l
Cl
R
2
OO
OCCl
3
Cl
R
2
+
Cl
O
O
Cl
Cl
Cl
O
O
O
l
Amide:Triphosgene 1:1
Amide:Triphosgene 1:0.33
- CO
2
Cl
Cl
+ HCl
DABCO
RCN
R
Cl
R
H
- HCl
2
- HCl
In the case of N-substituted formamides, the preferred dehydrating procedure is
that of Ugi, utilizing phosgene in the presence of a tertiary amine. Triphosgene
adds to the carbonyl group in exactly the same way as it does in the case of the
synthesis of a-chloroalkyl chloroformates (see Chapter 4.4 ''Chlorination''). If tri-
phosgene is catalytically decomposed by the tertiary amine, the reaction can also be
interpreted as a simple phosgenation.
