Chemistry Reference
In-Depth Information
O
O
Cl
R
+
R
3
N
:
Cl
Cl
Cl
N
R
R
129
RNCO
O
RCl
131
R
Cl
N
O
RCl
R
R
3
N
R
R
N
N
RCl
130
R
R
132
The availability of the nitrogen electron pair for bonding and the steric environ-
ment about the nitrogen center determine the reactivity of the various nitrogenous
functional groups.
This section deals with the reactions of amino compounds with phosgene gen-
erated from its common source or phosgene equivalents, by a direct transfer of the
chlorocarbonyl group, giving carbamoyl chlorides. Reactions of imines and other
unsaturated (cyclic) nitrogen compounds with phosgene will be discussed later.
4.2.2.1
Reactions with Primary Amines
The preparation of amides by the treatment of amines with acyl halides is a very
general reaction. When phosgene is the acyl halide, both aliphatic and aromatic
primary amines or their salts give carbamoyl chlorides, which can be regarded as
substituted chloroformamides, ClCONHR.
There are numerous solvent phosgenation procedures. If an excess of COCl
2
is
employed, or if the hydrogen chloride formed is removed as it is produced (for ex-
ample, by the use of a strong base, such as a tertiary amine), then the final product
will be the carbamoyl chloride. Performing the reaction at high temperatures in
the presence of an excess of phosgene results in the formation of isocyanate and
hydrogen chloride, consistent with the overall stoichiometry [69]; the carbamoyl
chlorides are easily dehydrochlorinated to the corresponding isocyanate upon
heating [70-73].
This is one of the most common methods for the preparation of isocyanates.
Various reported methods for obtaining isocyanates using phosgene or phosgene
equivalents are discussed in Section 4.3 of the present chapter.
The reactions of amines and amine hydrochlorides with phosgene in the gas-liquid
phase are reported to be catalyzed by numerous compounds such as morpholine
[74], boron trifluoride [75], aluminum chloride [76], N,N-dialkylcarbamates [77],
N,N-dialkylamides [78, 79], tetraalkylureas and thioureas [78-80], hexamethyl-
phosphoramide [78], as well as activated carbon [81].
Carbamoyl chlorides have been prepared by the vapor-phase reaction of phosgene
with primary amines [82-85]. The method is widely applicable, and its greatest
