Chemistry Reference
In-Depth Information
utility lies in the preparation of lower alkyl carbamoyl chlorides and the correspond-
ing isocyanates. Exposure of these alkyl carbamoyl chlorides to the conditions of
the solvent phosgenation procedure and conventional thermal dehydrohalogena-
tion techniques leads to extensive degradation and yield losses due to the forma-
tion of by-products (allophanoyl chlorides and isocyanate polymers).
The kinetics of the reaction of phosgene with several aromatic amines has
been examined [86-89] using a ''quenching-flow'' technique. The reactions follow
second-order kinetics, and an S
N
2 mechanism has been proposed.
One of the first phosgene-free transformations for the preparation of carbamoyl
chlorides was, surprisingly, accomplished with urea. Heating aniline hydrochloride
133 with urea at 370
C resulted in phenylcarbamoyl chloride 134 [90].
H
2
NNH
2
O
H
N
l
NH
2
.HCl
O
370°C
133
134
Chlorocarbonyl isocyanate 135 has been employed to obtain a chlorocarbonyl urea 137
from 2,6-diisopropyl aniline 136 [91].
OO
O
Cl
NCO
NH
2
HN
H
Cl
i
-Pr
i
-Pr
i
-Pr
135
i
-Pr
Et
2
O, 3h,
-50 to -30°C
136
137
The carbamoyl chloride 139 of enamine 138 has been obtained with triphosgene in
dichloromethane/pyridine at 25
C [92].
O
O
O
O
NHCOCl
NH
2
(CCl
3
O)
2
CO
Et
3
Si
Et
3
Si
O
O
Py, CH
2
Cl
2
, 25°C
40 min
O
O
O
O
138
139
Aromatic carbamoyl chloride 141 has been obtained with triphosgene from 3,4-
disubstituted anilines [93].
