Chemistry Reference
In-Depth Information
Tab. 4.12.
Hindered aryl isocyanates prepared by the Mitsunobu method [196].
Amine R
Isocyanate
yield (%)
a
Isocyanate
isolated
yield (%)
2,4,6-Me
3
C
6
H
2
100
92
2,6-Et
2
C
6
H
3
80
75
2-Et-6-MeC
6
H
3
77
72
2,6-iPr
2
C
6
H
3
100
89
2,4,6-trimethylbenzene-1,3-diamine
68
65
4,4
0
-methylenebis(2,6-dimethylaniline)
84
81
2-iPrC
6
H
4
<
2
b
C
6
H
5
<
2
c
a
Estimated by infrared spectroscopy;
b
A mixture of three products
including carbamoylhydrazine (311;Ar
¼
2-iPrC
6
H
4
),
dicarbamoylhydrazine (312;Ar
¼
2-iPrC
6
H
4
), and symmetrical urea
was formed;
c
A mixture of the carbamoylhydrazine (311;Ar
¼
Ph) and
dicarbamoylhydrazine (312;Ar
¼
Ph) was formed.
and dialkyl azodicarboxylate was 1:1.2:1.2. Both reaction vessels were cooled to
78
C prior to cannulation of the zwitterion solution into the carbamate salt so-
lution. The reaction mixture was allowed to slowly warm to room temperature over
a period of 30-60 min while maintaining a steady stream of CO
2
. The mixture was
then stirred overnight under 1 atm of CO
2
. IR analysis of the solution obtained,
made up to a specific volume with dichloromethane, was used to determine the
yield of in situ produced isocyanate. The isocyanates were isolated by either
fractional distillation or column chromatography. In reactions employing PBu
3
in
place of PPh
3
, an equimolar ratio of amine, PBu
3
, and DIAD was used, and the
reaction mixture was worked-up as soon as it reached ambient temperature.
O
H
NO
O
i-Pr
CH
2
Cl
2
1. -10 to -5°C
2. -20°C
3. -78°C, rt
ON
i-Pr
+
+
i-Pr
ON
NO
ArNH
2
+ CO
2
i-Pr
O
O H
Ar
311
PBu
3
O
Ar
HN
O
O
i-Pr
O
ON
i-Pr
O
O H
Ar
312
