Chemistry Reference
In-Depth Information
Tab. 4.13.
Conversion of carbamates to ureas via isocyanates formed with SiI
2
H
2
[206].
Entry
Carbamate
a
Urea
Isolated
yield (%)
Cy
R, R' = Bn, H
R, R' = Bn, allyl
R, R' =
i
-Pr
R, R' = Ph, H
84
76
57
68
NH
1a-d
CyNHBoc
O
NRR'
2
CyNHCbz
CyNHCONHBn
94
3
CyNHCO
2
Me
CyNHCONHBn
87
4
CyNHCO
2
Ph
CyNHCONHBn
89
5
78
NHBoc
NHCONHBn
6
Ph(CH
2
)
2
NHBoc
Ph(CH
2
)
2
NHCONHBn
91
CO
2
Et
CO
2
Et
Ph
Ph
7
74
NHBoc
NHCONHBn
8
PhNHBoc
PhNHCONHBn
83
OMe
OMe
9
78
Me
Me
NHBoc
NHCONHBn
Cbz
CO
2
Et
10
recovered SM (91%)
Me
a
Cy
ΒΌ
cyclohexyl
Carbon monoxide as a carbonylating reagent
Many examples of phosgene-free processes, mainly concerning the carbonylation
of aryl nitro derivatives, have been claimed. Some of the most representative, cited
in part in the Kirk-Othmer Encyclopedia of Chemical Technology (4th ed., vol. 19), are
illustrated below. 2,4-Dinitrotoluene undergoes reductive carbonylation with CO to
form 2,4-toluene diisocyanate (TDI) in the presence of palladium catalysts [209-
213]. A variation on this process involves capturing the isocyanate formed with
methanol, followed by thermolysis of the bis(carbamate) 326 [212].
Me
Me
Me
NO
2
NCO
NHCOOMe
1.
CO
, Pd catalyst
2. MeOH
thermolysis
NCO
NO
2
NHCOOMe
325
326
327
