Chemistry Reference
In-Depth Information
reactive towards the isocyanate, and hence it is harmless. The amino alcohol solu-
tion was added by injecting it under the surface of the di-tert-butyl tricarbonate
solution, in order to avoid turbidity of the reaction mixture due to the formation of
carbamic acid by reaction of the escaping carbon dioxide with the amino alcohol.
After decomposition of the unstable carbamic acid to the initial amino alcohol, the
latter reacts with already formed isocyanate, resulting in urea derivatives. This side
reaction distorts the perfect stoichiometry of the AB-type polymerization, and con-
sequently, limits the molecular weight of the polymer.
The formation of the a,o-isocyanato alcohols has been confirmed by IR and
1
H
NMR spectroscopy. In the IR spectrum of a solution of the product in chloroform,
a strong absorption due to the N
O stretch is seen at 2274 cm
1
.
1
H NMR
spectra show the absence of any side products and also prove the relative stability
of a,o-isocyanato alcohols in solution; however, concentration to dryness furnished
undefined products.
b
C
b
Boron trihalides as mild reagents for the conversion of carbamate esters to
isocyanates
Boron trihalides are known for their strong Lewis acid character, and for their
ability to cleave a wide variety of ethers, acetals, and esters under relatively mild
conditions [233].
The conversion of carbamate esters to isocyanates and diisocyanates of industrial
importance is possible using BCl
3
in the presence of Et
3
N [234]. The reaction is
simple in its execution and work-up, proceeding under mild conditions and af-
fording isocyanates 357 in excellent yields.
One such method (Scheme 4.3) involves the oxidative carbonylation of amines
[235-238], or the catalytic production of carbamate esters by reductive carbonylation
of nitro compounds [238-242], dealcoholysis of which gives isocyanates.
H
2
O
RNH
2
CO
1/2 O
2
R'OH
BCl
3
NEt
3
RNCO
357
RNHCO
2
R'
356
3 CO
R'OH
RNO
2
2 CO
2
Scheme 4.3.
Oxidative carbonylation of amines [235-238] and
reductive carbonylation of nitro compounds [238-242].
It has recently been shown that elimination of alcohol from carbamate esters
356 to yield isocyanates 357 can be facilitated using chlorocatecholborane 358 in
toluene, in the presence of triethylamine [243].
O
O
NEt
3
R
NCO
RNHCO
2
R'
B
Cl
B
OR'
O
O
356
358
359
357
