Chemistry Reference
In-Depth Information
Dimethyl carbonate as a carboxylating agent
Huels is reportedly operating a commercial route to isophorone diisocyanate
(IPDI) 422 based on the addition of isophorone diamine (IPDA) 420 to urea and
an alcohol followed by decomposition of the intermediate carbamate to IPDI
[290]. An alternative phosgene-free route to IPDI has been patented by Daicel
[291]. In this method, dimethyl carbonate (DMC) is reacted with IPDA to afford
isophorone dicarbamate (IPDC). The IPDC is decomposed in the liquid phase in a
high-temperature boiling solvent under reduced pressure to give high yields of
IPDI.
NH
2
NHCOOCH
3
(MeO)
2
CO
NH
2
NHCOOCH
3
420
421
NCO
+
2 CH
3
OH
NCO
422
The first step of the process, the addition of IPDA 420 to DMC, is base-catalyzed,
with alkali and alkaline earth metals being the preferred catalysts. In the patent,
the yield quoted for the first step is
98%. The second step of the reaction, crack-
ing the carbamate 421 to IPDI 422, is facilitated by a manganese, molybdenum,
tungsten or zinc catalyst. An interesting and economically important finding is
that the yield of IPDI obtained from cracking the IPDC depends significantly on
the time elapsed between synthesis of the IPDC and the cracking reaction. For
example, a standing time of just 8 h from the time of synthesis of IPDC to the
cracking step yields 74% IPDI, 19% monocarbamate (IPMI), and just 6% of high-
temperature boiling materials. In contrast, allowing molten IPDC to stand for 48 h
before cracking gives much reduced yields of 58% IPDI and 15% IPMC and in-
creases the proportion of unwanted high-temperature boiling materials to 26%.
The inventors suggest that the thermal energy needed to maintain IPDC in a
molten state (130
C) to facilitate handling leads to the formation of impurities,
which can lead to increased amounts of involatile materials in the ensuing crack-
ing stage.
>
4.3.1.4
Heterocyclic Isocyanates
In the synthesis of Cholecystokinin-B receptor antagonists, a benzodiazepin interme-
diate bearing an isocyanato group plays a key role. It is prepared from the corre-
sponding amine 423 by carbonylation with phosgene in 98% yield [292].
General procedure. 1-N-(Adamant-2-yl)-2,4-dioxo-3-isocyanato-5-N-phenyl-2,3,4,5-tetra-
hydro-1H-1,5-benzodiazepin [292]:
Phosgene (for a safe source, see Chapter 7) in tol-
