Chemistry Reference
In-Depth Information
lution and then with water at 5
C, followed by drying with sodium sulfate and
evaporation to give the pure carbonates 884.
A reagent for synthesizing carbamates and particularly asymmetric ureas (see
Sections 4.3.2 and 4.3.4) is o-(4-nitrophenylene) carbonate (NPC) 721 [504], a cyclic
carbonate obtained from 4-nitrocatechol and phosgene or triphosgene.
OH
COCl
2
O
O
DMAP
THF
2 h, rt
O
O
2
N
O
2
N
OH
721
720
Typical procedure. o-(4-Nitrophenylene) carbonate (NPC) 721 [504]:
To a stirred solu-
tion of 4-nitrocatechol (3.10 g, 20 mmol) in THF (250 mL), a solution of DMAP
(2.44 g, 20 mmol) in THF (100 mL) was added in a single portion. To this mixture,
a solution of phosgene (for a safe source, see Chapter 7) (20% in toluene, 50 mL, 84
mmol) was added dropwise with stirring over a period of 2 h and the reaction
mixture was stirred for a further 2 h at room temperature. A white precipitate be-
gan to form on addition of the phosgene solution. Nitrogen was bubbled through
the suspension for 15 min and then the precipitate was removed by filtration. The
filtrate was concentrated and the residue was crystallized from hexane to yield NPC
721 (1.49 g, 41%) as pale-yellow needles; mp 100-101
C.
Dropwise addition of a solution of triphosgene (3.96 g, 13.3 mmol) in THF (150
mL), instead of excess phosgene, over a period of 4 h at room temperature, fol-
lowed by stirring for 2 h at 50-60
C and the same work-up, gave 2.92 g (81%) of
NPC 721.
OH
COCl
2
O
2
NaOH (equivalent)
O
O
or Cat.
DPhC
885
cat = pyridine derivative
TiO
2
/ Al
2
O
3
HfO
2
Technical processes for the production of diphenyl carbonate (DPhC) 885 from
phenol and phosgene are covered in many patent applications (for the production
of polycarbonates on optical storage of electronic data, see Section 4.3.3.7). Such
processes are carried out in the presence of sodium hydroxide [643], pyridine de-
rivatives [644], TiO
2
and Al
2
O
3
[645], or HfO
2
[646].
A highly interesting synthetic route for forming intermediates en route to retinal
and their analogues involves a palladium-catalyzed transformation of an yne-carbo-
nate 888 into an allenyl enal 889. The carbonate 888 is generated by unsymmetrical
carbonylation of propargylic alcohol 886 and silyl enol ether 887 with phosgene
[647].
