Chemistry Reference
In-Depth Information
amines, with the generation of large amounts of inorganic salts, and these repre-
sent the main components of industrial waste.
Catalytic carbonylation of nitro compounds, with particular emphasis on nitro-
aromatic compounds, has been extensively studied, with the main focus being on
the production of isocyanates that have achieved great commercial importance in
the preparation of important industrial targets including ureas [768].
Various catalysts have been used to promote the process, including metal com-
plexes of groups 8-10 (mainly Pd, Ru, and Rh) combined with a Lewis acid co-
catalyst (mainly FeCl
3
, MoCl
5
,V
2
O
5
,orFe
2
O
3
) or a Brønsted acid (e.g. trime-
thylbenzoic acid). The methodology was particularly studied and developed with
the aim of producing methylenediphenyl isocyanates and phenyl diisocyanate,
which are of commercial importance in the manufacture of polyurethanes. Due to
the instability of monoisocyanates under the reaction conditions, the process was
better utilized in the production of N-phenylcarbamates 1054 by trapping iso-
cyanates with alcohols.
NCO
HH
NO
2
N
N
R
R'NH
2
R'
CO
1053
O
R
R
NHCOOR'
R'OH
1055
1052
R
1054
Furthermore, isocyanates can be converted in situ into symmetrical diphenyl ureas
by reaction with amines either added to the reaction mixture or produced in situ by
reduction of the nitro compound [238].
Diphenylurea (DPU) 1058 is synthesized in almost quantitative yield from ni-
trobenzene, aniline, and CO, in the presence of a Pd(p) complex with triphenyl-
phosphine (0.2 mol% with respect to aniline), dissolved in a non-polar solvent such
as toluene or xylene at 120
C.
Mechanistic studies with deuterated nitrobenzene suggest that the reaction in-
volves the carbamoyl complex [PhNCO(Pd)] 1057, although it is not clear as to how
important this pathway is [769-771].
Symmetrical para-substituted diaryl ureas can be obtained in satisfactory yields
(27-88%) by the reaction of accessible aromatic nitro compounds in the presence
of Ru
3
(CO)
12
in cis-cyclooctene as the solvent (substrate/catalyst ratio 25:1) [772].
Unsymmetrical substituted ureas are likewise synthesized by reductive carbony-
lation of 4-substituted nitrobenzenes with CO in the presence of an excess of an
aliphatic secondary amine using palladium acetate, bipyridyl, and copper tosylate
as co-catalyst; the best selectivity is obtained by continuously adding the aliphatic
amine during the course of the reaction (ca. 10 h) [773].
N,N
0
-Disubstituted ureas are obtained in fairly low yields (45-55%) using less
expensive catalysts by dioxygen-induced carbonylation of amines in the presence
