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Mechanistic Aspects of the Carbonylation of Nitrobenzene with Rhodium Catalysts.

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We interprete the low temperature of isomerization in
terms of a concerted mechanism. It is postulated that l a
first undergoes a Cope rearrangement[5.6Jto 3,4-divinyl1,s-cyclooctadiene 2 via l b (see Scheme 2). The intermediate 2 is presumably unstable at the reaction temperature,
and undergoes either a second Cope rearrangement to
yield 3 or a 1,3-shiftr7I to afford the cyclohexene derivatives 4 and 518'.
Received: March 18, 1982 [ Z 175 IE]
German version: Angew. Chem. 94 (1982) 710
The complete manuscript of this communication appears in:
Angew. Chem. Suppl. 1982, 1491- 1498
121 Satisfactory analytical and spectroscopic data have been obtained for 1
and 3-5: the structural assignment is based principally on 'H- and "CNMR studies.
[5] Review: H. M. Frey, R. Walsh, Chem. Rev. 68 (1968) 103.
[6] E. Vogel, Justus Liebigs Ann. Chem. 615 (1958) I :Angew. Chem. 74 (1962)
829; Angew. Chem. Int. Ed. Engl. 2 (1963) 1; H.-D. Martin, E. Eisenmann. M. Kunze, V. BonaciC-Koutecky, Chem. Ber. 113 1.1980) 1153: P.
A. Wender. S. McN. Sieburth, J. J. Petraitis, S. K. Singh, Tetrahedron 37
(1981) 3967.
171 J . Berson, P. B. Dervan, R. Malherbe, J. A. Jenkins, J. Am. Chem. SOC.98
(1976) 5937.
[XI V. Schull, H. Hopf, Tetrahedron Lett. 22 (1981) 3439.
Mechanistic Aspects of the Carbonylation of
Nitrobenzene with Rhodium Catalysts
By 3. Elleuch, Y. Ben Taarit. J . M. Basset*. and
J . Kervennal**
Nitro- o r dinitroarene compounds in the liquid phase
can be carbonylated by rhodium chlorides o r rhodium oxides at 120 to 190 "C under C O pressure1']. Interestingly,
the same catalysts as well as the Rho cluster Rh6(C0)16reduce nitroarenes with CO/H20 to the corresponding aminesl2I. We report here that the carbonylation of nitrobenzene 1 to form phenyl isocyanate 7 can also be performed
with metallic rhodium supported on A1,0,, and propose a
general redox mechanism based on IR studies. This mechanism involves the three common oxidation states of rhodium and the intermediacy of a nitrenerhodium surface
complex based on analogues in the coordination chemistry
of ciu~ters[~l.
Using 46 1 mg of the Rho/A1203[41
catalyst, nitrobenzene
(10 g) in 100 mL of ortho-dichlorobenzene in presence of
1 g pyridine is converted into phenyl isocyanate at 240 "C
and a CO pressure of 200 bar in 70% yield with 100%conversion after 405 min (1 h initiation time). Among the byproducts only azobenzene (0.25%) was identified.
Room temperature adsorption of carefully dried nitrobenzene 1 (1 torr) on the catalyst gives IR bands (Fig. 1) at
1350 (s) and 1520 (s) cm-', corresponding to the asymmetric and symmetric NO vibrations of adsorbed Ph- NO2, 1.
In addition, a multiplet of weak intensity (1585, 1605, 1620
and 1640 cm-') corresponding to the C C stretching bands
of the phenyl ring occurs. Upon heating the sample to
50 "C or above, the intensity of the v ( N 0 ) bands decreases
significantly. Subsequent adsorption of C O (20 torr) gives
Prof. Dr. J. M. Basset, B. Elleuch, Y. Ben Taarit
lnstitut d e recherches sur la catalyse
2, avenue Albert Einstein, 69626 Villeurbanne Cedex (France)
Dr. J. Kervennal
Societe Chimique Pechiney Ugine Kuhlman
Centre de recherches d e Pierre Benite
69310 Pierre Benite (France)
Angew. Chem. Int. Ed. Engl. 21 11982) No. 9
Fig. 1. IR-spectrum of Rh/A1203 a) after adsorption of P h N 0 2 1 ( I torr,
25 "C), b) after heating to 100 "C, c) after CO adsorption (5 torr) at 25 " C , d )
after heating to 50 "C, e) to 100 "C, 0 to 120 "C.
I R bands at 2125,2100, and 2029 cm-' corresponding to a
(band at 2125 cm-') and to a (CO),Rh' species. Simultaneously bands which appear at 1650,1454 and
1230 cm - indicate the formation of C02/carbonates.
Upon increasing the temperature to 12OoC, a band of
low intensity is observed at 2262 cm-', which is typical of
adsorbed phenyl isocyanate: the intensity of the bands at
2100 and 2029 c m - ' (Rh' species) increases, whereas two
bands appear at 2059 and 1860 cm-' corresponding to C O
coordinated to metallic
At 170°C the v(C0)
bands corresponding to (C0)Rh"' and (C0)2Rh' disap-
R h"- R hcR hcRhLR h" 2
+ co
' ;R
- co*
-Ph-N=C=O 7
Scheme 1.
0 Verlag Chemie GmbH, 6940 Weinheim. 1982
0570-0833/82/0909-0687 S 02.50/0
pear; only CO coordinated to Rh" and gaseous CO, can be
Similar behavior is observed if CO is adsorbed on the
catalyst prior to the addition of nitrobenzene. Bands corresponding to CO chemisorbed on Rh (2075 (s) and 1870 (br)
cm-') occur. The subsequent adsorption of PhNOz (1 torr)
at room temperature results in a drastic reduction of the
intensity of these bands, which are partially replaced by
the doublet characteristic of the (CO)2Rh' species. Introduction of CO (380 torr) at 100°C results in the simultaneous formation of Rh" coordinated to CO (bands at 2075
and 1870 c m - ' ) and of phenyl isocyanate (band at 2260
cm - I).
The results can be explained on the basis of the redox
mechanism shown in Scheme 1. Nitrobenzene 1 can oxidize Rho 2 to Rh"'/Rh' surface complexes 4. Simultaneously a phenyl nitrene complex 3 is formed with a structure similar to that already observed in Fe or Ru clusters['!
The Rh'/Rh"' surface complex 4 can be reduced at 25 "C
by CO to (CO)2Rh' 5 with simultaneous formation of C 0 2 .
Formation of phenyl isocyanate 7 is observed when 5 is
reduced to Rho surface complexes 8, which would favor
C O insertion into the R h N bond. Such insertion is probably the rate-determining step, and at low CO pressure, i. e.
under non-catalytic conditions, it does not occur.
Received: February 8, 1982 [Z 176 IE]
German version: Angew. Chem. 94 (1982) 722
- 2 7 . 0 kcal/mol
-4.2 kcal/mol
- 7 . 9 kcal/mol
Once again, experiment is confirmed by M N D O calculations: the orthogonal anion 0 - 3 O is about 7.9 kcal/mol
more stable than the planar anion 3'.
According to M N D O calculations, in the case of the anion of the cyclopentadienyl methyl ketone 2a the planar
conformation is slightly (4.2 kcal/mol) more stable than
the orthogonal one, so that (ester-)enolates of this type
should be conformationally labile. The 'H- and "C-NMR
spectra (Table I ) of 2a-c a Li confirm this prediction.
Table I. I3C-NMR spectra (6 values, referred to the solvent [D,]-tetrahydrowith 2 equivalents of hexamethylphosphoric
furan (THF)) of 2a-c'Li0
triamide (HMFT). Signal indices: without index=sharp; sb=slightly broadened: b = broadened; hb = highly broadened.
111 US-Pat. 4070391 (24. I. 78); W. B. Hardy, R. P. Bennett, Tetrahedron
Lett. 1967, 961.
[2] A. F. Iqbal, CHEMTEC 1977. 566.
131 F. L'Eplattenier, P. Matthys, F. Calderazzo, Inorg. Chem. 9 (1970) 342: S .
Aime, G. Gervasio, L. Milone, R. Rossetti, P. L. Stanghellini, J . Chem.
SOC.Dalton Trans. 1978. 534; M. A. Andrews, H. D. Kaesz, J . Am. Chem.
SOC.99 (1977) 6763.
141 Prepared by impregnating y-AIzO, (300 mz/g) with RhCII.nH20,drying
at 100 "C, and reduction at 500 "C in a stream of hydrogen. The size of
the Rh content of the
the Rho particles varies from between 15 to 20
catalyst is ca. 10%.
[ S ] A. K. Smith, F. Hugues, A. Theolier, J. M. Basset, R. Ugo, G. M. Zanderithi, J. L. Bilhou, W. F. Graydon, Inorg. Chem. 18 (1979) 3104.
Easy Rotation about the
Carbon-Carbon Bond in
Lithium Cyclopentadienyl (Ester-)Enolates**
By Gernot Boche*, Robert Eiben, and Walter Thiel
The CC "double bond" in (ester-)enolates is conformationally stable referred to the N M R time s ~ a l e [ ~its
~ ~douJ];
ble bond character has recently been confirmed by crystal
structure dataI4'. M N D O calculations are in agreement
with both findings[51:The planar acetone anion 1 ", e . g . , is
about 27 kcal/mol more stable than the orthogonal anion
0-1 rotated through 90" at the CC bond.
Interestingly, the situation is exactly the reverse in the
case of the acetyl-[9]annulene anion 3". The NMR signal
of C" (K@-salt in [D,]THF) appears at 6=206.7[51.This
finding is in no way compatible with a planar enolate
structure[3d1,but with a non-conjugated methyl ketone.
[*I Prof. Dr. G. Boche, R. Eiben
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (Germany)
Doz. Dr. W. Thiel
Fachbereich Physikalische Chemie der Universitat Marburg (Germany)
This work was supported by the Fonds der Chemischen Industrie.
0 Verlag Chemie GmbH, 6940 Weinheim. 1982
114.8 (hb)
120.7 115.6 (b) 114.0 (b)
120.6 115.4 (sb) 113.8 (sb)
107.7 113.7(hb) 113.2(hb)
1 1 1 . 1 (hb)
107.8 113.9 (sb) 113.0(sb) 111.5 (sb) Ill.O(sb) 168.5
111.0 (b)
1 1 1.4 (b) 110.4 (b)
1 11.1 (sb) 110.3 (sb)
Methyl enolate 2 a @ L i @ From
-60 to 55 "C the same
spectra are obtained as at 27 "C. The constantly sharp and
different signals of C2 and Cs allow only one interpretation: The rotation about the C ' C 5 bond is slow referred to
the NMR time scale.
tert-Butyl enolate 2b'Li ": At 27 "C, the atom pairs C 2
and Cs as well as C3 and C4 each give one sharp signal; at
- 21 "C these signals are broadened, and at - 34 "C each
are split into two lines, which, at -62 "C, are only slightly
broadened: C ' and C 6 , on the other hand, always show
sharp signals. This spectral behavior is compatible only
with a dynamic process due to rotation about the C'C6
bond. Other likely causes for such a behavior, namely protonation-deprotonation o r aldol-like reactions with any
tert-butylcyclopentadienyl ketone that may be present,
would involve all C atoms participating in the dynamic
process and, moreover, should also occur in the case of
2a Li @.
From the coalescence temperature of the signals
of C 2 and C 5 the values k=96.5 s - ' and AG' = 12.4k0.2
kcal/mol are obtained for the rotation at 21 f 4 "C. Since
the spectra of 2 b Q L i @in [D,]THF without HMPT are almost the same, aggregation and ion-pair effects are, remarkably, of no decisive importance in the rotation about
the C'C6 bond in 2b"Li@['4c1.The difference in behavior
of 2a 'Li and 2b Li@ stems from the steric hindrance in
the ground state of 2beLi@. The "transition states" o-
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Angew. Chem. b t . Ed. Engl. 21 (1982) No. 9
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carbonylation, nitrobenzene, mechanistic, aspects, rhodium, catalyst
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