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Metal carbonyl catalyzed reductive carbonylation of substituted nttrobenzenes in presence of alkenes as solvents
Journal of Molecular Catalysis,
60 (1990)
155-163
155
METAL CABBONYL CATALYZED REDUCTIVE CARBONYLATION OF SUBSTITUTED NITROBENZENES IN PRESENCE OF ALKENES AS SOLVENTS ANGELA
BABSOLI,
BRUNO
RINDONE*,
STEFANO
Dipartimento di Chimica Organica e Indwtriale, 20133 Milan (Italy) SERGIO
CENINI
and CORRADO
CROTTI
Dipartimento di Chimica Inorganica e Metallorganica Via Venezian 21, 20133Milan (Italy) (Received August 25,1989;
TOLLARI
Universit& di Milano, Via Venezian 21,
and CNR Center, Universit& di Milan,
accepted January 2,lSSO)
Several nitrobenzene derivatives were submitted to reaction with carbon monoxide in alkene solvents in the presence of catalytic amounts of RI&CO),,. Ureas and amines were formed in this reaction, and the secondary amine derived from insertion of the nitrene intermediate at the allylic position of the alkene solvent was a secondary product. The influence of the electronic factor of the substituents on the urea us. amine ratio is discussed.
Introduction
Arylnitrenes [ll and metal arylnitrene complexes 121 have several applications in organic synthesis. These species can be generated in many ways, often by the reduction of aromatic nitro compounds 131. Several intramolecular reactions of arylnitrenes generated from the triethylphosphite reduction of nitro derivatives [41 or from the metal carbonyl-catalysed reduction of these with carbon monoxide [5] have been reported. This reaction occurs in the presence of a multiple C=C bond or an aromatic or allylic C-H bond, yielding heterocycles as reaction products. Metal-assisted intermolecular nitrene insertions have been reported in only one case: the reaction of a nitrene metal complex with a carbon-carbon triple bond [61. Here we report attempts to perform this reaction by submitting aromatic nitro derivatives to reductive carbonylation, either in stoichiometric or in catalytic conditions with Fe(CO), and Ru~(CO)~~ respectively, using carbon monoxide as the reducing agent and operating in different alkenes as solvents. *Author to whom correspondence 0304-5102/90/$3.50
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
156
Experimental Nitroarenes, cis-cyclooctene, cyclohexene, camphene, acetonitrile, 1,5cis,cis-cyclooctadiene, stilbene, Ru,(CO),, and Fe(CO), were Merck reagents. Carbon monoxide was high purity grade. IR spectra tiere taken on a Perkin-Elmer 1310 spectrophotometer. ‘H NMR spectra were recorded on a Bruker WP 80 SJ spectrometer with SiMe, as internal standard (s = singlet; d = doublet; dd = doublet of doublets; m = multiplet; br = broad). Mass spectra were taken on a VG 7070 EQ spectrometer operating in electron impact at 70 eV or in chemical ionization, ionizing gas isobutane. Reaction a To a 0.41 M solution of nitrobenzene in cis-cyclooctene, 1 equivalent of iron pentacarbonyl was added and the mixture was refluxed under nitrogen for 72 h. After this time gas chromatographic analysis showed that the reaction was complete; the black suspension was filtered, the precipitate TABLE
1
Reactions, conversions and yields (calculated with respect to starting material) for the reduction of aromatic nitro derivatives Reaction ( substr. )
Cont.
Conversion
Insertion
Urea
Amine
(I)
(%I
(S)
(8)
a (la)”
(la) (lb) (lb) (la)
0.41 0.17 0.34
2 2 2
100 95 100
-
-
6a:2
tr
0.17 0.34
2 2
100 77
5a:4 Sa: 2 5a: 5 Sb: 6 5b:l
6a: 88 6b~40 6bz28
7% 1 7bz9
0.41
8
95
9:l
f3ar30
g (lb) h (la)
0.41 0.41
8
100
-
613:38
tr
i (lajb
0.41
100 95
-
-20 6az59
6ar 14
?a: 61
6~~36 -
7d: 27 -
b c d e f
(la)
Solvent
Of)
tr -
lo: 1
1 (la)
0.17
11 12 MeCN/lS
100
-
0.43 M m (lc) n (ld)’
0 (15P
p (15)’
0.40
2
81
0.40 0.40 0.40
2 2 2
70
100 50
5c5 5d:2 5a:4 5a: 3
Reaction conditions (unless otherwise specified): 7’ = 170 “C; t = 5 h; p (CO) = 60 atm; catalyst Rus(COJ1z, substrate to catalyst ratio = 25; tr = traces. a Reaction performed using a 1:l amount of Fe(CO), at reflux temperature for 72 h under nitrogen atmosphere. Also 25% of a nearly equimolecular mixture of azo-3 and azoxybenzene 4 was isolated. b A ruthenium complex was also isolated. ’ Small amounts of 4-nitrobenzoic acid were also recovered. d 8% of azobenzene 3 was also isolated. e Reaction performed at 120°C for 2 h.
157
washed with chloroform and methanol, and the resulting organic solution evaporated at reduced pressure and then chromatographed on silica gel Merck 0.05-0.2 mm (R = 100). Reactions b-p To a solution of the appropriate substrate in the alkene solvent in a glass liner, RUDER was added and the mixture put in a stainless steel autoclave. The air in the autoclave was replaced with dinitrogen by three freeze-pump-thaw cycles before introducing 60 atm carbon monoxide. The autoclave was heated at the desired temperature with a thermoregulated oil bath and magnetic stirring was applied. At the end of the reaction, the autoclave was rapidly cooled in an ice bath and vented. The reaction mixture was evaporated at reduced pressure and chromatographed on silica gel Merck 0.05-0.2 mm (R = 100). Compounds 3,4,6a-c, 7a-b and 7d were identified by comparison with authentic samples. Compound 5a Colourless oil; MS (electron impact, 70 ev): m/z = 201 (M+) (480/o), 158 (loo%),93 (60%),MS (chemical ionization, ionizing gas isobutane): m/z = 202 (M + 1') (loo%),158 (320/o), 94 (45%),‘H NMR spectrum, ClX&: 6 = 7.15 (m,2H), 6.62 (m,3H), 5.75 (m, lH), 5.3(m, lH), 4.3(brdd,lH), 3.5(br, lH, exchanges with DzO), 1.2-2.5 (m, 10H). Compound 5b Colourless oil; MS (electron impact, 70 eV): m/z = 215 (M+) (30%),172 (lOO%),107 (88%);‘H NMR spectrum, CDCl,: 6 = 7.3(s,3H), 7.05 (m,4H), 6.5 (m, lH), 5.2-6.0 (m,2H), 4.2 (br, lH, exchanges with D,O), 1.2-2.5 (m, 10H). Compound 9 Colourless oil; MS (electron impact, 70 eV): m/z = 173 (M+) (85%), 145 (70%), 93 (100%) ‘H NMR, CDCl,: 6 = 7.16 (m, 2H), 6.7 (m, 3H), 5.8 (br.m, 2H), 4.0 (br.s, lH), 3.6 (br, lH, exchanges with D,O), 0.8-2.1 (br, 6H). Compound 10 White crystals; MS (electron impact, 70 eV): m/z = 203 (M’) (98%), 173 (100%);‘H NMR, CDC13: 6 = 6.9-7.5 (m, 5H), 4.85 (br, lH, exchanges with DzO), 1.1-2.4 (m, 11H). Compound 5c Cohrless oil; MS (electron impact, 70 eV): m/z = 235 (M+) (45%),192 (loo%),127 (69%);‘H NMR, CDCl,: 6 = 7.05 and 6.50 (d, 2H), 5.22 (ddd, lH), 5.80 (ddd, lH), 4.20 (br, lH), 3.67 (br, lH, exchanges with D@), 1.10-2.40 (m, 10H).
158
Compound 5d Pale yellow crystals: MS (electron impact, 7OeV): m/z = 259 (M’) (57%), 228 (lo%), 216 (loo%), 135 (62%); ‘H NMF$ CDCl,: 6 = 7.80 and 6.50 (d, 2H), 6.80 (ddd, lH), 5.30 (ddd, lH), 4.30 (br, lH), 4.13 (br, lH, exchanges with DzO), 3.80 (s, 3H), 1.10-2.40 (m, 10H). Synthesis of N-(Z~yclooctenyl luniline 5a Cis-cyclooctene (18 mmol, 2 g) and N-bromosuccinimide (18 mmol, 3.2 g) were refluxed in 100 ml of carbon tetrachloride for 2 h; the resulting solution containing 2-bromocyclooctene was then filtered to eliminate succinimide and added slowly to a solution of aniline (18 mmol) and triethylamine (18 mmol). At the end of the reaction the mixture was evaporated and chromatographed on silica gel to give N-(2-cyclooctenyl)aniline 5a in 38% yield.
N=N
la:X=H
2
4
3
lb:X=CH, lc:X=CI ld:X=COOCH,
5a:X=H
Ga:X=H
5b:X=CH,
Gb:X=CH,
5c:X=CI
Gc:X=CI
Jd:X=COOCH, y2
x 7a:X=H 7b:X=CH, 7d: X=COOCH,
0 6
0%\ 9
159
N-(2-cyclooctenyl)4-methyl aniline 5b, the analogue 5c and N-(2cycloesenyl) aniline 9 have been synthesized by analogous procedures. Results
and discussion
The first attempt (reaction a) was performed using nitrobenzene la as substrate, cis-cyclooctene 2 as the solvent and a stoichiometric amount of Fe(CO)+ After 72 h at reflux temperature 100% conversion was attained, and 25% of a nearly equimolecular mixture of azo- 3 and azoxybenzene 4 could be isolated by silica gel column chromatography. A small amount, 4%, of compound 5a, deriving from the insertion of a nitrene intermediate on the allylic position of cyclooctene, was also present and its structure was conihmed by independent synthesis. When the reduction of nitrobenzene 1 was performed under pressure of carbon monoxide and in the presence of RuB(CO)rP as catalyst, a dependence of the conversion on the concentration of substrate was noted. In fact at the lower concentration of 0.17 M in cyclooctene (reaction b) 95% conversion resulted, and urea 6a was isolated by silica gel chromatography in 12% yield. Also the insertion compound 5a was isolated in 2% yield and was accompanied by traces of aniline 7a (see Table 1). A higher concentration of nitrobenzene la (0.34M) in the same conditions gave 100% conversion (reaction c). 88% of the urea 6a and 5% of the insertion product 5a could be isolated. A different situation resulted from the reaction of 4nitrotoluene lb in the same conditions. When the substrate was 0.17 M in cyclooctene (reaction d) a 100% conversion was noted and the urea 6b (40%), the amine 4-toluidine 7b (9%) and the insertion product 5b (6%) were isolated. This latter compound was confumed by independent synthesis. At the higher concentration of 0.34 M, a lower conversion (77%) resulted (reaction e), a slightly lower amount of urea 6b (28%) and a small amount of insertion product 5b (1%) were isolated by silica gel chromatography. The amine 7b was present as a trace. In control experiments aniline 7a was submitted to the conditions reported in reactions a-c and was essentially unconverted. In particular, neither the urea 6a nor the insertion product 5a was formed. Furthermore, no reaction of cyclooctene was noticed in reactions performed in the absence of the nitro derivative. The second alkene used as the solvent in these reaction was cyclohexene 8. Nitrobenzene la (0.41 M) gave in these conditions (reaction f) a conversion slightly lower than in cyclooctene, i.e. 95%, and silica gel chromatography allowed the isolation of the urea 6a (30%) and very low amounts of the insertion and carbonylation-insertion products 9 (1%) and 10 (l%), which were confirmed by independent synthesis. Again, a control experiment showed that these latter products did not derive from reaction of aniline 7a intermediately formed. 4-Nitrotoluene lb (0.41 M) under the same reaction conditions allowed isolation of only the urea 6b (reaction g) in 38% yield.
160
The third alkene used as a solvent was camphene 11. Using Rua(CO )i2 as the catalyst and nitrobenzene la (0.41 M) as the substrate, 100% conversion was observed and only the urea 6a was obtained (reaction h). The fourth alkene used was l,&cis,cis-cyclooctadiene 12. In these conditions nitrobenzene la (0.41 M) gave a 95% conversion, and 59% of the urea 6a could be isolated (reaction i). Small amounts of a ruthenium complex were also formed in this reaction, perhaps due to the higher coordinating ability of l&cis,ciscyclooctadiene compared with the other alkenes. An aromatic alkene was also used in this study, i.e. stilbene 13. An acetonitrile solution of nitrobenzene la (0.17 M) and stilbene in 1:2 ratio was submitted to the usual reaction conditions (reaction 1) and gave 100% conversion, allowing the isolation of 14% of urea 6a and 61% of aniline 7a. No insertion of nitrogen compounds into the alkene could be noted. The data thus far presented suggest a marked independence of the reaction from the nature of the alkene used as solvent. The urea is in most cases the preferred product, and the insertion products are confined to a few percent of the reaction mixture. This seems to suggest that the metal center acts as a template for the formation of the urea, which derives from a pathway involving two molecules of substrate and one molecule of carbon monoxide 171. Likewise, the formation of ureas had been noted also in the N,N-bis(salicylidene)ethylenediaminocobalt(II)-catalysed oxidative carbonylation of aromatic amines [Sl. The effect of the concentration of substrates la and lb on the conversion and the production of urea was complex. A higher concentration gave lower conversion and an accordingly lower formation of urea in the case of 4nitrotoluene lb in cyclooctene. Thus, an increase in concentration of the substrate lowered both the reactivity and the selectivity in urea. With nitrobenzene la, the conversion was almost quantitative in all the experiments, but the yield in urea was higher in runs performed at higher substrate concentration. Thus, an increase in concentration of the substrate did not affect significantly the reactivity, but markedly increased the conversion in urea. These results suggest that several competing mechanisms occur in these reactions. Other compounds gave insertion products in cis-cyclooctene: 4chloronitrobenzene lc gave 81% conversion, 36% yield of the urea 6c and 5% of the allylic amine 5c (reaction m). It should be noted that in the reactions of substrates la-c the IR spectra showed that some catalyst was present in the mixture after reaction. Methyl 4nitrobenzoate Id gave 27% of the amine 7d and 2% of the insertion product 5d. No urea was found in this reaction. The conversion was 70%. (reaction n). Also one black insoluble compound containing ruthenium was recovered and was shown by mass spectrometry to contain the amine 7d as a fragment and to have a polymeric structure. The products 5a-c, 9 and 10 resulting from insertion into a solvent molecule, were formed only in low amounts and seem to derive from a side
161
a@ R
ii
-
Q-J: H
16
Ar-N
19
18
Scheme 1.
process which occurs via hydrogen abstraction from the allylic position of the alkene, effected by an intermediate nitrene either free or bound to the metal center 16 to give an intermediate diradical species 17, followed by a recombination reaction to the final products 18 (Scheme 1). In the case of camphene 11 we couldn’t recover any insertion product. This might support the hypothesis of a radical mechanism, since the bridgehead allylic radical 14 is thought to be formed slowly. Lack of planarity and the impossibility of delocalizing the unpaired electron should be the reason for this; in fact, pyramidal structures for bridgehead radicals are rare and planarity is preferred if conjugation is present 191. An aziridine, 19, could also be intermediate in this reaction. This compound was never isolated, possibly due to the fact that aziridine formation is often found in the reaction of electron-poor nitrenes with nucleophilic alkenes 1101, but has never been reported to occur in metal nitrene-alkene systems. Accordingly, the reaction of nitrobenzene la in stilbene 9 could not give insertion products, owing to the lack of allylic hydrogens, and gave instead the urea 6a, the amine 7a and no aziridine. Similarly, in a recent report, the reaction of a molybdenum-nitroso complex and an alkene gave a metal-oxo derivative and allylamine [ill, and not the aziridine. These findings suggest that the Ru(O)-catalysed reductive cyclisation of o&e-nitrostilbenes to indoles [51 does not occur via the formation of
162
an intermediate aziridine 20 but via some ruthenium intermediate capable of the cyclisation reaction. The shift of the more electron-donating R' group from position 2 to 3 in the final indole 21 suggests a positive charge at position 3 in the transition state of the cyclisation. The simultaneous presence of the alkene and the nitrogen atom in the coordination sphere of ruthenium during the reduction was likely to occur in the reactions in alkene solvent. Ethylene is known to coordinate the Ru center in RUDER [121. This occurs also with trans-cyclooctene 1131 and with cyclohex-l-en-&one [ 141. The fact that the intramolecular reaction occurs readily [5], whereas the intermolecular reaction is only a side process, suggests important differences in the coordination sphere of the ruthenium center. Amine formation seems to be important in the ruthenium(O)catalysed reduction of nitro derivatives with carbon monoxide. The occurrence of small amounts of azobenzene in the reduction of nitrosobenzene 15 in the same conditions (reaction o) could derive from the reaction of the amine and the starting nitrosoderivative. A 100% conversion was obtained in this case, and from the very complex mixture only 4% of the insertion product 5a and 8% of azobenzene 3 could be isolated. A control experiment showed that the catalyst was necessary for the formation of the amine, thus excluding that the amine 5 could derive from an ‘ene-type’ reaction [ 151 between nitrosobenzene and cyclooctene. Moreover, a lower temperature (reaction p) gave a lower conversion of nitrosobenzene 15 (50%) and the isolation of 3% of compound 5a. These results showed again that insertion into the alkene is a side process. In conclusion, the catalytic reduction of substituted nitroaromatics in alkene solvents gives symmetric ureas as the main product, primary and secondary amines as side products, suggesting that the metal center acts as a template in the urea-determining step. This had previously been observed also in the Ru-catalysed reduction of nitroaromatics in the presence of alcohols 1161. Urea formation is suppressed when the strong electronwithdrawing substituent -COOMe is present in the paru position. In this case, the primary amine is the main reaction product. Thus, the selectivity in amine seems to be influenced by the electron-withdrawing effect of the substituent. This suggests that amine formation is due to hydride transfer on a ruthenium-nitrogen intermediate, and not to hydrogen atom abstraction. Hydride-transfer catalysis in Ru,(CO),, chemistry has been reported in other cases 1171 and also by us in the reduction of ortho-nitrophenyl ally1 ether with the same catalytic system [18]. The intermolecular insertion reaction into the alkene molecule seems to be a side process, and is not affected by the electronic characteristics of the substrate. Acknowledgements
We thank Mr. Marco Nicolini for experimental assistance. This work was supported by a grant of the Consiglio Nazionale delle Ricerche, Progetto Finalizzato Chimica Fine II.
163
References 1 B. Iddon, 0. Meth-Cohn, E. F. V. Striven, H. Suschitzyk and P. T. Gallagher, Angew. C&m. Znt. Ed. Engl., 18 (1979) 900. 2 S. Cenini, M. Pizzotti and C. Crotti, in R. Ugo (ed.), Aspects of Homogeneous Catalysis, Vo16, Reidel, Dordrecht, 1988. 3 W. Lwowski, Nitrenes, Interscience, New York, 1970. 4 R. J. Sundberg, J. Org. Chem., 30 (1965) 3604; ibid., 33 (1968) 487; R. J. Sundberg and T. Yamazaki, J. Org. Chem., 32 (1967) 290. 5 C. Crotti, S. Cenini, B. Rindone, S. Tollari and F. Demartin, J. Chem. Sot.. Chem. Commun., (1986) 784. 6 (a) S. Han, G. L. Geoffioy and A. L. Rheingold, Orgarwmetallics, 6 (1987) 2380; (b) M. Pizzotti, S. Cenini, C. Crotti and F. Demartin, J. Organometall. Chem., 375 (1989) 123. 7 G. D. Williams, R. R. Whittle, G. L. Geoffroy and A. L. Rheingold, J. Am. Chem. Sot., 109 (1987) 3936. 8 A. Bassoli, F. Chioccara, B. Rindone and S. ToIIari, J. Mol. Catal., 60 (1990) 41. 9 J. P. Lorand, S. D. Chodroff and A. W. Wallace, J. Am. Chem. Sot., 90 (1968) 5226; R. C. Fort and R. E. Frankhn, J. Am. C&m. Sot., 90 (1968) 5267; L. B. Humprey, B. Hodgson and R. E. Pinckock, Can. J. Chem., 46 (1968) 3099; A. Oberlinner and C. Riichardt, Tetrahedron Z.&t., (1971) 4685; R. C. Fort and J. Hiti, J. Org. Chem., 42 (1977) 3968. 10 0. Meth-Cohn, Act. Chem. Res., 20 (1987) 18. 11 L. S. Liebeskind, K. B. Sharpless, R. D. Wilson and J. A. Ibers, J. Am. Chm. Sot., 100 (1978) 7061. 12 J. G. Bentse and M. S. Wrighton, J. Am. Chem. Sot., 109 (1987) 4530; Y. M. Wuu, J. G. Bentsen, C. G. Brinkley and M. S. Wrigton, Znorg. Chem., 26 (1987) 530. 13 R. G. Ball, G. Y. Kiel, J. Takats, C. Kruger, E. Raabe, F. W. Grevels and R. Hoser, Organometallics, 6 (1987) 2260. 14 A. Basu, S. Baduri, K. Sharma and P. G. Jones, J. Organometall. Chem., 328 (1987) C34. 15 R. A. Abramovitch, S. R. ChaRand and Y. Yamada, J. Org. Chem., 40 (1975) 1541. 16 S. Cenini, C. Crotti, M. Pizzotti and F. Porta, J. Org. Chem., 53 (1988) 1243. 17 Y. Blum, D. Reshef and Y. Shvo, Tetrahedron L&t., (1981) 1541; M. Bianchi, G. Men&i, F. FrancaIanci and F. Piacenti, J. Organametull. C’hem., 188 (1980) 109; G. Des&es and J. Sabadie, J. Mol. Catal., 5 (1979) 415; L. Cottier, G. Descotes and J. Sabadie, J. Mol. Catal., 7 (1980) 337. 18 A. Bassoli, B. Rindone, S. Tollari, C. Cenini and C. Crotti, J. Mol. Catal., 52 (1989) I.47.
60 (1990)
155-163
155
METAL CABBONYL CATALYZED REDUCTIVE CARBONYLATION OF SUBSTITUTED NITROBENZENES IN PRESENCE OF ALKENES AS SOLVENTS ANGELA
BABSOLI,
BRUNO
RINDONE*,
STEFANO
Dipartimento di Chimica Organica e Indwtriale, 20133 Milan (Italy) SERGIO
CENINI
and CORRADO
CROTTI
Dipartimento di Chimica Inorganica e Metallorganica Via Venezian 21, 20133Milan (Italy) (Received August 25,1989;
TOLLARI
Universit& di Milano, Via Venezian 21,
and CNR Center, Universit& di Milan,
accepted January 2,lSSO)
Several nitrobenzene derivatives were submitted to reaction with carbon monoxide in alkene solvents in the presence of catalytic amounts of RI&CO),,. Ureas and amines were formed in this reaction, and the secondary amine derived from insertion of the nitrene intermediate at the allylic position of the alkene solvent was a secondary product. The influence of the electronic factor of the substituents on the urea us. amine ratio is discussed.
Introduction
Arylnitrenes [ll and metal arylnitrene complexes 121 have several applications in organic synthesis. These species can be generated in many ways, often by the reduction of aromatic nitro compounds 131. Several intramolecular reactions of arylnitrenes generated from the triethylphosphite reduction of nitro derivatives [41 or from the metal carbonyl-catalysed reduction of these with carbon monoxide [5] have been reported. This reaction occurs in the presence of a multiple C=C bond or an aromatic or allylic C-H bond, yielding heterocycles as reaction products. Metal-assisted intermolecular nitrene insertions have been reported in only one case: the reaction of a nitrene metal complex with a carbon-carbon triple bond [61. Here we report attempts to perform this reaction by submitting aromatic nitro derivatives to reductive carbonylation, either in stoichiometric or in catalytic conditions with Fe(CO), and Ru~(CO)~~ respectively, using carbon monoxide as the reducing agent and operating in different alkenes as solvents. *Author to whom correspondence 0304-5102/90/$3.50
should be addressed. @ Elsevier Sequoia/Printed
in The Netherlands
156
Experimental Nitroarenes, cis-cyclooctene, cyclohexene, camphene, acetonitrile, 1,5cis,cis-cyclooctadiene, stilbene, Ru,(CO),, and Fe(CO), were Merck reagents. Carbon monoxide was high purity grade. IR spectra tiere taken on a Perkin-Elmer 1310 spectrophotometer. ‘H NMR spectra were recorded on a Bruker WP 80 SJ spectrometer with SiMe, as internal standard (s = singlet; d = doublet; dd = doublet of doublets; m = multiplet; br = broad). Mass spectra were taken on a VG 7070 EQ spectrometer operating in electron impact at 70 eV or in chemical ionization, ionizing gas isobutane. Reaction a To a 0.41 M solution of nitrobenzene in cis-cyclooctene, 1 equivalent of iron pentacarbonyl was added and the mixture was refluxed under nitrogen for 72 h. After this time gas chromatographic analysis showed that the reaction was complete; the black suspension was filtered, the precipitate TABLE
1
Reactions, conversions and yields (calculated with respect to starting material) for the reduction of aromatic nitro derivatives Reaction ( substr. )
Cont.
Conversion
Insertion
Urea
Amine
(I)
(%I
(S)
(8)
a (la)”
(la) (lb) (lb) (la)
0.41 0.17 0.34
2 2 2
100 95 100
-
-
6a:2
tr
0.17 0.34
2 2
100 77
5a:4 Sa: 2 5a: 5 Sb: 6 5b:l
6a: 88 6b~40 6bz28
7% 1 7bz9
0.41
8
95
9:l
f3ar30
g (lb) h (la)
0.41 0.41
8
100
-
613:38
tr
i (lajb
0.41
100 95
-
-20 6az59
6ar 14
?a: 61
6~~36 -
7d: 27 -
b c d e f
(la)
Solvent
Of)
tr -
lo: 1
1 (la)
0.17
11 12 MeCN/lS
100
-
0.43 M m (lc) n (ld)’
0 (15P
p (15)’
0.40
2
81
0.40 0.40 0.40
2 2 2
70
100 50
5c5 5d:2 5a:4 5a: 3
Reaction conditions (unless otherwise specified): 7’ = 170 “C; t = 5 h; p (CO) = 60 atm; catalyst Rus(COJ1z, substrate to catalyst ratio = 25; tr = traces. a Reaction performed using a 1:l amount of Fe(CO), at reflux temperature for 72 h under nitrogen atmosphere. Also 25% of a nearly equimolecular mixture of azo-3 and azoxybenzene 4 was isolated. b A ruthenium complex was also isolated. ’ Small amounts of 4-nitrobenzoic acid were also recovered. d 8% of azobenzene 3 was also isolated. e Reaction performed at 120°C for 2 h.
157
washed with chloroform and methanol, and the resulting organic solution evaporated at reduced pressure and then chromatographed on silica gel Merck 0.05-0.2 mm (R = 100). Reactions b-p To a solution of the appropriate substrate in the alkene solvent in a glass liner, RUDER was added and the mixture put in a stainless steel autoclave. The air in the autoclave was replaced with dinitrogen by three freeze-pump-thaw cycles before introducing 60 atm carbon monoxide. The autoclave was heated at the desired temperature with a thermoregulated oil bath and magnetic stirring was applied. At the end of the reaction, the autoclave was rapidly cooled in an ice bath and vented. The reaction mixture was evaporated at reduced pressure and chromatographed on silica gel Merck 0.05-0.2 mm (R = 100). Compounds 3,4,6a-c, 7a-b and 7d were identified by comparison with authentic samples. Compound 5a Colourless oil; MS (electron impact, 70 ev): m/z = 201 (M+) (480/o), 158 (loo%),93 (60%),MS (chemical ionization, ionizing gas isobutane): m/z = 202 (M + 1') (loo%),158 (320/o), 94 (45%),‘H NMR spectrum, ClX&: 6 = 7.15 (m,2H), 6.62 (m,3H), 5.75 (m, lH), 5.3(m, lH), 4.3(brdd,lH), 3.5(br, lH, exchanges with DzO), 1.2-2.5 (m, 10H). Compound 5b Colourless oil; MS (electron impact, 70 eV): m/z = 215 (M+) (30%),172 (lOO%),107 (88%);‘H NMR spectrum, CDCl,: 6 = 7.3(s,3H), 7.05 (m,4H), 6.5 (m, lH), 5.2-6.0 (m,2H), 4.2 (br, lH, exchanges with D,O), 1.2-2.5 (m, 10H). Compound 9 Colourless oil; MS (electron impact, 70 eV): m/z = 173 (M+) (85%), 145 (70%), 93 (100%) ‘H NMR, CDCl,: 6 = 7.16 (m, 2H), 6.7 (m, 3H), 5.8 (br.m, 2H), 4.0 (br.s, lH), 3.6 (br, lH, exchanges with D,O), 0.8-2.1 (br, 6H). Compound 10 White crystals; MS (electron impact, 70 eV): m/z = 203 (M’) (98%), 173 (100%);‘H NMR, CDC13: 6 = 6.9-7.5 (m, 5H), 4.85 (br, lH, exchanges with DzO), 1.1-2.4 (m, 11H). Compound 5c Cohrless oil; MS (electron impact, 70 eV): m/z = 235 (M+) (45%),192 (loo%),127 (69%);‘H NMR, CDCl,: 6 = 7.05 and 6.50 (d, 2H), 5.22 (ddd, lH), 5.80 (ddd, lH), 4.20 (br, lH), 3.67 (br, lH, exchanges with D@), 1.10-2.40 (m, 10H).
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Compound 5d Pale yellow crystals: MS (electron impact, 7OeV): m/z = 259 (M’) (57%), 228 (lo%), 216 (loo%), 135 (62%); ‘H NMF$ CDCl,: 6 = 7.80 and 6.50 (d, 2H), 6.80 (ddd, lH), 5.30 (ddd, lH), 4.30 (br, lH), 4.13 (br, lH, exchanges with DzO), 3.80 (s, 3H), 1.10-2.40 (m, 10H). Synthesis of N-(Z~yclooctenyl luniline 5a Cis-cyclooctene (18 mmol, 2 g) and N-bromosuccinimide (18 mmol, 3.2 g) were refluxed in 100 ml of carbon tetrachloride for 2 h; the resulting solution containing 2-bromocyclooctene was then filtered to eliminate succinimide and added slowly to a solution of aniline (18 mmol) and triethylamine (18 mmol). At the end of the reaction the mixture was evaporated and chromatographed on silica gel to give N-(2-cyclooctenyl)aniline 5a in 38% yield.
N=N
la:X=H
2
4
3
lb:X=CH, lc:X=CI ld:X=COOCH,
5a:X=H
Ga:X=H
5b:X=CH,
Gb:X=CH,
5c:X=CI
Gc:X=CI
Jd:X=COOCH, y2
x 7a:X=H 7b:X=CH, 7d: X=COOCH,
0 6
0%\ 9
159
N-(2-cyclooctenyl)4-methyl aniline 5b, the analogue 5c and N-(2cycloesenyl) aniline 9 have been synthesized by analogous procedures. Results
and discussion
The first attempt (reaction a) was performed using nitrobenzene la as substrate, cis-cyclooctene 2 as the solvent and a stoichiometric amount of Fe(CO)+ After 72 h at reflux temperature 100% conversion was attained, and 25% of a nearly equimolecular mixture of azo- 3 and azoxybenzene 4 could be isolated by silica gel column chromatography. A small amount, 4%, of compound 5a, deriving from the insertion of a nitrene intermediate on the allylic position of cyclooctene, was also present and its structure was conihmed by independent synthesis. When the reduction of nitrobenzene 1 was performed under pressure of carbon monoxide and in the presence of RuB(CO)rP as catalyst, a dependence of the conversion on the concentration of substrate was noted. In fact at the lower concentration of 0.17 M in cyclooctene (reaction b) 95% conversion resulted, and urea 6a was isolated by silica gel chromatography in 12% yield. Also the insertion compound 5a was isolated in 2% yield and was accompanied by traces of aniline 7a (see Table 1). A higher concentration of nitrobenzene la (0.34M) in the same conditions gave 100% conversion (reaction c). 88% of the urea 6a and 5% of the insertion product 5a could be isolated. A different situation resulted from the reaction of 4nitrotoluene lb in the same conditions. When the substrate was 0.17 M in cyclooctene (reaction d) a 100% conversion was noted and the urea 6b (40%), the amine 4-toluidine 7b (9%) and the insertion product 5b (6%) were isolated. This latter compound was confumed by independent synthesis. At the higher concentration of 0.34 M, a lower conversion (77%) resulted (reaction e), a slightly lower amount of urea 6b (28%) and a small amount of insertion product 5b (1%) were isolated by silica gel chromatography. The amine 7b was present as a trace. In control experiments aniline 7a was submitted to the conditions reported in reactions a-c and was essentially unconverted. In particular, neither the urea 6a nor the insertion product 5a was formed. Furthermore, no reaction of cyclooctene was noticed in reactions performed in the absence of the nitro derivative. The second alkene used as the solvent in these reaction was cyclohexene 8. Nitrobenzene la (0.41 M) gave in these conditions (reaction f) a conversion slightly lower than in cyclooctene, i.e. 95%, and silica gel chromatography allowed the isolation of the urea 6a (30%) and very low amounts of the insertion and carbonylation-insertion products 9 (1%) and 10 (l%), which were confirmed by independent synthesis. Again, a control experiment showed that these latter products did not derive from reaction of aniline 7a intermediately formed. 4-Nitrotoluene lb (0.41 M) under the same reaction conditions allowed isolation of only the urea 6b (reaction g) in 38% yield.
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The third alkene used as a solvent was camphene 11. Using Rua(CO )i2 as the catalyst and nitrobenzene la (0.41 M) as the substrate, 100% conversion was observed and only the urea 6a was obtained (reaction h). The fourth alkene used was l,&cis,cis-cyclooctadiene 12. In these conditions nitrobenzene la (0.41 M) gave a 95% conversion, and 59% of the urea 6a could be isolated (reaction i). Small amounts of a ruthenium complex were also formed in this reaction, perhaps due to the higher coordinating ability of l&cis,ciscyclooctadiene compared with the other alkenes. An aromatic alkene was also used in this study, i.e. stilbene 13. An acetonitrile solution of nitrobenzene la (0.17 M) and stilbene in 1:2 ratio was submitted to the usual reaction conditions (reaction 1) and gave 100% conversion, allowing the isolation of 14% of urea 6a and 61% of aniline 7a. No insertion of nitrogen compounds into the alkene could be noted. The data thus far presented suggest a marked independence of the reaction from the nature of the alkene used as solvent. The urea is in most cases the preferred product, and the insertion products are confined to a few percent of the reaction mixture. This seems to suggest that the metal center acts as a template for the formation of the urea, which derives from a pathway involving two molecules of substrate and one molecule of carbon monoxide 171. Likewise, the formation of ureas had been noted also in the N,N-bis(salicylidene)ethylenediaminocobalt(II)-catalysed oxidative carbonylation of aromatic amines [Sl. The effect of the concentration of substrates la and lb on the conversion and the production of urea was complex. A higher concentration gave lower conversion and an accordingly lower formation of urea in the case of 4nitrotoluene lb in cyclooctene. Thus, an increase in concentration of the substrate lowered both the reactivity and the selectivity in urea. With nitrobenzene la, the conversion was almost quantitative in all the experiments, but the yield in urea was higher in runs performed at higher substrate concentration. Thus, an increase in concentration of the substrate did not affect significantly the reactivity, but markedly increased the conversion in urea. These results suggest that several competing mechanisms occur in these reactions. Other compounds gave insertion products in cis-cyclooctene: 4chloronitrobenzene lc gave 81% conversion, 36% yield of the urea 6c and 5% of the allylic amine 5c (reaction m). It should be noted that in the reactions of substrates la-c the IR spectra showed that some catalyst was present in the mixture after reaction. Methyl 4nitrobenzoate Id gave 27% of the amine 7d and 2% of the insertion product 5d. No urea was found in this reaction. The conversion was 70%. (reaction n). Also one black insoluble compound containing ruthenium was recovered and was shown by mass spectrometry to contain the amine 7d as a fragment and to have a polymeric structure. The products 5a-c, 9 and 10 resulting from insertion into a solvent molecule, were formed only in low amounts and seem to derive from a side
161
a@ R
ii
-
Q-J: H
16
Ar-N
19
18
Scheme 1.
process which occurs via hydrogen abstraction from the allylic position of the alkene, effected by an intermediate nitrene either free or bound to the metal center 16 to give an intermediate diradical species 17, followed by a recombination reaction to the final products 18 (Scheme 1). In the case of camphene 11 we couldn’t recover any insertion product. This might support the hypothesis of a radical mechanism, since the bridgehead allylic radical 14 is thought to be formed slowly. Lack of planarity and the impossibility of delocalizing the unpaired electron should be the reason for this; in fact, pyramidal structures for bridgehead radicals are rare and planarity is preferred if conjugation is present 191. An aziridine, 19, could also be intermediate in this reaction. This compound was never isolated, possibly due to the fact that aziridine formation is often found in the reaction of electron-poor nitrenes with nucleophilic alkenes 1101, but has never been reported to occur in metal nitrene-alkene systems. Accordingly, the reaction of nitrobenzene la in stilbene 9 could not give insertion products, owing to the lack of allylic hydrogens, and gave instead the urea 6a, the amine 7a and no aziridine. Similarly, in a recent report, the reaction of a molybdenum-nitroso complex and an alkene gave a metal-oxo derivative and allylamine [ill, and not the aziridine. These findings suggest that the Ru(O)-catalysed reductive cyclisation of o&e-nitrostilbenes to indoles [51 does not occur via the formation of
162
an intermediate aziridine 20 but via some ruthenium intermediate capable of the cyclisation reaction. The shift of the more electron-donating R' group from position 2 to 3 in the final indole 21 suggests a positive charge at position 3 in the transition state of the cyclisation. The simultaneous presence of the alkene and the nitrogen atom in the coordination sphere of ruthenium during the reduction was likely to occur in the reactions in alkene solvent. Ethylene is known to coordinate the Ru center in RUDER [121. This occurs also with trans-cyclooctene 1131 and with cyclohex-l-en-&one [ 141. The fact that the intramolecular reaction occurs readily [5], whereas the intermolecular reaction is only a side process, suggests important differences in the coordination sphere of the ruthenium center. Amine formation seems to be important in the ruthenium(O)catalysed reduction of nitro derivatives with carbon monoxide. The occurrence of small amounts of azobenzene in the reduction of nitrosobenzene 15 in the same conditions (reaction o) could derive from the reaction of the amine and the starting nitrosoderivative. A 100% conversion was obtained in this case, and from the very complex mixture only 4% of the insertion product 5a and 8% of azobenzene 3 could be isolated. A control experiment showed that the catalyst was necessary for the formation of the amine, thus excluding that the amine 5 could derive from an ‘ene-type’ reaction [ 151 between nitrosobenzene and cyclooctene. Moreover, a lower temperature (reaction p) gave a lower conversion of nitrosobenzene 15 (50%) and the isolation of 3% of compound 5a. These results showed again that insertion into the alkene is a side process. In conclusion, the catalytic reduction of substituted nitroaromatics in alkene solvents gives symmetric ureas as the main product, primary and secondary amines as side products, suggesting that the metal center acts as a template in the urea-determining step. This had previously been observed also in the Ru-catalysed reduction of nitroaromatics in the presence of alcohols 1161. Urea formation is suppressed when the strong electronwithdrawing substituent -COOMe is present in the paru position. In this case, the primary amine is the main reaction product. Thus, the selectivity in amine seems to be influenced by the electron-withdrawing effect of the substituent. This suggests that amine formation is due to hydride transfer on a ruthenium-nitrogen intermediate, and not to hydrogen atom abstraction. Hydride-transfer catalysis in Ru,(CO),, chemistry has been reported in other cases 1171 and also by us in the reduction of ortho-nitrophenyl ally1 ether with the same catalytic system [18]. The intermolecular insertion reaction into the alkene molecule seems to be a side process, and is not affected by the electronic characteristics of the substrate. Acknowledgements
We thank Mr. Marco Nicolini for experimental assistance. This work was supported by a grant of the Consiglio Nazionale delle Ricerche, Progetto Finalizzato Chimica Fine II.
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