3-(η 6 -Benzoyltricarbonylchromium)pyrrole. An example of how...
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3-(q6-Benzoyltricarbonylchromium)pyrrole. An example of how substitution isomerism in the ligand affects the type (a or 7 ~ ) of complex formed NIALL J. GOGAN, JAMES DOULL, AND JANE EVANS Department of Chetnistty. Metnorial Universio of Newfoundlnnd, St. John's, Njld., Canada AIB 3x7 Received March l I, 1985 NIALL J. GOGAN, JAMES DOULL, and JANE EVANS. Can. J. Chem. 63, 3147 (1985). The reaction of 3-benzoylpyrrole and Cr(C0)6 yields only the T-complex 3-(~h-benzoyltricarbonylchromium)pyrrole, in contrast to the previously reported reaction of 2-benzoylpyrrole and Cr(C0)6 that yielded only the a-chelate, tris(2-benzoyl- pyrrolato)chromium(III). The difference in behaviour is explained by a preference for chelate formation that is allowed by the stereochemistry of 2-benzoylpyrrole but is not allowed by the stereochemistry of 3-benzoylpyrrole. NIALL J. GOGAN, JAMES DOULL et JANE EVANS. Can. J. Chem. 63, 3147 (1985). La reaction du benzoyl-3 pyrrole avec le Cr(CO)h ne conduit qu'au complexe-T (qh-benzoyltricarbonylchrome)-3 pyrrole alors qu'il a kt6 rapport6 anterieurement que la reaction du benzoyl-2 pyrrole avec le Cr(CO)h ne conduit qu'i un chelate-a, le tris(benzoy1-2 pyrrolato)chrome(llI). On explique la diff6rence dans ces comportements par une pr6fkrence pour la formation du chelate qui est permise par la stereochimie du benzoyl-2 pyrrole mais qui n'est pas permise par la stkrkochime du benzoyl-3 pyrrole. [Traduit par le journal] Introduction Benzoylpyrroles, when reacted with Cr(CO),, have the potential of forming either Cr(ll1) a-coordinated complexes or T-C~(CO), complexes. We previously reported that 2-benzoyl- pyrrole forms the a-chelate, tris(2-benzoylpyrro1ato)chro- mium(IlI), when reacted with Cr(C0)6 (1). No evidence of T-complexation was found. At the time of our earlier study, it was not possible to examine the effect of changing the substi- tution position of the benzoyl group because no readily avail- able route to 3-benzoylpyrrole existed. Since then, Anderson, Loader and co-workers have developed successively better syn- thetic routes to 3-substituted pyrroles and have recently devel- oped a good method for preparing 3-benzoylpyrroles (2). We now wish to report the result of the reaction of 3-benzoyl- pyrrole with Cr(C0)6and contrast the result with that obtained earlier for 2-benzoylpyrrole. Experimental Melting points are uncorrected. Elemental analyses were performed by Canadian Microanalytical Service, Vancouver, B.C. The ir spectra were recorded on a Perkin-Elmer 283 spectrophotometer in CHCI, solution. The 'H nmr spectra were determined using a Bmker WP-80 Instrument and are expressed in ppm (6 valves) relative to tetramethyl- silane as internal reference in CDCI,. Mass spectra were recorded on a VG Micromass 7070 HS spectrometer. Reaction of he.racarbonylchromium(0) and 3-benzoylpyrrole Hexacarbonylchromium (0.297 g; 1.30 mmol) and 3-benzoyl- pyrrole (0.203 g; 1.18 mmol) were allowed to react in 50 mL of di-n-butyl ether at reflux under a nitrogen atmosphere for 6 h using the general method previously described (3). The resulting orange solu- tion was evaporated to yield an orange solid, which was recrystallized from methylene chloride/petroleum ether (60-80) to yield 0.258 g (7 1 % yield assuming 3-benzoylpyrrole Cr(CO),) of orange crystals, mp 166- 168°C. A small amount of decomposition product and some unreacted starting materials were the only other substances recovered from the reaction. Anal. calcd. for CI4H9O4NCr: C 54.72, H 2.95, N 4.56; found: C 54.57, H 2.71, N 4.58. Results and discussion The orange product shows two intense bands in the ir (CHCl,) spectrum at 1986 and 1914 cm-' characteristic of a Cr(CO), group T-bonded to a benzene ring (4). The ketonic vco is at 1636 cm-', virtually unchanged from the ligand (1635 cm-'), and v,, is unchanged at 3470 cm-I. This contrasts with tris(2-benzoylpyrrolato)chromium(lll), which has no intense bands in the 1850-2100 cm-' region, has the ketonic vco shifted from 1621 cm-' (ligand) to 1526 cm-' (complex), and has no VNH The product from 3-benzoylpyrrole has an 'H nmr spectrum with benzene proton resonances (6) at 5.36 (meta), 5.56 (para), and 6.10 (ortho) shifted upfield from the ligand values of 7.51, 7.39, and 7.85. These shifts are typical of a benzene ring T-bonded to a Cr(CO)3 group (5). The pyrrole proton resonances are essentially unchanged. The 'H nmr spec- trum of the a-complex from 2-benzoylpyrrole exhibits no sig- nals, no doubt owing to its paramagnetism = 3.9 BM). The elemental analyses agree with the values calculated for the T-complex of 3-benzoylpymole. The mass spectrum shows peaks at mle: 307 (13, Mf), 279 (15), 251 (26), 223 (loo), 171 (26), 94 (66), and 52 (75) corresponding to the molecular ion, successive loss of 3 COs, 3-benzoylpyrrole', C4H4NCOf, and Crf. From the above evidence, it is clear that the product from the reaction of 3-benzoylpyrrole and Cr(C0)6 is 3-(q6-benzoyltri- carbonylchromium)pyrrole and has the structure shown in (a), whereas the product from the reaction of 2-benzoylpyrrole and Cr(CO), has, in contrast, the structure shown in (b). This shows a dramatic difference in complexing behaviour, under the same reaction conditions, of ligands that only differ in the substi- tution position of the benzoyl group on the pyrrole ring. It can be explained by a strong preference for chelation through the pyrrole N and ketonic group 0 . This preference is allowed by the stereochemistry of 2-benzoylpyrrole, i.e. it has the correct "bite", but it is not allowed by the stereochemistry of Can. J. Chem. Downloaded from www.nrcresearchpress.com by 69.198.189.178 on 11/11/14 For personal use only.
Transcript of 3-(η 6 -Benzoyltricarbonylchromium)pyrrole. An example of how...
3-(q6-Benzoyltricarbonylchromium)pyrrole. An example of how substitution isomerism in the ligand affects the type (a or 7 ~ ) of complex formed
NIALL J. GOGAN, JAMES DOULL, A N D JANE EVANS Department of Chetnistty. Metnorial Universio of Newfoundlnnd, St. John's, Njld., Canada AIB 3x7
Received March l I , 1985
NIALL J. GOGAN, JAMES DOULL, and JANE EVANS. Can. J. Chem. 63, 3147 (1985). The reaction of 3-benzoylpyrrole and Cr(C0)6 yields only the T-complex 3-(~h-benzoyltricarbonylchromium)pyrrole, in
contrast to the previously reported reaction of 2-benzoylpyrrole and Cr(C0)6 that yielded only the a-chelate, tris(2-benzoyl- pyrrolato)chromium(III). The difference in behaviour is explained by a preference for chelate formation that is allowed by the stereochemistry of 2-benzoylpyrrole but is not allowed by the stereochemistry of 3-benzoylpyrrole.
NIALL J. GOGAN, JAMES DOULL et JANE EVANS. Can. J. Chem. 63, 3147 (1985). La reaction du benzoyl-3 pyrrole avec le Cr(CO)h ne conduit qu'au complexe-T (qh-benzoyltricarbonylchrome)-3 pyrrole
alors qu'il a kt6 rapport6 anterieurement que la reaction du benzoyl-2 pyrrole avec le Cr(CO)h ne conduit qu'i un chelate-a, le tris(benzoy1-2 pyrrolato)chrome(llI). On explique la diff6rence dans ces comportements par une pr6fkrence pour la formation du chelate qui est permise par la stereochimie du benzoyl-2 pyrrole mais qui n'est pas permise par la stkrkochime du benzoyl-3 pyrrole.
[Traduit par le journal]
Introduction Benzoylpyrroles, when reacted with Cr(CO),, have the
potential of forming either Cr(ll1) a-coordinated complexes or T-C~(CO), complexes. We previously reported that 2-benzoyl- pyrrole forms the a-chelate, tris(2-benzoylpyrro1ato)chro- mium(IlI), when reacted with Cr(C0)6 (1). No evidence of T-complexation was found. At the time of our earlier study, it was not possible to examine the effect of changing the substi- tution position of the benzoyl group because no readily avail- able route to 3-benzoylpyrrole existed. Since then, Anderson, Loader and co-workers have developed successively better syn- thetic routes to 3-substituted pyrroles and have recently devel- oped a good method for preparing 3-benzoylpyrroles (2). We now wish to report the result of the reaction of 3-benzoyl- pyrrole with Cr(C0)6 and contrast the result with that obtained earlier for 2-benzoylpyrrole.
Experimental Melting points are uncorrected. Elemental analyses were performed
by Canadian Microanalytical Service, Vancouver, B.C. The ir spectra were recorded on a Perkin-Elmer 283 spectrophotometer in CHCI, solution. The 'H nmr spectra were determined using a Bmker WP-80 Instrument and are expressed in ppm (6 valves) relative to tetramethyl- silane as internal reference in CDCI,. Mass spectra were recorded on a VG Micromass 7070 HS spectrometer.
Reaction of he.racarbonylchromium(0) and 3-benzoylpyrrole Hexacarbonylchromium (0.297 g; 1.30 mmol) and 3-benzoyl-
pyrrole (0.203 g; 1.18 mmol) were allowed to react in 50 mL of di-n-butyl ether at reflux under a nitrogen atmosphere for 6 h using the general method previously described (3). The resulting orange solu- tion was evaporated to yield an orange solid, which was recrystallized from methylene chloride/petroleum ether (60-80) to yield 0.258 g (7 1 % yield assuming 3-benzoylpyrrole Cr(CO),) of orange crystals, mp 166- 168°C. A small amount of decomposition product and some unreacted starting materials were the only other substances recovered from the reaction. Anal. calcd. for CI4H9O4NCr: C 54.72, H 2.95, N 4.56; found: C 54.57, H 2.71, N 4.58.
Results and discussion The orange product shows two intense bands in the ir
(CHCl,) spectrum at 1986 and 1914 cm-' characteristic of a Cr(CO), group T-bonded to a benzene ring (4). The ketonic vco
is at 1636 cm-', virtually unchanged from the ligand (1635 cm-'), and v,, is unchanged at 3470 cm-I. This contrasts with tris(2-benzoylpyrrolato)chromium(lll), which has no intense bands in the 1850-2100 cm-' region, has the ketonic vco shifted from 1621 cm-' (ligand) to 1526 cm-' (complex), and has no V N H The product from 3-benzoylpyrrole has an 'H nmr spectrum with benzene proton resonances (6) at 5.36 (meta), 5.56 (para), and 6.10 (ortho) shifted upfield from the ligand values of 7.51, 7.39, and 7.85. These shifts are typical of a benzene ring T-bonded to a Cr(CO)3 group (5). The pyrrole proton resonances are essentially unchanged. The 'H nmr spec- trum of the a-complex from 2-benzoylpyrrole exhibits no sig- nals, no doubt owing to its paramagnetism = 3.9 BM).
The elemental analyses agree with the values calculated for the T-complex of 3-benzoylpymole. The mass spectrum shows peaks at mle: 307 (13, Mf), 279 (15), 251 (26), 223 (loo), 171 (26), 94 (66), and 52 (75) corresponding to the molecular ion, successive loss of 3 COs, 3-benzoylpyrrole', C4H4NCOf, and Crf.
From the above evidence, it is clear that the product from the reaction of 3-benzoylpyrrole and Cr(C0)6 is 3-(q6-benzoyltri- carbonylchromium)pyrrole and has the structure shown in (a), whereas the product from the reaction of 2-benzoylpyrrole and Cr(CO), has, in contrast, the structure shown in (b). This shows
a dramatic difference in complexing behaviour, under the same reaction conditions, of ligands that only differ in the substi- tution position of the benzoyl group on the pyrrole ring. It can be explained by a strong preference for chelation through the pyrrole N and ketonic group 0 . This preference is allowed by the stereochemistry of 2-benzoylpyrrole, i.e. it has the correct "bite", but it is not allowed by the stereochemistry of
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3-benzoylpyrrole. Thus, in this latter case .rr-complexation oc- 1. C. S. DAVIES and N. J . GOGAN. J . Inorg. Nucl. Chem. 34, 2791 curs by default. (1972).
2. R. X. Xu. H. J . ANDERSON, N . J . GOGAN, C. E. LOADER, and Acknowledgements R. MCDONALD. Tetrahedron Lett. 22, 4899 (1981).
3. N. J. GOGAN and C.-K. CHU. J . Organometal. Chem. 132, 103 W e are grateful to the Natural Sciences and Engineering (1977).
Research Council of Canada for financial support and to Drs. 4. D, A. BROWN and H , SLOAN. I , Chem, sot, 3849 (1962). H. J. fhderson and C. E. Loader of this department for advice 5. E. 0 . FKCHER and H.-P. FRITZ. Angew. Chem. 73, 353 (1961). and assistance in the preparation of 3-benzoylpynole.
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