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Conformational Studies in Pyrrole Chemistry.

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Conformational Studies in Pyrrole Chemistry
By R. A . Jones[*]
Extensive participation of the zwitterionic limiting forms
( l a ) and (2a) in the structure of 2-formylpyrrole hinders
rotation about the C-C bond between the formyl group and
the pyrrole ring, so that two conformers can exist.
A study 111 of the dipole moment of 2-formylpyrrole [ ( I ) or
( 2 ) , R = HI showed that the conformer ( I ) is the more stable.
Further indications were obtained from IRIzl and NMR
studies [31. The NMR spectra show that the formyl-proton
couples with the proton on C-5 [41. Application of the zigzag
rule [5,61 indicates the predominance of structure ( I ) . Coupling with the proton on C-4 was not observed.
Spectroscopic comparison of the 1-alkyl-2-formylpyrroles171
[ ( I ) or ( 2 ) , R = CH3, CzH5. iso-C3H~,tert-C4H9] shows that
the predominating conformation remains unchanged through
the series in spite of the increasing size of the alkyl group.
The IR and UV spectra can be interpreted only if the carbony1 group is twisted slightly out of the plane of the pyrrole
ring. The effect of this twist on the coupling constants between the formyl-proton and the proton on C-5 is not
known 161, but a diminution of this constant is to be expected.
Since there is no coupling between the formyl-proton and the
proton o n C-4 it must be concluded that I-butyl-2-formylpyrrole also exists in form ( I ) .
These results can be explained by dipole-dipole interaction
[(3) and (4)I.This would lead to theenergycontentofsystem
( 3 ) being less than that of system ( 4 ) . Studies of the dipole
moment show that the “resonance moment” ( 5 ) is also important, but exact values are not available[sl.
The formyl group of I-formylpyrrole can rotate freely at
40 OC 191.
Spectroscopic examination of I-ethoxycarbonyl-2-formylpyrrole and 1-acetyl-2-formylpyrrole shows that an equilibrium exists between ( 6 ) and (7), the conformers ( 6 ) predominating to the extent of 70%, and assumption of dipoledipole interaction is in agreement with these results.
[VB 213 IEI
Lecture at Marburg (Germany), September 4, 1969
German version: Angew. Chem. 82, 1006 (1969)
Dr. R. A. Jones
School of Chemical Sciences, University of East Anglia
Norwich (England)
[l] A . Marinangelli, Ann. Chimica 44, 219 (1954).
[2] R. A . Jones and A . G . Moritz, Spectrochim. Acta 21, 295
[3] G. J . Karabatsos and F. M. Vane, J. h e r . chem. SOC.85,
3886 (1963).
[4] S. Gronowitz, A.-B. Hornfeldt, B. Gestblom, and R. A . Hoffman, Ark. Kemi 18, 133 (1961).
[ 5 ] C . N . Banwell and N . Sheppard, Discuss. Faraday SOC.34,
115 (1962).
161 S . Sternhall, Quart. Rev. (chem. SOC., London) 23,236 (1969).
[7] R. A . Jones and P. H . Wright, Tetrahedron Letters 1968,
[8] C.F. Candy, M. Sc. Thesis, University of East Anglia 1969.
[9] T. Mafsiio and H. Shosenji, J. chem. S O C . (London) D 1969,
The preparation of transition metal complexes by simultaneous
condensation of vaporized atoms of the elements Cr, Fe, Co,
and Ni with an excess of ligand has been achieved in good
yields in gram quantities by P . L. Timms. Thus Cr, Fe, C o ,
and Ni react with PF3 to give Cr(PF3)6, FeCPF3)5, along with
(PF3)3Fe(PFz)zFe(PF&, Coz(PF3)s, and Ni(PF3)4. Cocondensation of Cr and benzene gives a 60% yield of dibenzenechromium, that of Fe and Ni with cyclopentadiene
gives ferrocene and n-cyclopentadienyl-x-cyclopentenylnickel. Fe and benzene yield a highly reactive product which
explodes at temperatures above -5OOC and which affords
cyclohexane in the presence of H2, and ferrocene in the presence of cyclopentadiene. / Chem. Commun. 1969,1033 /-HB.
[Rd 103 IE]
between 0.1 and 40 torr on excitation with light of the wavelength 4358 A; it therefore follows that, under these conditions, collisions play no part in the transition to the triplet
system. The phosphorescence quantum yield decreased
below 0.1 torr; it was shown that this is extensively or exclusively due to radiationless deactivation of triplet molecules by collision with the walls. Those excited singlet molecules that are not deactivated by fluorescence all undergo
transition to the triplet state, the rate constant amounting to
10-8 s-1; direct radiationless deactivation of the excited
singlet to the ground state can be neglected. / J. chem.
Physics 51, 1551 (1969) /-Hz.
[Rd 105 IE]
The fluorescence and phosphorescence of biacetyl in the gas
phase at low pressures has been studied by C. S. Parmenter
and H . M. Poland with a view to gaining information about
the kinetics of the singlet-triplet transition. The quantum
yields of both kinds of emission were independent ofpressure
Angew. Chem. internat. Edit. J Vol. 8 (1969) / No. 12
The ESR spectrum of the acetonyl radical CH3-CO-CHz
has been investigated by G . Golde, K . Mobius, and W. Kaminski. The radical was generated by photolysis of an aqueous solution of acetone to which H202 had been added. The
temperature dependence of the ESR spectrum shows that the
rotation of the CH2 group about the C-C bond is hindered:
analysis of the spectrum at various temperatures afforded the
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chemistry, conformational, pyrroles, studies
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