摘要:

J.C.S. Perkin I1 Conformational lnterconversion in the I,I,4,4-Tetramethyltetralin-Z-ketyl Radical Anion by Electron Spin Resonance By Luigi Pasimeni, Marina Brustolon, and Carlo Corvaja," Institute of Physical Chemistry, University of Padova, 35100 Padova, Italy A moderately stable ketyl free radical containing two protons bonded to the carbon atom in the a-position of the carbonyl group has been prepared by alkali metal reduction of 1,I ,4,4-tetramethyl-2-tetralone. The e.s.r. spectrum shows a line broadening effect which is explained on the basis of the conformational interconversion of the radical anion between two equivalent conformations. The thermodynamic parameters of the process are obtained and their dependence on the counter ion is discussed. KETYLSwith two aromatic substituents are very stable species which can be obtained in high concentration by alkali metal reduction of the corresponding ketones1 When one or both substituents are alkyl groups the ketyls are stable only if no hydrogen atoms are bonded to the a-carbon atom.2 An exception to this observ- ation is provided by the relative stability of isopropyl t-butyl ketyl and of cyclopropyl phenyl ketyl which has been ascribed to a preferred conformation of the radical with the a-hydrogen in the nodal plane of the x system.3 A few e.s.r.spectra attributed to stable ketyl radicals with hydrogens at the a-carbon atom were later shown to be due to semidione radical anions produced by an oxidation pro~ess.~ By alkali metal reduction of 1,1,4,4-tetramethyl-Z-tetralone in ethereal solvents we have produced a radical species whose stability is sufficient to record the e.s.r.spectra at room temperature. The spectra indicate 1 N. Hirota, in ' Radical Ions,' eds. E. T. Kaiser and L. Kevan, Wiley-Interscience, New York, 1968. 2 J. E. Bennett, B. Mile, and A. Thomas, J, Chem. Soc. (A),1968. 298. that the species is an alkali metal-ketyl radical ion pair and this is the first observation of a relative stable alkyl ketyl with two a-hydrogen atoms. In this paper we discuss the e.s.r. results and the line- width effects displayed by the spectra, caused by intra-molecular motion, as well as the dependence on the counter ion. EXPERIMENTAL 1, 1,4,4-Tetramethyl-2- tetralone (Tetralone) was prepared by the method of Bruson et aZ.6 by addition of powdered aluminium chloride to a benzene solution of 2,2,5, ti-tetra-methyltetrahydrofuranone. The latter was synthesized from 2,5-dihydroxy-2,5-dimethylhex-3-yne. Tetralone-alkali metal ion pairs were prepared by treating tetralone with a metal mirror, or small pieces of lithium under vacuum.Standard vacuum techniques were G. A. Russell and G. R. Stevenson, J. Amer. Chem. Soc., 1971, 93, 2432.* G. A. Russell and E. R. Talaty, J. Amer. Chem. SOC.,1965,87, 4867. H. A. Bruson, F. W. Grant, and E. Bodko, J. Amer. Chem. Soc., 1958, 80, 3633. 1978 used. Solvent methyltetrahydrofuran (MTHF) was puri- fied by distillation over calcium hydride and stored under vacuum over potassium anthracenide.The e.s.r. spectra were recorded with a JEOL PE 3X spectrometer .operating in the X band and equipped with variable temperature accessories. RESULTS AND DISCUSSION The low temperature (-85") e.s.r. spectrum of tetralone reduced with potassium consists of a doublet of doublets with hyperfine separations aH1 32.67 and aH25.06G. This spectrum is consistent with a ketyl structure with two non-equivalent p-protons interacting with the unpaired electron. A possible semidione structure with two p-protons, produced by insertion of a CO group in the ring, as suggested for the reaction product of 2,2,4,4-tetramethyl~yclobutanone,~is ruled out by the high values of the hyperfine couplings. In fact the @-proton couplings are known to be given by relationship (1).' For cis-semidiones Bpc*, as obtained a=B = Bp,* cos2 8 (1) from the methyl proton hyperfine splitting of dimethyl semidione, is 14 G and this is the maximum value expected for p-protons of semidiones.In our case Bp,* was obtained by substituting aH1and aH2into equation (1)and taking into account the fact that the angles 8, and 8, for the two protons are related (0, = 8, 120"). We found Bpc*=33 G which is the typical value for ketyls radicals while the values of 8, and 8, are 8 and 112" respectively; thus the tetralone anion has a conform- ation with one of the two protons close to the nodal plane of the unpaired electron 2p orbital. On increasing the temperature the two inner lines of the spectrum broaden until they become undetectable and finally coalesce in a broad central line at close to room temperature.This behaviour reveals the occur- rence of internal motion with exchange of the two proton couplings.* The ion pairs with Li and Na behave in a similar way. Here the room temperature spectrum clearly reveals the alkali ion splitting. We attribute these spectral features to an interconversion between the two equivalent conformations A and B in the Figure. It is worth noting that both protons spend half their time in a position close to the nodal plane of the 9 orbital and this may be a cause of the relative stability * At temperatures >O "C each line is further split into a com- plex structure due to the hyperfine interaction with methyl protons and possibly by the cation nucleus.6 G. A. Russell, D. F. Lawson, H. L. Malkus, and P. R. Whittle, J. Chem. Phys., 1971, 54, 2164. of the radical, Although in this system the exchange process could be investigated at the fast exchange limit (coalesced lines) accurate measurements could be per- formed only in the slow exchange region because of the instability of the radical species at high temperatures. A Molecular structures of the tetralone radical anion. Inter-conversion of the two equivalent conformations A and B leads to exchange of the positions of protons H(l) and H(2) The e.s.r. spectra were computer simulated with a program in which the exchange between the equivalent sites was taken into account, employing the density matrix formalism.8 Calculations were performed for different values of the exchange rate in order to fit the experimental spectrum.The exchange frequencies thus obtained for each tem- perature were used to obtain the thermodynamic para- meters (Table) which show a dependence on the cation in the ion pair. Cation Li AEI kcal mol-1 5.5 A /MHz 9.2 x lo6 AH$/ AS $1 kcal mol-1 cal mol-1 K-1 4.9 -6.0 Na 4.0 6.5 x 104 3.3 -11.8 K 6.2 2.6 x 107 6.6 +5.7 Thermodynamic parameters for tetralone in MTHF. AE and A were obtained by the Arrhenius relation In ~-1= -@/RT + In A with 7-1 the exchange rate. For the evaluation of AHf and AS the relation In8 = AHt/R-l/T -ASr/R was used where 8 = RT/Nh. Thus by contrast with other case~,~~l~ in the case of tetralone there is a striking effect of the counter ion on the interconversion rate, which indicates that the latter is strongly involved in the process, This work was supported by the CNR through its Centro di studio sugli Stati Molecolari Radicalici ed Eccitati. [7/981 Received, 8th June, 19771 N. M. Atherton, ' Electron Spin Resonance,' ed. T. M. Sugden, Ellis Horwood, Chichester, 1973, ch. 3.* A. F. Neiva Correia, Ph.D. Thesis, University of Amsterdam, 1967. Q F. W. Pijpers, M. R. Arick, B. M. P. Hendriks, and E. de Boer, MoZ. Plzys., 1971, 22, 781. lo T. Takeshita and N. Hirota, J. Chem. Phys., 1969, 51, 2146.

ISSN:1472-779X    

DOI:10.1039/P29780000420

出版商:RSC    

年代:1978

数据来源: RSC