Abstraction of an allylic hydrogen atom in homobenzvalene (4) either in solurion by photolyticaßy generated tert-butoxyl radicals or in an adamantane matrix by X-rays produces the homobcnzvalenyl radical (5). which tbennally rearranps · to tbe tropylium ndical (1). In solution tbe activation cnergy for the rate determined step of the reaction sequence was detennined· to be 13.4 ± O.S kcal/mol

Christl, Manfred

Korth, H.-G.

Müller, W.

Sustmann, R.

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H.-G. Korth, W. Müller, R. Sustmann, M. Christi 1257 Rearrangements of Free Radicals, XII t) ESR Spectroscopic Study of the Ring Opening of the Homobenzvalenyl Radical Hans-Gert Kortb•, Wolfgang Müller•, Reiner Sustmann••, and Manfred Christlb Institut für Organische Chemie der Universität Essen•, Universitätsstraße 5, D-4300 Essen 1 Institut für Organische Chemie der Universität Würzburgb, Am Hubland, D-8700 Würzburg Received February 16, 1987 Abstraction of an allylic hydrogen atom in homobenzvalene (4) either in solurion by photolyticaßy generated Wt·b~to~yl rad-icals or in an adamantane matrix by X-rays produc:es the homo-bcnzvalenyl radical (5). whicb tbennally rearranps · to tbe tro-pylium ndical (1). In solution tbe activation cnergy for the rate determi.Dina stcp of the reaction sequence was detennincd· to be 13.4 ± O.S kcal/mol. Rearrangements of free radicals have been investigated thor-oughly in recent years 21• It is now weil recognized that free radicals show a behavior in rearrangement reactions which is not different in principle from that of other reactive intermediates. Particularly, studies in an adamantane matrix 11 have disproved the widespread opinion that free radicals do not rearrange as easily as carbocations or carbanions. The possibility to undergo reactions with low acti-vation barriers in solution, i. e. the recombination or dispropor-tionation of radicals, has obscured their rearrangement capabilities. Under conditions of long Iifetime (e. g. adamantane matrix) the lat-ter reactions are suppressed and others therefore become apparent. Recently, we have described rearrangements on the potential sur-face of C7H7 radicals 11• The most stable species in this series is the tropylium radical (7). Radical 5 derived from homobenzvalene is isomeric to bicyclo[3.2.0]heptadienyl radical l, 3-quadricyclanyl radical (2), and 7-norbornadienyl radical (3). (D 1 4 2 2-Adamantyl-Radical ______ .,.. Adamantane 3 6 Two methods were applied to generate 5. First, homobenzvalene (4) was exposed to X-rays in an adamantane matrix at -196°C. Aftertransfer of the matrix into the cavity of the ESR spectrometer at low temperature we observed at temperatures below -100°C only signals due to the 2-adamantyl radical 31• Close to -100°C the intensity of this signal decreased slowly and a new spectrum, iden-tified as that Of the tropyJium radica) (7) [aH ( -1 00°(') = 0.396 ± 0.004 mT], appeared. Thesesignals became visible already 5 min after heating the matrix from -196 to -100 °C, and their intensity increased steadily for three hours at -100°C. Spectral simulations, which included the 2-adamantyl radical, 7, and 5 (see below), gave evidence that also 5 is present at -100°C. However, the muJti-plicity of its lines prohibits an unambiguous identification. The appearance of 7 indicates hydrogen abstraction from the substrate homobenzvalene by the initially generated 2-adamantyl radicals to form the homobenzvalenyl radical (5), which obviously undergoes a facile rearrangement to the tropylium system, possibly via the intermediate 6. 5 6 7 J I .JIJ l J j J j ... I~ w Pr 'T f ~ " I~ 1 mT 0 0 0 0 1 mT Figure 1. ESR spectrum of the homobenzvaJenyl radical (5) in tri-chlorofluoromethane at -sooc (top) and in liquid paraffine at + 2 oc (bottom). Circles denote the hyperfine lines of the tropylium radical (7) Chem. Ber. 120, 1257-1258 (1987) © VCH Verlagsgesellschaft mbH, D-6940 Weinheim, 1987 0009-2940/87/0707-1257 $02.50/0 1258 In further experiments we studied the hydrogen abstraction by photolytically generated tert-butoxyl radicals in solution. In tri-chlorofluoromethane solution a high-resolution ESR spectrum of 5 could be recorded from -100 to 20°C (Figure 1). The spectrum is characterised by a large hyperfine coupling constant of 1.337 mT from two hydrogen atoms, typical for hydrogens in allylic position. The coupling of 0.320 mT is due to the centrat hydrogen atom in the allylic system and compares weil with other allylic radicals 41• Two more coupling constants ca,n be recognized (a" = 0.155 and a" = 0.032 mT), each due to the interaction of the unpaired elec-tron with two equivalent hydrogen atoms. The hyperfine coupling constant of 0.155 mT is assigned to the lateral bicyclobutane hy-drogen atoms by comparison with the ESR spectrum of the bi-cyclo[3.l.1]heptenyl radical (8t1• In 8 an additional coupling con-stant of 0.155 mT for two hydrogen atom atoms is observed, at-tributed to the anti hydrogen atoms at the methylene bridges 4•5l. The small hyperfine coupling constant of 0.032 mT (2H) in 5 must arise from the interaction with the two hydrogen atoms at the bicyclobutane bridgeheads. The corresponding coupling constant (syn hydrogen atoms at the methylene bridges) could not been de-tected in. the spectrum of 8 because of the line broadening effect of the adamantane matrix. In trichlorofluoromethane solution we could not observe the rearrangement of 5 to 7. The competing dimerisation, which can safely be assumed to be diffusion-controlled, is much faster than the isomerisation and thus reduces the lifetime of 5. fJowever, in the high er viscous liquid parafline the mean lifetime of 5 is increased and now the rearrangement to 7 at temperatures above ooc can be obscrved (Figure 1 ). With increasing temperature, radical 7 be-comes dominant in the ESR spectrum. A kinetic analysis of the reaction in paraffine solution, analogaus to that for other rearrangements 5•61, led to the determination of the activation barrier of the rate determining step of the isomerisation. Experimentally, the ratio of the concentration of the radicals S and 7 as a function of the temperature is measured and the activation energy is ob-tained from a plot of the logarithm of the concentration ratio vs. 1/T according to equation (1). lg([7]/[5]) = lg{Ar/(/·A,)112}- (Er- EJ2)/ 2.3RT (1) Here, Er denotes the Arrhenius activation energy for rearrange-ment and E1 the activation energy for termination (dimerisation). Ar and A1 stand for the corresponding frequency factors, I describes the rate of fonnation of radicals S, assumed lo be constant in the temperature range of our measurements Sl. For the analysis, the activation energy E1 for recombination of 5 in our solvent system is required. The recombination of sterically unhindered allylic rad-icals is diffusion-controlled 6.71, The activation energy of this process, therefore, is governed solely by the temperature dependence of the solvent viscosity. A similar situation should hold in our case. Ac-cording to the von Smoluchowski equation 8•91, the rate constant for diffusion-controlled reactions can be determined if the diffusion coefficient of the substrate is known. The latter can be approxi-mated by the dynamic viscosity of the solvent via the Stokes-Ein-H.-G. Korth, W. Müller, R. Sustmann, M. Christi stein equation 101• From the determination of f1 as a function of temperature we obtained a value of 7.4 ± 0.2 kcalfmol for E1• Using this value we calculated the activation energy for the rearrangement of S---.7 tobe 13.4 ± 0.5 kcaljmol. Although releasing considerable strain energy, the process s-6 may well be slightly endathennie since an unfavourable cyclopropyl radical is produced on the account of an allylic radical. Therefore, the driving force for the rearrangement must be provided by the subsequent formation of 7, the most stabilized member of the C1H1 family. Since conjugating substituents in position 2 or 3 of the cyclopropyl radical facilitate its electrocyclic ring opening 1•111, the norcaradiene radicat"6 should undergo t~e ring enlargement readily. Hence, the rate determining step should be the transformation 5 ....... 6 rather than 6---.7. This work was supported by the Deutsche Forschungsgemein-schaft and the Fonds der Chemischen Industrie. Experimental ESR measurements: Bruker ER-420 instrument, attached to a POP 11/34 computer 6•7l. For the measurements in solution we used either mixtures of trichlorofluoromethane or liquid paraffine 12l with di-tert-butyl peroxide and homobenzvalene 13l (4) in a v/v ratio of 10:5: 1. The solutions were thoroughly purged with argon to re-move oxygen. The adamantane matrix of homobenzvalenc was pre-pared by subliming 50 mg of adamantane (Aldrich Gold Label) in a sealed tube in the presence of 5 IJ.l of 4 and preparing from this material suitable pellets (5 mm diam.) in an IR press. X-ray irra-diation (Siemens Stabiliplan; 300 kV, Cu anode) was carried out according to ref.1l. The temperature dependence of the solvent viscosity of the ESR solution, determined with a Micro-Ubbelohde viscosimeter from 3 to 44°C, is described by the equation lg '7/cp = -(4.64 ± 0.13) + (7.34 ± 0.17 kcaljmol)/2.3 RT. The steady-state kinetics was meas-ured with 10:5: 1 vfv mixtures of paraffine oil/di-tert-butylperox-ide/substrate. The evaluation was performed by comparing suitable signals in. the experimental spectra with integrated computer-sim-ulated sp~ctra and plotting the determined concentration ratios as a function of 1/T. CAS ,Registry Nurobers 4: 3561~-58-7 /5: 108058-57-7 I) Part XI: R. Sustmann, D. Brandes, F. Lange, U. Nüchter, Chem. Ber. 118 (1985) 3500. 21 A. L. J. Beckwith, K. U. Ingold in Rea"angements in Ground and Excited States, ed. P. de Mayo, vol-. 1, p. 162, Academic Press, New York 1980. 3l R. V. Lloyd, S. DiGregorio, L. DiMauro, D. E. Wood, J. Phys. Cliem. 84 (1980) 2891. 41 H.-J. Dem, F. Lange, R. Sustmann, Chem. Ber. 116 (1983) 3316. 5l E. J. Hamitton jr., H. Fischer, Helv. Chim. Acta 76 (1973) 795. . 61 H.-G. Korth, H. Trill, R. Sustmann, J. Am. Chem. Soc. 103 (1981) 4483. 71 H.-G. Korth, P. Lommes, W. Sicking, R. Sustmann, lnt. J. Chem. Kinet. lS (1983) 267. 81 R. M. Noyes, Progr. React. Kinet. l (1961) 129. 91 H.-H. Schuh, H. Fischer, Helv. Chim. Acta 61 (1978) 2130. 10l A. Spernol, K. Wirtz, Z. Naturforsch., Teil A, 8 (1953) 522. 111 S. Sustmann, C. Rüchardt, Chem. Ber. 108 (1975) 3043. 12J Commercialliquid paraffine (Merck 7160) was used. 13l M. Christi, C. Herzog, D. Brückner, R. Lang, Chem. Ber. 119 (1986) 141. [46/87] Chem. Ber. 120, 1257-1258 (1987) 

Rearrangement of Free Radicals, XII. ESR Spectroscopic Study of the Ring Opening of the Homobenzvalenyl Radical

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Rearrangement of Free Radicals, XII. ESR Spectroscopic Study of the Ring Opening of the Homobenzvalenyl Radical

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