Photolysis of the carbonndash;hydrogen bond in pentamethylcyclopentadiene. Properties of the pentamethylcyclopentadienyl radical

摘要

J.C.S. Perkin I1 Photolysis of the Carbon-Hydrogen Bond in Pentarnethylcyclopentadiene. Pro pert ies of the Pentamet hylcyclopentad ienyl Radical By Alwyn G. Davies * and Janusz Lusztyk, Chemistry Department, University College London, 20 Gordon Street, London WC1 H OAJ Irradiation of pentamethylcyclopentadiene (A) in liquid solution with U.V. light results in homolysis of the ring @-H bond to give the pentamethylcyclopentadienyl radical (6)a (1 5 H) 6.4, a( la@)3.5 GI,and atomic hydrogen which abstracts hydrogen from a second molecule of (A) to give molecular hydrogen and a second radical (6). The radicals (6)self-react at a diffusion-controlled rate (2kt 2 x 1O' I mol--l s-in hexane at 25 "C) by two different routes. The first, which is thermally and photolytically reversible, is the combination to give the dehydrodimer (C), and the second, which is irreversible, is the disproportionation to give the parent cyclopentadiene (A) and the tetramethylfulvene (D).WE have recently shown that the cyclopentadienyl derivatives of certain metals are photosensitive and irradiation of these compounds in liquid solution with U.V.light in the cavity of an e.s.r. spectrometer shows the spectrum of the cyclopentadienyl radical equation (1)l. C,H,ML, -C,H,* +*ML, (1) M = Li,lY2 Na,2 SnlT, SnIV,193,Hgl PbTT, YbTv,4 TiI", Zrl" The derivatives of mercury and of tin 193 have been investigated most thoroughly. Introduction of one methyl group into the cyclopentadienyl ring of the tin compounds greatly reduces the photosensitivity, but the photolysis of bis (methylcyclopentadien yl) mercury st i11 shows a strong spectrum of the methylcyclopentadienyl radical.' In view of the increasing importance of the penta- methylcyclopentadieriyl group as a ligand for metals, we began a study of the photolysis of pentamethylcyclo- pentadienyl metal compounds, but it soon became apparent that the parent hydrocarbon itself is remark- ably, if not uniquely, photosensitive. In this paper we concentrate attention on the photoly- sis of pentamethylcyclopentadiene and on the properties of the pentamethylcyclopentadienyl radical which is formed.We hope to return to the question of the behaviour of the metal derivatiires at a later date. Generation of thc Yentamethylcyclo@entadie.tzylKadi-caZ.--The e.s.r.spectrum of the Me,C,* radical (I) was observed from the six different routes shown in Scheme 1. A suspension of pentaniethylcyclopenta-dienyl-lithium in tetrahyrirofuran showed a persistent MeSCSCOMe (1) Me,C, H SCHEMEZ weak spectrum of the hle,C,* radical trapped in the solid matrix, as soon as the sample was inserted into the cavity, and before it was irradiated. It is possible that the compound may be sensitive to the laboratory light, but, inore probably, the radical results from oxidation by adventitious oxygen. Irradiation of this suspension gave a very strong spectrum of the matrix-isolated radical (AHpp0.5 G) which persisted for some days at room temperature. Irradiation with U.V.light of a dilute solution of the lithium derivative in tetrahydro- furan showed a stronger spectrum of the radical (I), which disappeared immediately the light was sliuttered. For comparison, U.V. irradiation of a solution or suspen- sion of cyclopentadienyl-lithium shows tlie e.s.r. spec- trum of the C,H,* radical, but, in the presence of oxygen, the persistent spectrum of the fulvalene radical anion is observed.2 A stronger spectrum of the radical (I) was observed when bis (pentamet hylcyclopent adieny1)mercury was photolysed in toluene solution, and a mirror of mercury was deposited on the walls of the cell ; a typhI spectrum is illustrated in Figure 1. tioiis, neither cyclopentadiene nor niethylcyclopei ita-diene show the spectrum of tlie corresponding radical , but this relative insensitivity to photolysis is only a matter of degree: if the rate of removal of the C,H,* radical is greatly decreased by generating it by photo- lysis of cyclopentadiene as a single crystal or glass, the e.s.r.spectrum of the radical can be observed.2$18 This photolysis of pentamethylcyclopentadiene is considered in detail below. 1' _I1,f?t FIC;UHE E.s.r. spectrunl of the pentaniethylcyclopcntadienyl r, lical obtained from the photolysis of bis(pentamethy1cyclo-1 --pentadieny1)mercuryin tetrahydrofuran at 37 "C. The 13C satellites are arrowed U .v. irradiation of pent amet hylc yclopen tad ien yl me- thyl ketone showed the same spectrum of the radical (I), formed by Norrish I fragmentation of the ketone triplet.Similar treatment of cyclopentadienyl methyl ketone shows a relatively weak spectrum of the C,H,* radical.6 If the above systems were heated to 70-100 "C, none showed the spectrum of the Me,C,* radical before photo- lysis, but all showed the spectrum after photolysis. We present evidence below that this behaviour results from the thermal dissociation of the dehydrodimer (11) which is a decay product of the Me,C,* radical. Accordingly, pentamethylcyclopentadienyl-lithiumwas treated with iodine to give a product which showed a parent peak in the mass spectrum corresponding to the dehydrodimer and which, on heating or photolysis showed the e.s.r. spectrum of the Me,C,* radical. Cyclopentadienyl-lithium itself reacts with iodine to give bi(cyclopenta-2,4-dienyl)but this rearranges at room temperature to give bi( cyclopenta-1,3-dienyl) .' Bi(cyclohept a-2,4,6- trien yl) dissociates reversibly he-tween 120 and 140 "C to show the e.s.r.spectrum of the cycloheptatrienyl radical, and the bond dissociation energy has been measured to be ca. 146 kJ mol-1.8 The most interesting route to the radical (I),however, is the photolysis of the parent cyclopentadiene RIe,C,H as the neat liquid or in hydrocarbon solution ; an example of the spectrum which is observed is illustrated in our preliminary communication.9 Under the same condi- Properties of the Pentameth-ylcyclopentadie72ylRadical.-The e.s.r. spectrum of the pentamethylcyclopentadienyl radical shows (Figure 1) a (15 H) 6.4,a (13C) 3.5 G, g 2.0025.The inner 14 of the predicted 16 lines due to proton coupling could be identified, in the correct ratio of intensities; the first and sixteenth lines have an intensity of only l/F5i.e. 1/32 768 of that of the total spectrum, and could not be detected. The low value of the 13C hyperfine coupling confirms that, like cyclo- pentadienyl itself C,H,*, a (5H) 6.0, a (13C) 2.6 G, g 2.002 51 but unlike the cyclopropenyl analogue But,C,*, a(13C) 30.0 the Me,C,* radical has a x rather than a o configuration. No evidence for Jahn-Teller dis-tortion could be detected, nor would it be expected, at the temperature of our experiments. Our results support the assignment to the Me5C,.radical of the spectrum a(+13 H) 6.4 G which Monge and Schott observed when crystalline durene was irradiated first with X-rays and then with U.V. light equation (2).I2 The high concentration of the Me,C,-radical which can be obtained from the various routes in Scheme 1 might be taken to imply that the rate of the self-reaction of the radical is unusually low, perhaps because of steric hindrance. However, measurement of the rate constant for this reaction showed that the self-reaction is diffusion- controlled, with 2kt 2 x lo9 1 mol-l s-l in hexane at 25 *C, close to the value for the cyclopentadienyl radical itselfe2 The relatively high concentrations of the Me,C,* radical which can be achieved therefore reflect a high rate of formation rather than a low rate of removal, and seem best ascribed to weakening of the Me,C,-X bonds by conjugative and hyperconjugative stabilisation of the Me,C,* radical.Me Me Mer).. Me Analysis by g.1.c.-n1.s. of the products after photolysis showed the presence of the dehydrodimer (V) (Scheme 2) resulting from the combination of two pentamethyl-cyclopentadienyl radicals. If the solution after photo- lysis was heated to 70-120 "C, the dehydrodimer dis- sociated to restore the spectrum of the radical (I). The compound (V) has been obtained previously by Jutzi and Kohl from the reaction between bis(pentamethylcyc1o- pentadienyl) tin di-iodide and pyridine.13 The principal hydrocarbon which was isolated from the photolysis products, however, was the tetramethyl- fulvene (VI) which does not appear to have been re-ported previously.Presumably it is formed by the irreversible disproportionation of pairs of radicals (I) formed directly from pentame thylcyclopent adiene or indirectly by dissociation of the dehydrodimer (V). Me J.C.S. Perkin I1 as yet to detect the e.s.r. spectrum of the cyclopentadi- enylperoxyl radical C,H,OO., which by analogy with other cycloalkylperoxyl radicals would be expected to show hyperfine coupling to the y-proton, a(H,) 5-7 G. Photolysis ofthe C-H Bond.-As far as we are aware, there is no precedent for the photolysis of a hydrocarbon RH in solution showing the e.s.r. spectrum of the cor- responding radical Re.We have therefore investigated the photolysis of pentamethylcyclopentadiene in more detail to determine whether it involves simple unimole- cular homolysis of the C-H bond as illustrated in Scheme 2, or whether some more complex process is involved. All the evidence supports the picture of the simple homolysis. One alternative might be that photoexcited diene Me,C,H* could abstract hydrogen from unexcited Me,- C,H to give the radicals Me,C,H,* and Me,C,*. However, it seems most unlikely that the loss of the radical Me,C,H,* from solution could be faster than that of Me,C,* which we have shown (see above) to be diffusion- controlled, and no e.s.r. signals which could be ascribed Me CH ,CH; SCHEME3 to the radical Me5C5H2* were observed (Figure 1); furthermore, no products which might result from this radical could be detected.The spectrum of hydrogen atoms a(H) 503.8 GI could not be observed, but none would be expected. In their classic work on the electron irradiation of liquid (,,) Me hydrocarbons, Fessenden and Schuler observed the H* . spectrum only from liquid methane at -178 OC.14 Larger alkanes presumably scavenged the hydrogen atoms by the reactions RH + H*-R*+ H,, though pvMe Me Me MeMe Or' hV some alkenes showed the presence of radicals resulting bsol; Me Me Me Me Me SCHEME2 If the radical (I) was generated between -100 "C and room temperature in the presence of oxygen, a strong singlet was observed, g 2.015 1, AHpp 2.0 G, which persisted in the absence of light at low temperature and obviously is to be ascribed to the pentamethylcyclo- pentadienylperoxyl radical, Me,C,OO*.It is interesting that despite a number of attempts we have been unable from the addition of hydrogen to the double bond, e.g. CH,=CH, + He CH,-CH,*. We have given above the arguments for discounting the formation of the adduct Me,C,H,* in our system. The most probable fate of the hydrogen atoms therefore appeared to be their reaction with more parent hydro- carbon Me,C,H (or less likely, with the radical Me,C,*) to give molecular hydrogen and a second radical Me,C,e (Scheme 3). A sample of neat pentamethylcyclopentadiene was therefore sealed under vacuum and photolysed with U.V.light, and any gases which were formed were analysed by g.1.c. and by laser Raman spectroscopy. Hydrogen was identified by g.1.c. and this was confirmed beyond any question by the Raman spectrum shown in Figure 2. If the photolysis was carried out in liquid ethene as solvent, the hydrogen atam could be observed to react with the solvent, and at -120 to -80 "C a weak spec- trum of the ethyl radical (Scheme 3) could be observed. Although the photolysis of the C-H bond of a hydro-carbon in liquid solution by U.V. light does not appear to have been reported previously we believe that it does occur with certain other compounds, and have detected it, though more weakly, with cyc10heptatriene.l~ We hope that reactions of this type may render it possible to generate hydrogen atoms more readily in solution, and to investigate their reactions.EXPERIMEN TAL A'laterials .--Hexamethylbicyclo2.2.0hexa-2,Ei-Qiene (hexamethylDewar benzene) was converted into methyl pentameth ylcyclopen ta- 2,4-dienyl ketone, T(CC1,) 8.18 (2 Me, s, 6 H), 8.33 (6 H, s, 2 Me), 8.46 (3 H, s, Me), and 8.94 (3 H, s, Me),16 and thence into 1,2,3,4,5-pentamethyl-cyclopenta-1 ,3-diene l7 which, for photolytic experiments was purified by preparative g.l.c., ~(Ccl,) 7.60 (1 H, q), 8.25 (12 H, 4 Me), and 9.05 (3 H, d, J 7.5 Hz, Me). The diene was treated in tetrahydrofuran with butyl- lithium (1.6~ in hexane) to give a suspension of penta-met hylc yclopentad ienyl-li thium, which reacted with mercuric chloride yielding bis(pentamethylcyc1openta-dienyl)mercury, T(C,H,) 8.10 (12 H, s, 4 Me), 8.19 (12 H, s, 4 Me), and 8.73 (6 H, s, 2 Me).lB Jutzi and Kohl l3 have questioned the nature of the isolated product lS as a mercury derivative, pointing out that its characteristics are similar to those of bi(penta-methy lc yclopen ta- 2,4-dienyl).A suspension of Me,C,Li in tetrahydrofuran was treated with iodine. The lithium iodide which was precipitated was filtered off, and the solvent was removed under reduced . pressure yielding bi(pentamethylcyclopenta-2,4-dienyl)as a solid which was recrystallised from hexane, m/e 270 (M+). Technzques.--'H N.m.r. spectra were recorded at 60 MHz on a Perkin-Elmer R12 instrument, or at 200 MHz on a Varian XL 200 instrument.Samples for e.s.r. spectroscopy were sealed under vacuum in Suprasil silica cells, and thermolysed, or photolysed with light from a high pressure 500 W mercury arc. The solvents used were tetrahydrofuran for Me,C,Li, tetrahydro- furan, or toluene for (Me5C5),Hg, and cyclopropane, hexane, toluene, ethylene, or the neat liquid for Me,C,H. The kinetics of removal of the Me,C,- radical in solution were measured by computer-averaging of a series of signal decay curves produced by positioning a rotating sector in the light beam.19. 2o The Ratnan spectrum of hydrogen was recorded on a Spex 1401 spectrometer, using the 514.0 nm exciting Iine from a Ar+ ion laser CR3. G.1.c.-m.s. analysis was carried out with a Pye 204 chromatograph and V6 707OG double focusing mass spectrometer provided with a Finnigan-Ingos 2400 data system.Products of the Phdolysis of Pentamethylcyclopentadiene.-Pentamethylcyclopentadiene in hexane was photolysed in an e.s.r. cell for ca. 10 h. A sample of the supernatant gas was analysed by g.1.c. using helium as the carrier gas and a katharometer detector, and showed the unique negative peak due to hydrogen (Perkin-Elmer Sigma 2 instrument fitted with a 8 m column of 60-80 mesh Porapak QS at 25 "C). The Raman spectrum of the gas showed the characteristic vibrational-rotational spectrum illustrated in Figure 2. The liquid phase was analysed by g.1.c.-m.s. (OV 17 column ; temperature range 80-160 "C; electron energy 70 eV; source temperature 160 "C) and showed the presence of the following two products, bi(pentamethylcyc1openta-2,4-dieny1),I3 m/e 270 (3, hi?),255 (0.2), 137 (11.7), 136 2FIGURE Rotation and vibration-rotation spectrum of hydro-gen obtained from the photolysis of pentamethylcyclopenta-diene. Inset: reference spectrum of hydrogen at 1 atm.pressure (31.7), 135 (100, M/2), 134 (26.61, 119 (11.5), 105 (10.7), and 91 (6.2), and 1,2,3,4-tetramethylfulvene,mle 134 (52.6, Mj, 133 (14.4), 119 (loo), 91 (13.81, 79 (2.9), 77 (5.9), and 65 (3.9); this compound was recovered by preparative g.1.c. as a yellow liquid, T(CDC1,) 4.60 (2 H, s, CH,), 8.13 (6 H, s, 2 Me), and 8.18 (6 H, s, 2 Me). We are grateful to Dr. P. J. Garrett for the gift of a large sample of hexamethylDewar benzene, to Dr.J. R. M. Giles for assistance with the kinetic measurements, to Ih-. A. R. Burgess and Mr. K. W. Wheatley for carrying out the g.1.c. detection of hydrogen, and to Dr. R. J. H. Clark for advice and assistance with the Raman spectroscopy. This work was carried out during the tenure of a S.K.C. Research Assistantship by J. L. 0/1601 Received, 20th October, 19801 REFERENCES P. J. Barker, A. G. Davis, and M.-W. l'se, J. Chem. .Zoc., Perkin Trans. 2, 1980, 941. A. G.Davies, J. K.M. Giles, and J. Lusztyk, J. Chem Soc., Perkin Trans. 2, 1981, 747. 3 P. J. Barker, A. G. Davies, J. A.-A. Hawari, and M.-W. Tse, J. Chem. Soc., Perkin Trans. 2, 1980, 1488. A. G. Davies, C. Dowson, P. G. Harrison, and J. A.-.4.Hawari, unpublished work. 5 P. B. Brindley, A. G. Davies and J. A.-A. Hawari, un- published work. A. G. Davies and C. F. Ingold, unpublished work. E. Hedaya, Acc. Chem. Hes., 1969, 12, 367. a G. Vincow, H. J. Dauben, F. R. Hunter, and W. V. Volland, J. Am. Chem. Soc., 1969, 91, 2823. @ A. G. Davies and J. Lusztyk, J. Chem. Soc., Chem. Commun., 1980, 554. 10 G. R. Liebling and H. M. 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