J . Am. Chem. SOC.1988, 110, 2527-2538
2527
1,4-Perinaphthadiyl. Singlet- and Triplet-State Reactivity of a Conjugated Hydrocarbon Biradical Michael N. Richard Boothe,+ Ernest Clark,t Markus Cisin,* Hamdi M. Hassaneen,+Richard M. Pagni,*+ Gabriele Persy,: Robert J. Smith,+and Jakob Wirz*t Contribution from the Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996- 1600, and Institut fur Physikalische Chemie der Universitat Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland. Received August 14, 1987
Abstract: 1,4-Perinaphthadiyl (b) was generated from the azo precursor 1 by thermal elimination of nitrogen as well as by direct and sensitized photolysis. The biradical was persistent but light sensitive at low temperature in the triplet ground state (3b) and was characterized by optical and ESR spectroscopy; kinetic and spectrographic flash photolysis was used to monitor 'b at room temperature as a transient intermediate yielding naphthocyclobutane (2), divinylnaphthalene (3), and dimers (9, 10). The lifetime of 3b was reduced from T = 0.2 ms in degassed solution to 0.2 p s in air-saturated solution, and as a result, peroxides (4, 6) were formed at the expense of the hydrocarbon products. However, the dependence of the peroxide yield on oxygen pressure required that a second intermediate also interacted with oxygen. This was identified as the singlet state of b undergoing catalyzed intersystem-crossing (ISC) upon diffusional encounter with triplet oxygen; its lifetime at 300 K was estimated as 1 ns, and the energy difference between 'b and 'b as 5 kcal/mol. The efficiency of spontaneous ISC from 'b to 3b was found to increase with decreasing temperature, and the yields of the different final products (2-4, 6, 9 , 10) were strongly influenced by the conditions chosen for the formation of the biradical. The chemical properties of b are strikingly different from those of the related 1,3-perinaphthadiyls c and d due to the much shorter intrinsic lifetime of I C and Id.
The K,T biradical 1,8-naphthoquinodimethane(a) and several of its derivatives are persistent in the triplet state at or below 77 K in rigid glassy solutions and have been thoroughly characterized by a number of spectroscopic but little is known about their reactivity in solution, and there are no experimental data about the lowest singlet state of these biradicals. We have sought
a
b
C
d
to obtain such information by combining low-temperature and time-resolved spectroscopy with quantitative product analyses. Several reasons led us to choose 1,4-perinaphthadiyl (7,8,9,10tetrahydrocyclohepta [de]naphthalene-7,10-diyl, b) for this work naphthoquinodimethane biradicals are well suited for optical detection and hence for flash photolysis; several pathways (pyrolysis, direct photolysis, triplet sensitization) are available for the formation of b from its azo precursor 1,4-dihydro-l,4ethanonaphtho[de] [ 1,2]diazepine (l),which is also available with a stereochemical label ( l - d 2 ) ;finally, we were interested in comparing the reactivity of the tetramethylene biradical b with that of the related trimethylene biradical 1,3-perinaphthadiyI (c). Previous studies of cI0 and of its 2,2-dimethyl derivative d8 led to the conclusion that the collapse of these singlet 1,3 biradicals to allow for by ring closure or 1,2 H shift is too fast ( ? l o L 2 any intersystem-crossing (ISC) to the long-lived triplet ground state or for intermolecular trapping reactions. The results presented here indicate that, in contrast, singlet 1,4perinaphthadiyI (Ib) is a true intermediate that decays by several competing decay paths. The trapping of biradicals by molecular oxygen provides a useful diagnostic tool to determine the yield and lifetime of these reactive intermediates."J2 Is it safe to assume that the oxidation products are formed exclusively by the trapping of triplet biradicals, or do we need to consider the reaction of singlet biradicals with triplet oxygen as a real possibility? One may be tempted to dismiss the second alternative on the premises that, first, singlet biradical lifetimes are too short to allow for diffusional encounters with oxygen and that, second, peroxide formation from singlet biradicals University of Tennessee. *University of Basel.
with triplet oxygen is inhibited by the spin barrier. Actually, neither of these arguments is well-founded. We report our evidence for the efficient trapping of the singlet biradical Ib by molecular oxygen. The method should be of general value to obtain indirect evidence for the existence of singlet biradical intermediates with a lifetime of at least picoseconds.
Experimental Section General Procedures and Instrumentation. Experimental procedures for the photochemical and thermal nitrogen elimination from the azo precursor 1,4-dihydro-1,4-ethanonaphtho[de][ 1,2]diazepine (1) in degassed solutions and the yields of the resulting hydrocarbons 6b,7,8,8a-tetrahydrocyclobut [alacenaphthylene (naphthocyclobutane, 2) and 1,8-diethenylnaphthalene (3) as a function of temperature have been reported previo~sly.~~ The methods used to trap the biradical intermediates and (1)Review articles: (a) Platz, M. S. In Diradicals; Borden, W. T., Ed.; Wiley: New York, 1982. (b) Platz, M. S.; Carrol, G.; Pierrat, F.; Zayas, J.; Auster, S. Tetrahedron 1982,38, 777--785.(c) Rule, M.; M a t h , A. R.; Seeger, D. E.; Hilinski, E.F.; Dougherty, D. A,; Berson, J. A. Tetrahedron 1982,38,787-798.(d) Wirz, J. Pure Appl. Chem. 1984,56,1289-1300. (e) Berson, J. A. In Chemistry of Function41 Groups, Chemisrry of the Quinonoid Compounds, in press. (2)(a) Pagni, R. M.; Watson, C. R., Jr.; Bloor, J. E.; Dodd, J. R. J . Am. Chem. Soc. 1974,96,4064-4066.(b) Watson, C. R., Jr.; Pagni, R. M.; Dcdd, J. R.; Bloor, J. E.Ibid. 1976,98, 2551-2562. (e) Pagni, R. M.; Burnett, M. N.; Dodd, J. R. Ibid. 1977,99, 1972-1973. (3) Muller, J.-F.; Muller, D.; Dewey, H. J.; Michl, J. J . Am. Chem. SOC. 1978,100, 1629-1630. (4)Gisin, M.; Rommel, E.; Wirz, J.; Burnett, M. N.; Pagni, R. M. J . Am. Chem. SOC.1979,101, 2216-2218. (5)(a) Platz, M.S. J . Am. Chem. SOC.1979,101,3398-3399. (b) Platz, M. S.; Burns, J. R. Ibid. 1979,101,442554426,(c) Platz, M. S. Ibid. 1980, 102, 1192-1194. (d) Fritz, M. J.; Ramos, E. L.; Platz, M. S. J . Org. Chem. 1985,50, 3522-3526. (6)Chisholm, W. P.; Weissman, S. I.; Burnett, M. N.; Pagni, R. M. J . Am. Chem. SOC.1980,102, 7103-7104. (7)(a) Cofino, W. P.; van Dam, S. M.; Kamminga, D. A,; Hoornweg, G. Ph.; Gooijer, C.; MacLean, C.; Velthorst, N. H. Specrrochim. Acta, Parr A 1984,40A, 219-228. (b) Cofino, W. P.; van Dam, S . M.; Hoornweg, G. Ph.; Gooijer, C.; Maclean, C.; Velthorst, N. H.; Pagni, R. M. Ibid. 1984,40A, 251-257. (8) Hasler, E.;Gassmann, E.; Wirz, J. Helu. Chim. Acta 1985, 68, 777-788. (9)(a) Fischer, J. J.; Penn, J. H.; Dohnert, D.; Michl, J. J . Am. Chem. SOC.1986, 108, 1715-1716. (b) Fischer, J. J.; Michl, J. Ibid. 1987, 109, 583-584. (10) Pagni, R. M.; Burnett, M. N.; Hassaneen, H. M. Tetrahedron 1982, 38,843-851. ( 1 1) Wilson, R. M.In Organic Phorochemistry; Padwa, A,, Ed.; Marcel Dekker: New York, 1985;Vol. 7,Chapter 5, pp 339-466. (12)Adam, W.; Hannemann, K.; Wilson, R. M. J. Am. Chem. SOC.1986, 108,929-935.
0002-7863/88/1510-2527$01.50/00 1988 American Chemical Society
2528 J . Am. Chem. Soc., Vol. 110, No. 8. 1988 to determine the ensuing product distributions are described below. The results of all these experiments are collected in Table I (see Results). For preparative photolyses a medium-pressure mercury arc was used as a continuous source of irradiation. When high light powers were desired in order to increase the yield of biradical dimerization products, the samples were flashed repetitively with the output of a 2000-5 xenon flash lamp commercially available for photographic purposes. Suitable band-pass or cutoff glass filters were used with both light sources as indicated in the text. Photolyses at -78 OC were conducted by placing the sample tubes in a glass dewar containing a dry ice/acetone bath. Gas pressures were maintained as follows. Degassed samples were photolyzed in glass (often NMR) tubes that had been sealed after three 'freezepump-thaw" cycles. Solutions to be photolyzed under 1 atm oxygen were placed in open test tubes, and the gas was continuously bubbled through the solution. Photolyses under oxygen pressure were done with GriffinWorden pressure vessels from Kontes Scientific Glassware. Most thermolyses were conducted by placing the sample tube in an oil bath equipped with a thermostat. Gas pressures were maintained as described for the photolyses. For the application of gas pressures exceeding 5 atm, the sample was placed into a steel autoclave with external electric heating. With this setup only a very crude temperature control was possible, and the actual sample temperature could not be monitored directly. Flash spectroscopy of long-lived transient intermediates ( T >50 ps) was done with an apparatus of conventional design.', The excitation flash was produced by an electric discharge (1000 J, 20 p ) through two parallel quartz tubes of 16-cm length and 1-cm diameter, which were kept at a reduced pressure of ca. 6 kPa. The light flash was filtered through suitable cutoff glass filters. Evacuable quartz or glass sample cells of IO-cm length and 2-cm diameter were placed between the discharge tubes. Solute concentrations were chosen to give an absorbance of ca. 0.5 at the excitation wavelengths. Short-lived transients (20 ns < 7 < 100 ps) were monitored on a laser flash apparatus using the frequency-tripled (353-nm) or quadrupled (265-nm) 20-11s pulses of ca. 50 mJ from a Nd glass laser. The detection system has been described previousIy;'4'" it allowed for the simultaneous capture of the kinetic trace (at a given wavelength) and of the transient absorption spectrum (at a given time delay after excitation) in digital form and with nanosecond time resolution. The kinetic and spectrographic data were processed by computer. A solvent mixture known as EPA (diethyl ether/isopentane/ethanol, 5:5:2) was used to obtain rigid glassy solutions for optical spectroscopy at 77 K the experimental setup has been described.I" H NMR spectra are reported as follows: (solvent, magnetic field/MHz), downfield chemical shift 6/ppm vs internal tetramethylsilane (splitting, number of protons, coupling constants, assignment). Only the longest wavelength electronic absorption bands [A,,,/nm, c/M-' cm-l] and only the most characteristic infrared bands)aJn'-,,?,( and peaks of the mass spectrum (m/z, relative intensity) are reported for compound characterization. Syntheses. 1,4-Dihydro-1,4-ethanonaphtho[l,%-deI1,2]diazepine (1). This compound was prepared by the method of Watson et aLZbexcept that phenalene was prepared from phenalenone by the method of Boudjouk and J o h n s ~ n 'and ~ phenalenone was prepared from l-acetonaphthone by the method of Pagni et a1.I0 Chromatographic purification of the product 1 with chloroform on silica gel and slow precipitation induced by adding hot n-hexane to a concentrated chloroform solution followed by slow cooling to -20 OC gave pale yellow crystals: mp 123-125 "C (lit.2bglassy solid); UV (hexane) 410 nm ( e 1380), (acetonitrile) 410, ( e 1170), (methanol) 408 ( e 1170). ( 1a,4a, 1 1R *, 1 2 s *)- 1,4-Dihydro- 1,4-ethanonaphtho[ 1$-de I[ 1,2]diazepine-11,12-d2 (I-d2). Compound 1-d2 was prepared in the same
1H-phenalene-2,lOway as 1, starting from 2,3-dihydro-1,2,3-methenod2.I6 Integration of the bridgehead and methylene H NMR peaks showed the compound to be 94% deuteriated (d2) at the methylene groups. lH,4H-l,4-Ethenonaphtho(l,&deI1,2]dioxepin.A solution of 340 mg (1.9 mmol) of cyclohepta[de]naphthaleneZband 23 mg of meso-tetraphenylporphine in 5.5 mL of methylene chloride was photolyzed at -78 OC, while oxygen was bubbled through the solution. After 2 h of irradiation the solvent was removed by a rotary evaporator, and the solid was partly dissolved in methanol leaving the sparingly soluble sensitizer be(13) Porter, G . Techniques of Organic Chemistry, Parr 2; Weissberger, A,, Ed.; Interscience: New York, 1963; Vol. VIII, p 1055. (14) (al Lewa. E.: Platz. M. S.: Persv. G.: Wirz. J. J . Am. Chem. SOC. 1986, ids,' 37i3-3790. (b) 'Leyva,'E.; young, M . J: T.; Platz, M. S. Ibid. 1986. 108. 8307-8309. (15) Boudjouk, P.; Johnson, P. D. J . Org. Chem. 1978, 43, 3979-3980. (16) Turro, N. J.; Ramamurthy, V.;Pagni, R. M.; Butcher, J. A,, Jr. J . Org. Chem. 1977, 42, 92-96. ~
~~
Burnett
et
al.
hind. The filtrate was again evaporated and the residue chromatographed on silica gel. Elution with 5% ether in ligroin removed unreacted cyclohepta[de]naphthalene, while 10% ether in ligroin eluted the remaining sensitizer. Elution with 20% ether in ligroin afforded 273 mg (68%) of white flakes: mp 134.5-135.5 OC; NMR (CDCI,, 60 MHz) 6 7.07-7.90 (m, 6 H, aromatic), 6.65 (dd, 2 H, J = 3.2, 5.2 Hz, vinyl), 5.46 (dd, 2 H, J = 3.2, 5.2 Hz, bridgehead); UV (95% ethanol) 294 nm ( e 5400); MS (70 eV) 210 (4), 178 (loo), 152 (43). Anal. Calcd (C,,H,,O,) C 79.98; H, 4.79. Found: C, 78.42; H, 4.85. lH,4H-1,4-Ethanonaphtho[1,8-deI1,2ldioxepin (4). Compound 4 was prepared by the general procedure of Adam and Eggelte for the reduction of unsaturated peroxides." To a solution of 160 mg (0.76 mmol) of lH,4H-1,4-ethenonaphtho[l,8-de] [ 1,2]dioxepin in 20 mL of methylene chloride (distilled from calcium hydride) was added 3.3 g of dipotassium azodicarboxylate.'* While being stirred under nitrogen, the mixture was cooled to -78 OC, and a solution of 2 mL of glacial acetic acid diluted to 10 mL with methylene chloride was added dropwise. Following the addition, the mixture was returned slowly to room temperature and stirred overnight. The solids present were filtered from the mixture, and the solvent was removed on a rotary evaporator. The residue was chromatographed on silica gel with 6% ether in ligroin. The resulting oil was crystallized from ligroin, giving white needles of 4: mp 88-89 OC; NMR (CDCI,, 90 MHz) 8 7.81 (dd, 2 H, J = 7.9, 1.7 Hz,aromatic peri H), 7.26-7.52 (m, 4 H, aromatic), 5.3-5.4 (m, 2 H, bridgehead), 2.6-2.8 (m, 2 H, methylene), 1.94-2.15 (m, 2 H, methylene); UV (95% ethanol) 286 nm ( e 6900); MS (25 eV) 212 (83), 180 (48), 165 (100). Anal. (C,,Hi2021 C, H. Head-to-Head Dimer 5 of the Biradical b. The photodimer of cyclohepta[de]naphthalene, 7,8,15,16-tetrahydro-7,16:8,15-diethenocyclodeca[ 1,2,3-de:6,7,8-d'e'jdinaphthalene (120 mg),I9 was dissolved in 100 mL of chloroform and reduced with hydrogen in the presence of PtO,, until the starting material had reacted completely (TLC analysis on silica gel, eluting with 10% benzene in CCI,). The solvent was evaporated, and the solid residue was recrystallized twice from methylene chloride, yielding colorless crystals (90 mg) of the head-to-head dimer 7,8,15,16tetrahydro-7,16:8,15-diethanocyclodeca[ 1,2,3-de:6,7,8-d'eqdinaphthalene ( 5 ) : mp 280-281 "C; NMR (CDCI,, 90 MHz) 6 7.20 (dd, 4 H, J = 7.8, 1.8 Hz, aromatic peri H), 6.70-6.95 (m, 8 H, aromatic), 4.08 (br s, 4 H, bridgehead), 2.8 (br m, 4 H, methylene) 2.4 (br m, 4 H, methylene); MS (70 eV) 360 (M*, 4), 181 (50). 180 (20), 179 (100); UV (hexane) 295 nm ( e = lo4, slightly soluble). Photolyses and Thermolyses. Thermal and Photochemical Stability of the Products. Control experiments showed that the hydrocarbon products 2 and 3 were stable under all the photolysis and pyrolysis conditions used. The cyclic peroxide 4 was also inert to the photochemical reaction conditions at ambient temperature (A >380 nm, C6D6)but reacted slowly under the conditions used at -78 OC (A >340 nm); prolonged irradiation of 4 led to the formation of an unidentified ketone: NMR (C6D6,60 MHz) 6 8.5 (dd), 7.0-7.7 (m), 2.9 (s); IR (C6D6) 1690 cm-I. Heating a solution of 4 in C6D6 to 150 OC for 24 h led to no decomposition.
However, pyrolysis at 180 "C did result in a reaction yielding an unidentified product mixture that gave many poorly resolved H NMR peaks (C6D6)different from those observed after photolysis. The hydroperoxide 6 (vide infra) was stable in methanol and deuteriochloroform at room temperature for extended periods of time but decomposed completely in benzene within less than 12 h. It also decomposed slowly under the low-temperature irradiation conditions (A >340 nm) . Photolysis of 1 at Room Temperature under Oxygen. (a) Benzene, 1 atm 0 2 (Table I, Entry 2). Photolysis (A >380 nm) of 1 in reagent-grade benzene (typically 20 mg of 1 in 50 mL of reagent-grade benzene) gave a product mixture that by TLC analysis was found to contain the peroxide 4 in addition to the hydrocarbons 2 and 3. The relative product distribution was determined by 200-MHz H NMR after evaporation of the solvent and dissolution of the residue in C6D6. No resonances other than those due to compounds 2-4 were discernible in the spectrum. (b) Methylcyclohexane, 1 atm O2 (Table I, Entry 5). After the photolysis of 1 (A >380 nm; 11.5 mg in 90 mL of purified methylcyclohexane), the mixture was worked up and analyzed as described under a. The 200-MHz H NMR spectrum showed some very small unidentified resonances in addition to those of 2-4. (c) Benzene, 4.6 atm O2 (Table I, Entry 3). The product distribution was determined by 400-MHz H NMR spectroscopy (C6D,). The spec(17) Adam, W.; Eggelte, H. J. J . Org. Chem. 1977,42,3987-3988; Angew. Chem. 1977,89, 162; Angew. Chem., Int. Ed. Engl. 1977, 16, 713. Adam, W.; Erden, I . J . Org. Chem. 1978, 43, 2731-2738. (18) Werstiuk, N. H. Can. J . Chem. 1975, 53, 26-40. (19) Hazell, A. C.;Pagni, R. M.; Persy, G.; Rommel, E.; Wirz, J. Helu. Chim. Acta 1981, 64, 2830-2840.
J . Am. Chem. Soc., Vol. 110, No. 8, 1988 2529
1,l-Perinaphthadiyl Reactivity trum was essentially free of peaks not attributable to compounds 2-4. (d) Methanol, 1 atm O2 (Table I, Entry 10). After the photolysis of 1 (A >380 nm; 11.5 mg in 95 mL of reagent-grade methanol) and solvent removal, a pale yellow oil was obtained that solidified on standing. TLC analysis revealed the presence of one major and two minor unknown components in addition to the known products 2-4. The large spot was subsequently identified as 7-hydroperoxy-7,8-dihydrocyclohepta[de]naphthalene (6, vide infra). The product distribution was determined in CDC& by 200-MHz H NMR analysis; only a few small peaks remained unassigned. Photolysis of I d 2 at Room Temperature under Oxygen (Table I, Entry 4). The photolysis was performed under conditions identical with those used for 1 in benzene under 1 atm oxygen. The presence of deuterium simplified the H NMR spectra, and it was possible to deduce the product and deuterium distribution from 60-MHz NMR spectra taken before and after chromatographic separation of the products. Photolysis of 1 at -78 "C under Oxygen. (a) Methanol, 1 atm O2 (Table I, Entry 11). After the photolysis (A >340 nm) of 41.2 mg of 1 in 150 mL of reagent-grade methanol, the solution was warmed to room temperature, and the solvent was removed in vacuo. TLC analysis of the resulting pale yellow oil on silica gel with 16% ether in ligroin showed the presence of one major and three minor products; none of the known compounds 1-4 were among these components: IR (CH2CI2)3500 cm-I (-OOH); H NMR (CDCI,, 60 MHz) 6 7.1-7.9 (m, aromatic), 6.8 (d, J = 12 Hz, vinyl), 6.0 (m, vinyl), 5.4 (m, methine next to OOH), 3.0 (m, methylene), 1.8-2.8 (br m). The resonances at 6 6.8, 6.0, and 3.0 had relative areas of 1:1:2, while the resonance at 5.4 integrated for approximately 2. The peaks at 6 6.8, 6.0, 5.4 (in part), and 3.0 are attributed to 7-hydroperoxy-7,8-dihydrocyclohepta[de]naphthalene(6) on the basis of the results described below. Assuming that all the components in the mixture were 1$-disubstituted naphthalenes, greater than 50% of the material was 6. Other runs analyzed for up to 70% 6. Because the hydroperoxide did not survive column chromatography, the product mixture was reduced with 105 mg of LiAIH4 in 25 mL of ether at room temperature under N2 for 95 min. After the mixture was quenched with aqueous Na2S04, the ether was separated from the aqueous layer, dried over anhydrous MgS04, and removed in vacuo. TLC on silica gel with 16% ether in ligroin showed two spots of equal size above Rf 0, both of which moved more slowly than the hydroperoxide. The two components were separated by chromatography on alumina, eluting with ether in ligroin. The first component eluting from the column was vacuum distilled [lo0 "C (3 mm)] and yielded a colorless oil: MS (25 eV) 196 (M'), 168 (M+ - CH2CH2,base peak); H NMR (CDC13, 60 MHz) 6 7.55 (d, 2 H , arom.), 7.30 (t, 2 H, arom.), 7.09 (d, 2 H, arom.), 5.33 (m, 2 H, methine), 2.43 (m, 2 H, methylene endo to oxygen), 1.89 (m, 2 H, methylene exo to oxygen). The small amount of this compound could not be induced to crystallize, but it did have an H NMR spectrum identical with that of an authentic sample of 7,8,9,10tetrahydro-7,1O-epoxycyclohepta[de]naphthalene(7).20 The second component eluting from the column was also distilled in vacuo [lo0 "C (3 mm)] and yielded a colorless oil: MS (25 eV) 196 (M'), 178 (M+ - H20), 168, 167 (base peak); H NMR (CDCI,, 60 MHz) 6 7.15-7.92 (m, 6 H, aromatic), 6.83 (d, J = 12 Hz, vinyl next to naphthalene ring), 5.88 (dt, J = 12, 5 Hz, vinyl next to CHI), 5.16 (m, 1 H, methine), 2.83 (m, 2 H, methylene). TLC analysis on silica gel with 20% ether in ligroin showed one large spot and two tiny trailing spots; further chromatography on silica gel and Florisil did not improve this situation. The H NMR spectrum of this compound is identical with that reported by Meinwald and co-workers for 7,8-dihydrocyclohepta[d,e]naphthalen-7-ol(8)~20,21 (b) Methylcyclohexane, 1 atm O2 (Table I, Entry 6). The irradiation (A >340 nm) of compound 1 (15.6 mg) in 150 mL of purified and distilled methylcyclohexane at -78 "C led to the formation of a large amount of particulate matter. The solution was warmed to room temperature, and the solvent was removed in vacuo without heating. This left a yellow amorphous solid. TLC analysis on silica gel with 12% ether in ligroin showed that the starting material was gone and that no (or very little) hydrocarbon products (2 and/or 3) and no cyclic peroxide (4) had been formed. H NMR analysis confirmed the absence of 2-4 and revealed the presence of the hydroperoxide 6. Preparative Flash Photolysis of 1 in Degassed Methanol. A solution of 300 mg of 1 in 30 mL of degassed spectrograde methanol was photolyzed ( A >340 nm) with 120 flashes until the n** absorption band of 1, monitored by a 1-mm quartz cell attached to the sealed vessel, had disappeared completely. After removal of the solvent, the residue was (20) Ashworth, G.; Berry, D.; Smith, D. C. C. J . Chem. Soc., Perkin Truns.I 1979,2995-3000. We are grateful to Prof. D. Smith, University of Manchester, for providing the original NMR spectra of 7 and 8. (21)-Petty, R. L.; Ikeda, M.; Samuelson, G. E.; Boriack, C. J.; Onan, K. D.; McPhail, A. I.; Meinwald, J. J . Am. Chem. SOC.1978, 100,2464-2474.
dissolved in a small amount of carbon tetrachloride and chromatographed on silica gel. Ligroin (40-60 "C) eluted the monomer fraction (hydrocarbons 2 and 3, ca. 200 mg); a second fraction containing mainly dimers and some higher oligomers of b was eluted with ether (ca. 20 mg). TLC (silica gel, CCI4/hexane, 1:l) and HPLC (reversed phase, methanol/ water, 1O:l or silica gel, hexane) analyses of the dimer fraction indicated the presence of one major (TLC, Rf0.54; ca. 50%) and two minor components (TLC, Rf0.42 and 0.31; ca. 20% each) in addition to some very minor traces. Further separation of the components of the dimer fraction proved to be difficult. Repeated chromatography using a commercial silica gel column (Lobar 40-63 pm; Merck), thin-layer chromatography, and fractional sublimation in vacuo at 120-130 "C finally yielded small samples of each component, which were essentially pure by TLC and H NMR standards. Notably, the head-to-head sym dimer 5 was clearly not present in the reaction mixture; its Rfvalue on TLC was similar to that of the second component (Rf0.42), but it could not even be detected as a trace impurity in the H NMR spectra of that fraction. The first major dimer component was unambiguously identified as a single diastereomer of 7,8-dihydro-7-(7,8,9,lO-tetrahydrocyclohepta[de]naphthalen-7-yl)cyclohepta[de]naphthalene (9) based on the follow-
9 ing spectroscopic data: MS (75 eV) 360 (M', 4), 181 (48), 180 (26), 179 (100); H NMR [(CDCI,, 90 MHz with decoupling experiments, 400 MHz with decoupling experiments, 500 MHz). In the 400- and 500MHz spectra many protons were considerably broadened due to a coalescence phenomenon (presumably ring inversion in the tetrahydrocycloheptene moiety). The 400-MHz spectra were therefore taken at 50 "C, which led to sharp resonances for most protons. The assignments of chemical shifts and coupling constants refer to the structural formula for 9.1 6 7.67,7.71, 7.76,7.80 (each dd, 1 H each, J z 8, 1 Hz, aromatic peri H), 7.2-7.6 (m, 8 H, aromatic), 6.65 (dd, 1 H, J l , 2= 11.5 Hz, Jl,4 = 3 Hz, vinyl HI), 5.52 (ddd, 1 H, J1,2= 11.5 Hz, J2.3 = 8 Hz, J2,4 = 3.5 Hz,vinyl H2), 4.06 (dd, 1 H, Js,6 = 11.5 Hz, J6,7 = 5 Hz, bridgehead H6), 4.01 (ddd, 1 H, J5,6= 11.5 Hz, J3,s J4,s z 2 Hz, bridgehead H5), 3.32 (br, 1 H, HI,), 3.06 (ddd, 1 H, J i i , , 2 = 16 Hz, Jg.12 = 8 Hz, J10.12 = 2 Hz, Hi,), 2.60 (dddd, 1 H, J3,4 = 18 Hz, J2.4 = 3.5 Hz, J1,4 = 3 Hz, J4,5 2 Hz, H4), 2.24 (br, 1 H, H3), 1.69 (m, 2 H, H8 and Hlo), 1.31 (m. 2 H, H7 and H9); UV (hexane) 331 nm (e = 4000), 313 (c = SOOO), 301 ( e = 9000); fluorescence (hexane, excitation 313 nm) 345 nm (sh), 359 (max), 375 (sh). The second dimer (Rf 0.42) proved to be a single stereoisomer of pentadeca2,5-naphtho [ d e f l -1 1,13-naphtho[de] tricyclo[8.3 .2.03*9] 2,4,11,12-tetraene (10): MS (70 eV) 361 (M+ + 1, 28), 360 (M+, loo),
=
H21 \
HZ L / H 2 3
10
332 (18), 179 (31), 165 (33); H NMR [(CDCI,, 400 MHz) All geminal and vicinal coupling constants of protons on saturated carbons and of ortho and meta coupling constants of aromatic protons were assigned on the basis of decoupling experiments. CD2C12was used as a solvent to analyze couplings in the aromatic system. Assignments refer to the structural formula for 10; the assignments of protons H14-Hi6may have to be interchanged with Hl9-HI7.] 6 7.69 (dd, 1 H, J I 7 , I g = 8 Hz, J17,19 = 1 Hz, HI,), 7.67 (dd, 1 H, J15,~.5= 8 Hz, J14.16 = 1 Hz, Hi,), 7.52 (dd, 1 H, J22.23 = 8 Hz, J22.24 = 1 Hz, H22), 7.49 ( 4 1 H, J20.21 = 8 Hz, H21), 7.48 (br d, 1 H, J l g , 1 9 = 8 Hz, HI& 7.36 (dd, 1 H, J,7,1, = 8 Hz, J 1 8 , 1 g = 7 Hz, HI*), 7.35 (dd, 1 H, J14,is = 8 Hz, Ji,,,, = 1 Hz, Hid), 7.32 (dd, 1 H, J I S = , ~8 ~Hz, J14.15 = 8 Hz, His), 7.24 (4 1 H, J20,21 = 8 €32, H20), 7.20 (dd, 1 H, J22,23 = 8 Hz, J23,24 = 7.5 Hz, H23), 7.16 (br d, 1 H, J 2 , 2 4 = 7.5 Hz, Ji.24 = 1 Hz, H24), 4.52 (dd, 1 H, Jl1,13 = J12.13 = 4 Hz, HI,), 4.27 (ddd, 1 H, J 6 , 7 = 12 Hz, J7.g = 6 Hz, J 7 , 5 = 3 Hz, H7), 3.73 (ddd, 1 H, J 7 . g = 6 Hz, J8,p= 5 Hz, Jg.10 = 4 Hz, Hg), 3.52 (br ddd, 1 H, J 1 , 2 = 15 Hz, J1,4= 13 Hz, = 4 Hz, Hl), 2.83 (ddd, 1 H, JIs2= 15 Hz,
Burnett et al.
J. Am. Chem. SOC.,Vol. 110, No. 8, 1988
2530
= 4 Hz, J 2 . 3 = 3 Hz, H2), 2.49 (dddd, 1 H, J11.12 = 14 Hz, J9.11 = 8 Hz, J,o,li 5.5 Hz, J l i , l 3 = 4 Hz, Hit), 2.32 (dddd, 1 H, J 1 1 , 1 2 = 14
J2.4
Hz, Jl0,12 8 Hz, J9.12 7 Hz, J12.13 = 4 Hz, H12), 2.21 (dddd, 1 H, J,,,, = 14 Hz, J9.11 = 8 Hz, Js.9 = 5 Hz, J9.12 = 4 Hz, Hg), 2.15 (dddd, 1 H, J9.10 = 14 Hz, J l o , i 2 = 8 Hz, Jlo,l, = 5.5 Hz, Js,jo = 4 Hz, Hlo), 1.95 (ddddd, 1 H, J3,5 = 14 Hz, J3,4 8 Hz, J3,6 = 7 Hz, J1.3 = 4 Hz, J 2 . 3 3 Hz, H3), 1.76 (dddd, 1 H, J3,5 = 14 Hz, J 4 , 5 = 8 Hz, J5,j = 7 Hz, J5.7 = 3 Hz, H5), 1.60 (ddddd, 1 H, J l , 4 = 13 Hz, J4.6 = 11 Hz, J3.4 = J 4 . 5 = 8 Hz, J2.4 = 4 Hz, H4), 1.15 (dddd, 1 H, J6.7 = 12 Hz, J4,6 = 11 Hz, J5,6 = J3,6 = 7 Hz, H6); UV (hexane) 326 nm (t = IOOO), 303 (sh, e z 7000), 292 (t z 9000), 284 (sh, t z 8000); fluorescence (hexane, excitation 313 nm) 355 nm (max). The third component of the dimer fraction was somewhat unstable to prolonged chromatography and could not be identified: MS 361 (20), 360 (69), 181 (45), 179 (100). Thermolysis of 1 under Oxygen. (a) Toluene, 80 "C, 3 atm O2 (Table I, Entry 15). A solution of 16.3 mg of 1 in 10 mL of reagent-grade toluene under 3 atm oxygen was heated to 80 f 1 "C for 8 days. The solvent was removed and the product mixture analyzed by 200-MHz H NMR spectroscopy in CDC13. Except for some rather large unassigned peaks at B 2.4 (m) and 0.8-1.5 (m), the spectrum could be attributed to unreacted 1 and a mixture of compounds 2-4. (b) Tetrachloroethane, ca. 150 "C, 84 atm O2 (Table I, Entry 16). A solution of 20 mg of 1 in 5 mL of tetrachloroethane was put under 84 atm of oxygen pressure in a steel autoclave and was heated to ca. 150 "C for 2 h. This treatment gave a very complex reaction mixture containing at best a trace of any of the compounds 1-4 (TLC; 90-MHz H NMR). Similar pyrolysis under 90 atm nitrogen resulted in a clean conversion to the hydrocarbons 2 and 3; the latter products were stable to the pyrolysis conditions under 50 atm oxygen. Actinometry. Monochromatic light for irradiation was obtained by isolating the desired lines from a Hanau St. 41 stabilized mediumpressure mercury arc with the appropriate band-pass glass filters. The light flux entering the sample cells was determined by actinometry with azobenzene in methanol solution as described by Gauglitz and Hubig.22 It should be noted that in eq 14-16?2a used for the computer-aided analysis of the spectrophotometric data, the photokinetic factor should be defined as F = E'/( 1 - lo-='), the inverse of the definition of F given in eq 1.22a For actinometry at 405 nm the azobenzene solutions were preirradiated at 313 nm in order to produce larger absorbance changes upon irradiation at 405 nm. The following pseudo quantum yields Q = tft& t'c&t (ett and dCare the extinction coefficients of irons- and cis-azobenzene, respectively, at the wavelengths of irradiation) were used for actinometry:22bQ(313) nm) = 3700, Q(365 nm) = 130, Q(405 nm) = 530 M-' cm-I. For irradiations at 313 and 365 nm actinometry was also done with Aberchrome 540.23 The light fluxes determined by this method were about 10% lower than those obtained with azobenzene. Since we earlier found a quantum yield slightly in excess of 1 for nitrogen elimination from the azo precursor 11 of the biradical d by using Aberchrome 540,* the quantum yields reported here are based on azobenzene actinometry.
01
-
\y \, hi"'-
+
Results
Spectroscopic Detection of the Triplet Biradical 3ba t 77 K. The azo compound 1 was dissolved in an EPA solvent mixture (1.5 X M 1, diethyl ether/isopentane/ethanol,5:5:2) coaled to 77 K in an optical quartz dewar, and the rigid glassy solution was irradiated with monochromatic light of 405 nm. The progress of the photoreaction was monitored by recording the absorption spectra of the glass between the periods of irradiation; no spectral changes occurred during these dark intervals. Previous preparative work had shownzbthat irradiation of 1 in a rigid glass a t 77 K gave divinylnaphthalene (3) as the exclusive photoproduct and indeed, after exhaustive irradiation, the absorption spectrum was identical in shape and intensity with a spectrum of authentic 3 at a concentration of 1.5 X lo-' M. That the clean transformation 1 3 was, however, not a simple photoreaction is evident from the sequence of spectra displayed in Figure 1: there are no isosbestic points, and the spectra taken in the course of the reaction exhibit additional features not present in the spectra of either 1 or 3, namely, a weak, highly structured absorption band system in the visible (A, 512 nm) and a strong band system in the
-
(22) (a) Gauglitz, G.; Hubig, S. Z . Phys. Chem. (Munich) 1984, 139, 237-246. (b).Gauglitz, G.; Hubig, S. J . Photochem. 1985.30, 121-125. (c) Persy, G.; Wirz, J. EPA Newsletter 1987, 29, 45-46. (23) Heller, H . G.; Langan, J . R. J . Chem. Soc., Perkin Trans. 2 1981, 341-343.
(24) Malinowski, E. R.;Howery, D. G. Facfor Analysis in Chemistry; Wiley: New York, 1980. (25) Maeder, M.Anal. Chem. 1987.59, 530-533. We are most grateful to Dr. Maeder for valuable advice and support. (26) Borgen, 0. S.; Kowalsky, B. R. Anal. Chim. Acta 1985, 174, 1-26, Figures 17-20,
J . Am. Chem. SOC.,Vol. 110, No. 8, 1988 2531
1 .I-Perinaphthadiyl Reactivity 300
250
350
400
450
500 550600
475
500
21
20
525
550
I
0 2
I ' ,I
I
I
3 5
4 0
3 0
2 5
2 0
Figure 2. Absorption, fluorescence, and fluorescence excitation spectra of 3b in EPA at 77 K. The absorption and excitation spectra were
practically superimposable; therefore, only part of the latter is shown for clarity. The ordinate scale is based on the analysis described in the text (cf. Figure 3).
23
22
19
18
Figure 4. Resolution of the orthogonal components in the first absorption band of 3b by photoselection. The upper curve (absorbance parallel to the direction of polarization of the photolyzing light I >510 nm) is shifted upward by 0.1 absorbance unit.
Xl 1
8
6
4
2
0
-
Figure 3. Concentration profiles: the mole fractions X,of 3b and 3 are plotted vs the mole fraction of 1 during the photoreaction 1 3 in EPA at 77 K.
be analyzed as a mixture of 1 and 3 a t a total concentration that was equal to the initial concentration of 1. Thus, no optically transparent products or intermediates other than molecular nitrogen were formed. The light-sensitive intermediate (Figure 2) has been identified as the triplet biradical 1,4-perinaphthadiyl (3b)." A semiempirical calculation for the triplet-triplet absorption spectrum of the parent chromophore 3a predicted that the weak absorption band near 500 nm should contain two weak absorption bands of opposite polarization.*' This was confirmed experimentally by the method of photoselection:2s a sample of randomly oriented 3b in poly(methy1 methacrylate) glass a t 77 K was partially bleached through a polarizer and a cutoff filter (A >510 nm). The remaining sample of 3b was distinctly dichroic and could be decomposed into two orthogonal spectra (Figure 4). Similar results were obtained by the irradiation of 1 with polarized light, which directly produced a dichroic sample of 3b.6 Two other spectroscopic signals appeared when 1 was irradiated at 77 K. First, a green fluorescence emission was observed with a lifetime of 190 f 10 ns. The intermediate absorption and emission were clearly due to the same species: the sharp 0-0 transitions of absorption and emission nearly coincided at 51 2 nm, and the fluorescence excitation spectrum agreed well with the absorption spectrum (Figure 2). Second, ESR spectroscopy of irradiated frozen solutions of 1 in benzene, hexafluorobenzene, (27) Gisin, M.; Wirz, J. Helu. Chim. Acta 1983, 66, 1556-1568. (28) Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light. Solute Alignment by Photoselection, in Liquid Crystals, Polymers, and Membranes;VCH: New York, 1986. Michl, J.; Thulstrup, E. W. Arc. Chem. Res. 1987, 20, 192-199.
poly(methy1 methacrylate) glass, EPA, or ethanol a t 77 K gave a weak signal a t half-field and a broad spectrum centered near g = 2; the pattern was characteristic of randomly oriented molecules in the triplet state (zero-field splitting parameters ID/hcl = 0.018 cm-' and IE/hc( < 0.003 cm-I). All three signals of the intermediate (absorption, emission, ESR) were wiped out by further irradiation a t X >450 nm or when the sample was thawed. Naphthocyclobutane (2) was practically stable to irradiation with conventional light sources at 77 K. It exhibited strong fluorescence and phosphorescence emissions typical of an alkylsubstituted naphthalene. However, when 2 was photolyzed a t 77 K with a few high-intensity flashes from an excimer laser (248 nm, ca. 100 mJ/flash, flash width a t half-height ca. 20 ns), 3b was readily detected and unambiguously identified by its characteristic, long-lived and structured fluorescence emission a t wavelengths X 2 5 1 2 nm. Quantum Yield Determinations. Quantum yields +(-N2) for the photoelimination of nitrogen from the azo compound 1 a t ambient temperature were determined spectrophotometrically by monitoring the disappearance of the n a * absorption band of 1 a t ,,A 410 nm, where none of the photoproducts had any abM. Azobenzene sorption. Initial concentrations of 1 were ca. was used as an actinometer (cf. the Experimental Section).22 The direct irradiation of a hexane solution of 1 at 405 nm gave values within the range of +(-N2) = 0.56 f 0.02 (six determinations). Outgassing the solutions had no effect on the quantum yield of disappearance of 1, but it did change the product distribution (see below); the addition of 0.2 M dimethylmercury or piperylene (up to neat) did not affect either. A single determination of the quantum yield for irradiation of 1 a t 313 nm, Le., in the region of the naphthalene a** absorption band, gave a value of +(-N2) = 0.6. In order to determine the quantum yield +(-Nz) upon triplet sensitization of 1, a solution of 1 X M 1 with 2 X M benzophenone in degassed acetonitrile was irradiated a t 365 nm. Over 95% of the light was absorbed by the sensitizer, and control experiments by laser flash photolysis (excitation at 353 nm) showed that the triplet energy transfer from benzophenone to 1 was >99% efficient a t low conversions, since the benzophenone triplet (A, 530 nm) was quenched by 1 with a rate of ca. 8 X lo9 M-' s-l. Under these conditions, quantum yields in the range of +(-N2) = 0.32 f 0.02 were determined, again by monitoring the disappearance of 1 spectrophotometrically. Considering the possible systematic errors involved in actinometry at different wavelengths, we conclude that the photochemical quantum yields for direct irradiation in the n a * (405-nm) and H P * (313-nm) absorption
Burnett et al.
J . Am. Chem. SOC.,Vol. 110, No. 8, 1988
2532 Table I
product yields/% condn,b 02, solventa TI'C Platm 2 3 4 6 42/43 4ox/4wc 16 5.3 0 hu, 20 0 84 1 Ph 0.35 3.4 hu, 20 17 26 0 1 57 Ph 1 1.25 6.5 57 0 5.7 4.6 36.5 hu, 20 Ph 1 2.2 0.31 24c hu, 20 1 52' 24c 0 1-d2 Ph 0.91 hu, 20 12 47 0 3.4 1 41 1 Me-cC6HI, OD 0 0 1 Me-CC6Hl I hv, -78 1 0 yes 0 87 13 6.7 0 1 MeOH hu, 20 0 >98d >49 0 50 hu*, -78 11 1 MeOH 9.0 0 A, 160 0 90 10 12 1 1-ClNp 4.9 A, 140 0 0 0 0.2 83 17 13 1 1-C1Np 0 0 1 88 7.3 0 12 14 1 A, 160 1-C1Np A, 80 3 53d 41df 0 0.71 8.8 6* MePh 15 1 mg 84 0 0 16 1 A. -150 ClCld E E 'Ph = benzene, M ~ - c C ~ = H methylcyclohexane, I~ MeOH = methanol, 1-CINp = 1-chloronaphthalene, MePh = toluene. b h u = irradiation by a medium-pressure mercury arc through a UV cutoff filter (A >380 nm), hu = irradiation through pyrex (A >340 nm), A = pyrolysis. 'Yields refer to dideuterio derivatives of 2-4 (Scheme I). dThe quoted percent yields are relative due to the formation of other products. 'Taken from ref 2b. /The peroxide 4 was identified solely by H NMR, in particular by its characteristic peak at 6 5.4, as an authentic sample of 4 was not available at the time of the experiment. gCompounds 4 and 6 are unstable under these conditions. no. 1 2 3 4 5 6 7e
reactant
bands of 1 are identical within experimental error but that the value obtained for the sensitization of 1 is significantly lower. Flash Photolysis. The biradical 3b was formed within 50 ns of the excitation flash and was easily identified by its characteristic absorption bands a t 340 nm (strong) and 512 nm (weak). The rate law for the decay of the transient 3b was complex, consisting of both a second-order and a first-order component. When high concentrations of the transient were produced by intense light flashes, the decay was initially dominated by the second-order component. This was unavoidable when the very weak first absorption band of 3b was monitored in the visible region, because high concentrations were needed in order to obtain reasonably strong signals. Apart from this technical limitation, the decay kinetics a t 340 and a t 512 nm were the same, in further support of the previous claim that both of these bands were due to the same chemical species. In some experiments, up to 0.1 M piperylene was added to the solution in order to make sure that long-lived triplets such as, e.g., 32or 33did not contribute to the transient absorption. This had no effect on the decay kinetics. The decay approached first-order kinetics when the concentration of 3b was low, i.e., with flashes of low intensity or toward the end of the decay, and the decay rate 3kIsCwas then found to be close to 5 X lo3 s-l in degassed solvents of widely different polarity (hexane, benzene, acetonitrile, methanol). The second-order component was suppressed in highly viscous solvents; the lifetime of 3b was 260 ks in poly(methy1 methacrylate) glass and 180 ~s in glycerol a t ambient temperature. Therefore, glycerol was used to determine the temperature dependence of the first-order decay rate (Table 11). Second-order analyses of the initial parts of the decay curve gave k/ed 1 X lo6 s-l. The extinction coefficient t(512 nm) of 3b is close to lo00 M-' cm-I at 77 K; we have reduced this value to 500 M-I cm-l in order to account for the loss in fine structure a t ambient temperature. This gave a value of approximately 5 X lo9 M-l s-l for the absolute second-order decay rate of 'b; a value close to the diffusion-controlled limit was expected from the fact that this reaction could be effectively suppressed in viscous solvents. The observation of a fast second-order annihilation of 3b suggested that a t sufficiently high concentration a dimerization of the biradicals 'b was able to compete with the monomolecular decay to 2 and 3. A sample of 300 mg of 1 was therefore photolyzed by 120 flashes from the conventional discharge lamps. Chromatographic separation of the products showed that, in addition to the usual hydrocarbons 2 and 3 (200 mg), a new fraction (1 5 mg) was indeed formed that contained several dimers of b. Two new products, which amounted to ca. 70% of the dimer fraction, could be isolated and were identified by high resolution
Table 11. Constants for the Reactions of 'b and )b at Room Temperature (cf. Scheme 11) and Arrhenius Parameters
log
Eal
kcal (Ais-') mol-' =7 52 6.9 4.5
reaction svmbol rate' 3b 1krsc =l x 108 s-1 'b 3krsc 5 x 103 s-1 2.5 X 1O'O M-' s-' )02 )b + '02 'kox + '02 4 or 6 )kOx 2.4 X lo9 M-l s-' 3b + 'b 9 or 10 3kDim 2 5 X lo9 M-' s-' 'b-2 ikl =i x 109 s-l 13" =5 'k, =2 x 108 s-1 13' s7 'b+3 "The rate constant 'koxis assumed, and the values for 'kIsc, ' k z , and 'k3depend on this assumption; see text for explanations.
'b )b 'b )b
-+ --f
H NMR spectroscopy as one single diastereomer each of the dimers 9 and 10. It is noteworthy that the known [4 41 dimer SI9 was not present in the reaction mixture.
+
9( 50%) h
lO(20'M
5 ( 0%)
Figure 5 shows the difference in absorbance before and 12 ks after photolysis by a conventional discharge flash lamp of a deM solution of l in hexane containing l X gassed 3 X M piperylene. In addition to the characteristic sharp absorption band of 3b a t 340 nm we note the negative peak around 410 nm, which arises from the depleted absorbance of 1 in that region. An important conclusion may be drawn from this difference spectrum. It is apparent that the intensity of the transient absorbance by 3b a t 340 nm is comparable to that of the bleaching a t 410 nm. Since the extinction coefficient of 3b a t 340 nm is much larger than that of 1 a t 410 nm, it follows that the amount of triplet biradical 3b detected in solution corresponds to only a small fraction of the azo compound 1 depleted by photolysis. The spectrum was taken with a sufficiently small delay after the photolysis flash such that only a small fraction (525%) of 3b could escape detection. As noted above, the extinction coefficient is probably lower a t room temperature than the value of ~ ( ~340 b ,
J . Am. Chem. Soc., Vol. 110, No. 8, 1988 2533
I .I-Perinaphthadiyl Reactivity Scheme 1
H H
H
H
320
350
400
420
h/nm
Figure 5. Spectrographic flash photolysis of 1 in degassed methanol. The AA spectrum displays the difference in absorbance before and 12 ws after the peak of the flash.
nm) = 36000 M-' cm-' determined a t 77 K due to the usual blurring of the vibrational fine structure. If we use a reduced extinction coefficient of 20000 M-I cm-' and allow for a 25% loss of 3b at the time the spectrum was taken, we still calculate that the yield of 3b relative to the amount of 1 depleted is only 10%. This determination shows in a very direct way that the major path for nitrogen elimination from 1 upon direct irradiation a t room temperature does not proceed via the intermediate 3b, which is formed in a yield on the order of (but less than) 10%. Accordingly, the intensity of the transient absorption of 3b did not decrease when benzophenone was added as a sensitizer, despite the lower quantum yield for sensitized nitrogen elimination. The lifetime of 3b was reduced by 3 orders of magnitude to 200 ns in air-saturated solution. A plot of the observed first-order decay rate against the partial pressure of oxygen in equilibrium with a methanol solution of 1 was linear (5 points); assuming Henry's law and using a value of a = 1.02 X lo-* mol dm-3 atm-' for the solubility of oxygen in methanol at 20 0C,29the secondorder rate constant for the quenching of 3b by triplet oxygen was determined from the slope of the linear regression line as 3kox= (2.4 f 0.3) X lo9 M-I s-I. The fivefold higher value for 3kox reported previously had been determined by an inadequate techniq~e.~ Oxygen Trapping Experiments. (a) Photolyses. Since over 99% of the triplet biradicals 3b were quenched by triplet oxygen in aerated solution, it was important to know what the chemical consequences of this quenching were. Preliminary experiments, in which 60-MHz H N M R spectroscopy had been used to analyze the product mixtures, suggested that the effect of oxygen was to change the relative yield of the hydrocarbon products 2 and 3.30 Subsequent scrutiny showed that in fact a new compound had been formed in addition to 2 and 3. The new oxidation product could be separated from the hydrocarbons by thin-layer chromatography, but this was not feasible by column chromatography. The available spectroscopic data suggested that the new product was the cyclic peroxide 4, and this compound was therefore synthesized by an independent procedure.
4
It was then easy to confirm that photolysis of 1 in benzene at room temperature under an oxygen atmosphere indeed produced the same compound 4. Furthermore, it was obvious why 4 had not been detected in the 60-MHz spectrum of the product mixture; the resonances arising from 4 were buried by those of 2 and 3. (29) Schafer, K.; Lax, E. In Landoh-BGmstein, 6th ed.;Springer-Verlag: Berlin, 1962; Vol. 11, Part 2. Wilhelm, E.; Battino, R. Chem. Rev. 1973, 73, 1-9. Battino, R.; Rettich, T. R.; Tominaga, T. J . Phys. Chem. Ref. Data 1983, 12, 163-178. (30)Footnote 12 of ref 4.
1 1 CIS : -trans 2 2
D D
H
D
H
D
1
1
However, at 200 M H z and above, the bridgehead and methylene signals due to 2 and 4 and the terminal vinyl hydrogens of 3 were all resolved. Integration of these peaks in the product mixture showed that it contained 26% of the cyclic peroxide 4 in addition to the two hydrocarbons 2 and 3 (74%), which were formed in a ratio similar to that found in the absence of oxygen. No resonances due to compounds other than 2-4 were present in the H NMR spectrum, and control experiments showed that 2-4 were stable to the irradiation conditions used for their formation. Further oxygen trapping experiments, which were performed with different solvents, different oxygen pressures, and lower temperatures, were analyzed similarly as described in the Experimental Section. The results, together with some pertinent results of previous work:b are collected in Table I. A number of points need explicit comments. Previous work with the deuteriated azo compound 1-dz has shown that its thermal and photochemical decomposition proceeds through a planar or effectively (by conformational equilibration) planar intermediate.2b It was now of interest to know the effect of oxygen on the stereochemistry of the reaction. Photolysis of 1-d2 in benzene under an oxygen atmosphere at room temperature gave a product distribution similar to that observed with 1, except that the amount of divinylnaphthalene (3-d2)was abnormally high (24%). Owing to the small ambunt of deuteriated sample available, we have not attempted to establish whether this difference is real and does represent an isotope effect on the product distribution. The analysis of the products' NMR spectra after chromatographic separation did, however, leave no doubt that the deuterium label was randomized not only in the hydrocarbon products 2-d2 and 3-d, but also in the peroxide 4-d2,Le., half cis and half trans in 1,8-divinylnaphthalene and half exo-cis and half endo-cis in the other two products (Scheme I). One must conclude that an effectively planar transient is involved in the formation of all three products. In methanol solution, yet another oxidation product, the hydroperoxide 6,was formed in the presence of oxygen. This was found to be the major product (up to 70%) when an oxygenated solution of the azo compound 1 in methanol was irradiated (A >340 nm) at dry ice/acetone temperature. No hydrocarbons (2, 3) or cyclic peroxide ( 4 ) were formed under these conditions, and we believe that the unidentifiable 30% side products were formed by secondary photolysis of the new compound. These results should be contrasted to those obtained in the absence of oxygen where the isomeric hydrocarbons 2 and 3 and the dimers 9 and 10 were formed
+
gH 8
20%
2534
J . Am. Chem. Soc., Vol. 110, No. 8, 1988
Table 111. Comparison of H NMR Spectra of 6 and 8 in CDCll
Burnett et al. Scheme I1
n = 1 alcohol 18) n = ?. hydroperoiydr ( 6 )
hydroperoxide 6 alcohol 8 chem shift, multiplicity," chem shift, multiplicity," assignment d/ppm J/Hz G/ppm J/Hz 7.1-7.7 m aromatic 7.1-7.9 m 6.6 d (12) H, 6.75 dd (1 1.8, 2.4) 5.8 dt (12, 5) H2 5.91 dt (11.8, 3-4) 2.7 m H,, H, 3.00 m 5.0 m H5 5.40 m OH not assigned 3.0 br s 'm = multiplet, d = doublet, dd = doublet of doublets, dt = doublet of triplets, br s = broad singlet. The hydroperoxide 6 could not be isolated due to its instability on silica gel but was identified on the basis of spectroscopic data obtained with the crude product mixture and of chemical evidence as follows. The infrared spectrum of the photolysis mixture in CH2CI2showed an OH stretching band a t 3500 cm-I, a value typical for hydr~peroxides.~'The H N M R spectrum of the major component, whose resonances were easily distinguished from the much smaller resonances associated with impurities, was similar to the spectrum of the corresponding alcohol 8 (Table III).20*21 As the compound did not survive column chromatography, the reaction mixture was reduced with LiAIH,; this gave the cyclic ether 7 and the alcohol 8 as the major products. The mass spectra and H N M R spectra of these products were identical with those of authentic samples.20q2' The cyclic ether 7 most likely arose by acid-catalyzed cyclization of the alcohol 8 during the hydrolysis of the lithium salts formed in the reaction. The hydroperoxide 6 was stable in methanol and deuteriochloroform a t room temperature for extended periods of time but decomposed completely in benzene within less than 12 h. Thus, had the hydroperoxide been formed in the photodecomposition of the azo compound 1 in benzene under oxygen a t room temperature, it would probably not have been detected. The formation of 6 in benzene is, however, unlikely because all the resonances in the 200-MHz H N M R of the product mixture could be assigned to the known compounds 2-4. Control experiments have shown that both 4 and 6 are sensitive to short-wavelength irradiation but that 4 is not converted to 6 or vice versa. (b) Thermolyses in the Presence of Oxygen. Pyrolysis of the azo compound 1 in 1-chloronaphthalene in the presence of air at 140 O C and in oxygen-saturated 1-chloronaphthalene at 160 O C did not yield any oxygenated products; the two hydrocarbons 2 and 3 were formed in a ratio similar to that found upon thermolysis a t 160 O C in the absence of oxygen (Table I, entries 12-14). Heating a solution of 1 in toluene to 80 "C under 3 atm of oxygen pressure for 8 days yielded a complex product mixture that in addition to unreacted 1 and the hydrocarbons 2 and 3, contained an equal amount of the cyclic peroxide 4 (Table I, entry 15). The hydrocarbons 2 and 3 could not be detected in the product mixture formed by pyrolysis of 1 a t ca. 150 O C under 84 a t m of oxygen pressure, although both 2 and 3 could largely be recovered after exposure to such conditions. Thin-layer chromatograms indicated an extremely complex product mixture but were empty at low retentions where 2 and 3 would have appeared.
Discussion The goal of this work was to gain information about the reactivity of the singlet and triplet state of the biradical 1,4-perinaphthadiyl (b) and to study how its reactions depend on the solvent, the temperature, and the presence of trapping reagents. (3 1,) Nakanishi, K. Infrared Absorption Spectroscopy; Holden-Day: San
Francisco, 1962. Bcllamy, L. J. The Infrared Specrra of Complex Molecules, 2nd ed.; Chapman & Hall: London, 1980; Vol. 2.
N=N
g l 1
4
aooH +
6
Scheme I1 shows the most economical reaction mechanism, which can account for all the results and shall be defended below. The product yields obtained under various reaction conditions are collected in Table I and the rate constants determined or estimated for the reactions of singlet and triplet b a t room temperature are given in Table 11. Photoreactions at Low Temperature. 1,4-PerinaphthadiyI was first detected by its characteristic ESR spectrum, which was observed after the photolysis of 1 in rigid glassy solvents a t low temperature.2b Originally, the ESR signal intensities of samples of b, as well as those of the related biradicals ah and c , were ~ found not to be proportional to 1/ T, the temperature dependence expected for molecules with a triplet ground state (Curie's law). These findings were attributed to a temperature-dependent change in the Boltzmann population of the singlet and triplet state, and it was concluded that these biradicals have a singlet ground ~ t a t e . ~ . ~ However, an effort to detect corresponding changes in the absorption spectrum of b as a function of temperature has failed, and subsequent reinvestigations of the ESR spectra showed that the deviations observed previously must have been artifacts; provided that microwave powers below 10 FW were used, significant deviations from Curie's law were not observed down to ca. 10 K.4,5a,9aStrictly speaking, all of these observations still do not prove that the triplet state is the ground state of b; they are also consistent with the singlet- and triplet-state energies being equal to within a few tens of calories per mole. Such a close accidental degeneracy would, however, be quite fortuitous for any single compound in one solvent. Since the singlet state should be much more susceptible to substituent and solvent effects than the triplet state,Id it is highly improbable that such a degeneracy would occur for several derivatives and in various solvents of widely different polarity. Platz has reported Curie law behavior even for heteroatom-substituted derivatives of a.5b,5dWe conclude that the triplet state is the ground state of b. The exhaustive irradiation of 1 in a rigid glassy solution a t 77 K yields a single product, divinylnaphthalene (3). This reaction proceeds in two photochemical steps, as can be seen from the concentration profiles obtained by spectrophotometric analysis (Figure 3): The product of the first photoreaction is the triplet biradical 3b, which is stable in the dark at 77 K. The final product 3 is formed only after an "induction period", when the biradical intermediate 'b begins to accumulate. This explains the striking trend reversal found in the relative yield of products 2 and 3 as the reaction temperature is lowered. In fluid solution 3b decays thermally and yields predominantly 2, and the selectivity increases as the temperature is lowered. At 77 K, however, )b reacts photochemically giving 3. Thus, the product ratio of 2 to 3 increases from a.5 at 20 "C to >SO at -78 O C and then suddenly drops to zero a t 77 K (Table I ) . The preparative work then provides independent and more quantitative evidence for the qualitative spectroscopic observation that 3b is the only product
J . Am. Chem. Soc., Vol. 110, No. 8, 1988 2535
I , 4 - Perinaph thadiyl Reactivity formed by the irradiation of 1 at 77 K ana' that thefinal product 3 is formed only by secondary photolysis of 3b. The question, a t which point intersystem-crossing to the triplet manifold takes place, will be discussed below. At this point we note only that entirely different results were recently obtained with the related azo compound 11? The irradiation of 11 a t 77 K gave the naphthocyclopropane 12 directly, and the triplet biradical 1,3perinaphthadiyl (3d) was not formed (340 nm) used at -78 O C , the results of the product analyses from the reactions under oxygen (Table I) must be taken with a grain of salt. Nevertheless, two important and clear-cut trends are obvious from these data: (a) With sufficient oxygen pressure it is possible to suppress the formation of hydrocarbons entirely in both the photochemical and the thermal decomposition of 1 and over the whole temperature range investigated (+160 to -78 O C ) . At room temperature the yield of 4 increases from 26% at 1 a t m to 57% a t 4.6 a t m of oxygen pressure. (b) Under 1 a t m oxygen, the yield of oxidation products increases with decreasing temperature from virtually nil at 160 O C , to 2 5 5 0 % (depending on the solvent) at room temperature, and to near 100% a t -78 O C . These results cannot be attributed simply to the trapping of 'b by oxygen. If this were the only way oxygen interfered in the reaction, then we would expect a t most a 10% yield of peroxide, since the yield of 'b is only 10%. Furthermore, from the direct observation of 3b by flash photolysis, we know that oxygen quenching is by far the dominant decay process (99.9%) of 'b in air-saturated solution. Therefore, this process will not contribute to a higher trapping efficiency when the oxygen pressure is further increased. The observed increase in the yield of peroxides at elevated oxygen pressure requires that oxygen interacts with some further intermediate other than 3b. We have shown above that nitrogen elimination from the excited singlet state of 1 occurs on a subpicosecond time scale, too fast to allow for diffusional encounters of S l ( l )with oxygen under ordinary pressures, and in any case, S l ( l )cannot be involved in pyrolysis. On the other hand, there is evidence that the singlet biradical 'b is a true intermediate. An encounter between 'b and triplet oxygen could lead to the formation of peroxides either directly or by inducing ISC of 'b to 3b, followed by diffusional trapping of 'b by another molecule of oxygen, as shown in Scheme 11. Application of the steady-state approximation to the kinetic Scheme I1 gives the following expression for the ratio of the quantum yields of peroxide formation (4 and 6 ) to hydrocarbon formation (2 and 3) as a function of oxygen concentration:
-
_ (box 4HC
'~O~[OZI('~ +I'Sk C ~,[Od)
('h+ 1 k 3 ) ( 3 k ~+s ~3ko~[ozl)
In air-saturated solution and at higher oxygen pressures the lifetime of 3b is dominated by oxygen quenching, 'kSC
