J. Org. Chem. 1993,58, 1477-1482
1477
Determination of the Triplet Lifetimes of 1,3-Cyclopentadiyl Biradicals Derived from the Photodenitrogenation of Azoalkanes with Time-Resolved Photoacoustic Calorimetry Waldemar Adam,',? Herbert Platsch,t Jurgen Sendelbach,? and Jakob Wirz'J Institut fur Organische Chemie der Uniuersitat Wurzburg, Am Hubland, 0-8700Wurzburg, Germany, and Institut f u r Physikalische Chemie der Uniuerstiat Basel, Klingelbergstr. 80, CH-4056 Basel, Switzerland Received November 6, 1992
The triplet lifetime of the parent l,&cyclopentanediy1(2a) and the 4-methylene and the 1-phenyl derivatives 2b and 2c were determined by time-resolved photoacoustic calorimetry (PAC). For the biradical2b, PAC was the only suitable method to date for determining its hitherto inaccessible triplet lifetime. A comparison with results for biradicals 2a,c obtained in earlier studies shows that PAC is a reliable method for measuring triplet lifetimes of biradicals. The effect on the triplet lifetimes of allyl and benzyl conjugation at one radical center in 2b,c proved to be insignificant. In view of the large enhancement of the biradical lifetime as the result of benzylic conjugation at both radical sites, as in the diphenyl derivative 2d, the present results imply that the biradicals 2b,c are still flexible enough to enable effective intersystem crossing through pyramidalization. The much larger (ca. 200-fold) triplet lifetime of biradical2b compared to itagem-dimethyl derivative 28 suggests that geminal substitution is a general phenomenon for the effective reduction of the lifetimes of triplet biradicals through changes in the singlet-triplet energy gap. Introduction Biradicals are the focus of numerous mechanistic studies.' Especially popular are the l,&cyclopentanediyl biradicals,which are generated conveniently and efficiently by denitrogenation of cyclic azoalkanes (Scheme I) and have received recently much attention.lf-j. The corresponding triplet biradicals (T)are obtained by benzophenone-sensitized photolyses of the azoalkanes, whose lifetimes and the factors which govern them are of major concern in mechanistic discussions. Besides time-resolved spectroscopy (most frequently UV detection,Ie but also ESR,'d IR," Raman,'j etc.) indirect methods such as trapping with dioxygen (competition kinetics)lh and have been employed nitroxideslkor "free radical clocksn1f~2 to detect the transient triplet biradicals and to estimate their lifetimes. The latter methods have the advantage that biradicals, which lack a chromophore and thereby are "invisible" for transient spectroscopy, can be studied; however, detailed chemical analysis of the products and of the kinetics are necessary to extract the desired lifetime data, and the results have to rely on some assumption about the transferability of a benchmark rate constant (of ring opening or oxygen trapping). Institut fiir Organische Chemie, Wiirzburg. Institut fiir Physikalische Chemie, Basel. (1) (a) Dauben, W. G.; Salem, L.; Turro, N. Acc. Chem. Res, 1975,8, 41. (b) Wirz, J. Pure Appl. Chem. 1984,56,1289. (c) Borden, W. T., Ed. Diradicals; Wiley-Interscience: New York, 1982. (d) Buchwalter, S . L.; Closs, G.L. J. Am. Chem. SOC.1979, 101, 4688. (e) Caldwell, R. A. In Kinetics and Spectroscopy of Biradicals and Carbenes; Platz, M. S., Ed.; Plenum: New York, 1990. (0 Engel, P. S., Culotta, A. M. J.Am. Chem. SOC.1991,213,2686 and references cited therein. (g) Engel, P. S. Chem. Rev. 1980,80,99. (h) Adam, W.; Grabowski,S.; Wilson,R. M. Acc. Chem. Res, 1990,23,165 and references cited therein. (it Fisher, J. J.; Penn, J. H.; Dohnert, D.; Michl, J. J.Am. Chem. SOC.1986,108,1715. (j) Adams, J. S.; Weisman, R. B.; Enge1,P. S. J. Am. Chem. SOC.1990, 212,9115. (k) Adam, W.; Bottle, S. Tetrahedron Lett. 1991, 1405. Adam, W.; Bottle, S.; Finzel, R.; Kammel, T.; Peters, E. M.; Peters, K.; von Schnering, H. C.; Walz, L. J. Org. Chem. 1992, 56, 982. (2) (a) Adam, W.; Ganther, E.; Hbsel, P.; Platsch, H.; Wilson, R. M. Tetrahedron Lett. 1987.4407. (b) Adam. W.: Grabowski. S.: Scherhae. G.Tetrahedron Lett. 1988,5637. (c) Griller, R.; Ingold, K.U. ACC.Chem: Res. 1980, 23, 317. +
t
Scheme I
0"& X
8: X
- CH2, R
H, b: X ->C=CH*, R
-
5-2
3 R
1-2
10
H; C: X
CH2, R
Ph
I
Time-resolved photoacoustic calorimetry ( P A W is a direct method to determine lifetimes of high-energy transient intermediates, which does not require a chromophore for spectroscopicdetection. We have determined the lifetime of the triplet biradicals 2a-c in solution by PAC to assessthe effects of substituting one of the localized by radical centers in the parent 1,3-~yclopentanediyl(2a) a conjugating allyl or a benzyl function as in 2b,c and present herein our results. Some aspects of the biradical 2b have already been examined under matrix isolation4 and by dioxygen trapping;5thus, ESR studies4 have revealed a triplet ground state for 2b, while the singlet-triplet energy gap was estimated to be 2.7 kcal/mol and the activation barrier for intersystem crossing 2.2 kcal/mol.5 Results Synthesis of the Azoalkanes. Azoalkane la was readily available by following the published procedure.6a (3) (a) Braslavsky, S. E.; Heibel, G.E. Chem. Reu. 1992,92, 138. (b) Caldwell, R. A.; Melton, L. A.; Ni, T. J. Am. Chem. SOC.1989, 11, 457. (c) Goodman, J. L.; Herman, M. S . J.Am. Chem. SOC.1988, 110, 2681. (4) Dowd,.P.; Ham, S. W.; Chang, W.; Partian, C. Mol. Cryst. Li9. Cryst. 1989, 276, 13. (b) Dowd, P.; Ham, S . W.; Chang, W. J. J. Am. Chem. SOC.1989, 221,4130. ( 5 ) (a) Roth, W. R.; Bauer, F.;Braun, K.; Offerhaus, R. Angew. Chem. 1989,201, 1092; Angew. Chem., Znt. Ed. Engl. 1989,28,1056. (b) Roth, W. R.; Bauer, F.; Breuckmann, R. Chem. Ber. 1991,124, 2041.
0022-3263/93/1958-1477$04.00/00 1993 American Chemical Society
1478
Adam et al.
J,Org. Chem., Vol. 58, No.6, 1993 Scheme I1 i~ 140-160 ocio.ot
torr c
2. MTAD, CH2C12, -20 *C
coza5
4b‘
H1 Pd/C, EtOAC
-
L,N
99%): lH [2.1.O]pe11tane(3b)~~ NMR (250 MHz, toluene&) 6 0.68-0.75 (m, 2 H, CHd, 1.68-1.74 (m, 1H, CH), 1.84 ( d t , J = 13.3,1.8 Hz, 1H, CHz), 1.98-2.08 (m, 1 H, CH), 2.42 (dm, J = 13.3 Hz, 1HI CHzh4.44-4.48 (m, 1 H, HzC=), 4.70 (t,J = 1.8 Hz, HzC=); 13CNMR (50 MHz, C a d IS 14.0 (d), 19.4 (t), 25.3 (t),33.3 (d), 101.3 (t), 147.9 (8). Sensitized Photolysis of Azoalkane l b under Oxygen Gas Pressure. A solution of the azoalkane l b (142 mg, 1.31 mmol) and benzophenone (250 mg, 1.37 mmol) in 15 mL of CFC13 in a Griffin-Worden tube was cooled to -25 "C under oxygen gas pressure (7 atm). The solution was allowed to equilibrate thermally for 30 min prior to irradiation with the 364-nm line of the argon ion laser (UV output 0.9-1.0 W). After irradiation for 4 h, removal of the solvent at 0 OC/15 Torr, and silica gel chromatography at -25 "C (3:l dichloromethane/petroleum ether), 39 mg (27%) of the labile endoperoxide l l b was obtained IR (CCl,) 3040, 3000, 3960, 1455, 1435, 1250, 1230, 1180,1105,1030;'H NMR (200 MHz, -20 "C, CDCW ii 2.21 (d, J t 10.4 Hz, 1H, CHZ), 2.30 (dq,J = 10.4,2.2 Hz, 1H, CHd, 2.32 (dm, J = 16.9,l H, CHz), 2.54 (dq, J = 16.9, 2.2 Hz, 1H, CHd, 4.87 (br a, 1 HI CH), 4.90 (br 8, 1 H, CH), 4.93 (t, J = 2.2 Hz, 1 H, HzC-), 5.19 (t,J = 2.2 Hz, 1H, H2C-I; 13CNMR (63 MHz, CDCl3) 8 36.3 (t), 44.5 (t) 78.9 (d), 82.3 (d), 108.4 (t),145.5 (9); MS (70 eV) mlz 112 (25) [M+1,80 (8),79 (loo), 77 (17), 69 (6), 43 (25), 41 (28), 39 (39); calcd for CeHsOn 112.052, found 112.051 (MS).
Adam et al. Sensitized Photolysis of Bicyclopentane 3b under Oxygen Gas Pressure. Benzophenone-sensitizedphotolysisof houeane 3b in acetonitrile (0.018 M)under oxygen pressure (7 atm) as described above led to significant consumption of starting material within 20 min (monitored by GC). Thin-layer chromatographyon silica gel with dichloromethaneas eluent revealed at least one peroxide product (peroxidetest spray%). Due to this instability of 3b, the dioxygen trapping technique for determining the triplet lifetime of biradical2b could not be applied. General Procedure for the Photoacoustic Calorimetric Measurements. The front face irradiationcelland thealgorithm used for data analysis were those described by Melton, Caldwell, and co-workers.3bJ3 Excitation pulses from an excimer laser (248 or 308 nm, 25-11s pulse width) were attenuated to an energy of ca. 1mJ per pulse. The cell thickness was fixed to 0.2 mm with a Teflon spacer. The signal of a 1-MHz contact transducer (Panametrics, A103) was fed into a 2004 input of the transient digitizer (Tektronix 7912 AD). A dielectric mirror (>99.9% reflection, 90" incidence angle) was placed in front of the transducer to reduce the background signal. 2-Hydroxybenzophenone in acetonitrile was used as a calibration standard for the system response (T-wave). The absorbancesofthe reference and sample solutions were adjusted to the same value around 0.7 to within 0.001 absorbance units in a 1-mm cell. In the sample solutions >96% of the absorbance was due to the sensitizer benzophenone; small corrections were applied where necessary to account for the fraction of light absorbed directly by the azoalkane. A flow cell was used to ensure exposure of fresh solutions to each excitation pulse. Each trace was normalized to correct for the minor shot-to-shot variations (&2%) in the excitation pulse energy using the digital output from a pyroelectric energy meter (Laser Precision Co., Rj-7620 energy meter, RjP734 probe head).
Acknowledgment. We are grateful to the Swiss National Science Foundation, the Deutsche Forschungsgemeinschaft, and the Fonds der Chemischen Industrie for financial support of this work. H.P.thanks the Deutsche Forschungsgemeinschaft for a postdoctoral fellowship and J.S.the Fonds der Chemischen Industrie for a doctoral fellowship. (26) Johnson, R. A.; Nidy, E.G.J . Org. Chem. 1976,40, 1680.
