J . Am. Chem. SOC.1980, 102, 5297-5302
5297
pyridine and acetic acid, respectively. For 2-naphthol in water, photodissociation shows rate constants ranging from 4.1 X lo7l4 to 5.1 X lo7 s-I.l2 Furthermore, the temperature dependence of the time-resolved fluorescence due to 2* (Figure 8) suggests that the phototautomeric proton transfer is strongly viscosity dependent, in contrast to the dissociation of proton and the resulting ion-pair formation between N1- and the conjugated acid (pyridinium cation) in the excited state of 1. The long-wavelength fluorescence of 1 a t p H 11.5 (Figure 7) is probably due to a ca. 1:l mixture of N1 and N, a n i o n ~ . ~ - I ~
0 0
1
2
3
4
5
-& I447
Figure 9. A derivative-intensityplot of time-resolved kinetic parameter, according to the equation ((AIf/At)/Zf) + ( k , + k, + k,[CH,COOH]) = as a function of acetic acid concentration at 298 K for lumichrome in ethanol.
(dioxane). Although we have not deconvoluted the rise time of
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2*, the above values of k 3 are consistent with the slower rise time of 2* than the excitation pulse rise time, assuming f l.13 T h e fact that the fluorescence of 2* is not resolved within the nanosecond range examined (Figure 7) in aqueous solution of 1 a t p H 5.53 [this p H being lower than pK,*(Nl) = 3.615suggests that the photodissociation of the N l proton in aqueous solution is substantially slower than is the phototautomeric transfer of the proton from N I to Nloin dioxane and ethanol in the presence of
Conclusion The nanosecond time-resolved fluorescence spectra of lumichrome in dioxane and ethanol in the presence of pyridine and acetic acid, respectively, have been obtained by a P A R boxcar averaging system. Inspection of these spectra strongly suggests that the excited lumichrome (1*) undergoes a tautomeric proton shift from N l to Nlo,yielding the excited flavinic tautomer (2*) which emits maximally a t 540-545 nm. Both steady-state and time-resolved fluorescence data yield rate constants of 3-4.5 X lo8 M-' s-l for the phototautomeric reaction, and these rate constants are an order of magnitude lower than diffusion-controlled processes. The driving force for the phototautomeric proton shift is the redistribution of the electron density a t N l and N l oupon excitation of lumichrome., A strong temperature dependence, and that the photodissociation of the N I proton is substantially slower a t neutral and acidic pHs than is the phototautomerism in dioxane and ethanol in the presence of pyridine and acetic acid, respectively, has been observed. Acknowledgments. W e are grateful to Professor S. Georgiou for his valuable advice in constructing the nanosecond time-resolved spectrofluorometer used in this work. Funds for the construction of this equipment were provided by the Dean of the College of Arts and Sciences, Texas Tech University. (14) Weller, A. Prog. React. Kinet. 1961, I , 187. (15) Lasser, N.; Feitelson, J. Photochem. Photobiol. 1977, 25, 4 5 1
Organic Photochemistry with 6.7-eV Photons: (Endo Photoisomerization of Tricycle[ 3.2.1 .02~4]oct-6-ene and Exo) and Tricycle[ 3.2.2.02*4]non-6-ene(Endo and Exo) R. Srinivasan,*l8 Jose A. Ors, Karen H. Brown, Thomas Baum, Lloyd S. White, and Angelo R. Rossi*Ib Contribution from the IBM Thomas J . Watson Research Center, Yorktown Heights, New York 10598, and Chemistry Department, The University of Connecticut, Storrs, Connecticut 06268. Received October 26, 1979
Abstract: Photolysis of the title compounds at 185 nm in solution leads to internal addition of the olefinic group to the cyclopropane ring and cleavage of the cyclopropane to a bicyclic 1,4-diene. A theoretical analysis of the interactions between the *-orbitals of the double bond and the u orbitals of the cyclopropane in each of these compounds has been carried out. The effect of through-bond interactions superimposed upon the through-space effects when extended to the valence excited states gives rise to three low-lying excited states which are (in order of decreasing energy) uA uA* + x* (forbidden), us + x -+ x* + uA* (allowed), and x - us K* + uA* (allowed). The internal addition reaction is identified with the T - us K* + uA* state and the cleavage of the cyclopropane to yield a 1,4-diene with the us + 7 K* + uA* state. The low reactivity of the endo-tricycI0[3.2.2.0~-~]non-6-ene is believed to relate to a departure from the ordering of the excited states as described above.
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Introduction T h e tricyclic compounds 1-4 which incorporate an allylcyclopropane function in a rigid tricyclic framework have been of interest to both spectroscopists and photochemists. The inter0002-7863/80/ 1502-5297$01 .OO/O
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actions between the x electrons of the double bond and the Walsh orbitals of the cyclopropane have been examined in 1, 2, and 3 by photoelectron (PE) spectroscopy by Heilbronner and his coworkers2 and by Bruckmann and K l e ~ s i n g e r . ~These workers
0 1980 American Chemical Society
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Sriniuasan et al.
&dj+%a,
I
I
3
1
L
4
focused their attention on obtaining the ionization energies of higher lying filled levels and relating the splittings obtained to the interplay of through-space vs. through-bond interactions. Unfortunately, PE spectra do not produce direct information concerning the nature of the excited states in these strained molecules. Previous work on the photochemistry of these compounds has been concerned mainly with the internal addition reaction which in the case of 1 proceeds according to eq 1. Prinzbach and his
co-workers4 have discovered numerous examples of such internal additions and attempted to correlate the facility for such additions to the geometry of the molecule4b or the interaction energies between the olefin and the cyclopropane groups which were derived from their P E spectra. Reaction 1 was studied by Freeman and his co-workers5 with radiation >200 nm. They noted that it proceeded at half the chemical yield in the endo isomer 2. They also observed that both 1 and 2 have the same ultraviolet absorption maxima. Neither 3 nor 4 has been investigated photochemically previously, but from the photolysis of 54b,e and 6,6it had been found that internal addition of the olefin to the exooriented cyclopropane is overwhelmingly favored.
I
180
200
1
1
1
220 240 WAVELENGTH (nm)
260
Figure 1. Ultraviolet absorption spectra of 1 and 2. Solvent, pentane 2.64 X M; path 1.0 cm.
1
280
240
WAVELENGTH (nml
Figure 2. Ultraviolet absorption spectra of 3 and 4. Solvent, pentane M; path 1.0 cm. 3.07 X
I 400-
z
5
This work was motivated by our earlier study' of the photochemistry of bicycIo[4.l.O]hept-3-ene(7) at 185 nm which showed that >83% of the photon energy that was absorbed resulted in the decomposition of the cyclopropane ring. At the same time, there was no evidence for the internal addition of the olefin to the cyclopropane. It was considered desirable to extend the study to compounds 1-4 which incorporate 7 so that a comparison may be made of the photochemistry a t 185 nm of rigid systems with varying degrees of strain.
Results T h e ultraviolet absorption spectra of 1-4 in a hydrocarbon solvent are shown in Figures 1 and 2 . The spectra of pairs of isomeric compounds were obtained at identical concentrations in order that subtle differences may be detected by subtraction. These difference spectra are also shown in these figures. Both exo isomers show an absorption at -210 nm which is absent in (1) (a) IBM; (b) The University of Connecticut; Visiting scientist, IBM Thomas J. Watson Research Center, 1979. (2) P. Bischof, E. Heilbronner, H. Prinzbach, and H. D. Martin, H e h . Chim. Acta, 54, 1072 (1971). (3) P. Bruckmann and M. Klessinger, Angew. Chem., Inr. Ed. Engl., 11, 524 (1972). (4) (a) H. Prinzbach, W. Eberbach, and G. von Veh, Angew. Chem., In[. Ed. Engl., 4,436 (1965); (b) H. Prinzbach and D. Hunkler, Chem. Ber., 106, 1804 (1973); (c) H. Prinzbach, W. Auge, and M. Basbudak, ibid., 106, 1822 (1973); (d) H. Prinzbach, G. Sedelmeier, and H.-D. Martin, Angew. Chem., In?. Ed. Engl., 16, 103 (1977); (e) H . Prinzbach, M. Klaus, and W. Mayer, ibid., 8, 883 (1968). (5) (a) P. K. Freeman, D. G. Kuper, and V. N . Mallikarjuna Rao, Terruhedron Lett., 3301 (1965); (b) P. K. Freeman and D. M. Balls, J . Org. Chem., 32, 2354 (1967). (6) A. de Meijere, C. Weitemeyer, and 0 . Schallner, Chem. Ber., 110, 1504 (1977). (7) R. Srinivasan and J. A. Ors, J . Am. Chem. Soc., 101, 3411 (1979).
'0
30
60 90 120 TIME ( m i d
150
180
Figure 3. Rate of formation of products in the photolysis of 2 at 185 nm. Solvent, pentane; mercury resonance lamp with band-pass filter.
the endo isomers. The intensity of this absorption is weak in 1 but strong in 3. Photolysis of the endo isomer 2 at 185 nm in pentane solution gave the diene 8, the vinylcyclopropane 9, and the internal adduct 10. Compound 9 has been observed by Sauers and Shurpik* to
8
10
9
be a product of the direct or sensitized photolysis of 8. Since this reaction which is presumably a di-.rr-methane rearrangement gives 11 as well, a careful search for 11 was made in the photolysis of
8
sensitized
9
11
3. At low conversions, its presence could not be detected (
