6378
J. Am. Chem. SOC. 1982,104, 6378-6382
Photochemistry of Inter- and Intramolecular a-Diket0ne:Norbornene Systems Ronald R. Sauers,* Peter C. Valenti, and Charles A. Crichlow Contribution from the New Brunswick Department of Chemistry, Rutgers, The State University, New Brunswick, New Jersey 08903. Received March 22, 1982
Abstract: Photocycloadditions of biacetyl to norbornene and syn-7-methyl- and syn-7-tert-butylnorbomenehave been observed. The stereochemistry and quenching rates are consistent with increasing steric hindrance to the exo face of norbornene as the syn-7-substituent changes from H to CH, to t-C4H9. The intramolecular analogue of these reactions was studied via 1(5’-norbomene-2’-yl)-1,2-propanedione,which gave an oxetane on direct irradiation at 300 or 430 nm or on benzophenone-sensitized addition. The direct irradiation reactions are believed to proceed via singlet states as evidenced by estimated lifetimes derived from quenching by N,N-dimethylaniline. The effective molarity (EM) for these reactions is estimated to be >lo4 (S,)and > l o 5 (TI).
Introduction of the short lifetimes involved. By means of internal competition reactions singlet lifetimes of lo-” s were estimatedS6 In at least Photocycloaddition reactions between excited carbonyl comone case (1, R = C,&,) the rate of reaction successfully competed pounds and olefins are an important synthetic method for carwith intemal conversion and/or intersystem crossing of a naphthyl bon-carbon bond formation.’ This process, most commonly ketone. In this system we demonstrated that there were two known as the PatemeBiichi reaction, has been extensively studied reactive states, one of which was quenchable with dienes (T,) and from both a theoretical and an experimental point of view, and a more reactive upper state that was not (SI or T2). A reactivity their is general agreement on many of the mechanistic details of difference of ca. lo6 was estimated for these two states. these reactions. Thus, the initial stage of the reaction is believed In the present study we hoped to exploit the unique reactivity to involve exciplex formation as a consequence of (weak) of acylnorbornenes to probe the behavior of excited states of charge-transfer interactions.2 This conclusion is based on studies cu-diket~nes.~,~ There is evidence from vapor-phase photochemical involving quenching of excited carbonyl groups with olefins of studies that S2states of biacetyl can react competitively with varying ionization potential2 and on a detailed Arrhenius study internal conversion.s In addition, Lemairega has proposed that of phosphorescence quenching3 enolization of biacetyl takes place from S2 in solution phase in The intervention of biradical intermediates has been inferred contrast to the reactions of monoaryl diketonesgb Incorporation from kinetic studies4 and from stereochemical results in which olefin geometry is at least partly altered during c y c l o a d d i t i ~ n . ~ ~ ~ of a biacetyl chromophore into the norbornene system affords the opportunity to compare the reactivity of several states of the same Despite these efforts many facets of these reactions are not well molecule that differ in excitation energy’O (e.g., S2vs. SI)and/or understood. These include prediction of regiochemistry’ and multiplicity and that might lead to differing products depending quantum yields.’S6 Our own interest in this reaction has been on these factors. to extend the synthetic utility of the intramolecular oxetane reThese objectives were to be met by a study of the photochemical action6 and to further define the factors that govern the efficiency and photophysical behavior of 1-(5’-norbornene-2’-~1)-1,2of these reactions. For these and other reasons we have examined propanedione (3). This molecule is ideally suited for studies at the behavior of numerous endo-5-acylnorbornenes (I), many of which undergo clean cycloaddition to form oxetanes of structure 2 (eq 1). The preparative success of these reactions is a conse-
o+ 1
2
quence of very rapid intramolecular quenching of the excited carbonyl groups by the double bonds to the exclusion of competing processes. Attempts to measure excited-state lifetimes by standard quenching techniques, Le., Stern-Volmer analyses, failed because (1) (a) For a recent review that stresses synthetic applications see: Jones, G., I1 Org. Phorochem. 1981,5, 1. (b) Cowan, D. 0.;Drisko, R. L. “Elements of Organic Photochemistry”; Plenum Press: New York, 1976; pp 181-198. (c) Turro, N. J. “Modem Molecular Photochemistry”;Benjamin/Cummings: 1978; pp 432-452. (2) (a) Yang, N . C.; Hui, M. H.; Shold, D. M.; Turro, N. J.; Hautala, R. R.; Daws, K.; Dalton, J. C. J. Am. Chem. SOC.1977, 99, 3023. (b) Caldwell, R. A.; Sovocol, G. W.; Gajewski, R. P. Ibid. 1973, 95, 2549. (c) Kochevar, I . H.; Wagner, P. J. Ibid. 1970, 92, 3742. (3) Maharaj, U.; Winnik, M. A. J . Am. Chem. SOC.1981, 103, 2328. (4) Saltiel, J.; Neuberger, K. R.; Wrighton, M. J . Am. Chem. SOC.1969, 91, 3658. (5) (a) Turro, N. J., Wriede, P. A. J . Am. Chem. SOC.1970, 92, 320. (b) Yang, N . C.; Eisenhardt, W. Ibid. 1971, 93, 1277. (6) (a) Sauers, R. R.; Rousseau, A. D.; Byme, B. J . Am. Chem. SOC.1975, 97, 4947. (b) Sauers, R. R.; Lynch, D. C. J . Org. Chem. 1980, 45, 1286.
Po
c H3
4a, R = H b, R = CH, C, R=t-C,H,
3
different wavelengths and sensitized photochemistry. As model systems we examined the photochemistry and reactivity of norbornene and some syn-7-substituted homologues (4) with biacetyl S1 and T, states.”
Results and Discussion Two different synthetic sequences were devised for the preparation of 3, neither of which was completely satisfactory. The (7) For a recent thorough study of biacetyl-olefin cycloadditions see: Jones, G., 11; Santhanam, M.; Chiang, S.-H. J . Am. Chem. SOC.1980, 102, 6088. (8) For a summary of earlier work see: Calvert, J. G.; Pitts, J. N . , Jr.
‘Photochemistry”; Wiley: New York, 1966; pp 421-422. (9) (a) Irradiation of biacetyl below 310 nm is reported to lead to enol via the S2 state; see: Lemaire, J. J . Phys. Chem. 1967, 71, 2653. (b) For a contrasting situation, see: Wagner, P. J.; Zepp, R. G.; Liu, K.-C.; Thomas, M.; Lee, T.-J.; Turro, N. J. J . Am. Chem. SOC.1976, 98, 8125. (10) Loutfy, R. 0.;Dogra, S. K.; Yip, R. W. Con. J. Chem. 1979,57,342. (11) A preliminary account of some of this work has been published: Sauers, R. R.; Valenti, P. C.; Tavss, E. Tetrohedron Lett. 1975, 3129.
0002-7863/82/ 1504-6378$01.25/0 0 1982 American Chemical Society
J . Am. Chem. Soc., Vol. 104, No. 23, 1982 6379
a-Diket0ne:Norbornene Systems: Photochemistry Table I. UV Absorption Data for a-Diketones
solvent
compound
biacetylga C7H1, 6-heptene-2,3-di0ne*~ C,H,,
c-C,H,, c-C,H,,
exo-3 endo-3
hmax (E), nm 273 261 259 258
(17), 4 2 2 (22), 448 (21) (234), 431 (22), 4 2 2 (22) (358), 432 (15), 453 (13) (424). 430 ( 2 9 ) .453 (27)
original objective was to use the Corey-SeebachIza acylation procedure to form the ketodithiane 5 followed by hydrolysis (eq
bH
of biacetyl. We found that concentrations of up to 1 M norbornene did not affect the fluorescence intensity (*lo%) of biacetyl. Thus, the rate of quenching of biacetyl (Tf = 10.8 ns)I6 by norbornene is less than lo7 M-I s-l. Norbornenes. Photocycloaddition reactions were carried out between biacetyl and the olefins norbornene, syn-7-methylnorbornene (4, R = CH3), and syn-7-tert-butylnorbornene(4, R = t-C,H,). In all cases we isolated 1:l adducts that were shown to be acyloxetanes. Two major structural isomers, 7 and 8, were formed as a consequence of exo- and endo-cycloadditionpathways (eq 4). The structural assignments for these products were made
+
o=c
\
CI
4 7
5
2).12b Reproducible but poor yields were obtained from this procedure. The other procedure involved the novel procedure developed by Stetter12' in which acetaldehyde was condensed with 5401'bornene-2-carboxaldehyde(eq 3). a-Keto1 6 was oxidized to a
3=d--H
I
CH3-CHOH
6
diketone by means of bismuth oxide. Although the yields were acceptable, the reactions were accompanied by considerable epimerization as a result of the high basicity of the catalyst. The diketone 3 was an unstable yellow oil whose ultraviolet absorption spectrum showed a striking enhancement of the two principal absorption bands (Table I). In addition, we show data for the corresponding exo isomer and the acyclic y,b-unsaturated analogue 6-heptene-2,3-dione.13 It is interesting to note that the magnitude of the coupling of the chromophores in these systems is a function of both geometry and transition energy. The small enhancement (2-fold) and wavelength shift for the So SI absorption in endo-3 is typical of molecules in which the n orbital interacts directly with a 7c system.I4 The enhancement of the So S2transition (25-fold) is unusually large, and we are unaware of any comparable data. Presumably the better matching of orbital energies gives rise to an increased perturbation interaction. The coupling in exo-3 is attributed to interactions transmitted through the u framework15 and is also quite large as reflected by the So S2 transition (20-fold) but not by the So S1 transition. The fluorescence spectra of exo-3 and endo-3 were also quite different. Whereas exo-3 showed emission (A, 474 nm) of intensity comparable to that of biacetyl16 (A- 464 nm, 4f O.O029), the endo isomer showed virtually no emission (Aex 254-420 nm). Presumably the internal double bond of endo-3 intercepts the singlet state of the diketone moiety before the latter can fluoresce. It was of interest to carry out the intermolecular analogue of this experiment, namely, the effect of norbornene on the fluorescence
-
on the basis of the expected proton-proton coupling constants for the hydrogens labeled H, and HX.l7 The results are summarized in Table 11, from which it is seen that the stereochemistry of cycloaddition of norbornenes is controlled by the size of the syn-7-substituent. Two factors could be important in deciding the stereochemical outcome of these reactions, namely, the differential rates of exciplex (and/or biradical) formation and the differential partitioning of exo vs. endo biradical intermediates. We therefore attempted to evaluate the quenching rates of these olefins with biacetyl triplets on the assumption that the singlet state was not the photoactive state (vide supra). The phosphorescence of biacetyl was quenched by norbornene, and a Stern-Volmer analysis of the results gave a linear plot with a slope of 10.8. From this value one can calculate a k, of 2.4 X lo4 M-l s-', assuming 7p equals 4.6 X lo4 s.I6 Similar experiments with syn-7-tert-butylnorborneneand syn-7-methylnorbornene were more difficult to analyze experimentally because these olefins were very poor quenchers of biacetyl phosphorescence, giving rise to k T values of 1.1 and 0.6, respectively. The overall quenching rate 0?4c was estimated to be ca. 1.3 X lo3 M-' s-l. We assume that about half (between 40% and 66%) of this value, Le., 6.5 X lo2 M-' s-I, is attributable to exciplex formation and the other half to hydrogen abstraction since we also isolated a 34% yield of a product whose structure is believed to be 9 (eq 5). The quenching
4c
+
-
-+
(12) (a) Seebach, D.; Corey, E. J. J . Org. Chem. 1975, 40, 231. (b) A modification of a literature procedure was used in which copper(I1) chloride was substituted for silver(1) chloride: see: Corey, E. J.; Erickson, B. Ibid. 1971, 36, 3553. (c) Stetter, H.; Dambkes, G. Synthesis, 1980, 309. (13) Bishop, R.; Hamer, N. K. J . Chem. SOC.C 1970, 1197. (14) (a) For closely related examples, see: Cookson, R. C.; Wariyar, N. S. J . Chem. SOC.1956, 2302. (b) for other examples, see: Sauers, R. R.; Henderson, T. R. J . Org. Chem. 1974, 39, 1850. (c) For a discussion of theory see: Houk, K. N. Chem. Reu. 1976, 76, 1. (15) Cookson,R. C.; Henstock, J.; Hudec, J. J . Am. Chem. SOC.1966.88, 1060.
0 9
rate ratio of norbornene vs. syn-7-tert-butylnorborneneis thus ca. 37. These results stand in contrast to a related study by Turro and FarringtonIBain which quenching rates and stereochemical modes of addition were not parallel. In the present context three factors consistently point to a classical steric inhibition of these reactions by the tert-butyl group: exo:endo ratios, phosphorescence quenching rates,lsb and quantum yields of oxetanes.19 (16) (a) Almgren, M. Photochem. Photobiol. 1967.6, 829. (b) Turro, N. J.; Engel, R.; J . Am. Chem. SOC.1969, 91, 7 1 13. (17) Shigemitsu, Y.; Odaira, Y.; Tsutsumi, S . Tetrahedron Lett. 1967, 5 5 . Hara, M.; Odaira, Y.; Tsutsumi, S. Ibid. 1967, 2981. Flautt, T. J.; Erman, W. F. J . Am. Chem. Soc. 1963.85, 3212. Laszlo, P.; Schleyer, P. von R. Ibid. 1964,86, 1171. (18) (a) Turro, N. J.; Farrington, G. L. J . Am. Chem. SOC.1980, 102, 6056. (b) For another example of steric inhibition of an excited carbonyl olefin reaction, see: Maharaj, U.; Winnik, M. A. Tetrahedron Lett. 1981, 22, 517. (19) Strictly, the oxetane quantum yields should be normalized to reflect the efficiencies of the product formation from the triplet states that are actually trapped by the double bonds: &' = &lobad[k(4)/(k,(4) + T -')]-I. The correction is small for 4e (&' = 0.0036) but s i g h c a n t for 4c (Go; = 0.0012).
6380 J . Am. Chem. Soc., Vol. 104, No. 23, 1982 Table 11. Photocycloadditionsof Biacetyl to Norbornenes oxetane distributions oxetane olefin 4, R = ([71:[81) yields," % H
>24: 1
CH,
2.6: le
t-C,H,
