O X 1–X 2 4 5 6 7 O O R1 R1 O R1 R1 + O R1 R1 1 8b R1 R1 2 + 8a + hν ? O CR2 3 CR23 R2 CR2 3 O R2 R2 O CR2 3 CR2 3 + 2 3a R2 = H b R2 = D + 300 nm hν 1 2 3 4 5 6 7 8 10 9 9 : 10 = 11 : 1 11 : 12 = 2 : 1
Photochemistry of acetone in the presence of exocyclic olefins: an unexpected
competition between the photo-Conia and Patern`o–B ¨uchi reactions
Wen-Sheng Chung* and Chia-Chin Ho
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, ROC
When irradiated in the presence of several exocyclic olefins, acetone undergoes homoalkylation with the olefins to form a series of 4-cycloalkylbutan-2-ones (with quantum yields of 0.14 ± 0.01) rather than exhibiting the expected Patern `o– B ¨uchi reaction; in contrast, the photolysis of perdeuteriated acetone gave both types of products.
Carbonyl group photochemistry has been covered in many excellent reviews and books over the last four decades.1It has
long been known that when irradiated in the presence of olefins, aliphatic ketones undergo the Patern`o–B¨uchi reaction,2Norrish
type I or Type II reactions,2,3the former of which gives oxetane
products, and the latter, pinacols, alcohols and hydrocarbon dimers. In some rare cases they may also lead to ene-reaction products.4Results other than those described above have been
reported5but the reports have long been ignored.1For example,
the photochemistry of acetone with norbornene was reported5a
to proceed by chain addition of acetonyl radicals to norbornene, a process analogous to the addition of cyclohexanone to oct-1-ene5b and to cyclohexene.5c The same outcome, when
obtained thermochemically, is known as the Conia reaction.6
Thus, we shall call the latter reactions photo-Conia reactions. In the course of studying face selectivity in the photo-cycloaddition reaction of 5-substituted adamantan-2-ones
1–Xs, we have used many different olefins with nitrile or alkoxy
substituents and have found that they all give oxetane products in excellent yields.7 It is of interest to know whether the
photocycloaddition occurs when 1–X is replaced with methyl-eneadamantane 2 and the olefin is replaced with a ketone (Scheme 1). It is also surprising to note that the photochemistry of ketones in the presence of exocyclic olefins has not been
systematically studied.1We report here the photochemistry of
acetone 3a with methyleneadamantane 2 and a series of exocyclic alkenes, e.g. methylenecyclobutane 4, methylenecy-clopentane 5, methylenecyclohexane 6 and ethylidenecyclohex-ane 7.
Irradiation at 300 nm of a degassed solution of 0.1 g of 2 in 20 ml spectrograde acetone 3a at room temp. for 12 h leads to the formation of a novel photo-Conia6 adduct 9 as a major
product† (52% isolated yield), some oxetane 10 (5%), and many other minor reaction products (each less than 5% as determined by GC) (Scheme 2). The major product was first identified as a 1 : 1 adduct by GC–MS which indicated a molecular ion peak at
m/z 206. If oxetane 10 were the major product, ring metathesis
fragments3b,7would have been observed in the mass spectrum,
e.g. m/z at 176 (M+2 HCOH) and 148 (2), however, the major
peaks were observed at m/z 191 (M+2 Me), 188 (M+2 H 2O)
and 163 (M+2 COMe). This observation along with
informa-tion from 1H and 13C NMR spectra (vide infra) confirmed
structure 9 as the major product.
It has been reported7,8that hydrogen on the carbon a to the
oxygen of an oxetane ring has a chemical shift of d 4–5, whereas
hydrogen on the b-carbon atom has a chemical shift of d
2.5–3.6. The 1H NMR spectrum of 9 had a triplet at d 2.38 for
hydrogens a to carbonyl and a singlet at d 2.12 for Me, which
is incompatible with an oxetane structure. One expects to see only singlet oxetane ring protons no matter whether the oxetane is 8a or 10. The 13C NMR and DEPT signals of the major
product included 7 lines for adamantane and one methyl at d
29.80, two methylene carbons at d 26.57 and 42.04 due to the
3,4-carbons of adamant-4-ylbutan-2-one, and a quaternary carbon at d 209.63 due to carbonyl carbon, which provide
further evidence for the photo-Conia product 9.
In order to determine the proton source on C-2 of product 9, we also irradiated 2 in deuteriated [2H]acetone 3b for
comparison. The ratio of photo-Conia product to oxetane 9/10 was about 11 in acetone, but about 2 (11 : 12) in [2H
6]acetone.
Comparing the 1H NMR spectra of 11 with 9, three dramatic
changes are observed: (i) the triplet at d 2.38 for hydrogens a-to
carbonyl, (ii) the singlet at d 2.12 for Me, and (iii) the multiplet
at d 1.55 for proton at C-2 had all disappeared. These
deuteriated acetone 3b results indicate that the proton at C-2 was abstracted from acetone. Note that oxetane 12 has now been isolated in good yield, but was only trace when acetone 3a was used. The large deuterium isotope effect observed implies that
Scheme 1 Scheme 2
Chem. Commun., 1997 317
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O CR2 3 CR23 R2 CR2 3 O R2 R2 O CR23 CR2 3 + 13 (14%) + 3b R2 = D R1 R1 R2 300 nm hν n–3 n –3 trace 4 n = 4, R1 = H 3a R2 = H 5 n = 5, R1 = H 3a R2 = H 6 n = 6, R1 = H 3a R2 = H 7 n = 6, R1 = Me 3a R2 = H 7 n = 6, R1 = Me n–3 14 (12%) trace 15 (60%) trace 16 (17%) 17 (17%) 18 (5%) 19 (32%)
a C–H bond cleavage was involved in the transition state of this novel photo-Conia reaction.
Note that the photo-Conia reaction of 2 occurs only in neat acetone and deuteriated acetone, but not in other organic solvents such as acetonitrile, benzene and cyclohexane. Com-pound 2 would also neither react photochemically in dilute acetone solutions (@1 mol dm23 in organic solvents), nor
would it react with acetone in the dark. In order to explore the scope of this photo-Conia reaction, we also photolysed 2 in acetophenone, benzophenone and butan-2-one for 24 h. No reaction was found in the aryl ketones. Although the photo reaction in butan-2-one revealed evidence of formation of some Conia-type products under GC–MS analysis, they were too complex to be isolated.
We then turned our attention to the variation of 2 into a series of exocyclic olefins 4–7. The photolysis of cyclobutane 4, methylenecyclopentane 5 and methylene-cyclohexane 6 in acetone gives photo-Conia products 13–15 as
the only isolable products (Scheme 3).† Due to many possible secondary photochemical reactions, the yields from methyl-enecyclobutane 4 and methylenecyclopentane 5 are poor. Nevertheless, methylenecyclohexane 6 gave the homo-alkyla-tion product 15 as the major product in 60% yield. The expected oxetane product from the Patern`o–B¨uchi reaction was detected in trace by GC–MS but was not isolated. On the other hand, when a trisubstituted olefin such as 7 was photolysed in acetone, adducts 16 and 17 were obtained as a 1 : 1 mixture. The Patern`o– B¨uchi reaction product 19 became dominant when 7 was photolysed in deuteriated acetone 3b. The quantum yields for the photo-Conia reaction products of 2 and 6 in acetone (i.e.F
for 9 and 15) were determined‡ to be 0.15 and 0.13, respectively.
Kharasch5b,9suggested that in the reaction of aldehydes with
terminal olefins to form ketones, it is the acyl radical [R(ON)C·] that attacks the olefin. Acetone5aor cyclohexanone5b,c
under-going Type I cleavage would not, however, explain the observed photo-Conia products. Our results may be explained as follows: the rate-determining step involves an a-hydrogen
abstraction of acetone by another excited acetone to give an
a-keto radical,5cwhich is then added further to a molecule of
exocyclic olefin. In deuteriated acetone 3b, the C–D bond cleavage step is hampered with respect to that of a C–H bond, thus the Patern`o–B¨uchi reaction is comparable. Although the mechanism of this photo-Conia reaction is still unclear at present, it provides a novel and good-yield method for homo-alkylation,11 which has long been neglected in carbonyl
photochemistry.
We thank the National Science Council of the Republic of China for its financial support (Grant No. NSC 84-2113-M-009-002).
Footnotes
† Satisfactory spectral data were obtained for all products. Selected data for
9: colourless oil; 1H NMR (300 MHz, CDCl 3), d 2.38 (2 H, t, J 8.7 Hz), 2.12 (3 H, s) and 1.85–1.44 (17 H, m); 13C NMR (75.4 MHz, CDCl 3), d 209.63 (CNO), 43.96 (CH), 42.04 (CH2), 39.06 (CH2), 38.25 (CH2), 31.66 (CH), 31.43 (CH2), 29.80 (Me), 28.17 (CH), 27.93 (CH) and 26.57 (CH2); m/z 206 (M+, 2), 191 (5), 188 (24), 163 (20), 148 (100%), 106 (36) and 92 (52); (Found: M+206.1674. C
14H22O requires 206.1671). For 11: colourless oil; 1H NMR, d 1.88–1.81 (6 H, m), 1.72–1.65 (8 H, m) and 1.52–1.48 (2 H, m); 13C NMR, d 210.04 (CNO), 43.95 (CD), 38.98 (CH2), 38.27 (CH 2), 31.60 (CH), 31.45 (CH2), 31.33 (CH2), 28.18 (CH), 27.97 (CH) and 26.48 (CH2); m/z 212 (M+, 3), 194 (22), 192 (5), 166 (10), 148 (100%), 92 (48) and 80 (40); (Found: M+ 212.2054. C 14H16OD6 requires 212.2047). For 12: colourless oil, 1H NMR, d 4.19 (2 H, s), 2.30 (2 H, br s) and 1.82–1.60 (12 H, m); 13C NMR, d 87.87 (Cq), 75.59 (CH
2), 49.52 (Cq), 37.07 (CH2), 34.65 (CH2), 34.35 (CH2), 32.07 (CH), 26.79 (CH) and 26.55 (CH); m/z 212 (M+, 31), 194 (100%), 182 (95), 148 (7), 135 (28) and 65 (36). For 15: colourless oil; 1H NMR, d 2.44 (2 H, t, J 7.8 Hz), 2.14 (3 H, s), 1.71–1.63 (5 H, m), 1.50–1.43 (2 H, m), 1.26–1.15 (4 H, m) and 0.93–0.86 (2 H, m); 13C NMR, d 209.58 (CNO), 41.26 (CH2), 37.12 (CH), 32.99 (CH2), 31.10 (CH2), 29.73 (Me), 26.42 (CH2) and 26.13 (CH2); m/z 154 (M+, 15), 136 (11), 96 (77), 81 (65) and 55 (100%); (Found: M+154.1361. C 10H18O requires 154.1358). ‡ We used trans-stilbene as an actinometer when taking the quantum yield for its trans to cis isomerization as 0.32 at 300 nm light and measured the quantum yield for 9 and 15. For the use of this actinometer see ref. 10.
References
1 For reviews on the Patern`o–B¨uchi reaction see: D. R. Arnold, Adv. Photochem., 1968, 6, 301; J. C. Dalton and N. J. Turro, Ann. Rev. Phys. Chem., 1970, 21, 499; N. J. Turro, J. C. Dalton, K. Dawes, G. Farrington, R. Hautala, D. Morton, M. Niemczyk and N. Schore, Acc. Chem. Res., 1972, 5, 92; N. J. Turro, Modern Molecular Photo-chemistry, Benjamin, Menlo Park, 1978, ch. 10 and 11; G. Jones, II, in Organic Photochemistry, ed. A. Padwa, Wiley, New York, 1981, vol. 5. pp. 1–122; S. W. Schreiber, Science, 1985, 227, 858; H. A. J. Carless, in Synthetic Organic Photochemistry, ed. W. M. Horspool, Plenum, New York, 1984, pp. 425–487; M. Demuth and G. Mikhail, Synthesis, 1989, 145; A. G. Griesbeck, in Organic Photochemistry and Photo-biology, ed. W. M. Horspool and P.-S. Song, CRC, New York, 1994, p. 522; p. 550.
2 L. Patern`o and G. Chieffi, Gazz. Chim. Ital., 1909, 39, 341; G. B¨uchi, C. G. Inman and E. S. Lipinsky, J. Am. Chem. Soc., 1954, 76, 4327. 3 (a) R. G. W. Norrish and C. H. Bamford, Nature, 1936, 138, 1016; (b)
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5 (a) W. Reusch, J. Org. Chem., 1962, 27, 1882; (b) M. S. Kharasch, J. Kuderna and W. Nudenberg, J. Org. Chem., 1953, 18, 1225; (c) P. de Mayo, J. B. Stothers and W. Templeton, Can. J. Chem., 1961, 39, 488.
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10 T.-I. Ho, T.-M. Su and T.-C. Hwang, J. Photochem. Photobiol. A. Chem., 1988, 41, 293.
11 For other homoalkylation methods by metal oxides or organic peroxides see: Von A. Rieche, E. Schmitz and E. Gr¨undemann, Z. Chem., 1964, 4, 177; E. I. Heiba and R. M. Dessau, J. Am. Chem. Soc., 1971, 93, 524; A. Citterio, F. Ferrario and S. Bernardinis, J. Chem. Res. (S), 1983, 310, see ref. 6.
Received, 31st October 1996; Com. 6/07415D
Scheme 3
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