2860
J. Org. Chem., Vol. 38,No. 16, 1973
RYANG, SHIMA, AND SAKURAI
Photoaddition Reaction of Biacetyl HONG-SON RYANG,* KENSUKE SHIMA, AND HIROSHI SAKURAI The Institute of Scientific and Industrial Research, Osaka University, Suita, Osaka, Japan Received January 22, 1973 The photochemical reaction of biacetyl with various olefins has been investigated. Irradiations of biacetyl with indene, furan, and ethyl vinyl ether give oxetanes with higher orientational selectivity than that of monoketones. In the case of methyl-substituted olefins, it is found that novel prodhcts, 2-acetoethyl allyl ethers, accompany the oxetanes and are formed in good yield. The ratios of these products vary with the olefins used. The presence of biradical intermediates formed by addition of excited biacetyl to olefins is established by deuterium-labeling experiments. The quenching of biacetyl phosphorescence by an olefin indicates that the n-r* triplet state of biacetyl is involved in these reactions. The absence of adduct in the case of electron-deficient olefins indicates that the excited biacetyl is electrophilic in its reaction with olefins. The mechanism of these reactions is best described as an electrophilic attack of the n-r* triplet state of biacetyl to the olefins to give a biradical intermediate which undergoes competitive cyclization and disproportionation. The difference in the reactivity of biacetyl toward photoaddition relative to monoketones and o-quinones is discussed.
In recent years, the photochemical behavior of adiketones has attracted a great deal of attention. The results reported in the 1iterat~rel-l~ have produced considerable knowledge about the photochemical behavior of alkyl a-diketones. It is well known that o-quinones undergo photoaddition to olefins to produce oxetanes and dioxenes.15 Their formation is reasonably explained on the basis of a biradical intermediate. An interesting result, reported by Staab and Ipaktschi, was that irradiation of benzocyclobutanedione in the presence of olefins resulted in the formation of the spirolactonecyclopropane derivatives via carbene intermediates. l6 However, no report exists which describes photoaddition of acyclic alkyl a-diketones to olefins. Biacetyl and other alkyl a-diketones exhibit phosphorescence in solution at room temperature, in spite of the general lack of phosphorescence from monoketones, and abstract hydrogen atoms with a rate constant which is very small relative to monoketones.lZd Rubin and coworkers have demonstrated that photoreduction of o-quinones such as 9,lO-phenanthrenequinone gives 1,2 and 1,4 adducts, whereas camphorquinone gives only 1,2 adducts.” Furthermore, Gream (1) (a) W. H. Urry and D. J. Trecker, J. Amer. Chem. Sac., 84, 118 (1962); (b) W. H . Urry, D. J. Trecker, and D. A. Winey, Tetrahedron Lett., 609 (1962). (2) P. W. Jolly and P. de Mayo, Can. J. Chem., 42, 170 (1964). (3) W. G. Bentrude and K. R. Darnall, Chem. Commun., 810(1968). (4) R. Bishop and N. K. Hamer, J. Chem. Soc. C , 1193 (1970); 1197 (1970). (5) T.L. Burkoth and E. F. Ullman, Tetrahedron Lett., 145 (1969). (6) B.Akermark and N.-G. Johasson, i b i d . , 371 (1969). (7) S. P. Pappas, J. E. Alexander, and R . D. Zehr, Jr., J. Amer. Chem. SOC.,92, 6927 (1970). (8) (a) G. E. Gream, J. C. Paice, and C. C. R. Ramsay, Aust. J. Chem., 20, 1671 (1967); (b) G. E. Gream, J. C. Paice, and B. S. J. Usrynski, Chem. Commun., 895 (1970); (0) G. E. Gream, M. Mular, and J. C. Paice, Tetrahedron Lett., 3479 (1970). (9) W. M. Horspool and G. D. Khandelwal, Chem. Commun., 257 ( 1970). (10) (a) J. Lemaire, J. Phys. Chem., 71, 2653 (1967); (b) J. Lemaire, M. Niclause, X. Deglise, J. C. Andre, and G. Penson, and M. Bouchy, C. R. Acad. Sci., Ser. C , 267, 33 (1968). (11) R. G. Zepp and P. J. Wagner, J. Amer. Chem. Soc., 92, 7466 (1970). (12) (a) N. J. Turro and R. Engel, M o l . Photochem., 1, 143 (1969): (b) i b i d . , 1, 235 (1969); ( 0 ) J. Amer. Chem. Soc., 91, 7113 (1969); (d) N. J. Turro and T.-J. Lee, zbid., 91,5651 (1969); (e) ibid., QS, 7467 (1970). (13) (a) H. L. J. Baokstrom and K. Sandros, Acta Chem. Scand., 12, 823 (1958); (b) K. Sandros and H. L. J. Backstrom, %bid., 16, 958 (1962); (01 K. Sandros, zbid., 18,2355 (1964). (14) E . J. Baum and R. 0. C. Norman, J. Chem. Sac. B , 227 (1968). (16) G. P. Fundt and G. 0. Schenck in “1,4-Cycloadditions,” J. Hamer, Ed., Academic Press, New York, N. Y., 1967, p 345. (16) H. A. Staab and J. Ipaktschi, Chem. Ber., 101, 1457 (1968).
and coworkers have shown that the photoreactions of nonenolizable cyclic a-diketones with alcohols, amines, and olefins are not necessarily analogous to those of o-quinonesas These observations suggested to us that the photochemical behavior of acyclic alkyl adiketones toward olefins might be different from that of monoketones and o-quinones. Hence, we investigated the photoaddition of biacetyl to various olefins and tried to compare our results with those of other compounds.ls
Results All photochemical reactions described herein were carried out with a 350-W high-pressure mercury lamp in Pyrex glass under nitrogen filtered through an nhexane solution of naphthalene (X >320 nm) a t room temperature. Major products were 1: 1 adducts in each case. The various adducts were isolated by distillation and preparative vapor phase chromatography, and their structures were determined by ir, nmr, and mass spectra and elemental analysis as detailed in the Experimental Section. The ratios of the products were determined by vpc. The reactions of biacetyl with indene, furan, and ethyl vinyl ether gave oxetanes as main products. I n the case of indene, two oxetanes, 1 and 2 (1:2), in which the Cz position of indene was attached to the carbonyl oxygen of biacetyl, were obtained. The stereochemistry of 1 and 2 was assigned on the basis of the nmr spectra. Oxetane 3 and 4 were obtained from furan and ethyl vinyl ether, respe~tive1y.l~ These results suggest that the addition of biacetyl proceeds with higher orientational selectivity than that of monoketonesaZ0 I n no instance was there a detectable amount of dioxenes, and observation which contrasts with the results for 9,lO-phenanthrenequinone. l6 (17) (a) M , B. Rubin and P. Zwitkowits, Tetrahedron Lett., 2453 (1965); (b) M. B. Rubin and R . G. Labarge, J. Oru. Chem., Si, 3283 (1966); ( 0 ) M. R. Rubin and R. A. Reith, Chem. Commun., 431 (1966); (d) M. B. Rubin, Fortschr. Chem. Forsch., 18, 251 (1969); (e) M. €3. Rubin and Z. Hershitik, Chem. Commun., 1267 (1970). (18) For preliminary account6 of a portion of this work, see (a) H.-8. Ryang, K. Shima, and H. Sakurai, Tetrahedron Lett., 1091 (1970); (b) J . Amer. Chem. Soc., 98,5270 (1971). (19) Unfortunately, the configurations of 3 and 4 were not determined from present data. (20) For example, The photoreaction of acetone with ethyl vinyl ether gave both 3-ethoxyoxetane (70%) and 2-ethoxyoxetane (30%), see (a) 8. H. Sohroeter and C. M. Orlando, Jr., J. Ore. Chem., 34, 1181 (1969); (b) N. J. Turro and P. A. Wriede, d i d . , S4, 3562 (1969).
J. Org. Chem., Vol. 58, No. 16, 197s 2861
PHOTOADDITION REACTION OF BIACETYL H CH3
H COCHJ
~+C0CH3
%4-CH3
H
H 1
2
H COCH3
H
c22p'
&cH3 H
COCH, 4
3
Photoaddition reactions of biacetyl were also carried out with methyl-substituted olefins, i.e., 2-ethoxypropene (Sa), a-methylstyrene (Sb), isobutene (Sc), 2-methyl-2-butene (Sd), and 2,3-dimethyl-2-butene (Se). A new type of product, 1-acetoethyl allyl ethers 6,accompanied the oxetane isomers 7 and 8 (Table I).
R1,,c=c CH3
t CH3COCOCH,
/R2
5
3R'
5
arising from initial hydrogen abstraction from 5c by excited biacetyl and subsequent combination of the two radicals. No oxetane was isolated in the case of Seaz1 The formation of 6 is of interest in comparison with the results from monoketones and o-quinones. Photostability studies22and constant product ratios during the irradiations2* have shown that the adducts are the primary photochemical products. It is generally recognized that a-diketones undergo primary photochemical addition to olefins in competition with hydrogen abstraction, a cleavage, and enol formation. This suggested to us that 6 is formed through either (A) an attack of the excited carbonyl oxygen of biacetyl on olefin t o form a biradical intermediate followed by intramolecular hydrogen transfer or (B) initial hydrogen abstraction from olefin by the excited carbonyl of biacetyl followed by combination of the two radicals formedaZ4 I n order to determine the mechanism for the formation of 6, the photoreaction of biacetyl with CYmethyZ-da-styrene (Sb') was investigated. Sb' was prepared by the Wittig reaction of trideuteriomethyl phenyl ketone with methyltriphenylphosphonium bromide.2s Irradiation and isolation of the products were carried out as for Sb. No deuterium was introduced into the position C, of the allyl moiety in 6b'.26
R3
6
CH,CO CH3#-+ CH3 7 a,
Ph C ' -CH,
RIRz ;;&-+R3
Rl Rz
CD',
r
COCH, 8
= R, = H C,H,; R, = R3 = H CH,; R, = R, = H R, = CH,; R, = H R, = R, = CH,
R, = OC,H,; R,
b, R, = c, R, =
d, R, = e, R, =
---Product 6
yield, %7
27 36 21 2.1 27 41 7 1.8 5c 16 10 0.62 5d 54 21* 0.41 5e 70 0 a Irradiated at room temperature. Product distribution determined by vpc. b An isomer of oxetanes alone has been observed.
The ratios of these products vary with the olefins used. Two oxetanes were produced from Sa and Sb, whereas only one of the four possible oxetane isomers was isolated from 5d. Irradiation with 5c gave, in addition to an allylic ether and an oxetane, the corresponding pinacol 9 and an unsaturated alcohol 10
CH$OC-CCOCH,
I
I
CH3
CH,
I I CHz=C-CH,-CCOCH, I
bH 6 H 9
'
disproportionation
bH 10
fla
y
CD,= CCH20CDCOCH,
I
COCH,
I
Ph
CH3 6b'
>.
Ph CD,,
CH2
0
CH~C-'*CCH~ OD
Ph
CH,,
I
I
CD, =CCDzOCCCH3
. il
I
combination
+
+
5b
I I
Ph
Ratio of (7 8)/6
8
Sa
CH3 CH,
+
CHBCOCOCHB
CD,) 7 0 CH3
