7038 and error bars drawn on both sides (Figure 1) to reflect the error in k d . The sensitizers and their triplet energies, rates of deactivation (kd), and quantum yields for intersystem crossing are listed in Table 111.
Table 111. Data on Sensitizers ET,^
Sensitizer kcal/mol $ 8 d~ sec-1 @-Acetonaphthone 59.3 0.84 0.33 f 0.03 Phenanthrene 61.Xb 0.76 0.90f 0.08 Thioxanthone 65.5 1.06 1.3 f 0.1 Benzophenone 68.5 1.0 5.0f 2.01 Acetophenone 73.6 1.0 100.0f 200.01 Cyclopropyl phenyl ketone 74.4= 1.0 500.0=t200.01 Except where otherwise noted, from W. G. Herkstroeter, A. A. Lamola, and G. F. Hammond,J. Am. Chem. Soc., 86,4537 (1964). * E. Clar and M. Zander, Chem. Ber., 89,749 (1956). Reference 25. d Reference 7 except where otherwise noted. e This research. Reference 25. Reference 6 except where otherwise noted. 0
f
Control Experiments on Quenching Rates. To test the validity of our methods and the purities of our sensitizers and solvents, we determined quenching rates for a number of sensitizer triplets by trans-stilbene, and compared the results with those of Herkstroeter and Hammond.6 The procedure was identical with that above,
except that the quantum yield of cis-stilbene was determined by glpc. The rates obtained were: anthracene, 2.3 x IO5 M-1 sec-1 (lit.61.1 X lo5); acridine, 1.6 X IO6 (M65.8 X 106); and 1,2-benzanthracene, 7.1 X lo7(lit.6 3.3 X 105). The error in all three cases is less than that reporteda6for k d of the sensitizer. Intersystem Crossing Ratios. The method of Lamola and Hammond,? was used to determine intersystem crossing ratios ( $ 8 ~ ) for several sensitizers. The values obtained were 1,2-benzanthracene, 0.87 i 0.05; pyrene, 0.10 + 0.05; and thioxanthone, 1.00Z!Z 0.05.
Molecular Orbital Calculations. The extended Hiickel theory program described by Hoffmannlswas adapted for use on our IBM 7074 computer. Bond lengths in methyl azide were those determined by Livingston and Rao.13 Values for the Slater exponents were: hydrogen, 1.00;carbon, 1.625; nitrogen, 1.95.**Valencestate ionization potentials were those of Skinner and Pritchard.29 In one series of calculations the NIN2N3bond angle (e) was varied from 110 to 250” keeping other bond lengths and bond angles constant. In another, 0 was held constant at 140”and the effect of varying the MeNIN~Ns dihedral angle determined (Me and Nt had been kept coplanar in the first series of calculations). Acknowledgment. We are especially indebted to Dr. C. D. DeBoer for numerous helpful discussions and suggestions. (28) K. B. Wiberg, “Physical Organic Chemistry,” John Wiley & Sons, Inc., New York, N. Y., 1964, p 471. (29) H. H. Skinner and H. 0. Pritchard, Trans. Faraday Soc., 49, 1254 (1953); H. 0. Pritchard and H. H. Skinner, Chem. Reu., 55, 745 ( 1955).
Photocycloaddition of Thiocarbonyl Compounds to Olefins. The Reaction of Thiobenzophenone with Styrene and Substituted Styrenes A. Ohno, Y. Ohnishi, M. Fukuyama, and G , Tsuchihashi Contribution from the Sagami Chemical Research Center, Onuma, Sagamihara-shi, Kanagawa, 229, Japan. Received May 20, 1968 Abstract: Thiobenzophenone reacts with styrene, on irradiation with ultraviolet or visible light to give 2,2,3,3,5pentaphenyl-l&dithiane. The mechanism of this reaction has been studied kinetically. It is shown that the reaction proceeds through the addition of the n +T * triplet state of thiobenzophenone, which behaves like a thiyl radical, to styrene. The characteristic difference between the photocycloaddition of thiobenzophenone affording 1,Cdithianes and that of benzophenone to give oxetanes is discussed in terms of the reactivities of carbon radicals toward a thiocarbonyl sulfur and a carbonyl oxygen. The high reactivity of the intermediate composite radical toward the thioketone leads to 1,4-dithianes from thiobenzophenone. It is suggested that the use of photoexcited thiobenzophenone as a model of thiyl radicals simplifies the study of the addition reactions of thiyl radicals to olefins. From this viewpoint, the relative rates of the addition reactions of thiobenzophenone with unsubstituted and various para- and meta-substituted styrenes have been examined. Based on the absence of a substituent effect, it is proposed that the sulfur atom p to the radical center stabilizes it by forming a sulfur-bridged radical. The stabilizing effect of this type is strong enough to overshadow the effects of substituents in styrenes.
P
hotocycloaddition reactions of ketones to olefins to give oxetanes are well documented.’ O n the other hand, little is k n o ~ n about ~ - ~ the photochemistry of thioketones. A recent study on the photocycloaddition reactions of thiobenzophenone with various (1) For example (a) J. N. Pitts, Jr., and J. K. S. Wan, in “The Chemistry of the Carbonyl Group,” S. Patai, Ed., Interscience Publishers, New York, N. Y., 1966, Chapter 16; (b) N. J. Turro, “Molecular Photochemistry,” W. A . Benjamin, Inc., New York, N. Y., 1965, pp 208-21 1. (2) A.Schonberg and A. Mustafa, J . Chem. Soc., 275 (1943). (3) G.Oster, L. Citarel, and M. Goodman, J . Amer. Chem. Soc., 84, 703 (1962). (4) E. T. Kaiser andT. F. Wulfers, ibid., 86, 1897(1964).
olefins, by irradiation with a high-pressure mercury lamp, has shown that the only isolable products are 1,4-dithiane~.~Thus, reactions of thiobenzophenone with cyclohexene, 2,3-dihydropyran, ethyl vinyl ether, and styrene afford 3,3,4,4-tetraphenyl-2,5-dithiabicyclo[4.4.0]decane (I), 3,3,4,4-tetraphenyl-7-oxa-2,5-dithiabicyclo[4.4.0]decane (11), 2,2,3,3-tetraphenyl-5-ethoxy1,4-dithiane (111), and 2,2,3,3,5-pentaphenyl-1,4-dithiane (IV), respectively. We have found that the same (5)
G.Tsuchihashi, M. Yamauchi, and M. Fukuyama, Tetrahedron
Lert., 1971 (1967).
Journal of the American Chemical Society / 90:25 j December 4, 1968
7039
I
I11
I1
IV
reactions also take place by irradiation with visible light giving the products I-IV in more than 90% yields6 (better than uv irradiation). Each of these products consists of only one isomer : trans-ring junctions for I and 11, axial-ethoxy group in 111, and equatorial-phenyl group in IV. That the photocycloaddition of thioketone proceeds quite stereospecifically giving only one isomeric product in a remarkably high yield is in sharp contrast to the photocycloaddition of carbonyl compounds, the Paterno-Buchi reaction, where a mixture of oxetanes is obtained in fair yield. This has stimulated us t o study the mechanism of the reaction. Furthermore, in the course of the study, it has been found that the photoexcited thiobenzophenone can be regarded as a kind of thiyl radical (uide infra). An analogous relationship is observed in a pair of photoexcited benzophenone and t-butoxy radical for hydrogen-abstraction reactions.’ Thus, the study of photoexcited thiobenzophenone may shed light on the behavior of thiyl radicals in their addition t o olefins, because this approach will eliminate ambiguities caused by ionic reactions, polymerization of olefins, and uncertainty of the rate-determining step, which are always complications when thiols are used as thiyl radical source^.^^^ The mechanism and relative rates of the addition reactions of photoexcited thiobenzophenone with styrene and various substituted styrenes have accordingly been investigated.
Results The reaction of styrene affords IV in yields of 68% by a high-pressure mercury lamp and 94% by a photoreflector lamp. The reactions of p-methoxy-, pchloro-, p-cyano-, m-methoxy-, m-methyl-, m-chloro-, and m-cyanostyrenes also gave compounds of type IV on irradiation with a high-pressure mercury lamp, in average yields of around 65 %. Structures of products were confirmed by nmr spectroscopy. The rate of the reaction at 25” was followed by recording the decrease of 609-mp absorption of thiobenzophenone in the solution by a Cary-14 spectrophotometer. The reaction was found t o be zero order in thiobenzophenone t o more than 80% of complete reaction as illustrated in Figure 1. For the reactions with (6) A. Ohno, Y. Ohnishi, M . Yamauchi, and G. Tsuchihashi, Abstracts, 21st National Meeting of the Chemical Society of Japan, Suita, Osaka, Japan, April 2, 1968, p 2084. (7) C. Walling and M. J. Gibian, J . Amer. Chem. SOC., 86, 3902 (1964); 87,3361 (1965). (8) C. Walling, “Free Radicals in Solution,” John Wiley & Sons, Inc., New York, N . Y.,1957, pp 313-326. (9) W. A. Pryor, “Mechanisms of Sulfur Reactions,” McGraw-Hill Book Co., Inc., New York, N . Y., 1962, pp 16-93.
I
3.0
I
6.0
hr
Figure 1. Pseudo-zero-order kinetics for the reaction of thiobenzophenone with styrene.
substituted styrenes the light intensity was monitored by reference t o the rate of reaction in a standard cell containing both thiobenzophenone and styrene at the specified concentrations. The observed rate constant remains constant with change of the initial concentration of thiobenzophenone, whereas it increases with increasing concentration of styrene. Since thiobenzophenone itself is known to react to give 3,3,5,5-tetraphenyl-l,2,4-trithiacyclopentaneby ultraviolet irradiation,I0 the rate of this reaction was also studied. A cyclohexane solution of thiobenzophenone containing benzene instead of styrene was irradiated under the same conditions, and the zero-order rate constant was found t o be 3.40 X lesM sec-’. The true observed rate constant, kobsd, was derived by subtracting this minor value from each observed rate constant. The absorption spectrum of thiobenzophenone does not change whether styrene (A, 605 mp ( E 180)) or cyclohexane (A, 609 mp (E 180)) is used as a solvent and thus the possibility of the formation of a chargetransfer complex of thiobenzophenone with styrene prior to irradiation is ruled out. The results for styrene are summarized in Table I and those for substituted styrenes are listed in the second column of Table 11.
Discussion Mechanism of the Reaction. In view of the facts that the product consists of two molecules of thiobenzophenone and one molecule of styrene and that photoexcitation is zero order with respect to excited species in the present system, it is quite reasonable t o propose the mechanism shown in the following scheme (eq 1). In this mechanism, i is the rate for the formation of (10) A . Schonberg, 0. Schiitz, and S . Nickel, Chem. Ber., 61, 2175 (1928).
Ohno, et al.
1 Reaction of Thiobenzophenone with Styrene
7040
Table I. Kinetic Data for the Reaction of Thiobenzophenone with Styrene in Various Conditions
1.296 1.683 1.923 2.897 2.899 5.095 3.947" 3.947" 3.947"mb
0.772 0.127 0.459 2.16 0.594 0.134 0.490 2.04 0.520 0.143 0.537 1.86 0.345 0.140 0.706 1.40 0.345 0.138 0.638 1.57 0.196 0.161 0.898 1.10 0.253 0.132 1.23 0.253 0.069 1.13 0.253 0.133 1.13 a The reaction vessel is different from that used for the other runs. b 0.014 M of benzophenone was added to the solution.
[SIis the concentration of styrene, which remains almost constant throughout the reaction under the conditions employed. Since the kinetics is zero order in thiobenzophenone over a wide range of concentrations, the second term of the right-hand side in eq 2 can be neglected and the rate expression is simplified to t = k-l + k q [ V ] , - [VI)
(3)
2ik2[S]
Therefore
(4) or
Table 11.. Kinetic Data for the Reaction of Thiobenzophenone with Substituted Styrenes kobsd x 103,
M hr-'
Log
Log x y b (kzX/kzH) p-CH30 0.694 -0.346 0.02 m-CH3 0.627 -0.465 -0.10 m-CH30 0.526 -0.505 -0.14 0.03 p-c1 -0.332 0.706 m-C1 0.664 -0.376 0.01 0.687 -0.295 0.07 m-CN -0.315 p-CN 0.626 0.05 H -0.362c 0.00 a 0.13 M of thiobenzophenone was used. x = kobsd/[S]; y = 1/(2i - kabsd). c Average value. X
[SI, M 2.875 3.038 2.387 2.897 2.788 2.498 2.143
photoexcited thiobenzophenone" and mainly depends on the intensity of incident light. By the assumption S
By plotting l/kobsd against l/[S], values for k-l/kz and i should be obtained from the slope and the intercept of the line. This treatment is applied to the actual system as shown in Figure 2 . Since a nice straight line is obtained by this treatment, the proposed mechanism is plausible. A recent esr study on an alkaline solution of thiqbenzophenone has shown that irradiation with 3660-A light (r + T* band) produces a thioketyl radical identi$al with that produced by the irradiation of 5770-A light (n + T* band). l 3 Our results have shown that the compound IV is obtained in a better yield with visible light than with ultraviolet irradiation. These facts imply, as suggested by Heller,I3 that the n T* excited state of thiobenzophenone is responsible for the photochemical reaction, even in case of ultraviolet irradiation where the r + T* excited state of thiobenzophenone is formed. Therefore, the intermediate VI in eq 1 is assumed t o be n T* triplet state of thiobenzophenone. The i value calculated by least-squares method is 1.71 X lo-' M sec-'. Since the mechanism of photoexcitation is
-
-
v r
S
l*
1
VI
1
S
II
(Ph-C-Ph)ground
S
/I e l(Ph-C-Ph)r+r* ke
kc
+
kd
S
1
I/
VI1
t
Ph
ph
