4684
J. Org. Chem. 1995,60, 4684-4685
Remarkable Enhancements of Isomerization and Oxidation of Radical Cations of Stilbene Derivatives Induced by Charge-Spin Separation Sachiko Tojo, Kazuhiro Morishima, Akito Ishida, Tetsuro Majima," and Setsuo Takamuku" The Institute of Scientific and Industrial Research, Osaka University, Mihogaoka 8-1, Zbaraki, Osaka 567, J a p a n Received May 1, 1995
As typical reactions of radical cations of aromatic olefins, cis (c)-trans ( t )isomerization, dimerization, and addition of a nucleophile are well-kn0wn.l However, factors which control the relative rates of these reactions are unclear. No unimolecular c-t isomerization occurs in stilbene radical cation (SC+),2 while dimerization3 of St" with St occurs with the rate constant of kd = (3.53.9) x lo8 M-l 3b,c to yield a n-dimer radical cation (n-Stz.+)which converts to a a-dimer radical cation (aStz'+) decomposing to t-St'+ and St as final product^.^' I t is reported that t-St" reacts with superoxide molecule4 a t the rate constant of k,, = 1.9 x lo9 M-' s-l near to the diffusion rate constant,4c while t-St'+ has little reactivity toward 0 2 . 3 a , 4 , 5 On the other hand, we have found that both unimolecular c-t isomerization and oxidation of St'+derivatives substituted with a p-methoxy1 group @-CH30),as an electron-donating substituent, proceed with the rate constants of ki = 4.5 x 106-1.4 x lo7 s-l and ko, = (1.2-4.5) x lo7 M-l s-l, respectively (Scheme 1). On the basis of the two types of reactions (c-t isomerization and oxidation) of eight St'+derivatives (RCH=CHC~HS:1, R = C&; 2, R = 4-CH3CsH4; 3, R = 4-CH3OCsH4 (= An);4, R = 2,4-(CH3O)~CrjH3; 5, R 3,4( C H ~ O ) Z C ~6, H ~R; = 3,5-(CH3O)&H3; 7, AnCH=C(CH3)C6H5;8, A ~ C H = C H A ~ Iit) is , ~found that separation and localization of positive charge and an unpaired electron play an important role in increasing the reac(1)(a) Fox, M. A., Chanon, M., Eds. PhotoinducedElectronTransfer; Elsevier: Amsterdam, The Netherlands, 1988. (b) Mattes, S.L.; Farid, S. In Organic Photochemistry;Padwa, A,, Ed.; Marcel Deckker: New York, 1983;Vol. 6, pp 233-326. (c) Lewis, F. D. In Photoinduced Electron Transfer;Fox, M. A., Chanon, M., Eds.; Elsevier: Amsterdam, The Netherlands, 1988;Part C, Chapter 4. (d) Yoon, U.C.; Mariano, P. S. Acc. Chem. Res. 1992,25,233. (e) Parker, V.D. Acc. Chem. Res. 1984,17,243. (0Eberson, L. Adv. Phys. Org. Chem. 1982,18,79.(g) Schepp, N. P.; Johnston, L. J. J . Am. Chem. SOC.1994,116,6895and references cited therein. (2)(a)Lewis, F. D.; Bedell, A. M.; Dykstra, R. E.; Elbert, J. E.; Gould, I. R.; Farid, S. J . Am. Chem. SOC. 1990,112,8055and references cited therein. (b) Kuriyama, Y.; Arai, T.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett. 1990,173,253. (3) (a) Lewis, F. D.; Petisce, J. R.; Oxman, J. D.; Nepras, M. J. J . Am. Chem. SOC.1985, 107, 203. (b) Kuriyama, Y.; Sakuragi, H.; Tokumaru, K.; Yoshida, Y.; Tagawa, S. BUZZ. Chem. SOC.Jpn. 1993, 66, 1852 and references cited therein. (c) Tojo, S.; Morishima, K.; Ishida, A,; Majima, T.; Takamuku, S. Bull. Chem. SOC.Jpn. 1995,68, 958. (4)(a) Eriksen, J.; Foote, C. S. J . Am. Chem. SOC. 1980,102,6083. (b) Yamashita, T.; Tsurusako, T.; Nakamura, N.; Yasuda, M.; Shima, K. Bull. Chem. SOC.Jpn. 1993,66,857.(c) Tsuchiya, M.; Ebbesen, T. W.; Nishimura, Y.; Sakuragi, H.; Tokumaru, K Chem. Lett. 1987,2121. (d) Konuma, S.;Aihara, S.; Kuriyama, Y.; Misawa, H.; Akaba, R.; Sakuragi, H.; Tokumaru, K. Chem. Lett. 1991,1897. (5) Tokumaru and his co-workers proposed that the oxidation depends on the structures of olefins, since the rate constant of koz varied in the wide range, = 1.3x lo6,7.2 x lo5,and 2.6 x lo8 M-l s-l for t-1", t-8'+, and radical cation of (E)-2,3-diphenyl-2-butene, respectively, in laser flash photolysis of the olefin and 9-cyanoanthracene in (6)c- and t-2-6 were prepared by the Wittig reaction.
tivities and that the reactivities of radical cations in solution are explained in terms of distonic radical cati o n ~ . ~ ~ ~ The reactions of radical cations 1*+-&+were investigated using a pulse radiolysis technique in 1,2-dichloroethane (DCE) at room t e m p e r a t ~ r e . The ~ transient absorption spectra immediately after the electron pulse were assigned to c- and &la+-&+ formed from the capture of the hole generated in the initiation step (Table 1).cand t-2'+-8'' show two typical absorption bands in the range of 400-600 and 700-1000 nm assigned to DZ DOand D1 DOtransitions, respectively, similarly to l'+.'O However, the absorption peaks of both shorter and longer wavelength bands of 3+-3and 8'+ shift to longer wavelengths with higher intensities than those of l'+ (Table 1). c - 6 + shows the absorption peak with extremely low intensity similar t o that of 1,3-dimethoxybenzene radical cation." The rate constants of unimolecular c-t isomerization (k,)were calculated from the formation of t-l'+-&' bands at the absorption peaks of DZ(Amax) at 5 mM of c-1-8 where unimolecular c -t isomerization was little affected by dimerization of el*--&+ and c-1-8 (Table 1). The rate constants of dimerization (kd)were measured from the dependence of the decay of 1'+-&+ bands a t A,, on the concentration of 1-8 (5-100 mM), while the rate constants of oxidation (ko,) were obtained from the dependence of the decay of c- and t-l*+-&+ bands on the concentration of 0 2 (2-14 mM). These rate constants thus obtained are summarized in Table 1. Unimolecular c-t isomerization was observed in c-3'+5" andc-&+ withp-CH30 (k,= 4.5 x 106-1.4 x lo7 s-l), but not in el'+,c-2'+, and c - 6 + without p-CH,O. No isomerization or k, < l o 6 s-l was observed in c-7'+ with p-CH30 and a methyl substitution on the olefinic carbon. The oxidation with 0 2 was also observed in t-3'+-&+ and t-T+withp-CHSO (ko, = (1.2-4.5) x lo7 M-I s-l 1, but not in el'+, c-2'+, and c - 6 + without p-CH30. No oxidation or ko, < lo6 M-' s-l was observed in t-&+with two p-CH30. The dimerization was observed in lo+-3" and t - 6 - (kd = (2.0-4.3) x lo8 M-l Since the dimerization occurs even in 3'- with the large kd value but not in 4", 5 + , and 7'+, it is suggested that the dimerization involves initial formation of a n-complex with overlapping of two benzene rings and that the n-complex formation +
+
(7)(a) Yates, B. F.; Bouma, W. J.; Radom, L. Tetrahedron 1986,22, 6225 and references cited therein. (b) Hammerum, S. Mass Spectrom. Rev. 1988,7,123. ( c ) Stirk, K. M.; Kiminkinen, L. K. M.; Kenttamaa, H. I. Chem. Reu. 1992,92,1649. ( 8 ) Tojo, S.; Toki, S.; Takamuku, S. J. Org. Chem. 1991,56, 6240. (9)The sample solutions was irradiated with an 8-11s pulse of 28 MeV electrons.3c (10)Shida, T.; Hamill, W. J . Chem. Phys. 1966,44,2375. (11)Takamuku, S.;Komitsu, S.;Toki, S. Radiat. Phys. Chem. 1989, 34,553. (12)The dimerization was also observed in t-8'+ on the basis of electrochemical measurements, although kd = (3.4-5)x lo3 M-ls-l was too small to be observed under the present conditions in this study. Steckhan, E. J . Am. Chem. SOC.1978,100,3526. Burgbacher, G.; Schaefer, H. J. J.Am. Chem. SOC. 1979,101,7590.
0022-326319511960-4684$09.00/0 0 1995 American Chemical Society
J. Org. Chem., Vol. 60, No.15, 1995 4685
Communications
Table 1. Absorption Peak of D2 of 1'+-8'+ in DCE and Rate Constants for the Reactions of 1'+-8'+ in Pulse Radiolyses in DCE at Room Temperaturea cidc) trans ( t ) stilbene
1'+ 2'+ 3'+ 4'+ 5'+
6'+ 7'+ 8'+
I,,,/nm
ck/W1
515 520 530 530 540 520 450,500 500,570
18 000 30 900 13 200 d d 3270 8820,9710 d
cm-l
Kils-l
b b 4.5 x 1.3 x 1.4 x b b 5.5 x
lo6 lo7 lo7
lo6
kdlM-1 s-1
3.9 x 108 3.4 x 108 2.0 x lo8 d d d d d
ko~'M-1s-1
c c d d d C
2.5 C
107
480 490 470,500 480 480 460,510 440,490 480,540
65 000 80 900 39 200,74 800 10 5000 84 600 24 500,20 800 18 400,29 400 33 100,76 000
C
c ' c 4.5 4.0 1.2 x c 2.8
C
C
3.9 x 108 4.3 x 108 3.0 x io8 C C
3.4 x 108
107 107 107 107
The absorption peaks of D2 (d,,,) of 1'+-8'+ in DCE. The molar absorption coefficients (€1,) were estimated from the optical density a t A nm and the optical length of 1 cm with assumption of concentration of 8 x M for 2'+-8'+ which was measured for t-l'+ on the basis O f €480 = 6.5 x lo4 M-l cm-' in DCE. ki, calculated from the formation oft'+ a t 5 mM of c-1-8; k d , measured from the dependence of the decay of 1'+-8'+ on the concentration of 1-8; KO,, obtained from the dependence of the decay of c.+ or t'+ on the concentration of 0 2 . ki < lo6 s-l since the formation oft'+ was not observed. k d and KO, < lo6 M-' s-l since no dependence of decay of c*+or to+on the concentration of c, t , or 0 2 . Not determined.
f i - ~ Scheme 2
H
1;
0 0 CH30+
CH3O (A)
(6)
is inhibited by steric hindrance of substituents on benzene rings and olefinic carbons. On the basis of these experimental results of k, and KO, it is clearly shown that p-CH30 as an electrondonating substituent on the benzene ring changes remarkably the reactivities of l+-&+. It is considered that a resonance structure (B)as shown in Scheme 2 for c-3'+ contributes to the electronic states of 3'+-5'+, 7'+,and 8'+ with p-CH3O. Positive charge is more localized on oxygen of p-CH30, while the unpaired electron density is more localized on the olefinic @-carbonin 3'+-5'+, 7'+, and 8'+withp-CH30 (charge-spin separation) than in 1" and %+ without p-CH30. Alternatively, oxygen ofp-CH3O and the olefinic @-carbon participate considerably to SOMO of 3'+-5'+, 7'+,and 8'+.Since the energy barrier for the unimolecular c-t isomerizations decreases with increasing the single bond character in the central C=C double bond, the reactivity of the isomerization increases and 8'+.Since unpaired electron density in 3'+-5'+, 7'+, is localized on the olefinic @-carbon,the reactivity of the oxidation increases in 3'+-5'+, 7'+,and 8'+. Consequently, p-CH30 in 3'+-5'+, T*,and 8'+causes enhancement of the reactivity due to the charge-spin separation, like distonic radical cations carrying spatially separated positive charge and an unpaired electron. Product analyses were performed in y-radiolyses of c-3-5 in Ar- or 02-saturated DCE at room temperature.13 t-3-5 were formed in high yields in y-radiolyses of 12-34 in Ar-saturated DCE, and 4-methoxybenzyl phenyl ketone was regioselectively formed in y-radiolysis of t-3 in 02-saturated DCE, although the details will be published elsewhere. These results are consistent with the order of ki and K O , and the charge-spin separation induced by p-CH3O in 3"-5'+,7'+,and 8'+. (13) y-Radiolyses of the sample solutions were performed by a 6oCo y source; concentration of substrate, 10 mM.3C
The unimolecular c-t isomerization of c-7'+ or oxidation of t-W+ did not occur, although both 7'+and 8'+have p-CH30. These are possibly explained in terms of steric hindrance for the twisting and spin density on the olefinic carbon, respectively. Methyl substitution on the olefinic carbon in c-7'+ provides more steric hindrance for the twisting about the C=C double bond than in ~ - 3 ' The ~. density of unpaired electron on the olefinic carbon in 8'+ with two symmetrical p-CH30 is lower than in 3'+. The unimolecular c-t isomerization or the oxidation of c-&+ did not occur. It should be noted that c-W has the absorption peak at 520 nm with extremely low intensity similar to that of 1,3-dimethoxybenzeneradical cation. These results may be interpreted by a twisted structure with the 3,5-dimethoxyphenylring rotating against the C=C double bond, since a structure (B)is not possible in c-6" having m-CH30. Tokumaru and his co-workers reported ki = 3 x lo5 s-l for radical cations of ~-4,4'-dibromostilbene(c-@+) and and ki > 3 x l o 5 s-l for ~-4,4'-dimethylstilbene(el@+) c-8'+ 2b and proposed that c-t isomerization might be accelerated by reduction of the electron density on the unsaturated linkage induced by the substituents on the basis of lower coupling constants of the a-H of t-9 and t-10 havingp-substituents than that of t-1 itself in ESR studies.14 They also proposed that the oxidation depends on the structures of olefins, since the ko2 values varied in the over a wide range.4c,dThe ki values for c-3'+-5'+ and c-8" are 1or 2 orders larger than those for c-W+ and c-lo'+ 2b as shown in Table 1. Remarkable enhancements of the isomerization by p-CH30 are explained by chargespin separation in c-3'+-5'+ and c-8'+. The present work is the first example to clarify that the reactivities of stilbene radical cations are controlled predominantly by charge-spin separation induced by p-CH30, and it is suggested that distonic radical cations can be acceptable to explain reactivities of radical cations even in solution.
Acknowledgment. This work was partly supported by a Grant-in-Aid (Nos. 05453121 and 07455341) from the Ministry of Education, Science and Culture of Japan. J09507981 (14)Bonazzola, L.; Michaut, J.-P.; Roncin, J.; Misawa, H.; Sakuragi, H.; Tokumaru, K. Bull. Chem. SOC.Jpn. 1990,63, 347.
