7098
J . Am. Chem. SOC.1982, 104, 7098-7104
Acknowledgment. We are indebted to the Mass Spectrometry Service of t h e Area della Ricerca di Roma, C.N.R., Italy, for running t h e CIMS experiments. Registry No. [I]y (Y = S ) , 3331-14-4; [I]y (Y = 0),83416-52-8; [I]a
(Y = S),83416-50-6; [ I ] a(Y = O),59242-30-7; [I]P (Y = S),8341651-7; [IIP (Y = 0). 83416-53-9; 111 (Y = S), 12105-68-9; I11 (Y = O), 83416-54-0; MeC1, 74-87-3; MeF, 593-53-3; CH3FCH3+,64710-12-9; CH3C1CH3+, 24400-15-5; Me’, 14531-53-4; NMe3, 75-50-3; furan, 110-00-9; thiophene, 110-02-1; benzene, 71-43-2.
Time-Resolved Study of the Solvent and Temperature Dependence of Singlet Oxygen ( Ag) Reactivity toward Enol Ethers: Reactivity Parameters Typical of Rapid Reversible Exciplex Formation A. A. Corman,* I. R. Gould, and I. Hamblett Contribution from the Chemistry Department, University of Manchester, Manchester M13 9PL, U.K. Received March 18, 1982
Abstract: Pulsed nitrogen laser excitation has been used to examine and compare the reactivities of four enol ethers toward singlet oxygen (la,) in five solvents. T h e experimental values of AH* (0 1.0 kcal mol-’), AS* (-23 t o -34 eu), and the overall reaction rate constants ( 104-108 L mol-’ s-’) are characteristic of excited-state processes involving rapid reversible exciplex formation. Solvent effects a r e small but in some cases significant. It is concluded that such effects are the result of solvation entropy requirements a t the transition state leading from exciplex t o product, particularly in the case of dioxetane formation. The possibility is raised that the reversible exciplex mechanism operates generally for O2(IAJ reactions.
1. Introduction The n a t u r e of t h e reaction of singlet oxygen, O2 (‘Ag), with alkenes and enol ethers t o give “enen-type products continues t o be a subject of considerable discussion and argument.’” The controversy has centered around two mechanisms involving (a) a classical six-center transition s t a t e and (b) a perepoxide intermediate. The advent of theoretical calculations in this area has in no way decreased mechanistic controversy. Orbital correlation diagrams3and CNDO/2 C17and MIND0/38 calculations favour initial perepoxide formation. On the other hand GVB-C19 calculations are claimed t o rule o u t this intermediate in favor of a 1,4 biradical although a concerted mechanism is not excluded.I0 More recently STO-3G and unrestricted MIND013 (UM3) results have been taken t o exclude both perepoxide and biradical mechanisms and favor the concerted process.I2 Experimentally it has been shown that for molecules containing a trisubstituted double bond, both enol ethers, and simple olefins, there exists a pronounced preference for hydrogen migration from t h e most substituted side of t h e double bond.I3 This, together (1) (2) (3) (4)
C. S . Foote, Acc. Chem. Res., 1, 104 (1968). K. Gollnick, Adu. Photochem., 6, 1 (1968). D. R. Kearns, Chem. Reu., 71, 395 (1971). K. Gollnick and H. J. Kuhn in ref 5, Chapter 8. ( 5 ) ‘Singlet Oxygen”, H. H. Wasserman and R. W. Murray, Ed., Academic Press, New York, 1979. (6) A. A. Frimer, Chem. Reu., 79, 359 (1979). (7) S . Inagaki and K. Fukui, J . Am. Chem. SOC.,97, 7480 (1975). (8) M. J. S. Dewar and W. Thiel, J . Am. Chem. Soc., 97, 3978 (1975); 99, 2338 (1977). (9) L. B. Harding and W. A. Goddard, J . Am. Chem. Soc., 102, 439 (1980). (10) The biradical mechanism may be rejected on experimental grounds, particularly the proven suprafacial character of allylic hydroperoxide formation” and the lack of correlation of reactivity with radical ~tabi1ity.l.~ (1 1) M. Orfanopoulos and L. M. Stephenson, J . Am. Chem. Soc., 102, 1417 (1980). (12) K. Yamaguchi, S . Yabushita, T. Fueno, and K. N. Houk, J . Am. Chem. Soc., 103, 5043 (1981). (13) (a) K. H. Schulte-Eke, B. L. Muller, and V. Rautenstrauch, Helu. Chim. Acta, 61, 2777 (1978); (b) K. H. Schulte-Elte, B. L. Muller, and H. Pamingle, ibid., 62, 816 (1979); (c) K. H. Schulte-Elte, B. L. Muller, and G. Ohloff, Nouu. J . Chim., 2, 427 (1978); (d) M. Orfanopoulos, M. B. Grdina, and L. M. Stephenson, J. Am. Chem. SOC.,101, 275, (1979).
Scheme I
1
with small but apparently significant isotope effect^,",'^ led t o the suggesti~n’~ t h a t t h e initial reaction between O2(‘Ag) and olefin is irreversible formation of a complex, the geometry of which, and subsequently that of the products, is determined by secondary orbital effects. It has, however, been argued t h a t t h e steric preferences cited are consistent with ground-state conformational effects.16 Of particular interest have been reports of methanol trapping
of interrnediate~.’~,’~ In Scheme I are reproduced conclusions reached by Jefford and R i m b a u l t ” concerning t h e reaction of 2-methoxynorbornene (1) with O2(I&) in methanol. The isolation of the solvent addition product, shown not t o be derived from t h e (14) (a) K. R. Kopecky and J. H. Vander Sande, Can. J . Chem., 50,4034 (1972); (b) A. Nickon, V. T. Chaung, P. J. L. Daniels, R. W. Denny, J. B. DiGiorgio, J. Tsunetsugu, H. G. Vilhuber, and E. Werstiuk, J . Am. Chem. Soc.,94,5517 (1972); (c) L. M. Stephenson, D. E. McClure, and P. K. Sysak, ibid., 95, 7888 (1973); (d) S . B. Grdina, M. Orfanopoulos, and L. M. Stephenson, ibid., 101, 3111 (1979); J . Urg. Chem., 44,2936 (1979); ( e ) A. A. Frimer, P.D. Bartlett, A. F. Boschung, and J. G. Jewett, J. Am. Chem. Soc., 99, 7977 (1977). (15) L. M. Stephenson, Tetrahedron Lett., IO05 (1980). (16) (a) K. Schulte-Elte and V. Rautenstrauch, J . Am. Chem. Soc., 102, 1738 (1980); (b) K. N. Houk, J. C. Williams, P.A. Mitchell, and K. Yamaguchi, ibid., 103, 949 (1981). (17) C. W. Jefford and C. G. Rimbault, J. Am. Chem. Soc., 100, 6437 (1978). (18) E. W. H. Asveld and R. M. Kellogg, J . Am. Chem. SOC.,102, 3644 (1980).
0002-786318211504-7098$01.25/0 0 1982 American Chemical Society
J. Am. Chem. SOC.,Vol. 104, No. 25, 1982 7099
Reactivity Parameters of Singlet O2 corresponding dioxetane, was taken as evidence for a zwitterionic intermediate and claimed to support calculation^^^^^ in favor of perepoxide intermediacy in general. In this paper we report the determination of rate and activation parameters for the reaction in five solvents of O2(l Ag) with a series of enol ethers (1-4). Compounds 2 and 3 bear an obvious rela-
2
4
3
tionship to 2-methoxynorbornene (1) while 4 is typical of compounds shown'3d to exhibit a preference for H migration from the most substituted side of the double bond during allylic hydroperoxide formation. In terms of product distribution compounds 2-4 show behavior typical of enol e t h e r ~ , i.e., ~ ~ allylic J ~ ~ hydroperoxide formation in the less polar media, cyclohexane and toluene, with increasing proportions of dioxetane derived products on going from acetone to methanol to acetonitrile.z1 As mentioned, the products of reaction of 1 have been p~b1ished.l~It is generally acceptedZZ that most organic substrates, and certainly those of the class under consideration, react with 0, (lAJ exclusively in a chemical as opposed to a physical sense. In this work we therefore assume that rate constants for quenching of O2('Ag) by substrate reflect removal of the transient via chemical reaction.
2. Determination of Rate Parameters by Pulsed Laser Photolysis The extraction of rate parameters for decay and reaction of 0, ('Ag) in solution by pulsed laser photolysis is well documented;" in this work we have employed the 2.5-11s pulse of a nitrogen laser emitting at 337 nm. Absorption of such a pulse by an aerated mol L-I) solution of anthracene (A) as sensitizer (1-3 X results in O2 ('Ag) formation via channels 1-5. In the presence
lI A A* *
-+A PT
-+
3A* + A 3A*
+ O,('Zg-)
-+A
+ 0,('Ag)
(2) (3)
(4) (5)
of the monitor diphenylisobenzofuran (DPBF) and an additional quencher (Q) the decay of O2('Ag) is governed by channels 6-8, O2 (lAg)
2O2(32;)
+ Q k, 0 2 ('Ag) + DPBF 0 2
('Ag)
loss of 0 2 ('Ag)
-
4
kr
loss of DPBF
(7)
(8)
the rate parameters for which may depend to a lesser or greater extent on the medium. When the initial concentration of Oz ('Ag) (19) (a) S. Inagaki, S. Yamabe, H. Fujimoto, and K. Fukui, Bull. Chem. SOC.Jpn., 45,3510 (1972); (b) K. Yamaguchi, T. Fueno, and H. Fukutome, Chem. Phys. Lett., 22, 466 (1973).
(20) (a) W. Ando, K. Watanabe, J. Suzuki, and T. Migita, J . Am. Chem. Soc., 96, 6766 (1974); (b) G. Rousseau, P. Le Perchec, and J. M. Conia,
TetrahedronLett., 2517 (1977); Synthesis, 67 (1978); Tetrahedron, 34, 3475, 3483 (1978); (c) P. D. Bartlett, G . D. Mendenhall, and A. P. Schaap, Ann. N.Y. Acad. Sci., 171, 79 (1970); (d) E. C. Blossey, D. C. Neckers, A. L. Thayer, and A. P. Schaap, J . Am. Chem. SOC.,95, 5820 (1973). (21) A. A. Gorman, I. R. Gould, and I. Hamblett, to be published. (22) (a) G. Hollinden and R. B. Timmons, J . Am. Chem. SOC.,92,4181 (1970); (b) P. B. Merkel and D. R. Kearns, ibid., 97,462 (1975); (c) C. S. Foote and T.-Y. Ching, ibid., 97, 6209 (1975); (d) C. S. Foote in ref 5, Chapter 5; (e) R. H. Young and D. R. Brewer in ref 23, Chapter 6; (f) D. Bellus in ref 23, Chapter 9. (23) "Singlet Oxygen Reactions with Organic Compounds and Polymers", B. Ranby and J. F. Rabek, Ed., Wiley, New York, 1978. (24) (a) D. R. Kearns and R.Nilsson, J. Am. Chem. Soc.,94,7244 (1972); (b) D. R.Adams and F.Wilkinson, J . Chem. SOC., Faraday Trans. 2,68,586 (1972); (c) R. H. Young, D. Brewer, and R. A. Keller, J . Am. Chem. Soc., 95, 375 (1973).
is small compared to the initial concentrations of DPBF and Q the rate of loss of DPBF is given by eq 9, where Q may act via d(-[DPBF]) dt k,[DPBF] [ 0 2 ('Ag)lo exP[-(kd + k[DPBF1 + k,[Ql)tl ( 9 ) chemical or physical action and [O, ('A,)lo is the concentration of 0, ('Ag) at the end of the laser pulse. It is readily shown that a plot of In (D,- Dm)against time will be linear with a slope, k', given by eq 10, where Dland D, are respectively the DPBF optical k' kd + k,[DPBF] + k,[Q] (10) densities at time t and at completion of the bleaching event. The rate parameters, kd, k,, and k, can be evaluated from plots of k' against (1) DPBF concentration (in the absence of quencher) for kd and k, and (2) quencher concentration (at fixed DPBF concentration) for k,. An alternative O2 (lAg) monitor is the triplet state of P-car~ t e n e .The ~ ~ carotenoid quenches O2(l A,) via collisional energy transfer according to eq 11 and the subsequent triplet decav is
--
+ @-car k, 3p-car* + O2 (32;)
O2 (lA,)
3&car*
+
O2 (32,-) 3/?-car*
(11)
@-car+ 0, (jz;)
(12)
4
&car
(13) via channels 12 and 13. At sufficiently high carotene concentrations its triplet formation is rate limiting and therefore the rate constant for decay of 3p-car*, k', follows the decay of 0, ('Ag) and eq 10 is replaced by eq 14. Extraction of individual rate constants is as for DPBF bleaching. k' = kd
+ k,,[@car] + k,[Q]
(14)
3. Determination of Activation Parameters by Pulsed Laser Photolysis We have already demonstrated the temperature dependence of kd and k, using the pulse radiolysis technique26and compared these results with those previously reported.27 In order to determine the effect of temperature on k, for individual substrates in a particular solvent, it is not necessary to determine separately the temperature effect on kd and k , but only the effect on their composite contribution to k'. Thus, over the required temperature range, 0, ('Ag) quenching experiments (section 2) may be performed on a reaction solution which is identical, apart from the presence of quencher, with the solution for a set of "background" experiments. The difference in the two thus obtained k'v. temperature plots gives the variation in k,[Q] with temperature. Correction for changes in quencher concentration with temperature thus allows construction of the Arrhenius plot for k,. Either the DPBF or &carotene methods may be used. The former method has been employed to show that the quenching of O2 ('Ag) by furans and indoles in entropy controlled.26
4. Experimental Section 4.1. Pulsed Laser Photolysis. Experiments were carried out with the 2.5-11spulse (337 nm) of a Lambda Physik 1-MW nitrogen laser, at right angles to a pulsed 250-W xenon analyzing lamp and Hilger-Watt monochromator. Back-off circuitry, built in house, allowed small deviations from the initial high pulsed lamp intensity to be displayed. The period of exposure of reactants to the analyzing light was confined to
