4707
J . A m . Chem. SOC.1983, 105, 4701-4710 trans-BuOz when obtained a s a cross ozonide from unsymmetrical cis- a n d trans-alkenes h a s also been f ~ r w a r d e d . ~L~i k, ~e t h e situation for 1-alkenes, these proposals a r e useful starting points for a stereochemical analysis. Nevertheless, the ozonolyses in CHCIFz are a reminder t h a t t h e solvent environment may also play a role in determining t h e ozonide stereochemistry. The neglect of reaction-solvent coupling in present stereochemical Criegee models is a possible origin, at least in part, for s o m e of t h e limitations t h a t occur in their application.
Experimental Section IH N M R were obtained with a Briiker WM-360 spectrometer. Microwave spectra were recorded with a Hewlett-Packard 8460A spectrometer. Conventional vacuum-line techniques on a calibrated manifold were employed for most manipulations of reactants and products. A Welsbach Model T-408 ozonator was employed. Materials. C P grade propylene (Matheson), cis-2-butene (Matheson), trans-2-butene (Matheson), trans-2-pentene (Aldrich), acetaldehyde (Matheson Coleman and Bell), and solvents (isobutane, CHCIF,, and CH,CI) were normally dried by passage through cold traps. ( E ) Propene-1-dl (67% enriched) was prepared in our lab?6 and propene-2-dl (98% enriched) was purchased from Merck Co. Isotopic enrichments were checked by microwave spectroscopy. Ozonolysis Procedure. Reactions were conducted with flow rates of 0 . 1 4 2 mmol of O,/min by standard technique^.^^'^*^^ The solutions were 1.0 M in alkene (usually 2 mmol of alkene in 20 mmol of solvent), and 0.6-1.0 mmol of ozone was used. All the input ozone was consumed in the reaction vessel. After reaction, a fast warm-up procedure was employed.& The reaction mixture was then distilled through traps held at -78 and -196 "C. The ozonides were collected in the -78 "C trap. The total yields of ozonides were determined manometrically. They were around 60% from propylene, 35% from cis- or trans-2-butene, and less than 20% from trans-2-pentene. Relative Yields and cis -/trans -2-Butene Ozonide Stereochemistry, Ozonolysis of propylene produces the normal and cross ozonides (Scheme 11). The relative yields of these ozonides were analyzed by 360-MHz proton N M R . Figure 1 shows a part of the N M R spectrum of the ozonide mixture. Peak assignments were obvious from the relative integrations and the splittings. For cis- and trans-2-butene ozonides, a quartet arises from the methine hydrogen for each stereoisomer. The downfield quartet was assigned to the cis i s ~ m e r , * * ~ ~consistent ~ ' ~ * " with the GC/microwave results, which unequivocally determined the stereochemistry.' Integration of these peaks and the other resonances in this
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(34) Fliszir, S.;Renard, J.; Simon, D. 2.J . Am. Chem. SOC.1971, 93, 6953. (35) Rouse, R. A. J . Am. Chem. SOC.1974, 96, 5095. (36) Imachi, M.; Cant, N.; Kuczkowski, R. L. J . Cutal. submitted for publication.
region gave the relative ozonide yields. Although the methyl groups in the cis- and trans-2-butene ozonide mixture also produce two doublets at 6 1.40, they were too overlapped to be used for quantitative measurements. 'H N M R (room temperature, in CDC13): ethylene ozonide 6 5.082; propylene ozonide, 6 1.424 (d, J = 4.90 Hz, 3 H), 5.038 (d, J = 0.46 Hz, 1 H), 5.178 (s, br, 1 H), 5.256 (q, 1 H); cis-2-butene ozonide 6 1.404 (d, J = 4.98 Hz, 3 H), 5.335 (q, 1 H); trans-2-butene ozonide 6 1.402 (d, J = 4.88 Hz, 3 H), 5.290 (q, 1 H). KSIE of CH3CH=CH2/CH3CD=€H2 and CH,CH==CH,/CH$H= CHD. A mixture of approximately 50% propylene-do and 50% propylene-d, was prepared on a vacuum line and a portion saved for later analysis. The mixture was ozonized to 30% in isobutane at -78 "C followed by distillation through -78, -1 16, and -196 "C traps. Ozonide was collected at -78 "C, solvent at -1 16 OC, and unreacted propylene at -196 "C. The recovered and initial propylene as well as the ozonides were analyzed by 360-MHz N M R in CDCI, at room temperature. Propylene gives a complex spectrum with multiplets centered at 6 1.71, 4.93, 5.02, and 5.82. The peak at 6 5.82 arises from the methine hydrogen since its multiplicity appeared to be 16 and its intensity decreased with propene-2-d,. The peak at 4.93 could be assigned to the H trans to CH, from the intensity change with (E)-propene-1-d,. Multiplet regions were integrated and simple relationships were used to extract the do/dl ratios. For example, with mixtures of propene-do and -2-dl, the resonances at 5.82 arise from the dospecies while the multiplets at either 4.93 or 5.02 can be used to determine the doand 2-dl sum. For mixtures of propene-do and (E)-propene-1-d,, the resonances at 4.93 arise from the d, species while either of the other two multiplets downfield determine the do and d l sum. The relative intensities of the lo, 202and 1 2,, microwave transitions of the normal and both deuterated species of propylene were also used to determine the do/d, ratio^.'^,^' These transitions are split into partially resolved doublets due to methyl tunneling effects so the intensity ratios were evaluated by cutting and weighing. Base-line problems are accentuated by this procedure and contribute to some of the data scatter. The do/dl, N M R analysis of the propylene ozonide was carried out in a manner similar to that of propylene. These data were used in relationships previously discussed to determine kH/ kD.I5
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Acknowledgment. We a r e grateful t o F r a n k P a r k e r w h o w a s invaluable in obtaining NMR spectra, Dr. Misako I m a c h i who provided t h e (E)-propene-I-d,, a n d Dr. K u r t Hillig who advised on ozonolysis techniques. T h i s work was supported by G r a n t C H E - 8 0 0 5 4 7 1 f r o m t h e N a t i o n a l Science Foundation. Registry No. D,, 7782-39-0 CH,CHO, 75-07-0 propylene, 115-07-1; cis-2-butene, 590-18-1; trans-2-butene, 624-64-6; trans-2-pentene, 64604-8. (37) Lide, D. R., Jr.; Mann, D. E. J . Chem. Phys. 1961, 35, 1374
Chemistry of Singlet Oxygen. 43. Quenching by Conjugated Olefins Lewis E. Manring, Richard C. Kanner, and Christopher S . Foote* Contribution from t h e D e p a r t m e n t of Chemistry and Biochemistry, University of California, Los Angeles, California 90024. Received N o v e m b e r 26, 1982
Abstract: The interaction of singlet oxygen with the olefins 2,5-dimethyl-2,4-hexadiene (DMHD) and 1-methyl-2-phenylindene ( M P I ) leads to both reaction and quenching. Although the initial interaction with singlet oxygen is nearly independent of solvent and temperature, the relative amount of quenching depends on solvent in the case of D M H D and on both solvent and temperature in the case of M P I . T h e increased reaction rate for indenes a t lower temperatures is caused by a decrease in the fraction of the initial interaction which leads to quenching. A similar factor governs the increase in D M H D rates in going from aprotic to protic solvents.
T h e interaction of lo2with organic substrates has been studied I t is known t h a t three m a j o r paths a r e available 0002-7863/83/ 1505-4707$01.50/0
t o '0, in solution, a s shown in S c h e m e I.4 T w o of t h e three paths in S c h e m e I a r e dependent on t h e
0 1 9 8 3 A m e r i c a n Chemical Society
4708 J . Am. Chem. Soc., Vol. 105, No. 14, 1983
Manring, Kanner, and Foote
Scheme 1
Table I. Rates of Interaction and Rcaction of DMHD with '0,
3oz solvent
sensitizer, hv or chemical source
CH,OH CH,COCH, CH,Cl, CH,CN
s
10-6 2.6 i 0 . 2 O 3.6 i 1.2b 3.9 f 0.6b 5.2 t 1.1 5.2 t 0.43: 6.3 i l . l b
3.4 0.38 0.60 0.76
2.5 * 0.6 > k,. The problem is compounded if k, is solvent and/or temperature dependent, as has been shown presence of substrate (S): k, (nonreactive quenching) and k, to be the case for dialkyl sulfide-'02 reactions.8*21q22 The (oxidation of substrate by IO2). The third path, kd, is the rate availability of direct time-resolved 1.27 luminescence detection of radiationless decay and is solvent d e ~ e n d e n t . ' - ~ , ~ from '02provides a new and more precise method of measuring With the assumption that [ S ] ( k , k,) > kd,Le., the substrate (k, + kq).5*23-26In this paper we report that two substrates with interacts with a majority of the lo2formed, a number of possible conjugated olefinic double bonds do quench IO2 significantly and situations arise. First, when k, >> k,, product SO2 is formed on that the relative amount of quenching depends on solvent and every collision with IO2 which results in its removal. If k, k,, temperature. S is still oxidized to SO2,but the rate of SO2 appearance is less than the total rate of removal of IO2 because path k, leads to Experimental Section starting material and ground-state oxygen, causing no loss of Chemicals. Solvents were Mallinkrodt spectrograde CH3COCH3, substrate. Finally, if k, >> k,, SO2 is not formed appreciably, CH$N (distilled from €,O ' s), C&, and C H 3 0 H and AR CH2Clt. although IO2may be removed at a very high rate. All of the above CD3COCD3and DMHD were obtained from Aldrich. In early experisituations are known to occur in singlet oxygen c h e m i ~ t r y . ~ , ~ - ~ ments, the DMHD was purified by spinning band column distillation, and in later experiments by preparative GC. 2-Methyl-2-pentene (Aldrich) Quenching of singlet oxygen was first reported by Schenck and was passed through basic alumina immediately prior to use. 1,3-DiGollnick for nicotine.g Two mechanisms of quenching are known. phenylisobenzofuran (Aldrich) (DPBF) was recrystallized several times An energy-transfer process occurs with carotenes and certain other under dim lighting from benzene-ethanol, mp, 130-131 0C.27 The compounds with very low triplet energies.IO A second process synthesis of 1-methyl-2-phenylindene (MPI) has been described previinvolves formation of a charge-transfer complex between the o ~ s l y . ~ *Rose bengal (Eastman) was chromatographed on silica gel electron-rich substrate and lo2in which intersystem crossing plates with 2-butanone/H20 20: 1; tetraphenylporphine (Aldrich) (TPP) and S. This process occurs restrictions are relaxed, leading to 302 and mesoporphyrin IX-dimethyl ester (Sigma) were used without further with substrates such as amine^,^ sulfides,8 and phenols." purification. Rate Determinations. Values of ( k , + k,) were determined either from Quenching of lo2by simple olefins is believed to be unimportant,12 /3 (kd/(k, + k,)) or directly from the effect of added substrate on the IO2 although this has been firmly established for only a few cases.6 luminescence decay rate koM ( ( k , + k , ) [ S ] t kd). j3 values were deBecause olefins are not commonly expected to quench IO2,both termined by Young's technique2' (inhibition of the photooxygenation of ( k , k,)13-1sand k,16-lgvalues are often used as measures of DPBF followed by DPBF fluorescence) as described previously;30 lo-' relative olefin reactivities. Previously, relative ( k , + k,) values to 1 X 10" M DPBF were used. Values of M rose bengal and 5 X have usually been derived from measurements of /3, where /3 = ( k , + k ) were calculated by assumming kdMsoH= 1.05 X IOs kdaEmnc 2.17 X lo4 S-',' and kdMeCN= 1.54 X lo4 S-1.31 The methods k d / ( k , k,) and is calculated from the dependence of product formation on substrate c~ncentration,'~ or from a similar analysis for direct determination of ( k , + k,) by IO2luminescence are described based on competitive inhibition of a reactive s u b ~ t r a t e . ' ~ *Most '~ e l ~ e w h e r e . ' ~Mesoporphyrin ~~ IX-dimethyl ester, 5 X M , was used as a sensitizer in MeCN, acetone, CH2C12,and benzene. k, values are determined by the amount of product formed" or Olefin k , values were determined by the method of Higgins et aLzoby starting material consumedI6J8in competition with a substrate competition with 2-methyl-2-pentene (2M2P) (Aldrich). Values of k , whose reaction rate is well characterized.20 The use of both k, for 2M2P in all solvents were determined by the direct technique,' asand ( k , k,) values as reactivity measurements can obviously suming6 k,2MZP
