2683
J. Am. Chem. SOC.1991, 113, 2683-2686
Rates of Decarboxylation of Acyloxy Radicals Formed in the Photocleavage of Substituted 1 -Naphthylmethyl Alkanoates James W. Hilborn and James A. Pincock* Contribution from the Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, B3H 4J3 Canada. Received July 16, 1990
Abstract Rates of decarboxylation (kCgR) have been estimated for the acyloxy radicals 7a-f formed in the photolysis of substituted 1-naphthylmethyl alkanoates 6a-f. These rates are based on a proposed mechanism involving initial carbon-oxygen homolytic bond cleavage from the excited singlet state. The products are formed by two competing pathways: electron transfer in the radical pair to give an ion pair and decarboxylation. Measured product yields along with an estimate of the electron-transfer rate (kET)allow calculation of kco as a function of R. The values obtained are the following (R, k (lo9s-l)): CH), > kIX)and that competition between electron transfer (Km)and decarboxylation (kcol) then controls the product distribution. Since these are all esters of phenylacetic acid, the rate of decarboxylation is a constant (Le., radical clock”) so that the product ratio 3/5 = kET/kC02(R = PhCHz) is a measure of ET. systematic variation in kET is possible by variation of the substituents (X), which control the oxidation potential12 of the l-naphthylmethyl radical. These rates of electron transfer were well-rationalized by Marcus theory,” including the observation of the (8) Givens, R. S.; Matriszewski, B.; Neywick, C. V. J . Am. Chem. Soc. 1914, 96, 5547. (9) Cristol, S. J.; Bindel, T. H. Orgunic Photochemistry; Marcel Dekker: New York, 1983; Vol. 6, p 327. (10) Arnold, B.; Donald, L.; Jurgens, A.; Pincock. J. A. Can. J . Chem. 1985.63, 3140. (11) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1980, 13, 317. (12) Pincock. J. A.; Wayner, D. D. M. To be published. (13) Eberson, L. Electron Transfer Reactions in Organic Chemistry; Springer-Verlag: New York, 1987; p 32.
0002-7863/91/1513-2683$02.50/00 1991 American Chemical Society
2684 J . Am. Chem. SOC.,Vol. 113, No. 7, 1991
'inverted region", which predicts slower rates of electron transfer as the process becomes prohibitively exoergonic. To our knowledge, this is the first observation of this phenomenon in a caged radical pair. If this explanation is correct, a second consequence follows. If the rate of electron transfer is kept constant by keeping X constant, then product distribution will be controlled by the rate of decarboxylation. This rate can obviously be systematically varied by changing the carboxylic acid side of the ester. We now report results for the photolysis of the esters 6 that support this idea. As well, we report the first determination of rate constants for the decarboxylation of the radicals 7.
0
II
0-C-R
7
6 b,7~ R = CH3 6b,7b: R = CH3CHp 6c, 7 ~R:= (CHJ2CH 6d,7d: R = (CHJ3C 8.,70: R = PhCHp 6f, 71: R = PhCH2CH2
Experimental Section General Procedure. Ultraviolet (UV) spectra were obtained in methanol on a Varian Cary 219 spectrophotometer. Proton (IH NMR) and carbon ("C NMR) nuclear magnetic resonance spectra were obtained in CDCI, with TMS as an internal standard on a Nicolet NB 360 spectrometer. Mass spectra were obtained on a Hewlett-Packard 5890 GC/MS with a 5% phenylsilicone capillary column. Infrared spectra were obtained on a Pye Unicam S P 1000 spectrophotometer. Fluorescence spectra were obtained on a Perkin-Elmer MPF 66 spectrophotometer with samples in methanol that were degassed by three freezepump-thaw cycles; the optical densities of the samples were always less than 0.2, and quantum yields were obtained relative to a value of 0.2114 for 1-methylnaphthalene. Fluorescence lifetimes were obtained on the same samples with use of a PRA System 3000 instrument with a hydrogen flash lamp of pulse width 0.8 ns. Microanalysis were by Canadian Microanalytical Service Ltd., Delta, BC, Canada. Irradiations. Irradiations were done with a 200-W medium-pressure Hanovia mercury lamp in a standard immersion well with a Pyrex filter. Solutions were degassed with a slow nitrogen stream. Progress of the reaction was monitored by HPLC with a Waters system, operating in isocratic conditions (80/20 methanol/water), equipped with a Brownlee Spheri-IO reversed-phase column (25 X 0.46 cm). Detection was at 280 nm. Complete conversion of 200 mg of the esters took about 20 h. Product ratios remained constant throughout the course of the irradiation, and dark samples taken either before or during the irradiation showed no conversion. Yields were obtained by calibrating the HPLC detector with samples of the products of known concentration in methanol. Acetyl chloride, 2,2-dimethylpropanoyIchloride, propanoic anhydride, 1-(1'-naphthyl)ethanal, and 1-naphthylmethanol were obtained from Aldrich and were used without further purification. Pyridine was obtained from Analar and was distilled before use. 2-Methylpropanoyl chloride was prepared from 2methylpropanoic acid with use of the method of Kent and M c E l ~ a i n . 1-Cyanonaphthalene ~~ was synthesized by the method of Friedman and Shechter.16 1-(Methoxymethy1)naphthalene (8) was obtained by the procedure of Wright and Platz." Synthesis of Esters Qa,c-f. To a well-stirred solution of 1naphthalenemethanol (10 mmol) and 1 mL of pyridine in 50 mL of dry benzene was slowly added the corresponding acid chloride (IO mmol) in 30 mL of benzene at room temperature. The pyridinium hydrochloride salt precipitated, and after all of the acid chloride was added, the solution was stirred overnight. Then 50 mL of water was added, and the two layers were separated. The benzene layer was washed twice with 10% aqueous HCI, once with 5% aqueous NaOH, and finally with water. The organic layer was then dried (MgS04), filtered, and rotoevaporated to yield the crude ester. (14) Lentz, P.;Blunre, H.; Schutte-Frohlinde, D. Eer. Bunsen-Ges.Phys.
Chem. 1970, 74. 484.
(1 5 ) Kent, R. E.; McElvain, S.M. Organic Syntheses; Wiley: London, 1943: Collect. Vol. 111. D 490.
(16) Friedman, L.; Shechter, H. J. Org. Chem. 1961, 26, 2522. (17) Wright, B. B.; Platz, M. S. J . Am. Chem. Soc. 1984, 106, 4175.
Hilborn and Pincock The ester was then column chromatographed over silica gel with 50/50 hexane/methylene chloride as the eluent. The ester fractions were identified by TLC and then combined and concentrated. The esters were distilled under vacuum for purification. Synthesis of Ester 6b. To a 50" round-bottom flask were added 2.96 g (18.7 mmol) of 1-naphthylmethanol and 20 mL of pyridine. Then 2.40 mL (2.36 g, 18.2 mmol) of propanoic anhydride was introduced and the solution heated at 50 OC for 1 day and then stirred at room temperature for 12 h. The solution was poured into 50 mL of ether and washed sequentially with 20% HCI, 5% NaHC03, and water. The organic layer was dried (MgS04). filtered, and rotoevaporated to yield the crude ester as a pale yellow oil. The ester was purified as above. Chsracterization of Esters. I-Naphthylmethyl acetate (6a): yield 63%; 265 nm bp 52-53 OC (0.5 mmHg) [lit.lo bp 65 OC (1 mmHg)]; UV A, (e 6.54 x lo3), 275 (6.80 x lo3), 284 (4.57 x 10)); IR (neat) 3070, 2980, 1740 (C=O), 1340, 1240 (C-O), 1035,800,760 cm-I; IH NMR identical with that reported earlier;I0 "C NMR 6 170.9 (s, C=O), 133.7 (s), 131.5 (s), 131.4 (s), 129.2, 128.7, 127.4, 126.5, 125.9, 125.2, 123.5,64.5 (t, CH20, J = 146.4 Hz), 21.0 (9, C H 3 , J = 129.5 Hz); GC/MS, m / z 201 (7, M + l), 200 (52, M'), 158 (94), 141 (loo), 140 (62), 139 (36), 129 (50), 128 (29), 127 (29), 115 (47). I-Naphthylmethyl propanoate (6b): yield 65%; bp 86-88 OC (0.5 265 nm (e 6.56 X lo'), 275 (7.26 X lo'), 284 (5.10 mmHg); UV ,A, X IO'); IR (neat) 3015, 2950, 2915, 2870, 1735 (C=O), 1460, 1345, 1265, 1175 (C-O), 1070,1000,780,760 cm-I; ' H NMR 6 8.00 (d, 1 H, J = 8.05 Hz), 7.82-7.88 (m,2 H), 7.42-7.55 (m, 4 H), 5.57 (s, 2 H, CHZO), 2.38 (q, 2 H, CH2CH3, J = 7.56 Hz),1.15 (t, 3 H, CH2CH3, J = 7.54 Hz); ''C NMR 6 174.3 (s, C = O ) , 133.6 (s), 131.5 (s), 129.1, 128.9 (s),128.6, 127.3, 126.5, 125.9, 125.2, 123.5, 64.4 (t, CH20, J = 147.9 Hz), 27.6 (t, CH2CH3, J 127.6 Hz), 9.1 (q, CH2CH3, J = 127.3 Hz); GC/MS, m / z 215 (8, M + l), 214 (48, M+), 159 (12), 158 (96), 142 (18), 141 (loo), 140 (64). 139 (37). 129 (43), 128 (24), 127 (24), 115 (49), 57 (46). Anal. Calcd for CI4Hl4O2:C, 78.48; H, 6.59. Found: C, 78.04; H, 6.59. I-Naphthylmethyl 2-methylpropanoate (6c): yield 50%; bp ! 15 O C (0.5 mmHg) [lit.I8bp 127-128 OC (1 mmHg)]; UV A, 266 nm (a 5.37 X lo3), 276 (6.08 X IO3), 284 (4.40 X IO'); IR (neat) 3030, 2980, 2940, 2880, 1740 (C=O), 1465, 1195 (C-0), 1160,965,790, 775 cm-I; 'H NMR identical with that reported earlier;" "C NMR 6 176.9 (s, C=O), 133.6 (s), 131.6 (s), 131.6 (s), 129.1, 128.6, 127.2, 126.4, 125.8, 125.2, 123.5,64.5 (t, CH20, J = 148.8 Hz), 34.1 (d, CH(CH3)2, J = 129.7 Hz), 9.0 (q, CH(CH3)2,J = 127.6 Hz); GC/MS, m / z 229 (IO,M + I), 228 (49, M'), 158 (79), 142 (32), 141 (loo), 140 (46), 139 (37), 129 (24), 128 (23), 127 (24), 115 (79), 71 (24). I-Naphthylmethyl2,2-dimethylpropnoate(a): yield 62%; bp 89-9 1 265 nm OC (0.1 mmHg) [lit.I9 bp 108-110 OC (0.2 mmHg)]; UV ,A, (e 5.54 X lo3), 275 (6.47 X lo3), 284 (4.60 X lo3); IR (neat) 3060, 2990, 2965,2920,2885, 1730 (C=O), 1485, 1285, 1165 (C-O), 800,780 cm-'; IH NMR identical that that reported earlier;20I3C NMR 6 178.3 (s, C=O), 133.7 (s), 131.8 (s), 131.6 (s), 129.0, 128.6, 126.9, 126.4, 126.1, 125.8, 125.2, 123.6, 64.6 (t, CH20, J = 150.0 Hz), 39.0 (s, -C(CH3)3), 27.2 (q, C ( C H 3 ) 3 ,J = 126.7 Hz); GC/MS, m / z 243 (6, M + l), 242 (32, M'), 142 (24). 141 (loo), 139 (18), 127 ( I l ) , 115 (39), 57 (100). 1-Naphthylmethyl phenylacetate (6e): bp 130-132 OC (0.05 mmHg) [lit.21bp 212 OC (4-5 mmHg)]. I-Naphthylmethyl phenylpropanoate (62): bp 137-1 39 "C (0.05 mmHgZ2). Syntbesi of Akylnaphthalenes I0S-d. I-Ethylnaphthalene (loa) and I-propylnaphthalene (lob) were obtained by Wolff-Kishner reduction of 1-(1'-naphthy1)ethanal and 1-( 1'-naphthyl)propanone, re~pectively.~' 1-(1'-Naphthy1)propanoneand 2-methyl-1-(1'-naphthy1)propanone were prepared by reaction of the corresponding Grignard reagent with 12-Methyl-1-naphthylpropane(1Oc) was obtained ~yanonaphthalene.~~ via the method of West et aLzs 2,2-Dimethyl-l-naphthylpropane(IM) was prepared as described p r e v i o ~ s l y . ~These ~ hydrocarbons gave 'H NMR identical with those reported previously.2629 (18) Arnold, R. T.; Kulenovic, S.T.J . Org. Chem. 1980, 45, 891. (19) Winnik, M. A.; Pekcam, 0.; Egan, L. Polymer 1984, 25, 1767. (20) Holden, D. A.; Wang, P. Y.-K.; Guillet, J. E. Macromolecules 1980, 13, 295. (21) Chakravarti, R. N.; Dhar, R. C. J . Indian Chem. Soc. 1953,30,751. (22) DeCosta, D. P.;Pincock, J. A. To be published. (23) Tsuno, Y.; Sawada, M.; Fujii, T.; Yukawa, Y. Bull. Chem. Soc. Jpn. 1975,48. 3347. (24) Kloetzel, M. C.; Wildman, W. C. J . Org. Chem. 1946, 1 1 , 390. (25) West, C. T.; Donnelly, S.J.; Kooistra, D. A.; Doyle, M. P. J . Org. Chem. 38. -2615. . . . 1973. -. - (26) Bullpit, M.; Kitching, W. Synthesis 1977, 316. (27) Sadtler Handbook of Proton NMR Spectra; Simons, W. W., Ed.; Sadtler Research Laboratories: Philadelphia, 1978; Vol. 201.
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J . Am. Chem. SOC.,VO~. 113, NO. 7, 1991 2685
Rates of Decarboxylation of Acyloxy Radicals Tibk I. Results for the Photolysis of the Esters 6 in Methanol at 20 OC kco x I/R s-' 98 ND >2od
