3782
SAULG. COHENAND JACOBI. COHEN
M) acid independent above [CHsCOOH] g 6 X has a much lower value in an acid-free system. Such behavior may be due to salt effects on k3 upon increasing acid concentration. Further work, studying salts, solvent, and light intensity effects, may help to clear
the dismutation mechanism in both acid and neutral solutions. Acknowledgment. The authors wish to thank Dr. J. Rabani and Professor H. Linschitz for valuable discussions.
Photoreduction of Aminobenzophenones in Nonpolar Media. Effects of Tertiary Amines1 by Saul G . Cohen and Jacob I. Cohen Department of Chemistry, Brandeis University, Waltham, Massachusetts 04164
(Received M a y 7, 196%)
p-Aminobenzophenone (PAB) is photoreduced very inefficiently if a t all by 2-propanol and by 2-butylamine, cp < 0.001, due to a low-lying chemically unreactive charge-transfer (CT) triplet. PAB is photoreduced by triethylamine with quantum yields a t 313-360 mp varying from 0.22 to 0.57. Quantum yields are higher (i) a t higher light intensity, (ii) a t lower initial concentrations of PAB, and (iii) in dilute solution of amine in hydrocarbon. Reactivity of the tertiary amine arises from low polarity which leads to a chemically reactive n,n* triplet, and from heteroatom activation by N. Lower cp a t higher concentration of PAB arises from deactivation of PAB triplet by PAB ground state with efficiency half that of diffusion-controlled quenchers, in a process which may convert n,n* to C T triplet. Self-quenching is also observed in photoreduction of PAB by diisopropylamine, and in photoreduction of dimethylaminobenzophenone by triethylamine, which in general resembles that of PAB. Increase of cp by dilution with cyclohexane may arise from more efficient conversion to n,p* triplet. Dilution of triethylamine with acetonitrile prevents photoreduction; the dipolar aprotic solvent, like protic solvents, stabilizes and leads to the unreactive CT triplet. Photoreduction of PAB by cyclohexane has cp S 0.05 and leads to -90% pinacol. Photoreduction of PAB by diisopropylamine, cp g 0.040.09, leads to -100% pinacol; photoreduction in triethylamine leads t o 55% pinacol. Decrease of cp with decreasing concentratioii of triethylamine in cyclohexane is small and leads to k&, = 0.011 M . The effect of naphthalene in quenching photoreduction of PAB by triethylamine, when correction is made for self-quenching, leads to k,/k, = 500, k , S 1 X lo7 M-l see-l. Similar study in 1 M triethylamine in cyclohexane leads 4 X lo7M-lsec-l, k d = 4.6 X 106sec-'. The characteristics of photoreduction by amines, to k,/k, = 90, k , reactivity of bridgehead N, low kd/k,, lo-rv effectiveness of quenchers, moderate quantum yields, no N-deuterium, and small a-deuterium isotope effects, are consistent with rapid interaction of triplet with the nonbonding electrons of N, followed partly by transfzr of hydrogen and partly by regeneration of ketone and amine. Photoreduction of m-aminobenzophenone by triethylamine is about one-tenth as efficient as that of PAB; o-aminobenzophenone is not photoreduced significantly.
The photoreduction of aromatic ketones by primary and secondary alcohols,2 leading to pinacols and carb i n o l ~ is , ~a long known and useful reaction. Hydrocarbons4J and ethers6-8 were also found to be hydrogen donors, but they were generally less effective than alcohols. The n,a* excited triplet is thought to be the chemically reactive state of the k e t ~ n e . ~ ,There '~ are, however, groups of ketones which are either not photoreduced by alcohols or are photoreduced with low efficiency. (i) Ketones which have an appropriate ortho substituent may show hydrogen transfer from the substituent to the excited carbonyl, followed by reverse transfer, which leads finally to regeneration of the original ket~ne.'l-'~ (ii) Ketones which have a lowest The Journal of Physical Chemistry
triplet state of the a,a*rather than the n,a* type show low reactivity to alcohols.12 These may include fluore(1) For a preliminary report on part of this work, see S. G. Cohen and J. I. Cohen, J . Amer. Chem. SOC.,89, 164 (1967). (2) (a) G.Ciamician and P. Silber, Ber., 33, 2911 (1900); 34, 1541 (1901); (b) W. D. Cohen, Rec. Trav. Chim., 39, 243 (1920); (c) J. N. Pitta, Jr., R. L. Letsinger, R. P. Taylor, J. M. Patterson, G. Recktenwald, and R. B. Martin, J . Amer. Chem. Soc., 81, 1068 (1959). (3) W. E. Bachmann, ibid., 55, 355,391 (1933). (4) E.Paterno and G. Chieffi, Gazz. Chim. Ital., (11) 39, 415 (1909). (5) G.S.Hammond, W. P. Baker, and W. M. Moore, J . Amer. Chem. Soc., 83, 2795 (1961). (6) G. Ciamician and P. Silber, Ber., 44, 1554 (1911). (7) E. Bergmann and S. Fujise, A n n . , 483, 65 (1930).
PHOTOREDUCTION OF AMINOBENZOPHENONES IN NONPOLAR MEDIA
3783
nonela xanthone, l 4 naphthyl carbonyl compounds, 15, l6 polyalkyl-substituted acetophenones,l' and probably phenyl-substituted benzophenones.12 (iii) Ketones, including p-aminobenzophenone, which have a lowest triplet state in alcohols of the charge-transfer (CT) type will not show efficient photoreduction in such solvents. l8 I n a less polar medium, cyclohexane, the levels may be inverted and the chemically reactive n,T* triplet state may be formed, and photoreduction was observed. Doubt has been expressed as to the formation of a pinacol from the photoreduction of p-aminobenzophenone (PAB) by cyc10hexane.l~ We observedz0 that p-dimethylaminobenzophenone and 0- and p-aminobenzophenones are readily photoreduced by alcohol in the presence of mineral acid, which protonates the amino group, localizes the electron pair, and leads to the n,r* triplet. Quaternization of the p-amino group also leads to "normal" photoreduction in aqueous alcoholic media.20b$21We also observed that aliphatic amines are particularly effective photoreducing agents for aromatic ketones which are photoreduced by a l ~ o h o l s . Amines ~ ~ ~ ~ ~are less polar than alcohols and it appeared reasonable to examine them as photoreducing agents for ketones which lead to We now give a more T , T * and CT triplets in alcohols. detailed report' on the photoreduction of p-aminobenzophenone, PAB, and other aminobenzophenones by aliphatic amines. A study of the ketones which lead to T , T * triplets will be reported later.
grade. 2-Propanol, used in kinetics, was Eastman Spectrograde; that used for dilutions was Fisher reagent grade. Argon, Airco welding grade, was passed over Drierite and used. N,N-Dimethylformamide, Fisher reagent grade, formamide, Fisher reagent grade, and N-methylformamide, Eastman White Label, were used directly. 2,2'-Dinaphthyl, Gallard-Schlessinger, was recrystallized from cyclohexane, mp 187.5-188.5'. Naphthalene, Fisher, was recrystallized twice from methanol, mp 80'. trans-Stilbene, Pilot Chemical, was recrystallized twice from 2-propanol, mp 125-126'. Benzyl disulfide, Eastman White Label, was recrystallized twice from methanol, mp 69-70'. a-Toluenethiol, Eastman, was distilled, bp 71' (8 mm). Mesityl mercaptan was prepared as described p r e v i o u ~ l y . ~ ~ Analysis by glpc, carbowax column, column 185', detector 280', injector 250" showed one peak, retention time 6 min. 1,lO-Phenanthroline, Eastman, was used directly. Potassium ferrioxalate was prepared according to a reported26procedure. Diphenylamine barium sulfonate, Eastman White Label, was used directly. Irradiation Procedure. Aliquots, 4 or 5 ml, of the ketones in the solutions of varied composition, with and without the several additives, were photolyzed in Pyrex, either in 10-mm i.d. circular tubes or in 1 X 1 X 10 cm square tubes. The circular tubes, used in experiments with unfiltered light, were closed with ground-
Experimental Section
(8) (a) 5. G. Cohen and s. Aktipis, Tetrahedron Letters, No. 10, 579 (1965); (b) J. Amer. Chem. Sac., 88, 3587 (1966). (9) (a) G. S. Hammond and W.M. Moore, ibid., 81, 6334 (1959); (b) W. M. Moore, G. S. Hammond, and R. P. Foss, ibid., 83, 2789 (1961). (10) H. L. J. Backstrom and K. Sandros, Acta Chem. Scand., 14, 48 (1960).
Materials. o- and p-Aminobenzophenone, obtained from Aldrich, were treated with charcoal, recrystallized twice from 2-propanol, and dried under vacuum, mp 107-108' and 123-124", respectively. m-Aminobenxophenone was prepared by a reportedz4 procedure and recrystallized from benzene-cyclohexane, mp 85-86'. p-Dimethylaminobenzophenone, available from a previous study120 was recrystallized from ethanol, mp 89-90". Benzophenone, Fisher reagent grade, was Benzrecrystallized from ethanol-water, mp 47-48'. hydrol, Eastman White Label, was recrystallized from petroleum ether (bp 60-110'), mp 67-68'. Amines were distilled immediately before use: nbutylamine, Eastman, bp 78' ; 2-butylamine, Eastman, bp 63'; and diisopropylamine, Eastman, bp 83-84'. N-Ethyl diisopropylamine, Aldrich, was distilled from sodium hydride and redistilled before use, bp 127-128'. Triethylamine, Eastman, was distilled from sodium hydride and redistilled under argon before use, bp 89-90', Triethylenediamine, Aldrich, was sublimed twice under vacuum before use. Acetonitrile was from Matheson Coleman and Bell, Spectrograde. Acetone, benzene, cyclohexane, isooctane were Eastman Spectrograde. Diisopropyl ether, Fisher, was distilled from lithium aluminum hydride before use, bp 68-69'. Methanol was Fisher reagent
(11) (a) N. C. Yang and C. Rivas, J. Amer. Chem. Sac., 83, 2213 (1961); (b) E. F. Zwicker, L. I. Grossweiner, and N. C. Yang, ibid., 85, 2671 (1963). (12) J. N. Pitts, Jr., H. W. Johnson, Jr., and T. Kutawa, J. Phys. Chem., 66, 2456 (1962). (13) A. Beckett and G. Porter, Trans. Faraday Sac., 59,2051 (1963). (14) A. Schonberg and A. Mustafa, J. Chem. Sac., 67 (1949). (15) F. Bergmann and Y . Hirschberg, J. Amer. Chem, Sac., 65, 1429 (1943). (16) G. S. Hammond and P. A. Leermakers, ibid., 84, 207 (1962). (17) N. C. Yang, D. S. McClure, 8. L. Murov, J. J. Houser, and R. Dusenbery, ibid., 89, 5466 (1967). (18) G. Porter and P. Suppan, Trans. Faraday Sac., 61, 1664 (1965). (19) E. J. Baum, J. K. 8. Wan, and J. N. Pitts, Jr., J. Amer. Chem. Sac., 88, 2652 (1966). (20) (a) S. G. Cohen and M. N. Siddiqui, ibid., 86, 5047 (1964); 89,6409 (1967). (21) S. G. Cohen, R. Thomas, and M. N. Siddiqui, ibid., 89, 5845 (1967). (22) (a) S. G. Cohen and R. J. Baumgarten, ibid., 87, 2996 (1965); (b) 89, 3471 (1967). (23) 8. G. Cohen and H . M. Chao, ibid., 90, 165 (1968). (24) R. Geigy and W.Konigs, Ber., 18, 2401 (1885). (25) C. H. Wang and S. G. Cohen, J. Amer. Chem. Sac., 79, 1924 (1957). (26) C. G. Hatchard and C. A. Parker, Proc. Roy. Sac., A235, 518 (1956).
Volume 72, Number 11 October 1968
3784 glass stopcocks lubricated with silicone grease; the square tubes, used on the monochromator, were closed with Fisher-Porter Teflon valves. The tubes were cooled in Dry Ice-acetone, degassed in three freezepump-thaw cycles, and left under argon. I n experiments with unfiltered light the tubes were mounted on a turntable 4 or 8 cm from a G.E. H85IA3lUV mercury lamp. I n experiments with monochromatic light the tube was placed in a Cary-14 cell holder affixed to the exit slit of a Bausch and Lomb 38-86-01 monochromator, 3.2 mp/mm dispersion, slit width 3 mm in most experiments. The light source was an Osram SP-200 mercury point source lamp. A Corning 7-54 filter was placed between the exit slit and the cell holder. Tube contents were stirred magnetically and cooled by an air blower. The light intensity was monitored by ferrioxalate actinometryz6 or with benzophenonebenzhydrolgb as a secondary actinometer. I n experiments on the turntable with unfiltered light, the light intensity was monitored by concomitant photolysis of 0.1 M benzophenone in 2-propanol. The rate of benzophenone disappearance, 4 cm from the G.E. lamp, was 0.15-0.20 M h r l , cp G 1.1. I n all experiments irradiation tubes were selected in which rates of photoreduction of benzophenone in 2-propanol were the same. Progress of photoreduction was followed by deterof the long mination of absorbance at and near A, wavelength band of the ketone. I n some experiments aliquots of photolyzed and unphotolyzed solutions were diluted with 2-propanol in volumetric flasks and absorbances were measured in calibrated l-cm or l-mm quartz cells on a Beckman DU-2 or Gary 14 spectrophotometer. In other experiments a Pyrex irradiation tube was used to which a l-mm quartz absorbance cell was sealed. The solutions were irradiated for a measured period, removed from the irradiator, and examined in the spectrophotometer via the absoi Lance cell a t and near the appropriate A, and returned for further irradiation. Actinometry was carried out between the periods of irradiation. I n this way multiple values of rates and quantum yields were determined without use of separate samples. Values obtained by the two procedures were identical. 4,4'-Diaminobenzpinacol. A solution of 0.35 g (1.78 mmol, 0.025 M ) of p-aminobenzophenone, PAB, in diisopropylamine in a 100-ml Pyrex flask was degassed and irradiated under argon for 77.5 hr with the G.E. lamp. The solution was cooled to -20°, and the precipitate was collected and washed with cold diisopropylamine, 0.225 g, 6497, yield, mp 170-172" dec. The infrared spectrum in methylene chloride showed a weak multiplet centered a t ca. 3500 cm-l and strong bands a t 1625 and 1510 cm-'. The carbonyl band of PAB a t 1650 cm-I was absent. The product was dissolved in methylene chloride, decolorized with carThe Journal of Physical Chemistry
SAULG. COHENAND JACOB I. COHEN bon, and precipitated with petroleum ether, mp 180181" dec. Anal. Calcd for C Z ~ H ~ ~ NC, ~ O78.75; Z: H, 6.10; N, 7.07. Found: C, 78.57; H, 6.10; N, 7.28 (Schwarzkopf Laboratories). The ultraviolet spectrum of a 4 X M solution in methanol showed absorption: 230 mp, E 16,400; 244 mp(max), E 23,650; 260 mp, E 11,000; 286 mp(max), E 2940; 300 mp, E 1880; 334 mp, E 56. Addition of 1 drop 1 N hydrochloric acid discharged the maxima a t 286 and 244 mp and led to a new maximum at 260 mp, E 1470. Addition of 1 drop of 1 M sodium hydroxide regenerated the original spectrum. A solution of 0.01053 g (0.0266 mmol, 5.32 X M ) of the initially precipitated pinacol in 50 ml of 2propanol containing -0.3 ml of 5 M sodium hydroxide was allowed to stand overnight in the dark under argon, The absorbance was measured in a l-mm cell at 320, 334 (max), and 350 mp, and compared with that of a similar solution which contained no added alkali. Absorbances of 0.813, 0.995, and 0.832 at the three wavelengths indicated -99% yield of PAB from the decomposition of the pinacol. The absorbance of the blank, 0.066, indicated the presence of not more than 3.5% PAB in the precipitated pinacol. 4-Aminobenzhydrol. A solution of 1.0 g (5.1 mmol) of PAB in 50 ml of 2-propanol was reduced with 0.15 g (4.0 mmol) of sodium borohydride, leading to 0.70 g, 69% yield, of 4-aminobenzhydrol, mp 116-117" from petroleum ether, lit.27mp 121". The ultraviolet specM of the hydro1 in methanol showed trum of a 4 X absorption: 230 mp, E 8260; 244 mp(max), E 12,800; 260 mp, E 5750; 286 mp (Max), E 1630; 300 mp, E 1120; 334 mp, E 25. Addition of 1 drop of 1 N hydrochloric acid discharged the maxima at 244 and 286 mp and led to a new maximum at 260 mp, E 600. Sodium hydroxide restored the original spectrum. A 10-ml solution of 4 x 10-6 M 4-aminobenzhydrol in 1: 1 2-propanolwater containing 0.3 ml of 5 N NaOH, allowed to stand for a month, developed absorbance at 334 mp, equivalent to less than 1% conversion to PAB. E$ect of Alkali on Irradiated Solutions. (i) Aliquots of a solution of 0.01 M PAB in diisopropylamine were irradiated on the turntable for 30-60 min, the absorbances of appropriately diluted aliquots falling from an initial value of 0.738 at 334 mp to 0.368 and 0.117, respectively, corresponding to rates of photoreduction of 0.010 M hr-I and 0.0084 M hr-I for the two periods. Aliquots (15 ml) of the diluted unirradiated and irradiated solutions were treated with 0.5 ml of saturated sodium 2-propoxide in 2-propanol and allowed to stand in the dark for 10 days, at which time the absorbances at 334 mp were 0.733,0.547, and 0.410, respectively. The increase in absorbance following alkali treatment, due
(27) H. Kippenberg, Ber., 30, 1136 (1897).
PHOTOREDUCTION OF AMINOBENZOPHENONES IN NONPOLAR. MEDIA to PAB, corresponded to 99 and 95% formation of the pinacol in the photoreduction. (ii) Aliquots of a solution of 0.001 M PAB in cyclohexane were irradiated for 24 and 60 min on the turntable, and absorbances of aliquots, diluted with 2-propanol, were determined: (a) 0 min, 334 mp, 0.733, 350 mp, 0.597; (b) 24 min, 334 mp, 0.456, 350 mp, 0.386; (e) 60 min, 334 mp, 0.268; 350 mp 0.232. These values correspond to rates of photoreduction of 9.3 X and 6.4 X M hr-l for the two periods. Aliquots (15 ml) of the diluted solutions were treated with 0.2 ml of 5 M sodium hydroxide and absorbances were measured after 4 days: (a) 334 mp, 0.751, 350 mp, 0.602; (b) 334 mp, 0.588, 350 mp, 0.489; (e) 334 mp, 0.477, 350 mp, 0.402. The increase in absorbance following alkali treatment indicated 85% pinacol from the data a t 334 mp, 92% pinacol from the data at 350 (iii) A solution of -1 X lods M PAB in triethylamine was irradiated at 366 mp on the monochromator in a 1 em square Pyrex tube, to which was sealed a 1mm quartz cell in which the absorbance at 317.5 mp was measured after each period of irradiation with the aid of a neutral density filter: 0 min, 2.16; 15 min, 1.97; M 27 min, 1.84. This indicated a rate of 4.3 X hr-l, compared with a ferrioxalate rate of 2.48 X M hr-l, quantum yield 0.21. After 343 min of irradiation, the absorbance was 0.33, 88% reaction. An aliquot (1 ml) was treated with 0.2 ml of 5 m NaOH, allowed to stand for 3 days, diluted to 25 ml with 1: 1 2-propanol-water and examined in the ultraviolet. Absorbances of an unirradiated aliquot so treated were: 334 mp, 0.750; 350 mp, 0.610; of an irradiated aliquot diluted but not treated with alkali: 334 mp, 0.090; 350 mp, 0.073; and of the aliquot treated with alkali and diluted: 334 mp, 0.255; 350 mp, 0.207. Decomposition indicated at each wavelength 50% conversion of reduced PAB to pinacol.
Results Preliminary experiments were carried out on photoreduction of 0, m, and p-aminobenzophenones in a range of solvents. Solutions in Pyrex tubes on a turntable were irradiated with a G.E. H-85 lamp. Irradiation of 0.1 M solutions of the three aminobenzophenones in 2-propanol for 71 hr led to very little change in the ultraviolet spectra. Irradiation of a 1 X lo-* M solution of PAB in 2-propanol led to 38% decrease in absorbance after 12 hr, corresponding to a rate of photoM hr-l; benzophenone in reduction of -3 X 2-propanol was photoreduced a t the same time a t a rate of 0.16 M-l hr-l. Approximate rates of photoreduction by a variety of compounds are summarized in Table I. These numbers reflect a reduction of 0.01 M PAB in the primary amines to an extent of only 1-2% in 1020 hr. Disappearance of PAB was slightly more rapid
3785
Table I : Approximate Rates of Photoreduction in Pyrex, G.E. H851 A3 Lamp: p-Aminobeneophenone, PAB; o-Aminobenzophenone, OAB; m-Aminobeneophenone, MAB Rate,
-Ketone--Compd
108 X M hr-1
M
Solvent
PAB PAB PAB PAB PAB PAB PAB PAB PAB PAB PAB PAB
0.001 0.00005 0.01 0.01 0.01 0.01 0.01 0.001 0.001 0.02 0.02 0.02
2-Propanol 1 :1 2-Propanol-Hi0 n-Butylamine 2-Butylamine N-2-Propylacetamide Acetone Benzene Isooctane Cyclohexane Diisopropylamine Triethylamine N-Ethyldiisopropylamine
-0.03 -0. 02
OAB OAB OAB OAB
0.10 0.0002 0.01 0.01
2-Propanol 1:1 2-Propanol-HnO Cyclohexane Triethylamine
-0. 05 -0,003 N O , 001
MAB MAB MAB
0.10 0.001 0.01
2-Propanol Cyclohexane Triethylamine
N O . 02
~0.007
-0,005 -0.008 0.2 0.2 1 2 10 50 30
-0.06
-0.05 -0.50
in 2-propanol, and apparently even more rapid in acetone and benzene, though still quite slow. Photoreduction in isooctane and cyclohexane was more efficient, about 1% as fast as that of the standard, benzophenone in 2-propanol. Photoreduction by diisopropylamine was one-tenth as rapid as that of the standard, while the rate by triethylamine was about one-third that of the standard. Photoreduction of o-aminobenzophenone was very slow, showing no enhancement in cyclohexane and triethylamine. mAminobenzophenone was also quite unreactive, triethylamine showing some effectiveness, about 1% of that in the photoreduction of PAB. Product analysis indicated that photoreduction of PAB in diisopropylamine led to the pinacol, 4,4'-diaminobenzpinacol, which was isolated in 64% yield, mp 180-181' dec. It was readily distinguished from the possible monomolecular product, 4-aminobenzhydrol, mp 116-117", which was prepared by reduction of PAB by sodium borohydride. The ultraviolet spectra of the pinacol and hydrol are very similar, with molecular extinction coefficients for the pinacol approximately twice those of the hydrol. The spectra are those of an aniline,28with maxima at 244 and 286 mp which are discharged on protonation with HCI and regenerated with neutralization. The high extinction CT band of PAB at 334 mp, (28) J. G. Calvert and J. N. Pitts, Jr., "Photochemistry," John Wiley and Sons, Inc., New York, N. Y., 1966, (a) p 263; (b) p 298; (c) p 267.
Volume 78, Number 11 October 1968
SAULG. COHENAND JACOB I. COHEN
3786 B ~ 1 9 , 0 0 0is , absent in the pinacol and in the hydrol. Treatment of the pinacol in alcohol with alkali leads to quantitative generation of the ultraviolet spectrum of one mole of PAB per mole of pinacol. Decomposition to PAB and the hydrol occurs (eq l), as has been described for the base-catalyzed decomposition of benzpinaco13 and the pinacols from p-dimethylaminobenzophenoneZ0b and from p-benzoylphenyl-trimethylammonium chloride.Z1 Prolonged treatment of the hydrol
OH
OH /OH-
6H
0
with alkali led to essentially no PAB. This procedure for pinacol analysis was applied to irradiated solutions. for PAB Absorbance measurements at and near A,, before and after irradiation indicated the total quantity of PAB photoreduced. Base decomposition and increase of absorbance of PAB indicated the quantity which had been photoreduced to pinacol. When conversion of the ketone to pinacol is quantitative, decomposition by base regenerates one-half of the amount of ketone reduced. Other values are proportionate. Small corrections are made for the effects of alkali on the absorbances. This analysis may be general for derivatives of benzpinacol. It is not effective for pinacols from acetophenones which may be quite stable to alkali.2a Crossed coupling products which may be formed in photoreduction of benzophenones in some solvents are stable and do not appear to intsrfere. Some results of such analyses applied to photoreduction of PAB in a variety of solvents are given in Table 11. Photoreduction of PAB in diisopropylamine led to esTable 11: Yield of Pinacol in Photoreduction of PAB Reducing agent
Diisopropylamine Diisopropylamine Triethylamine Triethylamine 1 M Triethylamine in cyclohexane 1 M Triethylamine in cyclohexane Diisopropyl ether Cyclohexane Cyclohexane The Journal of Physical Chemistry
% Convn
Pinacol
0.02 0.001 0,001
50 84 79 88 41
102 95 53 55 56
0.001
84
56
0,001 0.001 0.001
21 38 63
98 90 88
PAB, M
0.01 0.01
%
sentially quantitative formation of the pinacol, while the more rapid photoreductions in triethylamine and in 1 M triethylamine in cyclohexane led to little more than 50% yield of the pinacol. This may be due to formation of the cross-coupling product from combination of the ketyl radical with the tertiary amine derived radical, as has been observed in the photoreduction of benzophenone by dimethylaniline.30 In one experiment prolonged irradiation of 0.02 M PAB in triethylamine, followed by dilution with an equal volume of methylene chloride, led to a material, mp 173-176" dec. It did not regenerate PAB on standing or refluxing with alkali; its infrared spectrum was similar to those of the pinacol and the hydrol, with additional C-H absorption. It decomposed on standing in air and may have been the mixed coupling product. Another material was obtained by glpc, the nmr and mass spectra of which were consistent with the dimer of the tertiary amine derived radical, N,NJN',N'-tetraetbyl-2,3-diaminobutane.31 Quantum yield measurements were carried out for the photoreduction of the aminobenzophenones, as affected by reaction conditions: light intensity, concentration of ketone, and reducing medium. The most detailed study was that of the PAB-triethylamine system. The quantum yields for photoreduction of 0.001 M PAB in neat triethylamine at 334 mp were determined by ferrioxalate actinometry as a function of light intensity. Light intensity was decreased by wire screens at the exit slit. Unexpectedly, the quantum yield fell from 0.42 at high light intensity ea. 1 X 10l6quanta/sec cm2 to 0.25 a t ea. 6 X 10la quanta/sec cm2. The actinometer and PAB solutions were opaque under all the reaction conditions. Results for ea. 15% photoreduction of PAB are given in Table 111. Table 111: Photoreduction of 0.001 M PAB in Triethylamine, 334 mp. Effect of Light Intensity on Quantum Yield cp ---PAR
Time
irradiation-M hr-1
25sec 1.49 X 10-2 16min 8 . 1 X 82min 5 . 4 X
7 -
Time
Actinometera----M hr-1
93sec 4.40 X 17min 3.27 X 121 min 2.61 X
Rel.
c
169 0.42 12.7 0.31 1 . 0 0.25
a Actinometer quantum yield, 1.23; highest light intensity, ca. 1 x 1016 quanta/sec cm2.
The efficiency of photoreduction of PAB and of pdimethylaminobenzophenone, DMAB, by triethylamine was then examined as a function of the initial concentrations of the ketones at constant light intensities. For PAB, irradiation was at 313 and 334 mp (29) D. Laufer, Ph.D. Thesis, Brandeis University, 1963. (30) R. 9. Davidson, Chem. Commun., 16, 576 (1966). (31) L. T. Allan and G. A. Swan, J . Chem. SOC.,4822 (1965).
3787
PHOTOREDUCTION OF AMINOBENZOPHENONES IN NONPOLAR MEDIA with benzophenone-benzhydrol as a secondary actin ~ m e t e r . Quantum ~~ yields for PAB-triethylamine are independcnt of wavelength over this range. Three concentrations of ketone were studied at each of the two wavelengths. For DJIAB, irradiation was at 334 mp and three concentrations of ketone were studied. Some results are given in Table IV. ~~
ca. 51 kcallmol.
This compound has substantial absorption at 334 mp, and the comparison was made with irradiation at 366 mp, and analysis at 350 mp. Some results with the two quenchers are given in Table
v.
Table V : Photoreduction of PAB by Triethylamine. Effects of Naphthalene, N, and trans-Stilbene, S
~
Table IV : Photoreduction of PAB and I>lIAB in Triethylamine. Effect of Initial Concentration of Ketone on Quantum Yield
PAR, M
----QuencherCompd
---Irradiation--
0.020 0.020 0.020 0,010
...
...
0,010
0.005
0.010 0.010
N N N N
0.0010
...
...
0.0010 0.0010
N N
0,010 0.021
Ketone
M
mp
PAB PAB PAB PAB PAB PAB I>l\IAB DIIAB DMAB
0,020 0,010
313 313 313 334 334 334 334 334 334
0.001
0.020 0,010 0.001 0.020 0,010 0.001
Time, min
Convn,
%
Rate, M hr-1
20 10
20 27 36 20 27 29 27 23 20
0.0121 0.0162 0.0207 0.00204 0.00263 0.00345 0.0158 0.0167 0.0197
1
120 60 5 20 8 0.6
(P
0.24 0.31 0.40 0.22 0.28 0.37
I n the photoreduction of PAB a 20-fold decrease in initial concentration leads to a 67% incrcase in quantum yield. While actinomctry was not carricd out concurrently with the photoreduction of DAIAB, the rates indicate a similar but somcwhat smaller effect, a 20-fold decrease in concentration lcading to a 25% increase in rate. The rate for the 0.001 M solution of DMAB corrcsponds to a quantum yield of ca. 0.4. The data for PRB in Tablc IV may be fitted to linear plots of l / p us. concentration of PAB with limiting quantum yiclds of 0.42 and 0.35 at the high and low light intensity, rcspectively, and with ratios of slope to intercept of 36 1V-l in both cases. This behavior is consistent with quenching by ground-state PRB or by an impurity in it. A sample of the PAB was treated further-recrystallized from 2-propanol, sublimcd, and recrystallized twice again. The quantum yield for photoreduction of a 0.02 AT solutions of this sample in triethylaminc was only 8% grcater than that obscrved in the normally used material, perhaps within experimental error, indicating that the concentration effect may not be due to impurity. The action of naphthalcne as a quencher for photoreduction of PA13 by tricthylaminc at 334 mp was examined at the same three concentrations of PAB as in Table IV. The energy of the lowcst triplet state of I'AB is -67 kcal mol in cyclohcxane and 63 kcal/mol in 2-propanol, somewhat lower than the corresponding valucs for benzophenone. l8 The triplct energy of naphthalene28"is about 61 lical, fairly similar to that of PAB, and it seemed desirable to includc comparison with another quencher, tmns-stilbenc, of lower triplet energy,
0.010
0.0020 0,0020 0.0020 0.0020 0.0020
M
c
...
...
N N
0.021 0.050
0.20 0.12 0.064
, . .
S S N N
0.30 0.23 0.21
0,010 0.015
0.18 0.17
0.020
0.38 0.23 0.16
...
0.31 0.22
0.0046 0,0094
0.19
0.23 0.18
0.0051
0,0091
The last set of data, Table V, indicates that ti.ans-sti1bene and naphthalene have equal quenching effectiveness. A single linear plot of l / p us. concentration of the two quenchers may be constructed, leading to a value of Ic,/k, (Et3N), the ratio of slope to intercept, of 69 M-' at 0.002 A I 1'AB in triethylamine. The first three sets of data, Table V, may be plotted similarly, leading to parallel lines, with ratios of slope to intercept of 40 A1-l at 0.02 M PAB, 45 M-' at 0.01 M PAB, and 67 M-' at 0.001 M PAB. These valucs, corresponding kr(Et3N)), to k,/k,(Et3W), or more properly I c , / ( k d should, however, be constant. The data of Table IV, for variations in quantum yield with concentration of PAB itself, also fitted a Stern-Volmer plot, with ratio of slope to intercept of 36 M-l, about half the value observed in these quenchcr studies at low (-0.001 )!A concentration of PAB, where the effect of the quencher is dominant and that of the PAB may be neglected. This indicated that the PAB may be treated as a quencher with half the efficiency of naphthalene and trans-stilbene. The varying concentration of PAB in the sets of data of Table V should be taken into account in the plots of l / c p us. concentration of quencher. Since PAB undergoes photoreduction better in nonpolar than in polar rnedia,l8 we examined the effect of dilution on the photoreduction by triethylamine. A few experiments were carried out in dilution with benzene and acetonitrile. A more detailed study was made
+
Volume 72, Number 1 1
October 1968
SAULG. COHENAND JACOB I. COHEN Table VI : Photoreduction of 0.001 M PAB by Triethylamine in Cyclohexane, 313 mp EtaN, M
----Quantum
7.14 (neat) 1.oo 0.100 0.051 0.014
OC
0.39" 0.57" 0.53" 0.46" 0.32" 0.052"
Table VII: Photoreduction of PAB by 1 M Triethylamine in Cyclohexane. Effect of Naphthalene
yield--
0.32'1 0.55'1 0.51'1 0.45'~ 0.32'1 0.055'1
" Higher light intensity, ferrioxalate rate, 0.030 M hr-1. Lower light intensity, ferrioxalate rate, 0.0054 A4 hr-1. Cyclohexane. on the effect of dilution with cyclohexane, and some of these results are given in Table VI. At 1 M triethylamine in cyclohexane the quantum yields are 46 and 72% greater than in neat triethylamine at the higher and lower light intensity, respectively. This difference arises largely from the difference in quantum yields in neat triethylamine. The quantum yields in the solutions in cyclohexane are essentially independent of light intensity, and a plot of l / c p us. l/(EtaN) leads to a single line for the solution data with ratio of slope to intercept = 0.011 M , and with the points for neat amine lying on the ordinate far above the intercept. The quantum yields in all the solutions are much greater than in neat cyclohexane, and triethylamine may be assumed to be the active reducing agent. Supporting evidence is found in analysis of the photolysates for pinacol by base decomposition, Table 11. Photoreduction of PAB in 1 M triethylamine in cyclohexane and in neat triethylamine leads to essentially the same extent of conversion to pinacol, 54 and 56%) respectively, values very different from that in cyclohexane, 89%, indicating that the cyclghexyl radical is not formed in the solution of the amine in cyclohexane. On this basis the ratio of slope to intercept,.O.Oll M , is Icd/lc,, the ratio of rate constants for deactivation of the excited ketone to that for abstraction by it of hydrogen from triethylamine. Experiments were carried out on the quenching by naphthalene of the photoreduction of PAB by 1 M triethylamine in cyclohexane. Irradiation was at 334 mp a t low light intensity, ferrioxalate rate 0.00312 M hr-l. Some results are given in Table VII. A plot of reciprocal quantum yield against the sum of the concentration of naphthalene and one-half the concentration of PAB is linear, and the ratio of slope to inter= 90. cept leads to IC&, Dilution of triethylamine with benzene also leads to increased quantum yield for photoreduction of PAB. Irradiation a t 313 mp of 0,001 M PAB in 1 M triethylamine in benzene led to photoreduction with zeroorder kinetics over the first 50% of reaction with quantum yield 0.56, the same as that in 1 M triethylamine in The Journal of Physical Chemistry
108M
Naphthalene, M
1.43 1.37 1.40 1.44
0.0105 0.0203 0.1006
PAB,
...
Irradn, min
Convn,
%
B
15 25 40 120
21.5 19.7 21.3 18.0
0.481 0.256 0.176 0.0518
cyclohexane and 40% greater than that in neat triethylamine at the same light intensity. On the other hand, dilution with a dipolar solvent, acetonitrile, leads to greatly decreased efficiency of photoreduction. Irradiation at 313 mp of 0.001 M PAB in 1 M triethylamine in acetonitrile led to very slow if any photoreduction, with rate perhaps about 1/300 as great as that in neat triethylamine. Irradiation of 5 X 10-5 M PAB in acetonitrile alone led to even slower photoreduction, -8 X M hr-l, about one-fifth that in 1 M triethylamine in acetonitrile. Quantum yields were also determined for photoreduction of PAB by N-ethyldiisopropylamine and triethylenediamine, by diisopropylamine and 1 M diisopropylamine in cyclohexane, by diisopropyl ether and by 1 M 2-propanol in cyclohexane, and for photoreduction of m-aminobenzophenone, MAB, in triethylamine. Some results are summarized in Table VIII. Photoreduction by ethyldiisopropylamine was somewhat less efficient than by triethylamine, cp = 0.22 as compared with 0.39 under similar conditions, Table VI. The solid bicyclic tertiary amine, triethylenediamine, with N atoms in bridgehead positions, used as an 0.05 M solution in cyclohexane, showed moderately high efficiency in photoreduction of PAB, cp = 0.21, somewhat less than half that of triethylamine under similar conditions. The secondary amine diisopropylamine was about an order of magnitude less efficient than the tertiary amines cp -0.02-0.04. Again photoreduction was more efficient at low concentration of ketone than at high, and in solution in cyclohexane than in the neat amine. Part of the increase may be due to photoreduction by the hydrocarbon, which is itself more effective (cp -0.05) than the secondary amine. The effectiveness of cyclohexane was markedly reduced by addition of 1 M 2propanol. Diisopropyl ether had efficiency similar to that of the hydrocarbon. m-Aminobenzophenone was photoreduced by 1 M triethylamine with low quantum yield, 0.07, about one-seventh that of PAB under similar conditions. Finally, it was of interest to inquire whether any correlation might be observed between the reactivity of the media in photoreduction of PAB and their effect on the charge-transfer absorption band. Absorption data were obtained on a Cary 14 or Beckman DU-2 spectro-
3789
PHOTOREDUCTION OF AMINOBENZOPHENONES IN NONPOLAR MEDIA Table VI11 : Quantum Yields for Photoreduction of p - and m-Aminobenzophenone
--
Irradiation----
Ketone-Medium
M
Compd
PAB PAB
0.002 0.001
PAB PAB PAB
0.050 0,001 0.001
PAB PAB PAB
0.001 0.001 0.001
MAB
0.006
Ethyldiisoprop ylamine 0.05 M Triethylenediamine in cyclohexane Diisoprop ylamine Diisopropylamine 1 M Diisopropylamine in cyclohexane Diisopropyl ether Cyclohexane 1 M 2-Propanol in cyclohexane 1 M Triethylamine in cyclohexane
Intens'"
mr
c
0.035 0.030
313 313
0.22 0.21
0.056 0.054 0.030
313 334 313
0.024
0.055 0.030 0.030
313 313 313
