A. Cu and A. C.
644
Testa
Photochemistry of the Nitro Group in Aromatic Heterocyclic Molecules A. Cu and A. C. Testa' Department of Chemistry, St. John's University, Jamaica, New York 11439 (Received May 13, 1974; Revised Manuscrip$Rece/vedDecember 13, 1974)
The 313-nm photolysis of 5-nitroquinoline in 50% isopropyl alcohol and water with varying concentrations of hydrochloric acid results in photoreduction with formation of 5-amino-6,8-dichloroquinoline.The photoreduction to an amine is similar to that observed for nitrobenzene and 1-nitronaphthalene and appears to proceed via the anion of the nitro compound to the hydroxylamine. The quantum yield for photoreduction increases with increasing HCl concentration and reaches an upper limit of 5.44 f 0.16 X 10-2 when (HCl) I 1M . Comparison of the behavior in HC1 and HzS04 and previously reported results for nitrobenzene, 1-nitronaphthalene, and 4-nitropyridine indicate that the primary process involves an electron transfer from the chloride ion to the triplet state of the nitro compound.
Introduction The photochemistry and excited state behavior of aro313 nm matic nitro compounds has been a subject of continuing interest in our laboratory. Following our initial studies on nit r ~ b e n z e n e l - and ~ nitronaphthalenes,4 we recently reported that the 4-nitropyridinium ion undergoes photoreduction via the 3n,7r* state to form 4-hydroxylaminopyridine as the only primary p h o t o p r ~ d u c t In . ~ that study a limiting quantum yield of 0.65 f 0.05 was measured for the acid concentration range 0.5-2.0 M HC1. The primary photochemical event was shown to involve electron transfer By comparison of our results with those for l-nitronaphthfrom the chloride ion and evidence for the anions of 4-nialene it seems reasonable to assign the reactive state as tropyridine and nitrobenzene were obtained from flash 37r,7r*. photolysis studies.6 As an extension of our photochemical The quantum yield for the photoreduction is observed to studies of 1-nitronaphthalene into heterocyclic systems we increase with increasing HC1 concentration and a summary have undertaken a photochemical study of 5-nitroquinoline of degassed data is presented in Figure 1. The increasing in aqueous alcohol solutions containing HC1. In addition to quantum yield observed in the lower acidity range is due to identifying the reactive excited state and the photoprodincreasing protonation of the ring nitrogen, while in the uct, the importance of electron transfer as a primary prohigher acidity range the limiting quantum yield indicates cess was worthy of investigation. Triplet yields of nitro complete protonation. The limiting quantum yield of 5.44 compounds are known for nitrobenzene (0.6713 and 1- and f 0.16 X lop2 is observed for (HCl) 1 1 M, a t which con%nitronaphthalene (0.63 and 0.83, respectivelyP and are centration the ring nitrogen is totally protonated. In order probably comparable in 4-nitropyridine and 5-nitroquinoto line. Although the triplet yield of pyridine may be ~ma11,~>8 establish whether or not the small quantum yield observed is due to a small triplet yield, the phosphorescence the large photochemical quantum yields for 4-nitropyridine yield was measured at 77'K in EPA. The phosphorescence observed in alcohol-aqueous HCl solutions indicate that at yield of 5-nitroquinoline relative to the value of 0.051 for least in some cases the triplet yields of substituted pyrinaphthalene in EPAQwas determined to be 0.27; consedines can be substantial, Le., >0.5. quently, it is clear that not all the available triplets are reacting. Photochemical depletion of 5-nitroquinoline was Results and Discussion determined by polarographic analysis in acetic acid-sodiM 5-niThe 313-nm irradiation of degassed 0.5 X um acetate buffers. The photoreduction yields for air-saturated solutions are approximately 25% lower, however, the troquinoline in aqueous alcohol solutions containing HC1 results in the decrease of its absorption band at 315 nm acid dependence is similar to that shown in Figure 1. The with an increasing absorption in the region 240-265 nm. importance of chloride in the primary process is reflected Upon prolonged photolysis a three peaked structure typical in the observation that the photochemical disappearance is of aminoquinoline hydrochlorides appears at 248, 255, and significantly lower in H2SO4. In 6 M HC1, when the ring 265 nm. It is thus evident that photoreduction to 5-aminoproton is totally protonated, @ = 5.9 X lop2, which dequinoline is occurring. The absorption increases a t 240 and creases to a value of 0.23 f 0.02 X in pure isopropyl al265 nm during the early stages of the photoreduction cohol. A similar effect is observed in the photochemistry of suggesting that the corresponding hydroxylaminoquinoline 4-nitropyridine where the photoreduction quantum yield in is the primary photoproduct. The photoproduct was identipure isopropyl alcohol is only 1.3 f 0.03 X lop2,and signifified from its spectra (uv, ir, nmr) and by comparison with a cantly smaller than the quantum yields measured in the sample of 5-aminoquinoline to be 5-amino-6,8-dichloroquipresence of HCl. Since the major impetus of this study was to investigate the importance of the chloride ion as an elecnoline. The overall process occurring is The Journal of Physical Chemistry, Voi. 79, No. 6, 1975
Photochemistry of the Nitro Group
645
TABLE I: Photoreduction Quantum Yields (Degassed Solutions) for Aromatic Nitro Compounds in 50% Isopropyl Alcohol-Water with Varying HCl Concentration
6
NC_ x
n
(HCl), M
W 2
F
4
NBuiC
4 -NPb,d
1
5- N Q ~ * ~
5
3
z
1.8 x lo-' 0.59 3.4 x 5.3 x 2.6 x lo-' 0.16 5.7 x 5.3 X 10" 0.14 0.06 12.8 x 5.9 x 10" 366-nm excitation. 313-nm excitation. Reference 2. Reference 5. e Reference 4.f This study.
b-
2
4
3 6
a 3 w
2
2
w 4
a U
03
0 0
2
0.4
0.2
(HCL)
6
4
, M.
Figure 1. Photoreduction quantum yields for 1 X IO-* M 5-nitroquinoline in degassed 50 % isopropyl alcohol-water vs. HCI concentration (A,, 313 nm; /a 2.5 X loi5 quantalsec).
-
tron donor to the triplet state of the aromatic nitro compound no analysis of photoproducts was performed for isopropyl alcohol solutions. The photoreduction of aromatic nitro compounds with alcohols is generally a very inefficient p r o c e s ~ In . ~50% ~ ~ isopropyl ~~ alcohol-water, 6 N in H2SO4, the disappearance quantum yield of 5-nitroquinoline is only 0.99 f 0.02 X due to the absence of the chlbride ion. In the absence of chloride ions and protons hydrogen abstraction from the alcohol is the only contributing photochemical process. By comparison of our results for 5-nitroquinoline with those for 4-nitropyridine, nitrobenzene, and 1-nitronaphthalene it seems very likely that electron transfer to the 3r,n* state, producing the anion, is the primary process, which is then followed by a rapid protonation. A representation for the photoreduction of 5-nitroquinoline, ArN02, is given in Scheme I. Scheme I
-
ArNOz -% ArN02*' AI-NO~*~
kisc
ArN0z*3
ArN02' ArN02H
+ H'
+ (CH,),CHOH
(2 1
ArNOz
- -
ArN02*3 + C1- c A r N b z -
-----t
+ C1*
ArN0,H
ArNHOH
(1)
(31 (41
ArNH2 (5)
(chlorinated aromatic amines)
The results demonstrate the involvement of chloride ion as an electron donor, since a sixfold decrease in photochemical disappearance is observed when 6 N HC1 is replaced with 6 N HzS04. Wubbels et a1.I0 have shown a similar effect in nitrobenzene. A summary of the photoreduction quantum yields for 5-nitroquinoline obtained in this study with those for previously studied aromatic nitro compounds in 50% isopropyl alcohol-water with varying concentration of HCl is presented in Table I for nitrobenzene (NB), 4-nitropyridine (4-NP), 1-nitronaphthalene (l-NN), and 5-nitroquinoline (5-NQ) in 2, 3, and 6 M HCl. Photolysis of and loe4 M 5-nitroquinoline did not exhibit any measurable differences. Previous results with
nitrobenzene and 4-nitropyridine indicate that there is no concentration or intensity dependence on the photoreduction quantum yield of aromatic nitro compounds.l,j Photolysis with 313 and 366 nm produces the same electronic state and thus the same photoreduction quantum yield. The primary photoproduct does not effectively absorb either of these wavelengths so that the quantum yield measurements are not affected by secondary events. No dark reactions were observed when the degassed cells, after photolysis, were left in the dark for 2 days. The data tabulated in Table I indicate that the photoreduction quantum yield of the nitro group in heterocyclics is generally higher than in the carbocyclic aromatic, and that the quantum yields for 1-nitronaphthalene and 5-nitroquinoline are an order of magnitude smaller than in the case of nitrobenzene and 4-nitropyridine, respectively. A similar behavior has been observed in the photoreduction quantum yields for phenyl ketones relative to naphthyl ketones.11 Porter and Suppanl2 have concluded that triplet r,r*states are -10% as reactive as triplet n,n* states in aromatic ketones. Similarly, in the case of aromatic nitro compounds the 3r,r*states (1-nitronaphthalene and 5 4 troquinoline) generally live longer than 3n,r* (nitrobenzene and 4-nitropyridine), both in solution and in frozen glasses,3 and exhibit lower reactivity as seen in the quantum yield results summarized in Table I. In the n,a* triplets of 1-nitronaphthalene and 5-nitroquinoline the excitation is localized primarily in the aromatic nucleus. In such cases the NO2 group is probably electron rich rather than electron poor as is the case in n,r* states. Consequently, radical-like reactivity a t oxygen may be considerably diminished. Thus, one possible interpretation for the above data is that anion formation for 1-nitronaphthalene and 5-nitroquinoline is less facile than for phenyl or pyridine type anions, i.e., electron transfer to a 3n,n* state is favored over transfer to a 37r,r*state. A referee has suggested the possibility that the energy of the triplet state may be significant in the electron transfer from C1- and the conversion of the triplet to the anion; however, we believe that electron deficiency is more important than energy of the triplet state. I t is very unlikely that variation of triplet yields in the four molecules is an important consideration in accounting for differences in the observed photochemical quantum yields. There is a decreasing quantum yield exhibited by 4-nitropyridine when (HC1) > 2 M , which is an exception to the trend shown in Table I. Nitrobenzene, 1-nitronaphthalene, and 5-nitroquinoline, on the other hand, show a general increasing quantum yield with increasing HC1 concentration. It appears that an acid-catalyzed process decreases the photochemical efficiency in this molecule; however, a satisfactory explanation for this behavior is still lacking. The Journai of Physical Chemistry, Voi. 79, No. 6, 1975
Chldhraru, Caragheorgheopol, Moraru, and Sahini
646
Experimental Section Materials. 5-Nitroquinoline and 5-aminoquinoline were obtained from Aldrich Chemical Co. and recrystallized twice from pentane and once from benzene before using. Spectrograde isopropyl alcohol and EPA were used as received. Glass distilled water and reagent grade HC1 and HzS04 were used for aqueous solutions. Equipment. All quantitative photolysis experiments were performed with 313-nm excitation in 50% isopropyl alcohol-water with varying acid concentrations, employing 1-cm spectrophotometric quartz cells. Other experimental procedures have been described elsewhere) Light intensities were typically 2.5 X l O I 5 quanta/sec as determined with the potassium ferrioxalate a~tin0meter.l~ The disappearance of 5-nitroquinoline was determined by polarographic analysis of buffered acetic acid-sodium acetate solutions (pH 4.7) before and after photolysis. There were no interfering waves associated with photoproducts generated during the photolysis. The polarographic potential was scanned from 0 to -0.8 V, and the half-wave potential appears at approximately -0.34 V vs. sce. Experiments were designed to keep the disappearance of 5-nitroquinoline at 115%. Large-scale photolyses were performed through a Pyrex sleeve with an Hanovia 450-W medium-pressure mercury immersion lamp (Type 679A-36). In a typical experiment, 1.50 g of 5-nitroquinoline was dissolved in 800 ml of 50% isopropyl alcohol-water, 0.15 M in HC1, and irradiating under nitrogen for approximately 9 hr. The originally colorless solution was red at the end of the experiment. The solution was then concentrated by evaporation in vacuo
and separation of the product was achieved with silica gel preparative TLC plates. The isolated photoproduct was established to be 6,8-dichloro-5-aminoquinoline by comparing its uv (Cary 14), nmr (Varian A60A), and ir (Beckman IR-8) spectra with that for 5-aminoquinoline. The photoproduct exhibits an amine band at 2.8 p and the amine protons, which appear at 4.2 6 in 5-aminoquinoline, are shifted to 5.2 6 due to chlorine substitution. The ring protons in the photoproduct appear between 7.5 and 9.0 6. Phosphorescence measurements of 4 X M 5-nitroquinoline at 77'K were made in EPA and in 50% isopropyl alcohol-water with varying amounts of HCl. The phosphorescence yields, using 313-nm excitation, for 5-nitroquinoline in EPA was determined relative to a value of 0.051 for naphthalene, which phosphoresces in the same wavelength region. References and Notes (1) R. Hurley and A. C. Testa, J. Amer. Chem. Soc., 88, 4330 (1966). (2) R. Hurley and A. C. Testa, J. Amer. Chem. SOC.,89, 6917 (1967). (3) R. Hurley and A. C. Testa, J. Amer. Chem. Soc., 90, 1949 (1968). (4) W. Trotter and A. C. Testa, J. Phys. Chem., 74, 845 (1970). (5) A. Cu and A. C. Testa, J. Phys. Chem., 77, 1487 (1973). (6) A. Cu and A. C. Testa, J. Amer. Chem. SOC.,96, 1963 (1974). (7) J. Lemaire, J. Phys. Chem., 71, 612 (1967). (8) R. 6. Cundall, s. Davies, and K. Dunniciiff in "The Triplet State," A. B. Zahian, Ed., Cambridge University Press, New York, N.Y., 1967, p 183. (9) S. P. McGlynn, T. Azumi, and M. Kinoshita In "Molecular Spectroscopy of the Triplet-State,'' Prentice-Hall, Englewood Cliff, N.J., 1969, p 272. ( I O ) G. 0. Wubbels, J. W. Jordan, and N. S. Mills, J. Amer. Chem. SOC.,95, 1281 (1973). (11) P. J. Wagner andG. S. Hammond, Advan. Photochem., 5, 99 (1966). (12) G. Porter and P. Suppan, Trans. Faraday Soc., 81, 1664 (1965). (13) C. G. Hatchard and C. A. Parker, Proc. Roy. Soc., Ser. A, 235, 518 (1956).
Electron Spin Resonance Study of the a-Keto lminoxy Radicals of Some Bicyclic Ketones H. CgldHraru, A. Caragheorgheopol, M. Moraru,' and V. E. Sahini" Department of Quantum Chemistry,Institute of Physical Chemistry, Bucharest 73, Romania (ReceivedAugust 72, 1974)
a-Keto iminoxy radicals from some cyclic and bicyclic ketones were studied by esr. Using properly substituted compounds of known conformation and configuration the proton hfs's have been assigned and their dependence on molecular geometry could be followed. For the a-anti proton hfs a cos2 8 type dependence involving hyperconjugation with the iminoxy carbon 2p, orbital was found. This is an indication of T-u polarization contributing to the spin density transmission in this class of u radicals.
Introduction For some years the iminoxy radicals (R1RZC=NO.) have been the subject of interest, the aim being to understand the hyperfine interaction mechanism in this class of u-type radicals. From a large amount of data Norman and Gilbert2 found evidence for a preferential syn interaction which is most efficient in the plane of u bonds and is highly favored by a W geometry. Our preliminary data on &-ketoiminoxy The Journal of Physical Chemistry, Voi. 79,No. 6, 1975
radicals3 showed however the unexpected feature that significant hfs from anti protons was involved. The same type of interaction was described in a recent work of Russell and Mackor4 who have undertaken a systematic study of cyclohexanone iminoxy radicals with new and important conclusions regarding the hyperfine interaction mechanism. We have studied a series of substituted bicyclic compounds with known configurations and conformations, and
