9568
J. Phys. Chem. 1996, 100, 9568-9572
Exploratory Photochemistry of Polyfluorinated 2-Naphthyl Azide Hong Bin Zhai and Matthew S. Platz* Department of Chemistry, The Ohio State UniVersity, 120 West 18th AVenue, Columbus, Ohio 43210 ReceiVed: April 2, 1996X
The photochemistry of perfluoro-2-naphthylazide was examined. At low temperature (77 K), photolysis of the azide produces a persistent triplet nitrene which can be characterized by UV-vis and EPR spectroscopy. Photolysis at ambient temperature releases a short-lived (τ < 1 ns) singlet nitrene which can be captured in neat pyridine to form an isolable ylide. Thus the lifetime of singlet perfluoro-2-naphthylnitrene is more similar to that of phenylnitrene where τ ) 0.01-0.1 ns than to perfluorophenylnitrene where τ has been deduced to be ≈20 ns. In the absence of concentrated pyridine, singlet perfluoro-2-naphthylnitrene forms a closed-shell azirine or ketenimine intermediate which forms intractable polymeric material.
Introduction
SCHEME 1
The sequence of intermediates produced upon photolysis of phenylazide 1 has been deduced and is depicted in Scheme 1.1 Photolysis releases singlet phenyl nitrene 2S which, depending upon the experimental conditions, (1) ring contracts to form vibrationally excited (q) cyanocyclopentadiene, which then forms the cyanocyclopentadienyl radical (3),2,3 (2) rings expands to form 1,2-dihydroazepeine (4),4 or (3) relaxes to form triplet phenyl nitrene 2T.5 Ketenimine 4 reacts with diethylamine to form azepine 5.6 There is no definitive evidence requiring the intermediacy of bicyclic azirine 6.
In this regard the photochemistry of 2-naphthyl azide differs from the parent system.1,7 Photolysis of 7 (Scheme 2) again releases a singlet nitrene (8S), but this nitrene interconverts with a bicyclic azirine intermediate (9a,b).7 This species can be
detected by matrix IR spectroscopy8 and reacts with diethylamine to give a different type of product (e.g., 10)7 than does ketenimine 4 with this reagent (Scheme 1).6 The bicyclic azirine can be converted into a mixture of ketenimines (11a,b) upon photolysis in argon at 10 K.8 Polyfluorination of the aromatic ring greatly assisted mechanistic analysis of the photochemistry of phenyl azide.5,8 Fluorine substitution extends the lifetime of singlet nitrene 13S by raising the barrier to ring expansion to ketenimine 14.1,9
SCHEME 2
This allowed convenient study of the photochemistry of 12 with nanosecond time resolution. These results encouraged us to examine the photochemistry of perfluoro-2-azidonaphthalene (15).
Herein we are pleased to report the results of this study which reveal important differences between the photochemistry of 12 and 15 and the limits of the fluorine effect. Experimental Section X
Abstract published in AdVance ACS Abstracts, May 15, 1996.
S0022-3654(96)00962-8 CCC: $12.00
General Methods. Melting points were obtained with an electrothermal melting point apparatus and are uncorrected. © 1996 American Chemical Society
Photochemistry of Polyfluorinated 2-Naphthylazide Fluorine nuclear magnetic resonance (NMR) spectra were recorded on a Bruker-250 spectrometer with deuteriochloroform as solvent. Infrared spectra were recorded on a Perkin-Elmer Model 1710 Fourier transform infrared spectrophotometer. Ultraviolet spectra were recorded on a Spectronic 3000 Array spectrophotometer. Mass spectra and exact masses were obtained on a Kratos MS-30 mass spectrometer at The Ohio State University Chemical Instrumentation Center. Laser Flash Photolysis. The laser flash photolysis system in use at The Ohio State University and the method of preparation of the samples have been reported elsewhere.10 Briefly, azide solutions were photolyzed in quartz cuvettes with a Lambda Physik excimer laser (308 nm, 17 ns, 150 mJ). The concentration of the azide was kept constant with optical density at 308 nm of about 1.0. Solutions were degassed prior to photolysis by bubbling with argon for 5 min. Because of large photochemical conversion of azide to product in each laser pulse, samples were changed after every laser shot. Matrix UV-Vis Spectroscopy. For measurements at 77 K a liquid nitrogen cooled cryostat (Oxford Instruments, Model DN1740) was used. UV-vis absorption spectra were obtained with a Spectronic 3000 Array spectrophotometer. Materials. The solvents (acetonitrile, benzene, carbon tetrachloride, cyclohexane, dichloromethane, Freon-113, methanol, pentane, tetrahydrofuran, pyridine, n-butyl sulfide, and disulfide) were dried by standard procedures prior to use. 2-Azido-1,3,4,5,6,7,8-heptafluoronaphthalene (15). A mixture of octafluoronaphthalene (1.83 g, 6.72 mmol) and hydrazine monohydrate (786 mg, 15.7 mmol) in 1,4-dioxane (16 mL) was stirred at room temperature for 4 h. The solvent was removed in vacuo, and the residue was dissolved in ether, washed with saturated sodium bicarbonate solution, water, and brine, dried over MgSO4, and concentrated. The solid was dissolved in acetonitrile (20 mL), and concentrated aqueous hydrochloric acid (2 mL) was added with stirring at 0 °C. The salt was collected and washed with ether. To a solution of the salt obtained above in 5 N HCl (10 mL), water (20 mL), and ether (60 mL) was added dropwise a solution of sodium nitrite (558 mg, 8.09 mmol), in water (5 mL), while the temperature was kept at 0-5 °C. The two layers were separated, and the aqueous layer was extracted with ether. The combined organic layer was washed with saturated sodium bicarbonate solution and brine, dried over MgSO4, and concentrated. The residue was chromatographed on a neutral alumina (40 g) column with hexanes as eluent to give 910 mg (46%) of the product as a brown solid; mp 37-38 °C. 19F NMR (250 MHz, CDCl3) δ -155.47 to -155.26 (m, 2 F, 2 CF), -148.16 to -147.85 (m, 1 F, CF), -146.92 to -146.34 (m, 3 F, 3 CF), -133.85 to -133.48 (m, 1 F, CF). MS (rel intensity) 295 (17, M+), 267 (60, M - N2), 248 (100, M - N2 - F). High-resolution MS calculated for C10F7N3 294.9981, found 294.9982. Pyridinyl-N-perfluoro-2-naphthylimine (20). A solution of perfluoro-2-azidonaphthalene (15, 152 mg, 0.515 mmol) in pyridine (2 mL) was degassed for 10 min by purging with argon and photolyzed with UV light (350 nm) at 4 °C for 1 h. Excess pyridine was removed in vacuo. The residue was chromatographed on a neutral alumina (20 g) column with hexanes, hexanes-ethyl acetate (1:3, v/v), and finally ethyl acetate as eluting solvents to give ca. 10 mg (5.6%) of the product as a yellow solid. 1H NMR (200 MHz, CDCl3) δ 7.41 (m, 1 H, CH), 7.43 (m, 2 H, 2 CH), 8.25-8.29 (m, 2 H, 2 CH). 19F NMR (250 MHz, CDCl3) δ -162.72 (m, 1 F, CF), -145.04 (m, 1 F, CF, -132.33 (m, 1 F, CF), -132.33 (m, 1 F, CF). UV (CH3CN) 418 nm. MS (rel intensity) 346 (89, M+), 327 (3, M - F), 267 (67, M - C5H5N), 248 (100, M - C5H5N - F), 79
J. Phys. Chem., Vol. 100, No. 22, 1996 9569
Figure 1. UV-vis difference spectrum obtained by exposing azide 15 in EPA glass at 77 K to 15 s of 300 nm light.
(74, C5H5N), 52 (31, C4H4). High-resolution MS calculated for C15H5F7N2 346.0342, found 346.0348. Results Low Temperature Spectroscopy. Photolysis of aromatic azides at cryogenic temperatures is known to favor triplet nitrene formation.5 Exposure of azide 15 in an ether-pentane-alcohol (EPA) glass to 300 ( 20 nm light at 77 K for 15 s leads to bleaching of the absorption spectrum of 15 and to the appearance of several new bands (Figure 1) at 349, 371, 400, 438, 599, and 651 nm. These bands are persistent for hours in the dark at -196 °C. Upon continued exposure to 300 nm light, these bands smoothly increase in intensity and no new bands are observed. The bands remain in the same ratio upon continued photolysis and thus are most likely due to a single species. The spectrum is similar in its general features to those of other aryl nitrenes1,5,11 and is therefore attributed to triplet nitrene 16T (Scheme 3).
This interpretation is supported by the observation of an intense magnetic resonance transition at 6746 G produced by 300 nm photolysis of 15 in EPA at 77 K. The EPR spectrum so obtained is characteristic of a triplet nitrene with D/hc ) 1.0052 cm-1. The D/hc value of 8T is, by comparison, 1.0083 cm-1.12 Laser Flash Photolysis Studies in the Absence of Pyridine. Azide 15 was studied by laser flash photolysis (LFP) techniques (XeCl, 308 nm, 17 ns). Transient spectra were recorded in acetonitrile, benzene, methanol, pentane, Freon-113 (CF2ClCFCl2), and methylene chloride. In each of these solvents the transient spectrum is very weak but there is a relativity sharp band at 360-370 nm and a broad ill-defined band at 420 nm which tails out to 500 nm (Figure 2). These spectral features are persistent. They show essentially no decay 50 ms after the laser pulse, and the bands are present in the same ratios in each solvent. Quantum yields for the decomposition of aryl azides are generally large.1 Thus the extinction coefficients and/or the yield of the species detected must be quite low. It seems likely that the weak sharp band at 360-370 nm is due to triplet nitrene 16T which is formed in solution at ambient temperature in poor yield. Thus we postulate that LFP of 15 releases singlet nitrene 16S which is not detected and which partitions between rearrangement either to an azirine (17 or
9570 J. Phys. Chem., Vol. 100, No. 22, 1996
Figure 2. Transient spectrum obtained upon 308 nm LFP of azide 15 in dichloromethane at ambient temperature. The spectrum was recorded 2.9 µs after the laser pulse over a window of 500 ns.
isomer) or a ketenimine type of intermediate (18 or isomer) and intersystem crossing (ISC) to the lower energy triplet state (Scheme 3). The azirine/ketenimine mixture may be responsible for the weak, broad absorption between 420 and 500 nm. We have previously reported that methanol catalyzes the ISC of singlet pentafluorophenylnitrene to the triplet state.9 However, LFP of 15 in methanol again produces only a weak transient spectrum of a triplet nitrene (Figure 3). No catalysis of ISC is observed in this system. Product studies were attempted in methanol, diethylamine, and hydrocarbons. Unfortunately photolysis (300 nm) in all solvents employed led mostly to polymer and a complex mixture of volatile materials. Laser Flash Photolysis in the Presence of Pyridine. Laser flash photolysis of aromatic azides frequently produces complex, overlapping transient spectra that are difficult, and sometimes impossible, to interpret. LFP of perfluorophenylazide 12 in the presence of pyridine produces a singlet nitrene, 13S, which is trapped to form ylide 21.9 This ylide is a stable compound
Zhai and Platz
Figure 3. Transient spectrum obtained upon 308 nm LFP of azide 15 in methanol. The spectrum was recorded 3 µs after the laser flash over a window of 600 ns.
Figure 4. UV-vis spectrum of an isolated sample of nitrene-pyridine ylide 20 at ambient temperature.
SCHEME 3
which can be isolated and characterized. It absorbs intensely at 390 nm and is a useful probe of the dynamics of 13S. Singlet nitrene 13S has a lifetime (τ ≈ 20 ns)1,9,13 that is too short, at present, to allow us to resolve the growth of pyridine ylide 21 by nanosecond spectroscopy. However, the lifetime of singlet p-carbomethoxytetrafluorophenylnitrene (23S) is sufficiently long to measure its lifetime (τ ≈ 200 ns) in methylene chloride and its absolute rate constant of reaction with pyridine (kpyr ) 3.1 ( 0.1 × 107 M-1 s-1) in the same solvent.13
Upon 350 nm photolysis of 15 in neat pyridine a deep yellow color is produced. Stable ylide 20 (Scheme 3) can be isolated from this sample in low (5%) yield and characterized by 1H
and 19F NMR spectroscopy and mass spectrometry. The UVvis spectrum of ylide 20 is given in Figure 4. Its spectrum is quite similar to that of ylide 21, although its absorption maximum is shifted to the red by 20-30 nm.9
Photochemistry of Polyfluorinated 2-Naphthylazide
J. Phys. Chem., Vol. 100, No. 22, 1996 9571
Figure 7. Plot of k′obs of the 530 nm absorbing transient versus [pyridine] in benzene at 323 K.
Figure 5. Transient spectrum observed upon LFP (308 nm) of 15 in neat pyridine. The spectrum was recorded 10 µs after the laser flash over a window of 800 ns.
Figure 6. Transient spectrum produced upon 308 nm LFP of azide 15 in acetonitrile containing 0.062 M pyridine. The spectrum was recorded 900 ns after the laser pulse over a window of 600 ns.
SCHEME 4
LFP (308 nm) of azide 15 in neat pyridine produces the weak transient spectrum of Figure 5, which clearly resembles that of stable ylide 20. However the relatively intense transient spectrum obtained by LFP of 15 in acetonitrile containing dilute (0.062 M) pyridine (Figure 6) does not at all resemble the absorption spectrum of ylide 20. We have previously reported that ketenimine 4 and its perfluorinated analogue 14 react rapidly with pyridine to produce ylides (e.g., 25, Scheme 4) which
absorb at much longer wavelength than that of nitrene-pyridine ylides 20 and 21. Thus we attribute the transient absorption of Figure 6 to ylide 19 formed by capture of a ketenimine (18 or isomer, Scheme 3) or azirine (17 or isomer) type of intermediate, followed (in the latter case) by electrocyclic ring opening to 19 (or isomer).
It is possible to resolve the pseudo-first-order growth of the transient absorbing at 530 nm. The pseudo-first-order growth rate constant, kobs, is linearly dependent on the concentration of pyridine (Figure 7). On this basis we confidently assign the 530 nm absorbing species as ylide 19 (Scheme 3). The slope of the plot of Figure 7 is k′pyr, the absolute rate constant of reaction of azirine/ketenimine (17/18, Scheme 3) with pyridine. The intercept of this plot is k′0 or 1/τ, where τ is the lifetime of azirine/ketenimine in the absence of pyridine and k′0 is the sum of all first-order and pseudo-first-order rate constants of all processes which consume 17/18 in the absence of pyridine. The azirine/ketenimine lifetime is controlled by reaction of the transient with azide, or in the case of methanol, with solvent (Vide infra). Values of k′pyr, τ, and Arrhenius activation parameters are given in Tables 1 and 2, respectively. The absolute rate constant of reaction 17/18 with pyridine (k′pyr) shows little variation with solvent (Table 1). It was not possible to resolve the growth of the 525 nm absorbing species following LFP of 15 in methanol. Clearly the lifetime of 17/18 in the nucleophilic solvent is rather short. This is not surprising as perfluoroketenimine 14 reacts with nucleophiles with rate constants on the order of 109 M-1 s-1.9 To estimate τ of 17/18 in methanol the optical yield of ylide (A525) was measured as a function of pyridine concentration. A double reciprocal treatment of the data (Figure 8) results in a linear plot. It can be shown that the ratio of the intercept to the slope of this plot is k′pyrτ, where τ is the lifetime of 17/18 in methanol in the absence of pyridine. If we assume that k′pyr is the same in methanol as in acetonitrile, we deduce that the lifetime of 17/18 in this solvent is 52 ns. It is important to note that singlet pentafluorophenylnitrene 13S is easily captured by 0.058 M pyridine in dichloromethane (Scheme 4). The fact that 16S requires neat pyridine to be intercepted in low (5%) yield indicates that this singlet nitrene is very much shorter lived than is 13S. On this basis we estimate
9572 J. Phys. Chem., Vol. 100, No. 22, 1996
Zhai and Platz
Figure 8. Double-reciprocal treatment of the yield of ylide (A525) versus pyridine.
TABLE 1: Effect of Solvent and Temperature on Kinetics of Ketenimine/Azirinepyridine Ylide (19) Formation following LFP of 2-Azido-1,3,4,5,6,7,8-heptafluoronaphthalene (15) entry
solvent
T (K)
k′0 (s-1)
k′pyr (M-1 s-1)
τ0 (ns)
1 2 3 4 5 6 7 8 9
C6H6 C6H6 C6H6 C6H6 C6H6 C6H12a CCl4 CH3CN CH3OH
286 295 300 307 315 295 295 295 295
1.48E6 2.47E6 3.70E6 4.19E6 6.38E6 2.79E6 2.21E6 1.92E7 2.44E8
2.18E9 2.63E9 2.85E9 3.05E9 3.72E9 3.81E9 2.16E9 2.57E9 2.57E9
676 405 270 239 157 358 452 52 52b
a Cyclohexane. b Deduced value obtained by assuming k′ pyr in methanol is equal to that in acetonitrile.
TABLE 2: Arrhenius Parameters for k′0(1/τ ) and k′pyr of the Pyridine Ylide 19 Produced by LFP (308 nm) of 2-Azido-1,3,4,5,6,7,8-heptafluoronaphthalene (15) in Benzene Ea (kcal/mol) A
k0(1/τ )
k′ pyr
8.84 8.79 × 1012 s-1
3.14 5.43 × 1011 M-1 s-1
that the lifetime of 16S is at least 100 times shorter than that of 13S and is subnanosecond. Thus the lifetime of 16S is closer to that of singlet phenylnitrene 2S (τ ≈ 10-100 ps)1 than to that of singlet perfluorophenylnitrene 13S (τ ≈ 20-50 ns)9. Discussion The short, subnanosecond lifetime of singlet perfluoro-2naphthylnitrene is the most remarkable finding of this work. The fluorine effect which extends the lifetime of singlet phenylnitrene (τ ≈ 0.01-0.1 ns)1 relative to perfluorophenylnitrene (τ ≈ 20-50 ns)9 is absent in this system. We have argued in the past13 for a molecular orbital interpretation of the fluorine effect. The lowest energy configuration of singlet phenylnitrene is open shell (σπ),14 but ring expansion likely proceeds through a closed-shell (π2) configuration by sliding an electron pair into a parallel, empty orbital.
We have argued that perfluorination reduces the accessibility of the π2 configuration by stabilizing the σ2 configuration due to π back-bonding from the fluorine substituents. This increases the barrier to ring expansion from 3 to 8 kcal/mol.
We postulate that the extended π system of naphthyl relative to phenyl should lower the energies of both simple closed-shell configurations. We speculate that the increased accessibility of σ2 and π2 configurations of perfluoronaphthyl relative to perfluorophenylnitrene increases the rate of ring expansion or cyclization. Hence the shorter lifetime of the larger nitrene. Of course thermodynamic effects may also be operative. Ring expansion and ring contraction reactions of 13S to form ketenimines or azirines means the complete loss of aromatic resonance energy of the aryl nitrene. The corresponding reactions of singlet perfluoro-2-naphthylnitrene 16S destroys aromaticity in only one ring while the second ring remains aromatic. As the resonance energy of benezene is greater then that of naphthalene (on a per ring basis) it is likely that the rearrangements of singlet perfluorophenyl nitrene 13S are less exothermic and slower then the rearrangements of the perfluoronaphthyl analogue. Conclusion Photolysis of perfluoro-2-naphthyl azide releases a singlet nitrene. At 77 K the singlet nitrene relaxes to a triplet nitrene. The triplet is persistent at 77 K and can be characterized by UV-vis and EPR spectroscopy. At ambient temperature the singlet nitrene can be captured by pyridine to form an isolable ylide. The singlet nitrene lifetime in solution at ambient temperature is subnanosecond. This short lifetime is closer to that of singlet phenylnitrene than that of singlet perfluorophenylnitrene. Acknowledgment. Support of this work by the National Institutes of Health (GM34823) is gratefully acknowledged. References and Notes (1) Schuster, G. B.; Platz, M. S. AdV. Photochem. 1992, 17, 69. (2) Wentrup, C. Tetrahedron 1974, 30, 1301. (3) (a) Collin, D. W.; Soundararajan, N. S.; Platz, M. S.; Miller, T. A. J. Phys. Chem. 1990, 94, 8890. (b) Collin, D. W.; Yu, L.; Williamson, J.; Platz, M. S.; Miller, T. A. J. Phys. Chem, 1990, 94, 3387. (4) Chapman, O. L.; LeRoux, J.-P. J. Am. Chem. Soc. 1978, 100, 282. (5) Leyva, E.; Platz, M. S.; Persy, G.; Wirz, J. J. Am. Chem. Soc. 1986, 108, 3783. (6) Doering, W. v. E.; Odum, R. A. Tetrahedron 1966, 22, 81. (7) (a) Schrock, A. K.; Schuster, G. B. J. Am. Chem. Soc. 1984, 106, 5234. (b) Leyva, E.; Platz, M. S. Tetrahedron Lett. 1987, 28, 11. (c) Hilton, S. E.; Scriven, E. F. V.; Suschitzky, H. J. Chem. Soc., Chem. Commun. 1974, 853. (d) Carrol, S. E.; Noy, B.; Seriven, E. F. V.; Suschitzky, H. Tetrahedron Lett. 1977, 943. (e) Rigaudy, J.; Igier, C.; Barcelo, J. Tetrahedron Lett. 1975, 3485. (8) Dunkin, I. R.; Thomson, P. C. P J. Chem. Soc., Chem. Commun. 1980, 499. (9) Poe, R.; Schnapp, K.; Young, M. F. T.; Grayzar, J.; Platz, M. S. J. Am. Chem. Soc. 1992, 114, 5054. (10) Soundararajan, N.; Platz, M. S.; Jackson, J. E.; Doyle, M. P.; Oon, S. M.; Liu, M. T. H.; Anand, S. M. J. Am. Chem. Soc. 1988, 110, 7143. (11) Reiser, A.; Bowes, G.; Horne, R. J. Trans. Faraday Soc. 1966, 62, 3162. (12) Wasserman, E. Prog. Phys. Org. Chem. 1971, 8, 319. (13) Marcinek, A.; Platz, M. S.; Chan, S. Y.; Floresca, R.; Rajagopalav, K.; Golinski, M.; Watt, D J. Phys. Chem. 1994, 98, 412. (14) (a) Kim, S. J.; Hamilton, T. P.; Schaefer, H. F., III J. Am. Chem. Soc. 1992, 114, 5349. (b) Hrovat, D. A.; Waali, E. E.; Borden, W. T. J. Am. Chem. Soc. 1992, 114, 8698. (c) Travers, M. J.; Cowles, D. C.; Clifford, E. P.; Ellison, G. B. J. Am. Chem. Soc., 1992, 114, 8699.
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