10186
J. Am. Chem. SOC. 1995,117, 10186-10193
Reactions of a Triplet Arylnitrenium Ion: Laser Flash Photolysis and Product Studies of N-tert-Butyl(2-acetyl-4-nitrophenyl)nitreniumIon Sanjay Srivastava and Daniel E. Falvey* Contribution from the Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742 Received June 20, 1 9 9 9
Abstract: Photolysis of l e (N-tert-butyl-5-nitro-3-methylanthraniliumion) (C104- or BF4- salt) generates the isomeric tert-butyl(4-nitro-2-acetylphenyl)nitrenium ion (2e). The latter further reacts to form iminium ion 4e and parent amine 5e as stable products. On the basis of triplet sensitization and quenching studies, it is shown that 4e is a product of the singlet state of 2e and 5e is a product of the triplet state of 2e. Laser flash photolysis experiments on l e give a short-lived (ca. 300 ns) transient spectrum having maxima at 390 and 540 nm which is assigned to the excited triplet state of le. It was not possible to directly detect nitrenium ion 2e under these conditions. However, its lifetime and reactivity could be studied through the use of a probe substrate. Triphenylmethane does not affect the decay of triplet le. However, its addition causes a new transient species to grow in over much longer time scales than the decay of triplet le. The new, longer lived species is identified as the triphenylmethyl radical (PhjC'). Its formation is attributed to hydrogen atom abstraction by the triplet state of 2e. Analysis of the growth kinetics for Ph3C' shows that 2e lives for ca. 2 ,us in CH2C12. On the basis of this observation as well as several other mechanistic and kinetic considerations, it is argued that arylnitrenium ion 2e has a triplet ground state.
Nitrenium are reactive intermediates that contain a divalent, positively charged nitrogen atom. The proposal that arylnitrenium ions are intermediates in the carcinogenic, DNAdamaging reactions of activated aromatic aminess-* has motivated much of the recent research in this area. Indeed, the past several years have seen significant advances in characterizing arylnitrenium ion lifetimes in aqueous solution?-12 their reactivity and selectivity toward nucleophile^,^^-^^ effects of ion pairing,16-18 and the mechanisms of their reactions with DNA and related nucleophiles.19-26 Abstract published in Advance ACS Abstracts, September 15, 1995. (1) Abramovitch, R. A.; Jeyaraman, R. In Azides and Nitrenes: Reactivity and Utility; Scriven, E. F. V., Ed.; Academic: Orlando, FL, 1984; pp 297357. (2)Gassman. P. G. Acc. Chem. Res. 1970, 3, 26-33. (3) Simonova, T. P.; Nefedov, V. D.; Toropova. M. A.; Kirillov, N. F. Russ. Chem. Rev. 1992, 61, 584-599. (4) Heller, H. E.; Hughes, E. D.; Ingold, C. K. Nature 1951, 168,909910. (5) Kadlubar, F. F. In DNA Adducts fdentijication and Signijicance; @
Hemminki, K., Dipple, A., Shuker, D. E. G., Kadlubar, F. F., Segerback, D., Bartsch, H., Eds.; University Press: Oxford, U.K., 1994; pp 199-216. (6) King, C. M.; Phillips, B. Science (Washington. D.C.) 1968, 159,
135 1- 1353. (7) Miller, J. A. Cancer Res. 1970, 30, 559-576. (8) Scribner, J. D.; Naimy, N. K. Cancer Res. 1975, 35, 1416-1421. (9) Fishbein, J. C.; McClelland, R. A. J. Am. Chem. Soc. 1987, 109. 2824-2825. (10) Davidse, P. A.; Kahley, M. J.; McClelland, R. A.; Novak, M. J. Am. Chem. Soc. 1994, 116,4513-4514. (1 1) Novak, M.; Kennedy, S. A. J. Am. Chem. Soc. 1995, 117, 574575. (12) Novak, M.; Kahley, M. J.; Eiger, E.; Helmick, J. S.; Peters, H. E. J. Am. Chem. Soc. 1993, 115, 9453-9460. (13) Robbins, R. J.; Yang, L. L.-N.; Anderson, G. B.; Falvey, D. E. J. Am. Chem. Soc. 1995, 117, 6544-6552. (14) Anderson, G. B.; Falvey, D. E. J. Am. Chem. Soc. 1993, 115,9870987 1. (15) Novak, M.; Kahley, M. J.; Lin, J.; Kennedy, S. A.; Swanegan, L. A. J. Am. Chem. Soc. 1994, 116, 11626- 11627. (16) Fishbein, J. C.; McClelland, R. A. J. Chem. Soc., Perkin Trans. 2 1995, 663-67 1. (17) Fishbein, J. C.; McClelland, R. A. J. Chem. Soc., Perkin Trans. 2 1995, 653-662.
Singlet
Triplet
Figure 1. Singlet and triplet nitrenium ions.
Less understood are the behavior and properties of triplet state arylnitrenium ions. Nitrenium ions are isoelectronic with carbenes and nitrenes and therefore have low-energy triplet and singlet states (Figure 1). Both t h e ~ r y ~and ~ , sophisticated ~* gas phase experiment^?^ agree that the simplest nitrenium ion, N&+, is a ground state triplet with a singlet-triplet energy gap of ca. +30 kcal/mol. For the phenyl- and most simple arylnitrenium ions ~ e m i e m p i r i c a l , ~ab~ initio,30*34.3s -~~ and density functional (18) Panda, M.; Novak, M.; Magonski, J. J. Am. Chem. Soc. 1989, 111, 4524-4525. (19) Novak, M.; Martin, K. A.; Heinrich, J. L. J. Org. Chem. 1989, 54, 5430-543 1. (20) Ohta. T.; Shudo, K.; Okamoto, T. Tetrahedron Lett. 1V8.23, 19831986. (21) Famulok, M.; Bosold, F.; Boche, G. Angew. Chem., Int. Ed. EngI. 1989,28, 337-338. (22) Famulok, M.; Boche, G. Angcw. Chem., Int. Ed. EngI. 1989. 28, 468-469. (23) Helmick, J. S.; Martin, K. A.; Heinrich, J. L.; Novak, M. J. Am. Chem. Soc. 1991, 113, 3459-3466. (24) Cardenelli, L.; Carloni, P.; Damiani, E.; Greci, L.; Stipa, P.; Rizzoli, C.; Sgarabotto, P. J. Chem. Soc., Perkin Trans. 2 1994, 1589-1595. (25) Abramovitch, R. A.; Beckert, J. M.; Chinnasamy, P.; Xiaohua, H.; Pennington, W.; Sanjivamurthy, A. R. V. Heterocycles 1989, 28, 623628. (26) Humphreys, W. G.; Kadlubar, F. F.; Guengerich, F. P. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 8278-8282. (27) Cramer, C. J.; Dulles, F. J.; Storer, J. W.; Worthington, S. E. Chem. Phys. Lett. 1994, 218, 387-394. (28) Peyerimhoff, S. D.; Buenker, R. J. Chem. P h y . 1979, 42, 167176. (29) Gibson, S. T.; Greene, J. P.; Berkowitz, J. J. Chem. Phys. 1985. 83, 43 19-4328.
0002-7863/95/15 17-10186$09.00/0 0 1995 American Chemical Society
J. Am. Chem. SOC.,Vol. 117, No. 41, 1995 10187
Reactions of a Triplet Arylnitrenium Ion
Scheme 1 a: X-H b: X-Br c: X-CI d: X-CH30: X=NO2
I 4
J
OR
3
theory (DFT)35predict singlet ground states. The predicted value for phenylnitrenium ion is -17.7 kcaymol from the DFT calculations, and the ab initio calculations give qualitatively similar values. Aryl substitution preferentially stabilizes the singlet state because the filled n-orbitals on the aromatic ring interact with and raise the energy of the out-of-plane p orbital relative to the in-plane 0 orbital. It follows then that n-donor substituents on the aromatic ring should further stabilize the singlet relative to the triplet. Likewise n-acceptors are predicted to stabilize the triplet relative to the singlet.30 The paucity of experimental information on triplet arylnitrenium ions can be attributed to their short lifetimes and the fact that they are usually inaccessible by ground state (thermal) generation. Photochemical methods make it possible to selectively generate triplet nitrenium ions and characterize their chemistry. Following earlier report^,^^,^' we found that photolysis of anthranilium salts 1 produces arylnitrenium ions 2 (Scheme 1).38,39 It was shown that the singlet state nitrenium ions '2 react via a 1,2 shift from the alkyl group to the nitrogen, giving iminium ion 4. Nucleophiles such as water and alcohols attack '2, giving ring adducts 3 and 6. The triplet nitrenium ion 32 reacts with H-atom donors to yield the parent amine 5. Two features of Scheme 1 are especially relevant to the present work. First, the photoproduct distributions often reflect the spin state first generated, rather than the lowest energy spin state. As with the arylcarbene~,4O-~~ both the singlet and triplet arylnitrenium ions are highly reactive and can be kinetically trapped before equilibrium between the spin states is achieved. Second, the precursor anthranilium ion 1 is not photolyzed instantaneously to 2. Rather, its excited singlet state '1* (30) Falvey, D. E.; Cramer, C. J. Tetrahedron Lett. 1992, 33, 1705i7nx -.
(31)Ford, G. P.; Scribner, J. D. J. Am. Chem. SOC. 1981, 103, 4281429 1. (32)Ford, G. P.; Herman, P. S . J. Mol. Struct. (THEOCHEM) 1991, 236, 269-282. (33) Glover, S. A.; Scott, A. P. Tetrahedron 1989, 45, 1763-1776. (34) Li, Y.; Abramovitch, R. A,; Houk, K. N. J. Org. Chem. 1989, 54, 291 1-2914. (35) Cramer, C. J.; Dulles, F. J.; Falvey, D. E. J. Am. Chem. SOC. 1994, 116, 9787-9788. (36)Doppler, T.; Schmid, H.; Hansen, H.-J. Helv. Chim. Acta 1979, 62, 304-3 13. (37) Haley, N. F. J. Org. Chem. 1977, 42, 3929-3933. (38) Anderson, G. B.; Yang,L. L.-N.; Falvey, D. E. J. Am. Chem. SOC. 1993, 115, 7254-7262. (39) Robbins, R. J.; Falvey, D. E. Tetrahedron Lett. 1994, 35, 49434946. (40) Schuster, G. B. Adv. Phys. Org. Chem. 1986, 22, 311-361. (41) Kirmse, W. Carbene Chemistry; Academic: New York, 1971. (42) Platz, M. S.; Maloney, V. M. In Kinetics and Spectroscopy of Carbenes and Biradicals; Platz, M. S . , Ed.; Plenum: New York, 1990; pp 239-352.
6
Scheme 2 R-H
+
+'
Ar-Y-tBu
-Re
.+
Ar-pBu H
R-H
-213.
Y+
Ar-Y-tBu H
partitions between ring opening (k,)and intersystem crossing to the triplet state 31*( k d . The partition ratio kisC/k,,depends on the particular anthranilium ion. This ratio is negligible for methyl derivative Id, but significant (ca. 0.25) for bromo derivative lb.'33'4 There are several unresolved issues regarding triplet arylnitrenium ions. The first is the mechanism of the formation of parent amine 5. This is the result of a net two-electron reduction of the arylnitrenium ion 2. On the basis of analogous behavior in the a r y l c a r b e n e ~ , ~we - ~proposed ~ that 5 was formed from two sequential H atom abstractions by 32(Scheme 2). However, the earlier experiments provided no direct evidence for the radical intermediates that would be formed in such a mechanism. Second, the possibility that the excited triplet anthranilium ion 3lreacted directly, rather than through 32,to give 5 was never excluded. While perhaps intuitively unlikely, this direct pathway was nonetheless consistent with available data. Herein are reported our investigations of the photochemistry of N-tert-butyl-3-methyld-nitroanthranilium ion (le). In addition to the issues noted above, work on this particular compound was also motivated by the prediction that strongly electron withdrawing groups on the aromatic ring would stabilize the triplet state relative to the singlet. Product analysis and laser flash photolysis (LFP) studies c o n f i i our proposed mechanism for the origin of 5 (Scheme 2). Specifically, radical coupling products from the H atom transfer reaction are isolated, and the intermediate radicals are detected by LFP. The excited triplet state of the precursor, 31e*,is also detected by LFP and is shown to be unreactive toward H atom donors. Finally, it is shown that the triplet state nitrenium ion 32e lives for ca. 2 ,us in CH2C12. This observation and several other mechanistic considerations suggest that 2e has a triplet ground state.
Results and Discussion
1. Synthesis and Ground State Reactivity. The anthranilium salt l e is prepared from the free base 5-nitro-3methylanthranil (7e) by treatment of the latter with tert-butyl alcohol in the presence of strong acid. The counterion of 1 is determined by the acid used to effect the alkylation. Thus, HC104 gives the perchlorate salt, and HBF4 gives the tetrafluoroborate salt. Preparation of 7e was accomplished by nitration of 3-methylanthranil (Scheme 3). In the dark, certain nucleophiles (e.g., CH3OH, H2O) add rapidly and reversibly to the C-3 position of anthranilium ions
Srivastava and Falvey
10188 J. Am. Chem. SOC.,Vol. 117, No. 41, 1995
Table 2. Effect of Triplet Sensitization and Quenching on Product Yields
Scheme 3
Table 1. Product Yields from Photolysis of Anthranilium Ion l e conditions
4e
5e
CH3CN CHXN/CHjOH (5 MY CH,CN/H20(5 M)" CH3CN/H20(27MY
35 38 31 34
63 CHZClz 61 CHzClz/CH30H (5 M)" 55 CH3OH" 48 THF"
4e
conditions
1. To avoid this potential artifact, nucleophilic trapping experiments were carried out with a small amount of acid added to the nucleophile. This issue is discussed in an earlier paper.I3 2. Direct Irradiation Photochemistry. Photolysis of l e (C104- or BF4- salt) (Xe arc lamp, L > 320 nm) in various solvents gives only two products: the parent amine 5e and the iminium ion 4e (eq 1). The latter undergoes facile hydrolysis. Although it can be detected in IH NMR spectra of the photolysis mixtures, it was not isolated. Instead, its hydrolysis product, amine 11 (eq 2), was isolated and fully characterized. The yields of the two stable photoproducts 4e and 5e depend on the reaction conditions as shown in Table 1.
hv
49
+
59
4e
5e
conditions
4e
5e
35 50
