Mendeleev Communications Electronic Version, Issue 1, 2000 (pp. 1–42) Selective photolysis of the azido groups in 2,4-diazidopyridines Sergei V. Chapyshev,*a Richard Waltonb and Paul M. Lahtib a Institute for Problems of Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russian Federation. Fax: +7 096 515 3588; e-mail: chap@icp.ac.ru b Department of Chemistry, University of Massachusetts, Amherst, MA 01003–4510, USA.Fax: +1 413 545 4490; e-mail: lahti@grond.chem.umass.edu DOI: 10.1070/MC2000v010n01ABEH001210 Selective photolysis of the a-azido group in 2,4-diazidopyridines has been determined by EPR spectroscopy and theoretically treated in terms of non-adiabatic photodissociation processes. The selective photochemical cleavage of chemical bonds in organic compounds is a very important problem in chemistry.1 In continuation of our research on selective derivatization of the azide groups in aromatic polyazides,2 an EPR study of the primary products formed in the photolysis of 2,4- and 2,6-diazidopyridines has been performed, the results of which are described here.Azides 1a–e, 3a,b, 5a–e and 8a,b were irradiated in degassed, frozen solutions of 2-methyltetrahydrofuran at l > 300 nm for 5 min at 77 K.† Upon irradiation all samples became blue and displayed X-band EPR spectra typical3–6 of triplet nitrene signals in the region of 6870–7100 G (Table 1).In order to differentiate the isomeric triplet pyridylnitrenes on the basis of their EPR characteristics, published data3–5 on the related EPR spectra were considered.In addition, PM3 computations‡ for a series of heteroarylnitrenes were carried out. Contrary to the case of parasubstituted phenylnitrenes,3,6 the zero-field splitting (zfs) D-values of heteroarylnitrenes correlate fairly well, although non-linearly, with electron-withdrawing properties of the heteroaryl substituents and with computed C–N bond lengths (Table 2).The adequacy of the PM3 computations is supported by an excellent fit of the geometrical parameters for triplet phenylnitrene calculated by this and CASSCF(8,8)/6-31G*8 methods (Figure 1). Using the correlation between the D-values and the PM3 C–N bond lengths for triplet heteroarylnitrenes, the EPR spectra of the primary products formed by photolysis of azides 1a–e, 3a,b, 5a–e and 8a,b were analysed.As can be seen in Table 1, among nitrenes 2a–e (generated from 1a–e), nitrenes 2b–d have the same D-values and C–N bond lengths, whereas 2a and 2e have lower D-values and shorter C–N bonds. An explanation of these effects comes from an analysis of the structures of 2a–e. The structures of 2b–d are better described by the resonance form I, those of 2a,e, by form II.The former has shorter C(5)–Cl and longer C(6)– NHAr bonds. The low D-value for 4a, formed by photolysis of 3a, is presumably associated with the presence of a strong electron- donating aziridine substituent at the pyridine ring of this nitrene. In comparison with 4a, dicyano derivative 4b has a higher D-value and longer C–N bond, due to the stronger electron-withdrawing character of the pyridine ring and the inability of a nitrene unit to conjugate with the para-chlorine atom. † The synthesis of azides 1a–e, 2a,b, 3a,b and 4a–e was described elsewhere. 13 The photolysis and EPR measurements were described previously. 14 ‡ The structures of azides, triplet nitrenes and excited states were calculated with the full optimization of geometrical parameters using the PM3 method (UHF, SCF level).15 The photolysis of 5a–d led to EPR spectra with two triplet nitrene signals.By analogy with 4a,b, the low-field signals were assigned to nitrenes 6a–e bearing the chlorine atom at the paraposition to the nitrene unit, whereas the high-field signals were assigned to nitrenes 7a–e. This assignment is supported by shorter C–N bond lengths in 6a–e, comparing with those for 7a–e (Table 1).The effect is quite small, but reasonably consistent. The nitrene peaks from photolysis of 5b,e showed shoulders, but were insufficiently resolved to distinguish two different nitrene signals. The small effect of substituents in the ArNH moieties on the D-value of 6a–e and 7a–e is likely explained by a weak conjugation of these moieties with the pyridine ring.The C(5)– C(4)–N–C(1' ) torsion angles in 6a–e and 7a–e are about 122°, while the related angles in 2a–e are about 20°. In contrast to 5a–e, diazides 8a,b upon irradiation displayed in the EPR spectra only one triplet nitrene, the line position of which was almost the same as that for nitrenes 2b–d. An answer to the question of which nitrenes, 2-nitreno 9a,b or/and 4-nitreno 10a,b, give the EPR signals in the experiment, was suggested by an analysis of the C–N bond lengths in 9a,b and 10a,b.The substantial difference in these bonds for 9a,b and 10a,b (Table 1) suggests that signals of these isomers should not coincide in the EPR spectra. Nitrenes 9a,b match better to the spectral line positions observed. N N N SCF PM3 CASSCF(8,8)/6-31G* MCSCF(8,8)/3-21G 3A2 1.326 1.434 1.387 1.402 1.338 1.425 1.386 1.404 1.402 1.404 1.391 1.396 3A2 3A2 Figure 1 The molecular geometry parameters of triplet phenylnitrene, computed by the SCF PM3, CASSCF(8,8)/6-31G*8 and MCSCF(8,8)/3-21G9 methods.aShoulder. Table 1 The line positions, D-values and computed C–N bond lengths of nitrenes 2a–e, 4a,b, 6a–e, 7a–e, 9a,b and 10a,b.Nitrene Field/G |D hc–1|/cm–1 C–N/Å 2a 7008 1.026 1.3291 2b 7045 1.040 1.3297 2c 7045 1.040 1.3297 2d 7105 1.063 — 2e 6911 0.989 1.3282 4a 6877 0.976 1.3282 4b 7020 1.031 1.3299 6a 6877 0.976 — 6b 6877 0.976 1.3260 6c 6877 0.976 1.3260 6d 6877 0.976 — 6e 6899 0.983 — 7a 7059 1.046 — 7b —a — 1.3264 7c 6989 1.019 1.3264 7d 6990 1.018 — 7e —a — — 9a 7040 1.038 1.3290 9b 7105 1.062 1.3293 10a — — 1.3277 10b — — 1.3278 Table 2 The D-values and computed C–N bond lengths of nitrenes.Nitrene |D hc–1|/ cm–1 C–N/Å Reference Py-3-N 1.005 1.3248 3 Py-2-N 1.051 1.3308 4 5-Trifluoromethylpyridyl-2-nitrene 1.087 1.3315 5 3,5-Bis(trifluoromethyl)pyridyl-2-nitrene 1.108 1.3333 5 Diazidocyanurylnitrene 1.44 1.3571 7Mendeleev Communications Electronic Version, Issue 1, 2000 (pp. 1–42) Both the C–N bond lengths and the D-values for 9a,b are almost equal to those for 2-nitreno species 2b–d, which have similar nitrene peak positions and D-values. The selective photolysis of the a-azido group in 8a,b can be rationalised from an analysis of the energies for two different excited T0 states of 8a,b (a-T0 and g-T0 states for dissociation of the a- and g-azido group, respectively).The photodissociation of chemical bonds in the molecules occurs as a result of energy transfer from the potentials of the excited states (curves 2 and 2' in Figure 2) onto the potential of the repulsive term (curve 3).10 The higher the energy of the excited states, the higher the probability that surfaces 2 or 2' will be crossed by surface 3.As has been recently shown,11 the most prominent feature of the azides in their excited T0 state is the considerable bending of the azido group (from about 170° in the ground state to about 128° in the T0 state) and a dramatic fall in the activation energy of N–N2 bond dissociation (from about 35 to 2 kcal mol–1, respectively). The computational distortion of the a- or g-azido group in 8a,b to 135° followed by the PM3 geometry optimization of the molecules in the T0 state allowed us to determine the energies of the a-T0 and g-T0 states of 8a,b.The energy of the former state was found to be higher by 4 kcal mol–1. This finding supports the idea that the a-excited states (T0, S1, T1, etc.)11 of 8a,b, indeed, lie more closely to the reaction coordinate, so that photodissociation of the a-azido groups in these compounds should be1,10 the preferable process.Note that the PM3 computation of the T0 state for 8a without preliminary bending of one of the azido groups was found to yield the a-T0 state by preference. Another interesting finding is the substantial difference in the orbital density distribution between the a- and g-azido groups in the HOMO of 8a,b (Figure 3).Assuming that the loss of stronger bonding interactions in the molecules leads to the formation of the less stable intermediates,12 the presence of a higher bonding orbital density at the Na and Nb atoms11 of the a-azido group in the HOMO of 8a,b explains fairly well the higher E G 1 2 3 2' 2'' Ed hn kd Figure 2 Schematic diagram of the electronic terms for non-adiabatic dissociation (1, 2, 2' and 2'' are electronic terms for the ground and excited states, respectively; 3 is the repulsive term; Ed is the energy of dissociation; kd is the dissociation rate constant).N HN N3 CN Cl Cl N HN N CN Cl Cl N HN N CN Cl Cl N HN N CN Cl Cl N N3 N3 R N R N N3 N R N R N N3 N CN NHC6H4R Cl N N3 N3 CN NHC6H4R Cl N N N3 CN NHC6H4R Cl N RC6H4HN N3 CN N Cl N RC6H4HN N3 CN N3 Cl N RC6H4HN N CN N3 Cl a R = p-OMe b R = p-Me c R = H d R = p-Br e R = m-NO2 1a–e 2a–e I II 1, 2, 5–7: 3a,b 4a,b R R R R a R = Cl b R = CN a R = p-Me b R = p-Br 3, 4: 8–10: hn hn hn hn hn hn 6a–e 5a–e 7a–e 8a,b 9a,b 10a,b d+ d+ d– d–Mendeleev Communications Electronic Version, Issue 1, 2000 (pp. 1–42) energies for the a-T0 state of these molecules.Figure 3 also demonstrates that the azido groups in the HOMO of 5b are equal (have almost no orbital density at the Na atoms). Thus, it is no surprise that the energies of the a- and a'-T0 states for 5b were also computed to be equal, and the photolysis of this diazide led to formation of both isomers 6b and 7b. It seems reasonable to assume that selective photolysis of the azido groups in polyazides can be possible only for those compounds in which the azido groups significantly differ in the HOMO orbital density distribution.Conversely, the computational modelling of the bonding orbital density will be helpful to predict the relative photolability of azido groups in aromatic polyazides. This work was supported by the U.S. National Science Foundation (grant nos.CHE-951595 and CHE-9740401). References 1 F. F. Crim, J. Phys. Chem., 1996, 100, 12725. 2 (a) S. V. Chapyshev, Mendeleev Commun., 1999, 164; (b) S. V. Chapyshev, Mendeleev Commun., 1999, 166; (c) S. V. Chapyshev, R. Walton and P. M. Lahti, Proceedings of International Workshop on Reactive Inter-mediates, IWRI’99, Szczyrk, Poland, 1999, p. 8. 3 E. 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Klopman, Wiley–Interscience, New York, 1974, ch. 3. 13 (a) S. V. Chapyshev and T. Ibata, Heterocycles, 1993, 36, 2185; (b) S. V. Chapyshev and N. V. Chapysheva, Khim. Geterotsikl. Soedin., 1994, 666 [Chem. Heterocycl. Compd. (Engl. Transl.), 1994, 30, 585]; (c) S. V. Chapyshev and V. M. Anisimov, Khim. Geterotsikl. Soedin., 1997, 1521 [Chem. Heterocycl. Compd. (Engl. Transl.), 1997, 33, 1315]. 14 (a) C. Ling, M. Minato, P. M. Lahti and H. van Willigen, J. Am. Chem. Soc., 1992, 114, 9959; (b) C. Ling and P. M. Lahti, J. Am. Chem. Soc., 1994, 116, 8784; (c) R. S. Kalgutkar and P. M. Lahti, J. Am. Chem. Soc., 1997, 119, 4771; (d) S. V. Chapyshev, R. Walton, J. A. Sanborn and P. M. Lahti, J. Am. Chem. Soc., in press. 15 (a) J. J. P. Stewart, J. Comput. Chem., 1989, 10, 221; (b) Spartan, version 4.0, Wavefunction, Inc., USA, 1995. Figure 3 The orbital density distribution in the HOMO of 5b and 8a. The higher bonding orbital density at the Na atom of the a-azido group in the HOMO of 8a indicates that a-excited states (T0, S1, T1, etc.) of 8a,b should lie more closely to the reaction coordinate. 5b 8a Received: 4th October 1999; Com. 99/1538