Comparative Study of the Photochemistry of the Azidopyridine 1-Oxides Kyle N. Crabtree,† Katherine J. Hostetler,† Tamara E. Munsch,‡ Patrik Neuhaus,‡ Paul M. Lahti,§ Wolfram Sander,‡ and James S. Poole*,† Department of Chemistry, Ball State UniVersity, Muncie, Indiana 47306, Lehrstuhl für Organische Chemie II, Ruhr-UniVersität Bochum, D-44780 Germany, and Department of Chemistry, UniVersity of Massachusetts, Amherst, Massacusetts 01003
[email protected] ReceiVed January 31, 2008
The photochemistry of azidopyridine 1-oxides was studied using an array of glass and matrix isolation techniques. As with room temperature, the photochemistry of 4-azidopyridine 1-oxide is dominated by triplet nitrene chemistry. However, in the case of the 3-azide, matrix photolysis indicates the formation of diazabicyclo[4.1.0]hepta-2,4,6-triene N-oxide and diazacycloheptatetraene N-oxide intermediates as well as triplet nitrene.
Introduction
SCHEME 1.
Photochemistry of Phenyl Azide
The chemistry of aryl azides has received a great deal of attention due to their ability to undergo 1,3-dipolar cycloaddition reactions and their ability to act as thermo- and photochemical precursors of aryl nitrene species. Much of aryl nitrene chemistry has been concerned with the utilization of such species as bioaffinity labels1 or as potential conductor materials,2 although these nitrenes may also be synthetically useful in the formation of azepine3 and carbazole4 systems. The photochemistry of simple aryl azides is now well-established and has been the subject of recent reviews by Platz et al.5,6 Using phenyl azide †
Ball State University. Ruhr Universität Bochum. University of Massachusetts. (1) (a) Meisenheimer, K. M.; Koch, T. H. Crit. ReV. Biochem. Mol. Biol. 1997, 32, 101. (b) Schnapp, K. A.; Platz, M. S. Bioconjugate Chem. 1993, 4, 178. (c) Schnapp, K. A.; Poe, R.; Leyva, E.; Soundararajan, N.; Platz, M. S. Bioconjugate Chem. 1993, 4, 172. (2) Meijer, E. W.; Nijhuis, S.; van Vroonhoven, F.C.B.M. J. Am. Chem. Soc. 1998, 110, 7209. (3) Doering, W.; Odum, R. A. Tetrahedron 1966, 22, 81. (4) (a) For example, see: Swenton, J.; Ikeler, T.; Williams, B. J. Am. Chem. Soc. 1970, 92, 3103. (b) Tsao, M-L.; Gritsan, N. P.; James, T. R.; Platz, M. S.; Hrovat, D. A.; Borden, W. T. J. Am. Chem. Soc. 2003, 125, 9343. ‡ §
10.1021/jo8001936 CCC: $40.75 2008 American Chemical Society Published on Web 04/10/2008
as an exemplar, the salient features of aryl nitrene photochemistry are shown in Scheme 1. Photodecomposition of the aryl azide yields a singlet nitrene species 2S, which at room temperature undergoes a ring-closure/ ring-expansion sequence to yield the didehydroazepine 4, for which a significant body of experimental evidence exists. The (5) PlatzM.J.InReactiVeIntermediateChemistry;Moss,R.A.,Platz,M.S.,Jones,M., Jr., Eds.; John Wiley & Sons:Hoboken, NJ,2004; pp 522–546.. (6) Gritsan, N. P.; Platz, M. S. Chem. ReV. 2006, 106, 3844.
J. Org. Chem. 2008, 73, 3441–3451 3441
Crabtree et al. TABLE 1.
Summary of Calculated Data for 4-Nitrenopyridine 1-Oxide and Photoproducts
method geometry
B3LYP/6-31G* B3LYP/6-31G* energy ZPEa Erel,0Kb (hartree) (hartree) (kcal/mol)
species 7T 7S TS (7S-8) 8 TS (8-9) 9
-377.5166 -377.5010f -377.4614 -377.4691 -377.4564 -377.4780
0.08422 0.08429f 0.08328 0.08419 0.08251 0.08432
-9.8 0.0e,f 24.2 19.9 26.9 14.4
CASSCF(8,8)/6-31G* CASSCF(8,8)/6-31G* energy (hartree)
ZPEc(hartree)
Erel,0Kd (kcal/mol)
-375.3795 -375.3706 -375.3314 -375.3344 -375.2945 -375.3338
0.08874 0.08852 0.08751 0.08870 0.08696 0.088587
-5.4 0.0e 24.0 22.9 46.8 23.2
CASSCF(10,9)/6-31G* CASSCF(8,8)/6-31G* energy Erel,0Kd (hartree) (kcal/mol) -375.3849 -375.3744 -375.3371 -375.3393 -375.3015g -375.3388
-6.5 0.0e 22.7 22.1 44.7g 22.4
CASSCF(8,8) MP2/6-31G* CASSCF(8,8)/6-31G* energy Erel,0Kd (hartree) (kcal/mol) -376.3426 -376.3294 -376.3031 -376.3065 -376.2689 -376.3096
-8.2 0.0e 15.9 14.5 37.0 12.5
a Unscaled ZPE energies, b Determined using ZPE scaled by the factor recommended by Scott and Radom, ref 23. c Unscaled ZPE energies calculated at CAS(8,8)/6-31G*level of theory. d Determined using unscaled ZPE corrections calculated at CASSCF(8,8)/6-31G*level of theory. e Energies for each species are determined relative to the energy of the singlet nitrene 7S (relative energy defined as 0). f Energies of singlet nitrenes are estimated from DFT calculations using the sum method described by Johnson et al., ref 16. g See the Supporting Information.
TABLE 2.
Summary of Calculated Data for 3-Nitrenopyridine-1-Oxide and Photoproducts
method geometry species 12T 12S TS (12S-13a) TS (12S-13b) 13a 13b TS (13a-14a) TS(13b-14b) 14a 14b
B3LYP/6-31G* B3LYP/6-31G* energy ZPEa Erel,0Kb (hartree) (hartree) (kcal/mol) -377.5004 -377.4741e,f -377.4483g
0.08359 0.08330e,f 0.08325g
-377.4713 -377.4840 -377.4627 -377.4638 -377.4853 -377.4749
0.08441 0.08491 0.08283 0.08277 0.08460 0.08403
-16.4 0.0e,f 16.1g 2.4 -5.3 6.8 6.1 -6.3 -0.1
CASSCF(8,8)/6-31G* CASSCF(8,8)/6-31G* energy ZPEc Erel,0Kd (hartree) (hartree) (kcal/mol) -375.3659 -375.3375 -375.3257 -375.3266 -375.3339 -375.3426 -375.2965 -375.2994 -375.3368 -375.3316
0.08754 0.08660 0.08683 0.08672 0.08898 0.08976 0.08708 0.08723 0.08897 0.08824
-17.2 0.0e 7.5 7.0 3.7 -1.2 26.0 24.3 1.9 4.7
CASSCF(10,9)/6-31G* CASSCF(8,8)/6-31G* energy Erel,0Kd (hartree) (kcal/mol)
CASSCF(8,8) MP2/6-31G* CASSCF(8,8)/6-31G* energy Erel,0Kd (hartree) (kcal/mol)
-375.3706 -375.3429 -375.3314 -375.3320 -375.3394 -375.3484 -375.3019 -375.3046 -375.3430 -375.3354
-376.3274 -376.3019 -376.2949 -376.2932 -376.3077 -376.3146 -376.2759 -376.2825 -376.3117 -376.3033
-16.8 0.0e 7.4 6.9 3.7 -1.5 26.1 24.4 1.4 5.7
-15.4 0.0e 4.6 5.5 -2.1 -6.0 16.6 12.6 -4.7 0.1
a Unscaled ZPE energies, b Determined using ZPE scaled by the factor recommended by Scott and Radom, ref 23. c Unscaled ZPE energies calculated at CAS(8,8)/6–31G* level of theory, d Determined using unscaled ZPE corrections calculated at CASSCF(8,8)/6–31G* level of theory. e Energies for each species are determined relative to the energy of the singlet nitrene 12S (Rel. energy defined as 0). f Energies of singlet nitrenes are estimated from DFT calculations using the sum method described by Johnson et al., ref 19. g The transition state found can serve both reaction paths as the imaginary frequency corresponds to an out-of-plane N vibration. However, given the nature of the computational method it cannot necessarily be described as a true bifurcation point, and the CASSCF calculations bear this out.
SCHEME 2. 1-Oxide (6)
Potential Photoproducts of 4-Azidopyridine
benzazirine 3 has not been observed for phenylnitrene6 but has been observed in polycyclic arylnitrenes.7 The ketenimine 4 is susceptible to nucleophilic attack, and this reaction may be used synthetically to yield amino- and alkylthioazepines.3,8 At low temperatures (
