J . Org. Chem., Vol. 42, No. 4, 1977
Photoelectron Spectra of Cyclic Azo N-Oxides and N,N'-Dioxides
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Photoelectron Spectra of Cyclic Azo N-Oxides and Azo N,N'-Dioxides Kevin E. Gilbert Department of Chemistry, University of Oregon, Eugene, Oregon 97403 Received February 11, 1976 The photoelectron spectra of 3,3,4,4-tetramethyldiazetine (11, 3,3,4,4-tetramethyldiazetine N-oxide (2), 3,3,4,4tetramethyldiazetine N,N'-dioxide (3), 3,3,6,6-tetramethyl-1,2-diazacyclohexene (4), 3,3,6,6-tetramethyl-l,Z-diazacyclohexene N-oxide (5), and 3,3,6,6-tetramethyl-1,2-diazacyclohexene N,N'-dioxide (6) have been measured and the ionization potentials have been interpreted in terms of molecular orbitals with the assistance of INDO calculations. The energy of the highest occupied P orbital increases along the series azo > azo oxide > azo dioxide, a result consistent with expectations based on simple perturbation theory. The relationships between the molecular orbital energies and the chemistry of azo N-oxides and azo N,N'-dioxides are discussed.
Azo N-oxides and azo N,N'- dioxides, formally obtained by stepwise oxidation of azo compounds, have many interesting physical and chemical properties such as the valence isomerization of azo N-oxides to oxadiaziridinesl and the dissociationldimerization of azo dioxidehitroso alkanese2 While studing these two reactions we prepared a number of cyclic azo, azo N-oxide, and azo N,N'-dioxide compounds. Herein we report the photoelectron spectra of these compounds and calculations of the electronic structures of azo N-oxides and azo N,N'- dioxides by the INDO-SCFMO method. Azo N-oxides, like azomethine ylides, azomethine imines, nitrones, azimines, and nitroalkanes, are 1,3 dipoles having nitrogen as the central atom3. In this series of 1,3 dipoles, reactivity in cycloaddition reactions varies from very high (azomethine ylides) to undetectable (nitroalkanes),with azo N-oxides being among the less reactive compounds. Rees and co-workers have reported that the reaction of benzo[c] cinnoline N-oxide and dimethyl acetylenedicarboxylate at 190 OC gives an ylide, presumably through rearrangement of an initially formed cycloadduct (eq l).4The reactivity of 1,3 di-
of the dimerizationldissociation process has been studied by Hoffmann, Mallory, and Gleiter; they suggested a non-leastmotion path for the dimerization reaction, the least-motion path being symmetry forbidderg Their ideas have received experimental support in the work of Greene and Gilbert on cyclic azo dioxides.* In those azo dioxides which do not dissociate, the chemistry of the azo dioxide functional group can be observed. Azo dioxides can be reduced to azo N-oxides and azo compounds, but they are resistant to further oxidation.BAzo dioxides react photochemically to give nitroxyls (loss of NO).8Jo In a thermal reaction, 3,3,4,4-tetramethyldiazetineN,N'-dioxide (3) gives 2,3-dimethyl-Z-buteneand Overall, azo dioxides are a relatively inert functional group. Herein we report the photoelectron spectra of a series of cyclic azo, azo N-oxide and azo N,N'-dioxide compounds and molecular orbital calculations on the electronic structures of these molecules. The reactivity of azo N-oxides in 1,3-dipolar cycloaddition reactions and the oxidation-reduction chemistry of azo N,N'-dioxides have received particular attention as we are interested in relationships between molecular orbital theory, photoelectron spectroscopy, and chemical reactivity.
2
1
poles in cycloaddition reactions has been correlated with the electronic structure of the 1,3 dipole,j as will be elaborated upon later. Azo N-oxides are thermally more stable than the corresponding azo compounds. In cases where the rates of extrusion of NzO and N2 from azoxy alkanes and azo alkanes have been measured the ratio of the rate constants is on the order of k,,,,,lk,,, = 10-1fi.6The difference in the rates of these reactions has been attributed to a symmetry-induced barrier to reaction in the azo N-oxide due to the perturbing effect of the oxygen on the MO's of the reactant.fi The chemistry of azo N,N'-dioxides has many aspects of interest, including the thermal dissociation of the azo dioxide to the corresponding nitroso alkane, in the formal sense cleavage of a double bond under very mild conditions (eq 2). Rates and equilibria of the reaction have been measured for a number of cyclic and acyclic system^.^^^^^ The mechanism
4
5
3
6
Experimental Section The spectra were run on a PS-18spectrometer (Perkin-Elmer) with a standard inlet for liquid and solid samples. The spectra were calibrated with argon as an internal standard. The ionization potentials reported in Table I are the average values from three or more spectra; single values were reproducible to f0.05 eV. All compounds were prepared as previously reported.8 For the solid samples the probe was heated to increase the count rate: 36 "C for 2 , 6 5 "C for 3,50 O C for 5 , and 80 O C for 6, all f 5 "C. PE Spectra. The PE spectra of the four-membered ring series 3,3,4,4-tetramethyldiazetine (1),3,3,4,4-tetramethyldiazetine N-oxide (21, and 3,3,4,4-tetramethyldiazetine N,N'-dioxide (3) are presented in Figure 1.The PE spectra of the six-membered ring series 3,3,6,6tetramethyl-1,2-diazacyclohexene (41, 3,3,6,6-tetramethyl-l,Z-di-
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J. Org. Chern., Vol. 42, No. 4,1977
Gilbert
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8
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10
11
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12
15
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14
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15
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I P (ev)
Figure 1. Photoelectron spectra of four-memberedring series (I, 2, and 3).
Compd
1st
2nd
3rd
1 2
8.77
10.24
3
8.52 7.85
11.5 10.8 10.6
4 5 6
9.73 10.7' 9.20b
8.19
9.58
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The spectra were calibrated with argon as a single internal standard (see Experimental Section).As a referee has pointed out, care must be taken when using this technique that the linearity of the spectral range not change with time; otherwise ionization potentials in the 8-eV region will be inaccurate. The reproducibility of our results suggests that we have not encountered this problem. (For a comparison, see ref 29, Table I). The first two bands overlap and a precise assignment is not possible; see text for discussion. Broad, diffuse band, assignment accurate to only a
f O . l eV.
8
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11
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Table 11. Geometries of 1-3 and the Models for 1-6 Calculated by the INDO Method
Table I. Ionization Potentials (eV) of Compounds 1-6"
9.82
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12 io 14 is is 11 I P (OV) Figure 2. Photoelectron spectra of six-membered ring series (4,5, and
I
Compd 1 7 7 2 8 8 36 9 9
