Transanulare Wechselwirkung des Azo‐Chromophors in isodrinanalogen Systemen¹⁾

Abstract

Die ungewohnliche transanulare Photoreaktivitat der Azoverbindung 5 spiegelt sich auch im Photoelektronenspektrum wider. Die Untersuchung der Verbindungen 5,13 und 14 laBt eine effektive Wechselwirkung (through-bond und through-space) von HQ = -0.62 eV zwischen und xcc in 5 erkennen. Dieser experimentelle Wert steht in ausgezeichneter Ubereinstimmung mit bekannten Parametern analoger isodrinartiger Systeme. An den Modellverbindungen 17 und 19 werden die angeregten Zustande mit Hilfe semiempirischer Rechnungen untersucht. Sowohl der n ~ n;als auch der kurnvellige n_ x', -&rgmg sind einer intramolekularen Photocycloaddition forderlich.Durch Rontgenbeugung wird der n~~,ncc-TransanuIarabstand zu 2.91 A ermittelt. Transanular Interactions of the Azo Chromophore in Isodrin-type Systems')The unusual transanular photoreactivity of the azo-compound 5 is also reflected in its photoelectron spectrum. The investigation of compounds 5, 13 and 14 reveals an effective interaction in 5 (through-bond and through-space) with HQ = -0.62 eV between XNN and RCC. This experimental value fits in well with known parameters of analogous isodrin-type systems. The excited states are looked into by means of semiempirical calculations with model compounds 17 and 19. Both the n n*+ and (at shorter wavelength) the n _ n: transition are conducive to an intramolecular photocycloaddition. X-ray diffraction has been used to determine the nNN,ncc-transanular distance (2.91 A).Spektroskopische Untersuchungen der Azo-Gruppe haben sich bislang hauptsachlich mit dem Studium von Wechselwirkungen im Chromophor selbst befafit, wie n,n-oder n,o-Inter-aktionen2). Prinzipiell sollten aber sowohl das n"-OrbitaI als auch die n + -bzw. n--Kombinationen zur Nachbargruppenbeteiligung und transanularen Wechselwirkung befahigt sein. Diese Erwartungen wurden bei der Untersuchung der Azoverbindungen 13), Z4), 3') und 45) bestatigt, in denen folgende homokonjugierte Wechselwirkungen zu erkennen sind: n -o f (1); n, -e, (2, 3, 4); ~N N -e, (2, 3); n--o (4).Eine nNN-ncc-Homokonjugation mit ungewohnlichen Konsequenzen liegt in der pentacyclischen Azoverbindung 5 vor, die lichtinduziert zu der Kafigverbindung 6 cyclisiert6). Da gewohnlich die Photoeliminierung von Nz dominiert, und die Einbeziehung des Azochromophors in das Photoprodukt eher die Ausnahme ist'), mun die hohe Effizienz der Photoreaktion 5 -+ 6 auf eine intensive Wechselwirkung zwischen Azogruppe und CC-Doppelbindung bereits im vertikalen angeregten Zustand (mit der Grundzustandsgeometrie) zuruckgefuhrt werden. Die in den Isodrinabkommlingen 7 und 8*v9) beobachteten, betrachtlichen, quantitativ erfanbaren transanularen Wechselwirkungen legten ahnliche Messungen fur 5 nahe. Wir berichten hier uber photoelektronenspektroskopische und strukturanalytische Untersuchungen.

Proximate,syn-Periplanar Bisdiazene Skeletons: Syntheses, Structures, Homoconjugate Reactivity and Photochemistry

“…From PE spectra of the corresponding NϭN/CϭC systems with their high propensity for [2ϩ2]photocycloaddition, [1] Martin and Hünig had determined significantly higher interaction parameters (-0.56 to -0.80 eV). [15] Clearly, doubts as to any correlation between these parameters and the efficiency of [2ϩ2]photocycloadditions are justified.…”

“…F 12 -Bisdiazenes (2) (Scheme 2): Ultimately, the successful route [4k] to 2 meant replacement in the Hünig procedure [15] of cyclopentadiene by 4H-pyrazoles 16 with the weakly reactive cis-1,4-dimethoxy-dihydrofuran 17 as dienophile. The simplest 4H-pyrazole not able to undergo tautomerization to the more stable 1H-pyrazole, the 4,4-dimethyl derivative 16a, trimerizes and hence has to be liberated by acids in order to undergo [π 4s ϩ π 2s ]cycloaddition.…”

“…[9] Insignificant as these differences within the F series might seem, such small geometrical discrepancies have, in our own experience, been found to totally divert the course of photochemical transformations. [10] The rather modest endothermicity of the steps F 11-22 Ǟ G [11][12][13][14][15][16][17][18][19][20][21][22] reflect the degree of strain inherent in the rigid skeletons F and, in principle, should all lie well within the energy range of π Ǟ π* and even n Ǟ π* excitations. [11] The exothermicity of the isomerizations G 11-22 Ǟ H 11-22 with 60-62 kcal mol -1 , is indicative of low thermal stability of the tetrazetidine photoproducts.…”

Correlation of Oxidation and Ionization Potentials for Azoalkanes

“…Azoalkanes constitute a typical class of substrates for which such a dilemma between the solution oxidation and gas phase ionization potentials obtains. Through intensive PES investigations in the 1970s, which persist to date, more than 100 ionization potentials for azoalkanes have been reported, ,− but since the one-electron oxidation of azoalkanes is mostly irreversible, , only a few peak oxidation potentials have been reported. − However, since in recent years mechanistic ,− and spectroscopic ,− studies on azoalkane radical cations have gained increasing attention, this unsatisfactory situation must be remedied. Hence, in the present work we introduce a convenient and useful correlation between the experimentally available oxidation and ionization potentials of azoalkanes, which allows us to estimate one if the other is known.…”

“…Azoalkanes constitute a typical class of substrates for which such a dilemma between the solution oxidation and gas phase ionization potentials obtains. Through intensive PES investigations in the 1970s, which persist to date, more than 100 ionization potentials for azoalkanes have been reported, 11,[20][21][22][23][24][25][26][27][28][29][30][31][32][33] but since the one-electron † Universita ¨t Basel. oxidation of azoalkanes is mostly irreversible, 34,35 only a few peak oxidation potentials have been reported.…”