ALBERT

Notes: Photochemical Generation and Reactions of Benzonitrile-benzylideThe low temperature irradiation of 2,3-diphenyl-2H-azirine (1) in DMBP-glass at -196° has been reinvestigated. It was possible to convert 1 nearly quantitatively into the dipolar species benzonitrile-benzylide (3, Φ3 = 0,78), which exhibits UV.-absorptions at 344 (∊ = 48000) and 244 nm (∊ = 28500) (Fig. 1, Tab. 1). Irradiation of 3 with 345 nm light at -196° resulted in almost complete reconversion to the azirine 1 (Φ = 0,15; Fig. 2). When the solution of 3 in the DMBP-glass was warmed up to about -160° a quantitative dimerization to 1,3,4, 6-tetraphenyl-2,5-diaza-1,3,5-hexatriene (8) occurred. This proves that 8 is not only formed by the indirect route 3 + 1 → 7 \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\longrightarrow }\limits^{hv} $\end{document} 11 → 8 known before (Scheme 1), but also by dimerization of 3 either by direct head to head coupling or via the intermediate e (p. 2675), followed by a fast thermal hydrogen transfer reaction.The occurrence of the dipolar intermediate 11 in the photochemical conversion of the bicyclic compound 7 to 8 could also be demonstrated by low temperature experiments: On irradiation at -196° 7 gave the cherry red dipolar intermediate 11 (λmax = 520 nm), which at -120° isomerizes to 8. It should be noted, that neither 7 nor 11 are formed by dimerization reactions of 3.Experiments carried out at room temperature demonstrate, that both processes for the formation of 8 may compete: Irradiation of a solution of 1 (DMBP, c = 8 × 10-4 to 5 × 10-3M) with 350 nm light of high intensity (which does not excite the bicyclic compound 7) leads to a relative high photostationary concentration of the dipolar species 3. Under these conditions the formation of 8 is due to dimerization of 3 (Φ8 = 0,19). With low light intensity only a very low stationary concentration of 3 can be obtained. Therefore the reaction of 3 with 1, leading to the bicyclic intermediate 7, becomes now predominant (Φ-1 = 1,55, which corresponds with the expected value of 2 × 0,8). Irradiation of 1 at -130° with 350 nm light of high intensity gives 8 with a quantum yield of 0,44. This is in agreement with the theoretical value Φ8 = 0,4 for an exclusive formation of 8 by dimerization of 3. The lower quantum yield for the formation of 8 at room temperature makes probable that under these conditions 3 not only dimerizes to 8, but also to another, so far unidentified dimer, e.g. 2,3,5,6-Tetraphenyl-2,5-dihydropyrazine.By flash photolysis of a solution of 1 (cyclohexane, c = 10-4M, 25°) the disappearance of 3 could directly be measured by UV.-spectroscopy: At relative high concentrations (c ≥ 10-7M) 3 disappeared according to a second order reaction with the rate constant k = 5 × 107M-1S-1. At lower concentrations (c ≤ 10-7M) the rate of disappearance of 3 follows first order kinetics. The rate constant of this pseudo first order reaction (3 + 1 → 7) has been determined to be 1 → 104M-1S-1.Using Padwa's table of relative rates for the cycloaddition of the dipolar species 3 to various dipolarophiles, including the azirine 1, an absolute rate constant of k ≈ 8 × 108M-1S-1 for the addition of 3 to the most active dipolarophile fumaronitrile could be estimated. In cyclohexane at room temperature, the diffusion controlled rate constant equals 6,6 × 109M-1S-1.In Table 1 the UV.-maxima of several nitrile-ylides, among them a purely aliphatic one, are given.

Notes: Mechanistic studies on the photoisomerization of 2-alkyl-indazoles into 1-alkyl-benzimidazoles. II. Primary photochemical processes and photophysical deactivation.In the previous paper [1] the structure of the intermediate in the photochemical indazole-benzimidazole-isomerization was discussed (3 in Scheme 1). In this communication experiments concerning the photochemical primary processes and photophysical deactivation of 2-alkyl-indazoles (1) are described.The quantum yield of the rearrangement 1 → 2 (ΦR) decreases with decreasing temperature while the fluorescence quantum yield (ΦF) increases and finally reaches a constant value ( ≠ 1) (Fig.10). This behaviour is inconsistent with the mechanism shown in Scheme 2. Photoreaction and fluorescence are both quenched, but not to the same extent, by freon 113 (Fig. 2). In addition the Stern-Volmer-plots are not linear. These observations are best explained by assuming the existence of two excited states in equilibrium (Scheme 3). The mechanism in Scheme 3 correctly explains the quenching experiments and the temperature dependence of ΦR and ΦF if the Arrhenius law holds for the two rate constants ksx and kR. However, for a quantitative calculation of ΦR, an additional branching of the reaction pathway must be postulated (Scheme 4). Two-dimensional drawings of hypothetical potential energy surfaces of the ground state and the first excited singlet state yielding a qualitative picture of the reaction and deactivation pathways of the discussed molecule are given in Fig. 15 a and b.

Notes: Mechanistic studies on the photochemistry of 2-alkylindazoles in aqueous solutions.The photochemistry of 2-alkylindazoles 1 in aqueous solutions is rather complex, the relative yields of different products being dependent on the pH-value of the irradiated solution: In neutral or basic solutions (pH 〉 7) as well as in most of the organic solvents isomerization to 1-alkyl-benzimidazoles 2 takes place. In dilute sulfuric acid (pH 2-4) this reaction is suppressed and the dihydro-azepinones 3 and 4 are formed. Irradiation in strongly acid solutions (pH 〈 1) yields the o-amino-acetophenones 5 (Scheme 1).The relative quantum yields of the photoproducts 2-5 have been measured as a function of the pH-value of the irradiated solution (Fig. 1). A comparison of these yields with the protonation equilibrium of the indazole in its first excited singlet state (pK = 2.8) suggests that 2 and 3 are both photoproducts of the neutral indazole molecule, whereas 4 as well as 5 are formed from the protonated indazole.The rearrangement of the indazole 1 to the benzimidazole 2 proceeds via an intermediate 6, which can be produced in high concentrations by monochromatic irradiation of 1 at low temperatures. The thermal reactivity of this intermediate in dilute sulfuric acid could be investigated: At pH 8 the only product is the benzimidazole 2. With decreasing pH-value increasing amounts of 3 are formed and at pH 〈 4 the formation of 2 is completely suppressed, the only product being the azepinone 3. Thus, 3 is a solvolysis product of the intermediate 6 (Scheme 2).The most probable primary product of singlet indazolium is the nitrenium ion 7. From this intermediate the formation of 5 can proceed in well-known thermal reactions. The formation of 4 is possibly due to a further protonation equilibrium nitrenium-nitrene. The nitrene 7 can be converted into the azepinone 4 via the azirine 8 (Scheme 3). The pK-values of different indazoles and intermediates are listed in the Table.

Notes: Mechanistic studies on the photoisomerization of 2-alkyl-indazoles into 1-alkyl-benzimidazoles. I. Structure and reactivity of an intermediate.2-Alkyl-indazoles (1) undergo photochemical isomerization to 1-alkyl-benzimidazole via previously unknown intermediates 3 (Scheme 1). In the present paper the structure and reactivity of these intermediates are discussed. Low-temperature irradiation (-60°) of 1 b with 300 nm light gives 3 b in quantitative yield. 3 b is transformed during warm-up to 1 b and 2 b (UV.-evidence). The formations of 1 and 2 show the same temperature dependence but their ratio is found to be temperature-independent. In contrast to the above behaviour, low-temperature irradiation with 250 nm light of 3 b yields 1 b only (no 2 b). These findings are consistent with the proposed reaction mechanism 2 c in Scheme 2.On the basis of spectroscopic properties and the described reaction pathways, it appears that the most suitable structure for intermediate 3 is a 7,8-diaza-tricyclo[4.3.0.07,9]nona-2,4,6(10)-trien (9). In Scheme 4 the reaction pathway for the iudazole-benzimidazole-rearrangement is summarized.

Notes: On the Mechanism of Radiationless Deactivation of the First Excited Singlet State of 2-Alkyl-indazolesThe temperature dependence of the quantum yields of fluorescence (ΦF), intersystem crossing (ΦT) and photoreactions (ΦR) have been measured for several neutral and protonated 2-alkyl-indazoles. The experimental data (Fig. 1 and 3) can be interpreted in terms of the reaction schemes 1 and 2 with temperature independent constants for fluorescence emission and intersystem crossing and temperature dependent photochemical primary processes. Whereas at low temperatures the sum of the quantum yields of fluorescence and intersystem crossing equals 1, at higher temperatures a very efficient radiationless deactivation process takes place (Table). Based on kinetic and photochemical data it is concluded that this deactivation proceeds via a hypersurface crossing (Fig. 2) or hypersurface touching (Fig. 4) in the course of the photochemical rearrangements 1 → 2 and 4 → 5 respectively.Similar mechanisms are expected to be responsible for the unusual internal conversion in other heterocyclic compounds.

Notes: Mechanistic studies on the photochemical reactions of benzfurazan.From other works it is known that irradiation of benzfurazan (1) in methanol gives the carbaminacid-ester 4, whereas in benzene the azepinederivative 3 is obtained (Scheme 1). The compounds 5-8 (Scheme 2) have been proposed as intermediates.In our investigations we detected and characterized by means of UV.- and IR.-spectroscopy the two species 5 and 8. Irradiation of 1 with 350 nm light at room temperature in a strongly polar solvent (e.g. H2O) yields exclusively 5 (Fig. 1) with a quantum yield of 0.48. In non polar solvents (e.g. hexane) 5 isomerizes in a second photochemical step to 8 (quantum yield 0.43) (Fig. 3). Thermally, 5 can be converted back to 1. The rate constant for this reaction at room temperature is 2 · 10-5s-1.The transformation 5 → 8 was also investigated at low temperature. There was no direct evidence for any intermediates of the type oxazirene (6) or nitrene (7). However, the formation of azepine 3 upon irradiation of 5 in benzene suggests as intermediate the nitrene 7 which could be converted into 8 in a fast thermal reaction (Scheme 3).

Notes: On the regioselectivity of cycloaddition reactions of photochemically generated benzonitrile-isopropylidesThis paper deals with the physical and chemical differences of zwitterionic benzo-nitrile-isopropylides, which differ by a p-substituent in the phenyl ring (H, F, OCH3; Scheme 2). These dipolar species (4-6) are produced by irradiation of the corresponding 2 H-azirines (1-3; Scheme 1) in a 2-methylpentane glass at -185°. Their UV. spectra are reproduced in the Figure. The spectra of 4 and 5 are characterized by an ‘aromatic band’ at short wavelength, and a longer wavelength band at approximately 275 nm, which is considered to be characteristic of the nitrile-ylide system. The UV. spectrum of the methoxy derivative 6, which shows a broad absorption at 260 nm, arises by an addition of the ‘nitrile-ylide band’ and the anisole band. The three dipolar species 4-6 do not show any significant differences in the regioselectivity of the cycloaddition with methyl α-methacrylate even though F and OCH3 have quite different σ-constants (Scheme 1). The addition according to modus A is very much preferred (B/A = 0,076). - It seems, that the substituents F and OCH3 do not affect the physical and chemical behaviour of the parent benzonitrile-isopropylide (4). All three dipolar species 4-6 react regiospecifically according to modus A with methyl trifluoroacetate (Scheme 1). The regioselectivity is reduced in the cycloaddition of 4 with methyl propiolate and ethyl phenylpropiolate (B/A = 0,04 and 0,28, respectively). The reduced regioselectivity in the latter case may be attributed to a reduced polarity of the triple bond in the dipolarophile.

Notes: Photochemistry of 2-Alkyl-indazoles at Low Temperatures2-Alkyl-indazoles 1 undergo a variety of photoreactions (Scheme 1) which have been shown to proceed via excited singlet states of the neutral and protonated indazole, respectively [6]. The quantum yields of these reactions decrease with decreasing temperature; around 150°K the 2-alkyl-indazoles are practically reactivity of both neutral and protonated indazoles is observed (Fig. 3), yielding 2-amino-acylbenzenes 7, obviously formed by hydrolysis of the corresponding imines 6 (Scheme 2). Yellow ‘intermediates’ (Figs. 1 and 2) which are observable only at low temperatures upon irradiation of neutral indazoles but are rather stable even at room temperature in acid solutions, could be identified as quinoid tautomers 8 of the imines (Scheme 4). It is assumed that the formation of the imines 6 proceeds via an arylnitrene or arylnitrenium-ion 13 (Scheme 5) and subsequent hydrogen abstraction, the nitrenes being formed directly in their triplet ground-state from the triplet state of the indazoles.