Benzo cinnoline, protonated

In strongly acidic media azobenzene is protonated and irradiation leads to a cis trans photostationary state of the cations. Prolonged irradiation induces a photochemical cyclodehydrogenation with the formation of benzo[c]cinnoline. Protonated azobenzene also shows intense fluorescence . 

Oxidative photocyclization of benzylideneaniline appears to proceed efficiently only in the presence of strong acid. The phenanthridine 31, however, has been prepared by irradiation of the imine 3228 few other examples of the photocyclization of arylimines have been reported.29 Strong acid is also required for successful photocyclization of azobenzenes to benzo-[c]cinnolines. Here, protonation is claimed to lower the reactive n, n excited state below the level of the unreactive n, it state. 2-Phenylazopyridine,... 

The ultraviolet spectrum of benzo[c]cinnoline in -hexane shows four band systems centred at approximately 250, 300, 350, and 400 nm. The first three of these are n-n bands, corresponding to those in the spectrum of phenanthrene, while the long-wavelength band, of low intensity, is due to an n-7T transition. In hydroxylic solvents this last band moves to shorter wavelengths and becomes obscured. Substituent effects, and those due to protonation and to A-oxidation, have been discussed in a series of papers by Holt and co-workers. ... 

The proton magnetic resonance spectrum of benzo[c]cinnoline in deu-teriochloroform shows three multiplets at t1.36, t1.57, and t2.18 due to protons H4,7, Hj iq, and H2,3,8,g, respectively. In hexadeuteriobenzene the last two multiplets show a marked shift to higher field whereas the first is unaffected. When Martin et al. first recorded the spectrum they made the... 

In ether solutions in the absence of air, benzo[c]cinnoline forms adducts with alkali metals containing two atoms of the metal. These give solutions of 6 on treatment with benzoic acid or other source of protons. When one equivalent of benzo[c]cinnoline is added to the violet solution of the disodium derivative (50), an equilibrium is set up and the dark green sodium derivative of the radical anion (52) precipitates [Eq. (13)]. Reaction of the dilithium adduct (51) with one equivalent of benzoyl chloride results in disproportionation into benzo[c]cinnoline and, after hydrolysis, 2,2 -dibenzamidobiphenyl [Eq. (14)]. Methylation of the dilithium derivative... 

AMI calculations have been performed on pyrazino[2.3-c]pyridazine and its N-protonated species.2 Energy levels and charge densities were calculated by Hiickel MO calculations.3 MNDO calculations have also been performed on a benzo-annulated pyrazino[2,3-c]pyridazine (quinoxalino[2,3-c]cinnoline).69... 

Despite some early claims to the contrary, it was concluded in CHEC-I <84CHEC-l(3B)l> that most aminopyridazines and cinnolines exist in the amino form. This generalization also extends to aminophthalazines, but 1,4-di(pyridylamino)phthalazines (29) are an exception and the presence of the imino tautomers is clearly shown by the unsymmetrical proton NMR spectrum with two distinct NH resonances, and a marked difference in the shifts of the peri benzo protons <83JHC345>. 

The quaternization of (benzo)pyridazines by alkyl halides (these systems are not readily susceptible to arylation) was reviewed in CHEC-I <84CHEC-l(3B)l>. Monoquaternization of pyridazines occurs more readily than other diazines but less readily than pyridine, reflecting the intermediate basicity/nucleophilicity of pyridazine. Diquaternization of pyridazine can only be achieved with oxonium salts, particularly Me30 BF4 . As with protonation and A-oxidation, mixtures of products are often obtained on quaternization of unsymmetrical pyridazines and have been the subject of theoretical studies. A number of 2-(ribofuranosyl)-3(2//)-pyridazinones have been prepared by stannic chloride catalyzed alkylation of 3-(trimethylsilyloxy)pyridazines with protected 1-0-ace-tylribofuranose <83JHC369>. The quaternization behavior of phthalazines is similar to that of pyridazines, but with cinnolines alkylation usually occurs at N-2, unless there is a particularly bulky substituent at C-3. 

In a reaction analogous to that of stilbene, protonated azobenzene iso-merizes at room temperature under irradiation with ( )e, z = 0.27 and = 0.25. Subsequently, the dehydro-photocyclization starts from the photo-stationary E/Z mixture to give protonated benzo[c]cinnoline with a yield of (j) = 0.02. This is parallel to the stilbene-to-phenanthrene reaction, and it proceeds not in concentrated sulfuric acid where the thermal Z -> E reisomerization is fast, but best in ca. 66% acid. Protonated benzo[c]cinnoline and hydrazobenzene are formed, and the latter undergoes the benzidine rearrangement. The complexation with metal ions in azobenzene crown ethers under participation of the n-electrons of the azo group leads to an increase of the E Z quantum yield from 0.25 of the metal free compound to 0.4 to 0.6 in the Ba and Ca "" complexes. The Z E yield decreases from 0.18 to 0.13 and 0.05. If the crown is larger, the values increase to 0.35 and (pz-iE = 0.45. " ... 

The photochemical and photophysical behavior of azobenzene adsorbed on polymer or silica films has been reported recently " azobenzene adsorbed in water-swollen Nafion-H+ (Nafion is a polymer consisting of a perfluorinated backbone and short pendant chains terminated by sulfonic acid groups) exhibited strong fluorescence. The irradiation resulted in the formation of benzo[c]cinnoline and benzidine in quantitative yield, while azobenzene incorporated into a methanol-swollen Nafion-H+ membrane did not emit fluorescence but merely underwent E,Z-isomerization. Thus, solvent-swollen Nafion is useful in the selective phototransformation of azobenzene. According to the authors, protonated azobenzene molecules are located at the fluorocarbon/water interface in water-swollen Nafion-H+ Z-Azobenzene formed on these membranes resulted in cychzation to give the observed products (Scheme 52). 

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