Pyridine photoisomerization

Arene oxides of heterocycles such as pyridine have often been postulated as photoisomerization intermediates (81H(15)1569,72JOC3592) but to date they have not been isolated or extensively studied. 

Although these enantioselective photoreactions are limited to amide or salt derived from achiral acid and chiral amine, one enantioselective photoisomerization reaction of cobaloxime coordinated with chiral axial ligands such as 1-methylpropylamine, l-(l-naphthyl)ethylamine, and 2-phenylglycinol has been reported. For example, finely powdered (2-cyanoethyl)cobaloxime (60), suspended in liquid paraffin and spread onto a Petri dish, was irradiated to give (S)-(-)-61 of about 80% ee after displacement of the chiral auxiliary of the complex with pyridine [32],... 

Styrylpyridine photochemistry has been important in one other system. The photoprocesses in the complexes W(CO)sX (X = pyridine, 2-styrylpyridine, and 4-styrylpyridine) have been investigated.129 Unlike the XRe(CO)3(taz s-styryl-pyridine)2 and Ru(II)-styrylpyridine complexes having lowest IL and CT states, respectively, the W(CO)5X complexes have lowest LF excited states with only a small contribution from W - pyridyl CT. The one-electron diagram for low-spin d6, Cnv complexes shown in Scheme 7 is appropriate here. Both photosubstitution and photoisomerization reactions are found for the W(CO)5X complexes and some quantum yield data are found in Table 23.129 The data show that substitution efficiencies for... 

The photoisomerization of certain pyridines to Dewar pyridines is described in Section 3.2.1.2.2 and the formation of bridged ring 6//-l,2-dihydro-3-pyridones in Section 3.2.1.10.4. 

Photoaquation of a number of iridium complexes has been observed, including transformations of cis-[IrCU(py)2] to give both fac-cis and mer-cis isomers, and also of [IrCl2(H20)(py)3] to give the cis, cis, traits isomer. Photoisomerization of cis- to tratts-[IrCl2(py)4] has been reported together with photo-pyridination of the three complexes IrCl3(H20)(py)2. ... 

Durr et al. have described some novel complex cinnoline derivatives which show photochromic properties. Albini et al. have provided some new evidence on the mechanism for the photoisomerization of heterocyclic N-oxides. Simple pyridine N-oxides are exceptional. Thus irradiation of pyridine N -oxide in aqueous base affords the ring-opened product (23). Aoyama et al. have described the unprecedented photocyclization of the amide (24) to the lactam (25). The first examples of C2a+2Tr] photoreactions of a three-membered ring and an azo-compound have been described by Hunig and Schmitt. Nicolaou et al. have prepared the first stable example of a 1,2-dithiethane the procedure involves (2i +2Tr) photodimerization of C=S groups. 

A Dewar isomer of unsubstituted pyridine was identified by Wilzbach, but it was very unstable.63 Other Dewar isomers of six-membered heterocycles, isolated and fairly stable, are substituted with perfluoroalkyl groups. However, 1,2-dihydroheterocyclic compounds such as a-pyrone or a-pyridone derivatives are easily photoisomerized to bicyclic Dewar isomers, which are useful for the synthesis of cyclobutadienes. The photochemistry of 1,2-dihydroheterocyclic compounds will be discussed, followed by those of fully aromatic compounds. 

Pyridine is photoisomerized to an unstable 1,4-bonded isomer which decays back to pyridine with a half-life time of 2.5 minutes at 25 On the other hand, pentakis(pentafluoro-ethyl)pyridine, a photoadduct of pentafluoropyridine with tetrafluoroethene, underwent photoisomerization under UV irradiation (> 220 nm) in perfluoropentane to give pen-takis(pentafluoroethyl)-l-azabicyclo[2.2.0]hexa-2,5-diene (19, 95%), which on further irradiation (> 220 nm) provided the corresponding 1 -azatetracyclo[2.2.0.0 .0 ]hexane (20) in 91 % yield. ... 

Methylsulfanyl-4-azatricyclo[3.3.2.0 ]deca-3,6,9-triene (17) was prepared from lactam 11 and phosphorus pentasulfide in pyridine which gave the thioamide 15 (68%), which on reaction with trimethyloxonium tetrafluoroborate gave the methylsulfanyl iminoether 16 (71%). Acetone sensitized photoisomerization of 16 produced 3-methylsulfanyl-4-azatricyclo-[3.3.2.0 ]deca-3,6,9-triene (17) in 93% yield when the photolysis of 16 was not carried out under conditions of triplet sensitization, only polymeric dark material was produced. ... 

Quantitative photoisomerizations of cis-dichlorobis(pyridine)platinum(II) in CHClj provide evidence for the simultaneous operations of several isomerization mechanisms ... 

The blue isomer obtained by irradiation of the well-known photochromic compound 2-(2,4-dinitrobenzyl)pyridine has been identified as the enamine (36) (More O Ferrall and Quirke). There has been a resurgence of interest in photochromic fulgides see e.g. Suzuki et al. Wilcox et al. have de bribed a new photolabile protecting group for amino-acids. An interesting example of intramolecular oxetane formation has been provided by the photoisomerization of indole (37) to (38) (Takechi et al.). 

The light-induced valence bond isomerization of six-membered heteroaromatic compounds has been observed with a number of systems.10 In 1970 the formation of Dewar pyridine from pyridine was reported,19 and this type of intermediate is tentatively suggested to be formed in the intriguing photoisomerization of 2-pyridylacetonitrile to anthranilonitrile (see Scheme 2).20 The quantum yield... 

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