Pyridine photochemical isomerisation

The conversion of 33 oto to 33 is performed in two steps 1) substitution of py by DMSO ana isomerisation, 2) substitution of DMSO by py. Its overall yield is above 80%. In order to perform a complete cycle, the photochemical reaction leading back to 33p oto was carried out in the usual way (irradiation performed at room temperature with a 1000 W xenon arc-lamp filtered by a water filter) in pyridine. It turned out to be virtually quantitative. 

Fig. 16 Thermal isomerisation of the photoisomer 33pJoto and the photochemical back reaction. The pyridine ligand moves from an external position to an intra-ring location while the - (CH2)i8 - fragment undergoes a folding/unfolding process...

Ultraviolet irradiation of pyridines can prodnce highly strained species that can lead to isomerised pyridines or can be trapped. The three picolines and the three cyano-substituted pyridines constitute photochemical triads irradiation of any isomer, in the vaponr phase at 254 nm, results in the formation of all three isomers. From pyridines and from 2-pyridones 2-azabicyclo[2.2.0]-hexadienes and -hexenones can be obtained in the case of pyridines these are nsnally nnstable and revert thermally to the aromatic heterocycle. Pyridone-derived bicycles are relatively stable, 4-alkoxy- and -acyloxy-pyridones are converted in particnlarly good yields. Irradiation of iV-methyl-2-pyridone in aqueous solution prodnces a mixture of regio- and stereoisomeric 4n pins 4n photo-dimers. ... 

The seminal report in this area documented the thermally-induced isomerisation of Tp Ir(CO)(>/ -C2H4) (229) to Tp = TrH(CH=CH2)(CO) (289), which proceeds cleanly at 60 °C. This thermodynamic preference is the reverse of that observed for the Cp and Cp analogues, which favour the j/ -alkene complex, as does the photochemically generated Tp RhH(CH=CH2)(CO) (290), which rapidly reverts to Tp Rh( -C2H4)(CO) (148) under ambient conditions. This disparity between the rhodium and iridium 71-alkene complexes has been widely observed. However, while the rhodium hydridovinyls are rarely isolated, the kinetic accessibility of Tp RhH(CH = CH2)(f/ -C2H4) (294) has been established. It was found that on treatment with NCMe or pyridine, Tp Rh( -C2H4)2 (118) cleanly converts to Tp Rh(CH2CH3)(CH = CH2)(L) (L = NCMe (156), py (293)),... 

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