Photochemistry and thermochemistry of picolyl chlorides

Abstract

Thermochemistry and photochemistry of picolyl chlorides were studied. The thermal reaction in benzene solution of 2-picolyl chloride afforded intermolecular condensation product, i.e., a quaternary salts. 2-Picolyl halide is known to give a self-condensation product. In the case of 3-picolyl chloride, polymers were obtained. The highly molecular weight polymer was not soluble in almost all the solvents. A cyclic hexamer was not formed because of the steric strain and the low reactivity. Thermolysis of 4-picolyl chloride gave not only a polymer but also a cyclic hexamer. The properties of the polymer was similar to that of the poly-3-chloromethyl pyridine, i.e., a highly molecular weight polymer. Absorption spectrum of a cyclic hexamer which was identified by NMR spectrum showed $\lambda_{\max}$ at 460 nm. This absorption data was similar to Berlin``s experiment(480 nm) in which the cyclic structure had not been proposed. The cyclic hexamer was cloven to the linear structure. It was soluble in water and methanol. Photolysis of 2-picolyl chloride at 253.7 nm gave a para-isomer followed by polymerization. Because chloromethyl group of 2-picolyl chloride has not a double bond character, it can not play a part in activating the hydrogens of the chloromethyl moiety. So aniline can not be produced via a Dewar pyridine. It is reasonable that the photoisomerization product is obtained by way of a prismane intermediate. The product favored the polymerization. 2-Methyl pyridines substituted $CH_3O$, iso-PrO, or EtO for H of the $CH_3$ group did not photoisomerized to the corresponding anilines or para-substituted pyridines within the range of the time used for 2-picolyl chloride. Thermolysis of picolyl chlorides in an acidic methanol solution did not afford any product.