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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 15, Number 10
BKCSDE 15(10)
October 20, 1994 

A Simple Huckel Approach to Intramolecular Photocyclization Reaction of N-(2-Chlorobenzyl)-Pyridinium, N-(Benzyl)-2-Chloropyridinium, and N-(2-Chlorobenzyl)-2-Chloropyridinium Salts
Gang Ho Lee*, Yong-Tae Park
We have calculated the π-electron density, atom self-polarizability, and free valence on each atom of N-(2-chlorobenzyl)-pyridinium, N-(benzyl)-2-chloropyridinium, and N-(2-chlorobenzyl)-2-chloropyridinium salts using a simple Huckel method in order to discuss their intramolecular photocyclization reaction in a qualitative method. Our calculation qualitatively predicts that photocyclization occurs through forming radicals as a reaction intermediate by breaking a C-Cl bond after photoexcitation into a triplet state via intersystem crossing from an initially excited singlet state. We noticed that this C-Cl bond breaking is aided by π-complex formation between a chlorine atom and the π -electrons of the neighboring ring in the triplet state and a stronger π-complex bond makes C-Cl bond breaking, i.e., radical formation, much easier. A chlorine atom will form a stronger π -complex bond to a benzyl ring of N-(benzyl)-2-chloropyridinium than a pyridinium ring of N-(2-chlorobenzyl)-pyridinium because the former can donate its π-electron more easily than the latter. The chlorine at position 15 of N-(2-chlorobenzyl)-2-chloropyridinium salt in the excited state also provides its π-electron to the benzyl ring. So this π-electron can increase the bond strength of the π-complex. Therefore, the strength of π-complex follows the order of N-(2-chlorobenzyl)-2-chloropyridinium, N-(benzyl)-2-chloropyridinium, and N-(2-chlorobenzyl)-pyridinium salts and thus the radical formation rate. This provides us with an intramolecular photocyclization reaction rate of the same order as given above.
857 - 860
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