Synthesis of 2-pyridinylbenzoxazole: Mechanism for the intramolecular photosubstitution of the haloarene with the carbonyl oxygen of the amide bond in basic medium

2-Pyridinylbenzoxazole derivatives have been synthesized by the intramolecular photosubstitution reaction of N-(2-halophenyl)pyridinecarboxamide (1 and 2) with its amide bond in basic medium. In neutral medium both intramolecular photosubstitution and photoreduction reactions occurred. In the photosubstitution reaction a singlet state of the o-haloarene is involved, whereas in the photoreduction a triplet state of the o-haloarene is involved; oxygen inhibited the photoreduction but not the photosubstitution. The relative rate studies showed that a base accelerates the photosubstitution reaction but decelerates the photoreduction. o-Iodoarenecarboxamide is more reactive than o-bromoarenecarboxamide, which in turn is more reacitive than o-chloroarenecarboxamide. UV-vis absorption change in the presence of a base showed that an imidol and/or imidolate anion is involved in the reaction. Several transient species, such as charge-transfer excited states and a cyclohexadienyl anion radical, have been identified from the photolysis of 1 and 2 in basic medium by laser flash photolysis. In neutral medium dibromide anion radical and a phenyl a radical were identified in addition to the above intermediates. On the basis of the photokinetic and laser flash photolysis studies, an intramolecular photosubstitution of N-(o-halophenyl)-pyridinecarboxamide with its amide bond occurs via an intramolecular S-N(ET)Ar* mechanism to afford 2-pyridinylbenzoxazole derivative, and the photoreduction proceeds via a free radical mechanism to give N-phenylpyridinecarboxamide.