Synthesis of 2-Pyridinylbenzoxazole:  Mechanism for the Intramolecular Photosubstitution of the Haloarene with the Carbonyl Oxygen of the Amide Bond in Basic Medium

Abstract

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 σ 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 SN(ET)Ar* mechanism to afford 2-pyridinylbenzoxazole derivative, and the photoreduction proceeds via a free radical mechanism to give N-phenylpyridinecarboxamide.

Direct photolysis mechanism of pesticides in water

“…25) The photoinduced cyclization of N-(2-halophenyl) pyridinecarboxamide to benzoxazole was examined by LFP at 266 nm, and the S N (ET) Ar* mechanism to form the anion radical was strongly supported. 26) LFP studies of pesticides using a Nd:YAG laser at 266 nm are summarized in Table 1. Sulfacetamide with a structure analogous to that of asulam showed three transient peaks at 330, 450, and 720 nm at pH 7 under N 2 .…”

“…A benzoxazole derivative was one of the main photoproducts of fenhexamid, with more rapid formation under an alkaline pH. 4,132) As compared to the photochemistry of N-(2-halophenyl)-pyridinecarboxamide, 26) cyclization is likely to start from an imidolate anion in S 1 , and the oxygen radical formed via intramolecular one-electron transfer attacks the anionic 2-chlorophenyl moiety, followed by the release of Cl − (S N (ET) Ar* mechanism), as shown in Fig. 2c.…”

Photoinduced Electron Transfer-Mediated Substitutions in Aqueous Media

“…In neutral medium both intramolecular photosubstitution and photoreduction reactions takes place [10]. In the photosubstitution reaction a singlet state of the o-haloarene is involved, whereas in the photoreduction a triplet manifold of the o-haloarene intervenes; oxygen inhibits the photoreduction but not the photosubstitution.…”

“…In the photosubstitution reaction a singlet state of the o-haloarene is involved, whereas in the photoreduction a triplet manifold of the o-haloarene intervenes; oxygen inhibits the photoreduction but not the photosubstitution. On the basis of the photokinetic and laser flash photolysis studies, an intramolecular photosubstitution of N-(ohalophenyl)-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 afford N-phenylpyridinecarboxamide [10].…”

Photodesulfurization of 2,4‐diaryl‐1,2, 4‐triazole‐3‐thiones

“…There are a few reports [5][6][7] on the intramolecular photocyclization of amides and thioamides. Synthesis of heterocyclic systems via photodesulfurization has also been reported [8][9][10].…”