Benzoxazole dimerization

The photolysis of 1,2-benzisoxazole in the absence of air in acetonitrile gave salicylonitrile and benzoxazole (67AHC(8)277). When air-saturated acetonitrile was employed, 2,2 -dimeriz-ation to (38) occurred, accompanied by benzoxazole. Photolysis of the 2,2 -dimer (38) and benzoxazole did not alter the ratio, thus indicating that neither one arose from the other. Selective excitation also ruled out dimer formation from benzoxazole under the reaction conditions (Scheme 9). This dimerization is similar to that observed for benzimidazole, except that in that series no 2,2 -dimerization was observed (74TL375). 

Heterocyclic compounds Heterocyclic C=N systems, such as benzoxazole and related thia-zoles, react with complex 2b to yield the ring-expanded adducts, e. g. complexes 90 and 91, by formal C-X (X = O, S) bond cleavage and coupling with the alkyne. In the case of benzisoxazole, the alkyne is not coupled but eliminated, and ring-enlargement of the ben-zisoxazole leads to the N-bridged dimer 92 [48]. 

The photochemical reaction of 2-phenylbenzoxazole (84CHEC(6)177) forms a head-to-tail dimer, 1,3-diazetidine, (393) (87CC578). Benzoxazole itself, in the presence of oxygen, gives a coupled product, (394) (74TL375). 

The majority of reversible photoinduced PT reactions takes place between a donor and an acceptor that are chemically different, being either different atoms or identical atoms in inequivalent positions of a molecule or a molecular assembly [12,13], Examples are systems in which the phenomenon was first discovered and which (sometimes in the form of related compounds and derivatives) continue to be the object of present work methyl-salicylate [6,7], n-salicylidene aniline [8], 3-hydroxyflavone [10], and dimers of 7-aza-indole [9], Other molecules include 2-(2 -hydro-xyphenyl)benzothiazole [163,164] and 2-(2 hydroxyphenyl)benzoxazole, which has been studied in detail by Grellmann and co-workers [165,166], and where in addition to the fast transfer in the excited singlet state, the slower transfer in the triplet manifold could be characterized by transient absorption measurements in solution at temperatures down to 55 K. 

N-alkylidene-2-hydroxyanilines (65) can be oxidized by dioxygen in the presence of CuCl in pyridine to 2 substituted benzoxazoles (66) in good yield [80]. The phenoxy radical formed via hydrogen atom abstraction attacks the adjacent double bond to bring about ring closure rather than dimerization ... 

As expected, benzoxazoles do not react as 1,3-dienes. However, (2 + 2)-cycloadditions have been observed, as shown by the photodimerization of 2-phenylbenzoxazole (11) giving rise to the dimer 12 [283] ... 

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