ИСТИНА

Recently, reaction of photocycloaddition of acetylenes and nitriles to mixed phosphonium–iodonium ylides (1) was opened up.[1,2] The photolysis of 1 gives the opportunity to synthesize novel oxazole (for nitriles) or furan (4) derivatives (for acetylenes) as well as exotic heterocycle λ5-phosphinoline (3) with relatively high yields (30–80% depending on 1 and 2) in a simple, one-pot and metal-free reaction under irradiation of the reagent mixture. (Scheme, the counterion BF4– is omitted for simplicity). Phosphonium salt (5) and PhI are formed also. The ratio of heterocyclic products 3 and 4 depends on the structure of acetylenic compound: 3 is a major product for phenylacetylene (PhA) and 4 – for acetylenes with an electron-rich substituent, 9-phenanthryl¬acetylene, for example. In this presentation we focused our attention on the reaction of ylide 1 with PhA, although other ylides and acetylenes will be also considered for comparison.The course of the reaction in the case of acetylenes is remarkable and raises many questions about the reaction mechanism and the nature of active intermediates. (i) The heterocyclic products are formed only at relatively high concentrations of 1 (> 0.025 M), when the solution of 1 in CH2Cl2 is microheterogeneous, with the size of an ylide particle attaining 800 nm at [1] = 0.1 M. (ii) The reaction has an induction time, starts on an ylide nanoparticle and then expands over the bulk solution.[3] (iii) The formation of 3 occurs even under irradiation by the light with λirr 450 nm on the very tail of the absorption band of 1. (iv) The formation of 3 (neutral compound) occurs from the cation (1) and is accompanied by elimination of H+. It is proposed that the reaction is acid-catalyzed. In fact, the addition of CF3COOH in concentrations [CF3COOH] ≤ [1] decreases the induction time and increases the rate of product formation. This allowed us to convert the photoinduced process to the acid-catalyzed reaction and to study the dependence of the induction time and the reaction rate on the reagent concentrations in kinetic mode by spectrophotometry. Since the ylides under study can be considered as iodonium salts, the proposed mechanism involves parallel homolytic and heterolytic dissociation of the C–I+Ph bond of 1 under irradiation to give active radicals and ionic transient species, followed by electrophilic addition of 2 to a cationic intermediate. The role of in situ generated iodine and the structure of relatively stable radicals monitored in the ESR study of the reaction mixture under irradiation will be also discussed.