Halogenation of Xanthenyl and Thioxanthenylallenes

Abstract

Reactions of acids with vinylidenexanthenes (or thioxanthenes) 1 and 9methoxy-9-vinylxanthenes (or thioxanthenes) 8 give xanthene-(or thioxanthene-) 9-spiro-1 '-indenes 7. With bromine or sulfuryl chloride they give the corresponding 2'-halogenoxanthene-(or thioxanthene-)9-~piro-1 '-indenes 5. Formation of such derivatives depends on preferential attack of the reagents on allenes 1 to give initially xanthylium (or thioxanthylium) ions 3 which cyclize. One 71 bond of the allenes 1 can be selectively reduced in acid media. Chemical and spectral evidence for these routes are presented.Recently we reported on the cyclization of vinylidenexanthenes (1, Y = 0, R = alkoxy) in acid media which give xanthene-9-spiro-1'-indenes (7, Y = 0) [l]. This cyclization revealed the preferential reactivity of the 71 bond of the involved xanthene residue. We have subsequently investigated reactions of equimolecular quantities of either bromine or sulfuryl chloride with vinylidenexanthenes and thioxanthenes (1, Y = 0 or S, R = alkoxy) in order to have further insight on the behaviour and reactivity of such polyarylallenes towards halogenation.Addition of bromine took place readily with instantaneous development of very deep blue-violet colour instead of the normal discharge of bromine colour. The same colour developed on addition of sulfuryl chloride to 1 ( Y = O or S) in carbon tetrachloride solution. This suggested that heteroatom in this type of compound would be responsible for development of such colour [l] as it may encourage migration of one halogen atom due to concerted electron-shift in 2 with formation of the coloured oxonium derivative 3. On the other hand, addition may take place through an initial cation attack on the activated central carbon atom of 1, rather than free radical addition to give 3 (Y = 0 or S). On heating solutions of 3 (Y = 0 or S, R = alkoxy) ready evolution of hydrogein halide occurred, affording mainly the corresponding 2'-bromo-6'-alkoxy-3'-aryl-xanthene-(or thioxanthene-)9-spiro-1 '-indenes 5 (Y = 0 or S, X = Br, R = alkoxy). When bromine was added to 1 (Y = 0, R = OMe) and the reaction mixture was left 2 h at RT., the deep blue-violet crystals isolated were identified as the tribromide: 4 (Y = 0, X = Br, R = OMe), in addition to the corresponding bromoindene 5. Compound 4 could be formed by addition of some liberated hydrogen bromide to 3. Formation of bro-Oo18-019)3/79/6/ 1872-06$0l.Oo/O 0 1979 Schweizerische Chemische Gesellschaft

Synthesis of Polysubstituted 2-Iodoindenes via Iodonium-Induced Cyclization of Arylallenes

“…Formation of an iodonium ion D upon further reaction of C with a second equivalent of cationic iodine atom followed by anti 5-endo nucleophilic attack of the aryl group gives the functionalized 2-iodoindene 2 . Considering the versatility of the carbocyclization reactions promoted by halogen electrophiles and prior reports in the literature illustrating the possibility to activate allene unsaturations with halogen electrophiles toward anti nucleophilic arylation reactions, we decided to investigate the formation of 2-iodoindenes 4 using an unprecedented methodology based on the electrophilic iodocarbocyclization of tri- and tetrasubstituted arylallenes 3 (Scheme , eq 2).…”