Reactions of 3-aryl-1-(trifluoromethyl)prop-2-yn-1-iminium salts with 1,3-dienes and styrenes

Iminium-substituted 2,3-dihydroisoxazoles: synthesis from acetylenic iminium salts and nitrones, and some transformations

Iminium-functionalized 2,3-dihydroisoxazoles (4-isoxazolines) were prepared in high yields by [3+2] cycloaddition of N-methyl-C-phenylnitrone and acetylenic iminium salts. Conversion of the very hygroscopic cycloadducts into neutral compounds was achieved by hydrolysis or hydride reduction of the iminium functional group. By a thermal rearrangement, the cycloadducts were converted into 2,3-dihydro-1H-imidazolium salts. With C,N-diphenylnitrone as the reaction partner, the expected cycloadduct, a [(2,3-dihydroisoxazol-4-yl)-1-(phenyl)methylene]-N,N-dimethylammonium triflate, could be prepared at −75°C and was characterized by NMR. This product was found to be thermally unstable above −30°C, being converted into an inseparable product mixture. Basic hydrolysis of this mixture afforded a structurally interesting acyclic enaminone/half-aminal, which is likely derived from a 2,3-dihydro-1,3-oxazole.

Reactions of 1-trifluoromethyl-prop-2-yne1-iminium salts with 2- and 3-aminopyridines

4-CF3-pyrido[1,2-a]pyrimidin-5-ium triflates could be prepared in fair to high yields from acetylenic iminium salts R-C≡C–C(CF3)=N+Me2·CF3SO3 − and 2-aminopyridinium triflate under thermal conditions. Their reactions with protic heteronucleophiles, leading to 4-CF3, 4-XR-substituted 1,4-dihydropyrido[1,2-a]pyrimidin-5-ium salts, have been studied and reversible addition as well as subsequent opening of the pyrimidine ring have been observed. The cyclocondensation of a 3-phenyl-propyniminium triflate and 3-aminoyridinium triflate required strong thermal activation and yielded a 4-CF3-and a 4-CF3,5-CH3-[1,7]naphthyridine. A 1-CF3-benzo[f][1,7]naphthyridine was obtained from the same propyniminium salt and 3-aminoquinolinium triflate.

1-Trifluoromethyl-prop-2-yne 1-iminium salts and 1-imines: reactions with the mesoionic „Nitron“

The heterocyclic mesoionic compound (1,4-diphenyl-1H-1,2,4-triazol-4-ium-3-yl)(phenyl)amide („Nitron“) has recently been found to exist in a prototropic equilibrium with minor amounts of a nucleophilic heterocyclic carbene of the 1,2,4-triazolyl-5-ylidene type. Here we report that Nitron reacts with 1-trifluoromethyl-substituted prop-2-yne iminium salts by conjugate nucleophilic addition of the anionic PhN‒ substituent in the mesoionic tautomer, whereas the nucleophilic triazolylidene form is involved in the reaction with 1-CF3-prop-2-yne imines. 3-(2,3-Dihydro-1H-benzo[c]azepin-5-yl)-1H-1,2,4-triazol-4-ium triflate salts were obtained in the former case and (Z)-9-arylidene-1,2,4,7-tetraazaspiro[4.4]nona-2,7-dienes in the latter.