Substituted 1-(pyridin-2-yl)-2,4,6-triphenylpyridinium perchlorates 1b-1e were converted with potassium ferricyanide and potassium hydroxide to sterically crowded 2-phenyl-3-[(Z)-1,3-diphenyl-3-oxopropenyl]imidazo[1,2-a]pyridines 2b-2e accompanied by minor isomeric 2-benzoyl-3,5-diphenyl-1-(pyridin-2-yl)pyrroles 3c-3e. 4-Phenyl-2,6-di(4-substituted phenyl)-1-(pyridin-2-yl)pyridinium salts 4a, 4b gave exclusively corresponding imidazo[1,2-a]pyridines 5a, 5b while the ferricyanide oxidation of 1-(5-iodo- and 5-cyanopyridin-2-yl)-2,4,6-triphenylpyridinium perchlorates 6a, 6b led to mixtures of major imidazo[1,2-a]pyridines 7a-7c and minor pyrroles 8a-8c. Some mechanistic aspects of the oxidation procedure are discussed in connection with a resistance of 2,6-diphenyl-1,3,5-trimethylpyridinium perchlorate (9c) towards the oxidizing agents.
用钾亚铁氰化钾和氢氧化钾将1-(吡啶-2-基)-2,4,6-三苯基吡啶盐酸盐1b-1e转化为空间位阻的2-苯基-3-[(Z)-1,3-二苯基-3-氧代丙烯基]咪唑[1,2-a]吡啶2b-2e,伴随着少量异构体2-苯甲酰基-3,5-二苯基-1-(吡啶-2-基)吡咯3c-3e。4-苯基-2,6-二(4-取代苯基)-1-(吡啶-2-基)吡啶盐4a、4b仅产生相应的咪唑[1,2-a]吡啶5a、5b,而对5-碘和5-氰基吡啶-2-基-1-(吡啶-2-基)-2,4,6-三苯基吡啶盐6a、6b的亚铁氰化氧化导致主要的咪唑[1,2-a]吡啶7a-7c和次要的吡咯8a-8c混合物。讨论了氧化程序的一些机理方面,涉及2,6-二苯基-1,3,5-三甲基吡啶盐酸盐9c对氧化剂的抵抗性。