Free radical reactions to generate alkenes and/or ionic reactions to generate hydroximoyl chlorides when β-nitrostyrenes react with triethylaluminium or diethylaluminium chloride
作者:Cheng-Ming Chu、Ju-Tsung Liu、Wen-Wei Lin、Ching-Fa Yao
DOI:10.1039/a807156j
日期:——
β-Nitrostyrenes 1 react with triethylaluminium or diethylaluminium chloride in diethyl ether solution and under nitrogen or argon to generate the alkenes 2 and the hydroximoyl chlorides 3 after work-up with ice-cold, conc. hydrochloric acid. The formation of the alkenes 2 is proposed to be a free-radical reaction via NO2/alkyl substitution since the yields of the alkenes 2 are increased in the presence of benzoyl peroxide (Bz2O2) and decreased in the presence of galvinoxyl. Only the alkenes 2 are produced with a high stereoselectivity for the E isomers when β-nitrostyrenes react with triethylaluminium in the presence of one to two equivalents of Bz2O2 as free-radical initiator. The mechanism of the generation of the hydroximoyl chlorides 3 is proposed to proceed through a 1,4-addition pathway to produce nitronates A, then the protonated nitronates B or the nitroso cations C are trapped by chloride ion to form the final products. The yields of compounds 3 are also improved by the presence of Lewis acids such as MgCl2. Medium to high yields of the hydroximoyl chlorides 3 and traces or low yields of the alkenes 2 are generated when triethylaluminium or diethylaluminium chloride react with β-nitrostyrenes in the presence of three equivalents of MgCl2 under argon.
β-硝基苯乙烯1在氮气或氩气氛下与三乙基铝或二乙基铝氯化物在醚溶液中反应,通过用冰冷浓盐酸处理后生成烯烃2和羟肟氯化物3。烯烃2的形成被认为是通过NO2/烷基取代的自由基反应,因为在苯甲酰过氧化物(Bz2O2)存在下,烯烃2的产率提高,而在加尔维诺基存在下则降低。当β-硝基苯乙烯在1到2等当量Bz2O2作为自由基引发剂的条件下与三乙基铝反应时,只产生具有高立体选择性的E异构体的烯烃2。羟肟氯化物3的生成机制被提出为通过1,4-加成途径生成亚硝酸盐A,然后质子化的亚硝酸盐B或亚硝基阳离子C被氯离子捕获形成最终产物。羟肟氯化物3的产率也因存在路易斯酸(如MgCl2)而提高。当三乙基铝或二乙基铝氯化物在氩气下与β-硝基苯乙烯在三等当量的MgCl2存在下反应时,产生中到高产率的羟肟氯化物3以及微量或低产率的烯烃2。