The thiocarbonyl ‘S’ is softer than thiolate ‘S’: A catalyst-free one-pot synthesis of isothiocyanates in water
作者:Latonglila Jamir、Abdur Rezzak Ali、Harisadhan Ghosh、Francis A. S. Chipem、Bhisma K. Patel
DOI:10.1039/b923336a
日期:——
Treatment of the preformed or the in situ generated aryl/alkyl dithiocarbamates triethylammonium salt (ArNHCSSâ.Et3NH+) with methyl acrylate in an aqueous medium gave solely arylisothiocyanate (ArNCS), whereas the in situ generated aryl dithiocarbamic acid (ArNHCSSâ.H+) yielded exclusively the thia-Michael adduct (ArNHCSSCH2CH2COOMe). This differential reactivity can be explained by two alternative mechanisms which is dependent both on the nature of the counter cation and on the pH of the reaction medium. Irrespective of the counter cations, the thiocarbonyl sulfur (=S) atom, having large orbital-coefficient, is softer compared to the thiol/thiolate sulfur (-SH/Sâ) in a dithiocarbamate salt and the former adds to the Michael acceptor by a 1,4-addition.
在水中介质中,用甲基丙烯酸处理预制的或原位生成的芳基/烷基二硫代氨基甲酸盐三乙基铵盐(ArNHCSS−.Et3NH+)仅得到芳基异硫氰酸酯(ArNCS),而原位生成的芳基二硫代氨基甲酸(ArNHCSS−.H+)仅生成硫代迈克尔加合物(ArNHCSSCH2CH2COOMe)。这种差异性反应活性可以通过两种替代机制来解释,这取决于反荷离子的性质和反应介质的pH值。不管反荷离子如何,与二硫代氨基甲酸盐中的硫醇/硫醇盐硫(−SH/S−)相比,硫羰基硫(=S)原子具有较大的轨道系数,更为柔软,前者通过1,4-加成与迈克尔受体加合。