亲核试剂的亲卤反应——2.冠醚促进的二氟二溴甲烷与硫酚钠的反应

HALOPHILIC REACTIONS OF NUCLEOPHILES——2.CROWN ETHER-PROMOTED REACTIONS OF CF2Br2 AND PhSNa

在苯、乙醚等溶剂中,CFBr与PhSNa不反应或反应极慢.加入催化量的二苯并-18-冠-6,则反应在数分钟内完成,主产物PhSCFBr的产率提高至75~85%,副产物PhSCFH及PhSSPh受到显著抑制.本体系明显优于最近文献报道的其他反应体系.讨论了反应机理,给出了表明反应主要是阴离子型链式机理的进一步证据:反应速度和产物分布对水、醇等给质子性杂质敏感,对氢原子供体型溶剂及光、氧、自由基捕获剂不敏感;在以CCl为溶剂的反应中,得到PhSCFCl和属于CCl的产物,而未检测到自由基型机理所预期的CClCCl.

In ether and benzene,sodium thiophenoxide reacts with dibromodifluoromethane either very slowly or not at all.However,catalytic amounts of dibenzo-18-crown-6 can accelerate the reactions dramatically.The reactions can be completed in a few minutes at room temperature,and more notably,they can give the desired product PhSCFBr(1)in high yield,whereas the formation of the by-products PhSOFH(2)and PhSSPh(3)is significantly suppressed.In ethereal solutions containing 2.8 mM of the crown ether,ranges of the yields of 1,2,and 3 were 78~85%,1~2%and 5~12%,respectively,depending on the purity of the solvents and the reagents used(see Table 2 in the Chinese text).The results represent significant improvements over those most recently reported.Possible reaction pathways were discussed and new evidence for an anionic chain mechanism with PhSCFas intermediate is presented,e.g.(1)moisture and other proton-donating species retard the reaction but increase the yields of 2 and 3;(2)premium-donating solvents,light,oxygen and di-tert-butyl nitroxide did not affect the reactions significantly;(3)with CClas the solvent,PhSCFGl,BrCCland HCClwere also formed,but not CCl,an expected product from reaction paths involving radicals.Evidently,PhSCFGl was formed from a chlorophilic attack by PhSCFon solvent molecules.