芴羟甲基化合成9-芴甲醇过程中脱水产物生成的影响因素及机理

Influencing Factors and Mechanism of Dehydration Product Formation During Hydroxylmethylation of Fluorene to 9-Fluorenylmethanol

以多聚甲醛为羟甲基化试剂,在碱催化下芴一步生成9-芴甲醇过程极易发生单分子共轭碱消除,脱水生成富烯化合物,导致9-芴甲醇的选择性下降.为实现对该过程的调控,本文考察了温度、碱的种类、乙醇和多聚甲醛的添加等对9-芴甲醇脱水反应的影响,并结合动力学计算和同位素示踪,推测了芴羟甲基化合成9-芴甲醇过程中副产物生成的机理.结果表明,升高温度、增加碱性和添加甲醛有利于脱水反应发生.乙醇通过氢键作用促进脱水反应,添加量大时会导致芴负离子和9-芴甲醇负离子发生质子化作用.中间体可能会发生分子内氢传递或者夺取芴和9-芴甲醇的9位氢,促进脱水反应发生.

During one-step synthesis process of 9-fluorenylmethanol with paraformaldehyde as hydroxymethylation reagent,9-fluorenylmethanol is easy to undergo base-catalyzedβ-elimination(E1cb)to yield dibenzofulvene,and the selectivity of 9-fluorenylmethanol decreased.To achieve the process of regulation and control,the factors affecting the dehydration of 9-fluorenylmethanol,such as temperature,the types of alkali and ethanol and paraformaldehyde were investigated.The mechanism of by-product formation in the process of hydroxymethylation of fluorene to form 9-fluorenylmethanol was speculated according to the experimental results,kinetic calculation and isotope tracing.The results showed that increasing the temperature and alkalinity,and adding formaldehyde promoted the dehydration reaction.Ethanol promotes dehydration reaction through hydrogen bonding,and a large amount of ethanol would lead to the internal protonation of fluorenyl anion and 9-fluorenylmethanol anion.The intermediate may have the behavior of intramolecular hydrogen transfer or intermolecular hydrogen capture from fluorene and 9-fluorenylmethanol,facilitating dehydration reactions.