Studies On Hydrochlorination,Hydration And Dimerization Reactions Of Alkynes

Alkynes, one of the highly important classes of organic compounds, have beenextensively used as important versatile synthons and building blocks in organicsynthesis. The reactions of alkynes are of considerable interest which provide varioussynthetic methods for many important products like vinyl halides, ketones, enynes,alkynols, and arene derivatives, etc. Among these important applications,hydrochlorination, hydration, and dimerization are practical and atom-economicalmethods for the synthesis of vinyl chloride, ketones, and conjugated enynes,respectively. These methods, however, suffer from some drawbacks, such as badselectivity and limited scope, which confine the application and development ofalkyne chemistry. Therefore, making further studies on these reactions remains animportant but challenging goal. This paper focuses on research abouthydrochlorination of alkynes in the absence of catalyst, Pd(0)-catalyzed hydration anddimerization of alkynes.In the first part, we studied the hydrochlorination reaction by use Ac2O as additivewithout the presence of catalyst. After optimization of the reaction conditions ofsolvent, temperature, and the amount of Ac2O, hydrochlorination reaction of alkynesproceeded quickly with high selectivity and gave vinyl chloride in91%yield. Wethen investigated the scope of hydrochlorination reaction of aromatic alkynes. Theresults indicated that effect of electron has greatly remarkable influence, positioneffect of substituents has also obvious influence, and the steric hindrance has littleinfluence on this reaction.In the second part, Pd(PPh3)4-catalyzed hydration reaction of alkynes was exploredwith the addition of hydrochloric acid. We screened the reaction parameters, includingthe amount of HCl and Pd(PPh3)4, temperature, and Br nsted acids. Under theoptimizing reaction conditions, hydration reaction of alkynes leads to ketonesfollowing Markovnikov’s rule under Pd(0)-catalyzed, for the first time, the highestyield is85%. The results of scope tests of hydration reaction of aromatic alkynesrevealed that electron effect, relative position effect of substituents, and steric effecthave all apparant influence on this reaction. Moreover, we firstly proposed the mechanism of Pd(0)-catalyzed hydration reaction of alkynes and trapped the keyintermediate-alkenyl palladium species, via Suzuki coupling reaction.The dimerization reaction of alkynes catalyzed by Pd(PPh3)4was examined in thepresence of HOAc in the third part. By optimizing reaction conditions, alkynesdimerized according to―head-to-tail‖mode and afforded1,3-disubstituted enynes,exclusively, with excellent chemo-, region-selectivity, and77%yield. Regretfully, theinfluence of electron effect on dimerization reaction is similarly pronounced, just likeon hydrochlorination and hydration reaction. But position effect of substituents hasfew influence and steric hindrance has no significant influence on this reaction. Wenot only proposed the mechanism Pd(0)-catalyzed dimerization reaction of alkynesaccording to―head-to-tail‖mode, the key intermediate-alkenyl palladium species, wastrapped via Suzuki coupling reaction. What’s more, we achieved high selective switchof reaction reactivity of hydration and dimerization reaction of alkynes enabled bycounterion for the first time.