溶剂对钯催化的叔丁基乙炔低聚反应化学选择性的调控作用

报道了溶剂对钯催化的叔丁基乙炔低聚反应化学选择性的调控作用. 反应可在苯-正丁醇双组分溶剂体系中顺利进行, 当双组分溶剂体系中苯占优势比例时, 反应发生递次的三分子炔烃顺式插入, 经由顺式s-烯钯中间体生成环三聚产物1,3,5-三叔丁基苯; 而当双组分溶剂中正丁醇组分上升至一定比例, 反应选择性生成(3Z,5Z)-2,2,7,7-四甲基- 3,6-二氯-3,5-辛二烯或(3Z,5Z)-2,2,7,7-四甲基-3,6-二溴-3,5-辛二烯, 这是由于正丁醇可显著加快C—Pd σ键的断裂, 并与叔丁基乙炔、σ-烯钯中间体形成弱氢键作用力, 同时也与Pd(II)和Cu(II)等离子存在配位效应. 在强极性质子溶剂H₂O中, 反应生成偶联双炔: 2,2,7,7-四甲基-3,5-辛二炔. 文中就反应溶剂体系、钯铜催化剂及反应可能机理等分别进行了探讨.

Solvent-Controlled Chemoselective Palladium-Catalyzed Oligomerization of tert-Butyl Acetylene

A novel solvent-controlled chemoselective palladium-catalyzed oligomerization of tert-butyl acetylene was reported in this paper. The reaction was carried out in benzene/n-BuOH binary solvent system smoothly. When benzene was preponderating component in the binary solvent system, a cyclotrimerization process occurred to give 1,3,5-tri-tert-butylbenzene via a mechanism of three acetylene molecular stepwise insertions forming σ-butadienyl-Pd and σ-hexatrienyl-Pd intermediates. While when the polar, protic and strong coordinating solvent, n-BuOH, which could help to cleave the C—Pd σ-bonds and solvate Pd(II) cation, Cu(II) cation, halo anion and σ-butadienyl-Pd inter-mediate etc., was increased to a certain ratio in the binary solvent system, the reaction proceed readily via n-BuOH promoted mechanism to give (3Z,5Z)-2,2,7,7-tetramethyl-3,6-dichloro-3,5-octadiene or (3Z,5Z)-2,2,7,7-tetramethyl-3,6-dibromo-3,5- octadiene, respectively, instead of cyclotrimerization product. Possible weak hydrogen bonds and n-π weak force between n-BuOH and tert-butyl acetylene (or σ-butadienyl-Pd intermediate) were also in favor of the n-BuOH promoted pathway. Mean-while, a coupling product 2,2,7,7-tetramethyl-3,5-octadiyne was given exclusively when the reaction was conducted in singular polar H₂O. Influences of the solvent, catalysts, and possible mechanism were discussed.