A highly selective system for palladium-catalyzed polycondensation of (E,E)-1,4-bis(2-bromoethenyl)benzene ((E,E)-1) with 2,5-dioctyloxybenzene-1,4-diboronic acid (2a) to give all-trans poly[( p-phenylenevinylene)-alt-(2,5-dioctyloxy-1,4-phenylenevinylene)] (all-trans 3) has been investigated using (E)-styryl bromide ((E)-4) and 2,5-dioctyloxybenzeneboronic acid (5a) as model compounds of (E,E)-1 and 2a, respectively. The reaction of (E)-4 and 5a in toluene in the presence of Pd(PPh3)4 catalyst and aqueous K2CO3 base affords considerable amounts of homocoupling products (i.e., 1,4-diphenylbutadiene (13%) and 2,2′,5,5′-tetraoctyloxybiphenyl (22%)), together with (E)-2,5-dioctyloxystilbene ((E)-6a) as the cross-coupling product (30%). The use of aqueous NaOH as a strong base and Bu4NBr as a phase-transfer catalyst notably reduces the homocoupling products, and the use of Pd(PBut3)2 instead of Pd(PPh3)4 results in almost perfect selectivity of the cross-coupling product (E)-6a. Under optimized catalytic conditions, the desired all-trans 3 has been successfully prepared without notable defects in the polymer chain.
使用(E)-
苯乙烯溴化物((E)-4)和 2,5-二辛基氧基-1,4-
苯乙烯(2a),研究了
钯催化(E,E)-1,4-双(2-
溴乙烯基)苯((E,E)-1)与 2,5-二辛基氧基-1,4-苯二
硼酸(2a)缩聚得到全反式聚[( 对苯基
乙烯)-alt-(2、(E)-
苯乙烯溴化物((E)-4)和 2,5-二辛基氧基苯
硼酸(5a)分别作为 (E,E)-1 和 2a 的模型化合物,研究了全反式聚[( 对亚苯基
乙烯)-alt-(2,5-二辛基氧基-1,4-亚苯基
乙烯)](全反式 3)。在 Pd(PPh3)4 催化剂和 K2CO3
水基存在下,(E)-4 和 5a 在
甲苯中发生反应,生成大量的均偶联产物(即、1,4-二苯基
丁二烯(13%)和 2,2′,5,5′-四辛基氧基
联苯(22%)),以及作为交叉偶联产物的 (E)-2,5- 二辛基氧基
二苯乙烯((E)-6a)(30%)。使用 NaOH
水溶液作为强碱和 Bu4NBr 作为相转移催化剂可以显著减少同偶联产物,而使用 Pd(PBut3)2 而不是 Pd(PPh3)4,可以几乎完全选择性地得到交叉偶联产物 (E)-6a。在优化的催化条件下,成功制备出了所需的全反式 3,聚合物链中没有明显的缺陷。