The aqueous tin-mediated Barbier reaction affords good to excellent yields and moderate syn diastereoselectivity under basic and acidic conditions. The high yields and stereoselectivity observed in the case of o-substituted aldehydes suggest a cyclic organotin intermediate or transition state in K2HPO4 solution. A practical and efficient aqueous tin allylation of methoxy- and hydroxybenzaldehydes can be carried out in HCl solution in 15 minutes to afford the corresponding homoallylic alcohols in high yields. Aliphatic aldehydes give moderate to excellent yields with reaction times ranging from 30 to 60 minutes. Under these conditions, crotylation gives exclusively the γ-product and the syn isomer is formed preferentially. For 2-methoxybenzaldehyde, an equilibration of the isomers to a syn/anti ratio of 1:1 can be observed after several hours. Control experiments with radical sources or scavengers give no support for radical intermediates. NMR studies suggest a mechanism involving an organotin intermediate. The major organotin species formed depends on the reaction medium and the reaction time. The use of acidic solution reduces the reaction times, due to the acceleration of the formation of the allyltin(IV) species.
水相
锡介导的Barbier反应在碱性和酸性条件下可实现良好至优异的产率和适度的顺式二构体选择性。对于邻位取代的
醛类所观察到的高产率和立体选择性表明在K₂HPO₄溶液中存在环状
有机锡中间体或过渡态。在HCl溶液中,可以在15分钟内高效地实现甲
氧基和羟基
苯甲醛的
水相
锡烯丙基化反应,获得相应的同
烯丙醇,产率很高。
脂肪醛的产率在中等到优异之间,反应时间为30至60分钟。在这些条件下,克罗提化仅产生γ-产物,并且更偏向形成顺式异构体。对于2-
甲氧基苯甲醛,经过几小时后可观察到异构体之间的平衡,顺/反的比例为1:1。使用自由基源或清除剂的对照实验并未支持自由基
中间体的存在。核磁共振研究表明该反应机制涉及
有机锡中间体。形成的主要
有机锡物种依赖于反应介质和反应时间。使用酸性溶液可以缩短反应时间,这是由于促进了
烯丙
锡(IV)物种的形成。