通过宝石的溴/金属交换反应生成锂和锌酸类胡萝卜素的立体化学已经研究了分别与BuLi和三有机锌酸锂形成的二溴化合物。已证明,衍生自1,1-二溴烯烃的锂和锌酸酯类化合物在低温下是构型稳定的,而在未反应的起始二溴烯烃的存在下,锂类化合物而不是锌酸酯类化合物在类胡萝卜素碳上容易进行异构化。在起始二溴环丙烷存在下,衍生自1,1-二溴环丙烷的Zincate类胡萝卜素在类胡萝卜素碳上缓慢进行异构化。事实证明,锂和锌酸盐类胡萝卜素的异构化反应是通过涉及类胡萝卜素与起始二溴化合物之间的溴/金属交换的机理进行的。在1,1-二溴烯烃在热力学条件下的反应中观察到锂类胡萝卜素的高度立体选择性形成。在动力学控制的条件下,溴/锂和溴/锌的交换反应均优先发生在起始位点的空间受阻溴原子上宝石-二溴化合物。通过交换机制合理化该观察,该交换机制涉及在受阻溴原子处的盐配合物形成或S N 2反应的线性过渡态,在这种情况下,由于碳-溴键的延长,应力会减轻。
Stereochemistry in carbenoid formation by bromine/lithium and bromine/zinc exchange reactions of 1,1-dibromoalkenes: higher reactivity of the sterically more hindered bromine atom
摘要:
Both lithium and zincate carbenoids (R1-(R2)C=C(Br)M; M = Li and Zn(Bu)2Li) generated by the halogen/metal exchange reaction of 1,1-dibromoalkene 1 with BuLi and (Bu)3ZnLi, respectively, are configurationally stable at low temperatures, but in the presence of excess 1, the lithium carbenoids undergo facile isomerization at the carbenoid carbons. Under kinetically controlled conditions, both the Br/Li and Br/Zn exchange reactions take place preferentially at the sterically more hindered bromine atom of 1.