New Lewis-Acidic Molybdenum(II) and Tungsten(II) Catalysts for Intramolecular Carbonyl Ene and Prins Reactions. Reversal of the Stereoselectivity of Cyclization of Citronellal
作者:Pavel Kočovský、Ghafoor Ahmed、Jiří Šrogl、Andrei V. Malkov、John Steele
DOI:10.1021/jo9821675
日期:1999.4.1
New Mo(II) complexes BnEt(3)N(+)[Mo(CO)(4)ClBr(2)](-) (A) and Mo(CO)(5)(OTf)(2) (B) and their W(II) congeners D and E have been developed as catalysts for the title reactions. Unlike other Lewis acids, the latter catalysts exhibit cis-stereoselectivity in the cyclization of citronellal (1 --> 3 with A and 1 --> 5 with B). Isotopic labeling allowed formulation of the reaction mechanism, according to
新的Mo(II)配合物BnEt(3)N(+)[Mo(CO)(4)ClBr(2)](-)(A)和Mo(CO)(5)(OTf)(2)(B)和它们的W(II)同系物D和E已被开发为标题反应的催化剂。与其他路易斯酸不同,后者的催化剂在香茅醛的环化反应中表现出顺式-立体选择性(A为1-> 3,B为1-> 5)。同位素标记可以确定反应机理,根据这些机理,这些络合物可充当庞大的路易斯酸,与羰基氧配位的eta(1)。立体化学似乎由突出的配体L(p)控制,其决定了船形过渡态III。可以通过[Mo(CO)(4)Br(2)](2)(C)或ZnCl(2)通过逆向烯反应平衡形成其反式受主2的动力学形成的顺式烯醇3。