MECHANISM OF THE WITTIG REACTION - THE ROLE OF SUBSTITUENTS AT PHOSPHORUS
MECHANISM OF THE WITTIG REACTION - THE ROLE OF SUBSTITUENTS AT PHOSPHORUS
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DOI：
10.1021/ja00220a037
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发表时间：
1988-06-08
影响因子：
15
通讯作者：
MARTH, CF
中科院分区：
文献类型：
--
作者：
VEDEJS, E;MARTH, CF
维蒂希反应立体化学的变化是由于占主导地位的动力学控制，在几乎所有的情况下。顺式或反式氧杂磷杂环己烷的形成是决定性的步骤，这是通过异步环加成发生的。1，2和1，3位阻相互作用的相互作用决定哪种非对映体氧杂磷杂环丁烷将是有利的。对于Ph_3P = CHCH_3，最佳过渡态是具有假赤道醛烷基R ′和假轴对称的褶皱四中心排列2a。α-甲基。这种几何结构是由于在环加成过程中有一个sp1 23 4-杂化原子（磷）作为反应中心之一所引起的不寻常的空间结果，当R ′体积较大时，这种几何结构越来越有利。如果最近的磷配体可以使紧密的面朝向醛，或者如果醛R '基团具有至少一个氢，则发生朝向反式非对映异构体5的趋势。这些条件是满足异常反式选择性叶立德10，甚至叔醛，由于键角的偏好和几何约束，由于5元环。相比之下，7被限制为通过顺式选择性褶皱几何结构反应，并仅提供Z烯烃。不稳定或“缓和”叶立德反应涉及早期过渡态有磷在扭曲的正方形金字塔几何形状。稳定的叶立德通过productlike几何反应，和动力学反式选择性是很容易理解的oxaphosphetane样的立体相互作用的基础上。
The variation in Wittig reaction stereochemistry is attributed to dominant kinetic control in nearly all cases. Formation of cis or trans oxaphosphetanes is the decisive step, and this occurs by an asynchronous cycloaddition. An interplay of 1, 2 and 1, 3 steric interactions decides which diastereomeric oxaphosphetane will be favored. For Ph3P= CHCH3, the best transition state is the puckered four-center arrangement 2a having a pseudoequatorial aldehyde alkyl group R'and pseudoaxia! a-methyl. This geometry results from the unusual steric consequences associated with having an sp1 23 4-hybridized atom (phosphorus) as one of the reacting centers in a cycloaddition process and is increasingly favored when R'is bulky. A trend toward the trans diastereomer 5 occurs if the nearest phosphorus ligand can orient a compact face toward the aldehyde, or if the aldehyde R'group has at least one-hydrogen. These conditions are satisfied in the exceptionally trans selective ylide 10, even for tertiary aldehydes, as a result of bond angle preferences and geometric constraints due to the 5-membered ring. In contrast, 7 is constrained to react via the cis-selective puckered geometry and affords exclusively the Z alkene. Nonstabilized or “moderated” ylide reactions involve early transition states having phosphorus in a distorted square-pyramidal geometry. Stabilized ylidesreact via a productlike geometry, and kinetic trans selectivity is easily understood on the basis of oxaphosphetane-like steric interactions.