THE STEREOCHEMISTRY OF HYDRIDE REDUCTIONS
THE STEREOCHEMISTRY OF HYDRIDE REDUCTIONS
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DOI：
10.1021/ja01592a069
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发表时间：
1956-01-01
影响因子：
15
通讯作者：
NOYCE, DS
中科院分区：
文献类型：
--
作者：
DAUBEN, WG;FONKEN, GJ;NOYCE, DS
研究表明，用氢化铝锂、硼氢化钠和异丙醇铝还原烷基环己酮会导致轴向（不稳定）异构体比例增加。还原步骤的机理是参考初始金属有机络合物形成的难易程度（空间方法控制）和络合物形成后产物形成的相对能量学（产品开发控制）来讨论的。酮的金属氢化物还原可产生非对映醇，这已成为许多研究的主题，并且此类研究已针对多种酮进行，包括无环、脂环和多环化合物。 1 许多研究者已经提出了立体化学结果的相关性1-6，并且在大多数情况下，这些总结的结果都是用构象概念来表达的，考虑到初始酮最可能的构象以及在还原过程中必然导致的变化。最具描述性的假设之一是 Cram 和 Greene7 的假设，其中认为氢化铝锂对无环酮的还原是通过在氢化物和羰基氧原子之间形成初始络合物而进行的，在该络合物中，氧“实际上成为分子中体积最大的基团，并且倾向于在连接到相邻不对称碳原子的两个体积最小的基团之间进行定向。”主要异构体“将通过进入基团从羰基络合物受阻最少的一侧进入而形成”。另一方面，Barton，10，仅仅总结了结果，并指出对于不受阻碍的酮，脂环族系列中的主要产物是赤道异构体。然而，其他先前的工作人员很少注意到一个惊人的事实，即任何系列中的主要产品通常都是更稳定的异构体。这种普遍性的一个很好的例子是在一系列 2-(哌嗪基)-苄基环己酮8的还原研究中发现，其中许多
It has been shown that reductions of alkylcyclohexanones with lithium aluminum hydride, sodium borohydride and alumi-num isopropoxide lead to increasing proportions of the axial (unstable) isomer. The mechanism of the reduction step is discussed with reference to the ease of formation of the initial metallo-organic complex (steric approach control) and to the relative energetics of the formation of the products once the complex is formed (product development control).The reduction by metal hydrides of ketones which can yield diastereomeric alcohols has been the sub-ject of numerous investigations and such studies have been carried out with a wide variety of ketones, including acyclic, alicyclicand polycyclic compounds. 1 Correlations of the stereochemical results havebeen proposed by numerous investiga-tors1-6 and for the most part the results of these summaries have been expressed in terms of con-formational concepts, considering the most likely conformation of the initial ketone and the changes which must result during the reduction proper. One of the most descriptive postulates is that of Cram and Greene7 in which it was considered that the reduction by lithium aluminum hydride of acy-clic ketones proceeded by the fonnation of an initial complex between the hydride and the carbonyl oxygen atom and in this complex, the oxygen “be-comes effectively the bulkiest group in the molecule and tends to orient itself between the two least bulky groups attached to the adjacent asymmetric carbon atom.” The predominant isomer “would be formed by approach of the entering group from the least hindered side of the carbonyl complex.” Barton, 10 on the other hand, has merely summa-rized the results and has pointed out that with an unhindered ketone, the predominant product in the alicyclic series is the equatorial isomer. Other pre-vious workers, however, have paid little attention to the striking fact that the predominant product in any series usually is the more stable isomer. An excellent example of this generality is to be found in the study of the reduction of a series of 2-(apiperazinyl)-benzylcyclohexanones8 in which many