Mechanism of the Kulinkovich cyclopropanol synthesis: Transfer-epititanation of the alkene in generating the key titanacyclopropane intermediate
Mechanism of the Kulinkovich cyclopropanol synthesis: Transfer-epititanation of the alkene in generating the key titanacyclopropane intermediate
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DOI：
10.1002/ejoc.200300588
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发表时间：
2003-12-15
影响因子：
2.8
通讯作者：
Gitua, JN
中科院分区：
文献类型：
--
作者：
Eisch, JJ;Adeosun, AA;Gitua, JN
研究了Kulinkovich环丙醇合成反应，即酯与烷基格氏试剂和Ti（OiPr）（4）的2：1或3：1混合物在低温下的相互作用，以确定哪些反应中间体参与反应以及它们是如何相互转化的。由于最终产物的性质，最明显的中间体之一是1，1-二异丙氧基-1-钛环丙烷，其源自从所用烷基格氏试剂释放的烯烃的环钛酸化。通过在-78 ℃和+25 ℃之间加热Et 2 Ti（OiPr）（2）或iPr（2）Ti（OiPr）（2）的醚溶液，尝试用酯或腈进行化学捕集，来寻找这种钛环的形成。以这种方式，表明在Et 2 Ti（OiPr）2的情况下形成这样的钛环，但在iPr（2）Ti（OiPr）（2）的情况下不形成这样的钛环。至于另外两种潜在中间体Ti（OiPr）（2）和R2 Ti（OiPr）（2）的作用，已证明在乙烯和酯存在下预形成的Ti（OiPr）（2）不形成相应的环丙醇。因此，在反应条件下，Ti（OiPr）（2）不能进行生产必需的钛酰环丙烷所必需的直接表观金属化。另一方面，iPr（2）Ti（OiPr）（2）或Et_2Ti（OiPr）（2）可以在低温下实现乙烯的转移-环钛酸化，从而与苯甲酸甲酯生成1-苯基-1-环丙醇。相比之下，iPr（2）Ti（OiPr）（2）和Et_2Ti（OiPr）（2）都不能在低温下转移-环钛酸酯丙烯。烯烃反应性的这种差异可以归因于空间因素在所提出的八面体过渡态的转移-表观位阻。最后，通过将游离乙烯引入到这样的Kulinkovich反应混合物中，通过乙烯气体本身或三分之一当量的EtMgX，环丙醇的分离产率比用1：2比率的Ti（OiPr）（4）/EtMgX获得的那些高一倍以上。从这一观察可以得出结论，游离乙烯通过与Et 2 Ti（OiPr）（2）配位催化引发Kulinkovich反应，并经历转移-环钛酸化以产生必需的钛环丙烷，从而释放乙烯，乙烯使反应持续进行。((C)Wiley-VCH Verlag GmbH & Co. KGaA，69451魏因海姆，德国，2003）
An investigation of the Kulinkovich cyclopropanol synthesis, the interaction of esters with 2:1 or 3:1 mixtures of alkyl Grignard reagents and Ti(OiPr)(4) at low temperatures, has been conducted, in order to ascertain which reactive intermediates are involved and how they are interconverted. Because of the nature of the ultimate product, one of the most obvious intermediates is the 1,1-diisopropoxy-l-titanacyclopropane stemming from the epititanation of the alkene set free from the alkyl Grignard reagent employed. A search for the formation of such a titanocycle by warming an ethereal solution of either Et2Ti(OiPr)(2) or iPr(2)Ti(OiPr)(2) between -78 degreesC and +25 degreesC was attempted by chemical trapping with either an ester or nitrile. In this manner it was shown that such a titanocycle was formed in the case of Et2Ti(OiPr)2 but not with iPr(2)Ti(OiPr)(2). As to the role of two other potential intermediates, Ti(OiPr)(2) and R2Ti(OiPr)(2), it was demonstrated that preformed Ti(OiPr)(2) in the presence of ethylene and an ester does not form the corresponding cyclopropanol. Thus, under the reaction conditions Ti(OiPr)(2) cannot perform the direct epimetallation necessary to produce the requisite titanacy-clopropane. On the other hand, either iPr(2)Ti(OiPr)(2) or Et2Ti(OiPr)(2) can achieve the transfer-epititanation of ethylene at low temperatures and hence with methyl benzoate yield 1-phenyl-1-cyclopropanol. In contrast, neither iPr(2)Ti(OiPr)(2) nor Et2Ti(OiPr)(2) can at low temperatures transfer-epititanate propylene. This difference in alkene reactivity can be ascribed to steric factors operating in the proposed octahedral transition state for transfer-epimetallation. Finally, by introducing free ethylene into such Kulinkovich reaction mixtures, either by ethylene gas itself or a third equivalent of EtMgX, the isolated yields of cyclopropanols were more than doubled over those obtained with a 1:2 ratio of Ti(OiPr)(4)/EtMgX. From this observation one can conclude that free ethylene catalytically initiates the Kulinkovich reaction by coordinating with Et2Ti(OiPr)(2) and undergoes transfer-epititanation to produce the requisite titanacyclopropane and thereby liberates ethylene, which perpetuates the reaction. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)