Regioselectivity of the base-induced ring cleavage of 1-oxygenated derivatives of cyclobutabenzene
Regioselectivity of the base-induced ring cleavage of 1-oxygenated derivatives of cyclobutabenzene
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DOI：
10.1002/hlca.19980810207
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发表时间：
1998-01-01
影响因子：
1.8
通讯作者：
Schiess, P
中科院分区：
文献类型：
--
作者：
Gokhale, A;Schiess, P
由1，2-二氢环丁二烯-1-酮I通过加入带电亲核试剂或由1-羟基-1，2-二氢环丁二烯2通过用碱去质子化产生的氧阴离子3通过经苄基碳负离子4和/或芳基碳负离子5的张力四元环的远端和/或近端裂解产生稳定的产物。对这一过程的系统研究揭示了两种异构碳负离子4和5的相对稳定性是决定开环反应过程的关键因素。虽然苄基碳负离子4可以用碳亲电试剂捕获，但用H+以外的亲电试剂捕获芳基碳负离子5的尝试失败了。在质子溶剂中，叔醇2的镁盐与其碱金属盐相比显示出增加的近端裂解速率。由此，我们得出结论，与苄基碳负离子4相反，游离芳基碳负离子5仅是暂时存在的。近端C，C-键裂解似乎发生通过质子化5从一个快速的，可逆的平衡3可逆箭头5，其中3强烈占主导地位，或在质子溶剂中，甚至可能通过限速质子化3在芳香族C-原子，绕过自由阴离子5。因此，除了异构碳负离子4和5的相对稳定性之外，其他因素在确定酮1和醇2中碱诱导的C，C-键断裂的区域化学中是重要的。
Oxy anions 3 generated from 1,2-dihydrocyclobutabenzen-1-ones I through addition of a charged nucleophile or from 1-hydroxy-1,2-dihydrocyclobutabenzenes 2 by deprotonation with base lead to stable products through distal and/or proximal cleavage of the strained four-membered ring via benzyl carbanion 4 and/or aryl carbanion 5. A systematic study of this process reveals the relative stability of the two isomeric carbanions 4 and 5 as a key factor in determining the course of the ring-cleavage reaction. While benzyl carbanions 4 can be trapped with carbon electrophiles, attempts at trapping aryl carbanions 5 with electrophiles other than H+ failed. In protic solvents, the magnesium salt of the tertiary alcohol 2 shows an increased rate of proximal cleavage as compared to its alkali salts. From this, we conclude that, in contrast to benzyl carbanions 4, free aryl carbanions 5 are of transient existence only. Proximal C,C-bond cleavage seems to occur either through protonation of 5 from a fast, reversible equilibrium 3 reversible arrow 5 in which 3 strongly predominates, or in protic solvents possibly even through a rate-limiting protonation of 3 at the aromatic C-atom, bypassing free anion 5 altogether. Thus, additional factors other than just the relative stability of isomeric carbanions 4 and 5 are of importance in determining the regiochemistry of the base-induced C,C-bond cleavage in ketones 1 and in alcohols 2.