Wittig modification of the Hofmann elimination reaction. Evidence for an .alpha.',.beta. mechanism
Wittig modification of the Hofmann elimination reaction. Evidence for an .alpha.',.beta. mechanism
复制标题
霍夫曼消除反应的维蒂希修改。
DOI：
10.1021/ja00779a069
复制
发表时间：
1972
影响因子：
15
通讯作者：
D. Andrzejewski
中科院分区：
文献类型：
--
作者：
R. D. Bach;K. Bair;D. Andrzejewski
反应时间为5小时。6所有值均针对MW二甲基环辛胺的同位素纯度进行校正。合成和氘分析如先前报道的那样进行。2 c这种同位素效应应根据抗E2消除量进行校正。d校正反式-环辛烯的量以将RLi添加到应变双键。反式烯烃在反应条件下不异构。涉及叶立德的初始形成，随后是分子内质子从β-碳原子上的提取，从而形成烯烃。α，β-消除的含义主要基于顺/反烯烃比。用甲基锂处理溴甲基二甲基环辛基溴化铵得到90/10的顺式/反式烯烃比的观察结果1为这种消除模式提供了令人信服的证据。活化苄基2的氮上的取代也提供95/5的顺式/反式烯烃比。此外，氧化胺3的科普消除通过分子内五元环状过渡态进行，仅提供顺式环辛烯。相反，通过氢氧化物热解的正常霍夫曼消除3和在液氨中KNH 2诱导的1消除1得到反式烯烃作为主要产物。在后面的实施例中，调用了E2消除。1 Coke 4最近在氘标记实验的基础上排除了Hofmann消除中的叶立德机制。然而，我们最近已经确定，在液氨中用KNH 2消除二甲基-正丁基环辛基碘化铵（2）仅通过反式消除进行，得到比例为32：68.2的顺式和反式环辛烯。原则上，如果涉及反式消除，则顺式/反式比例应该与所用的碱无关。在苯基锂和KNH 2中观察到的顺式/反式比例的差异促使我们研究霍夫曼消除的维蒂希修饰，以在氘标记实验的基础上建立机制。消除过程中碱基离子配对的最新报道
“Reaction time was 5 hr. 6 All values are corrected for the isotopicpurity of the MWdimethylcyclooctylamines. Syntheses and deu-terium analyses were carried out as previously reported. 2 c This isotope effect should be corrected for amount of anti E2 elimination. d The amount of fra/w-cyclooctene is corrected for addition of RLi to the strained double bond. The trans olefin is notisomerized under the reaction conditions. volved initial formation of an ylide followed by intra-molecular proton abstraction from the/3-carbon atom with resultantolefin formation. The implicationof an', ß elimination was based principally upon the cis/trans olefin ratio. Convincing evidence for this mode of elimination was provided by the observation1 that treatment of bromomethyldimethylcyclooctylammonium bromide with methyllithium gave a 90/10 cis/trans olefin ratio. Substitution on nitrogen of an ac-tivating benzyl group2 also affords a 95/5 cis/trans olefin ratio. In addition, the Cope elimination of the amine oxide, 3 which proceeds by an intramolecular five-membered cyclic transition state, affords exclusively ciscyclooctene. In contrast, the normal Hofmann elim-ination via pyrolysis of the hydroxide3 and the KNH2 induced elimination1 of 1 in liquid ammonia afforded the trans olefin as the dominant product. In the latter examples, an E2 elimination was invoked. 1 Coke4 has recently excluded an ylide mechanism in the Hofmann elimination on the basis of deuterium labelingexperi-ments. However, we have recently established that elimination of dimethyl-n-butylcyclooctylammonium iodide (2) in liquid ammonia with KNH2 proceeds exclusively by an'ß elimination to afford both cis-and trans-cyclooctene in a ratio of 32: 68.2 In principle, the cis/trans ratio should be independent of the base used if an a'ß elimination is involved. The discrepancy in cis/trans ratios observed with phenyllithium and KNH2 prompted us to investigate the Wittig modification of the Hofmann elimination to establish the mechanism on the basis of deuterium labeling experiments. Recent reports5 on base ion-pairing in elimination