REACTIVITY OF THE THIAZOLIUM C2 YLIDE IN APROTIC-SOLVENTS - NOVEL EXPERIMENTAL-EVIDENCE FOR ADDITION RATHER THAN INSERTION REACTIVITY
REACTIVITY OF THE THIAZOLIUM C2 YLIDE IN APROTIC-SOLVENTS - NOVEL EXPERIMENTAL-EVIDENCE FOR ADDITION RATHER THAN INSERTION REACTIVITY
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DOI：
10.1021/jo00017a010
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发表时间：
1991-08-16
影响因子：
3.6
通讯作者：
JORDAN, F
中科院分区：
文献类型：
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作者：
CHEN, YT;JORDAN, F
在C2位置合成了两种噻唑类化合物：3,4,5-三甲基[2-C-13]硝酸噻唑和3-苄基-5-（β -乙氧基乙基）-4-甲基[2-C-13]溴噻唑，并利用C-13 NMR研究了它们在强碱介质中的二聚化途径。当加入少于1等量碱时，n -甲基离子首先形成不对称二聚体，其中两个分子的C2原子相互结合，其中只有一个仍然携带氢；不对称二聚体在加入过量碱后，在C2处失去了剩余的氢，转化为数量几乎相等的顺二聚体和反二聚体的混合物。n -甲基衍生物上碱的加成顺序与共轭碱在C2位置上对第二个噻唑离子的亲核加成顺序一致。由于首先形成非对称二聚体，而不是对称二聚体，后者不能由两个共轭碱（酰化物）通过碳烯机制直接二聚化产生。相反，碳加成机理得到了两个实验的进一步支持。设计了一个“交叉”实验，在含有[2-C-13]-H和[2-C-12]-D的噻唑离子中加入限制性叔丁二氧化钾，在C2位置观察到很少H/D交换的条件下，在Me2SO中检测到不对称二聚体。同时，n -3-烯基噻唑离子也被合成，如果碳反应存在，就会生成环丙烷。相对于分子内反应，分子间不对称二聚体的形成与亲核加成反应一致。在n -苄基噻唑离子上添加碱后，通过C-13 NMR检测到的第一个产物是正/反对称二聚体混合物（再次通过C2原子键合），其中一个苄基从N3到C2发生了[1,3]-异位重排。根据混合后几分钟内的H-1核磁共振记录，这种盐的非对称二聚体也先于对称二聚体。由n -甲基和n -苄基噻唑离子衍生的C2基内酯的反应活性可以根据离子加成反应来解释，这意味着相关的维生素B1偶联碱（基内酯）具有类似的反应行为。
Two thiazolium compounds were synthesized specifically labeled at their C2 positions: 3,4,5-trimethyl[2-C-13]thiazolium nitrate and 3-benzyl-5-(beta-ethoxyethyl)-4-methyl[2-C-13]thiazolium bromide, with a view to examine their pathways leading to dimerization in strongly basic medium using C-13 NMR. On addition of less than 1 equiv of base the N-methyl ion first formed an unsymmetrical dimer in which the C2 atoms of two molecules were bonded to each other and only one of them still carried a hydrogen; that unsymmetrical dimer upon addition of excess base lost the remaining hydrogen at C2 and was converted to a mixture of syn and anti symmetrical dimers in nearly equal amounts. The sequence of observations on addition of base to the N-methyl derivative is consistent with nucleophilic addition of the conjugate base to a second thiazolium ion at its C2 position. Since the unsymmetrical dimer is formed first, rather than the symmetrical dimer, the latter cannot result from direct dimerization of two conjugate bases (ylides) by a carbene mechanism. Instead, a carbanion-addition mechanism was further supported by two experiments. A ''crossover'' experiment was designed in which unsymmetrical dimers were detected in Me2SO on addition of limiting potassium tert-butoxide to thiazolium ions containing [2-C-13]-H and [2-C-12]-D, under conditions such that there was little H/D exchange observed at the C2 position. Also, N-3-alkenylthiazolium ions were synthesized, that, if carbenic reactivity had existed, would have resulted in formation of cyclopropanes. In preference to the intramolecular reaction, intermolecular unsymmetrical dimers resulted in each case, consistent with nucleophilic addition. On addition of base to the N-benzylthiazolium ion, the first product to be detected by C-13 NMR was the syn/anti symmetrical dimer mixture (again bonded via the C2 atoms), that underwent a [1,3]-sigmatropic rearrangement of one of the benzyl groups from N3 to C2. According to H-1 NMR recorded within minutes of mixing, the unsymmetrical dimer precedes the symmetrical one for this salt as well. The reactivity of the C2 ylide derived from the N-methyl and N-benzylthiazolium ions can be rationalized according to an ionic addition reaction, implying that the related thiamin (vitamin B1) conjugate base (ylide) behaves similarly.