Kinetics, stereochemistry, and mechanisms of the silaalylic and silapropynylic rearrangements
Kinetics, stereochemistry, and mechanisms of the silaalylic and silapropynylic rearrangements
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硅杂酸和硅杂丙炔重排的动力学、立体化学和机制
DOI：
10.1021/ja00807a029
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发表时间：
1973
影响因子：
15
通讯作者：
H. Kwart
中科院分区：
文献类型：
--
作者：
J. Slutsky;H. Kwart
硅烯丙基重排的单分子气相反应动力学、完全不存在/3取代基速率效应、在硅反应中心苯基取代甲基对激活参数的影响不足，以及没有实现双分子异构化模式，都表明硅迁移的过渡状态是一致的、对称的。唯一观察到的取代基速率效应很容易与空间应变缓解（在烯丙基侧链的a位置）和共轭影响相关，共轭影响反映了过渡态中产物和反应物之间的能量差异。这些结果与在硫代烯基重排中发现的结果形成鲜明对比，其中d轨道参与了偶极反应中间体的形成。利用光学活性和氘标记对重排过程进行的立体化学研究表明，每一次迁移都伴随着硅构型的反转。这表明在轨道对称守恒的情况下，更倾向于利用3p轨道来桥接烯丙基结构。这是硅的1,3或1,5迁移的第一个例子，在这种情况下，对3p轨道利用的偏好已经被确定。在这些研究中，也首次实现了硅在丙基-烯基基团末端迁移的硅丙基重排。该反应的动力学特征表明，该反应是一个对称的、协调的过渡态，尽管在激活步骤中需要弯曲丙炔-烯基框架，但在所有方面与硅烯基重排相似。此外，立体化学研究证实，迁移过程再次发生在硅构型完全反转的情况下，显然没有受到任何可能导致保留和外消旋的竞争途径的污染。在硅烯基底物的简单烯烃类似物中，碳烯基的迁移没有足够的迁移率来与均裂断裂反应竞争。碳烯基相对于硅烯基的更大的激活势垒可能与信号化过程中关键键的解离能有关。
The unimolecular, gas-phase reaction kinetics of the silaallylic rearrangement, the complete absence of/3-substituent rate effects, the lack of influence on the activation parameters in substituting phenyl for methyl at the silicon reaction center, and the failure to realize a bimolecular isomerization mode all point to a concerted, sym-metrical transition state of silicon migration. The only substituent rate effects observed are readily correlated with steric strainrelief (in the a position of the allyl side chain) and conjugation influences which reflect in the transition state the energy differences between products and reactants. These results stand in sharp contrast to those noted in the thiaallylic rearrangement where d orbital involvement in the formation of a dipolar reaction intermediate has been implicated. Stereochemicalstudies using optically active as well as deuterium labeling of the course of rearrangement show that every act of migration is accompanied by inversion of the siliconconfiguration. This indicates a preference for utilizing a 3p orbital in bridging the allylic structure with conservation of orbital symmetry. It is the first case of 1, 3 or 1, 5 migration of silicon in which thepreference for 3p orbital utilization has been identi-fied. The correspondingsilapropynylic rearrangement involving silicon migration across the termini for a propargyl-allenyl grouping has also been realized (for the first time) in these investigations. The kinetic characteristics of this reaction indicate a symmetrical, concerted transition state, similar in all respects tothe silaallylic rearrangement despite the need for bending of the propargyl-allenyl framework in the activation step. Moreover, stereochemical studies confirm that the process of migration again takes place with complete inversion of the silicon con-figuration, apparently uncontaminated byany competing pathways which might lead to retention and racemiza-tion. A carboallylic migration in simple olefinic analogs of silaallylic substrates does not take place with sufficient mobility to compete with homolytic fragmentation reactions. The vastly greater activation barrier of the carboallylic vs. the silaallylic perhaps may be correlated with the dissociation energy of the critical bond of the sigmatropic process.