The origin of exo-selectivity in methyl cyanoformate addition onto the C=C bond of norbornene in Pd-catalyzed cyanoesterification
The origin of exo-selectivity in methyl cyanoformate addition onto the C=C bond of norbornene in Pd-catalyzed cyanoesterification
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DOI：
10.1039/c4dt00839a
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发表时间：
2014-01-01
影响因子：
4
通讯作者：
Nishihara, Yasushi
中科院分区：
文献类型：
--
作者：
Okuda, Yasuhiro;Szilagyi, Robert K.;Nishihara, Yasushi
已进行了计算研究，以阐明在钯催化的应变环烯烃，异戊烯和降冰片二烯的氰酯化反应的独家exo-selectivity的起源。一个混合密度泛函的理论水平与三重zeta质量基组，这是在早期的研究中提出的多边缘X-射线吸收光谱测量钯（II）和钯（IV）配合物提供一个实验健全的基态电子结构描述。考虑到氧化加成的产物可以被分离，我们专注于烯烃配位作为外选择性的最早可能来源。在BHandHLYP/def 2 TZVP/ PCM（toluene）水平上计算的trans-Pd（CN）（COOR）（PPh 3）（2）配合物的几何结构与晶体学测量的实验结构非常一致。在其膦配体之一解离时，配位不饱和反式异构体距离异构化过渡态仅17 kJ mol（-1），导致14电子顺式异构体的能量比反式异构体低17至37 kJ mol（-1）。无论烯烃配位的初始络合物如何，异戊烯络合物的外消旋异构体比相应的内消旋异构体低至少8 kJ mol（-1）。吉布斯自由能的这一显著差异的起源可以归因于在Pd-II中心处的顺式配体和顺式配体的亚甲基和亚乙基桥之间的显著不同的空间和无规氢相互作用。
A computational investigation has been carried out to elucidate the origin of the exclusive exo-selectivity in the Pd-catalyzed cyanoesterification of strained cyclic olefins, norbornene and norbornadiene. A hybrid density functional was selected for the level of theory with a triple-zeta quality basis set, which was proposed in an earlier study to provide an experimentally sound ground state electronic structure description for palladium(II) and palladium(IV) complexes from multi-edge X-ray absorption spectroscopic measurements. Given that the product of oxidative addition can be isolated, we focused on the olefin coordination as the earliest possible origin of exo-selectivity. The calculated geometric structure for the trans-Pd(CN)(COOR)(PPh3)(2) complex at the BHandHLYP/ def2TZVP/ PCM(toluene) level is in an excellent agreement with its experimental structure from crystallographic measurements. Upon dissociation of one of its phosphane ligands, the coordinatively unsaturated trans-isomer is only 17 kJ mol(-1) away from the isomerization transition state, leading to the 14-electron cis-isomers that are 17 to 37 kJ mol(-1) lower in energy than the trans-isomers. Regardless of the initial complex for olefin coordination, the exo-isomer for the norbornene complex is at least 8 kJ mol(-1) lower than the corresponding endo-isomer. The origin of this considerable difference in Gibbs free energy can be attributed to the remarkably different steric and agostic hydrogen interactions between the methylene and the ethylene bridges of the norbornene and the adjacent cis-ligands at the Pd-II center.