DO BARRIERS EXIST FOR NUCLEOPHILIC SUBSTITUTION AT TETRAVALENT SILICON IN THE GAS PHASE? AN AB INITIO AND ION CYCLOTRON RESONANCE STUDY
DO BARRIERS EXIST FOR NUCLEOPHILIC SUBSTITUTION AT TETRAVALENT SILICON IN THE GAS PHASE? AN AB INITIO AND ION CYCLOTRON RESONANCE STUDY
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DOI：
10.1002/chin.198515070
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发表时间：
1984-12
期刊：
影响因子：
--
通讯作者：
J. C. Sheldon;R. Hayes;J. Bowie
中科院分区：
文献类型：
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作者：
J. C. Sheldon;R. Hayes;J. Bowie
从头计算预测，氢与硅烷的低压气相反应产生H3Si~的途径是：(1)氢与氢的反应，(2)从通电的三角-双锥体中间H5Si中消去氢。对于MeO ‘ ’与硅烷（去除甲醇）也提出了类似的反应，并由ICR实验证实。任何亲核试剂对硅烷的攻击都是在没有能垒的情况下进行的。相反，计算表明，HO“和MeO”最初应通过甲基氢与甲基硅烷反应，生成[NuH—" CH2SiH3]类型的物质。这些配合物预计寿命很短，因为只有一个小屏障[20-35 kJ mol'1 (3-21G)]将它们与更稳定的三角-双锥体物质[Me (Nu) SiH3 ']分开。ICR实验未能检测到各种甲基硅烷与RO”或[RO”—HOR]之间的长寿命h键物种。这些反应和类似反应中的障碍不影响加合物的形成速率，因为观察到这些加合物的形成速率非常高，并且明显受到碰撞控制。从头算和MNDO计算预测，当亲核试剂和附着基团较大时（如I'/Sil4），五坐标硅加合物仍然稳定。因此，硅上的SN2反应机制与碳上的绝不相似。
Ab initio calculations predict that the low-pressure gas-phase reaction of H" with silane should produce H3Si~ by (i) reaction at hydrogen and (ii) by elimination of hydrogen from the energized trigonal-bipyramidal intermediate H5Si". Similar reactions are suggested for MeO" with silane (with the eliminationof methanol), and this is confirmed by ICR experiments. The attackof eithernucleophile on silane proceeds without energy barriers. In contrast, calculations indicate that HO" and MeO" should initially react with methylsilanes through a methyl hydrogento give species of the type [NuH---" CH2SiH3]. These complexes are expected to be short lived since only a small barrier [20-35 kJ mol'1 (3-21G)] separates them from the more stable trigonal-bipyramidal species [Me (Nu) SiH3"]. ICR experiments fail to detectlong-lived H-bonded species between various methylsilanes and RO" or [RO"---HOR]. Barriers in these and similar reactionsdo not affect the rates of formation of adducts since these are observed to be very high and apparently collision controlled. Ab initio and MNDO calculations predict that when the nucleophile and the attached groups are large (eg, I'/Sil4), five-coordinate silicon adducts are still stable. Thus the SN2 mechanism on silicon never resembles that of carbon.