Reactions of charged substrates .7. The methoxymethyl carbenium ion problem .2. A semiempirical study of the kinetic and thermodynamic stabilities of linear and cyclic oxo- and thiocarbenium ions generated from aldehyde hydrates, hemiacetals, acetals, and methyl ribosides and glucosides
Reactions of charged substrates .7. The methoxymethyl carbenium ion problem .2. A semiempirical study of the kinetic and thermodynamic stabilities of linear and cyclic oxo- and thiocarbenium ions generated from aldehyde hydrates, hemiacetals, acetals, and methyl ribosides and glucosides
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DOI：
10.1021/jo960748t
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发表时间：
1996-11-15
影响因子：
3.6
通讯作者：
Oppenheimer, NJ
中科院分区：
文献类型：
--
作者：
Buckley, N;Oppenheimer, NJ
利用半经验计算（AM1和PM3）研究了影响线性和环状醛水合物、半缩醛、缩醛以及甲基核糖苷和葡萄糖苷中碳 - 氧键断裂的因素。（对于某些体系，有低水平和高水平的从头算能量可与半经验结果进行比较。除了一个例外，这两种方法得到的结果在相对能量和反应性趋势方面显示出极好的一致性。）还研究了在氧鎓离子中用硫取代一个氧对反应性和稳定性的影响。一般来说，离去基团上孤对电子与潜在的氧鎓离子氧能够呈反式共平面排列的体系会自发断裂。然而，对离去基团相对于潜在氧鎓离子氧的各种构象的研究表明，对于甲醇作为离去基团，孤对电子排斥和空间因素比反式共平面效应对键断裂更为重要。所有潜在碳正离子中的α - 氧被硫取代的化合物，无论离去基团或化合物的结构如何，都会自发断裂。能量曲线、ΔH‡和ΔHR值表明，线性和环状硫鎓离子比相应的氧鎓离子稳定得多。将甲基核糖苷和葡萄糖苷的结果与相应的吡啶鎓底物的结果进行比较表明，两者都应通过A - 1机制水解。研究了以水合氢离子作为酸的广义酸催化。结合溶液结果，计算表明，具有良好离去基团或稳定氧鎓离子的底物，其反应是酸向离去基团的质子转移为限速步骤，但同时具有良好离去基团和稳定碳正离子的底物，其反应是质子转移和键断裂协同进行的。
Factors affecting the cleavage of the carbon-oxygen bond in linear and cyclic aldehyde hydrates, heimacetals, acetals, and methyl ribosides and glucosides have been investigated using semiempirical calculations (AM1 and PM3), (For some systems, low- and high-level ab initio energies are available for comparison with the semiempirical results, With one exception, the results obtained by the two methods show excellent agreement in relative energies and trends in reactivity,) The effects on reactivity and stability caused by substituting a sulfur for the a oxygen in the oxocarbenium ion were also studied. In general, systems that can have an antiperiplanar alignment of lone pairs on the leaving group and potential oxocarbenium ion oxygens undergo spontanteous cleavage. An examination of various conformers of the leaving group relative to the potential oxocarbenium oxygen shows, however, that lone pair repulsion and steric factors for MeOH as the leaving group are more important than the antiperiplanar effect for bond cleavage, All compounds in which the a-oxygen in the potential carbenium ion is replaced by sulfur undergo spontaneous cleavage regardless of the leaving group or structure of the compound. Energy profiles, Delta H-double dagger, and Delta H-R values show that linear and cyclic thiocarbenium ions are much more stable than the corresponding oxocarbenium ions. Comparison of results for methyl ribosides and glucosides with results for corresponding pyridinium substrates suggests that both should hydrolyze through an A-1 mechanism, General-acid catalysis with hydronium as the acid was studied, With solution results, the computations suggest that substrates with either a good leaving group or stable oxocarbenium ion react with rate-limiting proton transfer from the acid to the leaving group but that substrates with both a good leaving group and stable carbenium ion react with concerted proton transfer and bond cleavage.