SOLVOLYSIS OF D-GLUCOPYRANOSYL DERIVATIVES IN MIXTURES OF ETHANOL AND 2,2,2-TRIFLUOROETHANOL
SOLVOLYSIS OF D-GLUCOPYRANOSYL DERIVATIVES IN MIXTURES OF ETHANOL AND 2,2,2-TRIFLUOROETHANOL
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DOI：
10.1021/ja00526a043
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发表时间：
1980-01-01
影响因子：
15
通讯作者：
JENCKS, WP
中科院分区：
文献类型：
--
作者：
SINNOTT, ML;JENCKS, WP
用GLC法测定了α-和β-D-吡喃葡萄糖基氟化物、2，4-二硝基苯基β-D-吡喃葡萄糖甙以及β-D-吡喃葡萄糖基3-溴吡啶和β-D-吡喃葡萄糖基4-甲基吡啶离子的三氟甲磺酸盐在乙醇和三氟乙醇等摩尔混合物中的溶剂分解产物。在等摩尔的乙醇和三氟乙醇的混合物中，用三氟甲磺酸催化苯基α-和β-D-吡喃葡萄糖苷的溶剂分解的初始产物，以及β-D-吡喃葡萄糖基-对硝基苯基三氮烯的非催化溶剂分解的产物，也同样进行了检测。用于葡萄糖基氟化物的溶剂分解的介质的组成也从纯乙醇到纯三氟乙醇系统地变化。异头构型与原料相同的产物占8.1-88.5%，异头构型不变时，离去基团的变化或离去基团不变时，异头构型的变化导致不同的产物分布。因此，产物决定步骤的过渡态含有离去基团。与三氟乙醇相比，对乙醇攻击的偏好在0.9至20之间变化，在某种程度上显示保留或转化途径之间没有一般的系统区别。保留的亲核选择性降低阴离子离去基团，特别是氟化物，它优先稳定的过渡态containingtrifluoroethanol通过氢键。在α面的亲核攻击比在α面的亲核攻击更优选，并且表现出较低的选择性：这归因于2-羟基的氧原子与接近的醇的羟基之间的氢键。涉及可逆形成的离子对或相遇复合物的溶剂分解模型与仍然观察到的选择性不相容，其中离去基团的亲核性低于溶剂组分：涉及通过不可逆形成的离子对或相遇复合物在溶剂分子池的组分之间进行选择的模型需要一个难以置信的大池来解释观察到的特异性。因此，可以得出结论，所观察到的选择性是由溶剂的离去基团的离开，从反应中心的任一侧的促进的结果。
The products of solvolysis of a-and/3-D-glucopyranosyl fluorides, 2, 4-dinitrophenyl/3-D-glucopyranoside, and the trifluoromethanesulfonates of the/3-D-glucopyranosyl 3-bromopyridinium and-D-glucopyranosyl 4-methylpyridinium ions in an equimolar mixture of ethanoland trifluoroethanol buffered with~ 2 equiv of 2, 6-lutidine have been examined by GLC of their trimethylsilyl ethers. The initial products of the solvolyses of phenyl a-and/3-D-glucopyranosides catalyzed by trifluo-romethanesulfonic acid in an equimolar mixture of ethanol and trifluoroethanol, and the products of uncatalyzed solvolysis of/3-D-glucopyranosyl-p-nitrophenyltriazene, have been likewise examined. The composition of the medium for solvolysis of the glucosyl fluorides has also been systematically varied from pure ethanol to pure trifluoroethanol. Thepercentage of products with the same anomeric configuration as the starting material is in the range 8.1-88.5%; change of leaving group, at con-stant anomeric configuration, or of anomeric configuration, at constant leaving group, yields different product distributions. Therefore the transition state for the product-determining step contains the leaving group. The preference for attack by ethanol as compared with trifluoroethanol varies from 0.9 to 20 in a way which shows no general systematic distinction between pathways for retention or inversion. The nucleophilic selectivity for retention is lowered by anionic leaving groups, especially fluoride, which preferentially stabilizethe transition state containingtrifluoroethanol by hydrogen bonding. Nucleophilic at-tack at the a face is preferred over nucleophilic attack at the ß face, and exhibits a lower selectivity: this is ascribed to hydrogen bonding between the oxygen atom of the 2-hydroxyl groupand the hydroxyl group of the approaching alcohol. A model for sol-volysis involving a reversibly formed ion pair or encounter complex is incompatible with the selectivities still observed with leaving groups less nucleophilic than the solvent components: a model involving selection between the components of a pool of solvent molecules by an irreversibly formedion pair or encounter complex requires an implausibly largepool to explain ob-served specificities. It is therefore concluded that the observed selectivities are a consequence of the facilitation of the departure of the leaving group by the solvent, from either side of the reaction center.