The interplay of electronic, steric and stereoelectronic effects in hydrogen-atom abstraction reactions of SO4–˙, revealed by EPR spectroscopy
The interplay of electronic, steric and stereoelectronic effects in hydrogen-atom abstraction reactions of SO4–˙, revealed by EPR spectroscopy
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EPR 光谱揭示了 SO4–˙ 氢原子夺取反应中电子、空间和立体电子效应的相互作用
DOI：
10.1039/a903399h
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发表时间：
1999
期刊：
影响因子：
--
通讯作者：
A. Whitwood
中科院分区：
文献类型：
--
作者：
B. C. Gilbert;J. Smith;P. Taylor;S. Ward;A. Whitwood
用EPR光谱直接跟踪了SO4˙（由光解和单电子转移产生）对许多碳水化合物和一些模型化合物的攻击。SO4 -˙是一种反应性的亲电物质，但比HO˙更具选择性（反应性更低）。我们为极性效应和自由基稳定的重要性提供证据，特别是在确定选择性趋势方面。例如，在五元环（如THF、蔗糖、β- d -果糖呋喃糖）中，C-H萃取α优先于环状氧原子，以及从β- d -葡萄糖中提取轴向C1氢原子。SO4 -˙与α- d -葡萄糖的反应对C2、C5和C6位具有选择性。这反映了β-氧取代基的激活作用，其中自由基轨道可以遮蔽β-C-O键，提供SOMO -σ *相互作用，稳定了正在形成的自由基中心。对于肌醇，也可以从轴向的C-H键中提取出来，因为轴向的β-OH取代基稳定了自由基，但由于没有环氧原子，从赤道的C-H键中提取出来的速度更快。
Attack by SO4–˙ (generated by photolysis and one-electron transfer) on a number of carbohydrates and some model compounds has been followed directly by EPR spectroscopy.SO4–˙ is a reactive, electrophilic species but is more selective (less reactive) than HO˙. We present evidence for the importance of polar effects and radical stabilisation, especially in determining trends in selectivity. Examples include the preference for C–H abstraction α to a ring oxygen atom in a five-membered ring (e.g. THF, sucrose, β-D-fructofuranose) and abstraction of the axial C1 hydrogen atom from β-D-glucose.The reaction of SO4–˙ with α-D-glucose is selective towards the C2, C5 and C6 positions. This reflects the activating effect of a β-oxygen substituent where the radical orbital can eclipse the β-C–O bond providing a SOMO–σ* interaction which stabilises the developing radical centre. For myo-inositol, abstraction is also observed from axial C–H bonds for which the resulting radicals are stabilised by an axial β-OH substituent but, due to the absence of a ring oxygen atom, more rapid abstraction occurs from the equatorial C–H bond.