Freccero Mauro, Gandolfi Remo, Sarzi-Amadè Mirko, Rastelli Augusto
Dipartimento di Chimica Organica, Università di Pavia, V.le Taramelli 10, 27100 Pavia, Italy.
J Org Chem. 2005 Nov 11;70(23):9573-83. doi: 10.1021/jo0515982.
[Reaction: see text]. RB3LYP calculations, reported here, indicate that peroxy acid s-cis conformer is more stable than its s-trans counterpart, in agreement with experimental data. Difference in stability is the highest in the gas phase, but it falls considerably on going from the gas phase to moderately polar solvent. In the case of peroxy formic acid, the enthalpy (free energy) difference is about 3.4 (2.5) kcal/mol, respectively, in the gas phase but decreases to 1.2 (0.6) kcal/mol in dichloromethane solution. Introduction of an alkyl or aryl substituent on the peroxy acid, that is, on passing to peroxy acetic, peroxy benzoic (PBA), and m-chloroperoxy benzoic acid (MCPBA), adds a further significant (1.0-1.5 kcal/mol) favor to the s-cis isomer. RB3LYP/6-31+G(2d,p) calculations on the epoxidation of 2-propenol with peroxy formic and peroxy benzoic acids, respectively, suggest that the less stable peroxy acid s-trans conformer can compete with the more stable s-cis form in epoxidation reaction of these substrates. Transition structures arising from s-trans peroxy acids ("trans" TSs) retain both the well-established, for "cis" TS, perpendicular orientation of the O-H peroxy acid bond relative to the C=C bond and the one-step oxirane ring formation. These TSs collapse to the final epoxide via a 1,2-H shift at variance with the 1,4-H transfer of the classical Bartlett's "cis" mechanism. The "trans" reaction pathways have a higher barrier in the gas phase than the "cis" reaction channels, but in moderately polar solvents they become competitive. In fact, the "trans" TSs are always significantly more stabilized than their "cis" counterparts by solvation effects. Calculations also suggest that going from peroxy formic to peroxy benzoic acid should slightly disfavor the "trans" route relative to the "cis" one, reflecting, in an attenuated way, the decrease in the peroxy acid s-trans/s-cis conformer ratio. The predicted behavior for MCPBA parallels that of PBA acid.
[反应：见正文]。本文报道的RB3LYP计算表明，过氧酸的s-顺式构象比其s-反式构象更稳定，这与实验数据一致。稳定性差异在气相中最大，但从气相到中等极性溶剂时会显著降低。就过氧甲酸而言，在气相中焓（自由能）差分别约为3.4（2.5）kcal/mol，但在二氯甲烷溶液中降至1.2（0.6）kcal/mol。在过氧酸上引入烷基或芳基取代基，即转变为过氧乙酸、过氧苯甲酸（PBA）和间氯过氧苯甲酸（MCPBA）时，会进一步显著（1.0 - 1.5 kcal/mol）有利于s-顺式异构体。分别用RB3LYP/6 - 31 + G(2d,p)对2 - 丙烯醇用过氧甲酸和过氧苯甲酸进行环氧化反应的计算表明，稳定性较差的过氧酸s-反式构象在这些底物的环氧化反应中能与更稳定的s-顺式构象竞争。由s-反式过氧酸产生的过渡态结构（“反式”TSs）既保留了“顺式”TS中已确立的O - H过氧酸键相对于C = C键的垂直取向，也保留了一步形成环氧乙烷环的过程。这些TSs通过1,2 - H迁移分解为最终的环氧化物，这与经典巴特利特“顺式”机理的1,4 - H转移不同。“反式”反应途径在气相中的势垒比“顺式”反应通道高，但在中等极性溶剂中它们变得具有竞争力。实际上，“反式”TSs总是比其“顺式”对应物更能通过溶剂化效应得到显著稳定。计算还表明，从过氧甲酸到过氧苯甲酸，相对于“顺式”途径，“反式”途径应略有不利，以减弱的方式反映出过氧酸s-反式/s-顺式构象比的降低。MCPBA的预测行为与PBA酸相似。
