Dipartimento di Scienze e Tecnologie Chimiche, Università Tor Vergata, Via della Ricerca Scientifica, 1 I-00133 Rome, Italy.
J Org Chem. 2013 Jun 21;78(12):5909-17. doi: 10.1021/jo400535u. Epub 2013 Jun 6.
A time-resolved kinetic study in acetonitrile and a theoretical investigation of hydrogen abstraction reactions from N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) by the cumyloxyl (CumO(•)) and benzyloxyl (BnO(•)) radicals was carried out. CumO(•) reacts with both substrates by direct hydrogen abstraction. With DMF, abstraction occurs from the formyl and N-methyl C-H bonds, with the formyl being the preferred abstraction site, as indicated by the measured kH/kD ratios and by theory. With DMA, abstraction preferentially occurs from the N-methyl groups, whereas abstraction from the acetyl group represents a minor pathway, in line with the computed C-H BDEs and the kH/kD ratios. The reactions of BnO(•) with both substrates were best described by the rate-limiting formation of hydrogen-bonded prereaction complexes between the BnO(•) α-C-H and the amide oxygen, followed by intramolecular hydrogen abstraction. This mechanism is consistent with the very large increases in reactivity measured on going from CumO(•) to BnO(•) and with the observation of kH/kD ratios close to unity in the reactions of BnO(•). Our modeling supports the different mechanisms proposed for the reactions of CumO(•) and BnO(•) and the importance of specific substrate/radical hydrogen bond interactions, moreover providing information on the hydrogen abstraction selectivity.
在乙腈中进行了时间分辨动力学研究，并通过枯基氧自由基（CumO(•)）和苯氧自由基（BnO(•)）理论研究了从 N,N-二甲基甲酰胺（DMF）和 N,N-二甲基乙酰胺（DMA）中提取氢的反应。CumO(•)通过直接提取氢与两种底物反应。对于 DMF，从甲酰基和 N-甲基 C-H 键发生提取，甲酰基是优先提取的位置，这可以通过测量的 kH/kD 比和理论来证明。对于 DMA，优先从 N-甲基基团中提取，而从乙酰基中提取则是次要途径，这与计算的 C-H BDEs 和 kH/kD 比一致。BnO(•)与两种底物的反应最好通过 BnO(•)α-C-H 和酰胺氧之间形成氢键预反应复合物的限速来描述，然后是分子内氢提取。该机制与从 CumO(•)到 BnO(•)所测反应性的大幅增加一致，并且在 BnO(•)的反应中观察到 kH/kD 比接近 1。我们的建模支持了 CumO(•)和 BnO(•)反应提出的不同机制以及特定的底物/自由基氢键相互作用的重要性，此外还提供了关于氢提取选择性的信息。
