Wu Adam, Mader Elizabeth A, Datta Ayan, Hrovat David A, Borden Weston Thatcher, Mayer James M
Department of Chemistry, Campus Box 351700, University of Washington, Seattle, Washington 98195-1700, USA.
J Am Chem Soc. 2009 Aug 26;131(33):11985-97. doi: 10.1021/ja904400d.
Bimolecular rate constants have been measured for reactions that involve hydrogen atom transfer (HAT) from hydroxylamines to nitroxyl radicals, using the stable radicals TEMPO() (2,2,6,6-tetramethylpiperidine-1-oxyl radical), 4-oxo-TEMPO() (2,2,6,6-tetramethyl-4-oxo-piperidine-1-oxyl radical), di-tert-butylnitroxyl ((t)Bu(2)NO()), and the hydroxylamines TEMPO-H, 4-oxo-TEMPO-H, 4-MeO-TEMPO-H (2,2,6,6-tetramethyl-N-hydroxy-4-methoxy-piperidine), and (t)Bu(2)NOH. The reactions have been monitored by UV-vis stopped-flow methods, using the different optical spectra of the nitroxyl radicals. The HAT reactions all have |DeltaG (o)| < or = 1.4 kcal mol(-1) and therefore are close to self-exchange reactions. The reaction of 4-oxo-TEMPO(*) + TEMPO-H --> 4-oxo-TEMPO-H + TEMPO() occurs with k(2H,MeCN) = 10 +/- 1 M(-1) s(-1) in MeCN at 298 K (K(2H,MeCN) = 4.5 +/- 1.8). Surprisingly, the rate constant for the analogous deuterium atom transfer reaction is much slower: k(2D,MeCN) = 0.44 +/- 0.05 M(-1) s(-1) with k(2H,MeCN)/k(2D,MeCN) = 23 +/- 3 at 298 K. The same large kinetic isotope effect (KIE) is found in CH(2)Cl(2), 23 +/- 4, suggesting that the large KIE is not caused by solvent dynamics or hydrogen bonding to solvent. The related reaction of 4-oxo-TEMPO() with 4-MeO-TEMPO-H(D) also has a large KIE, k(3H)/k(3D) = 21 +/- 3 in MeCN. For these three reactions, the E(aD) - E(aH) values, between 0.3 +/- 0.6 and 1.3 +/- 0.6 kcal mol(-1), and the log(A(H)/A(D)) values, between 0.5 +/- 0.7 and 1.1 +/- 0.6, indicate that hydrogen tunneling plays an important role. The related reaction of (t)Bu(2)NO() + TEMPO-H(D) in MeCN has a large KIE, 16 +/- 3 in MeCN, and very unusual isotopic activation parameters, E(aD) - E(aH) = -2.6 +/- 0.4 and log(A(H)/A(D)) = 3.1 +/- 0.6. Computational studies, using POLYRATE, also indicate substantial tunneling in the (CH(3))(2)NO() + (CH(3))(2)NOH model reaction for the experimental self-exchange processes. Additional calculations on TEMPO(()/H), (t)Bu(2)NO(()/H), and Ph(2)NO(()/H) self-exchange reactions reveal why the phenyl groups make the last of these reactions several orders of magnitude faster than the first two. By inference, the calculations also suggest why tunneling appears to be more important in the self-exchange reactions of dialkylhydroxylamines than of arylhydroxylamines.
已经测量了涉及氢原子从羟胺转移（HAT）到硝酰基自由基的反应的双分子速率常数，使用稳定自由基TEMPO（）（2,2,6,6 - 四甲基哌啶 - 1 - 氧基自由基）、4 - 氧代 - TEMPO（）（2,2,6,6 - 四甲基 - 4 - 氧代 - 哌啶 - 1 - 氧基自由基）、二叔丁基硝酰基（（t）Bu₂NO（））以及羟胺TEMPO - H、4 - 氧代 - TEMPO - H、4 - MeO - TEMPO - H（2,2,6,6 - 四甲基 - N - 羟基 - 4 - 甲氧基 - 哌啶）和（t）Bu₂NOH。通过紫外 - 可见停流法监测反应，利用硝酰基自由基的不同光谱。氢原子转移反应的|ΔG⁰|均≤1.4 kcal mol⁻¹，因此接近自交换反应。4 - 氧代 - TEMPO（） + TEMPO - H → 4 - 氧代 - TEMPO - H + TEMPO（）的反应在298 K的乙腈中k(₂H,MeCN) = 10 ± 1 M⁻¹ s⁻¹（K(₂H,MeCN) = 4.5 ± 1.8）。令人惊讶的是，类似的氘原子转移反应的速率常数要慢得多：在298 K时k(₂D,MeCN) = 0.44 ± 0.05 M⁻¹ s⁻¹，k(₂H,MeCN)/k(₂D,MeCN) = 23 ± 3。在二氯甲烷中也发现了相同的大动力学同位素效应（KIE），为23 ± 4，这表明大KIE不是由溶剂动力学或与溶剂的氢键作用引起的。4 - 氧代 - TEMPO（）与4 - MeO - TEMPO - H（D）的相关反应也有大KIE，在乙腈中k(₃H)/k(₃D) = 21 ± 3。对于这三个反应，E(aD) - E(aH)值在0.3 ± 0.6至1.3 ± 0.6 kcal mol⁻¹之间，log(A(H)/A(D))值在0.5 ± 0.7至1.1 ± 0.6之间，表明氢隧穿起重要作用。（t）Bu₂NO（） + TEMPO - H（D）在乙腈中的相关反应有大KIE，在乙腈中为16 ± 3，以及非常不寻常的同位素活化参数，E(aD) - E(aH) = -2.6 ± 0.4，log(A(H)/A(D)) = 3.1 ± 0.6。使用POLYRATE进行的计算研究也表明在实验自交换过程的（CH₃）₂NO（） + （CH₃）₂NOH模型反应中有大量隧穿。对TEMPO（* / H）、（t）Bu₂NO（* / H）和Ph₂NO（* / H）自交换反应的额外计算揭示了苯基为何使这些反应中的最后一个比前两个快几个数量级。由此推断，这些计算还表明了为什么在二烷基羟胺的自交换反应中隧穿似乎比芳基羟胺的更重要。
