Nitroxyl radical plus hydroxylamine pseudo self-exchange reactions: tunneling in hydrogen atom transfer

Abstract

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.

Figure 1

Van't Hoff plot for reactions 2−4 in CD₃CN at 278−318 K.

Figure 2

(a) Overlay of UV-vis spectra for the reaction of 8.8 mM 4-oxo-TEMPO^• with 88 mM TEMPO-H (eq 2) in MeCN over 5 s at 298 K. (b) Absorbance at 460 nm showing the raw data (○) and first order A → B fit using SPECFIT (—).

Figure 3

Plot of pseudo first order k_obs versus [TEMPO-H(D)] for reaction 2 in MeCN (k_H/k_D = 23 ± 3) and in CH₂Cl₂ (k_H/k_D = 23 ± 4) at 298 K.

Figure 4

Eyring plots for (a) 4-oxo-TEMPO^• + TEMPO-H (reaction 2) and (b) ^tBu₂NO^• + TEMPO-H (reaction 4), both in MeCN and CH₂Cl₂.

Figure 5

(a) Transition structure for hydrogen transfer between (CH₃)₂NOH and (CH₃)₂NO^•. (b) H-bonded complex between (CH₃)₂NOH and (CH₃)₂NO^•. Bond lengths are in Ångstroms.

Figure 6

Bond lengths (Å), atomic spin densities (in green), and activation energies (kcal mol⁻¹) for the nitroxyl radicals, (a) ^tBu₂NO^•, (b) TEMPO^•, and (c) Ph₂NO^•, and the transition structures and CVT activation energies for their hydrogen self-exchange reactions with the corresponding hydroxylamines.

Figure 7

SOMOs of the transition structures for hydrogen exchange reactions: (a) ^tBu₂NO^• + ^tBu₂NOH, (b) TEMPO^• + TEMPO-H , and (c) Ph₂NO^• + Ph₂NOH.