Kinetic Studies on the 1,2-Sigmatropic Hydrogen Shift in the Photorearranged Intermediate ofN-Acetylpyrrole:  Tunneling Effects

Abstract

The rates for the 1,2-sigmatropic hydrogen and deuterium shifts in the ground state of the photorearranged intermediate of N-acetylpyrrole were directly measured by means of laser flash photolysis in several solvents; (e.g., 0.27 s-1 for 1,2-H shift and 0.12 s-1 for 1,2-D shift in nonpolar methylcyclohexane (MCH) at 293 K). The rate of the 1,2-hydrogen shift was remarkably increased by a basic catalyst, such as triethylamine, alcohols, and water. From the experimental results of temperature and isotope effects, it was shown that the 1,2-sigmatropic hydrogen (or deuterium) shift in MCH proceeds via quantum mechanical tunneling processes at two vibrational energy levels: E = 0 (v = v 0) and E = E v (=2.9 kcal mol-1 for the hydrogen shift or 3.3 kcal mol-1 for the deuterium shift) (v = v 1) under experimental conditions. The theoretical considerations for the tunneling mechanism were made by use of the tunnel effect theory proposed by Formosinho. The rates obtained by theoretical calculations were in good agreement with experimental ones. It is noteworthy that the 1,2-sigmatropic hydrogen (or deuterium) shift takes place via the intramolecular process at a low concentration of N-acetylpyrrole (1.7 × 10 -4 M) in dehydrated MCH.

Study of the Chemistry ofortho- andpara-Biphenylnitrenes by Laser Flash Photolysis and Time-Resolved IR Experiments and by B3LYP and CASPT2 Calculations

“…This interpretation has precedent in the work of Truhlar and co-workers. 37,38 Furthermore, Shizuka et al 39 have reported experimental and theoretical evidence for tunneling during their study of a similar 1,5-hydrogen shift in the photorearranged intermediate formed from N-acetylpyrrole. However, the very small H/D kinetic isotope effect of less than 3 at low temperatures does not seem to be consistent with a mechanism involving temperature-independent tunneling below 220 K. The temperature dependence of the observed rate constants (k obs ) for the decay of 305 nm transient absorption (Figure 16), produced by LFP of 1a and 1a-d 9 , is totally different from those for the disappearance of isocarbazole 4 and 4-d 9 (Figure 14).…”

Effects of Electronic Structures on the Excited-State Intramolecular Proton Transfer of 1-Hydroxy-2-acetonaphthone and Related Compounds

“…58 We have also reported similar catalytic effects of alcohols and triethylamine on the rate of sigmatropic hydrogen shifts in the photorearranged intermediate of phenyl acetate 59 and N-acetylpyrrole. 60 Therefore, it can be concluded that mutual proton or hydrogen atom transfer between solute and solvent molecules catalyzes the recovery process of 1H2AN′ into parent enol 1H2AN.…”

Tunneling effects on the 1,3-sigmatropic hydrogen shift in the photo-Fries rearranged intermediate of 2,4-dimethoxy-6-(p-tolyloxy)-s-triazine

“…The value of l was obtained as 1 ] 1012 s~1 by the best Ðtting of eqn. ( 7) and (19) to the data in Fig. 6.…”

“…By using these tunneling parameters and eqn. (19), the tunneling rate constants and at and for [1,3] hydrogen shift were obtained to be 0.22 and 8.92 ] 102 s~1, respectively. For the intramolecular [1,3] the theoretical values for the intramolecular [1,3] hydrogen and deuterium shifts at various temperatures were calculated as depicted by broken lines in Fig.…”

“…Similar catalytic e †ects due to alcohols have also been found for hydrogen shift reactions in the photorearranged intermediates of phenyl acetate18 and N-acetylpyrrole. 19 The observed rate for the 1,3-hydrogen shift in protic solvents can be written as…”