Hydrogen by Deuterium Substitution in an Aldehyde Tunes the Regioselectivity by a Nonheme Manganese(III)-Peroxo Complex

Abstract

Mononuclear nonheme Mn^III -peroxo complexes are important intermediates in biology, and take part in oxygen activation by photosystem II. Herein, we present work on two isomeric biomimetic side-on Mn^III -peroxo intermediates with bispidine ligand system and reactivity patterns with aldehydes. The complexes are characterized with UV/Vis and mass spectrometric techniques and reaction rates with cyclohexane carboxaldehyde (CCA) are measured. The reaction gives an unusual regioselectivity switch from aliphatic to aldehyde hydrogen atom abstraction upon deuteration of the substrate, leading to the corresponding carboxylic acid product for the latter, while the former gives a deformylation reaction. Mechanistic details are established from kinetic isotope effect studies and density functional theory calculations. Thus, replacement of C-H by C-D raises the hydrogen atom abstraction barriers and enables a regioselectivity switch to a competitive pathway that is slightly higher in energy.

Keywords: biomimetic models; density functional theory; hydrogen atom abstraction; kinetic isotope effects; oxidation.