The O-H Bond Dissociation Enthalpy(BDE) Of N-Oxyl Radical Precursors And Its Application In Catalytic Oxidation

N-oxyl radical,or nitroxide is an important class of compounds,and is a widly used in the field of catalysis,probes,bio signal release,and magnetic materials.The stability of nitroxide is determined by the precursor O-H bond dissociation energy(BDE),which determines the activity of these radicals in the catalytic reactions and even changes the mechanism of some reactions.The BDE of the nitroxide precursors,i.e.hydroxylamines and oximes,has been investigated in computational and experimental work.But these studies only included a limited amount of N-oxyl radicals that have been used as catalysts.There is a lack of knowledge about the BDEs of a broader scope of the precursors.In this paper,the BDEs of hydroxylamines and oximes were systematically studied via quantum chemical calculations.By employing various types of substituents,we obtained a relatively complete picture of the changes of BDEs in hydroxylamines and oximes.Based on the results,the dependence of activity of the N-oxyl radicals and the mechanism in the oxidation reaction on the BDE are studied.We discussed the influence of structure of hydroxylamines and oximes on the BDE.Previous studies have only dealt with carbonyl and trifluoromethyl substituted hydroxylamines,aromatic substituted N-hydroxy phthalimides(NHPIs),as well as a small number of cyclic hydroxylamines.We have calculated 120 hydroxylamines of different substituents and different rings,and quantitatively studied the relationship between structure and BDE.The electronic and ring-tension effects are equally important in determining the hydroxylamine BDE,while the influence of steric effect is smaller.It was also discovered that the difference in BDEs between open-chain and cyclic acyl hydroxylamine is due mainly to dipole-dipole repulsion.The relationship between structure and BDE has not been well established with oximes,especially the electronic effects.The only conclusion about the electronic effect is that the BDEs of amide oximes are particularly large,and some assumptions are given.In addition,a large amount of erroneous oxime BDEs were not corrected until recently.Firstly,by analysis of the molecular orbitals and the isotropic Fermi contact coupling constants of aldehydic hydrogen,it was found that most of the oxime radicals are a-radicals,and very few are π-radicals.Secondly,the electronic effect was discussed by using the inductive and resonance parameters.The electronic effect of oxime is opposite to that of hydroxylamine and is dominated by resonance effect,rather than inductive effect.The corresponding explanations were presented.Finally,a summary on the range of BDE due to various effects was presented for oximes and hydroxylamines.The activity of a N-oxyl radical towards the C-H bond is proportional to the precursor BDE.NHPI is the most commonly used catalyst,which possesses a large BDE and high activity.Furthermore,with the conjugated structure,modification can easily affect the reactive center.Therefore,NHPI is a good precursor for designing highly reactive catalysts.Former studies have improved the performance of NHPI by modifications.For example,esterification of hydroxyl group,or linking ionic groups via alkyl chains can improve solubility of NHPI,but the activity has little improvement;some researchers have designed NHPI analogues of two or three hydroxylamine groups,which achieves high activity,however,such molecules have fixed structures and have little chance for further modifications.We proposed a method that an ionic group is linked directly,or conjugated to the aromatic ring of NHPI.These modifications increase the BDE and reduce the reaction barrier of hydrogen abstraction by a N-oxyl radical.The mechanisms of N-oxyl radical catalyzed oxidation of alcohols have attracted heated discussion.On the basis of a series of studies,there are currently two recognized mechanisms.One mechanism is that a N-oxyl radical abstracts a-hydrogen atom to form a carbon radical,which then reacts with oxygen to form the products.This mechanism is called the radical mechanism.In the other mechanism,the N-oxyl radical goes through a single electron oxidation to the oxoammonium ion,NO+.The NO+then abstracts a-hydride of the alcohol to form the product.This mechanism is called the ionic mechanism.Different N-oxyl radicals catalyze via different mechanisms.Mechanisms for some of the catalysts were determined in former studies.However,each of these experiments is directed to a single catalyst.Few articals have included a number of different catalysts in a same reaction to compare their mechanisms,and none has given a standard to predict the mechanism.We proposed to predict the mechanism of alcohol oxidation by the BDE standard.The rate determining steps(RDSs)for the ionic mechanism and the radical mechanism were determined(for the ionic mechanism,we also considered the oxidation potential of reaction NO’→NO+),and relationships between the barrier of the RDSs and BDEs were established.Therefore,the BDE corresponding to the barrier that satisfy the condition for each mechanism can be obtained.The conclusion is that,for benzyl alcohol a catalyst with BDE>79 kcal/mol reacts with radical mechanism and catalytic activity is higher with larger BDE;a catalyst with BDE<74 kcal/mol can oxidize by ionic mechanism,and catalytic activity also increases with increasing BDE.For methanol,the catalyst BDE needs to be greater than 83 kcal/mol to satisfy the condition of radical mechanism,while to react by the ionic mechanism,the catalyst BDE needs to be less than 74 kcal/mol.By applying this model,we have found a simple method to predict the mechanism of alcohol oxidation by a N-oxyl radical,which provides a reference for future mechanistic research.In summary,quantum calculations were used to obtain the BDEs of substituted oximes and hydroxylamines,and the electronic effect,steric hindrance and ring tension effect were studied.We have also designed potentially highly reactive hydroxylamine catalysts utilizing electronic effect.Finally,we have explored the mechanism of N-oxyl-catalyzed oxidation of alcohols.The BDE of the precursor is a valid criterion to predict the mechanism of a N-oxyl radical catalyst in the oxidation of different alcohols.