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Density functional theory is used to explore the mechanisms of alkane hydroxylation for four synthetic non-heme iron(IV)-oxo complexes with three target substrates (Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde; J.-U.; Song, W. J.; Stubna, A.; Kim, J.; Münck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2004, 126, 472-473; Rohde, J.-U.; Que, L., Jr. Angew. Chem. Int. Ed. 2005, 44, 2255-2258.). The iron-oxo reagents possess triplet ground states and low-lying quintet excited states. The set of experimental and theoretical reactivity trends can be understood if the reactions proceed on the two spin states, namely two-state reactivity (TSR); an appropriate new model is presented. The TSR model makes testable predictions: (a) If crossing to the quintet state occurs, the hydroxylation will be effectively concerted; however, if the process transpires only on the triplet surface, stepwise hydroxylation will occur, and side products derived from radical intermediates would be observed (e.g., loss of stereochemistry). (b) In cases of crossing en route to the quintet transition state, one expects kinetic isotope effects (KIEs) typical of tunneling. (c) In situations where the two surfaces contribute to the rate, one expects intermediate KIEs and radical scrambling patterns that reflect the two processes. (d) Solvent effects on these reactions are expected to be very large.

The Account discusses the phenomenon of two-state reactivity (TSR) or multistate reactivity (MSR) in high-valent metal-oxo reagents, projecting its wide-ranging applicability starting from the bare species, through the reagents made by Que, Nam, and collaborators, to the Mn(V)-oxo substituted polyoxometalate, all the way to Compound I species of heme enzymes. The Account shows how the behaviors of all these variegated species derive from a simple set of electronic structure principles. Experimental trends that demonstrate TSR and MSR are discussed. Diagnostic mechanistic probes are proposed for the TSR/MSR scenario, based on kinetic isotope effect, stereochemical studies, and magnetic- and electric-field effects.

The paper outlines the fundamental factors that govern the mechanisms of alkane hydroxylation by cytochrome P450 and the corresponding barrier heights during the hydrogen abstraction and radical rebound steps of the process. This is done by a combination of density functional theory calculations for 11 alkanes and valence bond (VB) modeling of the results. The energy profiles and transition states for the various steps are reconstructed using VB diagrams (Shaik, S. S. J. Am. Chem. Soc. 1981, 103, 3692-3701. Shaik, S.; Shurki, A. Angew. Chem. Int. Ed. 1999, 38, 586-625.) and the DFT barriers are reproduced by the VB model from raw data based on C-H bond energies. The model explains a variety of other features of P450 hydroxylations: (a) the nature of the polar effect during hydrogen abstraction, (b) the difference between the activation mechanisms leading to the Fe(IV) vs the Fe(III) electromers, (c) the difference between the gas phase and the enzymatic reaction, and (d) the dependence of the rebound barrier on the spin state. The VB mechanism shows that the active species of the enzyme performs a complex reaction that involves multiple bond making and breakage mechanisms by utilizing an intermediate VB structure that cuts through the high barrier of the principal transformation between reactants and products, thereby mediating the process at a low energy cost. The correlations derived in this paper create order and organize the data for a process of a complex and important enzyme. This treatment can be generalized to the reactivity patterns of nonheme systems and synthetic iron-oxo porphyrin reagents.

Controlling the selectivity of a chemical reaction is a Holy Grail in chemistry. This paper reports theoretical results of unprecedented effects induced by moderately strong electric fields on the selectivity of two competing nonpolar bond activation processes, C-H hydroxylation vs C=C epoxidation, promoted by an active species that is common to heme-enzymes and to metallo-organic catalysts. The molecular system by itself shows no selectivity whatsoever. However, the presence of an electric field induces absolute selectivity that can be controlled at will. Thus, the choice of the orientation and direction of the field vis-à-vis the molecular axes drives the reaction in the direction of complete C-H hydroxylation or complete C=C epoxidation.

Cysteine dioxygenase (CDO) is a vital enzyme for human health involved in the biodegradation of toxic cysteine and thereby regulation of the cysteine concentration in the body. The enzyme belongs to the group of nonheme iron dioxygenases and utilizes molecular oxygen to transfer two oxygen atoms to cysteinate to form cysteine sulfinic acid products. The mechanism for this reaction is currently disputed, with crystallographic studies implicating a persulfenate intermediate in the catalytic cycle. To resolve the dispute we have performed quantum mechanics/molecular mechanics (QM/MM) calculations on substrate activation by CDO enzymes using an enzyme monomer and a large QM active region. We find a stepwise mechanism, whereby the distal oxygen atom of the iron(II)-superoxo complex attacks the sulfur atom of cysteinate to form a ring structure, followed by dioxygen bond breaking and the formation of a sulfoxide bound to an iron(IV)-oxo complex. A sulfoxide rotation precedes the second oxygen atom transfer to the substrate to give cysteine sulfinic acid products. The reaction takes place on several low-lying spin-state surfaces via multistate reactivity patterns. It starts in the singlet ground state of the iron(II)-superoxo reactant and then proceeds mainly on the quintet and triplet surfaces. The initial and rate-determining attack of the superoxo group on the cysteinate sulfur atom involves a spin-state crossing from singlet to quintet. We have also investigated an alternative mechanism via a persulfenate intermediate, with a realignment of hydrogen bonding interactions in the substrate binding pocket. However, this alternative mechanism of proximal oxygen atom attack on the sulfur atom of cysteinate is computed to be a high-energy pathway, and therefore, the persulfenate intermediate is unlikely to participate in the catalytic cycle of CDO enzymes.

The communication presents DFT calculations of 10 different C-H hydroxylation barriers by the active species of the enzyme cytochrome P450. The work demonstrates the existence of an excellent barrier-bond energy correlation. The so-obtained equation of the straight line is demonstrated to be useful for predicting barriers of related C-H activation processes, as well as for assessing barrier heights within the protein environment. This facility is demonstrated be estimating the barrier of camphor hydroxylation by P450cam.