Valence Bond Diagrams and Chemical Reactivity

Abstract

A unified description of chemical reactivity is made possible with valence bond (VB) diagrams, such as those shown for the a simple case (only reactants and products must be considered) and a more complex system (an intermediate state also plays an important role; RC = reaction coordinate). Analysis of reactivity and mechanistic problems in organic and organometallic chemistry exemplifies the generality of the VB paradigm: in situ DNA repair, C-F and C-H bond activation, S 2 mechanism, stepwise versus concerted cycloaddition, and a lot more.

An attempt to evaluate the effect of proton-coupled electron transfer on the H-abstraction step of the reaction between 1,1-dimethylhydrazine and cytochrome P450 compound I

“…To understand the electronic mechanism underlying the substrate-dependent reactivity differences in a chemical language, we performed an additional qualitative analysis using VB diagrams [34][35][36][37][38][39]. To describe each VB structure of a complex Cpd I system as concisely as possible, we shall employ the notation illustrated in Figure 4a (right).…”

On the identity and reactivity patterns of the “second oxidant” of the T252A mutant of cytochrome P450cam in the oxidation of 5-methylenenylcamphor

“…Thus, two bonds are broken, two new ones are formed, and the iron center is internally reduced from Fe IV to ferric (Fe III ). No wonder therefore that the corresponding barrier is large [56]. We shall attempt to conceptualize these changes by consulting the geometries in Fig.…”

“…7 shows that the barriers for the glycol path are, as anticipated [56], much lower by 14-15 kcal/mol, compared with those for the epoxide path, which involves extensive electronic reorganization. There is no question that the glycol path is more favorable electronically (simple radical coupling) and thermodynamically (compare the product stability in Fig.…”

Thermal hydrogen-atom transfer from methane: A mechanistic exercise

“…relatively weak C H bond, e.g. allylic or benzylic ones, can be activated by closed-shell, diamagnetic reagents, for example CrO 2 Cl 2 or [MnO 4 ] − , calculations strongly suggest that the electrons of the M = O unit must develop spin density at the accepting oxygen atom by electronic decoupling of one of the M = O bonds that occurs near the transition state to exhibit HAT reactivity [78,79]. If this penalty is too high, the closed-shell abstractor may switch to a protoncoupled electron-transfer (PCET) mechanism [47,59,60].…”