An initial review (PCET1) on proton-coupled electron transfer (PCET) by Huynh and Meyer appeared in Chemical Reviews in 2007. 1 This is a perennial review, a follow up on the original. It was intended for the special Chemical Reviews edition on Proton Coupled Electron Transfer that appeared in December, 2010 (Volume 110, Issue 12 Pages 6937-710). The reader is referred to it with articles on electrochemical approaches to studying PCET by Costentin and coworkers, 2 theory of electron proton transfer reactions by Hammes-Schiffer and coworkers, 3 proton-coupled electron flow in proteins and enzymes by Gray and coworkers, 4 and the thermochemistry of proton-coupled electron transfer by Mayer and coworkers. 5 Coverage for the current review is intended to be broad, covering all aspects of the topic comprehensively with literature coverage overlapping with the later references in PCET1 until late 2010. There is a growing understanding of the importance of PCET in chemistry and biology and its implications for catalysis and energy conversion. This has led to a series of informative reviews that have appeared since 2007. They include: "The possible role of Proton-coupled electron Transfer (PCET) in Water oxidation by Photosystem II" by Meyer and coworkers in 2007, 6 "Theoretical studies of proton-coupled electron transfer: Models and concepts relevant to bioenergetics" by Hammes-Schiffer and coworkers in 2008, 7 "Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer" by Costentin in 2008, 8 "Proton-Coupled Electron Transfer in Biology: Results from Synergistic Studies in Natural and Model Systems" by Nocera and Reece in 2009, 9 and "Integrating Proton-Coupled Electron Transfer and Excited States" by Meyer and coworkers in 2010. 10

Quantum chemical calculations of activation barriers and reaction energies for 1,3-dipolar cycloadditions by the high-accuracy CBS-QB3 method reveal previously unrecognized quantitative trends in activation barriers. The distortion/interaction model of reactivity explains why (1) there is a monotonic decrease of approximately 6 kcal/mol in the activation energy along the series oxides, imine, and ylide for the diazonium, nitrilium, and azomethine betaine classes of 1,3-dipoles; (2) nitrilium and azomethine betaines with the same trio of atoms have almost identical cycloaddition barrier heights; (3) barrier heights for the cycloadditions of a given 1,3-dipole with ethylene and acetylene have the same activation energies (mean absolute deviation of 0.6 kcal/mol) in spite of very different reaction thermodynamics (Delta DeltaH(rxn) range = 14-43 kcal/mol) and frontier molecular orbital (FMO) energy gaps. The energy to distort the 1,3-dipole and dipolarophile to the transition state geometry, rather than FMO interactions or reaction thermodynamics, controls reactivity for cycloadditions of 1,3-dipoles with alkenes or alkynes. A distortion/interaction energy analysis was also carried out on the transition states for the cycloadditions of diazonium dipoles with a set of substituted alkenes (CH2CHX, X = OMe, Me, CO 2Me, Cl, CN) and reveals that FMO interaction energies between the 1,3-dipole and the dipolarophile differentiate reactivity when transition state distortion energies are nearly constant.

Huisgen's 1,3-dipolar cycloaddition reactions, extraordinarily versatile methods for heterocycle synthesis, are now used in materials chemistry, drug discovery, and chemical biology. 1,2 The 1,3-dipoles range from the familiar atmospheric components, ozone and nitrous oxide, to the synthetically valuable azides of click chemistry fame. 1 Frontier molecular orbital (FMO) theory has provided a powerful model for the qualitative understanding of reactivity and regioselectivity, based upon electronic properties of isolated reactants. 3 Conceptual density functional theory has also provided a qualitative way to rationalize reactivity patterns. 4 Chemically accurate theoretical thermochemistries ((1 kcal/mol) are now available using the CBS-QB3 method. 5 Three of the most important classes of dipoles defined by Huisgen, the diazonium (DB), nitrilium (NB), and azomethine betaines (AB), shown below, were explored. 2 The CBS-QB3 ∆H q and ∆H rxn were computed for the reactions of 1-9 with ethylene and acetylene. 6

A single transition state may lead to multiple intermediates or products if there is a post-transition-state reaction pathway bifurcation. These bifurcations arise when there are sequential transition states with no intervening energy minimum. For such systems, the shape of the potential energy surface and dynamic effects, rather than transition-state energetics, control selectivity. This Minireview covers recent investigations of organic reactions exhibiting reaction pathway bifurcations. Such phenomena are surprisingly general and affect experimental observables such as kinetic isotope effects and product distributions.

We present evidence for Cp* being a sacrificial placeholder ligand in the [Cp*Ir(III)(chelate)X] series of homogeneous oxidation catalysts. UV-vis and (1)H NMR profiles as well as MALDI-MS data show a rapid and irreversible loss of the Cp* ligand under reaction conditions, which likely proceeds through an intramolecular inner-sphere oxidation pathway reminiscent of the reductive in situ elimination of diolefin placeholder ligands in hydrogenation catalysis by [(diene)M(I)(L,L')](+) (M = Rh and Ir) precursors. When oxidatively stable chelate ligands are bound to the iridium in addition to the Cp*, the oxidized precursors yield homogeneous solutions with a characteristic blue color that remain active in both water- and CH-oxidation catalysis without further induction period. Electrophoresis suggests the presence of well-defined Ir-cations, and TEM-EDX, XPS, (17)O NMR, and resonance-Raman spectroscopy data are most consistent with the molecular identity of the blue species to be a bis-μ-oxo di-iridium(IV) coordination compound with two waters and one chelate ligand bound to each metal. DFT calculations give insight into the electronic structure of this catalyst resting state, and time-dependent simulations agree with the assignments of the experimental spectroscopic data. [(cod)Ir(I)(chelate)] precursors bearing the same chelate ligands are shown to be equally effective precatalysts for both water- and CH-oxidations using NaIO4 as chemical oxidant.