The catalytic addition of water to alkynes (hydration) generates valuable carbonyl compounds from unsaturated hydrocarbon precursors. Traditional mercury(II) catalysts hydrate terminal alkynes with Markovnikov selectivity to methyl ketones. Much research has been devoted to finding catalysts based on less toxic metals, the most promising being gold(I), gold(III), platinum(II), and palladium(II). Catalytic anti-Markovnikov hydration of terminal alkynes to aldehydes has been realized in an efficient manner with ruthenium(II) complex catalysts. The present review article lists known hydration catalysts and discusses applications of catalytic hydration to different classes of substrates, with an emphasis on functional group tolerance and regioselectivity. 6 Reactions Related to Catalytic Alkyne Hydration 7 Conclusions and Outlook

The generation of a hidden Brønsted acid as a true catalytic species in hydroalkoxylation reactions from metal precatalysts has been clarified in case studies. The mechanism of triflic acid (CF(3)SO(3)H or HOTf) generation starting either from AgOTf in 1,2-dichloroethane (DCE) or from a Cp*RuCl(2)/AgOTf/phosphane combination in toluene has been elucidated. The deliberate and controlled generation of HOTf from AgOTf and cocatalytic amounts of tert-butyl chloride in the cold or from AgOTf in DCE at elevated temperatures results in a hidden Brønsted acid catalyst useful for mechanistic control experiments or for synthetic applications.

The first systematic steady-state and time-resolved emission study of firefly oxyluciferin (emitter in firefly bioluminescence) and its analogues in aqueous buffers provided the individual emission spectra of all chemical forms of the emitter and the excited-state equilibrium constants in strongly polar environment with strong hydrogen bonding potential. The results confirmed the earlier hypothesis that excited-state proton transfer from the enol group is favored over proton transfer from the phenol group. In water, the phenol-keto form is the strongest photoacid among the isomers and its conjugate base (phenolate-keto) has the lowest emission energy (634 nm). Furthermore, for the first time we observed green emission (525 nm) from a neutral phenol-keto isomer constrained to the keto form by cyclopropyl substitution. The order of emission energies indicates that in aqueous solution a second deprotonation at the phenol group after the enol group had dissociated (that is, deprotonation of the phenol-enolate) does not occur in the first excited state. The pH-dependent emission spectra and the time-resolved fluorescence parameters revealed that the keto-enol tautomerism reaction, which can occur in a nonpolar environment (toluene) in the presence of a base, is not favored in water.

The mysterious flashes of light communicated by fireflies conceal a rich and exciting solution spectrochemistry that revolves around the chemiexcitation and photodecay of the fluorophore, oxyluciferin. A triple chemical equilibrium by double deprotonation and keto-enol tautomerism turns this simple molecule into an intricate case where the relative spectral contributions of six chemical species combine over a physiologically relevant pH range, rendering physical isolation and spectral characterization of most of the species unmanageable. To disentangle the individual spectral contributors, here we demonstrate the advantage of chemical oriented multivariate data analysis. We designed a set of specific oxyluciferin derivatives and applied a multivariate curve resolution-alternating least squares (MCR-ALS) procedure simultaneously to an extensive set of pH-dependent spectroscopic data for oxyluciferin and the target derivatives. The analysis provided, for the first time, the spectra of the pure individual components free of contributions from the other forms, their pH-dependent profiles and distributions, and the most accurate to date values for the three equilibrium constants.

The 1,3-diaryl-imidazolium chlorides IPr·HCl (aryl = 2,6-diisopropylphenyl), IMes·HCl (aryl = 2,4,6-trimethylphenyl) and IXy·HCl (aryl = 2,6-dimethylphenyl), precursors to widely used N-heterocyclic carbene (NHC) ligands and catalysts, were prepared in high yields (81%, 69% and 89%, respectively) by the reaction of 1,4-diaryl-1, 4-diazabutadienes, paraformaldehyde and chlorotrimethylsilane in dilute ethyl acetate solution. A reaction mechanism involving a 1,5-dipolar electrocyclization is proposed.

Isolated and crystallized [Ti(TADDOLato)] complexes were used as catalysts for the first catalytic enantioselective (up to 90 % ee) C−F bond‐forming reaction, the fluorination of β‐ketoesters with the N‐fluorotriethylenediammonium salt F‐TEDA [Eq. (1)]. TADDOL=2,2‐dimethyl‐α,α,α′,α′‐tetraaryl‐1,3‐dioxolan‐4,5‐dimethanol.

Multienzymatic cascades are responsible for the biosynthesis of natural products and represent a source of inspiration for synthetic chemists. The Fe(II)/α-ketoglutarate-dependent dioxygenase AsqJ from Aspergillus nidulans is outstanding because it stereoselectively catalyzes both a ferryl-induced desaturation reaction and epoxidation on a benzodiazepinedione. Interestingly, the enzymatically formed spiro epoxide spring-loads the 6,7-bicyclic skeleton for non-enzymatic rearrangement into the 6,6-bicyclic scaffold of the quinolone alkaloid 4'-methoxyviridicatin. Herein, we report different crystal structures of the protein in the absence and presence of synthesized substrates, surrogates, and intermediates that mimic the various stages of the reaction cycle of this exceptional dioxygenase.

A complete understanding of the physics underlying the varied colors of firefly bioluminescence remains elusive because it is difficult to disentangle different enzyme-lumophore interactions. Experiments on isolated ions are useful to establish a proper reference when there are no microenvironmental perturbations. Here, we use action spectroscopy to compare the absorption by the firefly oxyluciferin lumophore isolated in vacuo and complexed with a single water molecule. While the process relevant to bioluminescence within the luciferase cavity is light emission, the absorption data presented here provide a unique insight into how the electronic states of oxyluciferin are altered by microenvironmental perturbations. For the bare ion we observe broad absorption with a maximum at 548 ± 10 nm, and addition of a water molecule is found to blue-shift the absorption by approximately 50 nm (0.23 eV). Test calculations at various levels of theory uniformly predict a blue-shift in absorption caused by a single water molecule, but are only qualitatively in agreement with experiment highlighting limitations in what can be expected from methods commonly used in studies on oxyluciferin. Combined molecular dynamics simulations and time-dependent density functional theory calculations closely reproduce the broad experimental peaks and also indicate that the preferred binding site for the water molecule is the phenolate oxygen of the anion. Predicting the effects of microenvironmental interactions on the electronic structure of the oxyluciferin anion with high accuracy is a nontrivial task for theory, and our experimental results therefore serve as important benchmarks for future calculations.