Reactive Intermediates

"Oxygen-containing mononuclear iron species–iron(III)-peroxo, iron(III)-hydroperoxo and iron(IV)-oxo–are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes. It has been difficult to generate synthetic analogues of these three active iron-oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron center during catalysis and the factors that control their chemical reactivities with substrates.

Here we report the high-resolution crystal structure of a mononuclear non-heme side-on iron(III)-peroxo complex, [Fe(III)(TMC)(OO)]+. We also report a series of chemical reactions in which this iron(III)-peroxo complex is cleanly converted to the iron(III)-hydroperoxo complex, [Fe(III)(TMC)(OOH)]2+, via a short-lived intermediate on protonation. This iron(III)-hydroperoxo complex then cleanly converts to the ferryl complex, [Fe(IV)(TMC)(O)]2+, via homolytic O–O bond cleavage of the iron(III)-hydroperoxo species.

All three of these iron species—the three most biologically relevant iron–oxygen intermediates—have been spectroscopically characterized; we note that they have been obtained using a simple macrocyclic ligand. We have performed relative reactivity studies on these three iron species which reveal that the iron(III)-hydroperoxo complex is the most reactive of the three in the deformylation of aldehydes and that it has a similar reactivity to the iron(IV)-oxo complex in C–H bond activation of alkylaromatics. These reactivity results demonstrate that iron(III)-hydroperoxo species are viable oxidants in both nucleophilic and electrophilic reactions by iron-containing enzymes."

Cathodic reduction of diazonium salts in acetonitrile led to the formation of azobenzenes, in good to moderate yields, and diarylamines as minoritary products. The reactions were carried out at the second reduction potential of the diazonium salts, involving aryl anions in the formation of the products.

Oxygen-containing mononuclear iron species—iron (iii)–peroxo, iron (iii)–hydroperoxo and iron (iv)–oxo—are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes 1, 2, 3, 4, 5, 6, 7. It has been difficult to generate synthetic analogues of these three active iron–oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution ...

Oxygen-containing mononuclear iron species—iron (iii)–peroxo, iron (iii)–hydroperoxo and iron (iv)–oxo—are key intermediates in the catalytic activation of dioxygen by iron-containing metalloenzymes 1, 2, 3, 4, 5, 6, 7. It has been difficult to generate synthetic analogues of these three active iron–oxygen species in identical host complexes, which is necessary to elucidate changes to the structure of the iron centre during catalysis and the factors that control their chemical reactivities with substrates. Here we report the high-resolution ...

A synthetic method is presented for the production of isotetrasilane, a higher order perhydridosilane, with the purity and volume necessary for use in extensive studies of the chemical vapor deposition (CVD) of epitaxial silicon (e-Si) thin films.  The chemical characteristics, thermodynamic properties and epitaxial film growth of isotetrasilane are compared with those of other perhydridosilanes. A film-growth mechanism distinct from linear perhydrodosilanes H(SiH2)nH, where n is ≤4, is re-ported.  Preliminary findings are summarized for CVD of both unstrained e-Si and strained e-Si doped with germanium (Ge) and carbon (C) employing isotetrasilane as the source precursor at temperatures of 500-550C.  The results suggest that bis(trihydridosilyl)silylene is the likely deposition intermediate under processing conditions in which gas-phase depletion reactions are not observed.

Lithium diisopropyl amide (LDA) is a very prominent reagent that plays a key role in organic synthesis, serving as a base par excellence for a broad range of deprotonation reactions. However, the state of aggregation in solution in the absence of donor bases was unclear. In this paper we solved this problem by employing DOSY NMR experiments based on a newly elaborated external calibration curve (ECC) approach with normalized diffusion coefficients.

Pyridoxal 5'-phosphate (PLP)-dependent enzymes represent about 4% of the enzymes classified by the Enzyme Commission. The versatility of PLP in carrying out a large variety of reactions exploiting the electron sink effect of the pyridine ring, the conformational changes accompanying the chemical steps and stabilizing distinct catalytic intermediates, and the spectral properties of the different coenzyme-substrate derivatives signaling the reaction progress, are some of the features that have attracted our interest to investigate the structure-dynamics-function relationships of PLP-dependent enzymes. To this goal, an integrated approach combining biochemical, biophysical, computational, and molecular biology methods was used. The extensive work carried out on two enzymes, tryptophan synthase and O-acetylserine sulfhydrylase, is presented and discussed as representative of other PLP-dependent enzymes we have investigated. Finally, perspectives of PLP-dependent enzymes functional g...

Both flash vacuum thermolysis (FVT) and matrix photolysis generate 2-diazomethylpyrazine (22) from 1,2,3-triazolo[1,5-a]pyrazine (24). FVT of 4-azidopyridine (18) as well as of 24 or 2-(5-tetrazolyl)pyrazine (23) affords the products expected from the nitrene, i.e., 4,4’-azopyridine and 2- and 3-cyanopyrroles. Matrix photolyses of both 18 and 24 result in ring expansion of 4-pyridylnitrene/2-pyrazinylcarbene to 1,5-diazacyclohepta-1,2,4,6-tetraene (20). Further photolysis causes ring opening to the ketenimine 27.

Precursors of 3-pyridylnitrene and 2- and 4-pyrimidinylcarbenes all afford mixtures of 2- and 3-cyanopyrroles on flash vacuum thermolysis, but 3-cyanopyrroles are the first-formed products. 3-Quinolylnitrenes and 4-quinazolinylcarbenes similarly afford 3-cyanoindoles. 2-Pyrimidinylcarbenes rearrange to 3-pyridylnitrenes, but 4-pyrimidinylcarbenes and 4-quinazolinylcarbenes do not necessarily rearrange to the corresponding 3-pyridylnitrenes or 3-quinolylnitrenes. The ring contraction reactions are interpreted in terms of ring opening of either the nitrenes or the diazacycloheptatetraenes to nitrile ylides.