Heme compound II models in chemoselectivity and disproportionation reactions

Heme compound II models bearing electron-deficient and -rich porphyrins, [Fe^IV(O)(TPFPP)(Cl)]⁻ (1a) and [Fe^IV(O)(TMP)(Cl)]⁻ (2a), respectively, are synthesized, spectroscopically characterized, and investigated in chemoselectivity and disproportionation reactions using cyclohexene as a mechanistic probe. Interestingly, cyclohexene oxidation by 1a occurs at the allylic C–H bonds with a high kinetic isotope effect (KIE) of 41, yielding 2-cyclohexen-1-ol product; this chemoselectivity is the same as that of nonheme iron(iv)-oxo intermediates. In contrast, as observed in heme compound I models, 2a yields cyclohexene oxide product with a KIE of 1, demonstrating a preference for C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C epoxidation. The latter result is interpreted as 2a disproportionating to form [Fe^IV(O)(TMP⁺˙)]⁺ (2b) and Fe^III(OH)(TMP), and 2b becoming the active oxidant to conduct the cyclohexene epoxidation. In contrast to 2a, 1a does not disproportionate under the present reaction conditions. DFT calculations confirm that compound II models prefer C–H bond hydroxylation and that disproportionation of compound II models is controlled thermodynamically by the porphyrin ligands. Other aspects, such as acid and base effects on the disproportionation of compound II models, have been discussed as well.

Disproportionation of Cpd II models depends on the electron-richness of the porphyrin ligand; Cpd II with an electron-deficient ligand is difficult to disproportionate, whereas Cpd II with an electron-rich ligand readily disproportionates to form Cpd I as a true oxidant.

Nonheme Iron Imido Complexes Bearing a Non-Innocent Ligand: A Synthetic Chameleon Species in Oxidation Reactions

High-valent iron-imido complexes can perform C-H activation and sulfimidation reactions, but are far less studied than the more ubiquitous iron-oxo species. As case studies, we have looked at a recently published iron(V)-imido ligand π-cation radical complex, which is formally an iron(VI)-imido complex [Fe^V (NTs)(TAML^+. )] (1; NTs=tosylimido), and an iron(V)-imido complex [Fe^V (NTs)(TAML)]^- (2). Using a theoretical approach, we found that they have multiple energetically close-lying electromers, sometimes even without changing spin states, reminiscent of the so-called Compound I in Cytochrome P450. When studying their reactivity theoretically, it is indeed found that their electronic structures may change to perform efficient oxidations, emulating the multi-spin state reactivity in Fe^IV O systems. This is actually in contrast to the known [Fe^V (O)(TAML)]^- species (3), where the reactions occur only on the ground spin state. We also looked into the whole reaction pathway for the C-H bond activation of 1,4-cyclohexadiene by these intermediates to reproduce the experimentally observed products, including steps that usually attract no interest (neither theoretically nor experimentally) due to their non-rate-limiting status and fast reactivity. A new "clustering non-rebound mechanism" is presented for this C-H activation reaction.

Investigating the Influence of Excipients on the Stability of Levothyroxine Sodium Pentahydrate

A range of tablet excipients were evaluated for their influence on the physical form and chemical stability of levothyroxine sodium pentahydrate (LSP; C₁₅H₁₀I₄NNaO₄·5H₂O). LSP-excipient binary powder blends were stored under two conditions: (a) in hermetically sealed containers at 40 °C and (b) at 40 °C/75% RH. By use of synchrotron X-ray diffractometry, the disappearance of LSP could be quantified and the appearance of crystalline levothyroxine (free acid) could be identified. Under hermetically sealed conditions (40 °C) hygroscopic excipients such as povidone induced partial dehydration of LSP to form levothyroxine sodium monohydrate. When stored at 40 °C/75% RH, acidic excipients induced measurable disproportionation of LSP resulting in the formation of levothyroxine (free acid). HPLC analyses of drug-excipient mixtures revealed that lactose monohydrate, microcrystalline cellulose, and croscarmellose sodium caused pronounced chemical decomposition of LSP. On the other hand, magnesium stearate, sodium stearyl fumarate, and alkaline pH modifiers did not affect the physical and chemical stability of the API following storage at 40 °C/75% RH. HPLC, being a solution based technique, revealed chemical decomposition of the API, but the technique was insensitive to physical transformations. Excipient properties such as hygroscopicity and microenvironmental acidity were identified to be critical determinants of both physical and chemical stability of LSP in a drug product. For drugs exhibiting both physical and chemical transformations, simultaneous solid-state and solution based analyses will enable comprehensive stability evaluation.

Imidazoline synthesis: mechanistic investigations show that Fe catalysts promote a new multicomponent redox reaction

Multicomponent reactions are attracting strong interest because they contribute to develop more efficient synthetic chemistry. Understanding their mechanism at the molecular level is thus an important issue to optimize their operation. The development of integrated experimental and theoretical approaches has very recently emerged as most powerful to achieve this goal. In the wake of our recent investigation of amidine synthesis, we used this approach to explore how an Fe-catalyzed aziridination can lead to an imidazoline when run in acetonitrile. We report that the synthesis of imidazoline by combination of styrene, acetonitrile, an iron catalyst and a nitrene precursor occurs along a new kind of multicomponent reaction. The formation of imidazoline results from acetonitrile interception of a benzyl radical styrene aziridination intermediate within Fe coordination sphere, as opposed to classical nucleophilic opening of the aziridine by a Lewis acid. Comparison of this mechanism to that of amidine formation allows a rationalization of the modes of intermediates trapping by acetonitrile according to the oxidation state Fe active species. The molecular understanding of these processes may help to design other multicomponent reactions.