Baldwin-Type Rules for Metal-Controlled Intramolecular Migratory Insertions. A Computational Study of Ni, Pd, and Pt Case

Transition metal (Ni, Pd, Pt) promoted carbocyclizations of alkenes and alkynes have been computationally investigated to rationalize the easiness or difficulty of such processes depending on the size of the forming ring. Special emphasis has been placed on measuring the activation barriers and the <i>exo</i> versus <i>endo</i> selectivities of the processes. The study leads us to propose some qualitative rules for metal controlled ring closures, noting some discrepancies with the classical Baldwin’s rules for nonmetallic cyclizations. Also, some unexpected results were found, like the extremely low activation barriers for 3-<i>exo</i>-dig and 3-<i>exo</i>-trig closures, contrasting with the scarcity of such experimental procedures, which can be computationally interpreted by the reversibility and endergonicity of cyclopropane/cyclopropene ring formation in the presence of these metals. The consistency of the present rules has been further checked by introduction of selected structural variations in the substrates

Baldwin-Type Rules for Metal-Controlled Intramolecular Migratory Insertions. A Computational Study of Ni, Pd, and Pt Case

Transition metal (Ni, Pd, Pt) promoted carbocyclizations of alkenes and alkynes have been computationally investigated to rationalize the easiness or difficulty of such processes depending on the size of the forming ring. Special emphasis has been placed on measuring the activation barriers and the <i>exo</i> versus <i>endo</i> selectivities of the processes. The study leads us to propose some qualitative rules for metal controlled ring closures, noting some discrepancies with the classical Baldwin’s rules for nonmetallic cyclizations. Also, some unexpected results were found, like the extremely low activation barriers for 3-<i>exo</i>-dig and 3-<i>exo</i>-trig closures, contrasting with the scarcity of such experimental procedures, which can be computationally interpreted by the reversibility and endergonicity of cyclopropane/cyclopropene ring formation in the presence of these metals. The consistency of the present rules has been further checked by introduction of selected structural variations in the substrates

Ti/Ni-Mediated Inter- and Intramolecular Conjugate Addition of Aryl and Alkenyl Halides and Triflates

In this work, we show that the unique combination of a nickel catalyst and Cp₂TiCl allows the direct conjugate addition of aryl and alkenyl iodides, bromides, and to a lesser extent, chlorides and triflates to α,β-unsaturated carbonyls at room temperature, without requiring the previous formation of an organometallic nucleophile. The reaction proceeds inter- and intramolecularly with good functional group compatibility, which is key for the development of free protecting group methodologies. Carbo- and heterocycles of five- and six-membered rings are obtained in good yields. Moreover, some insights about the mechanism involved have been obtained from cyclic voltammetry, UV–vis, and HRTEM measurements

Auswirkungen des Stabex-Systems auf die Stabilität der Exporterlöse - Eine empirische Analyse zum Nutzen partieller Stabilisierungselemente

Dyes with near-red emission are of great interest because of their undoubted advantages for use as probes in living cells. In-depth knowledge of their photophysics is essential for employment of such dyes. In this article, the photophysical behavior of a new silicon-substituted xanthene, 7-hydroxy-5,5-dimethyl-10-(<i>o</i>-tolyl)­dibenzo­[<i>b</i>,<i>e</i>]­silin-3­(5<i>H</i>)-one (<b>2-Me TM</b>), was explored by means absorption, steady-state, and time-resolved fluorescence. First, the near-neutral pH, ground-state acidity constant of the dye, p<i>K</i>_N‑A, was determined by absorbance and steady-state fluorescence at very low buffer concentrations. Next, we determined whether the addition of phosphate buffer promoted the excited-state proton-transfer (ESPT) reaction among the neutral and anion form of <b>2-Me TM</b> in aqueous solutions at near-neutral pH. For this analysis, both the steady-state fluorescence method and time-resolved emission spectroscopy (TRES) were employed. The TRES experiments demonstrated a remarkably favored conversion of the neutral form to the anion form. Then, the values of the excited-state rate constants were determined by global analysis of the fluorescence decay traces recorded as a function of pH, and buffer concentration. The revealed kinetic parameters were consistent with the TRES results, exhibiting a higher rate constant for deprotonation than for protonation, which implies an unusual low value of the excited-state acidity constant <i>pK</i>*_N‑A and therefore an enhanced photoacid behavior of the neutral form. Finally, we determined whether <b>2-Me TM</b> could be used as a sensor inside live cells by measuring the intensity profile of the probe in different cellular compartments of HeLa 229 cells

Ti(III)-Catalyzed Cyclizations of Ketoepoxypolyprenes: Control over the Number of Rings and Unexpected Stereoselectivities

We describe a new strategy to control the number of cyclization steps in bioinspired radical (poly)­cyclizations involving epoxypolyenes containing keto units positioned along the polyene chain. This approach provides an unprecedentedly straightforward access to natural terpenoids with pendant unsaturated side chains. Additionally, in the case of bi- and tricyclizations, decalins with <i>cis</i> stereochemistry have been obtained as a consequence of the presence of the ketone. The preferential formation of <i>cis</i>-fused adducts was rationalized using DFT calculations. This result is completely unprecedented in biomimetic cyclizations and permits the access to natural terpenoids with this stereochemistry, as well as to non-natural analogues

Ti(III)-Catalyzed Cyclizations of Ketoepoxypolyprenes: Control over the Number of Rings and Unexpected Stereoselectivities

We describe a new strategy to control the number of cyclization steps in bioinspired radical (poly)­cyclizations involving epoxypolyenes containing keto units positioned along the polyene chain. This approach provides an unprecedentedly straightforward access to natural terpenoids with pendant unsaturated side chains. Additionally, in the case of bi- and tricyclizations, decalins with <i>cis</i> stereochemistry have been obtained as a consequence of the presence of the ketone. The preferential formation of <i>cis</i>-fused adducts was rationalized using DFT calculations. This result is completely unprecedented in biomimetic cyclizations and permits the access to natural terpenoids with this stereochemistry, as well as to non-natural analogues

Pyrene-Containing <i>ortho</i>-Oligo(phenylene)ethynylene Foldamer as a Ratiometric Probe Based on Circularly Polarized Luminescence

In this manuscript, we report the first synthesis of an organic monomolecular emitter, which behaves as a circularly polarized luminescence (CPL)-based ratiometric probe. The enantiopure helical <i>ortho</i>-oligo­(phenylene)­ethynylene (<i>o</i>-OPE) core has been prepared by a new and efficient macrocyclization reaction. The combination of such <i>o</i>-OPE helical skeleton and a pyrene couple leads to two different CPL emission features in a single structure whose ratio linearly responds to silver­(I) concentration

Development of a New Dual Polarity and Viscosity Probe Based on the Foldamer Concept

Small molecular probes able to act as sensors are of enormous interest thanks to their multiple applications. Here, we report on the development of a novel supramolecular dual viscosity and polarity probe based on the foldamer concept, which increases the resolution limits of traditional probes at low viscosity values (0–4 mPa·s). The applicability of this new probe has been tested with a supramolecular organogel