Stretching the P–C Bond. Variations on Carbenes and Phosphanes

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene–pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene–phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PXphosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic

On the Organocatalytic Activity of N-Heterocyclic Carbenes: Role of Sulfur in Thiamine

The reaction energy profiles of the benzoin condensation from three aldehydes catalyzed by imidazol-2-ylidene, triazol-3-ylidene, and thiazol-2-ylidene have been investigated computationally. The barriers for all steps of all investigated reactions have been found to be low enough to indicate the viability of the mechanism proposed by Breslow in the 1950s. The most remarkable difference in the catalytic cycles has been the increased stability of the Breslow intermediate in case of thiazol-2-ylidene (by ca. 10 kcal/mol) compared to the other two carbenes, which results in lower energy for the coupling of the second aldehyde molecule, thus, increasing the reversibility of the reaction. Since the analogous transketolase reaction, being involved in the carbohydrate metabolism of many organisms, requires an initial decouplinga reverse benzoin condensationthis difference provides a reasonable explanation for the presence of a thiazolium ring in thiamine instead of the otherwise generally more available imidazole derivatives. The “resting intermediate” found by Berkessel and co-workers for a triazole-based catalyst was found more stable than the Breslow intermediate for all of the systems investigated. The (gas phase) proton affinities of several carbenes were compared, the relative trends being in agreement with the available (in aqueous solution) data. The hydrolytic ring-opening reaction of the thiazole-based carbene was shown to be different from that of imidazole-2-ylidenes

Moving on from Silicon to the Heavier Tetrels: Germyl- and Stannyl-Substituted Phosphole Derivatives

Germyl- and stannyl-substituted phospholes have been prepared and isolated. The increased reactivity of the tetrel carbon bond requires increased effort in purification by initial transformation to the chalcogen derivatives and subsequent reduction to the phosphole after subsequent to chromatographic purification for the germanium derivative. The photophysical properties of the germyl phosphole are comparable to that of its silyl analogue, whereas the stannyl phospholes turned out to be nonluminescent. All isolated compounds have been characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. Furthermore, single-crystal X-ray diffraction and density functional theory (DFT) calculations have been performed on selected compounds

Moving on from Silicon to the Heavier Tetrels: Germyl- and Stannyl-Substituted Phosphole Derivatives

Germyl- and stannyl-substituted phospholes have been prepared and isolated. The increased reactivity of the tetrel carbon bond requires increased effort in purification by initial transformation to the chalcogen derivatives and subsequent reduction to the phosphole after subsequent to chromatographic purification for the germanium derivative. The photophysical properties of the germyl phosphole are comparable to that of its silyl analogue, whereas the stannyl phospholes turned out to be nonluminescent. All isolated compounds have been characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. Furthermore, single-crystal X-ray diffraction and density functional theory (DFT) calculations have been performed on selected compounds

Bending Ferrocenes with Low Coordinated Bridging Units: The Investigation of Carbenes and Their Analogues with a Ferrocenophane Backbone

[3]ferrocenophanes with X–E–X ansa moieties containing a low coordinated center E stabilized by adjacent donor units X were studied by density functional theory methods. The cyclopentadienyl (Cp) rings favor an eclipsed position in most cases and exhibit a shortened C(1)–C(1′) distance compared to parent ferrocene. In case of bridges with the second row elements, the tilt of the Cp rings is more significant than that in case of third row elements; however, the estimated strain does not exceed 6 kcal/mol. The ansa unit has similar structural characteristics to the X–E–X-fragment in a six-membered saturated ring, with bond angles larger than that in the well-known heterocycles featuring five-membered cyclic systems. For compounds with X = PMe and E = C, Si, Ge, the non-planar coordination of the phosphorus atoms yields two symmetric minimum structures that are distinguished by trans and cis alignment of the PMe groups and are connected by a low-energy asymmetric transition structure with one planarized and one highly pyramidal phosphorus. In case of the analogous species with E = P+, this asymmetric structure was located as the sole minimum. A detailed analysis of the Kohn–Sham orbitals and the analysis of the electron density show that the electronic system of the ferrocene fragment is not mixing considerably with that of the low coordinated center of the ansa unit

Bending Ferrocenes with Low Coordinated Bridging Units: The Investigation of Carbenes and Their Analogues with a Ferrocenophane Backbone

[3]ferrocenophanes with X–E–X ansa moieties containing a low coordinated center E stabilized by adjacent donor units X were studied by density functional theory methods. The cyclopentadienyl (Cp) rings favor an eclipsed position in most cases and exhibit a shortened C(1)–C(1′) distance compared to parent ferrocene. In case of bridges with the second row elements, the tilt of the Cp rings is more significant than that in case of third row elements; however, the estimated strain does not exceed 6 kcal/mol. The ansa unit has similar structural characteristics to the X–E–X-fragment in a six-membered saturated ring, with bond angles larger than that in the well-known heterocycles featuring five-membered cyclic systems. For compounds with X = PMe and E = C, Si, Ge, the non-planar coordination of the phosphorus atoms yields two symmetric minimum structures that are distinguished by trans and cis alignment of the PMe groups and are connected by a low-energy asymmetric transition structure with one planarized and one highly pyramidal phosphorus. In case of the analogous species with E = P+, this asymmetric structure was located as the sole minimum. A detailed analysis of the Kohn–Sham orbitals and the analysis of the electron density show that the electronic system of the ferrocene fragment is not mixing considerably with that of the low coordinated center of the ansa unit

Toward a 1,4-Diphosphinine-Based Molecular CPS-Ternary Compound

Synthesis of the tricyclic 1,3-dithiole-2-thione-derived 1,4-dihydro-1,4-diphosphinine is presented using a base-induced ring formation protocol and chloro­(diethylamino)­(1,3-dithiole-2-thion-4-yl)­phosphane as the starting point. P-oxidation reactions of dihydrodiphosphinine by chalcogens led to bis­(P-oxide), bis­(P-sulfide), or bis­(P-selenide), respectively; all tricyclic compounds were obtained as cis/trans mixtures. 1,4-Dihydro-1,4-diphosphinine was converted into 1,4-dichloro-1,4-dihydro-1,4-diphosphinine. This compound is almost insoluble in organic solvents, furnished selectively the trans-bis­(amino) derivative upon a 2-fold P-substitution reaction with the weak nucleophile potassium bis­(trimethylsilyl)­amide, and reacted also with alcohols ROH (R = nBu, iPr, tBu) to give cis/trans mixtures of the corresponding bis­(alkoxy) derivatives. Furthermore, the dichloro derivative could be reduced to a 1,4-diphosphinine using PnBu3, but, unfortunately, the stubbornly insoluble product could be neither purified nor crystallized. Despite this, we achieved a thermal [4 + 2] cycloaddition reaction of this first CPS-ternary compound with diethylacetylene dicarboxylate to obtain the corresponding diphosphabarrelene, thus providing indirect evidence for the aromatic tricyclic diphosphinine. Detailed density functional theory studies on the formation of 1,4-diphosphinine provided insights into formation pathways as well as NMR, IR, and UV/vis data

Reversible Redox Chemistry of Anionic Imidazole-2-thione-Fused 1,4-Dihydro-1,4-diphosphinines

Anionic 1,4-dihydro-1,4-diphosphinines were synthesized from tricyclic 1,4-diphosphinines and isolated as blue powdery salts M­[2a–2c]. Reaction of solutions of these monoanions with iodomethane led to P-methylated compounds 3a–3c. An oxidation/reduction cycle was examined, starting from solutions of K­[2a] via P–P coupled product 4a and back to K­[2a], and the recyclability and redox chemistry of this cycle were confirmed by experimental and simulated cyclic voltammetry analysis, which is proposed as a potential 2-electron cathode for rechargeable cells. TD-DFT studies were used to examine species that might be involved in the process

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A simple imaging protocol for autofluorescence elimination and optical sectioning in fluorescence endomicroscopy: supplementary materia