Interplay between 1,3-Butadien-1,4-diyl and 2‑Buten-1,4-dicarbene Derivatives: The Quest for Nucleophilic Carbenes

By means of high level quantum chemical calculations, the influence of electron-donating heteroatomic groups (O, NH) was investigated on the 1,6-transannular ring closure of 1,6-cyclodecadiyne (<b>8a</b>). In the case of <b>8a</b>, the bicyclo[4.4.0]­deca-1,6-dien-2,7-diyl biradical <b>12</b> is generated. It was found that oxygen centers or NH groups next to the triple bond reduce the activation energy of the ring closure considerably. For the intermediate, a 2-buten-1,4-dicarbene derivative is predicted. The extension of the model calculations to two hydroxyl- or aminoacetylenes predicts the formation of the corresponding 1,3-butadien-1,4-diyl intermediates or the 2-buten-1,4-dicarbene derivatives, a member of the nucleophilic carbene family. Moreover, the calculations predict that two separated dimethoxyacetylenes are more than 7 kcal/mol less stable than the corresponding biradical and dicarbene, respectively. Possible reactions of the dicarbenes with transition metal compounds are discussed

Long Chalcogen–Chalcogen Bonds in Electron-Rich Two and Four Center Bonds: Combination of π- and σ‑Aromaticity to a Three-Dimensional σ/π-Aromaticity

Quantum chemical calculations were carried out by applying density functional theory to study the two center-three electron (2c-3e) bonds between the sulfur centers of cyclic dithio­ethers. Calculated were the S–S distance, the stabilization energy, and the energy of the σ → σ* transition. The extension of the calculations to two (2c-3e) bonds in one molecule shows that a rearrangement to one σ bond and two lone pairs on sulfur is usually more favorable. Exceptions are [H₂S₂ ⁺]₂, the dimer of the 1,2-dithia-3,5-diazolyl radical (<b>27a</b>), the dimer of the 1,2,4-trithia-3,5-diazolyl radical cation (<b>26a</b> ²⁺), and its Selena congeners and derivatives. In the case of [H₂S₂ ⁺]₂, the (4c-6e) bond between the chalcogen centers is a good description of this dimer. To describe the binding situation in the dimer <b>26a</b> ²⁺ and <b>27a</b>, the concept of a “simple” (4c-6e) bond was extended. Our calculations reveal a strong σ-aromaticity within the plane of the four sulfur centers in addition to a strong π-conjugation within the five-membered rings. The whole phenomenon can best be described as a three-dimensional σ/π-aromaticity within the 14π dimers

Switching Process Consisting of Three Isomeric States of an Azobenzene Unit

Azobenzene and its derivatives are among the most commonly used switching units in organic chemistry. The switching process consists of two states, in which the <i>trans</i> isomer has a stretched and the <i>cis</i> isomer a compact form. Here, we have designed a system in which all isomeric states of an azobenzene moiety (<i>trans</i> → <i>cis</i>-(<i>M</i>) → <i>cis</i>-(<i>P</i>)) are passed step by step. The first step involves a change in the distance between the benzene units, which is common for azobenzene derivatives. In the second step an inversion of the helicity (<i>M</i>→<i>P</i>) of the <i>cis</i> azobenzene unit takes place. The third step leads back to the stretched <i>trans</i> isomer. This switching cycle is achieved by coupling the azobenzene unit with two chiral clamps and with a further azobenzene switching unit

Enediyne Dimerization vs Bergman Cyclization

High-level quantum chemical calculations reveal that the dimerization of enediynes to 1,3-butadiene-1,4-diyl diradicals is energetically more favored than the corresponding Bergman cyclization of enediynes. Moreover, the activation barrier of both reactions can be drastically reduced by the introduction of electron-withdrawing substituents like fluoro groups at the reacting carbon centers of the triple bonds

Dimerization of Two Alkyne Units: Model Studies, Intermediate Trapping Experiments, and Kinetic Studies

By means of high level quantum chemical calculations (B2PLYPD and CCSD­(T)), the dimerization of alkynes substituted with different groups such as F, Cl, OH, SH, NH₂, and CN to the corresponding diradicals and dicarbenes was investigated. We found that in case of monosubstituted alkynes the formation of a bond at the nonsubstituted carbon centers is favored in general. Furthermore, substituents attached to the reacting centers reduce the activation energies and the reaction energies with increasing electronegativity of the substituent (<i>F</i> > OH > NH₂, Cl > SH, H, CN). This effect was explained by a stabilizing hyperconjugative interaction between the σ* orbitals of the carbon-substituent bond and the occupied antibonding linear combination of the radical centers. The formation of dicarbenes is only found if strong π donors like NH₂ and OH as substituents are attached to the carbene centers. The extension of the model calculations to substituted phenylacetylenes (Ph–CC–Y) predicts a similar reactivity of the phenylacetylenes: <i>F</i> > OCH₃ > Cl > H. Trapping experiments of the proposed cyclobutadiene intermediates using maleic anhydride as dienophile as well as kinetic studies confirm the calculations. In the case of phenylmethoxyacetylene (Ph–CC–OCH₃) the good yield of the corresponding cycloaddition product makes this cyclization reaction attractive for a synthetic route to cyclohexadiene derivatives from alkynes

Bio-inspired Herringbone Foldamers: Strategy for Changing the Structure of Helices

Cyclic oligomers of azole peptides were isolated from a multitude of marine organisms and were used for a large number of molecular machines. As shown previously, oligomers derived from <i>achiral</i> imidazole amino acids fold into <i>canonical</i> helices. Here we show that a minor change, the introduction of a methyl group in the δ position, results in a significant change in the secondary structure of the corresponding oligomers. Instead of a canonical helix, a noncanonical herringbone helix is formed. In the latter, the slope along the helix changes its sign at least twice per turn. This strategy allows a remarkable change of the secondary structure via a small modification. By means of enantiomerically pure amino acids, we were able to control, for the first time, both the helicity of the helix and the form of the herringbone. The investigation of the underlying herringbone basic element and its folding to a noncanonical helix were conducted by NMR and CD spectroscopy, as well as by X-ray crystallography and quantum chemical calculations

Bio-inspired Herringbone Foldamers: Strategy for Changing the Structure of Helices

Cyclic oligomers of azole peptides were isolated from a multitude of marine organisms and were used for a large number of molecular machines. As shown previously, oligomers derived from <i>achiral</i> imidazole amino acids fold into <i>canonical</i> helices. Here we show that a minor change, the introduction of a methyl group in the δ position, results in a significant change in the secondary structure of the corresponding oligomers. Instead of a canonical helix, a noncanonical herringbone helix is formed. In the latter, the slope along the helix changes its sign at least twice per turn. This strategy allows a remarkable change of the secondary structure via a small modification. By means of enantiomerically pure amino acids, we were able to control, for the first time, both the helicity of the helix and the form of the herringbone. The investigation of the underlying herringbone basic element and its folding to a noncanonical helix were conducted by NMR and CD spectroscopy, as well as by X-ray crystallography and quantum chemical calculations

<i>anti</i>-Diradical Formation in 1,3-Dipolar Cycloadditions of Nitrile Oxides to Acetylenes

By means of high level quantum chemical calculations (B2PLYPD and CCSD­(T)), the mechanisms of the reaction of nitrile oxides with alkenes and alkynes were investigated. We were able to show that in the case of alkenes, regardless of the chosen substituents, the concerted mechanism is always energetically favored as compared to a two-step process, which runs through an <i>anti</i>-diradical species. In the case of alkynes, the concerted mechanism is favored only for the reaction of alkyl-substituted acetylenes. For aryl-substituted acetylenes, the activation barrier toward the <i>anti</i>-diradical is equal to or lower than the activation barrier of the concerted reaction. This reversal of the reaction paths is not only limited to nitrile oxides as dipolarophiles. Conditions favoring the <i>anti</i>-diradical path are the presence of a triple bond in both the 1,3-dipole and the dipolarophile and additionally an aryl substituent attached to the alkyne. The featured energy relationships between the reaction paths are able to explain the experimentally observed byproducts of the reaction of nitrile oxides with arylacetylenes. The discovered differences for the preferred reaction path of 1,3-dipolar cycloadditions to acetylenes should be of considerable interest to a broader field of chemists

Model Studies on the Dimerization of 1,3-Diacetylenes

By means of high-level quantum chemical calculations (B2PLYPD and CCSD­(T)), the dimerization of 1,3-diacetylenes was studied and compared to the dimerization of acetylene. We found that substituted 1,3-diacetylenes are more reactive than the corresponding substituted acetylenes having an isolated triple bond. The most reactive centers for a dimerization are always the terminal carbon atoms. The introduction of a test reaction allows the calculation of the relative reactivity of individual carbon centers in phenylacetylene, phenylbutadiyne, and phenylhexatriyne. A comparison shows that the reactivity of the terminal carbon atoms increases with increasing numbers of alkyne units, whereas the reactivity of the internal carbon atoms remains very low independent of the number of alkyne units

Au(I)-Catalyzed Dimerization of Two Alkyne UnitsInterplay between Butadienyl and Cyclopropenylmethyl Cation: Model Studies and Trapping Experiments

In recent years, Au­(I)-catalyzed reactions proved to be a valuable tool for the synthesis of substituted cycles by cycloaromatization and cycloisomerization starting from alkynes. Despite the myriad of Au­(I)-catalyzed reactions of alkynes, the mono Au­(I)-catalyzed pendant to the radical dimerization of nonconjugated alkyne units has not been investigated by quantum chemical calculations. Herein, by means of quantum chemical calculations, we describe the mono Au­(I)-catalyzed dimerization of two alkyne units as well as the transannular ring closure reaction of a nonconjugated diyne. We found that depending on the system and the method used either the corresponding cyclopropenylmethyl cation or the butadienyl cation represents the stable intermediate. This circumstance could be explained by different stabilizing effects. Moreover, the calculation reveals a dramatic (>10¹²-fold) acceleration of the Au­(I)-catalyzed reaction compared to that of the noncatalyzed radical variant. Trapping experiments with a substituted 1,6-cyclodecadiyne using benzene as a solvent at room temperature as well as studies with deuterated solvents confirm the calculations. In this context, we also demonstrate that trapping of the cationic intermediate with benzene does not proceed via a Friedel–Crafts-type reaction