Mechanistic Diversity in Thermal Fragmentation Reactions: A Computational Exploration of CO and CO₂ Extrusions from Five-Membered Rings

The mechanisms of a variety of thermal pericyclic fragmentation reactions of five-membered heterocyclic rings are subjected to scrutiny at a density functional level by computation of transition state free energy barriers and intrinsic reaction coordinates (IRCs). The preferred computed products generally match those observed in flash vacuum thermolysis experiments. For certain reactions, which also have the highest reaction temperatures and computed barriers, a degree of multireference character to the wave function manifests in an overestimation of the DFT-computed barrier, with a more reasonable barrier obtained by a CASSCF single point energy calculation. Many of the IRCs exhibit “hidden intermediates” along the reaction pathway, but conversely reactions that could be considered to involve the formation of an intermediate nitrene prior to alkyl or aryl migration show no evidence of such an intermediate. Such exploration of the diversity of behavior in a class of compounds using computational methods with interactive presentation of the results within the body of a journal article is suggested as being almost a <i>sine qua non</i> for laboratory-based research on reactive intermediates

Catalytic and Computational Studies of N‑Heterocyclic Carbene or Phosphine-Containing Copper(I) Complexes for the Synthesis of 5‑Iodo-1,2,3-Triazoles

Two complementary catalytic systems are reported for the 1,3-dipolar cycloaddition of azides and iodoalkynes. These are based on two commercially available/readily available copper complexes, [CuCl­(IPr)] or [CuI­(PPh₃)₃], which are active at low metal loadings (PPh₃ system) or in the absence of any other additive (IPr system). These systems were used for the first reported mechanistic studies on this particular reaction. An experimental/computational-DFT approach allowed to establish that (1) some iodoalkynes might be prone to dehalogenation under copper catalysis conditions and, more importantly, (2) two distinct mechanistic pathways are likely to be competitive with these catalysts, either through a copper­(III) metallacycle or via direct π-activation of the starting iodoalkyne

A Molecular Complex with a Formally Neutral Iron Germanide Motif (Fe₂Ge₂)

We report the synthesis and isolation of a stable complex containing the formally neutral Fe₂Ge₂ motif, which is stabilized by the coordination of an N-heterocyclic carbene to the germanium and of carbon monoxide to the iron center. [(NHC^{<i>i</i>Pr₂Me₂})­GeFe­(CO)₄]₂ is obtained by reduction of the NHC^{<i>i</i>Pr₂Me₂}-coordinated dichlorogermylene adduct of Fe­(CO)₄, which in turn is obtained from the reaction of Fe₂(CO)₉ with GeCl₂·NHC^{<i>i</i>Pr₂Me₂} (NHC^{<i>i</i>Pr₂Me₂} = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). The solid-state structure of the title compound reveals two distinct coordination modes for the Fe­(CO)₄ fragments: bridging (π-type) and terminal (σ-type). In solution, the rapid equilibrium between the two modes was resolved by NMR at −35 °C. Reaction with propylene sulfide at room temperature affords the sulfide-bridged digermanium complex with two terminal Fe­(CO)₄ moieties

A Molecular Complex with a Formally Neutral Iron Germanide Motif (Fe₂Ge₂)

We report the synthesis and isolation of a stable complex containing the formally neutral Fe₂Ge₂ motif, which is stabilized by the coordination of an N-heterocyclic carbene to the germanium and of carbon monoxide to the iron center. [(NHC^{<i>i</i>Pr₂Me₂})­GeFe­(CO)₄]₂ is obtained by reduction of the NHC^{<i>i</i>Pr₂Me₂}-coordinated dichlorogermylene adduct of Fe­(CO)₄, which in turn is obtained from the reaction of Fe₂(CO)₉ with GeCl₂·NHC^{<i>i</i>Pr₂Me₂} (NHC^{<i>i</i>Pr₂Me₂} = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene). The solid-state structure of the title compound reveals two distinct coordination modes for the Fe­(CO)₄ fragments: bridging (π-type) and terminal (σ-type). In solution, the rapid equilibrium between the two modes was resolved by NMR at −35 °C. Reaction with propylene sulfide at room temperature affords the sulfide-bridged digermanium complex with two terminal Fe­(CO)₄ moieties

A Hückel Theory Perspective on Möbius Aromaticity

Heilbronner’s Hückel molecular orbital treatment of Möbius 4n−π annulenes is revisited. When uneven twisting in π-systems of small Möbius rings is accounted for, their resonance energies become comparable to iso-π-electronic linear alkenes with the same number of carbon atoms. Larger Möbius rings distribute π-twisting more evenly but exhibit only modest aromatic stabilization. Dissected nucleus independent chemical shifts (NICS), based on the LMO (localized molecular orbital)–NICS(0)_π index confirm the magnetic aromaticity of the Möbius annulenes considered

Total Synthesis of (+)-Lophirone H and Its Pentamethyl Ether Utilizing an Oxonium–Prins Cyclization

The first total synthesis of (+)-lophirone H (<b>1</b>) and its pentamethyl ether <b>29</b>, featuring an oxonium–Prins cyclization/benzylic cation trapping reaction, is described

Contraction and Expansion of the Silicon Scaffold of Stable Si₆R₆ Isomers

The reactivity of two stable Si₆R₆ clusters (<b>4</b> and <b>5</b>, R = 2,4,6-^<i>i</i>Pr₃C₆H₂) with unsymmetrical substitution patterns (including Si, SiR, and SiR₂ vertices) is reported. In order to account for the importance of such clusters as model systems for transient intermediates in the deposition of elemental silicon, we here propose the term “siliconoids” for silicon clusters with unsaturated valencies. With the hexasilaprismane <b>8a</b>, a saturatedi.e., non-siliconoidSi₆R₆ isomer is accessible from a suitable Si₃ precursor. Thermal redistribution of the substituents converts 1,1,2-trichlorocyclotrisilane <b>6</b> into the corresponding 1,2,3-derivative <b>7</b> prior to the requisite reductive coupling step leading to <b>8a</b>. On the other hand, a stable expanded Si₁₁-siliconoid <b>9</b> was isolated as a minor side product of the thermal isomerization of <b>4</b> to <b>5</b>, thus providing a first example of siliconoid cluster expansion in the condensed phase. In the solid-state structure, the two unsubstituted vertices of <b>9</b> strongly interact in a staggered propellane-like fashion. Oxidative cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid <b>5</b> with a large excess of iodine in refluxing toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane <b>11</b> in high yield. Conversely, chlorination of the isomeric <b>4</b> with BiCl₃ as a mild source of Cl₂ results in a complex mixture of products from chlorination of the unsubstituted vertices as well as σ-bonds of the cluster framework of <b>4</b>. The main product, 1,2-dichlorotricyclo­[2.2.0.0^2,5]­hexasilane <b>12</b>, undergoes thermal cluster contraction to give tricyclo­[2.1.0.0^2,5]­pentasilane <b>14</b> with an exohedral chlorosilyl group

Experimental and Computational Investigation of the Mechanism of Carbon Dioxide/Cyclohexene Oxide Copolymerization Using a Dizinc Catalyst

A detailed study of the mechanism by which a dizinc catalyst copolymerizes cyclohexene oxide and carbon dioxide is presented. The catalyst, previously published by Williams et al. (Angew. Chem. Int. Ed. 2009, 48, 931), shows high activity under just 1 bar pressure of CO₂. This work applies <i>in situ</i> attenuated total reflectance infrared spectroscopy (ATR-FTIR) to study changes to the catalyst structure on reaction with cyclohexene oxide and, subsequently, with carbon dioxide. A computational investigation, using DFT with solvation corrections, is used to calculate the relative free energies for various transition states and intermediates in the cycle for alternating copolymerization catalyzed by this dinuclear complex. Two potentially competing side reactions, sequential epoxide enchainment and sequential carbon dioxide enchainment are also investigated. The two side-reactions are shown to be thermodynamically disfavored, rationalizing the high selectivity exhibited in experimental studies using <b>1</b>. Furthermore, the DFT calculations show that the rate-determining step is the nucleophilic attack of the coordinated epoxide molecule by the zinc-bound carbonate group in line with previous experimental findings (ΔΔ<i>G</i>₃₅₃ = 23.5 kcal/mol; Δ<i>G</i>^‡₃₅₃ = 25.7 kcal/mol). Both <i>in situ</i> spectroscopy and DFT calculations indicate that just one polymer chain is initiated per dizinc catalyst molecule. The catalyst adopts a “bowl” shape conformation, whereby the acetate group coordinated on the concave face is a spectator ligand while that coordinated on the convex face is the initiating group. The spectator carboxylate group plays an important role in the catalytic cycle, counter-balancing chain growth on the opposite face. The DFT was used to predict the activities of two new catalysts, good agreement between experimental turn-over-numbers and DFT predictions were observed

Contraction and Expansion of the Silicon Scaffold of Stable Si₆R₆ Isomers

The reactivity of two stable Si₆R₆ clusters (<b>4</b> and <b>5</b>, R = 2,4,6-^<i>i</i>Pr₃C₆H₂) with unsymmetrical substitution patterns (including Si, SiR, and SiR₂ vertices) is reported. In order to account for the importance of such clusters as model systems for transient intermediates in the deposition of elemental silicon, we here propose the term “siliconoids” for silicon clusters with unsaturated valencies. With the hexasilaprismane <b>8a</b>, a saturatedi.e., non-siliconoidSi₆R₆ isomer is accessible from a suitable Si₃ precursor. Thermal redistribution of the substituents converts 1,1,2-trichlorocyclotrisilane <b>6</b> into the corresponding 1,2,3-derivative <b>7</b> prior to the requisite reductive coupling step leading to <b>8a</b>. On the other hand, a stable expanded Si₁₁-siliconoid <b>9</b> was isolated as a minor side product of the thermal isomerization of <b>4</b> to <b>5</b>, thus providing a first example of siliconoid cluster expansion in the condensed phase. In the solid-state structure, the two unsubstituted vertices of <b>9</b> strongly interact in a staggered propellane-like fashion. Oxidative cluster contraction of a siliconoid scaffold is observed upon treatment of siliconoid <b>5</b> with a large excess of iodine in refluxing toluene, thus providing access to a highly functionalized hexaiodocyclopentasilane <b>11</b> in high yield. Conversely, chlorination of the isomeric <b>4</b> with BiCl₃ as a mild source of Cl₂ results in a complex mixture of products from chlorination of the unsubstituted vertices as well as σ-bonds of the cluster framework of <b>4</b>. The main product, 1,2-dichlorotricyclo­[2.2.0.0^2,5]­hexasilane <b>12</b>, undergoes thermal cluster contraction to give tricyclo­[2.1.0.0^2,5]­pentasilane <b>14</b> with an exohedral chlorosilyl group

Experimental and Computational Investigation of the Mechanism of Carbon Dioxide/Cyclohexene Oxide Copolymerization Using a Dizinc Catalyst

A detailed study of the mechanism by which a dizinc catalyst copolymerizes cyclohexene oxide and carbon dioxide is presented. The catalyst, previously published by Williams et al. (Angew. Chem. Int. Ed. 2009, 48, 931), shows high activity under just 1 bar pressure of CO₂. This work applies <i>in situ</i> attenuated total reflectance infrared spectroscopy (ATR-FTIR) to study changes to the catalyst structure on reaction with cyclohexene oxide and, subsequently, with carbon dioxide. A computational investigation, using DFT with solvation corrections, is used to calculate the relative free energies for various transition states and intermediates in the cycle for alternating copolymerization catalyzed by this dinuclear complex. Two potentially competing side reactions, sequential epoxide enchainment and sequential carbon dioxide enchainment are also investigated. The two side-reactions are shown to be thermodynamically disfavored, rationalizing the high selectivity exhibited in experimental studies using <b>1</b>. Furthermore, the DFT calculations show that the rate-determining step is the nucleophilic attack of the coordinated epoxide molecule by the zinc-bound carbonate group in line with previous experimental findings (ΔΔ<i>G</i>₃₅₃ = 23.5 kcal/mol; Δ<i>G</i>^‡₃₅₃ = 25.7 kcal/mol). Both <i>in situ</i> spectroscopy and DFT calculations indicate that just one polymer chain is initiated per dizinc catalyst molecule. The catalyst adopts a “bowl” shape conformation, whereby the acetate group coordinated on the concave face is a spectator ligand while that coordinated on the convex face is the initiating group. The spectator carboxylate group plays an important role in the catalytic cycle, counter-balancing chain growth on the opposite face. The DFT was used to predict the activities of two new catalysts, good agreement between experimental turn-over-numbers and DFT predictions were observed