Synthetic Studies of Daphniphyllum Alkaloids: A New Method for the Construction of [7-5-5] All-carbon Tricyclic Skeleton

Daphniphyllum alkaloids have a complex molecular structure; thus, their synthesis can be challenging. A new method for the construction of the [7-5-5] tricyclic core of Daphniphyllum alkaloids was developed. The bicyclo[5.3.0]decane skeleton was constructed via the divinylcyclopropane rearrangement of a cyclopentenone derivative with a vinylcyclopropyl group at the β-position. After introducing a 2-iodoethyl group via a regioselective Michael addition with phenyl vinyl selenone, the [7-5-5] tricyclic system was formed by the intramolecular alkylation reaction of a cyclopentadienyl anion species.

Rh(I) Complexes in Catalysis: A Five-Year Trend

Rhodium is one of the most used metals in catalysis both in laboratory reactions and industrial processes. Despite the extensive exploration on “classical” ligands carried out during the past decades in the field of rhodium-catalyzed reactions, such as phosphines, and other common types of ligands including N-heterocyclic carbenes, ferrocenes, cyclopentadienyl anion and pentamethylcyclopentadienyl derivatives, etc., there is still lively research activity on this topic, with considerable efforts being made toward the synthesis of new preformed rhodium catalysts that can be both efficient and selective. Although the “golden age” of homogeneous catalysis might seem over, there is still plenty of room for improvement, especially from the point of view of a more sustainable chemistry. In this review, temporally restricted to the analysis of literature during the past five years (2015–2020), the latest findings and trends in the synthesis and applications of Rh(I) complexes to catalysis will be presented. From the analysis of the most recent literature, it seems clear that rhodium-catalyzed processes still represent a stimulating challenge for the metalloorganic chemist that is far from being over.

Designing Rh(I)-Half-Sandwich Catalysts for Alkyne [2+2+2] Cycloadditions

Metal-mediated [2+2+2] cycloadditions of unsaturated molecules to cyclic and polycyclic organic compounds are a versatile synthetic route affording good yields and selectivity under mild conditions. In the last two decades, in silico investigations have unveiled important details about the mechanism and the energetics of the whole catalytic cycle. Particularly, a number of computational studies address the topic of half-sandwich catalysts which, due to their structural fluxionality, have been widely employed, since the 1980s. In these organometallic species, the metal is coordinated to an aromatic ring, typically the ubiquitous cyclopentadienyl anion, C5H5 –(Cp) or to the Cp moiety of a larger polycyclic aromatic ligand (Cp′). During the catalytic process, the metal continuously ‘slips’ on the ring, changing its hapticity. This phenomenon of metal slippage and its implications for the catalyst’s performance are discussed in this work, referring to the most important computational mechanistic studies reported in literature for Rh(I) half-metallocenes, with the purpose of providing hints for a rational design of this class of compounds.1 Introduction2 Mechanism of Metal-Catalyzed Acetylene [2+2+2] Cycloaddition to Benzene and the Problem of the Indenyl Effect2.1 Acetylene-Acetonitrile [2+2+2] Co-cycloaddition to 2-Methylpyridine: Evidence of the Indenyl Effect2.2 Heteroaromatic Catalysts and the Evidence of a Reverse Indenyl Effect2.3 Booth’s Mechanistic Hypothesis and the Evidence of the Indenyl Effect3 Structure–Reactivity Correlation: The Slippage-Span Model4 Conclusions and Perspectives

Deriving the Vibronic Coupling Constants of the Cyclopentadienyl Radical with Density Functional Theory and G0W0

<div>The vibronic coupling constants of the cyclopentadienyl radical have been calculated with G₀W₀, HF, and DFT with various exchange-correlation functionals. The vibronic coupling constants for HF and DFT were derived using the gradients of the eigenvalues of the degenerate HOMOs of the closed-shell cyclopentadienyl anion, while the gradients of the corresponding quasiparticle energy levels were used in the case of G₀W₀. The differences between the linear vibronic constants obtained using HF and DFT were found to be small, and reduced further when the G₀W₀ correction is applied to HF and DFT. Finally, the linear vibronic coupling constants calculated with G₀W₀ were found to agree well with the values obtained using high level wave function methods in the literature, which suggests that G₀W₀ can be a useful tool towards the study of vibronic coupling.</div>

Deriving the Vibronic Coupling Constants of the Cyclopentadienyl Radical with Density Functional Theory and G0W0

<div>The vibronic coupling constants of the cyclopentadienyl radical have been calculated with G₀W₀, HF, and DFT with various exchange-correlation functionals. The vibronic coupling constants for HF and DFT were derived using the gradients of the eigenvalues of the degenerate HOMOs of the closed-shell cyclopentadienyl anion, while the gradients of the corresponding quasiparticle energy levels were used in the case of G₀W₀. The differences between the linear vibronic constants obtained using HF and DFT were found to be small, and reduced further when the G₀W₀ correction is applied to HF and DFT. Finally, the linear vibronic coupling constants calculated with G₀W₀ were found to agree well with the values obtained using high level wave function methods in the literature, which suggests that G₀W₀ can be a useful tool towards the study of vibronic coupling.</div>

In Silico Acetylene [2+2+2] Cycloadditions Catalyzed by Rh/Cr Indenyl Fragments

Metal-catalyzed alkyne [2+2+2] cycloadditions provide a variety of substantial aromatic compounds of interest in the chemical and pharmaceutical industries. Herein, the mechanistic aspects of the acetylene [2+2+2] cycloaddition mediated by bimetallic half-sandwich catalysts [Cr(CO)3IndRh] (Ind = (C9H7)−, indenyl anion) are investigated. A detailed exploration of the potential energy surfaces (PESs) was carried out to identify the intermediates and transition states, using a relativistic density functional theory (DFT) approach. For comparison, monometallic parent systems, i.e., CpRh (Cp = (C5H5)−, cyclopentadienyl anion) and IndRh, were included in the analysis. The active center is the rhodium nucleus, where the [2+2+2] cycloaddition occurs. The coordination of the Cr(CO)3 group, which may be in syn or anti conformation, affects the energetics of the catalytic cycle as well as the mechanism. The reaction and activation energies and the turnover frequency (TOF) of the catalytic cycles are rationalized, and, in agreement with the experimental findings, our computational analysis reveals that the presence of the second metal favors the catalysis.

A novel octa-nuclear 32-membered zirconocene macrocycle based on the aromatic selenite

A novel macrocyclic zirconocene(iv) aromatic selenite [(CpZr)8L16]·2(Cp4Zr2(μ-O)Cl2) (complex 1) (Cp = cyclopentadienyl anion; L = 4-fluorobenzeneseleninic acid) was prepared by the reaction of bis(cyclopentadienyl)zirconium dichloride with 4-fluorobenzeneseleninic acid and characterized by elemental analysis, infrared spectroscopy, 1H, 13C NMR spectroscopy, ESI-MS, XRD and X-ray diffraction.

Nucleus-independent chemical shift profiles along the intrinsic distortion path for Jahn-Teller active molecules. Study on cyclopentadienyl radical and cobaltocene

The aromatic/antiaromatic behavior of the cyclopentadienyl anion (Cp-), bis(?5-cyclopentadienyl)iron(II) (Fe(Cp)2), as well as of the Jahn-Teller (JT) active cyclopentadienyl radical (Cp?) and bis(?5-cyclopentadienyl)cobalt(II) (Co(Cp)2) has been investigated using Density Functional Theory (DFT) calculations of the Nuclear Independent Chemical Shifts (NICS). According to the NICS values, pentagon ring in Fe(Cp)2 is more aromatic than isolated Cp-. The NICS parameters have been scanned along the Intrinsic Distortion Path (IDP) for Cp? and Co(Cp)2 showing antiaromaticity, which decreases with increasing deviation from high symmetry D5h to low symmetry (LS) C2v. Changes in the NICS values along the IDP revealed that Co(Cp)2 in the LS nuclear arrangement has aromatic character, in contrast to the case of Cp?