Alessandro Bismuto

Two novel µ‐dinitrogen‐divanadium complexes with triamidoamine derivatives have been characterized spectroscopically and structurally. The protonation reactions of the µ‐N₂ ligands for them have given ammonia. Furthermore, the mixed valence Na⁺ adduct of the dinitrogen complex has been prepared and characterized. It is also evaluated by DFT calculations.

We recently reported the syntheses, crystal structures, and protonation reactions of dinitrogen divanadium complexes bearing triamidoamine ligands, ([{V(L^R )}₂(µ‐N₂)] (R = iBu (1), EtBu (2), and iPr₂Bn (3)). In this study, we have additionally synthesized two novel dinitrogen‐divanadium complexes with similar derivatives (R = Bn (4) and MeBn (5)), which are characterized spectroscopically and structurally. The X‐ray structures have both revealed a divanadium complex with a bridging N₂ ligand. The protonation reactions of the µ‐N₂ ligands for 4 and 5 in the presence of M⁺[C₁₀H₈]^– (M⁺ = Na⁺, K⁺) and proton source (HOTf or [LutH](OTf)) have given ammonia. Particularly, the NH₃ yield from the reaction of 5 with K⁺[C₁₀H₈]^– and [LutH](OTf) has been 341 %. Furthermore, mixed valence Na⁺ adduct of 1, [Na{V(LiBu)}₂(µ‐N₂)] (6), which is prepared by the reaction of 1 with sodium metal, has been prepared and its crystal structure has also been determined. Compound 6 is electrochemically and spectroscopically characterized, which is also evaluated by DFT calculation method.

Expect the unexpected! Quantum chemical activation strain analyses reveal that Lewis acids accelerate Diels–Alder reactions by reducing the Pauli repulsion between the reactants. This novel physical mechanism for the textbook Diels–Alder reaction challenges the wide‐held belief that the Lewis acids lower the LUMO energy of the dienophile and, in that way, make them more reactive toward the diene.

Abstract

The Lewis acid(LA)‐catalyzed Diels–Alder reaction between isoprene and methyl acrylate was investigated quantum chemically using a combined density functional theory and coupled‐cluster theory approach. Computed activation energies systematically decrease as the strength of the LA increases along the series I₂<SnCl₄<TiCl₄<ZnCl₂<BF₃<AlCl₃. Emerging from our activation strain and Kohn–Sham molecular orbital bonding analysis was an unprecedented finding, namely that the LAs accelerate the Diels–Alder reaction by a diminished Pauli repulsion between the π‐electron systems of the diene and dienophile. Our results oppose the widely accepted view that LAs catalyze the Diels–Alder reaction by enhancing the donor–acceptor [HOMO_diene–LUMO_dienophile] interaction and constitute a novel physical mechanism for this indispensable textbook organic reaction.

2+2=score! The diverse [2+2] cycloaddition of organic π‐systems with a silicon–metal double bond is demonstrated, leading to a range of four‐membered metallasilacycles. Notably, this process is reversible for ethylene, with the mechanism for both alkene and alkyne addition found to mimic that for the reaction of olefins with classical carbene–metal complexes.

Abstract

The versatile cycloaddition chemistry of the Si−Ni multiple bond in the acyclic (amido)(chloro)silylene→Ni⁰ complex 1, [(^TMSL)ClSi→Ni(NHC)₂] (^TMSL=N(SiMe₃)Dipp; Dipp=2,6‐iPr₂C₆H₄; NHC=C[(iPr)NC(Me)]₂), toward unsaturated organic substrates is reported, which is both reminiscent of and expanding on the reactivity patterns of classical Fischer and Schrock carbene–metal complexes. Thus, 1:1 reaction of 1 with aldehydes, imines, alkynes, and even alkenes proceed to yield [2+2] cycloaddition products, leading to a range of four‐membered metallasilacycles. This cycloaddition is in fact reversible for ethylene, whereas addition of an excess of this olefin leads to quantitative sp²‐CH bond activation, via a 1‐nickela‐4‐silacyclohexane intermediate. These results have been supported by DFT calculations giving insights into key mechanistic aspects.

A stimulus‐responsive receptor was prepared to control the ligand‐binding ability of three active sites with one effector. A stable supramolecular complex of the receptor and the effector was produced by the cooperative formation of multiple hydrogen and coordination bonds. As a result, the binding of a ligand to the active sites was inhibited in a competitive and allosteric mechanism.

Abstract

A stimulus‐responsive receptor 1 was designed and prepared to control the ligand‐binding ability of three active sites, two zinc tetraphenylporphyrin units (P1) and one zinc diethynyldiphenylporphyrin unit (P2), with one effector molecule 2. Bulky hexarylbenzene units were incorporated as shielding panels in the middle of the flexible side arms of 1. Spectroscopic titrations indicated that a stable supramolecular complex 1⋅2 (K _1⋅2 =6.7×10⁶  m ⁻¹) was produced by the cooperative formation of multiple hydrogen and coordination bonds. As a result, the binding of a ligand to P1 was inhibited by 2 in a competitive manner. Additionally, the formation of 1⋅2 brought about conformational restriction of the side arms to cover both faces of P2 with the shielding panels. The binding constant of 4‐phenylpyridine with P2 in 1⋅2 decreased to 8.9 % of that in 1. Namely, the ligand‐binding ability of P2 was inhibited according to an allosteric mechanism.

Aromaticity‐enhanced CO₂ capture is demonstrated via thorough DFT calculations on P/N‐based FLPs (amidophosphorane). Aromatic furan‐ or pyrrole‐bridged FLPs featured better performance on CO₂ capture, which could be attributed to the gradual gain of aromaticity in the process of activation. On the contrary, aluminacyclopentadiene‐bridged FLP gains anti‐aromaticity in the transition state, leading to relative higher activation energies for CO₂ capture.

Abstract

Carbon dioxide (CO₂, a common combustion pollutant) releasing continuously into the atmosphere is primarily responsible for the rising atmospheric temperature. Therefore, CO₂ sequestration has been an indispensable area of research for the past several decades. On the other hand, the concept of aromaticity is often employed in designing chemical reactions and metal‐free frustrated Lewis pairs (FLPs) have proved ideal reagents to achieve CO₂ reduction. However, considering FLP and aromaticity together is less developed in CO₂ capture. Here we report theoretical investigations on the aromaticity‐promoted CO₂ activation, involving heterocyclopentadiene‐bridged P/N‐FLPs. The calculations reveal that furan‐ and pyrrole‐bridged P/N‐FLPs can make CO₂ capture both thermodynamically and kinetically favorable (with activation energies of 5.4–7.7 kcal mol⁻¹) due to the aromatic stabilization of the transition states and products. Our findings could open an avenue to the design of novel FLPs for CO₂ capture.

Ch‐ch‐change S: A nickel‐catalyzed aryl thioether metathesis with high functional‐group tolerance was developed, with bis(dicyclohexylphosphino)ethane (dcype) being essential to promote the reaction. Synthetically challenging macrocycles were obtained in good yield in an unusual example of ring‐closing metathesis that does not involve alkene bonds. In‐depth organometallic studies support a reversible Ni⁰/Ni^II pathway to product formation.

Abstract

A nickel‐catalyzed aryl thioether metathesis has been developed to access high‐value thioethers. 1,2‐Bis(dicyclohexylphosphino)ethane (dcype) is essential to promote this highly functional‐group‐tolerant reaction. Furthermore, synthetically challenging macrocycles could be obtained in good yield in an unusual example of ring‐closing metathesis that does not involve alkene bonds. In‐depth organometallic studies support a reversible Ni⁰/Ni^II pathway to product formation. Overall, this work not only provides a more sustainable alternative to previous catalytic systems based on Pd, but also presents new applications and mechanistic information that are highly relevant to the further development and application of unusual single‐bond metathesis reactions.

Corrosion free: Two methods for the synthesis of vinyl halides by iridium‐catalyzed transfer hydrohalogenation of unactivated alkynes are described. The use of 4‐chlorobutan‐2‐one or tert‐butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents. This method leads to alkenyl halide compounds containing acid‐sensitive groups, such as tertiary alcohols, silyl ethers, and acetals.

Abstract

Described herein are two different methods for the synthesis of vinyl halides by a shuttle catalysis based iridium‐catalyzed transfer hydrohalogenation of unactivated alkynes. The use of 4‐chlorobutan‐2‐one or tert‐butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents, such as hydrogen halides or acid chlorides, thus largely improving the functional‐group tolerance and safety profile of these reactions compared to the state‐of‐the‐art. This method has granted access to alkenyl halide compounds containing acid‐sensitive groups, such as tertiary alcohols, silyl ethers, and acetals. The synthetic value of those methodologies has been demonstrated by gram‐scale synthesis where low catalyst loading was achieved.

Corrosion free: Two methods for the synthesis of vinyl halides by iridium‐catalyzed transfer hydrohalogenation of unactivated alkynes are described. The use of 4‐chlorobutan‐2‐one or tert‐butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents. This method leads to alkenyl halide compounds containing acid‐sensitive groups, such as tertiary alcohols, silyl ethers, and acetals.

Abstract

Described herein are two different methods for the synthesis of vinyl halides by a shuttle catalysis based iridium‐catalyzed transfer hydrohalogenation of unactivated alkynes. The use of 4‐chlorobutan‐2‐one or tert‐butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents, such as hydrogen halides or acid chlorides, thus largely improving the functional‐group tolerance and safety profile of these reactions compared to the state‐of‐the‐art. This method has granted access to alkenyl halide compounds containing acid‐sensitive groups, such as tertiary alcohols, silyl ethers, and acetals. The synthetic value of those methodologies has been demonstrated by gram‐scale synthesis where low catalyst loading was achieved.

Hydrogen activation: A comprehensive study (see graphic) provides the theoretical framework for the structures, mechanisms, and reactivities of several Lewis acid transition‐metal catalysts applied in H₂ activation.

Abstract

As a new type of bifunctional catalyst, the Lewis acid transition‐metal (LA‐TM) catalysts have been widely applied for hydrogen activation. This study presents a mechanistic framework to understand the LA‐TM‐catalyzed H₂ activation through DFT studies. The mer(trans)‐homolytic cleavage, the fac(cis)‐homolytic cleavage, the synergetic heterolytic cleavage, and the dissociative heterolytic cleavage should be taken as general mechanisms for the field of LA‐TM catalysis. Four typical LA‐TM catalysts, the Z‐type κ⁴‐L₃B‐Rh complex tri(azaindolyl)borane‐Rh, the X‐type κ³‐L₂B‐Co complex bis‐phosphino‐boryl (PBP)‐Co, the η²‐BC‐type κ³‐L₂B‐Pd complex diphosphine‐borane (DPB)‐Pd, and the Z‐type κ²‐LB‐Pt complex (boryl)iminomethane (BIM)‐Pt are selected as representative models to systematically illustrate their mechanistic features and explore the influencing factors on mechanistic variations. Our results indicate that the tri(azaindolyl)borane‐Rh catalyst favors the synergetic heterolytic mechanism; the PBP‐Co catalyst prefers the mer(trans)‐homolytic mechanism; the DPB‐Pd catalyst operates through the fac(cis)‐homolytic mechanism, whereas the BIM‐Pt catalyst tends to undergo the dissociative heterolytic mechanism. The mechanistic variations are determined by the coordination geometry, the LA‐TM bonding nature, the electronic structure of the TM center, and the flexibility or steric effect of the LA ligands. The presented mechanistic framework should provide helpful guidelines for LA‐TM catalyst design and reaction developments.

A crystalline monosubstituted carbene

A crystalline monosubstituted carbene, Published online: 15 October 2018; doi:10.1038/s41557-018-0153-1

Abstract

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