Alessandro Bismuto

A case of important cooperation: The semihydrogenation of alkynes catalyzed by a pyridonate borane complex is reported. While the catalyst can be described as an intramolecular frustrated Lewis pair, it is the change in the coordination mode of the pyridonate substituent upon H₂ bond activation that enables dissociation of the formed pyridone borane complex and hydroboration as the first C−H bond‐forming step. This mode of action is referred to as boron‐ligand cooperation.

Abstract

The metal‐free cis selective hydrogenation of alkynes catalyzed by a boroxypyridine is reported. A variety of internal alkynes are hydrogenated at 80 °C under 5 bar H₂ with good yields and stereoselectivity. Furthermore, the catalyst described herein enables the first metal‐free semihydrogenation of terminal alkynes. Mechanistic investigations, substantiated by DFT computations, reveal that the mode of action by which the boroxypyridine activates H₂ is reminiscent of the reactivity of an intramolecular frustrated Lewis pair. However, it is the change in the coordination mode of the boroxypyridine upon H₂ activation that allows the dissociation of the formed pyridone borane complex and subsequent hydroboration of an alkyne. This change in the coordination mode upon bond activation is described by the term boron‐ligand cooperation.

Ligand effects: The borane acceptor in diphosphinoborane ligands discriminates between the oxidation state Pd^II and Pd⁰, stabilizing the latter. The impact of the Pd→B interaction on inner‐sphere C−N bond reductive elimination of N,N‐dimethyl‐4‐nitroaniline was investigated (see scheme).

Abstract

The dative Pd→B interaction in a series of ^RDPB^R’ Pd⁰ and Pd^II complexes (^RDPB^R’=(o‐PR₂C₆H₄)₂BR’, diphosphinoborane) was analyzed using XRD, ¹¹B NMR spectroscopy and NBO/NLMO calculations. The borane acceptor discriminates between the oxidation state Pd^II and Pd⁰, stabilizing the latter. Reaction of lithium amides with [(^RDPB^R’)Pd^II(4‐NO₂C₆H₄)I] chemoselectively yields the C−N coupling product. DFT modelling indicates no significant impact of Pd^II→B coordination on the inner‐sphere reductive elimination rate.

Having your cake and eating it too: Functionalization of 9‐borafluorenes with trifluoromethyl groups makes them exceptionally easy to reduce while maintaining excellent stability towards hydrolysis. The systems also exhibit long‐lived fluorescence or thermally activated delayed fluorescence (TADF) depending on the exo‐aryl para‐substituent.

Abstract

Three different perfluoroalkylated borafluorenes ( ^F Bf) were prepared and their electronic and photophysical properties were investigated. The systems have four trifluoromethyl moieties on the borafluorene moiety as well as two trifluoromethyl groups at the ortho positions of their exo‐aryl moieties. They differ with regard to the para substituents on their exo‐aryl moieties, being a proton ( ^F Xyl^FBf, ^FXyl: 2,6‐bis(trifluoromethyl)phenyl), a trifluoromethyl group ( ^F Mes^FBf, ^FMes: 2,4,6‐tris(trifluoromethyl)phenyl) or a dimethylamino group ( p ‐NMe₂‐^FXyl^FBf, p‐NMe₂‐^FXyl: 4‐(dimethylamino)‐2,6‐bis(trifluoromethyl)phenyl), respectively. All derivatives exhibit extraordinarily low reduction potentials, comparable to those of perylenediimides. The most electron‐deficient derivative ^F Mes^FBf was also chemically reduced and its radical anion isolated and characterized. Furthermore, all compounds exhibit very long fluorescent lifetimes of about 250 ns up to 1.6 μs; however, the underlying mechanisms responsible for this differ. The donor‐substituted derivative p ‐NMe₂‐^FXyl^FBf exhibits thermally activated delayed fluorescence (TADF) from a charge‐transfer (CT) state, whereas the ^F Mes^FBf and ^F Xyl^FBf borafluorenes exhibit only weakly allowed locally excited (LE) transitions due to their symmetry and low transition‐dipole moments.

Divergent tetrazine synthesis: Described is a new strategy for the one‐pot synthesis of 3‐thiomethyltetrazines from carboxylic ester precursors, which provides a platform for the synthesis of unsymmetrical tetrazines through Pd‐catalyzed cross‐coupling and the first catalytic thioether reduction to access monosubstituted tetrazines.

Abstract

Since tetrazines are important tools to the field of bioorthogonal chemistry, there is a need for new approaches to synthesize unsymmetrical and 3‐monosubstituted tetrazines. Described here is a general, one‐pot method for converting (3‐methyloxetan‐3‐yl)methyl carboxylic esters into 3‐thiomethyltetrazines. These versatile intermediates were applied to the synthesis of unsymmetrical tetrazines through Pd‐catalyzed cross‐coupling and in the first catalytic thioether reduction to access monosubstituted tetrazines. This method enables the development of new tetrazine compounds possessing a favorable combination of kinetics, small size, and hydrophilicity. It was applied to a broad range of aliphatic and aromatic ester precursors and to the synthesis of heterocycles including BODIPY fluorophores and biotin. In addition, a series of tetrazine probes for monoacylglycerol lipase (MAGL) were synthesized and the most reactive one was applied to the labeling of endogenous MAGL in live cells.

A platinum(II)‐carbene Type II immunogenic cell death (ICD) inducer, PlatinER, promotes phagocytosis by immune cells via release of ICD‐associated DAMPs, including surface exposure of calreticulin and HSP‐90. By comparing its activity against a panel of structurally analogous Pt^II‐carbene analogues, we determined that PlatinER acts by invoking a partial endoplasmic reticulum (ER) stress response through localised generation of reactive oxygen species in the ER.

Abstract

Immunogenic cell death (ICD) is a rare immunostimulatory form of cell death that can improve the clinical outcomes of chemo‐immunotherapeutic combination regimens through the establishment of a long‐term cancer immunity. None of the clinically used DNA‐binding Pt^II complexes is considered a Type II ICD inducer. We generated a series of Pt^II‐carbene complexes by applying minor structural alterations to the scaffold of a Type II ICD inducer Pt‐NHC and compared their efficiency in triggering ICD‐related cellular responses and phagocytosis. We successfully identified PlatinER, a novel highly potent Pt^II candidate with superior ICD properties. Crucially, the magnitude of ICD‐associated phagocytosis induced upon exposure of cancer cells to Pt complexes was dependent on the levels of ER‐localized reactive oxygen species (ROS) generation, which underpins their mechanisms of action and provides a feasible approach for the design of more effective Type II ICD inducers.

The calix[4]pyrrolato aluminate enables a transfer of metal–ligand cooperativity from the d‐ into the p‐block. The compound traps carbonyls and delivers products that are showing highly dynamic–covalent behavior. Lithium ions provide full control over the formation and cleavage of C−C bonds. The adjustability in substrate activation enables a unique concept for rate control in catalysis.

Abstract

Metal‐ligand cooperativity (MLC) had a remarkable impact on transition metal chemistry and catalysis. By use of the calix[4]pyrrolato aluminate, [1]⁻, which features a square‐planar Al^III, we transfer this concept into the p‐block and fully elucidate its mechanisms by experiment and theory. Complementary to transition metal‐based MLC (aromatization upon substrate binding), substrate binding in [1]⁻ occurs by dearomatization of the ligand. The aluminate trapps carbonyls by the formation of C−C and Al−O bonds, but the products maintain full reversibility and outstanding dynamic exchange rates. Remarkably, the C−C bonds can be formed or cleaved by the addition or removal of lithium cations, permitting unprecedented control over the system's constitutional state. Moreover, the metal‐ligand cooperative substrate interaction allows to twist the kinetics of catalytic hydroboration reactions in a unique sense. Ultimately, this work describes the evolution of an anti‐van't Hoff/Le Bel species from their being as a structural curiosity to their application as a reagent and catalyst.

A diphosphine with an unsupported PP bond connecting two carbon‐free “inorganic” 1,3,2,4,5‐diazaphosphadisilolidine rings was prepared by reductive coupling of a P‐chloro‐substituted monocyclic precursor molecule. VT‐EPR studies revealed that the diphosphine exists in solution, like other compounds of this kind, in dynamic equilibrium with the corresponding phosphinyl radicals. Determination of the radical concentration from the EPR spectra permitted to calculate thermochemical parameters for the homolytic PP bond fission. The results disclose that both the enthalpy and entropy of dissociation are higher than in topologically related bi(diazaphospholidines). The impact of the entropy term allows explaining that, regardless of the presence of an energetically rather stable PP bond, the onset of dissociation is observable even at ambient temperature. Irradiation experiments showed that radical formation cannot only be induced thermally, but also by photolysis.

The first bismuth‐catalyzed carbonyl hydrosilylation is presented. The catalytically active electrophilic dicationic bismuth species [(Me₂NC₆H₄)Bi(L)₃]²⁺ (L=aldehyde/ketone) is characterized both in solution and in the solid state. Control experiments and computation studies support a carbonyl activation mechanism at bismuth followed by silane addition.

Abstract

Bismuth compounds are gaining importance as potential alternatives to transition‐metal complexes and electron deficient lighter p‐block compounds in homogeneous catalysis. Computational analysis on the two‐coordinate [(Me₂NC₆H₄)Bi]²⁺ possessing three electrophilic sites is experimentally evidenced by the isolation of [{Me₂NC₆H₄}Bi{OP(NMe₂)₃}₃][B(3,5‐C₆H₃Cl₂)₄]₂. These observations led us to generate dicationic organobismuth catalyst, [(Me₂NC₆H₄)Bi(L)₃]²⁺ (L=aldehyde/ketone), evidenced by NMR spectroscopy in solution and by single‐crystal X‐ray diffraction in the solid state. It efficiently catalyzes hydrosilylation of aldehydes and ketones resulting in silyl ethers as the only products in high yields. Our investigations support a carbonyl activation mechanism at the bismuth center followed by Si−H addition.

A highly electrophilic organobismuth dication was designed by controlling the energy of the acceptor orbitals. Structure of the dication polarizing three carbonyl moieties has been deduced by X‐ray structure analysis and NMR spectroscopy. Demonstration of catalytic carbonyl hydrosilylation is a proof of concept for designing desired level of electrophilicity at bismuth. More information can be found in the Communication by E. D. Jemmis, A. Venugopal, et al. on https://doi.org/10.1002/chem.202002006page 12717.

The fluoronitrenoid metal complexes FNCoF₂ and FNRhF₂ as well as the first ternary Rh^VI and Ir^VI complexes NIrF₃ and NRhF₃ are reported. They were obtained by the reaction of excited Group‐9 metal atoms with NF₃ and their IR spectra, isolated in solid rare gases, were recorded.

Abstract

The fluoronitrenoid metal complexes FNCoF₂ and FNRhF₂ as well as the first ternary Rh^VI and Ir^VI complexes NIrF₃ and NRhF₃ are described. They were obtained by the reaction of excited Group‐9 metal atoms with NF₃ and their IR spectra, isolated in solid rare gases (neon and argon), were recorded. Aided by the observed ^14/15N isotope shifts and quantum‐chemical predictions, all four stretching fundamentals of the novel complexes were safely assigned. The F−N stretching frequencies of the fluoronitrenoid complexes FNCoF₂ (1056.8 cm⁻¹) and FNRhF₂ (872.6 cm⁻¹) are very different and their N−M bonds vary greatly. In FNCoF₂, the FN ligand is singly bonded to Co and bears considerable iminyl/nitrene radical character, while the N−Rh bond in FNRhF₂ is a strong double bond with comparatively strong σ‐ and π‐bonds. The anticipated rearrangement of FNCoF₂ to the nitrido Co^VI complex is predicted to be endothermic and was not observed.

A series of α‐cationic phosphole ligands has been synthesized and their use as ancillary ligands explored in Au‐catalysis. If compared with non‐heterocyclic α‐cationic phospines, higher reaction rates and better control of the stereoselectivity are obtained.

Abstract

A series of structurally differentiated α‐cationic phospholes containing cyclopropenium, imidazolium, and iminium substituents has been synthesized by reaction of chlorophosphole 1 with the corresponding stable carbenes. Evaluation of the donor properties of these compounds reveals that their strong π‐acceptor character is heavily influenced by the nature of the cationic group. The coordination chemistry of these newly prepared ligands towards Au^I centers is also described and their unique electronic properties exploited in catalysis. Interestingly, α‐cationic phosphole containing catalysts were not only able to accelerate model cycloisomerization reactions, but also to efficiently discriminate between concurrent reaction pathways, avoiding the formation of undesired product mixtures.

A novel air‐stable Pd^I dimer is reported that triggers E‐selective olefin migration to enamides and styrene derivatives in the presence of multiple functional groups and with complete tolerance of air. The same dimer also triggers extremely rapid C−C coupling (alkylation and arylation) at room temperature in a modular and triply selective fashion.

Abstract

We report a new air‐stable Pd^I dimer, [Pd(μ‐I)(PCy₂ ^tBu)]₂, which triggers E‐selective olefin migration to enamides and styrene derivatives in the presence of multiple functional groups and with complete tolerance of air. The same dimer also triggers extremely rapid C−C coupling (alkylation and arylation) at room temperature in a modular and triply selective fashion of aromatic C−Br, C−OTf/OFs, and C−Cl bonds in poly(pseudo)halogenated arenes, displaying superior activity over previous Pd^I dimer generations for substrates that bear substituents ortho to C−OTf.