Jelte

AlCan Wrap: The Reaction of a seven‐membered cyclic alumanyl anion with a β‐diketiminato calcium tetraphenylborate provides facile access to a stable, but highly reactive, calcium alumanyl.

Abstract

A seven‐membered N,N′‐heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg−Al and Mg−Ca bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

The hyperpolarization of silanols and silanes was achieved using parahydrogen and an iridium catalyst. The resulting ²⁹Si NMR signal gains approach a factor of 3000 and were harnessed for the quantitative monitoring of a substitution reaction.

Abstract

Silanols and silanes are key precursors and intermediates for the synthesis of silicon‐based materials. While their characterization and quantification by ²⁹Si NMR spectroscopy has received significant attention, it is a technique that is limited by the low natural abundance of ²⁹Si and its low sensitivity. Here, we describe a method using p‐H₂ to hyperpolarize ²⁹Si. The observed signal enhancements, approaching 3000‐fold at 11.7 T, would take many days of measurement for comparable results under Boltzmann conditions. The resulting signals were exploited to monitor the rapid reaction of tris(tert‐butoxy)silanol with triflic anhydride in a T ₁‐corrected process that allows for rapid quantification. These results demonstrate a novel route to quantify dynamic processes and intermediates in the synthesis of silicon materials.

Double reduction of the THF adduct of 9H‐9‐borafluorene (1·THF) with excess alkali metal affords the dianion salts M2[1] in essentially quantitative yields (M = Li‐K). Even though the added charge is stabilized through π delocalization, [1]2‒ acts as a formal boron nucleophile toward organoboron (1·THF) and tetrel halide electrophiles (MeCl, Et3SiCl, Me3SnCl) to form B−B/C/Si/Sn bonds. The substrate dependence of open shell‐ vs. closed shell‐pathways has been investigated.

A matter of arrows: The concept of dative bonds has a long and controversial history, but also a bright future. Since the idea of two electrons coming from the same atom in a covalent bond is ubiquitous, the dative bond deserves its arrow.

Abstract

Dative Bond (IUPAC): “The coordination bond formed upon interaction between molecular species, one of which serves as a donor and the other as an acceptor of the electron pair to be shared in the complex formed… The distinctive feature of dative bonds is that their minimum‐energy rupture in the gas phase or in inert solvent follows the heterolytic bond cleavage path.” This definition encompasses an immense number of molecules such as Lewis adducts, transition‐metal complexes, supposedly hypervalent or hypovalent systems, and many molecules with multifaceted Lewis structures. Still, there is a large reticence to include dative bonds in the regular depiction of molecules, and even a larger unawareness of the dative bond arrow in many chemical circles. Herein we will discuss in simple chemical terms the past, present and future of such bonds. In addition, we will try to provide cleaner options to represent intricate molecules without sacrificing physical accuracy.

Cationic germanium: Chloride abstraction from the boryl/NHC stabilized Ge^II precursor (IPrMe)GeCl{B(NDippCH)₂} yields [(IPrMe){(HCNDipp)₂B}Ge=Ge{B(NDippCH)₂}(IPrMe)]²⁺ which can be viewed as an imidazolium‐functionalized digermene, and is cleaved in the presence of donor solvents yielding monomeric [Ge{B(NDippCH)₂}(IPrMe)(L)]⁺. The thf adduct is sufficiently labile that it can act as a convenient source of monomeric [Ge{B(NDippCH)₂}(IPrMe)]⁺, which undergoes oxidative bond‐activation chemistry.

Abstract

The synthesis of a boryl‐substituted germanium(II) cation, [Ge{B(NDippCH)₂}(IPrMe)]⁺, (Dipp=2,6‐diisopropylphenyl) featuring a supporting N‐heterocyclic carbene (NHC) donor, has been explored through chloride abstraction from the corresponding (boryl)(NHC)GeCl precursor. Crystallographic studies in the solid state and UV/Vis spectra in fluorobenzene solution show that this species dimerizes under such conditions to give [(IPrMe){(HCNDipp)₂B}Ge=Ge{B(NDippCH)₂}(IPrMe)]²⁺ (IPrMe = 1,3‐diisopropyl‐4,5‐dimethylimidazolin‐2‐ylidene), which can be viewed as an imidazolium‐functionalized digermene. The dimer is cleaved in the presence of donor solvents such as [D₈]thf or [D₅]pyridine, to give monomeric adducts of the type [Ge{B(NDippCH)₂}(IPrMe)(L)]⁺. In the case of the thf adduct, the additional donor is shown to be sufficiently labile that it can act as a convenient in situ source of the monomeric complex [Ge{B(NDippCH)₂}(IPrMe)]⁺ for oxidative bond‐activation chemistry. Thus, [Ge{B(NDippCH)₂}(IPrMe)(thf)]⁺ reacts with silanes and dihydrogen, leading to the formation of Ge^IV products, whereas the cleavage of the N−H bond in ammonia ultimately yields products containing C−H and B−N bonds. The facile reactivity observed in E−H bond activation is in line with the very small calculated HOMO–LUMO gap (132 kJ mol⁻¹).

Hype in science is commonplace, compounded by the hypocrisy of those who engage in or tolerate it while disapproving of the consequences. These are first steps along a slippery slope of hype, hypocrisy, data falsification, and dissemination of fake science, encouraged by systemic drivers in the contemporary structure of the science establishment. Collective, concerted intervention is required to discourage entry to this dangerous pathway; chemists must play an active role.

Abstract

In chemistry and other sciences, hype has become commonplace, compounded by the hypocrisy of those who tolerate or encourage it while disapproving of the consequences. This reduces the credibility and trust upon which all science depends for support. Hype and hypocrisy are but first steps down a slippery slope towards falsification of results and dissemination of fake science. Systemic drivers in the contemporary structure of the science establishment encourage exaggeration and may lure the individual into further steps along the hype‐hypocrisy‐falsification‐fakery continuum. Collective, concerted intervention is required to effectively discourage entry to this dangerous pathway and to restore and protect the probity and reputation of the science system. Chemists must play and active role in this effort.

Phosphine stabilized germaborenes featuring an unprecedented Ge=B double bond with short B···Ge contacts of 1.886(2) (4) and 1.895(3) Å (5) were synthesized starting from an intramolecular germylene‐phosphine Lewis pair (1). After oxidative addition of borontrihalides BX3 (X = Cl, Br) the addition products were reduced with magnesium and catalytic amounts of anthracene to give the borylene derivatives in a yield of 78% (4) and 57% (5). These halide substituted germaborenes were characterized by single crystal structure analyses and the electronic structures were studied by quantum chemical calculations. Deduced from NBO NRT analysis the dominating Lewis‐structure contains a Ge=B double bond. The germaborenes exhibit at room temperature a reversible photochemically initiated [2+2] cycloaddition with a phenyl moiety of a terphenyl substituent, forming a complex heterocyclic structure with Ge(IV) in a strongly distorted coordination environment.

An azobenzene setup was used to examine London dispersion in conjunction with solvent interactions between linear alkyl chains. ¹H NOESY NMR analysis indicated additional long‐range interactions with the opposite phenyl core in the Z‐isomer. This work provides rare insight into the stabilizing contributions of highly flexible groups in an intra‐ as well as an intermolecular setting.

Abstract

Interactions on the molecular level control structure as well as function. Especially interfaces between innocent alkyl groups are hardly studied although they are of great importance in larger systems. Herein, London dispersion in conjunction with solvent interactions between linear alkyl chains was examined with an azobenzene‐based experimental setup. Alkyl chains in all meta positions of the azobenzene core were systematically elongated, and the change in rate for the thermally induced Z→E isomerization in n‐decane was determined. The stability of the Z‐isomer increased with longer chains and reached a maximum for n‐butyl groups. Further elongation led to faster isomerization. The origin of the intramolecular interactions was elaborated by various techniques, including ¹H NOESY NMR spectroscopy. The results indicate that there are additional long‐range interactions between n‐alkyl chains with the opposite phenyl core in the Z‐state. These interactions are most likely dominated by attractive London dispersion. This work provides rare insight into the stabilizing contributions of highly flexible groups in an intra‐ as well as an intermolecular setting.

Single electron transfer to diphenyldiazomethane–borane adducts generates transient radical anions that effect C−H bond activations. Such reductions provide a strategy to stabilize weak and even undetectable donor–acceptor adducts.

Abstract

While (Ph₂CN₂)B(C₆F₅)₃ is unstable, single electron transfer from Cp*₂Co affords the isolation of stable products [Cp*₂Co][Ph₂CNNHB(C₆F₅)₃] 1 and [Cp*Co(C₅Me₄CH₂B(C₆F₅)₃)] 2. The analogous combination of Ph₂CN₂ and BPh₃ showed no evidence of adduct formation and yet single electron transfer from Cp*₂Cr affords the species [Cp*₂Cr][PhC(C₆H₄)NNBPh₃] 3 and [Cp*₂Cr][Ph₂CNNHBPh₃] 4. Computations showed both reactions proceed via transient radical anions of the diphenyldiazomethane–borane adducts to effect C−H bond activations.

The deadly nature of the puffer fish has been known to mankind since the dawn of civilization. However, the compound responsible for this fatal activity was isolated only in the early 20^th century and has been widely known to the scientific community as tetrodotoxin since then. This review traces its history, isolation, structural elucidation, biology, and synthesis. The syntheses of unnatural derivatives are covered as well.

Abstract

This review provides a comprehensive coverage of the history, biology and chemistry of tetrodotoxin (TTX). It traces the origin of this remarkable molecule all the way back to the ancient Chinese medicine records. The discovery of biological activity, isolation, and a brief overview of structure elucidation are summarized. Next, the biology of TTX is discussed, primarily in the context of its activity in the sodium channels, its anesthetic properties, and its potential use in cancer treatment or drug addiction. Biosynthesis of TTX is covered before the discussion of the total syntheses. All total, formal or partial syntheses are covered but those total syntheses that have been discussed in previous reviews are only briefly summarized. Finally, the synthesis of natural and unnatural derivatives is surveyed, and a conclusion and outlook are provided for this very extensive field of endeavor. To the best of our knowledge the literature coverage is complete up to December 2018.

A boragermene featuring a double bond between the germanium and boron atoms has been isolated for the first time from a cyclic (alkyl)(boryl)germylene–PMe₃ complex. This compound readily reacts with the Lewis base ^MeNHC to form the corresponding adduct on the boron center, and also undergoes hydrogenation with H₂ across the Ge=B bond.

Abstract

Boragermene 3 featuring a double bond between the Ge and dicoordinate B atoms has been synthesized for the first time by reacting the cyclic (alkyl)(boryl)germylene–PMe₃ adduct 1 with Cl₂BN(SiMe₃)₂ followed by reductive dehalogenation with KC₈. Addition of a Lewis base (^MeNHC) to 3 leads to the formation of the corresponding adduct 4, which shows double bond character between the Ge and tricoordinate B atoms. Compound 3 undergoes hydrogenation with H₂ concomitant with a complete scission of the Ge=B bond.

Nature Chemistry, Published online: 18 November 2019; doi:10.1038/s41557-019-0375-x

Despite the regular occurrence of high-profile accidents leading to serious injuries or deaths among lab personnel, the state of academic lab safety research has languished. Existing studies in this area are summarized and critiqued in this Review and suggestions are made for future research directions.

Publication date: September 2019

Source: Journal of Magnetic Resonance, Volume 306

Author(s): Eguchi Keiji

Publication date: September 2019

Source: Journal of Magnetic Resonance, Volume 306

Author(s): Gareth A. Morris

Publication date: September 2019

Source: Journal of Magnetic Resonance, Volume 306

Author(s): Gunnar Jeschke

Publication date: September 2019

Source: Journal of Magnetic Resonance, Volume 306

Author(s): Dmitry Budker

Publication date: September 2019

Source: Journal of Magnetic Resonance, Volume 306

Author(s): Bernhard Blümich

New phosphonium cations: Despite the bulky m‐terphenyl substituents that are needed to provide kinetic stabilization of this new compound class, various functional groups of different size and electronegativity may be chosen.

Abstract

Efforts to prepare an elusive donor‐free phosphenium ion, [R₂P]⁺, led us to synthesize functionalized fluorophosphonium cations of the type [R₂P(F)X]⁺ (X=SiEt₃, H, F), which were obtained from the related neutral fluorophosphines R₂PF and R₂PF₃ upon protonation and reaction with solvated [Et₃Si]⁺ ions (R=2,6‐Mes₂C₆H₃). The hypothetical reductive elimination of [R₂P(F)SiEt₃]⁺ and [R₂P(F)H]⁺ affording [R₂P]⁺, Et₃SiF and HF, respectively, was calculated to be endothermic by 40.1 and 190.6 kJ mol⁻¹.

The first diphosphaarsenium cation has been synthesised. This cation exhibits remarkable stability because of the delocalisation of a lone pair from a planar phosphorus centre into the vacant arsenic p‐orbital.

Abstract

Stable acyclic arsenium cations R₂As⁺, isoelectronic analogues of germylenes, are rare in comparison to the corresponding phosphenium cations. The first example of a diphosphaarsenium salt, [{(Dipp)₂P}₂As][Al{OC(CF₃)₃}₄]⋅1  PhMe, is described. This salt exhibits remarkable stability due to the delocalisation of a lone pair from a planar phosphorus centre into the vacant p‐orbital at arsenic; the bonding in 2 has been probed by DFT calculations. An attempt to synthesise an analogous diphosphaphosphenium salt unexpectedly generated the cyclic phosphonium salt [cyclo‐{(Mes)P}₂P(Mes)₂][BAr^F ₄]⋅CyMe through the cyclisation of a putative phosphine‐substituted diphosphene cation intermediate.

Colorimetric dermal sensors in the form of tattoos have been developed to measure pH, glucose, and albumin concentrations in an ex vivo porcine skin tissue model. The dermal sensors were injected into the dermis in an array format for multiplexed detection. Quantitative readouts were obtained with a smartphone camera.

Abstract

Tattooing is a ubiquitous body modification involving the injection of ink and/or dye pigments into the dermis. Biosensors in the form of tattoos can be used to monitor metabolites in interstitial fluid. Here, minimally invasive, injectable dermal biosensors were developed for measuring pH, glucose, and albumin concentrations. The dermal pH sensor was based on methyl red, bromothymol blue, and phenolphthalein, which responded to a pH range from 5.0 to 9.0. The dermal glucose sensor consisted of glucose oxidase, 3,3′,5,5′‐tetramethylbenzidine, and peroxidase that detected concentrations up to 50.0 mmol L⁻¹. The dermal albumin sensor consisted of 3′,3′′,5′,5′′‐tetrachlorophenol‐3,4,5,6‐tetrabromosulfophthalein to measure concentrations up to 5.0 g L⁻¹. The sensors were multiplexed in ex vivo skin tissue and quantitative readouts were obtained using a smartphone camera. These sensors can be used to manage of acid–base homeostasis, diabetes, and liver failure in point‐of‐care settings.

Measuring molecular forces: Calculations show that there is an almost perfect linear relationship between the bending of an alkyne chain and the change of the ¹³C NMR chemical shift of the outer acetylenic carbon atoms. NMR investigations of two molecular bows corroborate this remarkable correlation (see figure).

Abstract

Polyynes show a strictly linear relationship between the energy impact and the bending of the polyyne chain. The energy, which is necessary to bend the acetylenic chain, decreases with the increasing number of acetylene units. A deviation from linearity in polyynes can be realized in solution by violation of the mutual‐exclusion principle between IR and Raman spectra. However, there is still no possibility to measure the extent of the nonlinearity in solution. Herein, we show that the ¹³C NMR spectroscopy represents an appropriate tool for this as we found an almost perfect linear relationship between the bending of the alkyne chain and the change of the chemical shift of the outer acetylenic carbon atoms. By using molecular bows in which the alkyne chain can be bent by switching the azobenzene unit, this correlation can be proved experimentally. In the future, this correlation should enable the determination of the extent of the bending and the strain energy in polyynes. Consequently, polyynes could be employed as probes for measuring further molecular forces.

Aromaticity becomes germane; the germanium version of cyclopentadienide (Cp⁻): The potassium salt of a planar aromatic germacyclopentadienide was synthesized. Complexation of the potassium cation with crown ether triggers the structural switch to the non‐planar isomer with a localized structure.

Abstract

The reduction of a 1‐mesityl‐2,5‐bis‐trimethylsilylchlorogermole 8 with KC₈ is reported. While the reaction with one equivalent of KC₈ gave the dimer with a Ge−Ge bond 10, excess of KC₈ (four equivalents) resulted in the formation of the potassium salt of the germole dianion, 11 with reductive cleavage of the Ge−C bond. Careful reduction with two equivalents of KC₈ in THF provided the potassium salt of the planar germolide 5. Its solid‐state structure revealed contact ion pairs with the potassium ion η⁵‐coordinated to the germacyclopentadienide ring. The molecular structure of the anion indicates a high degree of cyclic electron delocalization, in agreement with results from DFT calculations. Separation of the ion pair by complexation of the potassium ions with 18‐crown‐6 triggers the isomerization to germolide 6, which is characterized by a pyramidal coordination sphere of the germanium atom and a localized diene structure. The isomers 5 and 6 represent a rare example for a structurally manifested switch between a non‐aromatic and an aromatic state induced by an external stimulus, in this case the complexation of the counter cation.

If Lewis acidic boranes, similar to those used in frustrated Lewis pair (FLP) chemistry, are exposed to H₂ in the presence of common reducing agents, instead of the Lewis bases required in FLP chemistry, then homolytic H₂ cleavage occurs. A series of intermediates formed during the H₂ cleavage process at a main group borane complex could be experimentally observed and structurally characterized.

Abstract

We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H₂ outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5‐dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical‐based, redox pathway involves the homolytic cleavage of H₂, in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.