Ruthenium Phenoxo Complexes: An Isolobal Ligand to Cp with Improved Properties.

Catalytic π-arene activation is based on catalysts that allow for arene exchange. To date, cyclopentadiene (Cp)-derived catalysts are the most commonly used in π-arene activation despite their low arene exchange rates. Herein, we report the synthesis, analysis, and catalytic application of Ru(II) complexes supported by phenoxo ligands, which are isolobal alternatives to Cp. The phenoxo complexes exhibit arene exchange rates significantly faster than those of the corresponding Cp complexes. The rate can be further increased through the choice of appropriate counterions. The mechanism of the arene exchange process is elucidated by kinetic and computational analyses. We demonstrate the utility of the new catalysts through an SNAr reaction between fluorobenzene and alcohols, including secondary alcohols that could not be used previously in related reactions. Moreover, the catalytic thermal decarboxylation of phenylacetic acids is presented.

Nitrate reduction enables safer aryldiazonium chemistry.

Aryldiazonium salts remain a staple in organic synthesis and are still prepared largely in accord with the protocol developed in the 19th century. Because of the favorable reactivity that often cannot be achieved with other aryl(pseudo)halides, diazonium chemistry continues to grow. Facile extrusion of dinitrogen contributes to the desired reactivity but is also reason for safety concerns. Explosions have occurred since the discovery of these reagents and still result in accidents. In this study, we report a diazonium chemistry paradigm shift based on nitrate reduction using thiosulfate or dihalocuprates as electron donors that avoids diazonium accumulation. Because nitrate reduction is rate-limiting, aryldiazoniums are produced as fleeting intermediates, which results in a safer and often more efficient deaminative halogenation in a single step from anilines.

Molybdenum(VI) Nitrido Complexes with Tripodal Silanolate Ligands. Structure and Electronic Character of an Unsymmetrical Dimolybdenum µ-Nitrido Complex Formed by Incomplete Nitrogen Atom Transfer.

In contrast to a tungsten nitrido complex endowed with a tripodal silanolate ligand framework, which was reported in the literature to be a dimeric species with a metallacyclic core, the corresponding molybdenum nitrides 3 are monomeric entities comprising a regular terminal nitride unit, as proven by single-crystal X-ray diffraction (SC-XRD). Their electronic character is largely determined by the constraints imposed on the metal center by the podand ligand architecture. 95Mo nuclear magnetic resonance (NMR) and, to a lesser extent, 14N NMR spectroscopy allow these effects to be studied, which become particularly apparent upon comparison with the spectral data of related molybdenum nitrides comprising unrestrained silanolate, alkoxide, or amide ligands. Attempted nitrogen atom transfer from these novel terminal nitrides to [(tBuArN)3Mo] (Ar = 3,5-dimethylphenyl) as the potential acceptor stopped at the stage of unsymmetric dimolybdenum µ-nitrido complex 13a as the first intermediate along the reaction pathway. SC-XRD, NMR, electron paramagnetic resonance, and ultraviolet-visible spectroscopy as well as magnetometry in combination with density functional theory allowed a clear picture of the geometric and electronic structure of this mixed-valent species to be drawn. 13a is formally best described as an adduct of the type [(Mo[O])+III-(µN)-III-(Mo[N])+VI], S = 1/2 complex with (Mo[O])+III in the low-spin configuration, whereas related complexes such as [(AdS)3Mo-(µN)-Mo(NtBuAr)3] (19; Ad = 1-adamantyl) have previously been regarded in the literature as mixed-valent Mo+IV/Mo+V species. The spin population at the two Mo centers is uneven and notably larger at the more reduced Mo[O] atom, whereas the only spin present at the (µN) bridge is derived from spin polarization.

NMR spectroscopy of a 18O-labeled rhodium paddlewheel complex: Isotope shifts, 103Rh-103Rh spin-spin coupling, and 103Rh singlet NMR.

Despite the importance of rhodium complexes in catalysis, and the favorable 100% natural abundance of the spin-1/2 103Rh nucleus, there are few reports of 103Rh nuclear magnetic resonance (NMR) parameters in the literature. In part, this is the consequence of the very low gyromagnetic ratio of 103Rh and its dismal NMR sensitivity. In a previous paper [Harbor-Collins et al., J. Chem. Phys. 159, 104 307 (2023)], we demonstrated an NMR methodology for 1H-enhanced 103Rh NMR and demonstrated an application to the 103Rh NMR of the dirhodium formate paddlewheel complex. In this paper, we employ selective 18O labeling to break the magnetic equivalence of the 103Rh spin pair of dirhodium formate. This allows the estimation of the 103Rh-103Rh spin-spin coupling and provides access to the 103Rh singlet state. We present the first measurement of a 18O-induced 103Rh secondary isotope shift as well as the first instance of singlet order generated in a 103Rh spin pair. The field-dependence of 103Rh singlet relaxation is measured by field-cycling NMR experiments.

Catalytic asymmetric cationic shifts of aliphatic hydrocarbons.

Asymmetric catalysis is an advanced area of chemical synthesis, but the handling of abundantly available, purely aliphatic hydrocarbons has proven to be challenging. Typically, heteroatoms or aromatic substructures are required in the substrates and reagents to facilitate an efficient interaction with the chiral catalyst. Confined acids have recently been introduced as tools for homogenous asymmetric catalysis, specifically to enable the processing of small unbiased substrates1. However, asymmetric reactions in which both substrate and product are purely aliphatic hydrocarbons have not previously been catalysed by such super strong and confined acids. We describe here an imidodiphosphorimidate-catalysed asymmetric Wagner-Meerwein shift of aliphatic alkenyl cycloalkanes to cycloalkenes with excellent regio- and enantioselectivity. Despite their long history and high relevance for chemical synthesis and biosynthesis, Wagner-Meerwein reactions utilizing purely aliphatic hydrocarbons, such as those originally reported by Wagner and Meerwein, had previously eluded asymmetric catalysis.

Elucidating the Electronic Nature of Rh-based Paddlewheel Catalysts from 103 Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations.

The tremendous importance of dirhodium paddlewheel complexes for asymmetric catalysis is largely the result of an empirical optimization of the chiral ligand sphere about the bimetallic core. It was only recently that a H(C)Rh triple resonance 103 Rh NMR experiment provided the long-awaited opportunity to examine - with previously inconceivable accuracy - how variation of the ligands impacts on the electronic structure of such catalysts. The recorded effects are dramatic: formal replacement of only one out of eight O-atoms surrounding the metal centers in a dirhodium tetracarboxylate by an N-atom results in a shielding of the corresponding Rh-site of no less than 1000 ppm. The current paper provides the theoretical framework that allows this and related experimental observations made with a set of 19 representative rhodium complexes to be interpreted. In line with symmetry considerations, it is shown that the shielding tensor responds only to the donor ability of the equatorial ligands along the perpendicular principal axis. Axial ligands, in contrast, have no direct effect on shielding but may come into play via the electronic c i s ${cis}$ -effect that they exert onto the neighboring equatorial sites. On top of these fundamental interactions, charge redistribution within the core as well as the electronic t r a n s ${trans}$ -effect of ligands of different donor strengths is reflected in the recorded 103 Rh NMR shifts.

Bi-Catalyzed Trifluoromethylation of C(sp2)-H Bonds under Light.

We disclose a Bi-catalyzed C-H trifluoromethylation of (hetero)arenes using CF3SO2Cl under light irradiation. The catalytic method permits the direct functionalization of various heterocycles bearing distinct functional groups. The structural and computational studies suggest that the process occurs through an open-shell redox manifold at bismuth, comprising three unusual elementary steps for a main group element. The catalytic cycle starts with rapid oxidative addition of CF3SO2Cl to a low-valent Bi(I) catalyst, followed by a light-induced homolysis of Bi(III)-O bond to generate a trifluoromethyl radical upon extrusion of SO2, and is closed with a hydrogen-atom transfer to a Bi(II) radical intermediate.

From the Glovebox to the Benchtop: Air-Stable High Performance Molybdenum Alkylidyne Catalysts for Alkyne Metathesis.

Molybdenum alkylidynes endowed with tripodal silanolate ligands belong to the most active and selective catalysts for alkyne metathesis known to date. This paper describes a new generation that is distinguished by an unprecedented level of stability and practicality without sacrificing the chemical virtues of their predecessors. Specifically, pyridine adducts of type 16 are easy to make on gram scale, can be routinely weighed and handled in air, and stay intact for many months outside the glovebox. When dissolved in toluene, however, spontaneous dissociation of the stabilizing pyridine ligand releases an active species of excellent performance and functional group tolerance. Specifically, a host of polar and apolar groups, various protic sites, and numerous basic functionalities proved compatible. The catalysts are characterized by crystallographic and spectroscopic means, including 95Mo NMR; their activity and stability are benchmarked in detail, and the enabling properties are illustrated by advanced applications to natural product synthesis. For the favorable overall application profile and ease of handling, complexes of this new series are expected to replace earlier catalyst generations and help encourage a more regular use of alkyne metathesis in general.

Taming secondary benzylic cations in catalytic asymmetric SN1 reactions.

Benzylic stereogenic centers are ubiquitous in natural products and pharmaceuticals. A potentially general, though challenging, approach toward their selective creation would be asymmetric unimolecular nucleophilic substitution (SN1) reactions that proceed through highly reactive benzylic cations. We now report a broadly applicable solution to this problem by identifying chiral counteranions that pair with secondary benzylic cations to engage in catalytic asymmetric C-C, C-O, and C-N bond-forming reactions with excellent enantioselectivity. The critical cationic intermediate can be accessed from different precursors via Lewis- or Brønsted acid catalysis. Key to our strategy is the use of only weakly basic, confined counteranions that are posited to prolong the lifetime of the carbocation, thereby avoiding nonproductive deprotonation pathways to the corresponding styrene.

The 103Rh NMR spectroscopy and relaxometry of the rhodium formate paddlewheel complex.

The nuclear magnetic resonance (NMR) spectroscopy of spin-1/2 nuclei with low gyromagnetic ratio is challenging due to the low NMR signal strength. Methodology for the rapid acquisition of 103Rh NMR parameters is demonstrated for the case of the rhodium formate "paddlewheel" complex Rh2(HCO2)4. A scheme is described for enhancing the 103Rh signal strength by polarization transfer from 1H nuclei, which also greatly reduces the interference from ringing artifacts, a common hurdle for the direct observation of low-γ nuclei. The 103Rh relaxation time constants T1 and T2 are measured within 20 min by using 1H-detected experiments. The field dependence of the 103Rh T1 is measured. The high-field relaxation is dominated by the chemical shift anisotropy mechanism. The 103Rh shielding anisotropy is found to be very large: |Δσ| = 9900 ± 540 ppm. This estimate is compared with density functional theory calculations.

Mechanistic Studies on the Bismuth-Catalyzed Transfer Hydrogenation of Azoarenes.

Organobismuth-catalyzed transfer hydrogenation has recently been disclosed as an example of low-valent Bi redox catalysis. However, its mechanistic details have remained speculative. Herein, we report experimental and computational studies that provide mechanistic insights into a Bi-catalyzed transfer hydrogenation of azoarenes using p-trifluoromethylphenol (4) and pinacolborane (5) as hydrogen sources. A kinetic analysis elucidated the rate orders in all components in the catalytic reaction and determined that 1 a (2,6-bis[N-(tert-butyl)iminomethyl]phenylbismuth) is the resting state. In the transfer hydrogenation of azobenzene using 1 a and 4, an equilibrium between 1 a and 1 a ⋅ [OAr]2 (Ar=p-CF3 -C6 H4 ) is observed, and its thermodynamic parameters are established through variable-temperature NMR studies. Additionally, pKa -gated reactivity is observed, validating the proton-coupled nature of the transformation. The ensuing 1 a ⋅ [OAr]2 is crystallographically characterized, and shown to be rapidly reduced to 1 a in the presence of 5. DFT calculations indicate a rate-limiting transition state in which the initial N-H bond is formed via concerted proton transfer upon nucleophilic addition of 1 a to a hydrogen-bonded adduct of azobenzene and 4. These studies guided the discovery of a second-generation Bi catalyst, the rate-limiting transition state of which is lower in energy, leading to catalytic transfer hydrogenation at lower catalyst loadings and at cryogenic temperature.

Oxidative Addition of Aryl Electrophiles into a Red-Light-Active Bismuthinidene.

The oxidative addition of aryl electrophiles is a fundamental organometallic reaction widely applied in the field of transition metal chemistry and catalysis. However, the analogous version based on main group elements still remains largely underexplored. Here, we report the ability of a well-defined organobismuth(I) complex to undergo formal oxidative addition with a wide range of aryl electrophiles. The process is facilitated by the reactivity of both the ground and excited states of N,C,N-bismuthinidenes upon absorption of low-energy red light.

Asymmetric Catalytic Friedel-Crafts Reactions of Unactivated Arenes.

Since its discovery more than a century ago, the Friedel-Crafts reaction has manifested itself as a powerful method for the introduction of carbon substituents to arenes. Despite its potential generality, the scope of the reaction is intrinsically limited by the arene's nucleophilicity, which has previously restrained the applicability of asymmetric variants to activated substrates. To overcome this fundamental limitation, we report herein an asymmetric Friedel-Crafts reaction of unactivated, purely hydrocarbon arenes, alkoxybenzenes, and heteroarenes with N,O-acetals to give enantioenriched arylglycine esters. Highly regio- and stereoselective C-C bond formation was achieved using strong and confined Brønsted acid organocatalysts, enabling the first asymmetric catalytic Friedel-Crafts reaction of simple alkylbenzenes.

Direct Regioselective Dehydrogenation of α-Substituted Cyclic Ketones.

We disclose a highly regioselective, catalytic one-step dehydrogenation of α-substituted cyclic ketones in the presence of 2,3-dichlorobenzo-5,6-dicyano-1,4-benzoquinone (DDQ). The high regioselectivity originates from a phosphoric acid-catalyzed enolization, selectively affording the thermodynamically preferred enol, followed by the subsequent oxidation event. Our method provides reliable access to several α-aryl and α-alkyl substituted α,ß-unsaturated ketones.

Toward a Formyl-to-Phenyl Conversion: An Unexpected Photochemical Fulvene Rearrangement.

Toward a conversion of aldehydes into arenes, we designed a sequence involving the initial reaction of an aldehyde to give a fulvene, followed by photochemical and platinum-catalyzed rearrangements into a Dewar benzene derivative, which finally isomerizes into the targeted arene. While computational studies support the plausibility of this route, we found that fulvene irradiation resulted in an unexpected isomerization into a spiro[2.4]heptadiene. This unusual photorearrangement has been investigated mechanistically and provides access to a variety of spiro[2.4]heptadienes with different substituents.

Bismuth-Catalyzed Amide Reduction.

In this article we report that a cationic version of Akiba's BiIII complex catalyzes the reduction of amides to amines using silane as hydride donor. The catalytic system features low catalyst loadings and mild conditions, en route to secondary and tertiary aryl- and alkylamines. The system tolerates functional groups such as alkene, ester, nitrile, furan and thiophene. Kinetic studies on the reaction mechanism result in the identification of a reaction network with an important product inhibition that is in agreement with the experimental reaction profiles.

Dihydrogen and Ethylene Activation by a Sterically Distorted Distibene.

Herein, we report the synthesis of a sterically distorted distibene ([4]2 ) and its transition-metal-like reactivity towards two fundamental feedstock chemicals: H2 and ethylene. Although [4]2 exhibits an unusually long Sb=Sb distance and noticeable backbone distortion in the solid state, NMR data suggest that [4]2 remains predominantly as a dimer in solution, even at high temperatures. However, it was proposed that the elusive reactivity of [4]2 toward H2 and ethylene results from reversible dissociation of [4]2 into the transient stibinidene ([4]), which could be observed by NMR spectroscopic techniques.

Bismuth radical catalysis in the activation and coupling of redox-active electrophiles.

Radical cross-coupling reactions represent a revolutionary tool to make C(sp3)-C and C(sp3)-heteroatom bonds by means of transition metals and photoredox or electrochemical approaches. However, the use of main-group elements to harness this type of reactivity has been little explored. Here we show how a low-valency bismuth complex is able to undergo one-electron oxidative addition with redox-active alkyl-radical precursors, mimicking the behaviour of first-row transition metals. This reactivity paradigm for bismuth gives rise to well-defined oxidative addition complexes, which could be fully characterized in solution and in the solid state. The resulting Bi(III)-C(sp3) intermediates display divergent reactivity patterns depending on the α-substituents of the alkyl fragment. Mechanistic investigations of this reactivity led to the development of a bismuth-catalysed C(sp3)-N cross-coupling reaction that operates under mild conditions and accommodates synthetically relevant NH-heterocycles as coupling partners.

Synthesis and isolation of a triplet bismuthinidene with a quenched magnetic response.

Large spin-orbit coupling (SOC) is an intrinsic property of the heavy elements that directly affects the electronic structures of the compounds. In this work, we report the synthesis and characterization of a monocoordinate bismuthinidene that features a rigid and bulky ligand. All magnetic measurements [superconducting quantum interference device (SQUID), nuclear magnetic resonance (NMR)] point to a diamagnetic compound. However, multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated (76%) by a spin triplet. The apparent diamagnetism is explained by an extremely large SOC-induced positive zero-field splitting of more than 4500 wavenumbers that leaves the MS = 0 magnetic sublevel thermally isolated in the electronic ground state.

A Catalytic Asymmetric Hydrolactonization.

Despite recent advancements in the development of catalytic asymmetric electrophile induced lactonization reactions of olefinic carboxylic acids, the archetypical hydrolactonization has long remained an unsolved and well-recognized challenge. Here, we report the realization of a catalytic asymmetric hydrolactonization using a confined imidodiphosphorimidate (IDPi) Brønsted acid catalyst. The method is operationally simple, scalable, and compatible with a wide variety of substrates. Its potential is showcased with concise syntheses of the sesquiterpenes (-)-boivinianin A and (+)-gossonorol. Through in-depth physicochemical and DFT analyses, we derive a nuanced picture of the mechanism and enantioselectivity of this reaction.