Sterically constrained tricyclic phosphine: redox behaviour,reductive and oxidative cleavage of P–C bonds,generation of a dilithium phosphaindole as a promising synthon in phosphine chemistry

The redox behaviour of sterically constrained tricyclic phosphine 3a was investigated by spectroelectrochemistry. The data suggested a highly negative reduction potential with the reversible formation of a dianionic species. Accordingly, 3a reacted with two equivalents of Li/naphthalene by reductive cleavage of a P–C bond of one of the PC₄ heterocycles. The resulting dilithium compound 5 represents a phosphaindole derivative with annulated aromatic C₆ and PC₄ rings. It is an interesting starting material for the synthesis of new heterocyclic molecules, as was shown by treatment with Me₂SiCl₂ and PhPCl₂. The structures of the products (6 and 7) formally reflect ring expansion by insertion of silylen or phosphinidene fragments into a P–C bond of 3a. Treatment of 3a with H₂O₂ did not result in the usually observed transfer of a single O atom to phosphorus, but oxidative cleavage of a strained PC₄ ring afforded a bicyclic phosphinic acid, R₂PO₂H.

Sterically constrained tricyclic phosphines with annulated five- and six-membered rings show fascinating chemical and redox reactivity as indicated by sophisticated in situ UV-vis CV and multi-pulse chronoamperometry.     

A reconstructed porous copper surface promotes selectivity and efficiency toward C2 products by electrocatalytic CO2 reduction

Electrocatalytic synthesis of multicarbon (C₂₊) products from CO₂ reduction suffers from poor selectivity and low energy efficiency. Herein, a facile oxidation–reduction cycling method is adopted to reconstruct the Cu electrode surface with the help of halide anions. The surface composed of entangled Cu nanowires with hierarchical pores is synthesized in the presence of I⁻, exhibiting a C₂ faradaic efficiency (FE) of 80% at −1.09 V vs. RHE. A partial current density of 21 mA cm⁻² is achieved with a C₂ half-cell power conversion efficiency (PCE) of 39% on this electrode. Such high selective C₂ production is found to mainly originate from CO intermediate enrichment inside hierarchical pores rather than the surface lattice effect of the Cu electrode.

The Cu electrode surface is reconstructed by a halide anion assisted method for promoting CO₂ reduction.     

Submonomer synthesis of peptoids containing trans-inducing N-imino- and N-alkylamino-glycines

The use of hydrazones as a new type of submonomer in peptoid synthesis is described, giving access to peptoid monomers that are structure-inducing. A wide range of hydrazones were found to readily react with α-bromoamides in routine solid phase peptoid submonomer synthesis. Conditions to promote a one-pot cleavage of the peptoid from the resin and reduction to the corresponding N-alkylamino side chains were also identified, and both the N-imino- and N-alkylamino glycine residues were found to favor the trans-amide bond geometry by NMR, X-ray crystallography, and computational analyses.

The use of hydrazones as a new type of submonomer in peptoid synthesis is described, giving access to peptoid monomers that are structure-inducing.     

Synthesis of N-aryl amines enabled by photocatalytic dehydrogenation

Catalytic dehydrogenation (CD) via visible-light photoredox catalysis provides an efficient route for the synthesis of aromatic compounds. However, access to N-aryl amines, which are widely utilized synthetic moieties, via visible-light-induced CD remains a significant challenge, because of the difficulty in controlling the reactivity of amines under photocatalytic conditions. Here, the visible-light-induced photocatalytic synthesis of N-aryl amines was achieved by the CD of allylic amines. The unusual strategy using C₆F₅I as an hydrogen-atom acceptor enables the mild and controlled CD of amines bearing various functional groups and activated C–H bonds, suppressing side-reaction of the reactive N-aryl amine products. Thorough mechanistic studies suggest the involvement of single-electron and hydrogen-atom transfers in a well-defined order to provide a synergistic effect in the control of the reactivity. Notably, the back-electron transfer process prevents the desired product from further reacting under oxidative conditions.

The synergy of SET, HAT, and BET enables a visible-light induced catalytic dehydrogenation for the synthesis of N-aryl amines.     

Robust selenium-doped carbon nitride nanotubes for selective electrocatalytic oxidation of furan compounds to maleic acid

Selective oxidation of biomass-derived furan compounds to maleic acid (MA), an important bulk chemical, is a very attractive strategy for biomass transformation. However, achieving a high MA selectivity remains a great challenge. Herein, we for the first time successfully designed and fabricated Se-doped graphitic carbon nitride nanotubes with a chemical formula of C_3.0N-Se_0.03. The prepared C_3.0N-Se_0.03 was highly efficient for electrocatalytic oxidation of various biomass-derived furan compounds to generate MA. At ambient conditions, the MA yield could reach 84.2% from the electro-oxidation of furfural. Notably, the substituents on the furan ring significantly affected the selectivity to MA, following the order: carboxyl group > aldehyde group > hydroxyl group. Detailed investigation revealed that Se doping could tune the chemical structure of the materials (e.g., C_3.0N-Se_0.03 and g-C₃N₄), thus resulting in the change in catalytic mechanism. The excellent performance of C_3.0N-Se_0.03 originated from the suitable amount of graphitic N and its better electrochemical properties, which significantly boosted the oxidation pathway to MA. This work provides a robust and selective metal-free electrocatalyst for the sustainable synthesis of MA from oxidation of biomass-derived furan compounds.

Biomass-derived furan compounds could be selectively electro-oxidized into maleic acid over a robust selenium-doped carbon nitride nanotube catalyst.     

A cyclopentadienyl functionalized silylene – a flexible ligand for Si- and C-coordination

The synthesis of a 1,2,3,4-tetramethylcyclopentadienyl (Cp⁴) substituted four-membered N-heterocyclic silylene [{PhC(NtBu)₂}Si(C₅Me₄H)] is reported first. Then, selected reactions with transition metal and a calcium precursor are shown. The proton of the Cp⁴-unit is labile. This results in two different reaction pathways: (1) deprotonation and (2) rearrangement reactions. Deprotonation was achieved by the reaction of [{PhC(NtBu)₂}Si(C₅Me₄H)] with suitable zinc precursors. Rearrangement to [{PhC(NtBu)₂}(C₅Me₄)SiH], featuring a formally tetravalent silicon R₂C Si(R′)–H unit, was observed when the proton of the Cp⁴ ring was shifted from the Cp⁴-ring to the silylene in the presence of a Lewis acid. This allows for the coordination of the Cp⁴-ring to a calcium compound. Furthermore, upon reaction with transition metal dimers [MCl(cod)]₂ (M = Rh, Ir; cod = 1,5-cyclooctadiene) the proton stays at the Cp⁴-ring and the silylene reacts as a sigma donor, which breaks the dimeric structure of the precursors.

A cyclopentadienyl functionalized silylene or its derivatives can be coordinated in all three forms: silylene (A), anion (B), and sila fulvene (C).     

Intramolecular Friedel–Crafts alkylation with a silylium-ion-activated cyclopropyl group: formation of tricyclic ring systems from benzyl-substituted vinylcyclopropanes and hydrosilanes

A trityl-cation-initiated annulation of benzyl-substituted vinylcyclopropanes (VCPs) with hydrosilanes is reported. Two Si–C(sp³) bonds and one C(sp²)–C(sp³) bond are formed in this process where an intramolecular 6-endo-tet Friedel–Crafts alkylation of a silylium-ion-activated cyclopropane ring is the rate-determining key step. The reaction mechanism is proposed based on computations and is in agreement with experimental observations. The new reaction leads to an unprecedented silicon-containing 6/6/5-fused ring system. A phenethyl-substituted VCP derivative yields another unknown tricycle having 6/6/6 ring fusion by reacting in a related but different way involving a 6-exo-tet ring closure.

Downstream to alkene hydrosilylation, the opening of the cyclopropane ring in benzyl-substituted VCPs is interlinked with an S_EAr of the aryl group.     

Figure-eight thermal hysteresis of aminomethylenehelicene oligomers with terminal C16 alkyl groups during hetero-double-helix formation

1 : 1 mixtures of aminomethylenehelicene (P)-tetramer and (M)-pentamer with terminal C₁₆ alkyl groups in fluorobenzene showed structural changes between hetero-double-helices B and C and random-coils 2A. Figure-eight thermal hysteresis appeared when the solution was cooled and heated at a constant rate and involved the crossing of cooling and heating curves in Δε/temperature profiles. This unusual thermal hysteresis emerged in the intermediate state between counterclockwise and clockwise thermal hystereses. This phenomenon arose from the competition between self-catalytic reactions to form B and C from 2A. Significant effects of terminal C₁₆ alkyl groups on the thermodynamic and kinetic phenomena are also described.

1 : 1 mixtures of aminomethylenehelicene (P)-tetramer and (M)-pentamer with terminal C₁₆ alkyl groups in fluorobenzene showed structural changes between hetero-double-helices B and C and random-coils 2A.     

The enantioselective total synthesis of laurendecumallene B

For decades, the Laurencia family of halogenated C₁₅-acetogenins has served as a valuable testing ground for the prowess of chemical synthesis, particularly as it relates to generating functionalized 8-membered bromoethers. Herein, we show that a readily modified and predictable approach that generates such rings and an array of attendant stereocenters via a bromenium-induced cyclization/ring-expansion process can be used to synthesize laurendecumallene B and determine the configuration of two of its previously unassigned stereocenters. In particular, this work highlights how the use of the bromenium source BDSB (Et₂SBr·SbCl₅Br) in non-conventional solvents is essential in generating much of the target''s complexity in optimal yields and stereoselectivity. Moreover, the final structural assignment of laurendecumallene B reveals that it has one element of bromine-based chirality that, to the best of our knowledge, is not shared with any other member of the class.

Use of two key reactions initiated by BDSB (Et₂SBr·SbCl₅Br) affords an efficient total synthesis of laurendecumallene B, establishing the configuration of its two bromine-bearing stereocenters.     

BNN-1,3-dipoles: isolation and intramolecular cycloaddition with unactivated arenes

The mono-base-stabilized 1,2-diboranylidenehydrazine derivatives featuring a 1,3-dipolar BNN skeleton are obtained by dehydrobromination of [ArB(Br)NH]₂ (Ar = 2,6-diphenylphenyl (Dpp), Ar = 2,6-bis(2,4,6-trimethylphenyl)phenyl (Dmp) or Ar = 2,4,6-tri-tert-butylphenyl (Mes*)) with N-heterocyclic carbenes (NHCs). Depending on the Ar substituents, such species can be isolated as a crystalline solid (Ar = Mes*) or generated as reactive intermediates undergoing spontaneous intramolecular aminoboration of the proximal arene rings via [3 + 2] cycloaddition (Ar = Dpp or Dmp). The latter reactions showcase the 1,3-dipolar reactivity toward unactivated arenes at ambient temperature. In addition, double cycloaddition of the isolable BNN species with two CO₂ molecules affords a bicyclic species consisting of two fused five-membered BN₂CO rings. The electronic structures of these BNN species and the mechanisms of these cascade reactions are interrogated through density functional theory (DFT) calculations.

The mono-base-coordinated 1,2-diboranylidenehydrazine derivatives exhibiting the BNN-1,3-dipolar reactivity toward unactivated arenes and CO₂ are reported.     

Unequivocal structure confirmation of a breitfussin analog by anisotropic NMR measurements

Structural features of proton-deficient heteroaromatic natural products, such as the breitfussins, can severely complicate their characterization by NMR spectroscopy. For the breitfussins in particular, the constitution of the five-membered oxazole central ring cannot be unequivocally established via conventional NMR methods when the 4′-position is halogenated. The level of difficulty is exacerbated by 4′-iodination, as the accuracy with which theoretical NMR parameters are determined relies extensively on computational treatment of the relativistic effects of the iodine atom. It is demonstrated in the present study, that the structure of a 4′-iodo breitfussin analog can be unequivocally established by anisotropic NMR methods, by adopting a reduced singular value decomposition (SVD) protocol that leverages the planar structures exhibited by its conformers.

Structural features of proton-deficient heteroaromatic natural products, such as the breitfussins, can severely complicate their characterization by NMR spectroscopy.     

Exploiting single-electron transfer in Lewis pairs for catalytic bond-forming reactions

A single-electron transfer (SET) between tris(pentafluorophenyl)borane (B(C₆F₅)₃) and N,N-dialkylanilines is reported, which is operative via the formation of an electron donor–acceptor (EDA) complex involving π-orbital interactions as a key intermediate under dark conditions or visible-light irradiation depending on the structure of the aniline derivatives. This inherent SET in the Lewis pairs initiates the generation of the corresponding α-aminoalkyl radicals and their additions to electron-deficient olefins, revealing the ability of B(C₆F₅)₃ to act as an effective one-electron redox catalyst.

Radical–ion pair generation from common Lewis pairs and its application to catalytic carbon–carbon bond formation.     

A germaaluminocene

The reactions of dipotassium germacyclopentadienediide with two Group 13 dichlorides, Cp*BCl₂ and Cp*AlCl₂, yield two structurally different products. In the case of boron a borole complex of germanium(ii) is obtained. The aluminium halide gives an unprecedented neutral germaaluminocene. Both compounds were fully characterised by multinuclear NMR spectroscopy supported by DFT computations. The molecular structure of the germaaluminocene was determined by XRD.

Boron vs. aluminum: the synthesis of a borole complex of Ge(ii) is reported. Changing just the element from boron to aluminum unexpectedly yields an unprecedented neutral germaaluminocene.     

A step-economic and one-pot access to chiral Cα-tetrasubstituted α-amino acid derivatives via a bicyclic imidazole-catalyzed direct enantioselective C-acylation

C^α-Tetrasubstituted α-amino acids are ubiquitous and unique structural units in bioactive natural products and pharmaceutical compounds. The asymmetric synthesis of these molecules has attracted a lot of attention, but a more efficient method is still greatly desired. Here we describe the first sequential four-step acylation reaction for the efficient synthesis of chiral C^α-tetrasubstituted α-amino acid derivatives from simple N-acylated amino acids via an auto-tandem catalysis using a single nucleophilic catalyst. The synthetic efficiency is improved via a direct enantioselective C-acylation; the methodology affords the corresponding C^α-tetrasubstituted α-amino acid derivatives with excellent enantioselectivities (up to 99% ee). This step-economic, one-pot, and auto-tandem strategy provides facile access to important chiral building blocks, such as peptides, serines, and oxazolines, which are often used in medicinal and synthetic chemistry.

The first four-step sequential reaction for the synthesis of C^α-tetrasubstituted chiral α-amino acid derivatives via auto-tandem catalysis has been developed.     

Silyloxymethanesulfinate as a sulfoxylate equivalent for the modular synthesis of sulfones and sulfonyl derivatives

An efficient protocol for the modular synthesis of sulfones and sulfonyl derivatives has been developed utilizing sodium tert-butyldimethylsilyloxymethanesulfinate (TBSOMS-Na) as a sulfoxylate (SO₂²⁻) equivalent. TBSOMS-Na, easily prepared from the commercial reagents Rongalite™ and TBSCl, serves as a potent nucleophile in S-alkylation and Cu-catalyzed S-arylation reactions with alkyl and aryl electrophiles. The sulfone products thus obtained can undergo the second bond formation at the sulfur center with various electrophiles without a separate unmasking step to afford sulfones and sulfonyl derivatives such as sulfonamides and sulfonyl fluorides.

An efficient protocol for the modular synthesis of sulfones and sulfonyl derivatives has been developed utilizing sodium tert-butyldimethylsilyloxymethanesulfinate (TBSOMS-Na) as a sulfoxylate (SO₂²⁻) equivalent.     

Iridium-catalyzed asymmetric trans-selective hydrogenation of 1,3-disubstituted isoquinolines

The development of the first asymmetric trans-selective hydrogenation of 1,3-disubstituted isoquinolines is reported. Utilizing [Ir(cod)Cl]₂ and a commercially available chiral Josiphos ligand, a variety of differentially substituted isoquinolines are hydrogenated to produce enantioenriched trans-tetrahydroisoquinolines in good yield with high levels of enantioselectivity. Directing group studies demonstrate that the hydroxymethyl functionality at the C1 position is critical for hydrogenation to favor the trans-diastereomer. Preliminary mechanistic studies reveal that non-coordinating chlorinated solvents and halide additives are crucial to enable trans-selectivity.

trans-Selective asymmetric hydrogenation of 1,3-disubstituted isoquinolines.     

Rapidly self-deoxygenating controlled radical polymerization in water via in situ disproportionation of Cu(i)

Rapidly self-deoxygenating Cu-RDRP in aqueous media is investigated. The disproportionation of Cu(i)/Me₆Tren in water towards Cu(ii) and highly reactive Cu(0) leads to O₂-free reaction environments within the first seconds of the reaction, even when the reaction takes place in the open-air. By leveraging this significantly fast O₂-reducing activity of the disproportionation reaction, a range of well-defined water-soluble polymers with narrow dispersity are attained in a few minutes or less. This methodology provides the ability to prepare block copolymers via sequential monomer addition with little evidence for chain termination over the lifetime of the polymerization and allows for the synthesis of star-shaped polymers with the use of multi-functional initiators. The mechanism of self-deoxygenation is elucidated with the use of various characterization tools, and the species that participate in the rapid oxygen consumption is identified and discussed in detail.

The rapidly self-deoxygenating Cu-RDRP in aqueous media is investigated.     

A pyrone remodeling strategy to access diverse heterocycles: application to the synthesis of fascaplysin natural products

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described. The pyrido[1,2-a]indole core was demonstrated to be a versatile scaffold that can be site-selectively functionalized. The utility of this novel annulation strategy was showcased in a concise formal synthesis of three fascaplysin congeners.

The synthesis of diverse N-fused heterocycles, including the pyrido[1,2-a]indole scaffold, using an efficient pyrone remodeling strategy is described.     

Structure and redox tuning of gas adsorption properties in calixarene-supported Fe(ii)-based porous cages

We describe the synthesis of Fe(ii)-based octahedral coordination cages supported by calixarene capping ligands. The most porous of these molecular cages has an argon accessible BET surface area of 898 m² g⁻¹ (1497 m² g⁻¹ Langmuir). The modular synthesis of molecular cages allows for straightforward substitution of both the bridging carboxylic acid ligands and the calixarene caps to tune material properties. In this context, the adsorption enthalpies of C₂/C₃ hydrocarbons ranged from −24 to −46 kJ mol⁻¹ at low coverage, where facile structural modifications substantially influence hydrocarbon uptakes. These materials exhibit remarkable stability toward oxidation or decomposition in the presence of air and moisture, but application of a suitable chemical oxidant generates oxidized cages over a controlled range of redox states. This provides an additional handle for tuning the porosity and stability of the Fe cages.

We describe the synthesis of Fe(ii)-based coordination cages whose stability and gas adsorption properties can be tuned through structural modifications and redox reactivity.     

Enantioselective total synthesis of (−)-myrifabral A and B

A catalytic enantioselective approach to the Myrioneuron alkaloids (−)-myrifabral A and (−)-myrifabral B is described. The synthesis was enabled by a palladium-catalyzed enantioselective allylic alkylation, that generates the C(10) all-carbon quaternary center. A key N-acyl iminium ion cyclization forged the cyclohexane fused tricyclic core, while vinyl boronate cross metathesis and oxidation afforded the lactol ring of (−)-myrifabral A. Adaptation of previously reported conditions allowed for the conversion of (−)-myrifabral A to (−)-myrifabral B.

A catalytic enantioselective approach to the Myrioneuron alkaloids (−)-myrifabral A and (−)-myrifabral B is described.