A Double Addition of LnH to a Carbon-Carbon Triple Bond and Competitive Oxidation of Ytterbium(II) and Hydrido Centers

Addition of two Ln?H bonds of an Yb(II) hydrido complex supported by bulky amidinate ligand to a C?C bond lead to the formation of 1,2-dianionic bibenzyl fragment. Both Yb(II) and hydrido centers are oxidized under the reaction conditions. The resulting Yb(II) -η(6) -arene interaction is surprisingly robust: the arene cannot be replaced from the metal coordination sphere when treated with Lewis bases.     

Chiral-Directing-Group-Assisted Rhodium(III)-Catalyzed Asymmetric Addition of Inert Arene C−H Bond to Aldimines with Subsequent Intramolecular Cyclization

By using a chiral directing group, an asymmetric rhodium(III)-catalyzed C−H bond addition to aldimines followed by intramolecular cyclization to form chiral isoindolinones has been achieved (up to 68 % yield, up to 93 % ee). A three-component variant that resembles Mannich reaction was also realized (41 % yield, 83 % ee). Product elaborations and preliminary mechanistic studies were described.     

Diverse Reactivity of bis(Silylene) and bis(Germylene) [LE−EL (E=Si and Ge: L=PhC(NtBu)2)] in the Oxidative Activation of C−F Bond: Si−Si Bond is Retained While the Ge−Ge Bond Is Cleaved

Herein we report the reactions of 3,4,5,6-tetrafluoroterephthalonitrile ( 1 ) with bis(silylene) and bis(germylene) LE−EL [E=Si ( 2 ) and Ge( 3 ): L=PhC(N^tBu)₂)]. The reaction of LSi−SiL (L=PhC(N^tBu)₂) ( 2 ) with two equivalents of 1 resulted in an unprecedented oxidative addition of a C−F bond of 1 leading to disilicon(III) fluoride {L(4-C₈F₃N)FSi−SiF(4-C₈F₃N)L}( 4 ), wherein the Si−Si single bond was retained. In contrast, the reaction of LGe−GeL (L=PhC(N^tBu)₂) ( 3 ) with one equivalent of 1 resulted in the oxidative cleavage of Ge−Ge bond leading to L(4-C₈F₃N₂)Ge ( 5 ) and LGeF ( 6 ). All three compounds ( 4 – 6 ) were characterized by NMR spectroscopy, EI-MS spectrometry, and elemental analysis. X-ray single-crystal structure determination of compound 4 unequivocally established that the Si^III−Si^III bond remains uncleaved.     

Deprotonation of a Seemingly Hydridic Diborane(6) To Build a B−B Bond

Deprotonation of the doubly arylene-bridged diborane(6) derivative 1 H₂ with (Me₃Si)₃CLi or (Me₃Si)₂NK gives the B−B σ-bonded species M[ 1 H] in essentially quantitative yields (THF, room temperature; M=Li, K, arylene=4,4′-di-tert-butyl-2,2′-biphenylylene). With nBuLi as the base, the yield of Li[ 1 H] drops to 20 % and the 1,1-bis(9-borafluorenyl)butane Li[ 2 H] is formed as a side product (30 %). In addition to the 1,1-butanediyl fragment, the two boron atoms of Li[ 2 H] are linked by a μ-H bridge. In the closely related molecule Li[ 3 H], the corresponding μ-H atom can be abstracted with (Me₃Si)₃CLi to afford the B−B-bonded conjugated base Li₂[ 3 ] (THF, 150 °C; 15 %). Li[ 1 H] and Li[ 2 H] were characterized by NMR spectroscopy and X-ray crystallography.     

Diaminomethylenemalononitrile as a Chiral Single Hydrogen Bond Catalyst: Application to Enantioselective Conjugate Addition of α-Branched Aldehydes

An improved diaminomethylenemalononitrile organocatalyst, bearing a N,N-disubstituted structure, promoted enantioselective conjugate addition reaction of α-branched aldehydes with vinyl sulfone, affording adducts with excellent enantioselectivities (up to 96% ee). Mechanistic studies revealed that the diaminomethylenemalononitrile motif holds the vinyl sulfone substrate using a single hydrogen bond accompanied by multiple weak interactions, including electrostatic C−H⋅⋅⋅O interactions.     

Mild and Efficient Michael Addition of Activated Olefins to Indoles using TBAB as a Catalyst: Synthesis of 3‐Substituted Indoles

A mild and efficient method for the synthesis of 3‐substituted indoles by treatment of activated olefins with indoles using TBAB as a catalyst afforded the corresponding products in excellent yields. The method is general for the preparation of a wide variety of 3‐substituted indoles.     

The Na⋅⋅⋅B Bond in NaBH3−: A Different Type of Bond

A newly introduced Na−B bond in NaBH₃⁻ has been a challenge for the chemical bonding community. Here, a series of MBH₃⁻ (M=Li, Na, K) species and NaB(CN)₃⁻ are studied within the context of quantum chemical topology approaches. The analyses suggest that M–B interaction cannot be classified as an ordinary covalent, dative, or even simple ionic interaction. The interactions are controlled by coulombic forces between the metals and the substituents on boron, for example, H or CN, more than the direct M–B interaction. On the other hand, while the characteristics of the (3, −1) critical points of the bonds are comparable to weak hydrogen bonds, not covalent bonds, the metal and boron share a substantial sum of electrons. To the best of the author's knowledge, the characteristics of these bonds are unprecedented among known molecules. Considering all paradoxical properties of these bonds, they are herein described as ionic-enforced covalent bonds.     

Stereochemistry of Phosphite Addition to Azomethine Bond of Achiral 2,6-Pyridinedicarbaldimines and Isophthalaldimines—A Comparative Study

Abstract

The addition of dialkyl H-phosphonates to isophthalaldimines 1a–d and pyridine-2,6-dicarboxaldimines 2a–d was investigated and led to the corresponding aminophosphonates. Diastereoselectivity of the addition to pyridine-2,6-dicarboxaldimines was lower than to isophthalaldimines. In contrast, addition of bis(trimethylsilyl) H-phosphonate to both groups of aldimines demonstrated that the diastereoselectivity in case of pyridine-2,6-dicarboxaldimines is comparable or even better than that for the isophthalic derivatives.     

Reductive Coupling of (Fluoro)pyridines by Linear 3d-Metal(I) Silylamides of Cr–Co: A Tale of C−C Bond Formation,C−F Bond Cleavage and a Pyridyl Radical Anion

Herein, we disclose the facile reduction of pyridine (and its derivatives) by linear 3d-metal(I) silylamides (M=Cr–Co). This reaction resulted in intermolecular C−C coupling to give dinuclear metal(II) complexes bearing a bridging 4,4′-dihydrobipyridyl ligand. For iron, we demonstrated that the C−C coupling is reversible in solution, either directly or by reaction with substrates, via a presumed monomeric metal(II) complex bearing a pyridyl radical anion. In the course of this investigation, we also observed that the dinuclear metal(II) complex incorporating iron facilitated the isomerisation of 1,4-cyclohexadiene to 1,3-cyclohexadiene as well as equimolar amounts of benzene and cyclohexene. Furthermore, we synthesised and structurally characterised a non-3d-metal-bound pyridyl radical anion. The reactions of the silylamides with perfluoropyridine led to C−F bond cleavage with the formation of metal(II) fluoride complexes of manganese, iron and cobalt along with the homocoupling or reductive degradation of the substrate. In the case of cobalt, the use of lesser fluorinated pyridines led to C−F bond cleavage but no homocoupling. Overall, in this paper we provide insights into the multifaceted behaviour of simple (fluoro)pyridines in the presence of moderately to highly reducing metal complexes.     

Photocatalyzed Hydrogen Atom Transfer Degradation of Vinyl Polymers: Cleavage of a Backbone C−C Bond Triggered by Radical Activation of a C−H Bond in a Pendant

In this work, we achieved a triggering degradation of polymers composed of carbon-carbon (C−C) bonded backbone without relying on introduction of labile heteroatom-based bond. The crucial point for the achievement is using vinyl ether (VE) as a comonomer in radical copolymerization of (meth)acrylate for introduction of the carbon-hydrogen (C−H) bonds active for photocatalyzed hydrogen atom transfer (HAT) as triggers in the pendant. Interestingly, methyl methacrylate (MMA)-n-butyl vinyl ether (NBVE) copolymer underwent degradation in acetonitrile in the presence of benzophenone (Ph₂CO) under UV irradiation at 80 °C. The degradation did not take place, when any one of UV, Ph₂CO, heat, and NBVE unit was removed or HAT-active solvent such as toluene and 1,4-dioxane was used. These control experiments strongly supported the HAT-triggering degradation. Furthermore, the degradation behaviors of the copolymers with other vinyl ethers such as tert-butyl vinyl ether and methyl isopropenyl ether indicated that the C−H bond neighboring to oxygen on the pendant is mainly responsible for the trigger leading to degradation. The HAT-triggering degradation was also demonstrated even with the acrylate-based copolymer.     

Influence of the Flexibility of Nickel PCP-Pincer Complexes on C−H and P−C Bond Activation and Ethylene Reactivity: A Combined Experimental and Theoretical Investigation

Using a pincer platform based on a bridgehead NHC donor with functional side arms, the combined effect of increased flexibility in six-membered pyrimidine-type heterocycles compared to the more often studied five-membered imidazole, and rigidity of phosphane side arms was examined. The unique features observed include: 1) the reaction of the azolium C_sp2−H bond with [Ni(cod)₂] affording a carbanionic ligand in [NiCl(PC_sp3^HP)] ( 8 ) rather than a carbene; 2) its transformation into the NHC, hydrido complex [NiH(PC^NHCP)]PF₆ ( 9 ) upon halide abstraction; 3) ethylene insertion into the Ni−H bond of the latter and ethyl migration to the N−C−N carbon atom of the heterocycle in [Ni(PC^EtP)]PF₆ ( 10 ); and 4) an unprecedented C−P bond activation transforming the P−C^NHC−P pincer ligand of 8 in a C−C^NHC−P pincer and a terminal phosphanido ligand in [Ni(PPh₂)(CC^NHCP)] ( 15 ). The data are supported by nine crystal structure determinations and theoretical calculations provided insights into the mechanisms of these transformations, which are relevant to stoichiometric and catalytic steps of general interest.     

A Three-Component Arene Difunctionalization: Merger of C(sp3)/(sp2)−H Bond Addition

A tandem three-component C−H bond addition involving the activation of an inert C(sp³)−H bond is reported. The process enables the direct regioselective synthesis of 1,2-difunctionalized arenes with the formation of C(sp³)− and C(sp²)−C(arene) bonds. 2-Iodobenzoic acid derivatives behave as masked bifunctional reagent (BFR) and react with 2-pyridyl-methyl sulfoximine (MPyS) protected aliphatic acids bearing α,α-disubstituted groups, and alkenes to produce β-aryl-δ-alkenyl amide derivatives in a single operation. The transformation involves Pd(II)/Pd(IV) and Pd(II)/Pd(0) catalytic systems. Detailed mechanistic studies, including density functional theory (DFT) calculations, reveal the formation of large T-shaped palladacycles and the onset of a 1,2-palladium migration via decarboxylation.     

A Novel Stereoseletive Approach to (E)-Vinylic Selenides

Vinylicselenidesareimportantintermediatesinorganicsynthesis.andtheirpromisingpotentialcouldbeanticipatedbythecombinationofthespecialreactivityofseleniumandthereactivit}'associatedwithcarbon-carbondoublebond.particularly'the}'areextremely"set'ulinhighlystereoselectiveC-Cbondformationprocess'.Variouspreparationsofthesecompoundshavebeenreported,butmostofthesemethodsinvolvedthedisposalofhighlytoxicselenides,suchasRSeX,RSeSeRandRSeH.Futhermore,manyofthesemethodsarenotstereocontrolledandgivemixt…     

A Novel Calorimetric Approach to the Redox Thermochemistry of Non-stoichiometric Oxides enthalpy of Oxidation of La1−yCayCrO3−δ

A novel calorimetric approach to the energetics of redox reactions in non-stoichiometric oxides is presented. The method utilizes a step-wise heated adiabatic calorimeter for determination of average enthalpies of oxidation. The measurement uncertainty is dominated by the uncertainty in the determination of the mass increment due to the oxidation. An experimental investigation of the re-oxidation energetics of reduced La_1–yCa_yCrO_3– is presented. The variation of the average formal oxidation state with temperature and oxygen fugacity is calculated from the experimentally determined enthalpy of oxidation and an estimated (lattice and configurational) entropy of oxidation. The calculated curves are in good agreement with experimental determinations reported in literature.This revised version was published online in November 2005 with corrections to the Cover Date.     

Addition of Organocuprates to Cyclobutenyl Esters. Synthesis of (±)-Grandisol

The conjugate addition of organocuprates to certain esters², methylene lactones³, and enol lactones⁴ has been demonstrated. We report herein that cyclobutenyl esters readily undergo conjugate addition with these reagents, and that this reaction has been used to synthesize the boll weevil pheromone (±)-grandisol (18).⁵     

Radical Addition of Dimethyl 2‐Ethynylcyclopropane‐1,1‐dicarboxylate to Electron‐Rich Olefins

The phenylseleno radical–catalyzed addition of dimethyl 2‐ethynylcyclopropane‐1,1‐dicarboxylate to electron–rich olefins has been achieved. This radical process leads to formation of highly substituted ethynylcyclopentane products through a [3+2] annulation.     

High Diastereoselectivity in the Conjugate Addition of Functionalized Alcohols to a Chiral(E)-Nitroalkene

Nitr0alkenesareversatilesyntheticintermediatesaspowerfulelectron-deficientreactantsincycloadditionandconjugatadditionreacti0ntoproducealargevarietyfunctionalizedcarbocyclicrings'.Purthermore,thenitrogrouPcanbeconvertedintoawiderangeoffiinctionalities2-Al0ughDiels-Alderreaction3andcyclopropanationoffunctionalizedcarbonnucleophiles4toY-chiralnitr0alkenehavebeenreportedrecently.Sofarthedetailedresearch0ftheconjugateadditionsoffimctionalizedalcoholst0chira1nitf0keneshasn0tbeenreported.Herewewish…     

Room-Temperature-Observable Interconversion Between Si(IV) and Si(II) via Reversible Intramolecular Insertion Into an Aromatic C−C Bond

An easily isolable silacycloheptatriene (silepin) 1 b was synthesized from the reaction of a N-heterocyclic imino (IPrN) substituted tribromosilane IPrNSiBr₃ with the sterically congested bis(trimethylsilyl)triisopropylsilyl silanide KSi(TMS)₂Si(ⁱPr)₃ (BTTPS). In solution, the Si(IV) silepin 1 b is in a thermodynamic equilibrium with the acyclic Si(II) silylene 1 a . The relative concentration of the Si(II) or Si(IV) isomers can be controlled by temperature variation and observed by variable temperature NMR and UV/Vis spectroscopy. DFT calculations show a small reaction barrier for the Si(II)⇌Si(IV) interconversion and a small energy gap between the Si(II) and Si(IV) species. The reactivity of 1 a/b is demonstrated on a variety of small molecules.     

Controlled Free-Radical Copolymerization of N-vinyl succinimide and n-Butyl acrylate via a Reversible Addition–Fragmentation Chain Transfer (RAFT) Technique

Summary : Copolymerization of N-vinyl succinimide and n-butyl acrylate in the presence of dibenzyl trithiocarbonate as a reversible addition-fragmentation chain transfer agent was investigated. The linear dependence of molecular mass on conversion and low values of polydispersity index confirmed pseudo-living mechanism of the process. For the first time the soluble copolymers of N-vinyl succinimide and n-butyl acrylate with high composition homogeneity have been synthesized by copolymerization in bulk. The copolymerization kinetics was studied by NMR ¹H spectroscopy; the reactivity ratios were determined: r_VSI = 0.11, r_BA = 2.54. The copolymer microstructure was estimated; it was shown that in conditions of RAFT polymerization gradient copolymers enriched with BA on the tails of the macromolecule and with VSI in the middle can be obtained. The method of elimination of trithiocarbonate fragment by the reaction with an excess of AIBN was proposed leading to formation of the simplest gradient structure of N-vinyl succinimide – n-butyl acrylate copolymer.     

A Novel Total Synthesis of (±)Shikonin

Shikonin 1 was a key component isolated from the traditional Chinese herbal medicine, Zicao(Lithospermum erythrorhizon Sieb.et Zucc), which possesses multiple biological activities1. There have been many synthetic routes to shikonin, however, all these ro…