Versatile Homoallylic Boronates by Chemo‐, SN2′‐, Diastereo‐ and Enantioselective Catalytic Sequence of Cu−H Addition to Vinyl‐B(pin)/Allylic Substitution

A highly chemo‐, diastereo‐ and enantioselective catalytic method that efficiently combines a silyl hydride, vinyl‐B(pin) (pin=pinacolato) and (E)‐1,2‐disubstituted allylic phosphates is introduced. Reactions, best promoted by a Cu‐based complex with a chiral sulfonate‐containing N‐heterocyclic carbene, are broadly applicable. Aryl‐, heteroaryl‐, alkenyl‐, alkynyl‐ and alkyl‐substituted allylic phosphates may thus be converted to the corresponding homoallylic boronates and then alcohols (after C−B bond oxidation) in 46–91 % yield and in up to >98 % S_N2′:S_N2 ratio, 96:4 diastereomeric ratio and 98:2 enantiomeric ratio. The reasons why an NHC−Cu catalyst is uniquely effective (vs. the corresponding phosphine systems) and the basis for different trends in stereoselectivity are provided with the aid of DFT calculations.     

Synthesis and Properties of 2,3‐Dihydro‐1H‐corannuleno[2,3‐cd]pyridine (=2,3‐Dihydro‐1H‐dibenzo[1,10 : 6,7]fluorantheno[3,4‐cd]pyridine) Derivatives: Heterocyclic peri‐Anellated Corannulenes

The anellation of a 6‐membered ring to the 2,3‐position of corannulene (=dibenzo[ghi,mno]fluoranthene; 1 ) leads to curved aromatic compounds with a significantly higher bowl‐inversion barrier than corannulene (see Fig. 1). If the bridge is −CH₂−NR−CH₂−, a variety of linkers can be introduced at the N(2) atom, and the corresponding curved aromatics act as versatile building blocks for larger structures (see Scheme). The locked bowl, in combination with an amide bond (see 9 and 10 ), gives rise to corannulene derivatives with chiral ground‐state conformations, which possess the ability to adapt to their chiral environment by shifting their enantiomer equilibrium slightly in favor of one enantiomeric conformer. Rim annulation of corannulene seems to display a significantly lower electron‐withdrawing effect than facial anellation on [5,6]fullerene‐C₆₀‐I_h, as determined by an investigation of the basicity at the N‐atom of CH₂−NR−CH₂ (see 4 vs. 15 in Fig. 2).     

2,4,6‐Triarylphosphinines versus 2,4,6‐Triarylpyridines: An Investigation of the Differences in Reactivity between Structurally Related Aromatic Phosphorus and Nitrogen Heterocycles

The novel atropisomeric pyridine derivative rac‐ 10 has been synthesized and structurally characterized. In contrast to its phosphorus analogue 3 , axially chiral 10 has a considerably lower rotational barrier as estimated by DFT calculations. However, the presence of the two enantiomers could be confirmed by means of chiral analytical HPLC analysis and by protonation experiments with a chiral acid. Compound rac‐ 10 could be further dehydrogenated by treatment with DDQ to the benzo(h)quinoline derivative rac‐ 12 . This conversion failed for the phosphorus analogue rac‐ 3 . Interestingly, although 2,4,6‐triarylphosphinines undergo facile C? H activation with [Cp*IrCl₂]₂ in the presence of NaOAc, this reaction does not proceed with the corresponding pyridine derivatives. On the other hand, the latter ones can be selectively ortho‐metalated with Pd(OAc)₂, leading to acetate‐bridged dimeric species, which could be unambiguously confirmed by means of X‐ray crystal structure analysis. The treatment of phosphinines with Pd(OAc)₂ led instead to the formation of the unusual cofacial oxidative coupling products 16 and 17 , which consist of a phosphorus‐containing cage structure.     

Short Syntheses of 4‐Deoxycarbazomycin B,Sorazolon E,and (+)‐Sorazolon E2

Short syntheses of 4‐deoxycarbazomycin B and sorazolon E were established through the condensation of cyclohexanone and commercially available 4‐methoxy‐2,3‐dimethylaniline, followed by Pd^II‐catalyzed dehydrogenative aromatization/intramolecular C−C bond coupling and deprotection. A chiral dinuclear vanadium complex (R _a,S,S )‐ 6 mediated the enantioselective oxidative coupling of sorazolon E, affording (+)‐sorazolon E2 in good enantioselectivity.     

Catalytic Enantioselective Synthesis of C1‐ and C2‐Symmetric Spirobiindanones through Counterion‐Directed Enolate C‐Acylation

A catalytic enantioselective route to C₁‐ and C₂‐symmetric 2,2′‐spirobiindanones has been realized through an intramolecular enolate C‐acylation. This reaction employs a chiral ammonium counterion to direct the acylation of an in situ generated ketone enolate with a pentafluorophenyl ester. This reaction constitutes the first example of a direct catalytic enantioselective C‐acylation of a ketone and provides an efficient and highly enantioselective route to axially chiral spirobiindanediones. These products can be diastereoselectively derivatized, offering access to a range of functionalized spirocyclic architectures.     

Structural Aspects of Palladium and Platinum Complexes With Chiral Diphosphinoferrocenes Relevant to the Regio‐ and Stereoselective Copolymerization of CO with Propene

A series of chiral diphosphinoferrocene ligands 3a – i , derived from josiphos (=(2R)‐1‐[(1R)‐1‐(dicyclohexylphosphino)ethyl]‐2‐(diphenylphosphino)ferrocene, formerly called {(R)‐1‐[(S)‐2‐(diphenylphosphino)ferrocenyl]ethyl}dicycloxexylphosphine) where the electronic properties of the ligand are systematically varied, were prepared. X‐Ray studies of five of these new ligands confirmed that these compounds display very similar conformations in the solid state and that no structural criteria could be found indicating the modified electronic properties. These ligands find application in the Pd‐catalyzed highly regio‐ and stereoselective CO/propene copolymerization reaction, where the electronic properties of the ligand show a great impact on the catalyst activity. Coordination‐chemical aspects of these diphosphinoferrocenes relevant to the CO/propene copolymerization reaction were addressed by the preparation and characterization of Pd‐ and Pt‐complexes of the general formula [PdCl₂(P−P)] ( 5 ), [PdMe₂(P−P)] ( 6 ), [PdClMe(P−P)] ( 7 ), [PdMe(MeCN)(P−P)]PF₆ ( 8 ), and [PtClMe(P−P)] ( 9 ) (P−P=chiral diphosphinoferrocene ligand ( 3a – h ), four of which were characterized by X‐ray crystallography.     

Single and Twofold Metal‐ and Reagent‐Free Anodic C−C Cross‐Coupling of Phenols with Thiophenes

The first electrochemical dehydrogenative C−C cross‐coupling of thiophenes with phenols has been realized. This sustainable and very simple to perform anodic coupling reaction enables access to two classes of compounds of significant interest. The scope for electrochemical C−H‐activating cross‐coupling reactions was expanded to sulfur heterocycles. Previously, only various benzoid aromatic systems could be converted, while the application of heterocycles was not successful in the electrochemical C−H‐activating cross‐coupling reaction. Here, reagent‐ and metal‐free reaction conditions offer a sustainable electrochemical pathway that provides an attractive synthetic method to a broad variety of bi‐ and terarylic products based on thiophenes and phenols. This method is easy to conduct in an undivided cell, is scalable, and is inherently safe. The resulting products offer applications in electronic materials or as [OSO]²⁻ pincer‐type ligands.     

Evaluation of the Catalytic Relevance of the CO‐Bound States of V‐Nitrogenase

Binding and activation of CO by nitrogenase is a topic of interest because CO is isoelectronic to N₂, the physiological substrate of this enzyme. The catalytic relevance of one‐ and multi‐CO‐bound states (the lo‐CO and hi‐CO states) of V‐nitrogenase to C−C coupling and N₂ reduction was examined. Enzymatic and spectroscopic studies demonstrate that the multiple CO moieties in the hi‐CO state cannot be coupled as they are, suggesting that C−C coupling requires further activation and/or reduction of the bound CO entity. Moreover, these studies reveal an interesting correlation between decreased activity of N₂ reduction and increased population of the lo‐CO state, pointing to the catalytic relevance of the belt Fe atoms that are bridged by the single CO moiety in the lo‐CO state. Together, these results provide a useful framework for gaining insights into the nitrogenase‐catalyzed reaction via further exploration of the utility of the lo‐CO conformation of V‐nitrogenase.     

Synthesis and properties of azobenzene‐containing poly(1‐alkyne)s with different functional pendant groups

1‐Alkynes containing azobenzene mesogenic moieties [HC?C(CH₂)₉? O? ph? N?N? ph? O? R; R = ethyl ( 1 ), octyl ( 2 ), decyl ( 3 ), (S)‐2‐methylbutyl ( 4 ), or (S)‐1‐ethoxy‐1‐oxopropan‐2‐yl ( 5 ); ph = 1,4‐phenyl] were synthesized and polymerized in the presence of a Rh catalyst {(nbd)Rh⁺[B(C₆H5)₄]^?; nbd = 2,5‐norbornadiene} to yield a series of liquid‐crystalline polymers in high yields (e.g., >75%). These polymers had moderate molecular weights (number‐average molecular weight ≥ 12,000), high cis contents in the main chain (up to 83%), good thermal stability, and good solubility in common organic solvents, such as tetrahydrofuran, chloroform, and dichloromethane. These polymers were thoroughly characterized by a combination of infrared, nuclear magnetic resonance, thermogravimetric analysis, differential scanning calorimetry, polarized optical microscopy, and two‐dimensional wide‐angle X‐ray diffraction techniques. The liquid‐crystalline behavior of these polymers was dependent on the tail group attached to the azobenzene structure. Poly‐ 1 , which had the shortest tail group, that is, an ethyl group, showed a smectic A mesophase, whereas poly‐ 2 , poly‐ 3 , and poly‐ 5 , which had longer or chiral tail groups, formed smectic C mesophases, and poly‐ 4 , which had another chiral group attached to the azobenzene structure, showed a chiral smectic C mesophase in both the heating and cooling processes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4532–4545, 2006     

Base‐Selective Five‐ versus Six‐Membered Ring Annulation in Palladium‐Catalyzed C–C Coupling Cascade Reactions: New Access to Electron‐Poor Polycyclic Aromatic Dicarboximides

Palladium‐catalyzed base‐selective annulation of dibromonaphthalimide to different aryl boronate esters by combined Suzuki–Miyaura cross‐coupling and direct C−H arylation afforded a series of new five‐ and six‐membered ring annulated electron‐poor polycyclic aromatic hydrocarbons. Cesium carbonate (Cs₂CO₃) as auxiliary base in these C−C coupling cascade reactions led exclusively to six‐membered ring annulation, while the use of organic base diazabicycloundecene (DBU) afforded the corresponding five‐membered ring annulated products. This base‐dependent selective mode of annulation is attributed to different mechanistic pathways directed by the applied base. The selective annulation was revealed by single crystal X‐ray analysis of the respective five‐ and six‐membered ring annulated products. The optical and redox properties of the new polycyclic aromatic dicarboximides were characterized by UV/Vis absorption and fluorescence spectroscopy and cyclic voltammetry.     

Majority‐Rules‐Type Helical Poly(quinoxaline‐2,3‐diyl)s as Highly Efficient Chirality‐Amplification Systems for Asymmetric Catalysis

A highly efficient majority‐rules effect of poly(quinoxaline‐2,3‐diyl)s (PQXs) bearing 2‐butoxymethyl chiral side chains at the 6‐ and 7‐positions was established and attributed to large ΔG_h values (0.22–0.41 kJ mol^?1), which are defined as the energy difference between P‐ and M‐helical conformations per chiral unit. A PQX copolymer prepared from a monomer derived from (R)‐2‐octanol (23 % ee) and a monomer bearing a PPh₂ group adopted a single‐handed helical structure (>99 %) and could be used as a highly enantioselective chiral ligand in palladium‐catalyzed asymmetric reactions (products formed with up to 94 % ee), in which the enantioselectivity could be switched by solvent‐dependent inversion of the helical PQX backbone.     

Synthesis of a Doubly Boron‐Doped Perylene through NHC‐Borenium Hydroboration/C−H Borylation/Dehydrogenation

Reaction of an N‐heterocyclic carbene (NHC)–borenium ion with 9,10‐distyrylanthracene forms four B−C bonds through two selective, tandem hydroboration–electrophilic C−H borylations to yield an isolable, crystallographically characterizable polycyclic diborenium ion as its [NTf₂]⁻ salt ( 1 ). Dehydrogenation of 1 with TEMPO radical followed by acidic workup yields a 3,9‐diboraperylene as its corresponding borinic acid ( 2 ). This sequence can be performed in one pot to allow the facile, metal‐free conversion of an alkene into a small molecule containing a boron‐doped graphene substructure. Doubly boron‐doped perylene 2 exhibits visible range absorbance and fluorescence in chloroform solution (Φ =0.63) and undergoes two reversible one‐electron reductions at moderate potentials of −1.30 and −1.64 eV vs. ferrocenium/ferrocene in DMSO. Despite sterically accessible boron centers and facile electrochemical reductions, compound 2 is air‐, moisture‐, and silica gel‐stable.     

Poly­[strontium(II)‐μ2‐aqua‐μ5‐4‐carboxyl­ato­phenoxy­acetato]

In the title three‐dimensional coordination polymer, [Sr(4‐CPOA)(H₂O)]_n (where 4‐CPOA²⁻ is the 4‐carboxylatophenoxy­acetate dianion, C₉H₆O₅), each Sr^II atom displays a bicapped triangular prismatic configuration, defined by five carboxyl and one ether O atom from five different 4‐CPOA²⁻ ligands, as well as two water mol­ecules. The Sr^II atoms are covalently linked by 4‐CPOA²⁻ ligands and water mol­ecules, giving rise to a three‐dimensional open framework. In previously studied polymers of this type, the 4‐CPOA²⁻ ligand shows a variety of binding modes to metal ions, from mono‐ to pentadentate. In the present Sr^II complex, a novel hexadentate bridging mode is observed.     

A two‐dimensional coordination polymer containing a two‐dimensional zinc–carboxylate layer constructed from helical chains: poly[(μ4‐benzene‐1,2‐dicarboxylato)zinc(II)]

The title compound, [Zn(C₈H₄O₄)]_n, consists of one Zn^II cation and one benzene‐1,2‐dicarboxylate dianion (BDC²⁻) as the building unit. The Zn^II cation is four‐coordinated by four carboxylate O atoms from four dianionic BDC²⁻ ligands in a distorted tetrahedral geometry. The Zn^II cations are linked by the BDC²⁻ ligands to generate a structure featuring two‐dimensional zinc–carboxylate layers containing left‐ and right‐handed helical chains. The two‐dimensional layers are stacked along the a direction. The thermal stability of the title compound has been studied.     

Oligonucleotide Analogues with a Nucleobase‐Including Backbone:, Part 6, 2‐Deoxy‐D‐erythrose‐Derived Phosphoramidites: Synthesis and Incorporation into 14‐Mer DNA Strands

Two modified DNA 14‐mers have been prepared, containing either a 2‐deoxy‐D ‐erythrose‐derived adenosine analogue carrying a C(8)−CH₂O group (deA*), or a 2‐deoxy‐D ‐erythrose‐derived uridine analogue, possessing a C(6)−CH₂O group (deU*). These nucleosides are linked via a phosphinato group between O−C(3′) (deA* and deU*) and O−C(5′) of one neighbouring nucleotide, and between C(8)−CH₂O (deA*), or C(6)−CH₂O (deU*) and O−C(3′) of the second neighbour. N⁶‐Benzoyl‐9‐(β‐D ‐erythrofuranosyl)adenine ( 3 ) and 1‐(β‐D ‐erythrofuranosyl)uracil ( 4 ) were prepared from D ‐glucose, deoxygenated at C(2′), and converted into the required phosphoramidites 1 and 2 . The modified tetradecamers 31 and 32 were prepared by solid‐phase synthesis. Pairing studies show a decrease in the melting temperature of 7 to 8 degrees for the duplexes 31 ⋅ 30 and 32 ⋅ 29 , as compared to the unmodified DNA duplex 29 ⋅ 30 . A comparison with the pairing properties of tetradecamers similarly incorporating deoxyribose‐ instead of the deoxyerythrose‐derived nucleotides evidences that the CH₂OH substituent at C(4′) has no significant effect on the pairing.     

A new three‐dimensional silver(I) coordination framework with a diamondoid topology constructed from 2‐(pyridin‐4‐yl)‐1H‐imidazole‐4,5‐dicarboxylic acid

The title compound, poly[[μ₄‐5‐carboxy‐4‐carboxylato‐2‐(pyridin‐4‐yl)‐1H‐imidazol‐1‐ido]disilver(I)], [Ag₂(C₁₀H₅N₃O₄)]_n, was synthesized by reacting silver nitrate with 2‐(pyridin‐4‐yl)‐1H‐imidazole‐4,5‐dicarboxylic acid (H₃PyIDC) under hydrothermal conditions. The asymmetric unit contains two crystallographically independent Ag^I cations and one unique HPyIDC²⁻ anion. Both Ag^I cations are three‐coordinated in distorted T‐shaped coordination geometries. One Ag^I cation is coordinated by one N and two O atoms from two HPyIDC²⁻ anions, while the other is bonded to one O and two N atoms from two HPyIDC²⁻ anions. It is interesting to note that the HPyIDC²⁻ group acts as a μ₄‐bridging ligand to link the Ag^I cations into a three‐dimensional framework, which can be simplified as a diamondoid topology. The thermal stability and photoluminescent properties of the title compound have also been studied.     

Non‐biaryl atropisomers. part 1. configurationally stable 1,2‐diaryl‐3‐methyl‐1,4,5,6‐tetrahydropyrimidinium salts

In the present communication we describe two examples of a new kind of configurationally stable non‐biaryl atropisomers in which the Ar‐N bond is the chiral axis, namely 1‐(o‐nitrophenyl)‐2‐aryl‐3‐methyl‐1,4,5,6‐tetrahydropyrimidinium iodides 1. Stereochemical features of such compounds are analyzed on the basis of their ¹H and ¹³C one‐ and two‐dimensional nmr spectra. A comparison is made with the corresponding amidines 2 .     

Water‐Accelerated Organometallic Chemistry: Alkyne Carboalumination – Sulfinimine Addition and Asymmetric Synthesis of Allylic Amines

Carboalumination of alkynes in the presence of catalytic Cp₂ZrCl₂ and H₂O affords vinyl‐alane intermediates, which serve as nucleophiles in the subsequent addition to enantiomerically enriched (tert‐butyl)‐ and (para‐tolyl)sulfinimines. This new in situ protocol produces two new C C bonds. Chiral allylic sulfinamides are obtained in high diastereoselectivity and in good yield. Cleavage of the chiral auxiliary leads to synthetically useful allylic amine building blocks, and facile oxidative degradation of the alkene moiety can be used as an approach toward amino acid derivatives and for assignment of absolute configuration.     

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern,Photophysical Studies,and Enantioselective Catalysis by a Chiral Sensitizer

The intramolecular [2+2] photocycloaddition of four 4‐(but‐3‐enyl)oxyquinolones (substitution pattern at the terminal alkene carbon atom: CH₂, Z‐CHEt, E‐CHEt, CMe₂) and two 3‐(but‐3‐enyl)oxyquinolones (substitution pattern: CH₂, CMe₂) was studied. Upon direct irradiation at λ=300 nm, the respective cyclobutane products were formed in high yields (83–95 %) and for symmetrically substituted substrates with complete diastereoselectivity. Substrates with a Z‐ or E‐substituted terminal double bond showed a stereoconvergent reaction course leading to mixtures of regio‐ and diastereomers with almost identical composition. The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for the title compounds, which–in contrast to simple alkoxyquinolones such as 3‐butyloxyquinolone and 4‐methoxyquinolone–decayed rapidly (τ≈1 ns) through cyclization to a triplet 1,4‐diradical. The diradical can evolve through two reaction channels, one leading to the photoproduct and the other leading back to the starting material. When the photocycloaddition was performed in the presence of a chiral sensitizer (10 mol %) upon irradiation at λ=366 nm in trifluorotoluene as the solvent, moderate to high enantioselectivities were achieved. The two 3‐(but‐3‐enyl)oxyquinolones gave enantiomeric excesses (ees) of 60 and 64 % at ?25 °C, presumably because a significant racemic background reaction occurred. The 4‐substituted quinolones showed higher enantioselectivities (92–96 % ee at ?25 °C) and, for the terminally Z‐ and E‐substituted substrates, an improved regio‐ and diastereoselectivity.     

Poly[(μ5‐3‐carboxybenzene‐1,2‐dicarboxylato)lead(II)]: a helical lead(II) coordination polymer

In the title polymer, [Pb(C₉H₄O₆)]_n, the asymmetric unit contains a monomer of a Pb^II cation with a doubly deprotonated 3‐carboxybenzene‐1,2‐dicarboxylate dianion (1,2,3‐Hbtc²⁻). Each Pb^II centre is seven‐coordinated by seven O atoms of bridging carboxy/carboxylate groups from five 1,2,3‐Hbtc²⁻ ligands, forming a distorted pentagonal bipyramid. The Pb^II cations are bridged by 1,2,3‐Hbtc²⁻ anions, yielding two‐dimensional chiral layers. The layers are stacked above each other to generate a three‐dimensional supramolecular architecture via a combination of C—H...O interactions. The thermogravimetric and optical properties are also reported.