Oxidative Cyclization Reaction of 2‐Aryl‐Substituted Cinnamates To Form Phenanthrene Carboxylates by Using MoCl5

The oxidative cyclization reaction of 2‐aryl cinnamates and derivatives thereof can be easily performed with MoCl₅ as the oxidant. This powerful reagent allows oxidative coupling reactions for which other reagents fail. The best results are obtained when the 2‐phenyl substituent of the cinnamate is equipped with two methoxy groups. Even iodo moieties in the bay region of phenanthrene are tolerated under the reaction conditions. If naphthalene moieties are involved, a rearrangement of the skeleton occurs, providing an elegant route to highly functionalized angular arenes. The cyclization is demonstrated for 15 example substrates with isolated yields of up to 99 % for the phenanthrene derivative. The broad scope of the reaction underlines the usefulness of MoCl₅ and MoCl₅/TiCl₄ in the oxidative coupling reaction.     

Iodo‐substituted triphenylene‐based discogens: by metal‐mediated oxidative cross‐coupling

New monoiodopentakis(alkoxy)triphenylene derivatives were synthesized and characterized. The metal‐mediated oxidative coupling of 2‐iodoalkoxybenzene with 3,3′,4,4′‐tetraalkoxybiphenyl by MoCl₅ as oxidizing reagent yields 2‐iodopentakis(alkoxy)triphenylene. These products were purified by repeated column chromatography; their liquid crystalline properties were studied by polarizing optical microscopy and differential scanning calorimetry. All the derivatives exhibit a columnar mesophase over a wide temperature range.     

Powerful Fluoroalkoxy Molybdenum(V) Reagent for Selective Oxidative Arene Coupling Reaction

We introduce the novel fluoroalkoxy molybdenum(V) reagent 1 which has superior reactivity and selectivity in comparison to MoCl₅ or the MoCl₅/TiCl₄ reagent mixture in the oxidative coupling reactions of aryls. Common side reactions, such as chlorination and/or oligomer formation, are drastically diminished creating a powerful and useful reagent for oxidative coupling. Theoretical treatment of the reagent interaction with 1,2‐dimethoxybenzene‐type substrates indicates an inner‐sphere electron transfer followed by a radical cationic reaction pathway for the oxidative‐coupling process. EPR spectroscopic and electrochemical studies, X‐ray analyses, computational investigations, and the experimental scope provide a highly consistent picture. The substitution of chlorido ligands by hexafluoroisopropoxido moieties seems to boost both the reactivity and selectivity of the metal center which might be applied to other reagents as well.     

Inversion of Configuration at the Phosphorus Nucleophile in the Diastereoselective and Enantioselective Synthesis of P‐Stereogenic syn‐Phosphiranes from Chiral Epoxides

Nucleophilic substitution results in inversion of configuration at the electrophilic carbon center (S_N2) or racemization (S_N1). The stereochemistry of the nucleophile is rarely considered, but phosphines, which have a high barrier to pyramidal inversion, attack electrophiles with retention of configuration at P. Surprisingly, cyclization of bifunctional secondary phosphine alkyl tosylates proceeded under mild conditions with inversion of configuration at the nucleophile to yield P‐stereogenic syn‐phosphiranes. DFT studies suggested that the novel stereochemistry results from acid‐promoted tosylate dissociation to yield an intermediate phosphenium‐bridged cation, which undergoes syn‐selective cyclization.     

Unusual redox play of Mo(V/IV) during oxidative aryl–aryl coupling

The oxidative treatment of a suitable 1,3-diarylpropene precursor by MoCl₅ causes a series of redox steps yielding a dimer of dibenzo[a,c]cycloheptene. After the oxidative aryl–aryl bond formation, a C,H activation occurs providing a tropylium intermediate. Upon aqueous workup the metal waste acts as reductive media generating the dimer in an almost quantitative manner. The oxidative generation of the tropylium species as well as the subsequent redox play by the metal waste is unique and unprecedented. The dimeric compound can be oxidatively cleaved and subsequently decarboxylated providing the key intermediate of a previous synthesis of metasequirin-B derivatives.     

Chiral M3L2 Self‐Assembled Capsules through Metal Coordination of Enantiopure Ligating Benzocyclotrimers: NMR Spectroscopic and ESI Mass Spectrometric Investigation

The synthesis of enantiopure (+)‐benzotricamphor syn‐ 5 , an important chiral C₃‐symmetric rigid building block for supramolecular applications, was studied in detail to reduce the number of steps and to increase the diastereoselectivity and overall yield. The new synthetic procedure allowed larger amounts of syn‐ 5 to be obtained and used for the preparation of new derivatives, such as the corresponding tris‐trifluoromethanesulfonate syn‐ 12 , which was efficiently transformed into (+)‐benzotribornenetrinitrile syn‐ 1 and (+)‐benzotribornenetris(ethynyl‐4‐pyridine) syn‐ 2 . The previously reported (+)‐benzotricamphortrioxime syn‐ 6 was transformed into tris‐nitrile syn‐ 3 by Beckman reaction. Compounds syn‐ 1 – 3 were employed as multidentate ligands for silver(I) and platinum(II) centres in apolar solvents. The linear coordination geometry of Ag^I and square‐planar geometry of cis‐chelated Pt^II in combination with the chiral tripodal ligands syn‐ 1 – 3 led to the formation of chiral enantiopure capsules with M₃L₂ stoichiometry, as confirmed by 2D NMR NOESY and DOSY experiments as well as ESI mass spectrometry.     

Synthesis of 3‐Fluoropyrrolidines and 4‐Fluoropyrrolidin‐2‐ones from Allylic Fluorides

Various 3‐fluoropyrrolidines and 4‐fluoropyrrolidin‐2‐ones were prepared by 5‐exo‐trig iodocyclisation from allylic fluorides bearing a pending nitrogen nucleophile. These bench‐stable precursors were made accessible upon electrophilic fluorination of the corresponding allylsilanes. The presence of the allylic fluorine substituent induces syn‐stereocontrol upon iodocyclisation with diastereomeric ratios ranging from 10:1 to > 20:1 for all N‐tosyl‐3‐fluoropent‐4‐en‐1‐amines and amides. The sense and level of stereocontrol is strikingly similar to the corresponding iodocyclisation of structurally related allylic fluorides bearing pending oxygen nucleophiles. These results suggest that the syn selectivity observed upon ring closure involves I₂–π complexes with the fluorine positioned inside.     

Oxidative coupling of aromatic compounds with trimethylbenzylsilane by the action of MoCl5

Conclusions The benzylation of aromatic compounds was carried out efficiently under mild conditions by the action of trimethylbenzylsilane in the presence of MoCl₅, AlCl₃, or WCl₆ with the formation of the corresponding diarylmethanes. The benzylation in the presence of MoCl₅ conforms to behavior typical of electrophilic substitution.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1868–1870, August, 1987.     

Factors that regulate the conformation of m-benziporphodimethene complexes: agostic metal-arene interaction, hydrogen bonding, and η2,π coordination

Group 12 and silver(I) tetramethyl‐m‐benziporphodimethene (TMBPDM) complexes with phenyl, methylbenzoate, or nitrophenyl groups as meso substituents were synthesized and fully characterized. The dimeric silver(I) complex displays an unusual η²,π coordination from the β‐pyrrolic C?C bond to the silver ion. All of the complexes displayed a close contact between the metal ion and the inner C(22)? H(22) on the m‐phenylene ring. The downfield chemical shifts of H(22) and large coupling constants between Cd^II and H(22) strongly support the presence of an agostic interaction between the metal ion and inner C(22)–H(22). Crystal structures revealed that the syn form is the predominant conformation for TMBPDM complexes. This is distinctively different from the exclusive anti conformation observed in m‐benziporphyrin and tetraphenyl‐m‐benziporphodimethene (TPBPDM) complexes. Evidently, intramolecular hydrogen‐bonding interactions between axial chloride and methyl groups stabilize syn conformations. Unlike the merely syn conformation observed in the solid‐state structures of TMBPDM complexes that contain an axial chloride, in solution these complexes display highly solvent‐ and temperature‐dependent syn/anti ratio changes. The observation of dynamic ¹H NMR spectroscopic scrambling between syn and anti conformations from the titration of chloride ion into the solution of the TMBPDM complex suggests that axial ligand exchange is a likely pathway for the conversion between syn and anti forms. Theoretical calculations revealed that intermolecular hydrogen‐bonding interactions between the axial chloride and CHCl₃ stabilizes the anti conformation, which explains the increased ratio for the anti form when dichloromethane or chloroform was used as the solvent.     

Synthesis and Characterization of Poly(2,2-Dipropargyl-1,3-Propylene Benzaldehyde Acetal) Containing a Bulky Acetal Structure

Abstract

Poly(2,2-dipropargyl-1,3-propylene benzaldehyde acetal) (PDPBA), a cyclopolymer containing a bulky acetal functional group, was prepared with transition metal catalysts. MoCl₅ was found to be the most effective catalyst, and any cocatalyst effect was hardly observed. Polymerization was quantitatively carried out under mild conditions (30°C, M₀ = 0.125) with MoCl₅-based catalysts. The structure and physical properties of the resulting polymer were investigated. The spectral data indicated that the polymer contains alternating double and single bonds along the polymer backbone and a cyclic recurring unit with spiro acetal group. The polymer was completely soluble in common organic solvents. The M _n value of the polymer was in the 70,000 to 80,000 range relative to polystyrene standards by GPC. In addition, the polymer possessed good thermal and oxidative stability. The electrical conductivity of the undoped polymer was 10^?12 S/cm.     

Mild and Efficient Deoxygenation of Amine‐N‐oxides with MoCl5/NaI System

The MoCl₅/NaI system was found to be a new reagent for deoxygenation of various amine‐N‐oxides to the corresponding amines in good to excellent yields under mild conditions.     

Preparation of C3‐Symmetric Homochiral syn‐Trisnorbornabenzenes through Regioselective Cyclotrimerization of Enantiopure Iodonorbornenes

C₃‐symmetric homochiral (?)‐syn‐trisoxonorbornabenzene 1 possessing a rigid cup‐shaped structure was synthesized through a novel regioselective cyclotrimerization of enantiopure iodonorbornenes catalyzed by palladium nanoclusters. The yield of the cyclotrimerization was dependent on the stability of the palladium clusters, which was ascertained from the appearance and TEM images of the reaction mixtures. The efficient preparation of (?)‐syn‐ 1 was established in short steps, including the newly developed cyclotrimerization reaction. The thus‐prepared homochiral (?)‐syn‐ 1 can serve as a key intermediate for the synthesis of C₃‐symmetric homochiral cup‐shaped molecules with a helical arrangement of substituents. Introduction of several types of substituents was well demonstrated through palladium‐catalyzed coupling reactions with the corresponding phosphate and triflate of (?)‐syn‐ 1 .     

Oligonucleosides with a Nucleobase‐Including Backbone,Part 7

The conformational analysis of 7 was carried out in (D₆)DMSO and in mixtures of (D₆)DMSO and CDCl₃ to evaluate the syn/anti conformations, relevant to the pairing propensity of this type of nucleotide analogue. The HO−C(5′) of unit I and of unit II of the dimer 7 form an intramolecular H‐bond to N(3). In (D₆)DMSO, the C(5′)−OH⋅⋅⋅N(3) H‐bond in unit I is partially broken, while that in unit II persists to a larger extent. The syn conformation prevails for unit I and particularly for unit II. The furanosyl moieties adopt predominantly a 2′‐endo conformation that is largely independent of the solvent.     

Structural Diversity in Cyclometalated Diiridium(III) Complexes with Bridging syn and anti μ2-Oxamidato and μ2-Dithioxamidato Ligands

Six new diiridium complexes containing 2-methyl-6-phenylpyridyl as the cyclometalating ligand with a μ₂-oxamidato or a μ₂-dithioxamidato ligand as the bridge have been synthesized in 60–73 % yields. These complexes were revealed by multinuclear NMR spectroscopy to contain inseparable mixtures of diastereomers (rac, ΔΔ/ΛΛ and meso, ΔΛ) with bridges in anti and syn configurations. The remarkable variety of isomers present was confirmed by X-ray crystallography on single crystals grown from mixtures of each complex. In one complex with a N,N’-bis(4-trifluoromethylphenyl)-μ₂-oxamidato bridge, two single crystals of anti and syn isomers were structurally determined. Two single crystals of the μ₂-dithioxamidato bridge complex were found to contain rac and meso forms of the syn isomer. Hybrid DFT computations on the four isomers of each diiridium complex revealed negligible energetic preferences for one isomer despite the methyl groups in the 2-methyl-6-phenylpyridyl cyclometalating ligands being close to the neighboring methyl groups and the bridge, thus supporting the experimental findings of isomer mixtures. Two distinct broad emissions with maxima at 522–529 nm and at 689–701 nm observed in these complexes in dichloromethane are attributed to mixed metal-ligand to ligand charge transfer (MLLCT) excited states involving the pyridyl and bridge moieties respectively with the aid of electronic structure computations.     

Ternary molybdenum(III) chlorides

The phase diagrams of ACl/MoCl₃ (A=Na, K, Rb, Cs) were elucidated by DTA measurements in sealed quartz ampoules in the range of 0–40 mol% MoCl₃. The samples were prepared from alkali metal chlorides and the compounds A₃MoCl₆ or A₃Mo₂Cl₉. The 31 compounds withA=Na, Rb, Cs were obtained by sintering mixtures of 3ACl+MoCl₃; the enneachlorides A₃Mo₂Cl₉ withA=K, Rb, Cs were precipitated from solutions of MoCl₃·3H₂O and ACl in formic acid. Congruently melting compounds A₃MoCl₆ exist in all four systems, incongruently melting enneachlorides A₃Mo₂Cl₉ in systems withA=K, Rb, Cs. Still unknown structures were determined by analog-indexing powder patterns according to known structure families. Especially Cs₃MoCl₆ is isotypic with the recently found Cs₃CrCl₆ structure. Additionally, the unit cell parameters were determined for the compounds A₃MoCl₅·H₂O (A=K, Rb, Cs) analogous to Cs₂TiCl₅·H₂O, whose structure was determined by single crystal measurements.Dedicated to Prof. Menachem Steinberg on the occasion of his 65th birthday     

Stereodivergent Organocatalytic Intramolecular Michael Addition/Lactonization for the Asymmetric Synthesis of Substituted Dihydrobenzofurans and Tetrahydrofurans

A stereodivergent asymmetric Lewis base catalyzed Michael addition/lactonization of enone acids into substituted dihydrobenzofuran and tetrahydrofuran derivatives is reported. Commercially available (S)‐(?)‐tetramisole hydrochloride gives products with high syn diastereoselectivity in excellent enantioselectivity (up to 99:1 d.r._syn/anti, 99 % ee_syn), whereas using a cinchona alkaloid derived catalyst gives the corresponding anti‐diastereoisomers as the major product (up to 10:90 d.r._syn/anti, 99 % ee_anti).     

Trisphosphine‐Chelate‐Substituted Molybdenum and Tungsten Nitrosyl Hydrides as Highly Active Catalysts for Olefin Hydrogenations

Reaction of [M(NO)Cl₃(NCMe)₂] (M=Mo, W) with (iPr₂PCH₂CH₂)₂PPh (etpⁱp) at room temperature afforded the syn/anti‐[M(NO)Cl₃(mer‐etpⁱp)] complexes (M=Mo, a ; W, b ; 3 a,b (syn,anti); syn and anti refer to the relative position of Ph(etpⁱp) and NO). Reduction of 3 a,b (syn,anti) produced [M(NO)Cl₂(mer‐etpⁱp)] ( 4 a,b (syn)), [M(NO)Cl(NCMe)(mer‐etpⁱp)] ( 5 a,b (syn,anti)), and [M(NO)Cl(η²‐ethylene)(mer‐etpⁱp)] ( 6 a,b (syn,anti)) complexes. The hydrides [M(NO)H(η²‐ethylene)(mer‐etpⁱp)] ( 7 a,b (syn,anti)) were obtained from 6 a,b (syn,anti) using NaHBEt₃ (75 °C, THF) or LiBH₄ (80 °C, Et₃N), respectively. 7 a,b (syn,anti) were probed in olefin hydrogenations in the absence or presence of a hydrosilane/B(C₆F₅)₃ mixture. The 7 a,b (syn,anti)/Et₃SiH/B(C₆F₅)₃ co‐catalytic systems were highly active in various olefin hydrogenations (60 bar H₂, 140 °C), with maximum TOFs of 5250 h^?1 ( 7 a (syn,anti)) and 8200 h^?1 ( 7 b (syn,anti)) for 1‐hexene hydrogenation. The Et₃SiH/(B(C₆F₅)₃ co‐catalyst is anticipated to generate a [Et₃Si]⁺ cation attaching to the O_NO atom. This facilitates NO bending and accelerates catalysis by providing a vacant site. Inverse DKIE effects were observed for the 7 a (syn,anti)/Et₃SiH/(B(C₆F₅)₃ (k_H/k_D=0.55) and the 7 b (syn,anti)/Et₃SiH/(B(C₆F₅)₃ (k_H/k_D=0.65) co‐catalytic mixtures (20 bar H₂/D₂, 140 °C).     

Intermolecular Palladium‐Catalyzed Oxidative Fluorocarbonylation of Unactivated Alkenes: Efficient Access to β‐Fluorocarboxylic Esters

A novel palladium‐catalyzed intermolecular oxidative fluorocarbonylation of alkenes has been developed, in which employment of a cooperative process with electrophilic ArIF₂‐meidated alkenes activation and palladium‐catalyzed carbonylation is crucial for the successful catalytic transformation. The current transformation presents the first convenient method to generate β‐fluorinated carboxylic acid derivatives under mild reaction conditions from simple alkenes with excellent regioselectivity.     

Chiral Calcium–BINOL Phosphate Catalyzed Diastereo‐ and Enantioselective Synthesis of syn‐1,2‐Disubstituted 1,2‐Diamines: Scope and Mechanistic Studies

A highly enantioselective, chiral, Lewis acid calcium–bis(phosphate) complex, Ca[ 3 a ]_n, which catalyzes the electrophilic amination of enamides with azodicarboxylate derivatives 2 to provide versatile chiral 1,2‐hydrazinoimines 4 is disclosed. The reaction gives an easy entry to optically active syn‐1,2‐disubstituted 1,2‐diamines 6 in high yields with excellent enantioselectivities, after a one‐pot reduction of the intermediate 1,2‐hydrazinoimines 4 . The geometry and nature of the N‐substituent of the enamide affect dramatically both the reactivity and the enantioselectivity. Although the calcium–bis(phosphate) complex was a uniquely effective catalyst, the exact nature of the active catalytic species remains unclear. NMR spectroscopy and MS analysis of the various calcium complexes Ca[ 3 ]_n reveals that the catalysts exist in various oligomer forms. The present mechanistic study, which includes nonlinear effects and kinetic measurements, constitutes a first step in understanding these calcium–bis(phosphate) complex catalysts. DFT calculations were carried out to explore the mechanism and the origin of the enantioselectivity with the Ca[ 3 ]_n catalysts.     

Elucidating the Reactivity of Vicinal Dicarbenoids: From Lewis Adduct Formation to B−C Bond Activation

The reactivity of the diaminoacetylene Pip‐C≡C‐Pip (Pip=piperidyl=NC₅H₁₀) towards phenyldichloro‐ and triphenylborane is presented. In the case of the less Lewis acidic PhBCl₂, the first example of a double Lewis adduct of a vicinal dicarbenoid is reported. For the more Lewis acidic triphenylborane, coordination to the bifunctional carbene leads to a mild B?C bond activation, resulting in a syn‐1,2‐carboboration. Ensuing cis/trans isomerization yields a novel ethylene‐bridged frustrated Lewis pair (FLP). The compounds were characterized using multinuclear NMR spectroscopy, structural analysis, and mass spectrometry. Reactivity studies of both isomers with the N‐heterocyclic carbene 1,3‐dimethylimidazol‐2‐ylidene (IMe) aided in elucidating the proposed isomerization pathway. DFT calculations were carried out to elucidate the reaction mechanism. The rather low free energy of activation is consistent with the observation that the reaction proceeds smoothly at room temperature.