Diastereo‐ and Enantioselective Intramolecular [2+2] Photocycloaddition Reactions of 3‐(ω′‐Alkenyl)‐ and 3‐(ω′‐Alkenyloxy)‐Substituted 5,6‐Dihydro‐1H‐pyridin‐2‐ones

3‐(ω′‐Alkenyl)‐substituted 5,6‐dihydro‐1H‐pyridin‐2‐ones 2 – 4 were prepared as photocycloaddition precursors either by cross‐coupling from 3‐iodo‐5,6‐dihydro‐1H‐pyridin‐2‐one ( 8 ) or—more favorably—from the corresponding α‐(ω′‐alkenyl)‐substituted δ‐valerolactams 9 – 11 by a selenylation/elimination sequence (56–62 % overall yield). 3‐(ω′‐Alkenyloxy)‐substituted 5,6‐dihydro‐1H‐pyridin‐2‐ones 5 and 6 were accessible in 43 and 37 % overall yield from 3‐diazopiperidin‐2‐one ( 15 ) by an α,α‐chloroselenylation reaction at the 3‐position followed by nucleophilic displacement of a chloride ion with an ω‐alkenolate and oxidative elimination of selenoxide. Upon irradiation at λ=254 nm, the precursor compounds underwent a clean intramolecular [2+2] photocycloaddition reaction. Substrates 2 and 5 , tethered by a two‐atom chain, exclusively delivered the respective crossed products 19 and 20 , and substrates 3 , 5 , and 6 , tethered by longer chains, gave the straight products 21 – 23 . The completely regio‐ and diastereoselective photocycloaddition reactions proceeded in 63–83 % yield. Irradiation in the presence of the chiral templates (?)‐ 1 and (+)‐ 31 at ?75 °C in toluene rendered the reactions enantioselective with selectivities varying between 40 and 85 % ee. Truncated template rac‐ 31 was prepared as a noranalogue of the well‐established template 1 in eight steps and 56 % yield from the Kemp triacid ( 24 ). Subsequent resolution delivered the enantiomerically pure templates (?)‐ 31 and (+)‐ 31 . The outcome of the reactions is compared to the results achieved with 4‐substituted 5,6‐dihydro‐1H‐pyridin‐2‐ones and quinolones.     

Stabilization of the Low‐Spin State in a Mononuclear Iron(II) Complex and High‐Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding

The tetrapyridyl ligand bbpya (bbpya=N,N‐bis(2,2′‐bipyrid‐6‐yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)₂] ( 1 ) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai’s domain model, and powder X‐ray diffraction measurements, 1 is low‐spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T_1/2=418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong N?H???S intermolecular H‐bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1 . DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N′‐di(pyrid‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)₂] ( 2 ). Periodic DFT calculations suggest that crystal‐packing effects are significant for compound 2 , in which they destabilize the HS state by about 1500 cm^?1. The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.     

Enantioselective Visible‐Light‐Induced Radical‐Addition Reactions to 3‐Alkylidene Indolin‐2‐ones

The title compounds underwent a facile and high‐yielding addition reaction (19 examples, 66–99 % yield) with various N‐(trimethylsilyl)methyl‐substituted amines upon irradiation with visible light and catalysis by a metal complex. If the alkylidene substituent is non‐symmetric and if the reaction is performed in the presence of a chiral hydrogen‐bonding template, products are obtained with significant enantioselectivity (58–72 % ee) as a mixture of diastereoisomers. Mechanistic studies suggest a closed catalytic cycle for the photoactive metal complex. However, the silyl transfer from the amine occurs not only to the product, but also to the substrate, and interferes with the desired chirality transfer.     

Structures of N—（2,3,4,6—Tetra—O—acetyl—β—D—glycosyl） thiocarbamic Benzoyl Hydrazine

The crystal structure of N-(2,3,4,6-tetra-O-acetyl-β-D-gly-cosyl)-thiocarbamic benzoyl hydrazine(C22H27N3O9S) was determined by X-ray diffracton method.The hexopyranosyl ring adopts a chair conformation.All the ring substituents are in the equatorial positions.The acetoxyl-methyl group is in synclinal conformation.The S atom is in synperiplanar conformation while the benzoyl hydrazine moiety is anti-periplanar.The thiocarbamic moiety is almost companar with the benzoyl hydrazine group.There are two intramolecular hydrogen bonds and one intermolecular hydrogen bond for each molecule in the crystal structure.The molecules form a network structure through intermolecular hydrogen bonds.     

The Strained Sesquiterpene β‐Caryophyllene as a Probe for the Solvent‐Assisted Epoxidation Mechanism

In our attempt to synthesize β‐caryophyllene oxide in food‐compatible conditions, we observed the uncatalyzed and highly selective epoxidation of β‐caryophyllene, a strained bicyclic sesquiterpene, in ethanol with aqueous H₂O₂ under radical‐suppressing conditions without the addition of a catalyst. The unusual reactivity of β‐caryophyllene allowed us to use it as a probe for the mechanism of the solvent‐assisted epoxidation in a wide range of organic solvents. A kinetic study was performed to investigate the epoxidation mechanism; an excellent correlation was found between the observed epoxidation rates in different solvents and the Abraham’s hydrogen bond formation parameters of these solvents. By means of computational analysis, it was found that the main role of the solvent consists of the stabilization of the elongated O?O bond of H₂O₂ in the transition state through hydrogen‐bond donation to the leaving OH moiety of H₂O₂. α‐Humulene was found to possess similar reactivity as β‐caryophyllene whereas isocaryophyllene—the unstrained isomer of β‐caryophyllene—was unreactive.     

The Role of Hydrogen‐Bonding Interactions in the Ultrafast Relaxation Dynamics of the Excited States of 3‐ and 4‐Aminofluoren‐9‐ones

The dynamics of the excited states of 3‐ and 4‐aminofluoren‐9‐ones (3AF and 4AF, respectively) are investigated in different kinds of solvents by using a subpicosecond time‐resolved absorption spectroscopic technique. They undergo hydrogen‐bonding interaction with protic solvents in both the ground and excited states. However, this interaction is more significant in the lowest excited singlet (S₁) state because of its substantial intramolecular charge‐transfer character. Significant differences in the spectroscopic characteristics and temporal dynamics of the S₁ states of 3AF and 4AF in aprotic and protic solvents reveal that the intermolecular hydrogen‐bonding interaction between the S₁ state and protic solvents plays an important role in its relaxation process. Perfect linear correlation between the relaxation times of the S₁ state and the longitudinal relaxation times (τ_L) of alcoholic solvents confirms the prediction regarding the solvation process via hydrogen‐bond reorganization. In the case of weakly interacting systems, the relaxation process can be well described by a dipolar solvation‐like process involving rotation of the OH groups of the alcoholic solvents, whereas in solvents having a strong hydrogen‐bond‐donating ability, for example, methanol and trifluoroethanol, it involves the conversion of the non‐hydrogen‐bonded form to the hydrogen‐bonded complex of the S₁ state. Efficient radiationless deactivation of the S₁ state of the aminofluorenones by protic solvents is successfully explained by the energy‐gap law, by using the energy of the fully solvated S₁ state determined from the time‐resolved spectroscopic data.     

Combined Experimental and Computational Studies on the Physical and Chemical Properties of the Renewable Amide, 3‐Acetamido‐5‐acetylfuran

The pK_a of 3‐acetamido‐5‐acetylfuran (3A5AF) was predicted to be in the range 18.5–21.5 by using the B3LYP/6‐311+G(2d,p) method and several amides as references. The experimental pK_a value, 20.7, was determined through UV/Vis titrations. Its solubility was measured in methanol‐modified supercritical CO₂ (mole fraction, 3.23×10^?4, cloud points 40–80 °C) and it was shown to be less soluble than 5‐hydroxymethylfurfural (5‐HMF). Dimerization energies were calculated for 3A5AF and 5‐HMF to compare hydrogen bonding, as such interactions will affect their solubility. Infrared and ¹H nuclear magnetic resonance spectra of 3A5AF samples support the existence of intermolecular hydrogen bonding. The highest occupied molecular orbital, lowest unoccupied molecular orbital, and electrostatic potential of 3A5AF were determined through molecular orbital calculations using B3LYP/6‐311+G(2d,p). The π–π* transition energy (time‐dependent density functional theory study) was compared with UV/Vis data. Calculated atomic charges were used in an attempt to predict the reactivity of 3A5AF. A reaction between 3A5AF and CH₃MgBr was conducted. As 3A5AF is a recently developed renewable compound that has previously not been studied extensively, these studies will be helpful in designing future reactions and processes involving this molecule.     

1,2,4‐Triazol‐3‐ylidenes with an N‐2,4‐Dinitrophenyl Substituent as Strongly π‐Accepting N‐Heterocyclic Carbenes

The synthesis and characterisation of a series of new Rh and Au complexes bearing 1,2,4‐triazol‐3‐ylidenes with a N‐2,4‐dinitrophenyl (N‐DNP) substituent are described. IR, NMR, single‐crystal X‐ray diffraction and computational analyses of the Rh complexes revealed that the N‐heterocyclic carbenes (NHCs) behaved as strong π acceptors and weak σ donors. In particular, a natural bond orbital (NBO) analysis revealed that the contributions of the Rh→C_carbene π backbonding interaction energies (ΔE_bb) to the bond dissociation energies (BDE) of the Rh? C_carbene bond for [RhCl(NHC)(cod)] (cod=1,5‐cyclooctadiene) reached up to 63 %. The Au complex exhibited superior catalytic activity in the intermolecular hydroalkoxylation of cyclohexene with 2‐methoxyethanol. The NBO analysis suggested that the high catalytic activity of the Au^I complex resulted from the enhanced π acidity of the Au atom.     

Phosphorus Ligands for Iron(III)‐Mediated Atom Transfer Radical Polymerization of Methyl Methacrylate

The iron(III)‐catalyzed atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) was successfully employed using tributylphosphine (TBP) and trimethylphosphite (TMP) as ligands in the absence of a reducing agent. The effects of solvent and initiator on polymerization of MMA were investigated. Most of the polymerizations with these ligands were well controlled with a linear increase in the number average molecular weights ( ) versus conversion and relatively low molecular weight distribution ( = 1.2–1.4) throughout the reactions, and the measured weights matched with the predicted values. The ethyl 2‐bromoisobutyrate (EBriB) initiated ATRP of MMA with the FeBr₃/TBP or FeBr₃/TMP catalytic system was better controlled in toluene than in the other solvents used in this study at 80 °C.

     

On‐Surface Self‐Assembly of a C3‐Symmetric π‐Conjugated Molecule Family Studied by STM: Two‐Dimensional Nanoporous Frameworks

The on‐surface self‐assembled behavior of four C ₃‐symmetric π‐conjugated planar molecules ( Tp , T12 , T18 , and Ex ) has been investigated. These molecules are excellent building blocks for the construction of noncovalent organic frameworks in the bulk phase. Their hydrogen‐bonded 2D on‐surface self‐assemblies are observed under STM at the solid/liquid interface; these structures are very different to those in the bulk crystal. Upon combining the results of STM measurements and DFT calculations, the formation mechanism of different assemblies is revealed; in particular, the critical role of hydrogen bonding in the assemblies. This research provides us with not only a deep insight into the self‐assembled behavior of these novel functional molecules, but also a convenient approach toward the construction of 2D multiporous networks.     

Unerwartete Reduktion von [Cp*TaCl4(PH2R)] (R = But,Cy, Ad,Ph, 2,4,6‐Me3C6H2; Cp* = C5Me5) durch Reaktion mit DBU – Molekülstruktur von [(DBU)H][Cp*TaCl4] (DBU = 1,8‐Diazabicyclo[5.4.0]undec‐7‐en)

Unexpected Reduction of [Cp*TaCl₄(PH₂R)] (R = Bu^t, Cy, Ad, Ph, 2,4,6‐Me₃C₆H₂; Cp* = C₅Me₅) by Reaction with DBU – Molecular Structure of [(DBU)H][Cp*TaCl₄] (DBU = 1,8‐diazabicyclo[5.4.0]undec‐7‐ene) [Cp*TaCl₄(PH₂R)] (R = Bu^t, Cy, Ad, Ph, 2,4,6‐Me₃C₆H₂ (Mes); Cp* = C₅Me₅) react with DBU in an internal redox reaction with formation of [(DBU)H][Cp*TaCl₄] ( 1 ) (DBU = 1,8‐diazabicyclo[5.4.0]undec‐7‐ene) and the corresponding diphosphane (P₂H₂R₂) or decomposition products thereof. 1 was characterised spectroscopically and by crystal structure determination. In the solid state, hydrogen bonding between the (DBU)H cation and one chloro ligand of the anion is observed.     

Tetraalkylammonium Salts as Hydrogen‐Bonding Catalysts

Although the hydrogen‐bonding ability of the α hydrogen atoms on tetraalkylammonium salts is often discussed with respect to phase‐transfer catalysts, catalysis that utilizes the hydrogen‐bond‐donor properties of tetraalkylammonium salts remains unknown. Herein, we demonstrate hydrogen‐bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich‐type reactions. The structure and the hydrogen‐bonding ability of the new ammonium salts were investigated by X‐ray diffraction analysis and NMR titration studies.     

Ultrafast Relaxation Dynamics of the Excited States of 1‐Amino‐ and 1‐(N,N‐Dimethylamino)‐fluoren‐9‐ones

The dynamics of the excited states of 1‐aminofluoren‐9‐one (1AF) and 1‐(N,N‐dimethylamino)‐fluoren‐9‐one (1DMAF) are investigated by using steady‐state absorption and fluorescence as well as subpicosecond time‐resolved absorption spectroscopic techniques. Following photoexcitation of 1AF, which exists in the intramolecular hydrogen‐bonded form in aprotic solvents, the excited‐state intramolecular proton‐transfer reaction is the only relaxation process observed in the excited singlet (S₁) state. However, in protic solvents, the intramolecular hydrogen bond is disrupted in the excited state and an intermolecular hydrogen bond is formed with the solvent leading to reorganization of the hydrogen‐bond network structure of the solvent. The latter takes place in the timescale of the process of solvation dynamics. In the case of 1DMAF, the main relaxation pathway for the locally excited singlet, S₁(LE), or S₁(ICT) state is the configurational relaxation, via nearly barrierless twisting of the dimethylamino group to form the twisted intramolecular charge‐transfer, S₁(TICT), state. A crossing between the excited‐state and ground‐state potential energy curves is responsible for the fast, radiationless deactivation and nonemissive character of the S₁(TICT) state in polar solvents, both aprotic and protic. However, in viscous but strong hydrogen‐bond‐donating solvents, such as ethylene glycol and glycerol, crossing between the potential energy surfaces for the ground electronic state and the hydrogen‐bonded complex formed between the S₁(TICT) state and the solvent is possibly avoided and the hydrogen‐bonded complex is weakly emissive.     

Amine‐bis(phenolate) Iron(III)‐Catalyzed Formal Hydroamination of Olefins

A practical synthesis of highly functionalized amines by the formal hydroamination reaction of alkenes with nitroarenes catalyzed by an air stable amine‐bis(phenolate) iron(III) complex is reported. The reaction uses an easily handled silane, low catalyst loadings, and mild reaction conditions. A wide range of substrates are transformed with synthetically useful yields (21 examples).     

Chemoselectivity Control: Gold(I)‐Catalyzed Synthesis of 6,7‐Dihydrobenzofuran‐4(5H)‐ones and Benzofurans from 1‐(Alkynyl)‐7‐oxabicyclo[4.1.0]heptan‐2‐ones

New and chemoselective gold(I)‐catalyzed transformations of 1‐(arylethynyl)‐7‐oxabicyclo[4.1.0]‐ heptan‐2‐ones were developed. Two completely different products—6,7‐dihydrobenzofuran‐4(5H)‐ones and benzofurans—could be obtained from the same starting material. The selectivity is determined by the ligand of the gold catalyst: triphenylphosphine delivers 6,7‐dihydrobenzofuran‐4(5H)‐ones, and 1,3‐bis(diisopropylphenyl)imidazol‐2‐ylidene leads to benzofurans. Eleven examples of each case are provided. The mechanistic suggestions for the pathways to both product types are supported by isotope labeling experiments.     

Weak C—H...O and C—H...π hydrogen bonds and π–π stacking interactions in a series of four N′‐[(E)‐(aryl)methylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazides

Oxazolidin‐2‐ones are widely used as protective groups for 1,2‐amino alcohols and chiral derivatives are employed as chiral auxiliaries. The crystal structures of four differently substituted oxazolidinecarbohydrazides, namely N′‐[(E)‐benzylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₂H₁₂N₃O₃, (I), N′‐[(E)‐2‐chlorobenzylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₂H₁₂ClN₃O₃, (II), (4S)‐N′‐[(E)‐4‐chlorobenzylidene]‐N‐methyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₂H₁₂ClN₃O₃, (III), and (4S)‐N′‐[(E)‐2,6‐dichlorobenzylidene]‐N,3‐dimethyl‐2‐oxo‐1,3‐oxazolidine‐4‐carbohydrazide, C₁₃H₁₃Cl₂N₃O₃, (IV), show that an unexpected mild‐condition racemization from the chiral starting materials has occurred in (I) and (II). In the extended structures, the centrosymmetric phases, which each crystallize with two molecules (A and B) in the asymmetric unit, form A+B dimers linked by pairs of N—H...O hydrogen bonds, albeit with different O‐atom acceptors. One dimer is composed of one molecule with an S configuration for its stereogenic centre and the other with an R configuration, and possesses approximate local inversion symmetry. The other dimer consists of either R,R or S,S pairs and possesses approximate local twofold symmetry. In the chiral structure, N—H...O hydrogen bonds link the molecules into C(5) chains, with adjacent molecules related by a 2₁ screw axis. A wide variety of weak interactions, including C—H...O, C—H...Cl, C—H...π and π–π stacking interactions, occur in these structures, but there is little conformity between them.