A theoretical study on the complete catalytic cycle of the hetero-Pauson-Khand-type [2+2+1] cycloaddition reaction of ketimines,carbon monoxide and ethylene catalyzed by iron carbonyl complexes

The [2+2+1] cycloaddition reaction of 1,4-diazabutadienes, carbon monoxide and ethylene catalyzed by iron carbonyl complexes produces pyrrolidin-2-one derivatives. Only one of the two imine moieties is activated during the catalysis. The mechanism of this cycloaddition reaction is studied by density functional theory at the B3LYP/6-311++G(d,p) level of theory. In accordance with experimental results, a [(diazabutadiene)Fe(CO)(3)] complex of square-pyramidal geometry is used as the starting compound S of the catalytic cycle. Based on experimental experience, the reaction with ethylene is considered to take place before any interaction with carbon monoxide. According to the computational results, the reaction does not proceed by ligand dissociation followed by addition of ethylene and subsequent intramolecular activation steps but by the approach of an ethylene molecule from the base of the square-pyramidal complex. This reaction yields an intermediate I(4) in which ethylene is coordinated to the iron centre and a new C-C bond between ethylene and one of the imine groups is formed. The insertion of a terminal carbon monoxide ligand into the metal-carbon bond between ethylene and iron produces the key intermediate I(7). The reaction proceeds by metal-assisted formation of a lactam P. The catalytic cycle is closed by a ligand-exchange reaction in which the diazabutadiene ligand substitutes P with reformation of S. This reaction pathway is found to be energetically favored over a reductive elimination. It leads to the experimentally observed heterocyclic product P and a reactive [Fe(CO)(3)] fragment.     

Exo-pi-bonding to an ortho-carborane hypercarbon atom: systematic icosahedral cage distortions reflected in the structures of the fluoro-, hydroxy- and amino-carboranes, 1-X-2-Ph-1,2-C2B10H10 (X=F, OH or NH2) and related anions

The structures of derivatives of phenyl-ortho-carborane bearing on the second cage hypercarbon atom a pi-donor substituent (F, OH, O-, NH2, NH- and CH2-) were investigated by NMR, X-ray crystallography and computational studies. The molecular structures of these compounds, notably their cage C1-C2 distances and the orientations of their pi-donor substituents (OH, NH2, NH- and CH2-) show remarkable and systematic variations with the degree of exo pi-bonding, which varies as expected with the pi-donor characteristics of the substituent.     

Design of Thermally Stable Amine–Imine Nickel Catalyst Precursors for Living Polymerization of Ethylene: Effect of Ligand Substituents on Catalytic Behavior and Polymer Properties

Nickel complexes bearing amine–imine ligands with various backbone substituents were synthesized and employed as ethylene polymerization catalysts on activation with Et₂AlCl. The substituent on the backbone carbon atom of the amine moiety is decisive for the living nature of ethylene polymerization. A bulky amine–imine nickel precursor with a tert‐butyl group on the carbon atom of the amine group can polymerize ethylene in a living fashion at an elevated temperature of 65 °C, which is the highest temperature of living polymerization of ethylene with late transition‐metal catalysts. The wide applicable temperature range for living polymerization and sensitivity of the branch structure of the polyethylene to temperature enable precise synthesis of di‐ and triblock polyethylenes featuring different branched segments by sequential tuning of the polymerization temperature.     

A DFT study on the regioselectivity of the reaction of dichloropropynylborane with isoprene

This theoretical study deals with the reaction of isoprene and dichloropropynylborane. We report the results of the DFT calculations applied to the two processes involved, Diels-Alder cycloaddition and 1,4-alkynylboration. The boron influences both the chemoselectivity and the regioselectivity of this reaction through secondary orbital interactions (SOI hereafter) that give rise to transition structures with strong [4 atom + 3 atom] character. The "meta" regioselectivity observed experimentally for the reaction between 2-substituted 1,3-dienes and alkynyldihaloboranes has been explained as a result of the higher stabilization of these transition structures with "meta" orientation. Intrinsic reaction coordinate calculations were performed to determine connectivities and established the remarkable result that the geometrically very similar transition structures leading to both regioisomeric 1,4-alkynylboration products correspond to different pathways. For the "meta" orientation a direct alkynylboration of the diene through a concerted transition structure was found.     

(Z)‐7‐Chloro‐3‐[(3‐chloro­phenyl)­methyl­idene]‐4‐p‐tosyl‐3,4‐di­hydro‐2H‐1,4‐benzoxazine

In the title compound, C₂₂H₁₇Cl₂NO₃S, the mol­ecule is a substituted 3,4‐di­hydro‐2H‐1,4‐benzoxazine compound which has three phenyl rings which are essentially planar. The 3,4‐di­hydro‐2H‐oxazine part of the mol­ecule is fused to the benzo ring and has a half‐boat conformation; the dihedral angle between the planar part of the oxazine ring and the benzo ring is 10.2 (2)°. The (3‐chloro­phenyl)­methyl­idene substituent has a Z configuration in relation to the ring N atom of the oxazine moiety. Interestingly, the p‐toluenesulfonyl (p‐tosyl) substituent on the ring N atom protrudes away from the 3‐­chloro­phenyl substituent thus avoiding any steric interaction.     

The mutual influence of the imine substituents of terephthal-bis-imines concerning their reactivity towards Fe2(CO)9

The reaction of terephthal-bis-imines with Fe₂(CO)₉ proceeds via a C---H activation reaction in the ortho position with respect to one of the imine functions. The corresponding hydrogen atom is shifted towards the former imine carbon atom producing a methylene group instead. The dinuclear iron complexes formed by this reaction sequence and showing no coordination of the second imine group were isolated from reactions of bis-imines with both phenyl and cyclohexyl substituents at the imine nitrogen atoms. In addition, we observed three different reaction pathways of the second imine substituent of the starting material which is obviously thus influenced by the fact that the first one is coordinating an Fe₂(CO)₆ moiety. If the organic substituent at the imine nitrogen atoms is a phenyl group the formation of a trinuclear complex is achieved in which an additional Fe(CO)₃ group is coordinating the CN double bond and one of the carbon---carbon bonds of the central phenyl ring in an η⁴-fashion. The same reaction leads to the isolation of a tetranuclear iron---carbonyl compound in which both imine substituents were transformed via the pathway described above, each building up dinuclear subunits. In contrast to this the reaction of a bis-imine with cyclohexyl groups at the imine nitrogen and thus an enhanced nucleophilicity leads to the formation of a tetranuclear complex in which only one imine group reacts under C---H activation with subsequent hydrogen migration towards the former imine carbon atom. The second imine substituent also shows a C---H activation reaction in the ortho position with respect to the imine group but the corresponding hydrogen atom is transferred to one of the aromatic carbon atom of the central phenyl ring of the ligand. The C=N double bond remains unreacted and only coordinates the second Fe₂(CO)₆ moiety via the nitrogen lone pair.     

Electronic and steric control in regioselective addition reactions of organolithium reagents with enaldimines

A reaction mode of imines derived from naphthalene-1-carbaldehyde and acyclic alpha,beta-unsaturated aldehydes with organolitium reagents was dependent on the characteristic nature of a substituent on the imine nitrogen atom. An imine having an electron-withdrawing aryl group on the nitrogen atom behaves as a 1,2-directing imine toward organolithium reagents. In contrast, an imine bearing an alkyl or a bulky aryl group favors 1,4-addition of organolithium reagents. Electronic and steric tuning of a substituent on the imine nitrogen atom for a reaction mode was rationalized on the basis of molecular orbital calculations.     

Regioselective CC bond formation between ethylene and α-naphthylcarbaldimines catalyzed by Ru3(Co)12: NMR spectroscopic investigations on the proceeding of the reaction

The reaction of ethylene with imines derived from α-naphthylcarbaldehyde catalyzed by Ru₃(CO)₁₂ leads to the selective and quantitative formation of products in which one molecule of ethylene has been inserted into the CH bond in ortho position with respect to the exocyclic imine substituent. The stoichiometric reaction of the same ligands with Ru₃(CO)₁₂ leads to dinuclear ruthenium carbonyl complexes showing the same regioselectivity of CH activation but the hydrogen atom is shifted in an intramolecular hydrogen transfer reaction towards the former imine carbon atom. If the catalytic alkylation of α-naphthylcarbaldimines is monitored by NMR the occurrence of the dinuclear product of the stoichiometric reaction is observed before the reaction again quantitatively yields the imines bearing an ethyl group in 2-position of the naphthalene core. This proofs that there must be an equilibrium between the dinuclear ruthenium carbonyl complex which is also observed if α-naphthylcarbaldimines are treated with an equimolar amount of Ru₃(CO)₁₂ and another ruthenium compound where the ethylene might be inserted catalytically into a ruthenium carbon bond.     

Stoichiometric Reactions of Enamines Derived from Diphenylprolinol Silyl Ethers with Nitro Olefins and Lessons for the Corresponding Organocatalytic Conversions – a Survey

The stoichiometric reactions of enamines prepared from aldehydes and diphenyl‐prolinol silyl ethers (intermediates of numerous organocatalytic processes) with nitro olefins have been investigated. As reported in the last century for simple achiral and chiral enamines, the products are cyclobutanes ( 4 with monosubstituted nitro‐ethenes), dihydro‐oxazine N‐oxide derivatives ( 5 with disubstituted nitro‐ethenes), and nitro enamines derived from γ‐nitro aldehydes ( 6 , often formed after longer reaction times). The same types of products were shown to be formed, when the reactions were carried out with peptides H‐Pro‐Pro‐Xaa‐OMe that lack an acidic H‐atom. Functionalized components such as alkoxy enamines, nitro‐acrylates, acetamido‐nitro‐ethylene, or hydroxylated nitro olefins also form products carrying the diphenyl‐prolinol silyl ether as a substituent. All of these products must be considered intermediates in the corresponding catalytic reactions; the investigation of their chemical properties provided useful hints about the rates, the conditions, the catalyst resting states or irreversible traps, and/or the limitations of the corresponding organocatalytic processes. High‐level DFT and MP2 computations of the structures of alkoxy enamines and thermodynamic data of a cyclobutane dissociation are also described. Some results obtained with the stoichiometrically prepared intermediates are not compatible with previous mechanistic proposals and assumptions.     

Enantioselective intramolecular [2 + 2 + 2] cycloaddition of 1,4-diene-ynes: a new approach to the construction of quaternary carbon stereocenters

The intramolecular [2 + 2 + 2] cycloaddition of various 1,4-diene-ynes was examined using a chiral rhodium catalyst. In the case of 1,4-diene-ynes with a substituent at the 2-position of the 1,4-diene moiety, tricyclic compounds possessing a strained bicyclo[2.2.1]heptene skeleton with two quaternary carbon stereocenters were obtained in high enantiomeric excess. On the other hand, in the case of 1,4-diene-ynes with no substituent at this position, bicyclic cyclohexa-1,3-dienes with a quaternary carbon stereocenter were obtained probably by carbon-carbon bond cleavage of the reaction intermediate.     

Single‐Step Modular Synthesis of Unsaturated Morpholine N‐Oxides and Their Cycloaddition Reactions

A single‐flask procedure for the generation of α‐keto‐N ‐alkenylnitrones through a Chan–Lam coupling and subsequent spontaneous 6π electrocyclization of these intermediates for the synthesis of 2H ‐1,4‐oxazine N ‐oxides has been developed for a variety of α‐ketooximes and alkenylboronic acids. This transformation provides a new approach to C‐substituted unsaturated morpholine derivatives that are poised to undergo further functionalization for the preparation of a diverse array of novel heterocyclic structures. The scope of the new method for the synthesis of 2H ‐1,4‐oxazine N ‐oxides is discussed, in addition to initial studies describing the cycloaddition reactivity of these new heterocyclic intermediates.     

Analysis of the Electronic Structure of 1-(4-Aminobutyl)-3-methylimidazolium Chloride via DFT Method

Density functional theory and natural bond orbital analysis are applied to study the microelectronic structural properties of 1-(4-aminobutyl)-3-methylimidazolium chloride(C_4NH_2C_1imCl). We obtain 48 stable cation conformers at the B3LYP/6-311++G(d, p) level of theory. When the Cl anion is located at different positions around the two stable cation conformers,optimization and frequency are calculated at the same level of theory, and seven stable C_4NH_2C_1imCl structures are obtained. Ion-pair association energies, natural population analysis(NPA) charges, and the second-order stabilization energies are also calculated. Results show that three energy-degenerated states are present in seven conformers. The calculated ion-pair association energy, NPA charges, and second-order stabilization energy values are the same as those with the same energy level conformers. Finally, H bond formation in the molecule is analyzed by the interactions of frontier molecular orbitals. The lone pair of N(11) atom in the amino group exhibits higher reactivity than that of the N(4) atom in the ring.     

Reaction of <Emphasis Type="Italic">N</Emphasis>-sulfonyl-1,4-benzoquinone imines with enamines

N-Sulfonyl derivatives of 1,4-benzoquinone imine reacted with enamines to give 1,4-addition products and products of their subsequent cyclization, substituted 5-aminobenzofurans and 5-aminoindoles, depending on the solvent nature, electron-withdrawing power of the substituent on the quinone imine nitrogen atom, and enamine structure. The presence of strong electron-withdrawing trifluoromethanesulfonyl group on the quinone imine nitrogen atom favors formation of 1,4-addition products and benzofuran derivatives.     

Conjugated schiff's bases.20 cycloaddition of heterocomulenes to some 1,3-heterodienes involving unusual assistance of methyl group

Highly substituted 1,Δ-diazabutadienes react with aroyl isothiocyanates in a 1,3-dipolar cycloaddition node yielding five-membered thiohydantoin-type heterocycles. The cycloaddition is accompanied by 1,4 shift of hydrogen from a methyl group attached to C2 of the 1,C-diazabutadiene moiety. The mechanism of this reaction is discussed in comparison with similar cycloadditions with aryl isocyanates.     

Nonmetathetic activity of ruthenium alkylidene complexes: 1,4-hydrovinylative cyclization of multiynes with ethylene

An efficient 1,4-hydrovinylative cyclization reaction of triynes and tetraynes catalyzed by ruthenium alkylidene complexes under ethylene is described. The regioselectivity of vinyl group incorporation can be controlled by the nature of the substituent on the alkyne, and the Grubbs second-generation catalyst is the most effective among typical ruthenium alkylidene complexes.     

Diastereoselective addition of zincated hydrazones to alkenylboronates and stereospecific trapping of boron/zinc bimetallic intermediates by carbon electrophiles

Zincated hydrazones possessing a tert-butyl group on the zinc atom undergo addition to (E)- or (Z)-alkenylboronic acid pinacol esters to produce alpha-alkylated-gamma-boryl-gamma-zinciohydrazone intermediates with good to excellent diastereoselectivity (ds). The 1,1-organodimetallic intermediate possessing a boron atom and a zinc atom in the position gamma to the hydrazone group undergoes further C-C bond formation with a carbon electrophile to give a gamma-boryl hydrazone possessing several contiguous stereogenic centers with up to 99% ds. The (S)-1-amino-2-methoxymethylpyrrolidine hydrazone shows a high level of asymmetric induction in the addition/trapping sequence. Density functional theory calculations on the pathways of the addition reaction revealed a metallo-ene mechanism consisting of the formation of a pi complex between a zincated hydrazone and a vinylborane followed by a six-centered bond reorganization of a highly ordered boat conformation transition state. The calculations indicated that the use of the zinc atom together with the imine or hydrazone is the key for the success of the olefinic variant of the aldol reaction that has long been considered not to take place because of the endothermicity of the reaction and has never been examined with any seriousness by chemists. The steric repulsion caused by the bulky tert-butyl ligand on the zinc atom and the pinacol moiety of the vinylboronate substrates in the highly ordered transition structures gives rise to the observed high ds of the present carbozincation reaction.     

Mechanism of the reaction of 2-methoxybenzo[d][1,3,2]dioxaphosphinin-4-one with chloral by quantum-chemical calculations

Quantum-chemical calculations of various stereoisomers, intermediates, and transition states of the reaction of 2-methoxybenzo[d][1,3,2]dioxaphosphinin-4-one with chloral, leading to formation of 2-methoxy-3-(trichloromethyl)benzo[e[1,4,2 ⁵]dioxaphosphepin-2,5-dione, were carried out by the density functional theory (DFT) method with the PBE functional and Triple z basis, using the Priroda program. The first step of the reaction is [1+2] cycloaddition of phosphorus to the chloral C=O bond to form an intermediate with a five-coordinate phosphorus atom via a transition state in which the positive and negative charges are strongly localized on phosphorus and chloral oxygen, respectively. Calculations of the internal reaction coordinate from all transition states were carried out.     

Gold nanoparticle based NSET for monitoring Mg2+ dependent RNA folding

Understanding the mechanism of how RNA molecules fold into their native structures are vital to their functional properties. Here we report for the first time that gold nanoparticle based NSET can be used for probing the transition states of an RNA unfolding reaction. Our result shows that time-dependent NSET can clearly distinguish structural transitions between unfolded to folded states. Our experimental observation point out that NSET can be used for the design of an optical based molecular ruler to track RNA folding transition states at distances more than double the distances achievable using traditional dipole-dipole Coulombic energy transfer based methods.     

CF2-containing acetylenephosphonates in heterocyclization reactions: the first synthesis of 2-difluoromethyl azaxanth-3-ylphosphonates

Acetylenephosphonates carrying the XCF(2) group have been studied in a base-mediated heterocyclization reaction with selected 2-amino-3-formylchromones to give 2-difluoromethyl azaxanth-3-ylphosphonates. The presence of the fluorinated substituent determined the regioselectivity as well as the reactivity of this process.     

Nickel-mediated regioselective [2 + 2 + 2] cycloaddition of carboryne with alkynes

Transition metal-benzyne complexes have found many applications in organic synthesis, mechanistic studies, and the synthesis of functional materials. In sharp contrast, the chemistry of transition metal-carboryne complexes, especially late transition metal complexes, is virtually unknown. This communication reports a novel nickel-mediated regioselective [2 + 2 + 2] cycloaddition reaction of carboryne with alkynes via the Ni-carboryne intermediate (eta2-C2B10H10)Ni(PPh3)2. Because of the bulkiness of the carborane moiety, a high regioselectivity was achieved in the reactions involving unsymmetrical alkynes. This work furnishes a novel method for the preparation of highly substituted benzocarboranes which are difficult to obtain by other methods.