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.     

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.     

Isomerization of an imine intermediate in a reductive amination reaction over metal catalysts

The reductive amination reaction of acetone by cyclohexylamine over hydrogenation metal catalysts was investigated. The study is focused on the formation of side products in the reaction. It was verified that the formation of amines having unusual combinations of alkyls is caused by the metal-catalyzed rearrangement of the double bond around the nitrogen atom in an imine intermediate and consequent reactions of the isomeric imine. It was found that the isomerization reactions occur over virtually all of the hydrogenation catalysts studied, while their respective activities for the imine isomerization decreases in the order Ni = Co > Ru > Pt = Rh > Pd.     

Quantum-chemical modelling of proton transfer in the Beckmann rearrangement

A model of proton transfer in the Beckmann rearrangement was developed by density functional theory (DFT) for the example of acetaldoxime. It was concluded that the Beckmann rearrangement of acetaldoxime may consist of three stages: Transfer of a proton from the imine carbon atom to the nitrogen atom with the formation of an acetaldoxime cation; formation of an enolic imine by an intermolecular mechanism; formation of acetamide (the final product of the Beckmann rearrangement) also by an intermolecular mechanism. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 3, pp. 144–147, May–June, 2008.     

Autoxidation of hydrazones. Some new insights

Autoxidation of hydrazones is a generally occurring reaction, leading mostly to the formation of alpha-azohydroperoxides. All structural kinds of hydrazones, having at least one hydrogen atom on nitrogen, are prone to autoxidation; however, there are marked differences in the rate of the reaction. Hydrazones of aliphatic ketones are 1-2 orders of magnitude more reactive than analogous derivatives of aromatic ketones. Even less reactive are the hydrazones of chalcones, which function also as efficient inhibitors of autoxidation of other hydrazones. These differences can be attributed to the reduction of the rate of the addition of oxygen to a hydrazonyl radical, which is a reversible reaction. In the case of conjugated ketones, it becomes endothermic, making this elementary step slow down and the chain termination reactions become important. Substituents influence the stability of hydrazonyl radicals and, consequently, the bond dissociation energies of the N-H bonds. In acetophenone phenylhydrazones, the substituents placed on the ring of hydrazine moiety exhibit a higher effect (Hammett rho = -2.8) than those on the ketone moiety (rho = -0.82), which denotes higher importance of the structure with spin density concentrated on nitrogen in delocalized hydrazonyl radical. Electronic effects of the substituents also affect the transition state for the abstraction of hydrogen atom by electrophilic peroxy radicals; NBO analysis display a negative charge transfer of about 0.4 eu from hydrazone to a peroxy radical in the transition state.     

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.     

Mechanistic investigation of imine formation in ruthenium‐catalyzed N‐alkylation of amines with alcohols

Imines are observed frequently in ruthenium‐catalyzed N‐alkylation of amines with alcohols. Herein, nitrogen–phosphine functionalized carbene ligands were developed and used in ruthenium‐catalyzed N‐alkylation to explore the mechanism of imine formation. The results showed that strongly electron‐donating ligands were beneficial for imine formation and alcohol dehydrogenation to generate acid. In addition, with an increase of electron density of nitrogen atom in substituted amines, the yield of imines in N‐alkylation was improved. At the same time, with electron‐rich imines as substrates, the transfer hydrogenation of imines became difficult. It is suggested that strongly electron‐donating ligands and substrates caused an increase of electron density on the ruthenium center, which resulted in the elimination of hydrogen atoms in active species [LRuH₂] as hydrogen gas rather than transfer onto the imine coordinated with the ruthenium center.     

Structural Studies of a Copper(II) Complex with Inequivalent 2‐Acetylpyridine 3‐Piperidylthiosemicarbazonato Ligands and a Nickel(II) Complex with Different Modes of Coordination by the Two Thiosemicarbazonato Moieties of 1‐Phenylglyoxal Bis{N(4)‐diethylthiosemicarbazone}

The crystal structure of a copper(II) complex of 2‐acetylpyridine 3‐piperidylthiosemicarbazone, [Cu(Acpip)₂], indicates a tridentate, monoanionic ligand (i. e., pyridine nitrogen, imine nitrogen and thiolato sulfur atoms) and a bidentate, monanionic ligand (i. e., imine nitrogen and thiolato sulfur atoms). The stereochemistry approaches square pyramidal with the bidentate ligand occupying an apical (imine nitrogen atom) and basal (thiolato sulfur atom) position. The structure of a nickel(II) complex of 1‐phenylglyoxal N(4)‐diethylthiosemicarbazone, [Ni(Pg4DE)], has a 4‐6‐5 trichelate system rather than the 5‐5‐5 system common to bis(thiosemicarbazones). Coordination of the hydrazinic nitrogen atom of the “phenyl arm” along with the thiolato sulfur atom provides the 4‐membered chelate ring.     

聚苯胺的质子酸掺杂机制的研究

木文用FT-IR、ESR、XPS等研究了聚苯胺的质子酸掺杂机制。结果表明聚苯胺掺杂时的质子化反应优先发生在分子链中的醌亚胺结构单元的氮原子上,并产生了分子内氧化还原反应而形成阳离子自由基。质子所带的电荷由于共轭作用能较好地离域到邻近苯环及对位氮原子上。     

Rhenium-catalyzed regio- and stereoselective addition of imines to terminal alkynes leading to N-alkylideneallylamines

The reaction of terminal alkynes with imines using ReBr(CO)(5) as a catalyst results in the production of N-alkylideneallylamines and not the conventional propargylamines. The substituent on the imine nitrogen is important, and a diphenylmethyl group gave the best result. The catalytic cycle of this regioselective C-C bond forming reaction appears to involve the formation of an alkynyl rhenium species and subsequent nucleophilic attack of the alkynyl β-carbon atom on the imine carbon to give a vinylidene rhenium species.     

Novel N-phosphonio imine-catalyzed epoxidation reactions

A new type of acyclic N-phosphonio imine catalyst for selective epoxidations has been synthesized. The activity of these imine catalysts can easily be modulated by varying its substituents. The substituent attached to the imine nitrogen atom is particularly important for an efficient oxygen transfer.     

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.     

Generation of 1-aryl-3-methoxycarbonylnitrilimines and their reactions with unsaturated hydrocarbons

Thermolysis of 1-(4-methoxyphenyl)- and 1-(4-fluorophenyl)heptamethoxycarbonyl-3a,7a-dihydroindazoles at 135?C140 °C resulted in the elimination of hexamethyl benzenehexacarboxylate and in the generation of 1-aryl-3-methoxycarbonylnitrilimines, which were trapped by alkenes and dienes to give the corresponding (1) pyrazolines via 1,3-dipolar cycloaddition and (2) 2-oxoalken-1-oic acid hydrazones via allylic proton migration to the nitrogen atom. The reaction with tetramethylethene unexpectedly yielded substituted 5-(1-hydroxy-1-methylethyl)- or 5-acetylpyrazolinecarboxylates as the main products. The formation of the latter compounds suggests initial abstraction of the H atom from tetramethylethene and probable participation of a water molecule that supplies one more oxygen atom to the reaction products.     

Synthesis of hydrazones derived from 2-substituted-10-carboxyphenothiazine hydrazides

2-Substituted-10-carboxyphenothiazine hydrazides 3 were found to yield hydrazones 5 on reaction with aromatic aldehydes. However, a similar reaction of hydrazides 3 with aliphatic ketones in refluxing ethanol failed to give the expected hydrazones 6 . The corresponding phenothiazines 1 were isolated in this reaction, apparently through alcoholysis of the amide bond to the ring nitrogen atom in 3 .     

Crystallographic report: Tris{2‐[2‐(1‐methyl)imidazolyl]methyliminoethyl}aminezinc(II) dihexafluorophosphate

The cation in the title compound has crystallographic threefold symmetry. The zinc atom is in a distorted octahedral geometry, being coordinated by three nitrogen atoms of the imine and three nitrogen atoms of imidazole. Copyright © 2004 John Wiley & Sons, Ltd.     

Activation of the Si-B linkage: copper-catalyzed addition of nucleophilic silicon to imines

Activation of the Si-B bond through copper-catalyzed transmetalation quickly developed into a practical method to generate Cu-Si reagents. These silicon nucleophiles cleanly add to aldehyde-derived imine electrophiles to form α-silylated amines in protic media, and no carbon-to-nitrogen Brook-type rearrangement of the intermediate anion is observed. Aside from electron-withdrawing groups at the imine nitrogen atom, for example, SO(2)Tol and P(O)Ph(2), previously delicate nitrogen substituents such as phenyl or benzhydryl are tolerated. The same protocol also allows the unprecedented addition to representative ketone-derived imines.     

The variation of coordination modes of aromatic imines in iron carbonyl complexes: Is there a correlation between bond lengths in organometallic model compounds and the reactivity of the ligands in catalytic C-C bond formation reactions?

The reaction of aromatic imines with Fe₂(CO)₉ proceeds via a two-step reaction sequence. A C-H activation reaction in ortho-position with respect to the exocyclic imine function is followed by an intramolecular hydrogen transfer reaction towards the former imine carbon atom. The resulting dinuclear iron carbonyl complexes show an aza-ferra-cyclopentadiene ligand which is apically coordinated by the second iron tricarbonyl moiety. Comparing the bond lengths of 43 different compounds, which were synthesized and structurally characterized in our group shows that the iron iron bond length correlates with one of the iron carbon bond lengths. The longer the iron carbon bond between the apically coordinated iron atom and the carbon atom next to the former imine carbon atom is, the shorter is the iron iron bond. The same ligands may be used as the substrates in ruthenium catalyzed C-C bond formation reactions. Whereas most of the imines react via the formal insertion of CO and/or ethylene into the C-H bond in ortho-position to the imine function, the ligands that show the longest iron carbon bond lengths in the model compounds under the same reaction conditions produce different types of isoindolones.     

Ethylene polymerization on titanium phenoxyimine complexes with different structures

The kinetics of ethylene polymerization in the presence of catalytic systems based on methylaluminoxane-activated titanium bis(phenoxyimine) complexes with different structures has been investigated in the temperature range 30–70°C. The structures of the complexes have different substituents at the imine nitrogen atom and in the phenoxy group in the ligand, which affect the activity of the system and the molecular weight of polyethylene resulting from polymerization over at least 1 h. The polymerization kinetics is most sensitive to the structure of the substituent at the imine nitrogen atom and to bulky substituents in the ortho position of the phenoxy group. The results obtained are explained. An attempt is made to classify the influence of the substituents in the ligands. Process conditions ensuring living polymerization have been found. The physicochemical properties and structural features of the polyethylenes obtained have been determined.     

Triazolines 24. Permanganate-catalyzed low temperature thermolysis of 5-(4-pyridyl) substituted 1,2,3-triazolines

Potassium permanganate oxidation of 1-aryl-5-(4-pyridyl)-1,2,3-triazolines I in a benzene-water two-phase system using the phase-transfer catalyst tetrabutylammonium chloride yields the corresponding 1H-1,2,3-tri-azoles II . However, when the reaction is run in a single phase in anhydrous benzene alone, the products are not triazoles, but imines III that can normally be obtained only by high-temperature thermolysis of the tri-azolines. The presence of both potassium permanganate and tetrabutylammonium chloride to yield the benzene soluble tetrabutylammonium permanganate ion pair appears essential for imine formation, although only in catalytic amounts. Thus a reaction pathway is proposed, which involves the initial coordination of the pyridyl nitrogen with the manganese atom of the permanganate ion leading to IV , followed by loss of nitrogen and regeneration of the permanganate ion via the dihydropyridine intermediates, V and VI , to yield the enamine VII , which tautomerizes to the imine III . Supportive evidence for the formation of IV is derived from the failure of 1,5-diaryl- and l-aryl-5-(3-pyridyl)triazolines to yield the respective imines; they lack the structural requirements necessary to comply with the proposed mechanism, and are recovered unchanged. Unlike the high temperature pyrolysis, the permanganate catalyzed low temperature thermolysis reactions provide cleaner products in better yields. Thus, the low temperature thermolysis may afford a route for the synthesis of clean samples of l-arylimino-l-ethyl-4-pyridines, especially when the triazolines are on hand.     

Isocyanate acting as a carbonyl precursor: pyridyl group-assisted formation of 4H-pyrido[1,2-a]pyrimidin-4-ones from ketimines and isocyanates

By the reactions of ketimines bearing a pyridyl or a picolyl group on a nitrogen atom of the imine moiety with tosylisocyanate, 4H-pyrido[1,2-a]pyrimidin-4-one derivatives could be obtained in quantitative yields. In these reactions, tosylisocyanate acts as a carbonyl precursor. The pyridyl or picolyl group is a key functional group because it is not only the constituent structure of the 4H-pyrido[1,2-a]pyrimidin-4-one framework but also the promoter of the formation of a ketene intermediate.