Ketimine Complexes Research Articles

Selective Generation of Aldimine and Ketimine Tautomers of the Schiff Base Condensates of Amino Acids with Imidazole Aldehydes or of Imidazole Methanamines with Pyruvates-Isomeric Control with 2- vs. 4-Substituted Imidazoles.

The Schiff base condensation of 5-methyl-4-imidazole carboxaldehyde, 5Me4ImCHO, and the anion of an amino acid, H2N-CH(R)CO2- (R = -CH3, -CH(CH3)2 and -CH2CH(CH3)2), gives the aldimine tautomer, Im-CH=N-CH(R)CO2-, while that of 5-methylimidazole-4-methanamine, 5MeIm-4-CH2NH2, with a 2-oxocarboxylate anion, R-C(O)-CO2-, gives the isomeric ketimine tautomer, Im-CH2-N=C(R)CO2-. All are isolated as the neutral nickel(II) complexes, NiL2, and are characterized by single crystal structure determination, IR, and positive ion ESI MS. In the cases of the 4 substituted imidazoles, either 5MeIm-4-CHO or 5MeIm-4-CH2NH2, both the aldimine and ketimine complexes are isolated cleanly with no evidence of an equilibrium between the two tautomers under the experimental conditions. The aldimines are blue while the tautomeric ketimines are green. In contrast, for the 2-substituted imidazoles, with either Im-2-CHO or Im-2-CH2NH2, the isolated product from the Schiff base condensation is the ketimine, which in the solid is green, as observed for the 4-isomer. These results suggest that for the 2-substituted imidazoles, there is a facile equilibrium between the aldimine and ketimine tautomers, and that the ketimine form is the thermodynamically favored tautomer. The aldimine tautomers of the 4-substituted imidazoles have three stereogenic centers, the nickel (Δ or Ʌ) and the two alpha carbon atoms (R or S). The observed pair of enantiomers is the ɅRR/ΔSS enantiomeric pair, suggesting that this pair is lower in energy than the others and that this is in general the preferred chiral correlation in these complexes.

The Biosynthetic Pathway of Ergothioneine in Culinary-Medicinal Winter Mushroom, Flammulina velutipes (Agaricomycetes).

Ergothioneine is a natural 2-thiol-amidazole amino acid that plays an important role in inflammation, depression, and cardiovascular disease. Flammulina velutipes is a common basidiomycete mushroom rich in ergothioneine (EGT). However, the biosynthetic pathway of EGT in F. velutipes is still unclear. In this study, the F. velutipes ergothioneine biosynthetic gene 1 (Fvegtl), F. velutipes ergothioneine biosynthetic gene 2 (Fvegt2), and F. velutipes ergothioneine biosynthetic gene 3 (Fvegt3) were cloned and expressed, and the activities of the proteins encoded by these three genes (FvEgt1, F. velutipes ergothioneine biosynthase 1; FvEgt2, F. velutipes ergothioneine biosynthase 2; and FvEgt3, F. velutipes ergothioneine biosynthase 3) were identified. The results showed that FvEgtl not only has the function of methyltransferase, but also has the function of hercynlcysteineteine sulfoxide (Hersul) synthase, which can catalyze the production of Hersul from histidine and cysteine in F. velutipes. FvEgt2 and FvEgt3 are two functionally different cysteine desulfurase enzymes. Among them, FvEgt2 is a cysteine-cysteine desulfurase-which catalyzes the activation of the S-H bond on cysteine, while FvEgt3 is a pyridoxal phosphate (PLP)-dependent cysteine desulfurase responsible for catalyzing the production of ketimine complex. Our results show that FvEgt1/FvEgt2/FvEgt3 can simultaneously catalyze the production of EGT by histidine, cysteine, and pyridoxal phosphate. Collectively, the in vitro synthesis of EGT in the edible fungus F. velutipes was first achieved, which laid the foundation for the biological production of EGT.

Mechanistic Insight into the Ring-Opening Polymerization of ε-Caprolactone and L-Lactide Using Ketiminate-Ligated Aluminum Catalysts.

The reactivity and the reaction conditions of the ring-opening polymerization of ε-caprolactone (ε-CL) and L-lactide (LA) initiated by aluminum ketiminate complexes have been shown differently. Herein, we account for the observation by studying the mechanisms on the basis of density functional theory (DFT) calculations. The calculations show that the ring-opening polymerization of ε-CL and LA are rate-determined by the benzoxide insertion and the C–O bond cleavage step, respectively. Theoretical computations suggest that the reaction temperature of L–LA polymerization should be higher than that of ε-CL one, in agreement with the experimental data. To provide a reasonable interpretation of the experimental results and to give an insight into the catalyst design, the influence of the electronic, steric, and thermal effects on the polymerization behaviors will be also discussed in this study.

CO2/Epoxide Coupling and the ROP of ε-Caprolactone: Mg and Al Complexes of γ-Phosphino-ketiminates as Dual-Purpose Catalysts

γ-Phosphino-ketimines Ph2PC[C(Me)O][C(Me)NAr] (Ar = 2,6-diisopropylphenyl, L1H; Ar = 2,6-difluorophenyl, L2H) were synthesized by treating deprotonated ketimines with PPh2Cl. Their Mg complexes [(L1)2(THF)Mg] (1) and [(L2)2(THF)2Mg] (2) were obtained in excellent yields from a reaction between LH and di-n-butylmagnesium in THF. Addition of a slight excess of trimethylaluminum to LH in toluene yielded the Al complexes [L1AlMe2] (3) and [L2AlMe2] (4). Complexes 1 and 3 displayed high catalytic activity in the synthesis of cyclic carbonates from CO2 and epoxides and also in the ring-opening polymerization (ROP) of e-caprolactone. However, complexes 2 and 4, which contain fluoro substituents, showed poor activity in the synthesis of cyclic carbonates and could not initiate the ROP reaction. The pentacoordinated Mg complex 1 was found to be a better catalyst than the aluminum complex 3. It was also observed that the complexes 1 and 3 were more efficient than the unsubstituted ketiminate complexes reported in t...

Four-coordinate six-membered ketiminate and diketiminate boron complexes. Synthesis, structure, reactivity, and application in the design of nitrogen heterocycles

The data on ketiminate and diketiminate boron complexes are first reviewed. The present review is focused on the synthesis and chemical transformations of these chelates. The special attention is paid to the application of these boron chelates in the synthesis of nitrogen heterocycles including CF3-substituted derivatives.

Metal-ligand cooperative activation of nitriles by a ruthenium complex with a de-aromatized PNN pincer ligand.

The pincer complex (PNN)RuH(CO), with a de-aromatized pyridine in the ligand backbone, is shown to react with nitriles in a metal-ligand cooperative manner. This leads to the formation of a series of complexes with new Ru-N(nitrile) and C(ligand)-C(nitrile) bonds. The initial nitrile cycloaddition products, the ketimido complexes 3, have a Brønsted basic (nitrile-derived) Ru-N fragment. This is able to deprotonate a CH2 side-arm of the pincer ligand to give ketimine complexes (4) with a de-aromatized pyridine backbone. Alternatively, the presence of a CH2 group adjacent to the nitrile functionality can lead to tautomerization to an enamido complex (5). Variable-temperature NMR studies and DFT calculations provide insight in the relative stability of these compounds and highlight the importance of their facile interconversion in the context of subsequent nitrile transformations.

Methoxyaryl substituted aluminum ketiminate complexes and its activity in ring opening polymerization processes

The synthesis of dimethyl aluminum (LoAlMe2) and chloromethyl aluminum (LoAlMeCl) complexes containing ketiminate ligand [(2-MeO)C6H4]N(H)C(Me)=CHC(Me)=O (LoH) is reported along with the preparation of Lo3Al from LoH and LiAlH4, and ionic [Lo′Al(thf)3]I from LoAlMe2 and I2. In the later complex the pseudooctahedral geometry of the aluminum ion by terdentate ligand {[(2-O)C6H4]NC(Me)=CHC(Me)=O}2− (Lo′), originated from Lo ligand by the methyl group abstraction, and three THF molecules are observed. Structures of these complexes were determined by X-ray techniques. The activity of LoAlMe2 and LoAlMeCl as initiators of ring opening polymerization of ε-caprolactone, trimethylene carbonate and l-lactide, and (co)polymerization of cyclohexene oxide and carbon dioxide was studied.

Synthesis and reactivity of cationic niobium and tantalum methyl complexes supported by imido and β-diketiminato ligands

The synthesis and reactivity of the cationic niobium and tantalum monomethyl complexes [(BDI)MeM(N(t)Bu)][X] (BDI = [Ar]NC(CH(3))CHC(CH(3))N[Ar], Ar = 2,6-(i)Pr(2)C(6)H(3); M = Nb, Ta; X = MeB(C(6)F(5))(3), B(C(6)F(5))(4)] was investigated. The cationic alkyl complexes failed to irreversibly bind CO but formed phosphine-trapped acyl complexes [(BDI)(R(3)PC(O)Me)M(N(t)Bu)][B(C(6)F(5))(4)] (R = Et, Cy) in the presence of a combination of trialkylphosphines and CO. Treatment of the monoalkyl cationic Nb complex with XylNC (Xyl = 2,6-Me(2)-C(6)H(3)) resulted in irreversible formation of the iminoacyl complex [(BDI)(XylN[double bond, length as m-dash]C(Me))Nb(N(t)Bu)][B(C(6)F(5))(4)], which did not bind phosphines but would add a methide group to the iminoacyl carbon to provide the known ketimine complex (BDI)(XylNCMe(2))Nb(N(t)Bu). Further stoichiometric chemistry explored i) migratory insertion reactions to form new alkoxide, amidinate, and ketimide complexes; ii) protonolysis reactions with Ph(3)SiOH to form thermally robust cationic siloxide complexes; and iii) catalytic high-density polyethylene formation mediated by the cationic Nb methyl complex.

Aldimine 2,6-bis(imino)pyridine iron(II) and cobalt(II)/methyl aluminoxane catalyst systems for polymerization of tert-butylacrylate

Aldimine 2,6-bis[(imino)methyl]pyridine iron(II) (1, 4, and 6) and cobalt(II) (3 and 5) complexes bearing bulky cycloaliphatic (bornyl and myrtanyl) or aromatic (naphthyl) terminal groups have been applied successfully, after activation with methyl aluminoxane (MAO), as catalysts for the polymerization of tert-butylacrylate. For comparison reasons, complex 2 that contains the ketimine ligand, 2,6-bis[(−)-cis-myrtanylimino)ethyl]pyridine (BMEP), has also been utilized. All studied complexes showed moderate polymerization activities, and they produced high molar mass syndiorich-atactic polymers. Surprisingly, the aldimine-based catalyst systems showed comparable activities compared with the corresponding ketimine complex (2), and they produced high molar mass polymers. In addition, complexes with bulky terminal cycloaliphatic substituents on the tridentate aldimine ligands showed higher polymerization activity compared with the aromatic ones (6). Polymerization activity and polymer molar masses are dependent on the ligand framework.

Ketimine synthesis in the coordination sphere of thallium (I)

[AuTl(C 6F 5) 2(en)] (en = ethylenediamine) reacts with cyclic ketones as cyclopentanone (Cy 5 O), cyclohexanone (Cy 6 O) or cycloheptanone (Cy 7 O) in 1:1 or 1:2 molar ratio leading to products of stoichiometry [AuTl(C 6F 5) 2{Cy x N(CH 2) 2NH 2}] ( x = 5 1, 6 2 or 7 3), or [AuTl(C 6F 5) 2{Cy x N(CH 2) 2N Cy x }] ( x = 5 4, 6 5 or 7 6). Addition of ethylenediamine to the ketimine complexes in chloroform regenerates [AuTl(C 6F 5) 2(en)], the starting material, and the free ketimines, as their NMR and mass spectra evidenced. The ketimine complexes display luminescence in solid state at room temperature and at 77 K at higher wavelengths than the diamine starting product (505 nm). The excited states responsible for this behaviour are assigned to orbitals due to the gold–thallium interactions.

Synthesis and Structural Characterization of Magnesium Ketiminate Complexes: Efficient Initiators for the Ring-Opening Polymerization of l-Lactide

A series of dinuclear magnesium complexes of the type [LMg(μ-OBn)]2 (where L = NNO-tridentate ketiminate ligand) have been prepared. Their activities toward the ring-opening polymerization of l-lactide have been investigated. Experimental results indicate that the reactivity of [LMg(μ -OBn)]2 is dramatically affected by the steric and electronic effect of the substituents on the ketiminate ligand. On the basis of the melting point depression and variable-temperature 1H NMR studies, it is believed that these magnesium complexes exist as an equilibrium between a monomeric and a dimeric species in solution in which the monomeric species is the most active species during polymerization. The polymerization probably proceeds with the coordination of the lactide to the monomeric species giving a five-coordinated magnesium complex as an intermediate.

Nonracemic 3°-Carbamines from the Asymmetric Allylboration of N-Trimethylsilyl Ketimines with B-Allyl-10-phenyl-9-borabicyclo[3.3.2]decanes

The simple and efficient asymmetric synthesis of 3 degrees -carbamines 7 from N-TMS enamines (3) and either enantiomeric form of beta-allyl-10-(phenyl)-9-borabicyclo[3.3.2]decane (1) is reported. The high reactivity (<1 h, -78 degrees C) and enantioselectivity (60-98% ee) of these substrates can be attributed to the fact that the complexation of 3 with 1 facilitates its isomerization to the corresponding syn-N-TMS ketimine complex from which allylation can occur. In addition to providing the homoallylic amines 7 with predictable stereochemistry, the procedure also permits the efficient recovery of the chiral boron moiety (50-65%) as air-stable crystalline pseudoephedrine complexes 8, which are directly converted back to 1 with allylmagnesium bromide in ether (98%).

BF3‐Mediated Additions of Organolithiums to Ketimines: X‐Ray Crystal Structures of BF3—Ketimine Complexes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Binding of C5-Dicarboxylic Substrate to Aspartate Aminotransferase: Implications for the Conformational Change at the Transaldimination Step,

The mechanism for the reaction of aspartate aminotransferase with the C4 substrate, l-aspartate, has been well established. The binding of the C4 substrate induces conformational change in the enzyme from the open to the closed form, and the entire reaction proceeds in the closed form of the enzyme. On the contrary, little is known about the reaction with the C5 substrate, l-glutamate. In this study, we analyzed the pH-dependent binding of 2-methyl-l-glutamate to the enzyme and showed that the interaction between the amino group of 2-methyl-l-glutamate and the pyridoxal 5'-phosphate aldimine is weak compared to that between 2-methyl-l-aspartate and the aldimine. The structures of the Michaelis complexes of the enzyme with l-aspartate and l-glutamate were modeled on the basis of the maleate and glutarate complex structures of the enzyme. The result showed that l-glutamate binds to the open form of the enzyme in an extended conformation, and its alpha-amino group points in the opposite direction of the aldimine, while that of l-aspartate is close to the aldimine. These models explain the observations for 2-methyl-l-glutamate and 2-methyl-l-aspartate. The crystal structures of the complexes of aspartate aminotransferase with phosphopyridoxyl derivatives of l-glutamate, d-glutamate, and 2-methyl-l-glutamate were solved as the models for the external aldimine and ketimine complexes of l-glutamate. All the structures were in the closed form, and the two carboxylate groups and the arginine residues binding them are superimposable on the external aldimine complex with 2-methyl-l-aspartate. Taking these facts altogether, it was strongly suggested that the binding of l-glutamate to aspartate aminotransferase to form the Michaelis complex does not induce a conformational change in the enzyme, and that the conformational change to the closed form occurs during the transaldimination step. The hydrophobic residues of the entrance of the active site, including Tyr70, are considered to be important for promoting the transaldimination process and hence the recognition of the C5 substrate.

BF3-Mediated Additions of Organolithiums to Ketimines: X-ray Crystal Structures of BF3−Ketimine Complexes

[reaction: see text] Additions of lithium acetylides and n-BuLi to N-alkyl ketimines mediated by BF(3)-Et(2)O in THF afford hindered tert-alkylamines in moderate to good yields. Stereochemical results and crystal structures of three BF(3)-imine complexes suggest that allylic strain strongly influences conformation and may be an important determinant of reactivity and selectivity.

Fluoroaryl-substituted ketiminate complexes of aluminum

The ketiminate complex AlCl[OC(Me)CHC(Me)N( p-C 6H 4F)] 2 ( 4) has been prepared from the β-aminoenone, O C(Me)CH C(Me)N(H)( p-C 6H 4F) ( 3) by lithiation of 3 with n-BuLi, followed by reaction with AlCl 3 and by the reaction of 3 with Me 2AlCl. A second compound, [AlCl 2{O C(Me)CH C(Me)N(H)( p-C 6H 4F)} 4][AlCl 4] ( 5), was also isolated from the AlCl 3 reaction. The structures of 4 and 5 were determined by X-ray diffraction analysis.

Reduction of Ketimines by Samarium(II) Complexes. Isolation and Structural Characterization of Samarium(III) η1-Amine/η1-Ketimido and η2-Ketimine Complexes

The reaction of (C5Me5)2Sm(THF)2 with 2 equiv of benzophenone imine (Ph2CNH) in THF at room temperature gave the samarocene(III) amine/ketimido complex (C5Me5)2Sm(NCPh2)(NH2CHPh2) (1) in 58% isolated yield. The use of 1 equiv of Ph2CNH also afforded 1 as the only isolable product, albeit in a lower yield (ca. 20%). Deuterium-labeling experiments suggest that 1 is formed via hydrogen abstraction by the in-situ-generated imine radical anion species, followed by acid−base reaction between the resulting amido species and another molecule of Ph2CNH. The reactions of 2 equiv of Sm(OAr)2(THF)3 (Ar = C6H2-tBu2-2,6-Me-4) with N-phenyl benzophenone imine (Ph2CNPh) and N-phenyl fluorenone imine (C12H8CNPh) yielded the Sm(III) η2-imine-dianion/aryloxide complexes Sm(η2-Ph2CNPh)(OAr)(THF)3 (3, 65%) and Sm(η2-C12H8CNPh)(OAr)(THF)3 (4, 77%), respectively, together with the byproduct Sm(OAr)3. The reaction of 1 equiv of Sm(OAr)2(THF)3 with Ph2CNPh also gave the imine-dianion complex 3, whereas an imine radical anion spec...

Apple 1-Aminocyclopropane-1-carboxylate Synthase in Complex with the Inhibitor l-Aminoethoxyvinylglycine: EVIDENCE FOR A KETIMINE INTERMEDIATE

The 1.6-A crystal structure of the covalent ketimine complex of apple 1-aminocyclopropane-1-carboxylate (ACC) synthase with the potent inhibitor l-aminoethoxyvinylglycine (AVG) is described. ACC synthase catalyzes the committed step in the biosynthesis of ethylene, a plant hormone that is responsible for the initiation of fruit ripening and for regulating many other developmental processes. AVG is widely used in plant physiology studies to inhibit the activity of ACC synthase. The structural assignment is supported by the fact that the complex absorbs maximally at 341 nm. These results are not in accord with the recently reported crystal structure of the tomato ACC synthase AVG complex, which claims that the inhibitor only associates noncovalently. The rate constant for the association of AVG with apple ACC synthase was determined by stopped-flow spectrophotometry (2.1 x 10(5) m(-1) s(-1)) and by the rate of loss of enzyme activity (1.1 x 10(5) m(-1) s(-1)). The dissociation rate constant determined by activity recovery is 2.4 x 10(-6) s(-1). Thus, the calculated K(d) value is 10-20 pm.

Four- and five-coordinate aluminum ketiminate complexes: synthesis, characterization, and ring-opening polymerization.

A series of aluminum complexes featuring with the ketiminate ligand, OCMeCHCMeNHAr (Ar = 2,6-(i)Pr(2)C(6)H(3), 1), have been prepared and characterized spectroscopically and structurally. Reactions of 1 with trialkylaluminum in 1:1 or 1:2 molar ratio generate four- and five-coordinated aluminum complexes (OCMeCHCMeNAr)AlR(2) (R = Me (2); R = Et (3)) and (OCMeCHCMeNAr)(2)AlR (R = Me (4); R = Et (5)) in high yields. Similarly, reaction of AlCl(3) with 1 or 2 equiv of the lithiated 1 in toluene afforded bis(ketiminate) aluminum chloride complex, (OCMeCHCMeNAr)(2)AlCl (6) or (OCMeCHCMeNAr)AlCl(2) (7). Surprisingly, reacting 6 with 1 equiv of AgBF(4) in methylene/acetonitrile mix-solvents generates (OCMeCHCMeNAr)(2)AlF (8) in moderate yield. The structures of complexes 2-6 and 8 have been determined by X-ray crystallography. Complexes 2 and 3 both exhibit tetrahedron structures with the aluminum atom surrounded by oxygen and nitrogen atoms of chelating ketiminate and two alkyl groups. The mono- and bis-ketiminate aluminum complexes 2-5 have shown moderate activity toward the ring-opening polymerization of epsilon-caprolactone.

Synthesis of the dimeric μ2-O aminoalkoxy complexes of rhenium [(CO)3Re(μ2-O∩NH2)]2 and their condensation products with acetone [(CO)3Re(μ2-O∩N=CMe2)

The allyl rhenium complex Re(CO)4(η3-C3H5) (1) reacts with the amino alcohols 2-amino ethanol, 1-amino-propane-2-ol, 2-amino phenol and 2-amino benzyl alcohol (2a–d) to give the dimeric μ2-O-bridged tricyclic aminoalkoxy complexes of rhenium of the type [(CO)3Re(μ2-O∩NH2)]2 (3a–d). The acidic NH2 ligand undergoes a condensation reaction with acetone at r.t. forming the corresponding ketimine complexes [(CO)3Re(μ2-O∩N=CMe2)]2 (4a–d). The compounds have been characterized by spectroscopic (IR, NMR, MS), analytical (C, H, N) and X-ray diffraction analyses (4a–d).