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PubMed2005

Isomerization and deuterium scrambling evidence for a change in the rate-limiting step during imine hydrogenation by Shvo's hydroxycyclopentadienyl ruthenium hydride.

Hydroxycyclopentadienyl ruthenium hydride 5 efficiently reduces imines below room temperature. Better donor substituents on nitrogen give rise to faster rates and a shift of the rate-determining step from hydrogen transfer to amine coordination. Reduction of electron-deficient N-benzilidenepentafluoroaniline (8) at 11 degrees C resulted in free amine and kinetic isotope effects of k(OH)/k(OD) = 1.61 +/- 0.08, k(RuH)/k(RuD) = 2.05 +/- 0.08, and k(RuHOH)/k(RuDOD) = 3.32 +/- 0.14, indicative of rate-limiting concerted hydrogen transfer, a mechanism analogous to that proposed for aldehyde and ketone reduction. Reduction of electron-rich N-alkyl-substituted imine, N-isopropyl-(4-methyl)benzilidene amine (9), was accompanied by facile imine isomerization and scrambling of deuterium labels from reduction with 5-RuDOH into the N-alkyl substituent of both the amine complex and into the recovered imine. Inverse equilibrium isotope effects were observed in the reduction of N-benzilidene-tert-butylamine (11) at -48 degrees C (k(OH)/k(OD) = 0.89 +/- 0.06, k(RuH)/k(RuD) = 0.64 +/- 0.05, and k(RuHOH)/k(RuDOD) = 0.56 +/- 0.05). These results are consistent with a mechanism involving reversible hydrogen transfer followed by rate-limiting amine coordination.

Aniline Compounds↗
PubMed2001

Mechanisms of acid-catalyzed Z/E isomerization of imines.

The kinetics and mechanism of acid-catalyzed Z/E isomerization of O-methylbenzohydroximoyl chloride (1Za and 1Ea), methyl O-methylbenzohydroximate (1Zb and 1Eb), ethyl O-methylbenzohydroximate (1Zc and 1Ec and five para and meta substituted derivatives), O-methylcinnamohydroximoyl chloride (2Za and 2Ea), and methyl O-methylcinnamohydroximate (2Zb and 2Zb) have been investigated. The kinetics of Z/E isomerization of these imines have been studied in glacial acetic acid (1Ea and 1Zc) and in dioxane solutions containing HCl, trifluoromethanesulfonic acid, or tetrafluoroboric acid (1Ea, 1Zb, 2Ea, and 2Zb). The isomerization takes place by either (a) rotation about the carbon-nitrogen double bond of the protonated imine (iminium ion rotation) or (b) nucleophilic attack on the protonated imine to form a tetrahedral intermediate that undergoes stereomutation and loss of the nucleophile (nucleophilic catalysis). The hydroximoyl chlorides 1Eaand 2Ea only isomerize by the nucleophilic catalysis mechanism. The hydroximate 1Zb appears to be capable of isomerizing by either mechanism. The hydroximate 2Zb may be isomerizing only by iminium ion rotation. Theoretical calculations support the notion that increased conjugation in the protonated imine increases the rate of iminium ion rotation.

Acids↗
PubMed2001

Studies toward gymnodimine: development of a single-pot Hua reaction for the synthesis of highly hindered cyclic imines.

[structure: see text]. In studies directed toward gymnodimine and related marine toxins, a single-pot variation of the Hua cyclic imine synthesis has been developed. The reaction involves generation of N-trimethylsilyl lactams in situ followed by alkyllithium addition leading directly to cyclic imines. Importantly, this reaction proceeds efficiently with highly hindered alpha,alpha-dialkyl lactams, provided 1,2-dimethoxyethane (DME) is used as solvent, leading to stable cyclic imines. Overall, this transformation allows a one-pot coupling of an alkyliodide and a lactam to give a cyclic imine.

Animals↗
PubMed1998

Thioether adducts of a new imine reactive intermediate of the pneumotoxin 3-methylindole.

Cytochrome P450 enzymes can potentially oxygenate 3-methylindole to form 2,3-epoxy-3-methylindoline which could rearrange to the stable metabolite 3-methyloxindole or open to form 3-hydroxy-3-methylindolenine, a putative electrophilic imine. The purpose of the current work was to determine if the imine was formed, and to characterize it via its adducts with thiol nucleophiles. Thiols were added to incubations of goat lung microsomes with 3-methylindole and deuterated analogues of 3-methylindole to trap the imine intermediate as its thioether conjugates. The N-acetylcysteine conjugate of 3-hydroxy-3-methylindolenine was detectable by LC/MS, but a molecular ion was not observed because the adduct rapidly dehydrated to form the 2-substituted indole. However, the imine was S-alkylated, and the intermediate carbinol was intramolecularly trapped using thioglycolic acid as a trapping agent that induced cyclocondensation to a lactone. The retention of one atom of deuterium from [2-2H]-3-methylindole and three from 3-[2H3-methyl]indole substantiated the mechanism in which the lactone adduct was produced by sulfur addition to either 3-hydroxy-3-methylindolenine or the epoxide. Tandem mass spectrometry of the lactone adduct produced a daughter ion spectrum consistent with this adduct. These studies demonstrated the existence of a new reactive intermediate of 3-methylindole, 3-hydroxy-3-methylindolenine, which may play a role in the pneumotoxicity of this chemical.

Animals↗
PubMed2001

Carbinolamines, imines, and oxazolidines from fluorinated propranolol analogs. (19)F NMR and mass spectral characterization and evidence for formation as intermediates in cytochrome P450-catalyzed N-dealkylation.

Formation of carbinolamine, imine, and oxazolidines from the reactions of desisopropylpropranolol (5), its O-methyl ether (10), and 3-(1-naphthoxy)propylamine (11) with trifluoroacetone and trifluoroacetaldehyde methyl hemiacetal was investigated by (19)F NMR and tandem mass spectrometry. Products from the metabolism of the related secondary amine substrates trifluoropropranolol (7), its O-methyl ether (23), and its N-trifluoroethyl-O-methyl ether analog (24) in the presence of rat liver microsomes and CYP1A2 were examined to determine whether these species were formed. The (19)F NMR experiments showed the presence of carbinolamine and imine species from these primary amines and fluorinated carbonyl compounds in solution. Mass spectral experiments under atmospheric pressure chemical ionization and electrospray ionization-ion trap conditions showed formation of imine metabolites (and/or oxazolidine from 7) as well as products of N-dealkylation and aromatic hydroxylation when the secondary amine substrates were incubated with rat liver microsomes or CYP1A2. In spite of mass spectral evidence for these imines as metabolites, we were unable to detect the carbinolamines under the conditions used in these studies. Their presence is inferred from the results of the (19)F NMR experiments.

Animals↗
PubMed2001

Bioactivation of diclofenac via benzoquinone imine intermediates-identification of urinary mercapturic acid derivatives in rats and humans.

The metabolism of diclofenac has been reported to produce reactive benzoquinone imine intermediates. We describe the identification of mercapturic acid derivatives of diclofenac in rats and humans. Three male Sprague-Dawley rats were administered diclofenac in aqueous solution (pH 7) at 50 mg/kg by intraperitoneal injection, and urine was collected for 24 h. Human urine specimens were obtained, and samples were pooled from 50 individuals. Urine samples were analyzed by liquid chromatography-tandem mass spectrometry (LC/MS/MS). Two metabolites with MH(+) ions at m/z 473 were detected in rat urine and identified tentatively as N-acetylcysteine conjugates of monohydroxydiclofenac. Based upon collision-induced fragmentation of the MH(+) ions, accurate mass measurements of product ions, and comparison of LC/MS/MS properties of the metabolites with those of synthetic reference compounds, one metabolite was assigned as 5-hydroxy-4-(N-acetylcystein-S-yl)diclofenac and the other as 4'-hydroxy-3'-(N-acetylcystein-S-yl)diclofenac. The former conjugate also was detected in the pooled human urine sample by multiple reaction-monitoring LC/MS/MS analysis. It is likely that these mercapturic acid derivatives represent degradation products of the corresponding glutathione adducts derived from diclofenac-2,5-quinone imine and 1',4'-quinone imine, respectively. Our data are consistent with previous findings, which suggest that oxidative bioactivation of diclofenac in humans proceeds via benzoquinone imine intermediates.

Acetylcysteine↗
PubMed1987

Mechanism of formation and quantitation of imines, pyrroles, and stable nonpyrrole adducts in 2,5-hexanedione-treated protein.

The condensation of gamma-diketones with protein epsilon-amino moieties to yield alkylpyrrole adducts has been demonstrated in many in vitro and in vivo systems, although certain features of this reaction remain unclear. The present in vitro study was designed to examine additional aspects of gamma-diketone-protein interactions, including the possible formation of imine intermediates and stable nonpyrrole products, and the potential for conformational changes in pyrrolylated protein. Values for total, stable covalent binding were consistently higher than p-dimethylaminobenzaldehyde (DMAB)-detectable pyrrole adduct concentrations when bovine serum albumin (BSA) was incubated (24 hr, 37 degrees C) with [14C]-2,5-hexanedione (2,5-HD) at diketone:lysine ratios greater than or equal to 5:1 (at pH 9.5) or 1:1 (at pH 7.4). Treatment of pyrrolylated BSA with proteases before the DMAB assay decreased but did not eliminate the difference between these parameters. Quantitative amino acid analysis of pyrrolylated BSA revealed molar decreases in lysine content equivalent to DMAB-detectable pyrrole adduct concentrations; no other amino acids were significantly altered. Cleavage of disulfide bonds in pyrrolylated BSA by dithiothreitol resulted in an apparent decrease in DMAB-detectable pyrrole, which was reversible upon subsequent protease treatment. A similar decrease was not seen with pyrrolylated concanavalin A, a protein that lacks disulfide linkages. Samples of BSA were incubated with [14C]-2,5-hexanedione for 2-144 hr and a portion of each incubation mixture treated with NaCNBH3 to selectively reduce imines to stable amines. Substantial levels of an imine intermediate were detected at 2, 6, and 24 hr but not at 144 hr. The above findings support proposed mechanisms involving imine intermediates in the pyrrolylation reaction. In addition, evidence for the formation of stable nonpyrrole adducts at high diketone:amine molar ratios has been provided. Results consistent with potential conformational alterations in pyrrolylated protein have also been demonstrated.

Amino Acids↗
PubMed1994

New NO-donors with antithrombotic and vasodilating activities, V: Oligonitroso sydnone imines.

Nine 4,4'-bis- and four 4,4'-tris-N-nitroso syndrome imines were synthesized. The sydnone imine moiety is connected either by aromatic 1,3-phenylene or 1,3,5-benzene or aliphatic methylene or propylene bridges. Compared to the corresponding sydnone imines the collagen induced platelet aggregation inhibiting activity is increased by several orders of magnitude by the nitroso derivatives. The most potent compound bears a hexyl substituent in 3-position (1d: IC50 = 0.05 mumol/L). These data show that aromatic bridges (1d, 2d) are more favourable than aliphatic ones (4b). This indicates the mutual influence of the nitroso-imino moieties via the aromatic bridges. In the series of 3,3'-bis-nitrososydnone imines (13 compounds) mostly additive effects of the nitroso groups are seen. The activities range from IC50 = 0.2 mumol/L (5i; 1,3-xylene bridge) to IC50 = 8 mumol/L (5b; trimethylene bridge). The differences suggest different affinities to the platelet membrane.

Animals↗
PubMed2003

Useful dual Diels-Alder behavior of 2-azetidinone-tethered aryl imines as azadienophiles or azadienes: a beta-lactam-based stereocontrolled access to optically pure highly functionalized indolizidine systems.

Imines derived from 4-oxoazetidine-2-carbaldehydes have been found to be versatile Diels-Alder reagents in that they exhibit two reactivity patterns. 2-Azetidinone-tethered imines undergo diastereoselective reaction with Danishefsky's diene in the presence of different Lewis acids. The effect of the amount of catalyst on the conversion rate as well as on the product ratio has been studied. Under standard reaction conditions, indium(III) chloride and zinc(II) iodide provided the best yields, and indium(III) triflate the highest diastereoselectivity in the Lewis acid promoted aza-Diels-Alder cycloaddition. Treatment of the aforementioned imines with cyclopentadiene, 2,3-dimethyl-1,3-butadiene or 3,4-dihydro-2 H-pyran led to cycloadducts arising from inverse electron-demand condensation involving the beta-lactam-tethered aryl imine as the heterodiene component. In addition, the first methodology for preparing indolizidines from beta-lactams has been developed. This process involves amide bond cleavage of the beta-lactam ring in the aza-Diels-Alder cycloadducts with concomitant cyclization. Full chirality transfer occurs when the reaction is performed with an enantiomerically pure substrate.

Journal Article↗
PubMed2003

Highly diastereoselective additions to polyhydroxylated pyrrolidine cyclic imines: ready elaboration of aza-sugar scaffolds to create diverse carbohydrate-processing enzyme probes.

Representative diastereomeric, erythritol and threitol polyhydroxylated pyrrolidine imine scaffolds have been rapidly elaborated to diversely functionalized aza-sugars through highly diastereoselective organometallic (RM) additions (R=Me, Et, allyl, hexenyl, Ph, Bn, pMeO-Bn). The yields for these additions have all been substantially enhanced from previously optimised levels (<58 %) for normal additions using a reverse addition procedure (e.g. R=Ph; 44 % normal mode --> 78 % reverse mode). The high diastereoselectivities (>98 % de for all except R=Me) are consistent with additions that are controlled by the configuration of the C-2 centre adjacent to the azomethine imine carbon and the conformation of the pyrrolidine imine. The high potential of this method was demonstrated by concise syntheses of 1-epi- and 2-epi-desacetylanisomycins. In addition, the late stage addition of hydrophobic substituents, which this imine addition methodology allows, enabled the preparation of novel aza-sugars with enhanced inhibitory potential. This was highlighted by the screening of a representative selection of these "hydrophobically-modified" aza-sugars against a diverse panel of 12 non-mammalian and human carbohydrate-processing enzymes. This identified a novel nanomolar alpha-galactosidase inhibitor (IC(50)=250 nM) and a novel highly selective glucosylceramide synthase inhibitor (IC(50)=52 microM, no alpha-glucosidase inhibition at 1 mM). Furthermore, analysis of the structure-activity relationships of racemic series of inhibitors allowed some validation of Fleet's mirror-image enzyme active site postulate.

Journal Article↗
PubMed2006

Mechanistic investigations of imine hydrogenation catalyzed by cationic iridium complexes.

Complexes [IrH2(eta6-C6H6)(PiPr3)]BF4 (1) and [IrH2(NCMe)3(PiPr3)]BF4 (2) are catalyst precursors for homogeneous hydrogenation of N-benzylideneaniline under mild conditions. Precursor 1 generates the resting state [IrH2{eta5-(C6H5)NHCH2Ph}(PiPr3)]BF4 (3), while 2 gives rise to a mixture of [IrH{PhN=CH(C6H4)-kappaN,C}(NCMe)2(PiPr3)]BF4 (4) and [IrH{PhN=CH(C6H4)-kappaN,C}(NCMe)(NH2Ph)(PiPr3)]BF4 (5), in which the aniline ligand is derived from hydrolysis of the imine. The less hindered benzophenone imine forms the catalytically inactive, doubly cyclometalated compound [Ir{HN=CPh(C6H4)-kappaN,C}2(NH2CHPh2)(PiPr3)]BF4 (6). Hydrogenations with precursor 1 are fast and their reaction profiles are strongly dependent on solvent, concentrations, and temperature. Significant induction periods, minimized by addition of the amine hydrogenation product, are commonly observed. The catalytic rate law (THF) is rate = k[1][PhN=CHPh]p(H2). The results of selected stoichiometric reactions of potential catalytic intermediates exclude participation of the cyclometalated compounds [IrH{PhN=CH(C6H4)-kappaN,C}(S)2(PiPr3)]BF4 [S = acetonitrile (4), [D6]acetone (7), [D4]methanol (8)] in catalysis. Reactions between resting state 3 and D2 reveal a selective sequence of deuterium incorporation into the complex which is accelerated by the amine product. Hydrogen bonding among the components of the catalytic reaction was examined by MP2 calculations on model compounds. The calculations allow formulation of an ionic, outer-sphere, bifunctional hydrogenation mechanism comprising 1) amine-assisted oxidative addition of H2 to 3, the result of which is equivalent to heterolytic splitting of dihydrogen, 2) replacement of a hydrogen-bonded amine by imine, and 3) simultaneous H delta+/H delta- transfer to the imine substrate from the NH moiety of an arene-coordinated amine ligand and the metal, respectively.

Journal Article↗
PubMed2003

Iminophosphorane mediated imine metathesis.

The iminophosphorane Cl(3)P[double bond]NAr (1a, Ar = 2-fluorophenyl) reacts metathetically with imines at 80 degrees C to produce [double bond]NR exchange products. Compound 1a effectively catalyzes imine/imine and imine/carbodiimide cross-metathesis. The observation of [double bond]NR exchange products as well as spectroscopic evidence for the existence of diazaphosphetidine type intermediates suggests that a [2 + 2] addition/elimination mechanism is the primary pathway for substrates with N-alkyl substituents and a secondary pathway for N-aryl imines. In contrast to previously studied carbodiimide systems, the resting state of the catalyst is the iminophosphorane and not the diazaphosphetidine. For N-aryl imines, Lewis-acid catalysis appears to be the dominant mechanism, not addition/elimination. For N-alkyl imines, a decomposition pathway, involving HCl elimination from a phosphorus intermediate, is competitive in some cases.

Journal Article↗
PubMed2004

Insertion reactions of trans-Mo(dmpe)2(H)(NO) with imines.

The insertion chemistry of the hydride complex trans-Mo(dmpe)(2)(H)(NO) (1) (dmpe = bis(dimethylphosphino)ethane) with imines has been investigated. It was found that disubstituted aromatic imines RCH[double bond]NR' (R, R' = Ar) insert into the Mo-H bond of 1, while a series of various mono- and other disubstituted imines do not react. The insertion products trans-Mo(dmpe)(2)(NO)[NR'(CH(2)R)] (R = R' = Ph (2); R = Cp(2)Fe, R' = Ph (3); R = Ph, R' = Cp(2)Fe (4); R = 1-naphthyl, R' = Ph (5)) have been isolated and fully characterized by elemental analysis, IR and NMR spectroscopy, and mass spectrometry. The imine PhCH[double bond]NC(10)H(7) (C(10)H(7) = 1-naphthyl) reacted with 1 establishing an equilibrium to produce the nonisolable complex trans-Mo(dmpe)(2)(NO)[NC(10)H(7)(CH(2)Ph)] (6). The equilibrium constant for this reaction has been derived from VT-NMR measurements, and the Delta H and Delta S values of this reaction were calculated to be -48.8 +/- 0.4 kJ.mol(-1) and -33 +/- 1 J.K(-1).mol(-1) reflecting a mild exothermic process and its associative nature. Single-crystal X-ray diffraction analyses were carried out on 2-5.

Journal Article↗
PubMed1996

A Bis(azo-imine)palladium(II) System with 10 Ligand pi Electrons. Synthesis, Structure, Serial Redox, and Relationship to Bis(azooximates) and Other Species.

The first azo-imine chelate system, Pd(N(H)C(R)NNPh)(2) (Pd(RA)(2)), has been isolated in the form of diamagnetic solids by the 6e(-)-6H(+) reduction of bis(phenylazooximato)palladium(II), Pd(N(O)C(R)NNPh)(2) (abbreviated Pd(RB)(2)), with ascorbic acid in a mixed solvent (R = Ph, alpha-naphthyl). Selected spectral features are described. The X-ray structures of Pd(PhA)(2) and Pd(PhB)(2) have revealed trans-planar geometry consistent with metal oxidation state of +2. Bond length trends within the chelate rings are rationalized in terms of steric and electronic factors. In Pd(PhA)(2) a total of 10 ligand pi electrons are present, each formally monoanionic ligand contributing five. Model EHMO studies have revealed that the filled HOMO (a(u)) in Pd(RA)(2) is a bonding combination of two ligand pi orbitals with large azo contributions. The LUMO (b(g)) is roughly the corresponding antibonding combination. The outer pi-electron configuration of Pd(RA)(2) is (a(u))(2)(b(g))(0). Four successive voltammetric responses, two oxidative and two reductive, are observed. The E(1/2) range is -1.3 to +0.8 V vs SCE for Pd(PhA)(2) in a 1:9 MeCN-CH(2)Cl(2) mixture (Pt electrode). EPR and electronic spectra of the electrogenerated one-electron-oxidized complex Pd(PhA)(2)(+) are described. The azo-imine system is compared with imine-imine and azo-azo systems. Crystal data for the complexes are as follows. Pd(PhA)(2): crystal system monoclinic; space group C2/c; a = 18.167(5) Å, b = 7.420(3) Å, c = 16.527(6) Å; beta = 92.70(3) degrees; V = 2225(1) Å(3); Z = 4; R = 2.61%, R(w) = 3.58%. Pd(PhB)(2): crystal system monoclinic; space group P2(1)/n; a = 5.735(5) Å, b = 10.797(6) Å, c = 18.022(11) Å; beta = 97.73(6) Å; V = 1105(1) Å(3); Z = 2; R = 3.37%; R(w) = 3.40%.

Journal Article↗
PubMed1997

Migratory Transmetalation in Diphenoxo-Bridged Cu(II)M(II) Complexes of a Dinucleating Macrocycle with N(amine)(2)O(2) and N(imine)(2)O(2) Metal-Binding Sites.

A phenol-based heterodinucleating macrocycle (H(2)L), comprised of two 2-((methylamino)methyl)-6-(iminomethyl)-4-bromophenolate entities bridged by the -(CH(2))(2)- groups between amine nitrogens and by the -(CH(2))(3)- groups between imine nitrogens, displays dissimilar N(amine)(2)O(2) and N(imine)(2)O(2) metal-binding sites sharing the phenolic oxygens and is prepared as a Cu(II)Pb(II) complex [CuPb(L)](ClO(4))(2). An analogous complex [CuPb(L)(BzO)(dmf)]ClO(4) crystallizes in the triclinic space group P&onemacr;, (No. 2), with a = 13.998(2) Å, b = 15.568(2) Å, c = 8.699(2) Å, alpha = 95.32(1) degrees, beta = 92.23(1) degrees, gamma = 82.81(1) degrees, V = 1871.9(5) Å(3), and Z = 2. Refinements based on 3788 reflections with I > 3.00sigma(I) converged with R = 0.058 and R(w) = 0.069. The Cu(II) resides at the N(imine)(2)O(2) site and assumes a planar geometry. The Pb(II) resides at the N(amine)(2)O(2) site and assumes a seven-coordinate geometry by further addition of a dmf molecule and a bidentate benzoate group. The Cu--Pb separation, doubly bridged by the phenolic oxygens, is 3.466(2) Å. The reaction of [CuPb(L)](ClO(4))(2) with metal(II) sulfate salts provides Cu(II)M(II) complexes [CuM(L)](ClO(4))(2).nH(2)O (M = Mn (n = 2), Co (n = 2), Ni (n = 0), Cu (n = 0), Zn (n = 1)). The CuZn complex, [CuZn(L)(AcO)]ClO(4), crystallizes in the triclinic space group P&onemacr;, (No. 2), with a = 12.290(3) Å, b = 13.402(4) Å, c = 11.501(2) Å, alpha = 95.10(2) degrees, beta = 116.68(2) degrees, gamma = 112.00(2) degrees, V = 1491.8(9) Å(3), and Z = 2. Refinements based on 2497 reflections with I > 3.00sigma(I) converge with R = 0.046 and R(w) = 0.034. The Cu(II) is bound at the N(amine)(2)O(2) site and the Zn(II) is bound at the N(imine)(2)O(2) site with a Cu--Zn separation of 2.942(2) Å. Further, the acetate group bridges the two metal ions providing a five-coordinate geometry about both metal ions. The CuM complexes except for the CuZn complex show significant antiferromagnetic spin-exchange within each dinuclear unit. The CuNi complex shows the ESR spectrum of the spin-doublet ground state which demonstrates the delocalization of the unpaired electron over the CuNi core.

Journal Article↗
PubMed2003

Syndiospecific living propylene polymerization catalyzed by titanium complexes having fluorine-containing phenoxy-imine chelate ligands.

The propylene polymerization behavior of a series of Ti complexes featuring fluorine-containing phenoxy-imine chelate ligands is reported. The Ti complexes combined with methylalumoxane (MAO) can be catalysts for living and, at the same time, stereospecific polymerization of propylene at room temperature or above. DFT calculations suggest that the attractive interaction between a fluorine ortho to the imine nitrogen and a beta-hydrogen of a growing polymer chain is responsible for the achievement of room-temperature living propylene polymerization. Although the Ti complexes possess C(2) symmetry, they are capable of producing highly syndiotactic polypropylenes. (13)C NMR is used to demonstrate that the syndiotacticity is governed by a chain-end control mechanism and that the polymerization is initiated exclusively via 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization. (13)C NMR spectroscopy also elucidated that the polypropylenes produced with the Ti complexes possess regio-block structures. Substitutions on the phenoxy-imine ligands have profound effects on catalytic behavior of the Ti complexes. The steric bulk of the substituent ortho to the phenoxy oxygen plays a decisive role in achieving high syndioselectivity for the chain-end controlled polymerization. Over a temperature range of 0-50 degrees C, Ti complex having a trimethylsilyl group ortho to the phenoxy oxygen forms highly syndiotactic, nearly monodisperse polypropylenes (94-90% rr) with extremely high peak melting temperatures (T(m) = 156-149 degrees C). The polymerization behavior of the Ti complexes can be explained well by the recently proposed site-inversion mechanism for the formation of syndiotactic polypropylene by a Ti complex having a pair of fluorine-containing phenoxy-imine ligands.

Journal Article↗
PubMed2002

Ruthenium-catalyzed reaction of alpha,beta-unsaturated imines with carbon monoxide and alkenes leading to beta,gamma-unsaturated gamma-butyrolactams: involvement of direct carbonylation at olefinic C[bond]H Bonds as a key step.

The reaction of alpha,beta-unsaturated imines with CO and alkenes in the presence of Ru(3)(CO)(12) as a catalyst results in a three-component coupling reaction that gives alpha,alpha-disubstituted beta,gamma-unsaturated gamma-butyrolactams. The reaction proceeds via a two-step sequence involving the initial formation of ketone derivatives by catalytic carbonylation at the beta-olefinic C-H bonds of alpha,beta-unsaturated imines, followed by the (uncatalyzed) intramolecular nucleophilic attack of the imine nitrogen on the ketonic carbon to generate a tetrahedral intermediate, which then undergoes a 1,2-ethyl migration. The reaction of a cyclic unsaturated imine, derived from the reaction of (1R)-(-)-myrtenal with tert-butylamine, gives a beta-aminocyclopentene derivative, which is formed by an aldol-type condensation of the initially formed ketone, indicating the initial formation of ethyl ketone.

Journal Article↗
PubMed2005

Efficient catalytic effects of Lewis acids in the 1,3-dipolar cycloaddition reactions of carbonyl ylides with imines.

[reactions: see text] 1,3-Dipolar cycloaddition reactions between imines and carbonyl ylides generated by tandem intramolecular carbenoid-carbonyl cyclizations were found to be effectively catalyzed by Lewis acids (10 mol %). The Rh2(OAc)4-catalyzed reactions of o-(methoxycarbonyl)-alpha-diazoacetophenone with imines such as N-[2-(benzyloxy)benzylidene]aniline in the absence of Lewis acid gave no 1,3-dipolar cycloaddition products, but rather the dimeric product of the corresponding carbonyl ylide. In contrast, in the presence of Lewis acids such as Yb(OTf)3, the 1,3-dipolar cycloaddition reactions of the corresponding 1-methoxy-2-benzopyrylium-4-olate proceeded smoothly with several imines, giving in most cases exo-selectivity and no formation of the dimeric product. When Yb(OTf)3 was used as a Lewis acid catalyst, a fundamental catalytic effect was also observed in the cycloaddition reactions of imines with carbonyl ylides generated from 1-diazo-5-phenyl-2,5-pentanedione, 1-diazo-2,5-hexanedione and diazomethyl 2,3,4,5-tetrachloro-6-methoxycarbonylphenly ketone. This efficient catalytic effect can be satisfactorily explained in terms of energetics of the cycloaddition in the absence and the presence of Lewis acid by calculations using the ONIOM (B3LYP/6-31G(d):PM3) method.

Journal Article↗