Organoaluminum Complexes Research Articles

Synthesis of binuclear phenoxyimino organoaluminum complexes and their use as the catalyst precursors for efficient ring-opening polymerisation of ε-caprolactone

A series of novel binuclear phenoxyimino organoaluminum complexes of the type [(RN=CH)C6H3OAlMe2]2 [R = C6H5 (2a), 2,6-(i)Pr2C6H3 (2b), 2,6-Ph2C6H3 (2c), adamantyl (2d), (t)Bu (2e)] have been prepared in high yields, and these complexes were identified by (1)H, (13)C NMR and elemental analysis. Structural analysis for 2a-e revealed that these complexes have a distorted tetrahedral geometry around Al and both the Al-O and the Al-N bond distances were considerably influenced by substituents in the imino groups. The complexes were tested as catalyst precursors for ring-opening polymerisation (ROP) of ε-caprolactone (CL) in the presence of BnOH, and their catalytic activities were strongly affected by the catalyst structures and polymerisation conditions. An efficient living ROP has been achieved using the 2b/BnOH system.

Structural diversity and versatility for organoaluminum complexes supported by mono- and di-anionic aminophenolate bidentate ligands

The present contribution describes the synthesis and structural characterization of structurally diverse organoaluminum species supported by variously substituted aminophenolate-type ligands: these Al complexes are all derived from the reaction of AlMe 3 with aminophenols 2-CH 2NH(R)-C 6H 3OH ( 1a, R = mesityl (Mes); 1b, R = 2,6-di-isopropylphenyl (Diip)) and 2-CH 2NH(R)-4,6- t Bu 2-C 6H 2OH ( 1c, R = Mes; 1d, R = Diip). The low temperature reaction of AlMe 3 with 1a– b readily affords the corresponding Al dimeric species [μ-η 1,η 1-N,O-{2-CH 2NH(R)-C 6H 4O}] 2Al 2Me 4 ( 2a– b), consisting of twelve-membered ring aluminacycles with two μ-η 1,η 1- N,O-aminophenolate units, as determined by X-ray crystallographic studies. Heating a toluene solution of 2a (80 °C, 3 h) affords the quantitative and direct formation of the dinuclear aluminium complex Al[η 2-N; μ,η 2-O-{2-CH 2N(Mes)-C 6H 4O}](AlMe 2) ( 4a) while species 2b, under the aforementioned conditions, affords the formation of the Al dimeric species [η 2-N,O-{2-CH 2N(Dipp)-C 6H 4O}AlMe] 2 ( 3b), as deduced from X-ray crystallography for both 3b and 4a. In contrast, the reaction of bulky aminophenol pro-ligands 1c– d with AlMe 3 afford the corresponding monomeric Al aminophenolate chelate complexes η 2- N,O-{2-CH 2NH(R)-4,6- t Bu 2-C 6H 2O}AlMe 2 ( 5c– d; R = Mes, Diip; Scheme 3) as confirmed by X-ray crystallographic analysis in the case of 5d. Subsequent heating of species 5c– d yields, via a methane elimination route, the corresponding Al-THF amido species η 2- N,O-{2-CH 2N(R)-4,6- t Bu 2-C 6H 2O}Al(Me)(THF) ( 6c– d; R = Mes, Diip). Compounds 6c– 6d, which are of the type {X 2}Al(R)(L) (L labile), may well be useful as novel well-defined Lewis acid species of potential use for various chemical transformations. Overall, the sterics of the aminophenol backbone and, to a lesser extent, the reaction conditions that are used for a given ligand/AlMe 3 set essentially govern the rather diverse “structural” outcome in these reactions, with a preference toward the formation of mononuclear Al species (i.e. species 5c– d and 6c– d) as the steric demand of the chelating N,O-ligand increases.

Aluminum speciation in the bulk and rhizospheric soil solution of the species colonizing an abandoned copper mine in Galicia (NW Spain)

The present study was carried out to identify and quantify the aluminum species present in the bulk and rhizospheric soil solution of the spontaneous vegetation colonizing the dump (Calluna vulgaris, Erica cinerea) and slope (C. vulgaris, E. cinerea, Salix atrocinerea) of an abandoned Cu mine in Touro (Galicia, NW Spain). Total dissolved aluminum was speciated into reactive Al (Alr) and acid-soluble Al (Alsa). Acid-soluble aluminum comprises colloids, polymers and/or organo-aluminum complexes, whereas Alr comprises non-labile monomeric aluminum (AlnL) and labile monomeric aluminum (AlL). The latter fraction included Al3+, Al–OH complexes, Al–F complexes, and Al–SO4 complexes. The results show that Ericaceae modified the pH of the rhizosphere from 6.6 to 4.0 (C. vulgaris) in the slope, whereas S. atrocinerea only grew in close-to-neutral soil conditions and increased the rhizosphere pH by 0.8 units. The total concentration of Al in the soil solution was low ( 40% of the AlL) and in the C. vulgaris rhizosphere in the slope (>52% of the AlL); the Al–F complexes were more abundant in the E. cinerea rhizosphere in the slope (>40% of the AlL), and Al–OH complexes predominated in all samples associated with S. atrocinerea (>70%). In the mine dump, the Ca/AlL ratio changed from 0.07 (bulk soil) to higher than 1.4 in the root zone of the Ericaceae thus decreasing the risk of toxicity by this element. Labile aluminum is the prevailing fraction in all dump samples and in the rhizosphere of Ericaceae growing on the slope. Al3+ is the predominant species in the most acidic samples with the highest concentration of labile Al and the highest AlL/F ratio. In areas where there may be a risk of Al toxicity, Ericaceae accumulated Ca in the vicinity of the roots, thus increasing the Ca/AlL ratio relative to that in the bulk soil and preventing absorption of Al and the associated phytotoxicity.

ChemInform Abstract: Asymmetric Cyanosilylation of Aldehydes Promoted by a Novel Chiral Lewis Acid, Organoaluminum Complex of Acyclic Dipeptide Esters or . alpha.-Amino Amides Containing a Phenolic Schiff Base.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Synthesis and Structural Characterization of Five-Coordinate Aluminum Complexes Containing Diarylamido Diphosphine Ligands

A series of five-coordinate aluminum complexes supported by o-phenylene-derived amido diphosphine ligands, [N(o-C(6)H(4)PR(2))(2)](-) ([R-PNP](-); R = Ph, (i)Pr) and [N(o-C(6)H(4)PPh(2))(o-C(6)H(4)P(i)Pr(2))](-) ([Ph-PNP-(i)Pr](-)), have been prepared and structurally characterized. Alkane elimination reactions of trialkylaluminum with H[Ph-PNP] (1a), H[(i)Pr-PNP] (1b), and H[Ph-PNP-(i)Pr] (1c) in toluene at -35 degrees C respectively produced the corresponding dialkyl complexes [Ph-PNP]AlR(2), [(i)Pr-PNP]AlR(2), and [Ph-PNP-(i)Pr]AlR(2) (R = Me (2a-c), Et (3a-c), (i)Bu (4a-c)) in high isolated yield. The dihydride complexes [Ph-PNP]AlH(2) (6a), [(i)Pr- PNP]AlH(2) (6b), and [Ph-PNP-(i)Pr]AlH(2) (6c) were prepared in one-pot reactions of in situ prepared dichloride precursors (5a-c) with LiAlH(4) in THF at room temperature. X-ray diffraction studies of 2a-c, 3b-c, 5b, and 6b revealed a distorted trigonal-bipyramidal structure for these molecules in which the two phosphorus donors are mutually trans. The solution structures of these organoaluminum complexes were all characterized by (1)H, (13)C, and (31)P NMR spectroscopy. The NMR data are indicative of solution C(2) symmetry for [Ph-PNP](-) and [(i)Pr-PNP](-) complexes, whereas they are indicative of C(1) for [Ph-PNP-(i)Pr](-) derivatives. The (1)H NMR spectra of 3a-c and 4a-c revealed diastereotopy for the alpha-hydrogen atoms in these molecules.

Regio- and Stereoselective Ferrier Reaction of O-1,3-Dienyl Acetals Promoted by Organoaluminum Complexes

The Ferrier reaction of O-1,3-dienyl acetals promoted by organoaluminum complexes such as methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD) is shown to proceed with a high degree of regio- and stereoselectivity to afford the corresponding alpha-alkenyl-substituted beta-alkoxy aldehydes in good yields. The mechanistic origin of the high regiocontrolling ability of MAD is elucidated. This method, coupled with the easy availability of the requisite substrates, expands the synthetic scope of the Ferrier reaction. [reaction: see text].

Efficient Asymmetric Catalysis of Chiral Organoaluminum Complex for Enantioselective Ene Reactions of Aldehydes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Efficient asymmetric catalysis of chiral organoaluminum complex for enantioselective ene reactions of aldehydes

Chiral organoaluminum complex 1 efficiently catalyzed the asymmetric hetero-ene reaction of commercially available 2-methoxypropene (2) with aldehydes under mild conditions to give the corresponding β-hydroxymethyl ketones 3 in good to excellent chemical yields with high enantiomeric excesses. The asymmetric catalysis of 1 was further applied to the carbonyl addition of methallylsilanes, where exclusive formation of the optically active allylic silanes 5 was achieved.

Efficient asymmetric catalysis of chiral organoaluminum complex for enantioselective ene reactions of aldehydes

Chiral organoaluminum complex 1 efficiently catalyzed the asymmetric hetero-ene reaction of commercially available 2-methoxypropene ( 2 ) with aldehydes under mild conditions to give the corresponding β-hydroxymethyl ketones 3 in good to excellent chemical yields with high enantiomeric excesses. The asymmetric catalysis of 1 was further applied to the carbonyl addition of methallylsilanes, where exclusive formation of the optically active allylic silanes 5 was achieved.

Design of Multinuclear Chiral Organoaluminum Complexes with (R)-Binaphthol Derivatives

Design of Multinuclear Chiral Organoaluminum Complexes with (<i>R</i>)-Binaphthol Derivatives

Synthesis and Characterization of Organoaluminum Complexes Containing Bi- or Tridentate-Substituted Pyrrole Ligands

Reactions of AlCl3 with 1 equiv of Li{C4H2N(CH2NMe2)2-2,5} or 2 equiv of Li{C4H3N(CH2NMe2)-2} in diethyl ether at −78 °C afforded AlCl2{C4H2N(CH2NMe2)2-2,5} (1) and AlCl{C4H3N(CH2NMe2)-2} (2), respectively. Alkylation of 1 with MeLi in diethyl ether generates the aluminum dimethyl complex 3. Compound 2 is thermally unstable and was converted to uncharacterized product in CHCl3 within 2 days at 80 °C. Compounds 1, 2, and 3 were characterized by NMR spectroscopy and X-ray structure determination.

Electroluminescence of Organoaluminum Complexes in the Presence of Silica Gel

It was demonstrated that light-emitting materials can be produced by the photochemical dissolution of aluminum in composites based on a cresol–formaldehyde novolak resin and CBr4in the presence of triphenylmethane dyes such as aluminon, pyrocatechol violet, and malachite green. The electroluminescence (EL) is due to the formation of organoaluminum complexes. The SnO2(anode)/novolak resin + pyrocatechol violet/Al (cathode) diodes exhibited the highest EL intensity. Compositions based on aluminon are characterized by weak EL, which increased by a factor of 30 upon the additional introduction of silica gel into the composition. As a result of supporting a PVA film on the diode surface (on the cathode side), the EL increased by a factor of 1.5 and became as high as ∼10 cd/m2. This value is comparable to the EL of pyrocatechol violet–containing diodes. The possible mechanism of the influence of silica gel and the importance of a PVA film for increasing the EL intensity are discussed. In the aluminon-based compositions, the dependence of EL on the diode storage time both with and without an applied voltage was measured.

Photochemical production of electroluminescent image

A polymer containing donorN-epoxypropylcarbazolyl groups and the acceptor tris(8-quinolinolato) aluminum (Alq3) was shown to form a light-sensitive, acid-producing polymeric composition. The acid in the presence of 8-quinolinol (8-hydroxyquinoline) ensured photochemical dissolution of aluminum in the polyeric layer, yielding additionally organic aluminum complexes, such as Alq3, Alq2+, and Alq 2 + , and Alq3-con-taining polymers. This led to a emergence of electroluminescence (EL) upon application of a potential difference to a sandwich diode representing transparent anode/exposed polymer layer/aluminum cathode. Electroluminescence did not appear at an Alq3 concentration of 6 wt % and less in the composition, and only partial dissolution of the aluminum film after its deposition on the exposed polymeric layer provided the emergence of EL whose intensity increased with increasing the dose absorbed by the polymer. The addition of the chemical sensitizer dimethylaminobenzaldehyde to the composition triggered the conventional process of chemical amplification which consists in thermal buildup of the acid in exposed area. This provided a significant increase in concentration of organoaluminum complexes and enhancement of EL intensity.

Polymerization of t-butyl acrylate using organoaluminum complexes and correlation between main-chain tacticity and glass transition temperature of the obtained polymers

The polymerization of tert-butyl acrylate (tBA) was performed with triethylaluminum-(−)-sparteine, triethylaluminum–tertiary phosphine, triethylaluminum–n-BuLi and bis(2,6-di-tert-butyl-4-methylphenoxide)methylaluminum–t-BuLi complexes in toluene. The poly(tBA)s obtained with these initiators were rich in syndiotactic diad as observed by 1H NMR analysis. On the basis of the triad and tetrad stereochemical information of the poly(tBA)s obtained using triethylaluminum-(−)-sparteine and triethylaluminum–n-BuLi, the polymerization reactions were found to obey Bernoullian statistics. Glass transition temperatures of the poly(tBA)s were measured by differential scanning calorimetry (DSC). A syndiotactic poly(tBA) (racemo (r) diad content: 65%) and an isotactic one (r=19%) exhibited higher glass transition temperature (50 and 43°C, respectively) than an atactic one (r=50%,Tg=21°C).

Screening of Optically Active Nickel Initiators for Enantioasymmetric Polymerization of γ-Benzyl Glutamate-N-Carboxyanhydride

The incorporation of only L-amino acids into DNAencoded proteins imparts a great biological significance to the chirality of amino acids. This importance also translates to compounds synthesized from amino acid components. As with all chiral materials, it is typically more difficult to synthesize amino acids as pure enantiomers rather than as racemic mixtures. One method of utilizing the racemate is to kinetically resolve it into optically pure product and unreacted enantiomer.1 We have explored the polymerization of R-amino acid-Ncarboxyanhydrides (NCAs) to form block copolypeptides for applications as biomedical materials.2 In such materials, the absolute configuration of the amino acid monomers is crucial for both structural development and biological activity.3 To utilize synthetic, racemic amino acids in these materials, we pursued the use of optically active initiators to develop enantioasymmetric polymerizations. We now report the use of chiral pyridinyl oxazoline ligands in nickel initiators to enantioasymmetrically polymerize γ-benzyl-glutamate NCA. Enantioasymmetric ring-opening polymerizations have been developed for many chiral cyclic monomers such as epoxides and episulfides.4 There also have been attempts to prepare optically active polypeptides from racemic NCA mixtures. Interest in this area has arisen from the pharmaceutical value of the optically active products as well as possible relevance to the origins of handedness in biological macromolecules.5 Some of the different chiral initiators used to induce asymmetry in NCA polymerizations include optically active amines6 and polypeptides,7 organoaluminum complexes,8 and chiral nickel carboxylates.9 These initiators generally displayed moderate-to-poor efficiencies and gave complex polymerizations whose propagation steps were poorly understood. All systems gave broad molecular weight distributions and poor control of polymer molecular weights and displayed reasonable enantioselectivities only at very low conversions of monomer. We have developed nickel initiators that allow the controlled polymerization of NCAs.2 Using this methodology, polypeptides can be prepared with defined chain lengths and with narrow molecular weight distributions. A key feature of this system is that all polymer formation occurs with identical nickel-containing reactive species. The straightforward nature of this system provides a superior starting point for the development of an enantioasymmetric polymerization system compared to previous methods used to polymerize NCAs. In particular, asymmetric initiators can be prepared by direct substitution of the achiral 2′,2′bipyridyl (bpy) ligand in our nickel system with a suitable optically active ligand (Figure 1). The choice of a suitable chiral ligand required some consideration. Most common chiral ligands are bidentate, since the chelate structure provides both a rigid environment as well as asymmetry around the metal.1 A large number of chiral chelating ligands are based on bisarylphosphines,10 which unfortunately form ineffective initiators when complexed with nickel due to their poor donating ability, as we have previously shown for PPh3. For this reason, we focused our attention on N-donor ligands that are similar in character to bpy, which forms a very efficient initiator.11 We initially explored diimine and bisoxazoline ligands (e.g., 1-4) (Figure 2), which have been used extensively in metalmediated asymmetric aziridinations,12 Diels-Alder reactions,13 and olefin polymerizations.14 These ligands were found to bind only weakly to zerovalent nickel and were thus also generally ineffective in generating active NCA polymerization initiators. To increase coordinating ability, we decided to study hybrid ligands consisting of pyridine coupled with either a chiral imine or oxazoline group (Figure 3). To test the efficiency of such ligands, we prepared achiral 5 and evaluated its ability to form an active initiator. When 5 was mixed with Ni(COD)2 in tetrahydrofuran (THF), a stable blue complex, 5Ni(COD), was formed. This compound displayed NCA polymerization activity that was virtually identical to that of bpyNiFigure 1. Strategy for formation of chiral nickel NCA polymerization initiators.

Kaolin cements in limestones; potential indicators of organic-rich pore waters during diagenesis

ABSTRACT Kaolin (dickite and/or kaolinite) cement occurs locally in the Chalk Group (Upper Cretaceous-Lower Tertiary) in the North Sea and the Derbyhaven Beds (Lower Carboniferous) on the Isle of Man, United Kingdom. Stable-isotope and petrographic data indicate that the kaolin cements in both the Chalk Group and Derbyhaven Beds precipitated during late diagenesis in either marine or modified marine pore waters. Kaolin cements in both formations are associated with hydrocarbons, which suggests that organic complexing of aluminum may be responsible for precipitation of kaolin cement. Published experimental studies have demonstrated that the mobility of aluminum is greatly increased by complexing with organic acids. In such systems aluminum would be released and made available for clay-mineral precipitation when the organo-aluminum complexes are destabilized. Increased whole-rock 13C values in Chalk Group limestones from the Eldfisk Field, where kaolin cement was widespread, provides corroborating evidence for the late diagenetic decarboxylation of organic acids, which would have released complexed aluminum. The presence of kaolin cement may thus be an indicator of the former presence of organic-rich pore fluids in limestones and provide an additional tool for determination of the pore-water history of limestones and sedimentary basins.

An unusual reaction of Me 3Al with the macrobicyclic ligand C 22H 28N 2O 3 (C 22H 28N 2O 31,15-diaza-3,4:12,13-dibenzo-5,8,11-trioxabicyclo[13,3,1]nonadecane) : Synthesis and crystal structure of a novel μ3-oxygen-bridged tetranuclear organoaluminum complex (Me 3Al)(C 22H 28N 2O 3)(AlMe 2) 2(AlMe 3)( μ3-O)

The reaction of AlMe 3 with the macrobicyclic ligand 1,15-diaza-3,4:12,13-dibenzo-5,8,11-trioxabicyclo[13,3,1]nonadecane (C 22H 28N 2O 3) in a 4:1 ratio in the presence of a trace of water resulted in the formation of a novel tetranuclear organoaluminum complex (Me 3Al)(C 22H 28N 2O 3)(AlMe 2) 2(AlMe 3)( μ 3-O) in a nearly quantitative yield. The complex has been characterized by 1H-NMR, EI mass spectra and elemental analyses. A single crystal X-ray diffraction study reveals a μ 3-oxygen atom bridging three different alkyl aluminum centers in a trigonal planar geometry, and the fourth aluminum center is bonded to the ether oxygen of the macrobicycle with a dative bond length.

Organoaluminum-promoted selective addition to fluorinated carbonyl compounds via pentacoordinate trialkylaluminum complexes

A new organoaluminum-promoted selective alkylation with several nucleophiles has been developed which involves the chelation-controlled addition to fluoro carbonyl compounds with trialkylaluminums via pentacoordinate organoaluminum complexes.

Novel Oxo-Bridged Blue Luminescent Organoaluminum Complexes: Al4(CH3)6(μ3-O)2(dpa)2 and Al3(7-azain)4(OCH(CF3)2)2(CH3)(μ3-O) (dpa = Deprotonated Di-2-pyridylamine, 7-azain = Deprotonated 7-Azaindole)

Two novel blue luminescent organoaluminum complexes Al4(CH3)6(μ3-O)2(dpa)2 (1) and Al3(7-azain)4(OCH(CF3)2)2(CH3)(μ3-O) (2) (dpa = deprotonated di-2-pyridylamine, 7-azain = deprotonated 7-azaindole) have been synthesized and characterized structurally. The unusual stability of these compounds is attributed to the presence of the triply-bridging oxo ligand.

Structure and Stability of Aluminum Alkyl Cocatalysts in Ziegler−Natta Catalysis

The structure of organoaluminum(halide) complexes, used as cocatalysts in Ziegler−Natta catalysis, is investigated by experimental far-infrared spectroscopy and by quantum simulations. Experimental...