Alkoxide Species Research Articles

Controlled Postgrafting of Titanium Chelates for Improved Synthesis of Ti-SBA-15 Epoxidation Catalysts

A new synthesis procedure that is based on the grafting of titanium alkoxide species chemically modified by acetylacetone (acac) on the surface of a P123/SBA-15 composite material is proposed to prepare Ti-SBA-15 catalysts and studied by a combination of elemental analysis, Fourier transform infrared spectroscopy (FTIR), solid-state 13C (CP) NMR, thermogravimetric analysis (TGA), X-ray diffraction (XRD), diffuse reflectance UV−vis (DR-UV−vis), and N2 physisorption at −196 °C. In the absence of the acac chelates, the observed formation of anatase TiO2 onto the surface of the material demonstrates that the coordinating ligand acts as an inhibitor for the crystallization of anatase. Furthermore, FTIR and 29Si NMR results show that the chelated titanium alkoxide precursor interacts strongly with the silanol groups, which, in turn, greatly enhances the dispersion of the titanium species in the mesoporous silica matrix. Moreover, a decrease of the temperature applied for the postgrafting and an increase of both the acac/Ti ratio and pH are shown to favor the retention of titanium on the materials surface without affecting the titanium dispersion. According to the X-ray photoelectron spectroscopy (XPS) results, a maximal titanium content of 13.8% can be well-dispersed on the surface of the mesopores without formation of an excess on the external surface of the solids. However, the results of the DR-UV−vis analyses and the catalytic epoxidation of cyclohexene reveal that the maximal concentration of titanium species in tetrahedral coordination is obtained for materials with a Ti/Si ratio of 5.6%. Even if materials with higher titanium content do not show higher conversion of cyclohexene, they do exhibit remarkably low catalytic deactivation during the recycling tests. A higher hydrothermal stability is suggested as an explanation for the lower deactivation of Ti-SBA-15 at high titanium content.

Three-coordinate late transition metal fluorinated alkoxide complexes

Homoleptic fluorinated alkoxide complexes have been prepared from KOC4F9, 1, via salt metathesis routes. One four-coordinate K{K(18C6)}[Co(OC4F9)4], 2, and four three-coordinate complexes: {K(18C6)}[Fe(OC4F9)3], 3, {K(18C6)}[Co(OC4F9)3], 4, {K(18C6)}[Cu(OC4F9)3], 5, and {K(18C6)}[Zn(OC4F9)3], 6, have been prepared and all except 5 have been characterized crystallographically. Compounds 3, 4, and 6 are very rare examples of monomeric, trigonal alkoxide complexes. All compounds have been characterized with UV-vis and IR spectroscopy, solution magnetic susceptibility, and elemental analysis. In solution, compound 2 exists in an equilibrium with 4 and 1, which has been probed with cyclic voltammetry, supporting energetically different Co2+/Co3+ potentials in the three-coordinate (E(p,a) = approximately 1.2 V vs Fc/Fc+) and four-coordinate (E(p,a) = approximately 0.9 V) geometries. The ligand field engendered by the perfluoro-t-butoxide ligand has been studied with DFT calculations on 4 and the hypothetical [Co(OC4H9)3]- as well as the previously reported [Co(mes)3]- and [Co{N(TMS)2}3]- showing significant -type interactions in the xy plane as well as above and below for the two alkoxide species.

Continuous tubular flow reactor for XAFS studies of organometallic reactions: Possibilities and limitations for studies of the Soai reaction

A computer-controlled continuous tubular flow reactor system has been commissioned that permits time-resolved in situ XAFS measurements of organometallic reactions. The system was commissioned by Zn K-edge measurements of products formed during the Soai reaction. XANES data are shown that illustrate the quality of the data that can be achieved. The XANES spectra are compatible with the presence of dimer, trimer or other oligomeric alkoxide species in the Soai process. It is shown how heterogeneity in the Soai reaction system leads to considerable complications with the measurements due to the formation of floating particles of the aldehyde/iPr2Zn adduct formed in the reaction; additionally, decomposition of iPr2Zn with residual air and moisture leads to deposits on cell walls.

Transformation of Ethylzinc Species to Zinc Acetate Mediated by O2 Activation: Reactive Oxygen‐Centered Radicals Under Control

To date, the formation of alkyl perox-ide (path a, Scheme 1), alkoxide (path b), and oxo spe-cies (path c) in the oxygenation of Zn R species has beenwell-documented. To our knowledge, homolytic MO ORbond cleavage to give the corresponding metal carboxylatespecies (path d, scheme 1) has not been considered as a modeof decomposition of both zinc and other metal alkylperoxides.

Replacing Chloride by Alkoxide: Cp2Zr(H)OR, Searching for Alternatives to Schwartz's Reagent

Attempts to synthesize zirconocene hydrido alkoxide species, Cp2Zr(H)OR, via several synthetic pathways consistently failed to yield the targeted complexes, which seem prone to undergo disproportionation into Cp2ZrH2 and a bisalkoxide complex. Reaction of Cp2ZrH2 with HOR, Me3SiOR, or Ph2CO yielded the corresponding bisalkoxide complexes Cp2Zr(OR)2, of which Cp2Zr{OC(H)Ph2}2 was characterized by an X-ray diffraction study. Insertion of vinyl ethers into Cp2ZrH2 yielded Cp2Zr(OR)2 and Cp2Zr(Et)OR. Attempted hydrogenation of Cp2Zr(Me)OR or reaction of Cp2Zr(BH4)OR with NEt3 did not yield the targeted complexes. Cp2Zr(H)OC6H5 and Cp2Zr(H)OC6F5 could be obtained, however, as transient species by β-H elimination from Cp2Zr(tBu)OR, ligand exchange between Cp2ZrH2 and Cp2Zr(OC6F5)2, or protonation in the presence of olefin to yield the corresponding hydrozirconation products. In hydrozirconation reactions of styrene both species are highly regioselective. The alkoxide substituent failed to inhibit β-H eliminatio...

Effect of a conservative mutation of an active site residue involved in substrate binding on the hydride tunneling reaction catalyzed by choline oxidase

The reaction of alcohol oxidation catalyzed by choline oxidase was previously shown to occur through the tunneling of a hydride ion from the α-carbon of an activated alkoxide species to the N(5) atom of FAD within a highly preorganized enzyme-substrate complex [G. Gadda, Biochemistry 47 (2008) 13745–13753]. In the present study, a glutamate residue (312) that participates in substrate binding has been replaced with aspartate by site-directed mutagenesis, and the effect of the substitution on the kinetic parameters of the enzyme and the mechanism of hydride ion transfer were investigated using stopped-flow spectrophotometry. The thermodynamic parameters associated with the enzymatic reaction catalyzed by the mutant enzyme and the temperature dependence of the kinetic isotope effects are consistent with a hydride transfer reaction occurring either through environmentally assisted tunneling or a classical over-the-barrier transition state, but not within a preorganized enzyme-substrate complex. In contrast, less than threefold changes in the K d values for choline were observed in the mutant enzyme with respect to the wild-type enzyme. Thus, the conservative substitution of an active site residue that is involved in substrate binding, but not directly in catalysis, affects primarily the k red value that reports on catalysis and minimally the K d value that reports on binding, thereby providing an example of the interplay that these kinetic parameters may have in enzymatic reactions.

Oxidant‐Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid–Base Sites on Alumina

A gamma-alumina-supported silver cluster catalyst--Ag/Al(2)O(3)--has been shown to act as an efficient heterogeneous catalyst for oxidant-free alcohol dehydrogenation to carbonyl compounds at 373 K. The catalyst shows higher activity than conventional heterogeneous catalysts based on platinum group metals (PGMs) and can be recycled. A systematic study on the influence of the particle size and oxidation state of silver species, combined with characterization by Ag K-edge XAFS (X-ray absorption fine structure) has established that silver clusters of sizes below 1 nm are responsible for the higher specific rate. The reaction mechanism has been investigated by kinetic studies (Hammett correlation, kinetic isotope effect) and by in situ FTIR (kinetic isotope effect for hydride elimination reaction from surface alkoxide species), and the following mechanism is proposed: 1) reaction between the alcohol and a basic OH group on the alumina to yield alkoxide on alumina and an adsorbed water molecule, 2) C-H activation of the alkoxide species by the silver cluster to form a silver hydride species and a carbonyl compound, and 3) H(2) desorption promoted by an acid site in the alumina. The proposed mechanism provides fundamental reasons for the higher activities of silver clusters on acid-base bifunctional support (Al(2)O(3)) than on basic (MgO and CeO(2)) and acidic to neutral (SiO(2)) ones. This example demonstrates that catalysts analogous to those based on of platinum group metals can be designed with use of a less expensive d(10) element--silver--through optimization of metal particle size and the acid-base natures of inorganic supports.

Hydride Transfer Made Easy in the Reaction of Alcohol Oxidation Catalyzed by Flavin-dependent Oxidases

Choline oxidase (E.C. 1.1.3.17; choline-oxygen 1-oxidoreductase) catalyzes the two-step, four-electron oxidation of choline to glycine betaine with betaine aldehyde as enzyme-associated intermediate and molecular oxygen as final electron acceptor. Biochemical, structural, and mechanistic studies on the wild-type and a number of mutant forms of choline oxidase from Arthrobacter globiformis have recently been carried out, allowing for the delineation at molecular and atomic levels of the mechanism of alcohol oxidation catalyzed by the enzyme. First, the alcohol substrate is activated to its alkoxide species by the removal of the hydroxyl proton in the enzyme-substrate complex. The resulting activated alkoxide is correctly positioned for catalysis through electrostatic and hydrogen bonding interactions with a number of active site residues. After substrate activation and correct positioning are attained, alcohol oxidation occurs in a highly preorganized enzyme-substrate complex through quantum mechanical transfer of a hydride ion from the alpha-carbon of the chelated, alkoxide species to the N(5) atom of the enzyme-bound flavin. This mechanism in its essence is shared by another class of alcohol oxidizing enzymes that utilize a catalytic zinc to stabilize an alkoxide intermediate and NAD(P)(+) as the organic cofactor that accepts the hydride ion, whose paradigm example is alcohol dehydrogenase. It will be interesting to experimentally evaluate the attractive hypothesis of whether the mechanism of choline oxidase can be extended to other flavin-dependent enzymes as well as enzymes that utilize cofactors other than flavins in the oxidation of alcohols.

On the Role of Histidine 351 in the Reaction of Alcohol Oxidation Catalyzed by Choline Oxidase

Choline oxidase catalyzes the four-electron, flavin-linked oxidation of choline to glycine betaine with transient formation of an enzyme-bound aldehyde intermediate. The recent determination of the crystal structure of choline oxidase to a resolution of 1.86 A established the presence of two histidine residues in the active site, which may participate in catalysis. His466 was the subject of a previous study [Ghanem, M., and Gadda, G. (2005) Biochemistry 44, 893-904]. In this study, His351 was replaced with alanine using site-directed mutagenesis, and the resulting mutant enzyme was purified and characterized in its mechanistic properties. The results presented establish that His351 contributes to substrate binding and positioning and stabilizes the transition state for the hydride transfer reaction to the flavin, as suggested by anaerobic substrate reduction stopped-flow data. Furthermore, His351 contributes to the overall polarity of the active site by modulating the p K a of the group that deprotonates choline to the alkoxide species, as indicated by pH profiles of the steady-state kinetic parameters with the substrate or a competitive inhibitor. Surprisingly, His351 is not involved in the activation of the reduced flavin for reaction with oxygen. The latter observation, along with previous mutagenesis data on His466, allow us to conclude that choline oxidase must necessarily utilize a strategy for oxygen reduction different from that established for glucose oxidase, where other authors showed that the catalytic effect almost entirely arises from a protonated histidine residue.

Organo[2‐(hydroxymethyl)phenyl]dimethylsilanes as Mild and Reproducible Agents for Rhodium‐Catalyzed 1,4‐Addition Reactions.

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On the mechanism of the catalytic destruction of 1,2-dichloroethane over Ce/Zr mixed oxide catalysts

This study has been undertaken to investigate the efficiency of ceria, zirconia, and Ce x Zr 1− x O 2 mixed oxides as catalysts for the 1,2-dichloroethane destruction in dry air. Mixed oxides exhibit promoted redox and acidic properties, which result catalytically relevant for the oxidation of this chlorinated compound. Considering all compositions it was observed that catalytic activity varies as a function of zirconia content, being Ce 0.5Zr 0.5O 2 the sample which showed a better performance. The notable improvement in catalyst activity of CeO 2 can be achieved through structural doping with Zr ions. Likewise, the present study has been focused on the elucidation of the pathway involved in the progressive oxidation of the chlorinated hydrocarbon as a function of the temperature by means of FTIR. It is postulated that the destruction of 1,2-dichloroethane at low temperatures proceeds through dehydrochlorination into vinyl chloride in the presence of acid sites. This compound can be protonated in the presence of OH surface species forming carbonium ions that can be attacked by nucleophilic oxygen species from the catalyst to form chlorinated alkoxide species. These intermediates readily decompose to generate acetaldehyde, which can be further oxidised into acetates and finally degraded to CO x .

Theoretical study of two pathways of double-bond isomerization of pentene catalyzed by zeolites

The double-bond isomerization of 1-pentene over zeolites has been investigated by using density functional theory at the B3LYP/6-31G(d,p) level. The calculated results indicate that the double-bond isomerization may proceed via either a stepwise or a concerted reaction pathway. The stepwise reaction consists of two elementary steps: firstly, the formation of an alkoxy intermediate by addition of a proton from zeolites, and then the decomposition of this alkoxy intermediate to yield an adsorbed 2-pentene. The calculated activation barriers are 17.9 and 31.5 kcal/mol for the first and second step, respectively. The concerted reaction occurs via one-shift proton transfer, which avoids the formation of highly stable alkoxide species. The resulting activation energy, 20.5 kcal/mol, is close to that of the first step of the stepwise reaction. From an energetic point of view, it is expected that at low temperatures, the concerted reaction mechanism dominates the overall isomerization reaction due to the lower activation energy, while at higher temperatures, the two reaction pathways compete against each other because the alkoxy formation will occur relatively easily. The results can well explain the experimental phenomena observed.

Organo[2-(hydroxymethyl)phenyl]dimethylsilanes as Mild and Reproducible Agents for Rhodium-Catalyzed 1,4-Addition Reactions

Stable and reusable tetraorganosilicon reagents, alkenyl-, aryl-, and silyl[2-(hydroxymethyl)phenyl]dimethylsilanes, undergo 1,4-addition reactions to alpha,beta-unsaturated carbonyl acceptors under mild rhodium-catalysis. The reaction tolerates a diverse range of functional groups and is applicable to gram-scale synthesis. Use of a chiral diene ligand allows the achievement of the corresponding enantioselective transformations using the tetraorganosilicon reagents, providing the silicon-based approach to optically active ketones and substituted piperidones that serve as synthetic intermediates of pharmaceuticals. A rhodium alkoxide species is suggested to be responsible for a transmetalation step on the basis of the observed kinetic resolution of a racemic chiral phenylsilane in the enantioselective 1,4-addition reaction under the rhodium-chiral diene catalysis.

Computational Studies on the Stabilities of trans-[Ir(OMe)(CO)(PPh3)2] and trans-[Ir(CH2Me)(CO)(PPh3)2] toward β-H Elimination

The relative stabilities of trans-[Ir(XMe)(CO)(PR3)2] species (X = O, CH2) toward β-H elimination have been studied via combination of density functional and hybrid density functional/Hartree−Fock calculations. For both small (R = H) and full (R = Ph) model systems β-H elimination from the methoxide species is found to be disfavored both kinetically and thermodynamically compared to that from the analogous ethyl complexes. This is consistent with the greater stability of alkoxide species seen experimentally (R = Ph). In all cases the major contribution to the activation barrier is phosphine dissociation, and for the alkyl systems this leads directly to an agostically stabilized intermediate from which β-H transfer readily occurs. In contrast, with the trans-[Ir(OMe)(CO)(PR3)2] species a π-stabilized intermediate is formed and a further isomerization barrier must be overcome before β-H transfer can be accessed. Further calculations were performed on the acetophenone complex [Ir(H)(η2-OC(Me)Ph)(CO)(PPh3)], ...

A new synthetic approach to biaryls of the rhazinilam type. Application to synthesis of three novel phenylpyridine-carbamate analogues

The synthesis of three novel racemic phenylpyridine-carbamate analogues of rhazinilam and their biological evaluation as inhibitors of microtubule assembly and disassembly by interaction with tubulin are described. The sterically hindered ortho-disubstituted biaryl unit as the challenging key structural element is first obtained by a sequential regiocontrolled nucleophilic addition of a lithium ortho-lithiohomobenzylic alkoxide species to 3-bromo-5-oxazolyl pyridine as the electrophile and a subsequent oxidation step. The incorporation of the amino group by replacement of the bromide has been achieved using a Buchwald-Hartwig amination coupling. Ultimate deprotection steps furnished free-amino and free-hydroxyl appendages which were connected by phosgenation to furnish the nine-membered median carbamate ring.

A quantum chemical study of the mechanism of action of Vitamin K carboxylase (VKC): III. Intermediates and transition states

A reaction path including transition states is generated for the Dowd mechanism [P. Dowd, R. Hershlne, S.W. Ham, S. Naganathan. Vitamin K and energy transduction: a base strength amplification mechanism. Science 269 (2005) 1684–1691] of action for Vitamin K carboxylase (VKC) using quantum chemical methods (B3LYP/6-311G**). VKC, an essential enzyme in mammalian systems, catalyzes the conversion of hydroquinone form of Vitamin K to the epoxide form in the presence of oxygen. An intermediate species of the oxidation of Vitamin K, an alkoxide, acts apparently to abstract the gamma hydrogen from specifically located glutamate residues. We are able to follow the Dowd proposed path to generate this alkoxide species. The geometries of the proposed model intermediates and transition states in the mechanism are energy optimized. We find that the most energetic step in the mechanism is the uni-deprotonation of the hydroquinone – once this occurs, there is only a small barrier of 3.5 kcal/mol for the interaction of oxygen with the carbon to be attacked – and then the reaction proceeds downhill in free energy to form the critical alkoxide species. The results are consistent with the idea that the enzyme probably acts to facilitate the formation of the epoxide by reducing the energy required to deprotonate the hydroquinone form.

Catalytic activity of Brønsted acid sites in zeolites: Intrinsic activity, rate-limiting step, and influence of the local structure of the acid sites

The catalytic activity of Brønsted acid sites in zeolites was studied by the monomolecular conversion of propane over zeolites with varying framework topologies and Si/Al ratios. The rates and apparent activation energies of cracking and dehydrogenation were determined. The activity of the Brønsted acid sites depends on the rate-limiting step of the reaction. In the cracking reaction, the protonation of the alkane is the rate-limiting step, and the heat of reactant adsorption dominates the differences in the observed activity. The similar intrinsic activities over the different zeolites show that the ability of zeolitic Brønsted acid sites to transfer a proton to an alkane does not vary significantly, suggesting that the acid sites that participate in the reaction have very similar strengths. In the dehydrogenation reaction, the rate-limiting step is the desorption of the alkoxide species. The rate is determined by the stability of the alkoxide species, which is influenced by the local geometric and electronic structure of the Brønsted acid site and is affected by zeolite structure and Si/Al ratio. Implications of these conclusions are related to other reactions, such as catalytic cracking and alkylation.

Synthesis and Reactions of Group 4 Imido Complexes Supported by Cyclooctatetraene Ligands

The reactions of the pseudo-two-coordinate titanium imido complexes [Ti(NtBu)(COT)] (1) (COT = η8-C8H8), [Ti(NtBu)(COT‘ ‘)] (2) (COT‘ ‘ = η8-1,4-C8H6(SiMe3)2), and [Ti(NAr)(COT)] (3) (Ar = 2,6-iPr2C6H3) with a variety of organic substrates are reported. Reaction of 1 with CO2, tBuNCO, or ArNCO and reaction of 3 with CO2 or tBuNCO afforded the organic products tBuNCO, tBuNCNtBu, tBuNCNAr, ArNCO, and ArNCNtBu, respectively, and a titanium oxo species. These reactions proceeded via an initial [2 + 2] cycloaddition to form an N,O-bound intermediate of the type [Ti{N(R)C(O)R‘}(COT)], which subsequently underwent a retrocycloaddition to give an organic product and the titanium oxo species. In contrast, reaction of 3 with ArNCO led to the formation of the N,N-bound [2 + 2] cycloaddition product [Ti{N(Ar)C(O)N(Ar)}(COT)] (7). In general, the reactions of 1 and 3 with CS2 and isothiocyanates also resulted in an initial [2 + 2] cycloaddition to form an N,S-bound intermediate of the type [Ti{N(R)C(S)R‘}(COT)], which also subsequently underwent a retrocycloaddition to give an organic product and a metal sulfide species. However, the N,S-bound compound [Ti{N(Ar)C(S)S}(COT)] (10) was stable to retrocycloaddition and was isolated. Proton transfer reactions occurred between pinacol and compounds 1−3 to form the bis(alkoxide) species [Ti{OC(Me)2C(Me)2O}(COT)] (11) (from 1 or 3) or [Ti{OC(Me)2C(Me)2O}(COT‘ ‘)] (12) (from 2) and the corresponding free amine. The reactions between 1−3 and 2 equiv of the thiols tBuSH and HS-2,4,6-iPr3C6H2 all resulted in the oxidation of the thiol to the disulfides tBuS−StBu and (2,4,6-iPr3C6H2)S−S(2,4,6-iPr3C6H2). Treatment of 1 with tBuNC in the presence of 1,3,5,7-cyclooctatetraene led to formal nitrene group transfer and the formation of the Ti(II) species [Ti(COT)(η4-C8H8)] (13) and tBuNCNtBu. The analogous reactions between 2 and 3 and tBuNC resulted in the formation of the adducts [Ti(NtBu)(COT‘ ‘)(CNtBu)] (15) and [Ti(NAr)(COT)(CNtBu)] (17), and similarly the reaction between 1 and pyridine led to the isolation of the adduct [Ti(NtBu)(COT)(py)] (19) (py = pyridine). Complex 19 was crystallographically characterized. DFT studies indicated that the interaction between pyridine and the Ti center in 19 and tBuNC and the Ti center in 17 was electrostatic in nature. Complexes of the type [Ti(NR)(COT)(AlMe3-xClx)] (R = tBu, x = 0 (20); R = Ar, x = 0 (21); R = tBu, x = 1 (22); R = Ar, x = 1 (23)) were formed through the reactions of 1 and 3 with AlMe3 and AlMe2Cl, and DFT studies indicated that they contained four-membered metallacyclic rings. Attempts to prepare monomeric zirconium imido cyclooctatetraene complexes through the reactions of [Zr2(μ-NR)2Cl4(THF)x] (R = tBu, x = 3; R = 2,6-Me2C6H3(Ar‘), x = 4) with K2COT, Li2COT‘ ‘·1.8(THF), or Li2[COT*] (COT* = 1,4-C8H6(SiMe2tBu)2) were unsuccessful. Only the crystallographically characterized dimeric species [Zr2(μ-NAr‘)2(COT‘ ‘)2] (24) was isolated.

Ring‐opening polymerization of ε‐caprolactone initiated by rare earth alkoxides and borohydrides: a comparative study

AbstractThe polymerization of ε‐caprolactone initiated by trivalent rare earth (Ln) derivatives, namely the isopropoxide ‘Ln(OiPr)3’ opposed to the borohydride Ln(BH4)3(THF)3 (Ln = La, Nd, Sm) and (Cp*)2Sm(BH4)(THF) (Cp* = (η‐C5Me5)), has been evaluated. The comparison, based on the structure of the initiator and on the polymerization features (molar mass, molar mass distribution, side reactions) reveals some differences especially in the number of active polymer chains per metal and in the occurrence of some transfer reactions with Ln(BH4)3(THF)3. Polymerizations performed with borohydride derivatives lead to a gel as a result of van der Waals interactions, while the reaction medium remains fluid with the alkoxide species. A typical coordination‐insertion mechanism with oxygen‐acyl bond cleavage of the monomer prevails with both types of initiators. However, while the alkoxide complexes lead to α,ω‐hydroxyalkoxypoly(ε‐caprolactone), the borohydride compounds allow the synthesis of α,ω‐dihydroxypoly(ε‐caprolactone). Both polymerization processes are well controlled under specific conditions. Copyright © 2006 Society of Chemical Industry

Ring opening polymerization initiated by methylaluminoxane/AlMe 3 complexes

The ROP of cyclic ethers, carbonates and esters in the presence of commercially available methylaluminoxane/trimethylaluminum system has been studied. MALDI-ToF end groups analysis indicates that in a majority of systems considered, the polymerization process is initiated by insertion of a monomer into the Al–O–Al bond, generating alkoxide species, which are active sites in coordination polymerization. The polymerization of six-membered carbonates proceeds selectively, forming linear polydiols with high yields at moderate temperatures. The polymerization of oxiranes and lactones is, however, accompanied by back-biting reactions leading to cyclic oligomers. The interaction of oxirane with aluminoxane electrophilic sites causes also the formation of cationic species, which initiate the polymerization of THF. The cationic species formed in those systems were trapped by triphenylphosphine and identified by 31P NMR spectra.