Alkyl Transfer Reactions Research Articles

Highly Active Immobilized Catalyst for Ethylene Polymerization: Neutral Single Site Y(III) Complex Bearing Bulky Silylallyl Ligand.

Exploring the surface organometallic chemistry on silica of highly electrophilic yttrium complexes is a relatively uncommon endeavor, particularly when focusing on tris-alkyl complexes characterized by Y-C σ-alkyl bonds. A drawback with this class of complexes once grafted on silica, is the frequent occurrence of alkyl transfer by ring opening of siloxane groups, resulting in a mixture of species. Herein, we employed a more stable homoleptic yttrium allyl complex bearing bulky η3-1,3-bis(trimethylsilyl)allyl ligand to limit this transfer reaction. This strategy has been validated by comparing the reactivity between [Y{ η3-1,3-C3H3(SiMe3)2}3] and [Y(o-CH2PhNMe2)3] with SiO2-700, where the undesired alkyl transfer reaction occurred for [Y(o-CH2PhNMe2)3] leading to a bipodal [(≡SiO)2Y(o-CH2PhNMe2)] as major surface species, 2, while [Y{ η 3-1,3-C3H3(SiMe3)2}3] resulted selectively in a monopodal species, [(≡SiO)Y{η3-1,3-C3H3(SiMe3)2}2], 1. The materials obtained were characterized by DRIFT, solid state NMR, mass balance analysis and EXAFS. Catalyst 1 showed high activity compared to 2 in ethylene polymerization. The catalytic performance of this neutral catalyst 1 was extended to pre-industrial scale in the presence of hydrogen and 1-hexene. An unprecedented activity, up to 7400 gPE gcat -1 h-1 was obtained even with very low concentration of scavenger AliBu3 (TIBA/Y=1.2). The obtained HDPE exhibited desired spherical particle morphology with broad molar mass distribution.

Analytical Py-GC/MS study on conversion behaviors and reaction mechanism of biochar-mediated catalytic upgradation of primary pyrolytic products from lignocellulosic biomass

This study aims to demonstrate the general carbocatalytic conversion behaviors and reaction mechanisms of representative biomass-derived primary pyrolytic products towards product upgradation. Specifically, 4-Ethylguaiacol (4-EGC) and 5-hydroxymethylfurfural (5-HMF) were selected as lignin- and cellulose/hemicellulose-derived model compound, respectively, to elucidate the origin of products under different temperatures and biochar-reactant mixing ratios. Three reaction routes were distinguished, including side-chain elimination, ring opening and recombination, with each route exhibiting unique characteristics in terms of initiation temperature and selectivity. For 4-EGC, elimination of side-chains began at 500 °C, accompanied by alkyl transfer reactions to generate multi-alkyl-substituted phenols, while ring-opening started at 700 °C, and aromatics could be simultaneously produced. Biochar did not change the initiation temperature of each route but significantly influenced the product distribution. Comparing the biochar-reactant mixing ratio of 0 and 10:1, at 600 °C the selectivity towards phenols increased from 20.52 % to 54.98 %, and at 800 °C the selectivity towards aromatics increased from 14.28 % to 53.01 %. For 5-HMF, the elimination of side chains already began at 400 °C, and the ring-opening started at around 600 °C, but recombination products (phenols and aromatics) would not notably emerge until 700 °C. Biochar could both lower the initiation temperature of each reaction route and change the conversion efficiency for 5-HMF owing to the lower stability of furan ring. However, the impact of biochar on selectivity towards aromatics was less pronounced compared to 4-EGC. Even at 800 °C, the relative content of aromatics only reached 28.88 % with a biochar-reactant mixing ratio of 10:1 for 5-HMF. Comparative investigations on biochar structure before and after co-pyrolysis reactions further revealed that, at 600 °C, biochar maintained its structural integrity, but at 800 °C, it underwent significant changes, including amorphous carbon deposition covering the surface. Raman spectra analysis supported these findings, showing that biochar becomes more amorphous after reactions at 600 °C, while at 800 °C, larger aromatic ring systems form on the biochar, reducing defect sites.

Thermal rearrangement in thermal cascade reaction polymers via ortho-carbonate ester functionalization of polyimides and their gas separation performance

Thermal Rearrangement Polymers have a strong potential regarding to gas separation membrane materials, especially for the purification of methane containing gas streams, due to their inter-connected bottleneck-type pores. However, most TR polymers reveal high permeabilities and high conversions only at high temperatures of 450 °C. Our study demonstrates a method to enhance the permeability at lower annealing temperatures by tailoring the thermally triggered decomposition and reaction cascade via modification and temperature protocol. In this study a set of seven carbonate ester modifications was prepared and thermo-analytically studied by means of DSC and TGA-FTIR on-line analysis combined with structure determination methods and quantum mechanical simulations. Two decarboxylative alkyl-transfer reaction mechanisms were formulated for the decomposition reaction of the carbonate ester group. A correlation of the size and branching of the carbonate ester connected alkyl group and its effect on the film properties and gas separation performance was studied. High polyimide to polybenzoxazole conversions were determined for the modified polymers and a 2008 upper bound performance was obtained for all materials, after annealing at 400 °C. Furthermore, the ethyl carbonate ester (ECO3PI-1) polymer entered the target zone above an ideal selectivity of CO2/CH4 of 30 after aging, making it an attractive membrane material.

Bicyclo[2.2.0]hexene derivatives as a proaromatic platform for group transfer and chemical sensing

Here we report the design, preparation, synthetic utility, and sensing application of a class of proaromatic structures, namely bicyclo[2.2.0]hexene (BCH) derivatives. Building on a valence isomerism concept, they feature modular and easy synthesis as well as high thermal stability, and can be oxidatively activated under mild conditions. New alkyl transfer reactions using BCHs as a radical donor have been developed to showcase the utility of their proaromaticity. Moreover, the redox-triggered valence isomerization of a quinoline-derived BCH led to colorimetric and fluorescent responses toward vapors of electrophilic reagents in solution and solid phase, respectively. This optical response was shown to involve a 1,3-cyclohexadiene structure that possesses an intramolecular charge transfer excited state with interesting aggregation induced emission (AIE) character. Thus, the potential of BCHs has been demonstrated as a versatile platform for the development of new reagents and functional materials.

Elucidating the mechanism of cob(I)alamin mediated methylation reactions by alkyl halides: SN2 or radical mechanism?

Methyl transfer reactions mediated by cobalamins (Cbls) have been considered as one of the most important biologically relevant molecular transformations for many enzymatic reactions catalyzed by Cbl-dependent enzymes. The exact mechanism of methyl transfer reactions involving Cbls is still poorly understood. To investigate the mechanistic details of Cbl mediated methylations by alkyl halides, density functional theory (DFT) along with the polarizable continuum model (PCM/water) for solvation has been applied. Two different mechanisms have been examined, namely SN2 and radical-based electron transfer (ET) to elucidate the methyl transfer reaction. The calculations have suggested that the methyl transfer from methyl halides proceeds through SN2 nucleophilic displacement. However, with more bulky alkyl substrate, namely isopropyl and tert-butyl halides the reaction followed the ET-based radical pathway which is associated with an ET from diradical form of cob(I)alamin to alkyl halides. Our proposed mechanism for alkyl transfer reaction corroborates with the experimental findings, which reported a mechanistic switch from a two-electron (SN2-type) to a one-electron mechanism for sterically demanding alkyl halides. The present theoretical contribution provides more in-depth insight into the methyl transfer reaction catalyzed by corrinoid-dependent methyltransferases.

Alkyl transfer reactions on solid acids. The disproportionation of ethylbenzene and toluene on H-mordenite and HY zeolites

ABSTRACTThe disproportionation of ethylbenzene and toluene have been compared on two zeolites, H-mordenite and HY. The online examination of products as a function of time on stream reflect the different processes occurring on the two zeolites. The relative activities expressed on a unit aluminium center basis, are in line with their relative cracking (n-heptane) activities reported by others, showing that no extraneous factors affect activity. The data set provides a baseline for the activity of these two zeolites for future use in comparing other more complex alkylation reactions involving mononuclear and binuclear aromatic molecules.

Preparation of Paving Asphalt by Co-pyrolysis of Coal Tar Pitch and FCC Slurry

Coal tar pitch was co-pyrolyzed with FCC slurry. The co-pyrolysis temperature is at 420oC with the mixture of coal tar pitch and FCC slurry mixing ratio of 1:1. The penetration, softening point, ductility and other properties meets the specifications of BZ200. The co-pyrolysis asphalt was analyzed using FTIR, NMR etc. The results show that there occurs hydrogen transfer and alkyl transfer reaction of FCC slurry and coal tar pitch during co-pyrolysis, leading to the improvement of molecular structure of modified asphalt.

Alkyl transfer reactions on solid acids. The transalkylation of benzene with mesitylene on H-mordenite and HY zeolites

The catalytic transalkylation chemistry encountered with mesitylene interacting with benzene has been revealed working with the acid zeolites, H-mordenite and HY. Preferential adsorption on the active site of the alkyl group containing-aromatic molecule was very advantageous for transalkylation reactions since otherwise the smaller benzene would be the dominant adsorbed molecule; it was also noted that when the alkyl acceptor was a small molecule, formation of the transition state was favourable. The study has shown that transalkylation was favoured over disproportionation at higher temperatures and this was in line with the earlier conclusions that disproportionation would only be effective at low temperatures. Reaction temperature influences the selectivity of alkyl-transfer reactions. The feed composition seemed to play a significant role in transalkylation and its manipulation may result in effective removal of alkyl groups from the alkyl-containing molecule. The window of operation is reduced somewhat by the finding that working at high temperatures is associated with undesirable higher deactivation rates.

Improving Ultra-Deep Desulfurization Efficiency by Catalyst Stacking Technology

The catalyst for diesel ultra-deep desulfurization is selected on the basis of several factors, including feedstock composition, liquid hourly space velocity, and operation pressure. Owing to their specific processing purposes, different catalyst systems, such as straight distillation, coking distillation, blends of straight distillation, and secondary processing oil, are applied for various feeds because of their considerable difference in terms of sulfur content, nitrogen content, aromatics constitute, and cetane number. This study aims to determine the most appropriate catalyst and grading scheme by evaluating the deep desulfurization efficiency of different schemes when processing various feeds. The results showed that the W–Mo–Ni/Mo–Co catalyst stacking is the most effective among various schemes because of the following reasons: (1) loading W–Mo–Ni catalyst on the upper bed of reactor not only benefits the saturation of polyaromatics even in the middle or at the end of the run but also provides low nitrogen feed for the bottom bed; (2) loading Mo–Co type catalyst with alkyl transfer performance in the bottom bed at high temperature reaction zone facilitates alkyl transfer reaction and mitigates the effect of thermodynamic equilibrium limitations in the middle and at the end of the run, consequently improving the HDS efficiency under high temperatures.

Determinants of the CmoB carboxymethyl transferase utilized for selective tRNA wobble modification

Enzyme-mediated modifications at the wobble position of tRNAs are essential for the translation of the genetic code. We report the genetic, biochemical and structural characterization of CmoB, the enzyme that recognizes the unique metabolite carboxy-S-adenosine-L-methionine (Cx-SAM) and catalyzes a carboxymethyl transfer reaction resulting in formation of 5-oxyacetyluridine at the wobble position of tRNAs. CmoB is distinctive in that it is the only known member of the SAM-dependent methyltransferase (SDMT) superfamily that utilizes a naturally occurring SAM analog as the alkyl donor to fulfill a biologically meaningful function. Biochemical and genetic studies define the in vitro and in vivo selectivity for Cx-SAM as alkyl donor over the vastly more abundant SAM. Complementary high-resolution structures of the apo- and Cx-SAM bound CmoB reveal the determinants responsible for this remarkable discrimination. Together, these studies provide mechanistic insight into the enzymatic and non-enzymatic feature of this alkyl transfer reaction which affords the broadened specificity required for tRNAs to recognize multiple synonymous codons.

ChemInform Abstract: Regioselective Alkyl Transfer from Phosphonium Ylides to Functionalized Polyfluoroarenes.

AbstractThe regioselective alkyl transfer reaction is further applied in the synthesis of the pesticide tefluthrin (XII).

Nucleophilic reactivity of Zinc-bound thiolates: subtle interplay between coordination set and conformational flexibility

During the past decades it has been established that zinc-bound cysteines in proteins can react with various electrophiles to perform physiological functions such as alkyl transfer reactions or oxidative stress sensing. Electrophiles targeting especially vulnerable structural zinc fingers have also been proposed as therapeutic agents against cancers or HIV. However, the nucleophilic reactivity of zinc fingers remains poorly understood. In this article, we investigate the nucleophilic reaction of zinc finger model peptides with H2O2 in order to get deeper insight into the factors governing the reactivity of zinc-bound cysteines of zinc finger sites. We use a set of nine peptides belonging to two different peptide families with (Cys)4, (Cys)3(His) and (Cys)2(His)2 coordination sets. One family is derived from the consensus peptide of classical ββα zinc fingers and the other one derived from the zinc finger site of the oxidative stress sensor Hsp33 that adopts a loosened zinc ribbon fold. The coordination properties and the structural behaviors of the new members of the latter family were carefully characterized. The rate constants of the reaction of the nine zinc finger models with H2O2 were measured at various temperatures to determine the activation parameters. In all cases, the reaction is characterized by a small enthalpy of activation, which shows that the nucleophilic reaction of zinc-bound cysteines is easy, and large unfavorable negative entropy of activation. Neutral Zn(Cys)2(His)2 cores are intrinsically less reactive than negatively charged Zn(Cys)4 and Zn(Cys)3(His). Interestingly, we observe that the more flexible zinc finger sites are the more reactive. Indeed the entropies of activation are strongly linked to the conformational behavior of the peptide in solution. This work reveals important factors that govern the reactivity of zinc-bound cysteines in these two structural classes of zinc fingers and can be used to identify reactive zinc fingers in proteins.

ChemInform Abstract: Enantioselective Addition of Organometallic Reagents to Carbonyl Compounds: Chirality Transfer, Multiplication and Amplification

Nucleophilic addition of organometallic reagents to carbonyl substrates constitutes one of the most fundamental operations in organic synthesis. Modification of the organometallic compounds by chiral, nonracemic auxiliaries offers a general opportunity to create optically active alcohols, and the catalytic version in particular provides maximum synthetic efficiency. The use of organozinc chemistry, unlike conventional organolithium or -magnesium chemistry, has realized an ideal catalytic enantioselective alkylation of aldehydes leading to a diverse array of secondary alcohols of high optical purity. A combination of dialkylzinc compounds and certain sterically constrained β-dialkylamino alcohols, such as (–)-3-exo-dimethylaminoiso- borneol [(–)-DAIB], as chiral inducers affords the best result (up to 99% ee). The alkyl transfer reaction occurs via a dinuclear Zn complex containing a chiral amino alkoxide, an aldehyde ligand, and three alkyl groups. The chiral multiplication method exhibits enormous chiral amplification: a high level of enantioselection (up to 98%) is attainable by use of DAIB in 14% ee. This unusual nonlinear effect is a result of a marked difference in chemical properties of the diastereomeric (homochiral and heterochiral) dinuclear complexes formed from the dialkylzinc and the DAIB auxiliary. This phenomenon may be the beginning of a new generation of enantioselective organic reactions.

Reactions of Aromatic N-Heterocycles with d0fn-Metal Alkyl Complexes Supported by Chelating Diamide Ligands

Aromatic heterocycles are a prominent feature within natural products and pharmaceuticals and considerable efforts are directed toward their synthesis and functionalization. These molecules also appear as unwanted impurities in carbon-based fuels, and processes that fragment them are of increasing interest. Early transition metal-carbon bonds show diverse reactivity toward aromatic heterocycles: researchers have reported both functionalization, relevant to synthetic efforts, and ring opening, relevant to their removal from fuels. In particular, chelating ferrocene-diamides possess unique electronic characteristics as ancillary ligands that enable a wide range of reactivity behaviors for the resulting metal complexes. In this Account, we describe our efforts to understand the reactivity of group 3 metal and uranium alkyl complexes supported by these organometallic ligands toward aromatic N-heterocycles. Two geometrically related ancillary ligands were investigated: 1,1'-ferrocenylene-diamides and pincer-type pyridine-diamides. A substrate-dependent behavior was observed. For example, all the benzyl metal complexes cleaved 1-methylimidazole. In the case of pyridines, differences in reactivity were identified: C-H activation and C-C coupling occurred with substituted pyridines, while alkyl transfer predominated with isoquinoline and chelating pyridines. The products of the C-C coupling or the alkyl-transfer reactions underwent subsequent hydrogen transfer: within the same ring for the substituted pyridines and between two different heterocycles for isoquinoline and chelating pyridines. The comparison between yttrium and lutetium benzyl complexes supported by ferrocene- or pyridine-diamide ligands indicated that similar reactions occurred for specific substrates (1-methylimidazole, 2-picoline, and isoquinoline). A broader range of reaction types and a larger substrate scope were identified, however, for the ferrocene than for the pyridine-type complexes. Based on the reactions discussed in this Account and on isolated examples drawn from the literature, we conclude that the ferrocene-diamides represent a versatile ligand framework. We propose that iron's ability to accommodate changes in the electronic density at the metal center more readily than classical supporting ligands leads to the privileged status of these organometallic ancillary ligands.

Trihaloacetaldehyde N, O-acetals: useful building blocks for dihalomethylene compounds

The reaction of trifluoroacetaldehyde N, O-acetals with more than 2 equiv of alkyllithiums at −78 °C resulted in regiospecific defluorinative alkylation with unusual regioselectivity to give α,α-difluoroketone N, O-acetals in excellent yield. In contrast, under similar conditions, trichloroacetaldehyde N, O-acetals gave simple mono-dechlorinated product without the alkyl transfer reaction from alkyllithiums to the generated intermediates.

Dearomatization Reactions of N-Heterocycles Mediated by Group 3 Complexes

Group 3 (Sc, Y, Lu, La) benzyl complexes supported by a ferrocene diamide ligand are reactive toward aromatic N-heterocycles by mediating their coupling and, in a few cases, the cleavage of their C-N bonds. When these complexes reacted with 2,2'-bipyridine or isoquinoline, they facilitated the alkyl migration of the benzyl ligand onto the pyridine ring, a process accompanied by the dearomatization of the N-heterocycle. The products of the alkyl-transfer reactions act as hydrogen donors in the presence of aromatic N-heterocycles, ketones, and azobenzene. Experimental and computational studies suggest that the hydrogen transfer takes place through a concerted mechanism. An interesting disproportionation reaction of the dearomatized, alkyl-substituted isoquinoline complexes is also reported.

A New Reaction for Organoselenium Compounds: Alkyl Transfer from Diorganoselenium(IV) Dibromides to Alkenoic Acids To Give γ- and δ-Lactones

Various 1-[[bis(1-methylethyl)amino]carbonyl]-2-(dibromo-n-alkylseleno)ferrocene derivatives reacted with 2 equiv of 4-pentenoic acid or 5-hexenoic acid to give the corresponding γ- or δ-lactones, respectively, from transfer of the alkyl group bound to selenium to the terminal alkene carbon. In the lactone cyclization, 1-[[bis(1-methylethyl)amino]carbonyl]-2-(bromoselenenyl)ferrocene (25) is formed and reacts with a second equivalent of the alkenoic acid to give either 5-[1-[[bis(1-methylethyl)amino]carbonyl]ferrocene-2-selenenylmethyl]tetrahydro-2-furanone (23) or 6-[1-[[bis(1-methylethyl)amino]carbonyl]ferrocene-2-selenenylmethyl]tetrahydro-2H-pyran-2-one (26) as a mixture of diastereomers in yields comparable to the corresponding γ- or δ-lactone. The addition of 1-[[bis(1-methylethyl)amino]carbonyl]-2-(dibromo-n-butylseleno)ferrocene to 3-butenoic acid gave alkyl transfer to the internal alkene carbon and formation of the γ-lactone. Density functional theory (DFT) geometry optimizations were performed on several idealized n-alkyldibromselenoferrocene structures to examine the role of iron in the alkyl transfer and the role of various structural features on the alkyl carbon−selenium bond length. The results of the computations suggested that the ferrocenyl iron was not involved in the alkyl transfer reactions.

The effect of substitution on the utility of piperidines and octahydroindoles for reversible hydrogen storage

Substituted piperidines and octahydroindoles are compared in terms of their usability as reversible organic hydrogen storage liquids for hydrogen-powered fuel cells. Theoretical Gaussian calculations indicate which structural features are likely to lower the enthalpy of dehydrogenation. Experimental results show that attaching electron donating or conjugated substituents to the piperidine ring greatly increases the rate of catalytic dehydrogenation, with the greatest rates being observed with 4-aminopiperidine and piperidine-4-carboxamide. Undesired side reactions were observed with some compounds such as alkyl transfer reactions during the dehydrogenation of 4-dimethylaminopiperidine, C–O and C–N cleavage reactions during hydrogenation and/or subsequent dehydrogenation of 4-alkoxy and 4-amino indoles, and disproportionation during the hydrogenation of 4-aminopyridine.

Electrochemical Alkyl Transfer Reaction from Trialkylboranes to Polyhalo Compounds.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Electrochemical alkyl transfer reaction from trialkylboranes to polyhalo compounds

Reduction of decyl dichloro- and trichloroacetate, under mild electrolysis conditions by using the sacrificial anode process, affords α-chlorocarbanions which readily react with trialkylboranes to give alkylated products in a one step reaction.