Mechanistic approaches to asymmetric synthesis of aziridines from guanidinium ylides and aryl aldehydes

To approach more realistic mechanisms for asymmetric aziridine synthesis from guanidinium ylides and aryl aldehydes, reactions were systematically carried out by using a variety of p-substituted benzaldehydes under modified conditions. Two kinds of reaction mechanisms controlled by the nature of the p-substituents of aryl aldehydes is proposed for the two-steps aziridine synthesis composed of a C-C bond formation by nucleophilic addition of guanidinium ylides to aryl aldehydes (step 1) and the fragmentation of intermediate adducts to aziridine products by intramolecular nucleophilic substitution (step 2). A S_Ni-like mechanism via cationic-like transition state is proposed for step 2 in the asymmetric synthesis using EDG-substituted benzaldehydes, whereas with EWG-substituted benzaldehydes, a S_N2-like mechanism is proposed. Hammett analysis, based on the diastereomeric ratio in the aziridine products, is consistent with the proposed rate-determining steps in these two mechanisms. A second Hammett analysis, based on the enantiomeric ratio of the aziridine products, clearly reveals the difference in the susceptibilities to the electronic substituents effect between step 1 and step 2.     

tert-Butylnitrite as a convenient and easy-removable oxidant for the conversion of benzylic alcohols to ketones and aldehydes

The oxidation of primary and secondary benzylic alcohols was achieved by using tert-butyl nitrite (t-BuONO) as a stoichiometric oxidant. Various substrates were effectively converted into the corresponding ketones or aldehydes in good to excellent yields. The reaction presumably proceeded by a nitrosyl exchange and a subsequent thermal decomposition of benzylic nitrites. This process would realize an oxidation of alcohols with oxygen in theory by combining with a reproduction of alkyl nitrites from NO and alcohols under an O₂ atmosphere. In addition, almost pure oxidized products were readily obtained by simple evaporation of the reaction mixtures since t-BuONO produced only volatile side products.     

The stobbe condensation with dimethyl homophthalate. II

The condensation of some heterocyclic aldehydes with dimethyl homophthalate in the presence of either sodium hydride or potassium t-butoxide as condensing agents gave either the (E)-or the (Z)-half-ester (1b) as the predominant product. The structure and configuration of the products has been established by both chemical and spectroscopic tools.     

Electronic structure and disorder in NaxCoO2 and SrRh2O4

We discuss the electronic structure of Na_xCoO₂ from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5 + x leading to a nearly full Co t_2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the e_g bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from a_g symmetry states and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh₂O₄. Like Na_xCoO₂ it shows a large crystal field gap between narrow t_2g and e_g manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.     

Catalytic allylic transfer reactions of functionalized aldehydes promoted by BINOL-Ti(IV) with synergistic reagent

Practical and efficient catalytic asymmetric allylic transfer reactions of tin reagents promoted by BINOL-Ti[OCH(CF₃)₂]₂ complex by the utilization of t-BuSBEt₂ are successful with a variety of functionalized aldehydes containing ketone, aldehyde, ester, amide, or carbamoyl functionality and affords products in high levels of enantioselectivity.     

Bi2O3-catalyzed oxidation of aldehydes with t-BuOOH

A variety of aromatic, aliphatic and conjugated aldehydes were transformed to the corresponding carboxylic acids with 70% t-BuOOH solution (water) in the presence of catalytic amounts of Bi₂O₃. This method possesses a wide range of capabilities, does not involve cumbersome work-up, exhibits chemoselectivity and proceeds under mild conditions. The resulting products are obtained in good yields within reasonable time. The overall method is green.     

Development of chiral dinitrones as modular Lewis base catalysts: asymmetric allylation of aldehydes with allyltrichlorosilanes

Chiral dinitrones were synthesized by the condensation of a C₂-symmetrical chiral dihydroxylamine with various aldehydes. The electronic and steric properties of the dinitrones can be modified by changing the aldehyde component. The activity of dinitrones as Lewis base catalysts was examined for the asymmetric allylation of aldehydes with allyltrichlorosilanes. Using DMPU as an additive in chloroform, the reaction proceeded at room temperature to afford allylated products in good yields and good enantioselectivities.     

Efficient method for the oxidation of aldehydes and diols with tert-butylhydroperoxide under transition metal-free conditions

An efficient, mild, and simple protocol is presented for the oxidation of aldehydes and diols to carboxylic acids utilizing 70% aq TBHP as oxidant and t-BuOK as additive. The oxidation of aldehydes could be achieved by two methods under aqueous medium. Excellent yields of products were obtained in short reaction times. Notably, the products were isolated by simple filtration technique and do not involve chromatographic separation. These reactions may prove to be valuable alternatives to traditional metal-mediated oxidations. Oxidation does not require any transition metals or organic solvents in reaction, making this protocol green.     

Vinyl-copper derivatives—XI: Reactivity of Z-alkenyl cuprates towards various electrophiles. Application to the synthesis of some natural products

Z-Alkenylcuprates 1 and 2, prepared in situ by addition of acetylene to alkylcuprates, react with a variety of electrophiles (epoxides, carbon dioxide, aldehydes) and give conjugate addition products with α,β-unsaturated aldehydes, ketones and esters, and with activated cyclopropanes. They also add across the triple bond of some alkynes. The synthesis of natural products (7,9,22) is described.     

Interrelations between electronic structure and spatial geometry of specific ligands in the functioning active site of some pyridoxal-p-dependent enzymes

Using a spectrokinetic approach the absorption spectra of the short-lived transient products in the enzymatic reaction of the glutamate decarboxylase with natural substrate are determined for the first time. The quantum-chemical calculations of the electronic structure and spectra of various ionic species of numerous vitamin B₆ derivatives allowed a hypothesis on the nature of the intermediate products detected to be suggested. A model of the enzyme functioning taking into account the charge equilibria and some electronic–conformational relations is proposed.     

tert-Butyl esters of tripeptides based on Pro-Phe as organocatalysts for the asymmetric aldol reaction in aqueous or organic medium

The enantioselective aldol reaction between ketones and aldehydes constitutes one the most common reaction models for the evaluation of novel organocatalysts. The last few years, it has been shown that the organocatalytic aldol reaction can be performed in water. A family of tripeptides consisting of proline, phenylalanine, and tert-butyl esters of amino acids was successfully employed in this asymmetric transformation. The products of the reaction between various ketones and aldehydes were obtained in high yields (up to 99%) with excellent diastereo- (up to 97:3 dr) and enantioselectivities (up to 99% ee). The C-terminal amino acid determines the ability of the tripeptide (Pro-Phe-AA-O^tBu) to act efficiently in aqueous or organic medium.     

Prediction of retention indices. VI: Isothermal and temperature-programmed retention indices,methylene value,functionality constant,electronic and steric effects

The Kováts retention index system with n-alkanes as reference standards has properties not fully explored when single, isolated or stand-alone analytes are analyzed by isothermal gas chromatography. When a homologous series of analytes are analyzed by either linear or non-linear temperature-programmed gas chromatography, the retention data of the entire series can be treated systematically to produce an I vs. Z plot that is linear, thereby giving insight into the relationship between chemical structure and retention index. Dead time t_M is both instrument and temperature dependent. With no dead time t_M adjustment, the retention indices of analytes calculated from experimental retention times by the method of either linear or logarithmic interpolation give statistically identical values. Linear regression analysis of the data shows the slope as methylene value (A) and intercept as functionality constant or group retention factor (GRF) of the homologous series. The A and (GRF) values vary with chemical structures, intermolecular electronic and steric interactions, and polarity of column liquid phases, and can link gas chromatographic retention index to chemical structure. Examples of the influence of molecular electronic effects and steric effects on retention index are given and discussed.     

Stereocontrolled addition of boron enolates to trans α,β-aziridine aldehydes. A new route to anti-1,2-amino alcohols

A study of the addition of boron enolates of methyl ketones to trans α,β-aziridine aldehydes is reported. The reaction proceeds with excellent anti stereoselectivity furnishing functionalized products, capable of other controlled transformations, some of which are described.     

Chiral N,N′-dioxide-Yb(III) complexes catalyzed enantioselective hydrophosphonylation of aldehydes

Chiral N,N′-dioxide-Ytterbium(III) complexes promoted the asymmetric addition of diethyl phosphate to aldehydes, giving the corresponding products with good yields and enantioselectivities. The addition of pyridine favored both reactivity and enantioselectivity. A possible catalytic cycle was proposed to explain the mechanism of the asymmetric hydrophosphonylation of aldehydes.     

Asymmetric acyloin condensation catalysed by phenylpyruvate decarboxylase. Part 2: Substrate specificity and purification of the enzyme

Phenylpyruvate decarboxylase from Achromobacter eurydice was used to catalyse the asymmetric acyloin condensation of phenylpyruvate 1 with various aldehydes 2 to produce optically active acyloins PhCH₂COCH(OH)R 3. The specific activity of the phenylpyruvate decarboxylase enzyme was increased by a factor of 332 after its purification. The molecular weight of the purified enzyme was shown to be 150 kDa by gel filtration chromatography, while SDS gel electrophoresis showed two sub-units with molecular weights of 90 and 40 kDa. The acyloin condensation yield decreased with increasing chain length for straight chain aliphatic aldehydes from 76% for acetaldehyde to 24% for valeraldehyde. The e.e.s of the acyloin products were 87–98%. Low yields of acyloin products were obtained with chloroacetaldehyde (13%) and glycoaldehyde (16%). Indole-3-pyruvate was a substrate of the enzyme and provided acyloin condensation product 3-hydroxy-1-(3-indolyl)-2-butanone 5 with acetaldehyde in 19% yield, while benzoylformate was not a substrate for the enzyme.     

Synthesis of 4-substituted pyrano[4,3-b]pyran-2,5-diones in an ionic liquid

The mildly basic ionic liquid N,N,N,N-tetramethylguanidinium triflate (TMGTf) was found to be a very effective solvent for the reaction between 4-hydroxy-6-methyl-2H-pyran-2-one, Meldrum's acid, and aldehydes to afford some novel pyrano[4,3-b]pyran-2,5-diones in high yields at room temperature. The stages, through which these reactions might proceed, depend largely on the nature of the aldehyde substrates. The reaction of aliphatic aldehydes has given access to some novel carboxylated products.     

Electronic structure of saturated hydrocarbons in the semi-empirical equivalent orbital method

The problem of obtaining the matrix elements of Hartree-Fock Hamiltonians for alkanes using the EO method is considered. It has been shown that the data on the electronic structure of diamond together witht ₁/e splitting in the neopentane photoelectron spectrum are helpful to produce such EO method parameter scale which involves even “through space” interactions. In terms of the EO method the photoelectron spectra of propane, butane, isobutane, and neopentane are interpreted. The valence band structure of polyethylene in analytical form is obtained.     

Diasteroselective synthesis of oxazolidines and imidazolidines via the Lewis acid catalyzed C-C cleavage of aziridines

In this work, cis-2,5-disubstitutedoxazolidines were efficiently constructed via a regioselective C-C bond cleavage of N-tosylaziridine 2,2-dicaboxylates and a subsequent [3+2] cycloaddition with aromatic aldehydes in the presence of Zn(OTf)₂. The reactions were highly diastereoselective to form oxazolidines in cis configurations. trans-2,5-Disubstituted imidazolidines were also diastereoselectively synthesized in the similar manner using imines as substrates and AgOTf as catalyst. Based on the detailed investigation of the substrate diversity for both reactions, including the electronic effects and the steric effects of the substituted groups on the aziridines, aldehydes, and imines, a stepwise mechanism was postulated for the diastereoselective formation of cis-2,5-disubstitutedoxazolidines and trans-2,5-disubstituted imidazolines.     

[Ir(COD)Cl]2/tris(2,4-di-t-butylphenyl)phosphite-catalyzed addition reactions of arylboronic acids with aldehydes

[Ir(COD)Cl]₂/tris(2,4-di-t-butylphenyl)phosphite-catalyzed addition reactions of arylboronic acids with aldehydes were described. The Ir(I) catalyst, generated from [Ir(COD)Cl]₂ and tris(2,4-di-t-butylphenyl)phosphite, was an efficient catalyst system for the addition reactions of a variety of arylboronic acids with aromatic and aliphatic aldehydes. The easy availability of the catalyst and good yields make these reactions potentially useful in organic synthesis.     

Polymerization of methylmethacrylate by organometallic compounds—VIII. Initiation by n-, i-, s- or t-butylmagnesium compounds—evidence for an eneidic pseudoanionic mechanism

The molar-mass distributions of PMMA initiated by various n-, s-, i- and t-butylmagnesium compounds were investigated. In most cases the distributions were polymodal. In some cases the different peaks were shown to have different chain configurations. The position of the peaks of the distribution was independent of THF concentration in the solvent mixture, but the relative amount of polymer in each peak changed, as did the steric triad composition of the whole sample. It is postulated that the molar-mass distribution and stereospecificity of the polymer are controlled by an eneidic mechanism in which persistent, independent growth-centres of different reactivity and stereospecificity are formed in the initiation step. The character of the polymer produced is primarily determined by conditions prevailing in the initiation stage: structure of the butyl group; presence or absence of halide; solvent composition and temperature.The concentrations of the various growth sites (down to the μM range, with the more reactive) and their reactivities have been calculated, subject to certain assumptions concerning the mechanism. Some conclusions about the structures of some of them can be drawn, but, in general, no direct evidence is obtainable.In cases where the mechanism was examined in detail, the propagation reaction was most easily explained in terms of the formation of a monomer-complex at the growth site which proceeds through a four-centre orientation to insert in a covalent MgC bond. The mechanism is probably more accurately classified as pseudoanionic rather than anionic.