Study of the isomerization of optically active 2-bromo-1-propyl pivalate

Conclusions In a study of the isomerization of optically active 2-bromo-1-propyl pivalate in aprotic solvents, the reaction was found to proceed with the retention of the stereochemical configuration of the starting compound. This finding indicates the formation of cyclic acyloxonium intermediate during the isomerization reaction.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 724–725, March, 1989.     

Stereospecific nickel-catalyzed cross-coupling reactions of alkyl ethers: enantioselective synthesis of diarylethanes

Secondary benzylic ethers undergo stereospecific substitution reactions with Grignard reagents in the presence of nickel catalysts. Reactions proceed with inversion of configuration and high stereochemical fidelity. This reaction allows for facile enantioselective synthesis of biologically active diarylethanes from readily available optically enriched carbinols.     

An unprecedented memory of macromolecular helicity induced in an achiral polyisocyanide in water

We have found an unprecedented memory of macromolecular helicity induced in an achiral sodium salt of poly(4-carboxyphenyl isocyanide) (poly-1-Na). Poly-1-Na folds into a one-handed helix through configurational isomerization around the C=N backbones by interactions with optically active amines in water. The helix remains when the optically active amines are completely removed, and further modifications of the side group to carboxy and esters can be possible without loss of the macromolecular helicity memory.     

Hydrogen-transfer polymerization of acrylamide and methacrylamide with optically active basic catalysts

Hydrogen-transfer polymerization of acrylamide and Methacrylamide with an optically active amyl alcoholate or n-amyl alcoholate (sodium, calcium, magnesium, barium, and aluminum) was investigated to 100°C in toluene. The initiation ability of the metal ion of the initiator increased in the order, sodium > barium > calcium > magnesium > aluminum. The optically active polymer was obtained by the polymerization of methacrylamide with an optically active alcoholate (barium or calcium), but was not obtained by the other alcoholates and by the polymerization of acrylamide with the optically active alcoholate. The specific rotation of the optically active polymer obtained was about +1.1° ～ +1.3°. The hydrolyzed product of the optically active polymer was α-methyl β-alanine having optical activity (+1.0°). The initiation mechanisms of the polymerization were thought to be the dehydrogenation of the monomer of the negative ion and the Michael addition reaction with the monomer of the negative ion and the catalyst, and it was confirmed that the optically active polymer was prepared by intermolecular hydrogen transfer mechanism. In the polymerization of MMA with menthol barium and borneol barium as the optically active catalyst, the optically active polymer was obtained.     

有机钛化学——Ⅳ.某些新型双和单(烯基环戊二烯基)钛衍生物的合成与催化烯烃异构化

用几种不同路线设计和合成了16个新型的烯基取代环戊二烯及其钛衍生物。取代环戊二烯基钛催化剂在烯烃异构化反应中具有较高的活性和选择性。对反应机理进行了初步的研究。     

CpRu(PN) complex-catalyzed isomerization of allylic alcohols and its application to the asymmetric synthesis of muscone

Highly efficient isomerization of allylic alcohols into saturated carbonyls is accomplished using the catalyst system of Cp*RuCl[Ph2P(CH2)2NH2-kappa2-P,N]-KOt-Bu (Cp* = eta5-C5(CH3)5) under mild conditions. Mechanistic consideration based on isotope-labeling experiments indicated the present reaction is applicable to the asymmetric isomerization of racemic sec-allylic alcohols with a prochiral olefin via dynamic kinetic resolution. A concise asymmetric synthesis of muscone has been achieved, where the asymmetric isomerization using an optically active ligand is a key reaction.     

New Polymer Structures

A number of new polymers have been studied. They include polymers obtained by chain reaction polymerization of olefins and epoxides with functional groups where the polymerizable group was separated from the functional group by an inert spacer group, the preparation of head-to-head polymers, and the preparation of optically active polymers based on the macromolecular asymmetry.     

Enantioselective borodeuteride reduction of aldimines catalyzed by cobalt complexes: preparation of optically active deuterated primary amines

[reaction: see text] The enantioselective borodeuteride reduction catalyzed by optically active beta-ketoiminato cobalt complexes was applied to N-(di(o-tolyl)phosphinyl)aldimines to afford the corresponding optically active deuterated primary amines in high yields with high enantiomeric excesses after simple deprotection. The present deuteride reduction of aldimines is in the opposite sense of the enantioselective for the previously reported borohydride reduction of ketones or diphenylphosphinyl aldimines. The stereochemical course in these enantioselective reductions is discussed.     

Formation of supported size-controlled nanoparticles of sulfated zirconia

Methods of the preparation of catalysts for alkane skeletal isomerization based on uniform nanoparticles of sulfated zirconia anchored to different supports were investigated. These catalysts were characterized by using the ICP, HRTEM and BET techniques. The activities of the catalysts in the reaction of n-butane isomerization were measured and compared with those of bulk catalysts.     

Mechanistic switch leading to highly efficient chirality transfer in Pd(0)-catalyzed coupling-cyclization of aryl iodides with 1:1 acid-base salts of 2,3-allenoic acids and L-(-)-cinchonidine or D-(+)-/L-(-)-alpha-methylbenzylamine. Enantioselective synthesis of highly optically active 3-aryl polysubstituted butenolides

An efficient methodology provides an easy access to highly optically active polysubstituted butenolides starting from aryl halides and 1:1:salts of optically active 2,3-allenoic acid-base via an oxidative addition-coordinative cyclization-reductive elimination mechanism, which led to the high efficiency of this chirality isomerization reaction.     

α-Hydroxy carboxylic acids as ligands for enantioselective addition reactions of organoaluminum reagents to aromatic and aliphatic aldehydes

The first examples of the enantioselective titanium-mediated trialkylaluminum additions to aromatic and aliphatic aldehydes catalyzed by optically active α-hydroxy acids are presented. The reactions proceed with very good yields and in good asymmetric induction. Enantioselectivities up to 92% are obtained depending on the ligand and aldehyde used. A stereochemical model for the reaction is proposed.     

α-Hydroxy carboxylic acids as ligands for enantioselective diethylzinc additions to aromatic and aliphatic aldehydes

The first examples of the enantioselective titanium-mediated diethylzinc additions to aromatic and aliphatic aldehydes catalyzed by optically active α-hydroxy acids are presented. The reactions proceed with very good yield and good asymmetric induction. Enantioselectivities up to 90% are obtained depending on ligand and aldehyde used. A stereochemical model for the reaction is proposed.     

Self-supported heterogeneous titanium catalysts for enantioselective carbonyl-ene and sulfoxidation reactions

A new strategy for the heterogenization of chiral titanium complexes was developed by the in situ assembly of bridged multitopic BINOL ligands with [Ti(OiPr)4] without using a support. The assembled heterogeneous catalysts (self-supported) showed excellent enantioselectivity in both the carbonyl-ene reaction of alpha-methylstyrene with ethyl glyoxylate (up to 98 % ee) and the oxidation of sulfides (up to >99 % ee). The catalytic performance of these heterogeneous catalytic systems was comparable or even superior to that attained with their homogeneous counterparts. The spacers between the two BINOL units of the ligands in the assembled catalysts had significant impact on the enantioselectivity of the carbonyl-ene reaction. This demonstrates the importance of the supramolecular structures of the assemblies on their catalytic behavior. In the catalysis of sulfoxidation, the self-supported heterogeneous titanium catalysts were highly stable and could be readily recycled and reused for over one month (at least eight cycles) without significant loss of activity and enantioselectivity (up to >99.9 % ee). The features of these self-supported catalysts, such as facile preparation, robust chiral structure of solid-state catalysts, high density of the catalytically active units in the solids, as well as easy recovery and simple recycling, are particularly important in developing methods for the synthesis of optically active compounds in industrial processes.     

Enantioselective Aza-Diels Alder reaction catalyzed by clay supported Schiff base complex†

Clay supported catalysts based on 2- hydroxy naphthaldehyde and amino acids, their copper and titanium complexes have been described. The catalysts were found to be active toward the synthesis of pyrano-, furanoquinolines through Aza-Diels Alder reaction. The catalysts gave good yield and high enantioselectivity. The catalysts were environmentally friendly, economical, synthetically robust and reusable.     

Enantioselective sulfa-Michael addition of thioacids to α,β-unsaturated ketones with bifunctional organocatalyst

Organocatalytic conjugate addition of thioacids to α,β-unsaturated ketones has been studied in the presence of cinchona alkaloid derived urea catalyst. Both the enantiomers of products are accessible with the same level of enantioselectivity using pseudoenantiomeric quinine/quinidine derived catalysts. The catalytic process provides optically active thioesters with high chemical yields (up to 99%) and useful enantioselectivity (up to 83% ee). The reaction was performed with 1 mol % of catalyst in toluene at room temperature. A transition state model has been proposed to explain the stereochemical outcome of the reaction.     

Highly chemo-, diastereo-, and enantioselective reduction of 1,2-dialkyl-3-aryl-1,3-diketones for preparation of aldol-type compounds

[reaction: see text]. Highly chemo-, diastereo-, and enantioselective borohydride reduction of 2-substituted-1,3-diketones was achieved in the presence of the optically active beta-ketoiminato cobalt complex catalysts to afford the optically active 2-substituted-3-hydroxyketones. The present catalytic and enantioselective reduction could provide an alternative potential for preparation of optically active anti-aldol-type compounds.     

Stereo- and enantioselective polymerization of hexa-1,5-diene on heterogeneous Ziegler-Natta catalysts obtained with the use of optically active internal donors

Pure racemates and individual enantiomers of diethyl 2,3-diisopropylsuccinate ((+,−)-DIPS and (−)-DIPS) and 2,3-diisopropyl-1,4-dimethoxybutane ((+,−)-DPDMB and (+)-DPDMB) were prepared. The synthesized compounds and diethyl phthalate were used as internal donors for the preparation of heterogeneous Ziegler-Natta catalysts. Polymerization of hexa-1,5-diene was carried out on these catalysts. The obtained samples of poly(methylene-1,3-cyclopentane) were studied by ¹³C NMR spectroscopy and polarimetry. The induction of optical activity to polymer is observed with (−)-DIPS as the internal donor. The induction value makes up 15–20% from the value recorded in the case of optically active metallocene catalysts. The emergence of induction may be associated with the presence of close contact between the titanium atom and the donor molecule within the active site of catalyst, as well as with the fact that the donor molecule either deactivates a part of the stereospecific titanium centers or influences the structure of the titanium centers during their formation and development.     

The Isocyanide–Cyanide Rearrangement; Mechanism and Preparative Applications

Until recently the isocyanide–cyanide rearrangement was of interest almost solely as an example of a unimolecular gas-phase reaction, and kinetic studies had been carried out in only a few simple cases. Kinetic measurements in solution were made possible only by the discovery and suppression of a parallel free-radical chain process which leads to the same products. The rate of the isomerization is almost independent of the structure of the starting material and of the substituents present. An exception is provided by extreme steric hindrance in three dimensions which, as in tris-α-substituted triptycyl isocyanides, leads to a considerable increase in the activation energy. The results can be interpreted in terms of a purely sigmatropic mechanism, as predicted by ab initio calculations. The preparative application of this rearrangement reaction requires the suppression of side reactions and can best be carried out by flash pyrolysis; yields are then almost quantitative. Allyl isocyanides react without allyl isomerization, optically active isocyanides with complete retention of configuration. New, economically interesting syntheses for the known nonsteroidal anti-inflammatory drugs ibuprofen and (S)-naproxene are described. The application of the useful synthetic building blocks, the optically active β-acyloxy cyanides, which are formed from optically active α-amino acids, will be demonstrated.     

Stereospecific Cyclopolymerization with Group 4 Metallocenes

Homogeneous Ziegler-Natta catalysts are stereoselective cyclopolymerization catalysts for non-conjugated dienes. Cyclopolymerization of 1,5-hexadiene affords poly(methylene-l,3-cyclopentane) (PMCP), a polymer for which four structures of maximum order are possible. A variety of metallocene catalyst precursors have been investigated; the molecular weight and microstructure of the polymers are sensitive to the structure of the catalyst precursor as well as the reaction conditions. The selectivity for cyclization depends on reaction conditions; decreasing the olefin concentration and increasing the temperature of the reaction favor cyclization. The stereochemistry of cyclopolymers can also be controlled with appropriate choice of catalyst precursor. Diastereoselective cyclopolymerization of 1,5-hexadiene with achiral catalysts yields atactic trans-PMCP and cis-PMCP, depending on the catalyst precursor. Enantioselective cyclopolymerization with optically active catalysts yields optically active poly(methylenecyclopentane), a novel example of a polymer which is chiral by virtue of its main-chain stereochemistry.     

Studies on Ziegler-Natta catalysts. Part V. Stereospecificity of the active center

Experimental results on Ziegler-Natta catalysts, based on observations made with the electron microscope, and a qualitative comparison of the stereospecificity of various catalyst combinations are given. The polymerization of olefin in these experiments is performed in the gas phase on dry catalysts in the absence of solvent or excess aluminium alkyl. The crystallographic structure of the lateral faces of α-TiCl₃ is established by electron microscopy and electron diffraction. The electron micrographs of α-TiCl₃–AlMe₃ catalysts show that the active centers, which are revealed by the dotwise formation of polymer, are located along the growth spirals, on lateral faces, and on surface defects. These regions of the surface are the only regions in which the surface titanium atoms are incompletely coordinated. The presence of chlorine vacancies and exposed titanium atoms is therefore an essential condition for the formation of active centers. However, the number of active centers is small in comparison to the number of incompletely coordinated titanium atoms, and hence it is concluded that the normally occurring α-TiCl₃ sites with one vacancy do not yield active centers on reaction with aluminum alkyl. It is proposed that the reaction with aluminum alkyl on such sites leads ultimately to a bimetallic complex which fills the original vacancy on the titanium atom. That the complexation is reversible and that the deblocked alkylated site, which is of the type proposed by Cossee, is an active center is not excluded. Such a center would, however, give atactic polymer. Similar complex formation on a TiCl₃ site having originally two vacancies would leave one vacancy on the titanium atom. This is believed to be the center of stereospecific polymerization. A model of this active center and a mechanism of polymer growth on it are proposed.