Stereoselective synthesis of syn-β-amino esters using the TiCl4/R3N reagent system

The reactions of benzaldehyde imines and esters with the TiCl₄/R₃N reagent system give syn-β-amino esters as the major products in 38-87% yields.     

Regioselective reaction of quinones with Reformatsky reagent: (BrZnCH2CO2Et·THF)2

Reformatsky reactions of p-quinones with crystalline reagent (BrZnCH₂CO₂Et·THF)₂ were investigated and took place successfully, providing β-hydroxy esters in high yield. Notably, in the case of 2,6-disubstituted-p-quinones, regioselective Reformatsky reactions occurred to give corresponding β-hydroxy esters in good yields.     

Transesterifications mediated by t-BuNH2

A mild protocol for transesterification of simple esters is described. The method is based on the use of t-BuNH₂/ROH (R = Me, Et, i-Pr, t-Bu) with or without LiBr. The scope of the procedure was explored for aliphatic and aromatic esters. The protocol is particularly useful when going from higher to lower hindered esters and harsh reaction conditions are needed for the reversal process. A rationalization of the mechanism is presented. The scope and limitation of this transformation are also described.     

Efficient cleavage of carboxylic tert-butyl and 1-adamantyl esters, and N-Boc-amines using H2SO4 in CH2Cl2

A new procedure for the deprotection of carboxylic tert-butyl and 1-adamantyl esters, and N-Boc-amines using H₂SO₄ in CH₂Cl₂ is described. The proposed method is simple, cheap, eco-friendly and represents a valid alternative to existing ones, with special significance in large scale applications.     

Amidation of esters assisted by Mg(OCH3)2 or CaCl2

Magnesium methoxide (Mg(OCH₃)₂) and calcium chloride have been shown to facilitate the direct aminolysis of esters by ammonia to primary amides. Methyl, ethyl, isopropyl, and tert-butyl esters were converted to the corresponding carboxamides in good yields. Reactions have been run on a larger scale and without the safety liability inherent in the use of magnesium nitride (Mg₃N₂). Ammonium chloride and amine hydrochlorides have been used successfully in the place of ammonia with magnesium methoxide.     

Fluorolactonization of norbornenecarboxylic acids and their methyl esters with F-TEDA-BF4 and XeF2

The selective fluorolactonization was achieved by treatment of cis-5-norbornene-2,3-endo-dicarboxylic acid or its monomethyl and dimethyl esters with F-TEDA-BF₄ or XeF₂. The reactions of 5-norbornene-endo-2-carboxylic acid and its monomethyl ester with F-TEDA-BF₄ or XeF₂ proceed in a non-selective manner to give fluorolactonization, addition and rearrangement products. The basic factor responsible for selectivity of the fluorolactonization is the presence of two endo-oriented carboxyl groups in the substrate molecule. The electrophilicity and type of the fluorinating agent is of secondary importance in this regard. It is postulated that the fluorolactonization of norbornenecarboxylic acids and their methyl esters with F-TEDA-BF₄ or XeF₂ is realized mainly via “open” fluoronorbornyl carbocation intermediates which in the reaction with XeF₂ are postulated as the tight ion pairs.     

Solvent complexes of the type [FeIII(CN)5L]n−

The title complexes have been prepared in situ by photolyzing the [Fe(CN)₅NO]^2? ion in some aliphatic alcohols, formamide and its derivatives, pyridine, acetonitrile and dimethylsulphoxide. Characterization of the complexes was made on the basis of three pieces of evidence: (i) the spectral properties, (ii) the presence of the titratable hydrogen in the complexes with protogenic solvents, and (iii) the anation reaction with the N₃^? and NCS^? ions.     

RuH2(CO)(PPh3)3-catalyzed arylation of aromatic esters using arylboronates via C-H bond cleavages

The RuH₂(CO)(PPh₃)₃-catalyzed C-H functionalization of aromatic esters with 5,5-dimethyl-2-aryl-[1,3,2]dioxaborinanes (arylboronates) gave the ortho arylation products. This coupling reaction can be performed with various combinations of isopropyl benzoate derivatives and arylboronates. Introduction of CF₃ group in the aromatic ring increased the reactivity of the esters. Pinacolone effectively served as an acceptor of a hydride generated by C-H bond cleavage, and the amount of pinacolone used also affected the yield of the arylation product.     

The first one-pot synthesis of alkoxycyclopropanes via cyclometalation of styrene with ClnAlEt3−n and RCO2R′ mediated by Cp2ZrCl2

The one-pot cyclopropanation of styrene using Cl_nAlEt_3−n (Et₂AlCl, EtAlCl₂, AlCl₃) and carboxylic esters in the presence of Cp₂ZrCl₂ as catalyst gives rise to alkoxycyclopropanes.     

N-tert-Butoxycarbonylation of amines using H3PW12O40 as an efficient heterogeneous and recyclable catalyst

The commercially available heteropoly acid H₃PW₁₂O₄₀ (0.5 mol %) is a very efficient and environmentally benign catalyst for N-tert-butoxycarbonylation of amines (primary, secondary) with di-tert-butyl dicarbonate at room temperature in short reaction times (<10 min). No competitive side products such as isocyanates, ureas, N,N-di-tert-butoxycarbonyls, O-Boc and oxazolidinones were observed. Chiral α-amino alcohols and esters afforded the corresponding N-Boc derivatives chemoselectively in excellent yields.     

Polymerization of methylmethacrylate with cetyl pyridinium bromide (CPB)-benzoyl peroxide (BZ2O2) combination as a redox initiator and with CPB as a lone photoinitiator

Polymerization of MMA was carried out in near-bulk and in fairly dilute solutions in DMF at 40° with (1) CPB-BZ₂O₂ combination as the redox initiator in the dark and (2) CPB as the sole photoinitiator in visible light. R_p was proportional to ([CPB] [BZ₂O₂])^0.18 for the redox polymerization and to [CPB]^0.35 for the photopolymerization both in near-bulk (10% DMF, v/v) and in high dilution (50% DMF, v/v). Polymerization was inhibited by hydroquinone in each case indicating a radical mechanism. Effect of several solvents/additives on the polymerization revealed that dimethyl formamide, acetonitrile and pyridine acted as rate enhancing solvents in the redox polymerization while in the photopolymerization they acted as normal diluents; benzene, methanol, acetone and chloroform exhibited inert nature while formamide and acetamide behaved as retarding additives in both types of polymerization. Initiator transfer of the degradative type reasonably explains kinetic nonidealities.     

AlCl3-NaI(NaBr)-t-BuOH: mild, chemo- and stereoselective reagents for hydrohalogenation of propiolic derivatives

(Z)-β-Iodo-propenamides and -β-iodo-propenoic esters were selectively prepared in high yields, at room temperature, through reaction of 2-propynamides and 2-propynoic esters, respectively, with AlCl₃ and NaI in the presence of t-BuOH in dichloromethane. These experimental conditions are compatible with the presence of acid sensitive acetal groups. Alternative use of EtOH or H₂O in place of t-BuOH was investigated. (Z)-Bromo-propenamides and corresponding esters were prepared according to a similar procedure using sodium bromide in refluxing acetonitrile.     

Synthesis, characterization and thermal properties of small R2R2NX-type quaternary ammonium halides

Twenty-one R₂R^′₂N⁺X⁻ -type (R=methyl or ethyl, R^′=alkyl, X=Br or I) quaternary ammonium (QA) halides have been prepared by using a novel one-pot synthetic route in which a formamide (dimethyl-, diethylformamide, etc.) is treated with alkyl halide in the presence of sodium or potassium carbonate. The formation of QA halides was verified with ¹H-NMR, ¹³C-NMR, MS and elemental analysis. The crystal structures of four QA halides (two bromide and two iodide) were determined using X-ray single crystal diffraction, and the powder diffraction method was used to study the structural similarities between the single crystal and microcrystalline bulk material. The thermal properties of all compounds were studied using TG/DTA and DSC methods. The smallest compounds decomposed during or before melting. The decreasing trend of melting points was observed when the alkyl chain length was increased. The liquid ranges of 120-180 °C were observed for compounds with 5-6 carbon atoms in the alkyl chain. The low melting points and wide liquid ranges suggest potential applicability of these compounds for example as ionic liquids precursors.     

Silphos [PCl3−n(SiO2)n]: a heterogeneous phosphine reagent for the conversion of epoxides to β-bromoformates or alkenes

Silphos [PCl_3−n(SiO₂)_n] as a heterogeneous phosphine reagent is efficiently applied for the transformation of epoxides to β-bromoformates in the presence of bromine or N-bromosuccinimide in dimethyl formamide at 0 °C. The combination of Silphos and iodine was also found suitable for the room temperature preparation of alkenes. The use of Silphos provides the advantage of easy separation of the phosphine oxide by-product from the reaction mixture.     

Excess molar enthalpies of formamide + some alkan-1-ols (C1-C6) and their correlations at 298.15 K

Excess molar enthalpies, H^E for the binary systems formamide+methanol, + ethanol, + propan-1-ol, + butan-1-ol, + pentan-1-ol, and + hexan-1-ol have been measured at 298.15 K and atmospheric pressure with a Paar 1455 solution calorimeter. All the system present endothermic events and showed maximum positive H^E values around 0.40-0.50 mole fraction of formamide. The H^E values increases in the order: methanol<ethanol<propan-1-ol<butan-1-ol<pentan-1-ol<hexan-1-ol. Experimental showed insolubility of hexan-1-ol in formamide around x≅0.5 mole fraction of formamide. The excess enthalpies of the above mentioned binary systems, were used to discuss interaction between the alkan-1-ols and formamide molecules. The results are interpreted to gain insight into the changes in molecular association equilibria and structural effects in these systems through O···HO hydrogen bonding. The experimental data have been correlated using Redlich-Kister polynomials. In this research work, the thermodynamics models were also tested: NRTL, Wilson models and their parameters were calculated. The correlation of excess enthalpy data in the systems using NRTL model provides good results.     

A simple, convenient method for the synthesis of maleic anhydrides from α-keto esters and alkanoic acid anhydrides using the TiCl4/n-Bu3N reagent system

Reaction of α-keto esters with alkanoic acid anhydrides using the TiCl₄/n-Bu₃N reagent system gives the corresponding maleic anhydrides in 62-95% yields.     

Reductive esterification of aromatic aldehydes using Zn/Ac2O/imidazole or Zn/Yb(OTf)3/(RCO)2O system

Benzaldehydes are reduced by metallic zinc in the presence of Ac₂O and imidazole, giving the corresponding benzyl acetates in good yields. Reductive esterification of aromatic aldehydes is also carried out via gem-diacetoxy compounds. Carbonyl compounds are readily converted to the gem-diacyloxy compounds in excellent yields on treatment with 2 molar amounts of acid anhydride and 10 mol% of Yb(OTf)₃ in MeCN at room temperature. Thus-formed diacyloxy compounds derived from aromatic aldehydes are reduced in situ by metallic zinc to afford the corresponding esters.     

Synthesis of cis-2-aryl-3-pyrrolidine carboxylic esters via diastereoselective cyclization of γ-imino esters using a TiCl4/Et3N reagent system

Facile synthesis of cis-2-aryl-3-pyrrolidine carboxylates from readily accessible γ-imino esters by intramolecular cyclization mediated by a TiCl₄/Et₃N reagent system is described.     

Zirconium-catalyzed cyclopropanation of α-olefins mediated by R′CO2R″ and ClnAlEt3-n

A Cp₂ZrCl₂-catalyzed one-pot cyclopropanation method has been developed to afford alkoxycyclopropanes and cyclopropanols from α-olefins involving esters of carboxylic acids and Cl_nAlEt_3-n.     

Polymer-mounted N3P(MeNCH2CH2)3N: a green, efficient and recyclable catalyst for room-temperature transesterifications and amidations of unactivated esters

Merrifield resin-supported N₃P(MeNCH₂CH₂)₃N shows excellent activity in the transesterification of higher esters such as glyceryl tribenzoate to methyl esters. The catalyst was successfully cycled 20 times (albeit with an increase in reaction time) without compromising yield up to the 20th cycle. The catalyst also showed good performance in amidation reactions of unactivated esters with amino alcohols.