DELETEile chiral bidentate ligands derived from alpha-amino acids: synthetic applications and mechanistic considerations in the palladium-mediated asymmetric allylic substitutions

A new class of chiral amidine-phosphine hybrid ligands 7a,b, which are readily accessible from the corresponding alpha-amino acids, were developed. A versatility for construction of new ligands is desirable, by which a variety of reactions and substrates become applicable. Indeed, a variety of modifications, such as exchange reactions to other amino groups in the amidine skeleton and the production of other types of ligands, are possible using the precursor compounds of 7a. Thus, novel chiral ligands 7c,d, 8, 11, and 13, which provide sterically and electronically different chiral circumstances, were prepared and used for the palladium-mediated asymmetric allylic substitutions of both acyclic and cyclic compounds. In these reactions, high levels of asymmetric induction were achieved for both substrates. A marked advancement of reactivity and enantioselectivity in palladium-catalyzed asymmetric allylations of 1,3-diphenylpropen-2-yl pivalate 14a was attained by examination of electronic substituent effects in a new series of chiral P-N and S-N hybrid ligands 8 and 11. Mechanistic views concerning the enantiodiscriminating step were demonstrated, in which a good correlation between a novel Pr/Mr concept and the absolute configuration of allylation products are discussed for the prediction of enantioselecting direction. The use of ketene silyl acetals as nucleophiles was investigated and compared with the corresponding harder anionic carbon nucleophiles. The former nucleophiles afforded higher enantioselectivity in asymmetric allylic transformations of 14a.     

Shifting the equilibrium of a biocatalytic cascade synthesis to enantiopure epoxides using anion exchangers

Hydroxide-loaded anion exchangers have been successfully employed to shift the equilibrium of a one-pot, two-step, two-enzyme cascade reaction affording enantiopure epoxides starting from prochiral α-chloroketones. The α-chloroketones were asymmetrically reduced employing an alcohol dehydrogenase and then transformed further to the corresponding epoxides employing halohydrin dehalogenases. Each epoxide enantiomer could be obtained with up to 93% conversion in enantiomerically pure form (>99% ee). In contrast to previous studies the amount of hydride donor (2-propanol) could be reduced due to favoured halohydrin formation in the reduction of α-chloroketones.     

Synthesis of Cyclic Carbonates Catalysed by Aluminium Heteroscorpionate Complexes

New aluminium scorpionate based complexes have been prepared and used for the synthesis of cyclic carbonates from epoxides and carbon dioxide. Bimetallic aluminium(heteroscorpionate) complexes 9 – 14 were synthesised in very high yields. The single‐crystal X‐ray structures of 12 and 13 confirm an asymmetric κ²‐NO‐μ‐O arrangement in a dinuclear molecular disposition. These bimetallic aluminium complexes were investigated as catalysts for the synthesis of cyclic carbonates from epoxides and carbon dioxide in the presence of ammonium salts. Under the optimal reaction conditions, complex 9 in combination with tetrabutylammonium bromide acts as a very efficient catalyst system for the conversion of both monosubstituted and internal epoxides into the corresponding cyclic carbonates showing broad substrate scope. Complex 9 and tetrabutylammonium bromide is the second most efficient aluminium‐based catalyst system for the reaction of internal epoxides with carbon dioxide. A kinetic study has been carried out and showed that the reactions were first order in complex 9 and tetrabutylammonium bromide concentrations. Based on the kinetic study, a catalytic cycle is proposed.     

Asymmetric Conjugate Additions and Allylic Alkylations Using Nucleophiles Generated by Hydro‐ or Carbometallation

This Minireview discusses catalytic asymmetric conjugate addition and allylic alkylation reactions where the nucleophiles were generated in situ by hydrometallation or carbometallation. This exciting recent trend in asymmetric catalysis promises to expand the range of transformations available for the rapid and selective assembly of complex, functional molecules for both academic and industrial research. This Minireview aims to serve as a reference for studies reported to date and discusses the current state‐of‐the‐art, scope and limitations of these processes.     

Acid Catalyzed Multicomponent Reaction to Access Functionalized N-Benzhydryl Amides: A Tandem Ritter Reaction

Acid catalyzed multicomponent reaction (MCR) for the synthesis of N-benzhydryl amide derivatives from aldehydes, N,N-disubstituted arylamines and nitriles is reported. The reaction is compatible with electronically differentiating aryl/heteroaryl aldehydes/acetals, different nucleophiles (cyclic and acyclic N,N-disubstituted arylamines,β-naphthols, 1,3 dicarbonyl, 1,3,5-trimethoxy benzene), alkyl nitriles, aryl/heteroaryl nitriles in catalytic TFA/TfOH through tandem Ritter reaction. The one-pot MCR with broad substrate scope generated a wide variety of sterically hindered N-substituted amides and is successfully applied for the synthesis of isoindolinone.     

Reductive spiroannulation of nitriles with secondary electrophiles

The scope of reductive decyanation and spiroannulation reactions has been expanded to include secondary electrophiles for potentially useful transformations. Secondary phosphates and chlorides, as well as terminal epoxides, cyclize in a stereospecific fashion. Both endo and exo modes of cyclization were observed with terminal epoxides.     

Diastereoselective nucleophilic substitution reactions of oxasilacyclopentane acetals: application of the "inside attack" model for reactions of five-membered ring oxocarbenium ions

The additions of nucleophiles to oxocarbenium ions derived from oxasilacyclopentane acetates proceeded with high diastereoselectivity in most cases. Sterically demanding nucleophiles such as the silyl enol ether of diethyl ketone add to the face opposite the C-2 substituent. These reactions establish the syn stereochemistry about the newly formed carbon-carbon bond. Small nucleophiles such as allyltrimethylsilane do not show this same stereochemical preference: they add from the same face as the substituent in C-2-substituted oxocarbenium ions. The stereoselectivities exhibited by both small and large nucleophiles can be understood by application of the "inside attack" model for five-membered ring oxocarbenium ions developed previously for tetrahydrofuran-derived cations. This stereoelectronic model requires attack of the nucleophile from the face of the cation that provides the products in their lower energy staggered conformations. Small nucleophiles add to the "inside" of the lower energy ground-state conformer of the oxocarbenium ion. In contrast, sterically demanding nucleophiles add to the inside of the envelope conformer where approach is anti to the C-2 substituent of the oxocarbenium ion, regardless of the ground-state conformer population.     

Asymmetric catalytic anhydride openings via carbon-based nucleophiles

The asymmetric desymmetrization of cyclic anhydrides via the addition of carbon-based nucleophiles has been the focus of considerable levels of interest because it leads to optically active products. Over the past 20 years, a variety of different catalytic asymmetric alkylation reactions have been developed for the desymmetrization of cyclic anhydrides using different metal reagents as nucleophiles and using chiral ligands. The purpose of this review is to provide an overview of significant developments in this field. ~ 2013 Fen-Er Chen. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.     

Geminal dicarboxylates as carbonyl surrogates for asymmetric synthesis. Part II. Scope and applications.

An enantioselective synthesis of allylic esters has been achieved by a novel asymmetric alkylation of allylic gem-dicarboxylates. The catalyst derived from palladium(0) and R,R-1,2-di(2'-diphenylphosphinobenzamido)cyclohexene efficiently induced the alkylation process with a variety of nucleophiles to provide allylic esters as products in good yield. High regio- and enantioselectivities were observed in the alkylation with most nucleophiles derived from malonate, whereas a modest level of ee's was obtained in the reactions with less reactive nucleophiles such as bis(phenylsulfonyl)ethane. In the latter case, a slow addition procedure proved effective, leading to significantly improved ee's. The utility of the alkylation products was demonstrated by several synthetically useful transformations including allylic isomerizations, allylic alkylations, and Claisen rearrangements. Using these reactions, the chirality of the initial allylic carbon-oxygen bond could be transferred to new carbon-oxygen, carbon-carbon, or carbon-nitrogen bonds in a predictable fashion with high stereochemical fidelity. The conversion of gem-diesters to chiral esters by the substitution reaction is the equivalent of an asymmetric carbonyl addition by stabilized nucleophiles. In conjunction with the subsequent reactions that occur with high stereospecificity, allylic gem-dicarboxylates serve as synthons for a double allylic transformation.     

Catalytic activity of halohydrin dehalogenases towards spiroepoxides

A novel activity of halohydrin dehalogenases towards spiroepoxides has been found. The enzyme from Arthrobacter sp. (HheA) catalysed highly regioselective azidolysis of spiroepoxides containing 5, 6 and 7-membered cycloalkane rings, while the enzyme from Agrobacterium radiobacter (HheC), besides high regioselectivity, also displayed moderate to high enantioselectivity (E up to >200) that can be applied for the kinetic resolution of chiral spiroepoxides. The orientations of spiroepoxides in the active site of halohydrin dehalogenases were studied by quantum-chemical calculations and docking simulations. Analyses of the complexes obtained revealed the origins of diastereoselectivity and enantioselectivity of the investigated biotransformations.     

Selected Copper‐Based Reactions for C−N,C−O,C−S,and C−C Bond Formation

Metal‐catalyzed cross‐coupling reactions belong to the most important transformations in organic synthesis. Copper catalysis has received great attention owing to the low toxicity and low cost of copper. However, traditional Ullmann‐type couplings suffer from limited substrate scopes and harsh reaction conditions. The introduction of several bidentate ligands, such as amino acids, diamines, 1,3‐diketones, and oxalic diamides, over the past two decades has totally changed this situation as these ligands enable the copper‐catalyzed coupling of aryl halides and nucleophiles at both low reaction temperatures and catalyst loadings. The reaction scope has also been greatly expanded, rendering this copper‐based cross‐coupling attractive for both academia and industry. In this Review, we have summarized the latest progress in the development of useful reaction conditions for the coupling of (hetero)aryl halides with different nucleophiles. Additionally, recent advances in copper‐catalyzed coupling reactions with aryl boronates and the copper‐based trifluoromethylation of aromatic electrophiles will be discussed.     

Enantioselective organocatalytic fluorination using organofluoro nucleophiles

Synthetic fluorinated compounds are enormously useful in areas such as materials, agrochemicals, pharmaceuticals and fine chemicals. While methods of electrophilic fluorination have been extensively developed to stereoselectively install fluorine atoms onto molecules, nucleophilic fluorination is a much less explored approach. Recently, several organofluoro reagents have been designed and used as nucleophiles in the asymmetric synthesis of fluorinated compounds, significantly expanding the scope of enantio-enriched fluorine-containing compounds that can be synthesised. Such organofluoro nucleophiles are particularly useful in organocatalytic transformations. In this review, recent advances in the application of organofluoro nucleophiles in organocatalysis are summarised.     

Cr(Salen)] as a 'bridge' between asymmetric catalysis,Lewis acids and redox processes

An overview focused on the synthesis, structural features and catalytic applications of chiral [Cr(Salen)] complexes is presented. Key aspects of modern organic chemistry such as Lewis acids, asymmetric catalysis and redox processes are strictly connected with chronium-Schiff base complexes. Among the asymmetric transformations mediated by [Cr(Salen)] complexes, Diels-Alder and hetero-Diels-Alder reactions, ARO of epoxides, kinetic resolution of meso-epoxides and Nozki-Hyama reactions are taken into account.     

Enantioselective conjugate addition of diethylzinc to cyclic enones catalyzed by chiral copper complexes containing a new phosphorus ligand with an H8-binaphthoxy moiety

A new chiral phosphorus amidite ligand with an H₈-binaphthoxy moiety was synthesized and applied to the copper-catalyzed asymmetric conjugate addition of diethylzinc to cyclic enones with high enantioselectivity. The results gave support to the generality of the advantage of the sterically more demanding H₈-binaphthyl backbone in biaryl chiral ligands.     

ZrCl4 as a new and efficient catalyst for the opening of epoxide rings by amines

Zirconium(IV) chloride catalyses the nucleophilic opening of epoxide rings by amines leading to the efficient synthesis of β-amino alcohols. The reaction works well with aromatic and aliphatic amines in short times at room temperature in the absence of solvent. Exclusive trans stereoselectivity is observed for cyclic epoxides. Aromatic amines exhibit excellent regioselectivity for preferential nucleophilic attack at the sterically less hindered position during the reaction with unsymmetrical epoxides. However, in case of styrene oxide, selective formation of the benzylic amine was observed during the reactions with aromatic amines.     

Mild and general conditions for the cross-coupling of aryl halides with pentafluorobenzene and other perfluoroaromatics

New reaction conditions are described that enable the direct arylation of pentafluorobenzene with sterically encumbered aryl bromides and aryl chlorides. These reactions occur in high yield and under mild conditions. Notably, the reactions can be performed at 80 degrees C in isopropyl acetate with a catalyst generated by the in situ mixing of Pd(OAc)(2) and S-Phos. The enhanced scope of these transformations should further reduce the need to use pentafluorophenylboronic acid in the construction of perfluoroarenes. [reaction: see text]     

Ammonium-directed olefinic epoxidation: kinetic and mechanistic insights

The ammonium-directed olefinic epoxidations of a range of differentially N-substituted cyclic allylic and homoallylic amines (derived from cyclopentene, cyclohexene, and cycloheptene) have been investigated, and the reaction kinetics have been analyzed. The results of these studies suggest that both the ring size and the identity of the substituents on nitrogen are important in determining both the overall rate and the stereochemical outcome of the epoxidation reaction. In general, secondary amines or tertiary amines with nonsterically demanding substituents on nitrogen are superior to tertiary amines with sterically demanding substituents on nitrogen in their ability to promote the oxidation reaction. Furthermore, in all cases examined, the ability of the (in situ formed) ammonium substituent to direct the stereochemical course of the epoxidation reaction is either comparable or superior to that of the analogous hydroxyl substituent. Much slower rates of ring-opening of the intermediate epoxides are observed in cyclopentene-derived and cycloheptene-derived allylic amines as compared with their cyclohexene-derived allylic and homoallylic amine counterparts, allowing for isolation of these intermediates in both of the former cases.     

Rhodium-catalysed hydroboration employing new Quinazolinap ligands; an investigation into electronic effects

As part of an ongoing effort to improve the efficiency and substrate scope of our Quinazolinap ligand series in the rhodium-catalysed asymmetric hydroboration of vinyl arenes, 2-(p-trifluoromethylphenyl)-Quinazolinap and 2-(p-methoxyphenyl)-Quinazolinap have been synthesised and resolved in good yield. These, along with the previously reported 2-(2-pyridyl)-Quinazolinap and 2-(2-pyrazinyl)-Quinazolinap, form part of an electronic series of Quinazolinap ligands synthesised in order to explore electronic effects in this ligand class. The application of this series of ligands to the rhodium-catalysed asymmetric hydroboration of a range of vinylarenes is described. Good conversions and regioselectivities as well as excellent enantioselectivities up to 97% were obtained. 2-(p-Methoxyphenyl)-Quinazolinap demonstrated consistently high enantioselectivities in the hydroboration of sterically demanding vinylarenes.     

A pragmatic procedure for predicting regioselectivity in nucleophilic substitution of aromatic fluorides

The scope and limitations of a method for predicting the regioisomer distribution in kinetically controlled nucleophilic substitution reactions of aromatic fluorides have been investigated. This method is based on calculating the relative stabilities of the isomeric σ-complex intermediates using DFT. A wide set of substrates and anionic nucleophiles have been investigated. Predictions from this method can be used quantitatively—these agree to an average accuracy of ±0.5 kcal/mol with experimental observations in eleven of the twelve investigated reactions.     

Mild and general methods for the palladium-catalyzed cyanation of aryl and heteroaryl chlorides

[reaction: see text] New methods for the palladium-catalyzed cyanation of aryl and heteroaryl chlorides have been developed, featuring sterically demanding, electron-rich phosphines. Highly challenging electron-rich aryl chlorides, in addition to electron-neutral and electron-deficient substrates, as well as nitrogen- and sulfur-containing heteroaryl chlorides can all undergo efficient cyanation under relatively mild conditions using readily available materials. In terms of substrate scope and temperature, these methods compare very favorably with the state-of-the-art cyanations of aryl chlorides.