Excellent Nucleophiles Research Articles

Photoinduced Side‐Chain Functionalization, Grafting, Cross‐Linking, and Chain Extension by Thiol‐Epoxide Click Reaction Using Photo Base Generator

AbstractIn this work, photoinduced thiol‐epoxy click reaction for side‐chain functionalization, chain‐extension, grafting, and cross‐linking of polymers prepared by atom transfer radical polymerization by using 1,5‐diazabicyclo[5.4.0]undec‐5‐ene thioxanthone acetate (Tx‐DBU) as a photo base generator (PBG) is described. Irradiation of Tx‐DBU at 365 nm leads to the generation of 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in neutral form as a super base. DBU is capable of abstracting protons from thiols and thus formed thiolates are excellent nucleophiles that exhibit high affinity toward electrophiles such as epoxide rings. Appropriately selected thiols are successfully used for the desired reactions. The precursors and resulting polymers are characterized using 1H‐NMR, Fourier transform infrared, gel permeation chromatography, and differential scanning calorimetry measurements. This study shows that the use PBGs can further be extended to the synthesis of various complex macromolecular structures by the described post modification processes.

Stereoselective Allylic Alkylations of Amino Ketones and Their Application in the Synthesis of Highly Functionalized Piperidines.

Chelated ketone enolates are excellent nucleophiles for allylic alkylations. Electron-withdrawing groups on the allyl moiety allow subsequent intramolecular Michael additions giving rise to piperidines with up to five stereogenic centers.

Stereoselective Modification of N-(α-Hydroxyacyl)-glycinesters via Palladium-Catalyzed Allylic Alkylation.

N-(α-Hydroxyacyl)-glycinesters can be used as excellent nucleophiles in Pd-catalyzed allylic alkylation. The method allows for the stereoselective introduction of a wide range of side chains, including highly functionalized ones. Both diastereomers can be accessed through variation of the reaction conditions. Furthermore, the use of stannylated carbonates introduces vinylstannane motifs, which are eligible for subsequent C-C coupling reactions.

Tertiary Enamides: Versatile and Available Substrates in Synthetic Chemistry.

Enamines and their variant enamides as powerful and versatile synthons have attracted great attention in synthetic chemistry. Enamides display unique stability and reduce enaminic reactivity in view of the electron-withdrawing effect of N-acyl group. A great deal of satisfactory achievements in the synthesis and application of enamides has been made in recent years. Especially, tertiary enamides without N-H bond regarded as low reactivity of compounds in the past can act as excellent nucleophiles to react with electrophiles for the construction of various nitrous molecules. This review focuses on recent advances on tertiary enamides in the synthetic strategies and applications including addition, coupling reaction, functionalization and electro- or photo-chemical reaction. Tertiary enamides as electron-deficient nucleophiles display a satisfactory balance between stability and reactivity to offer multiple opportunities for the construction of various functionalized nitrogencontaining compounds. Further exploration of the reactive mechanisms involved tertiary enamides and the development of novel and efficient transformations to generate ever more complex building blocks starting from tertiary enamides are particularly worth pursuing.

C- and N-Metalated Nitriles: The Relationship between Structure and Selectivity

Metalated nitriles are exceptional nucleophiles capable of forging highly hindered stereocenters in cases where enolates are unreactive. The excellent nucleophilicity emanates from the powerful inductive stabilization of adjacent negative charge by the nitrile, which has a miniscule steric demand. Inductive stabilization is the key to understanding the reactivity of metalated nitriles because this permits a continuum of structures that range from N-metalated ketenimines to nitrile anions. Solution and solid-state analyses reveal two different metal coordination sites, the formally anionic carbon and the nitrile nitrogen, with the site of metalation depending intimately on the solvent, counterion, temperature, and ligands. The most commonly encountered structures, C- and N-metalated nitriles, have either sp3 or sp2 hybridization at the nucleophilic carbon, which essentially translates into two distinct organometallic species with similar but nonidentical stereoselectivity, regioselectivity, and reactivity preferences. The hybridization differences are particularly important in SNi displacements of cyclic nitriles because the orbital orientations create very precise trajectories that control the cyclization selectivity. Harnessing the orbital differences between C- and N-metalated nitriles allows selective cyclization to afford nitrile-containing cis- or trans-hydrindanes, decalins, or bicyclo[5.4.0]undecanes. Similar orbital constraints favor preferential SNi displacements with allylic electrophiles on sp3 centers over sp2 centers. The strategy permits stereoselective displacements on secondary centers to set contiguous tertiary and quaternary stereocenters or even contiguous vicinal quaternary centers. Stereoselective alkylations of acyclic nitriles are inherently more challenging because of the difficulty in creating steric differentiation in a dynamic system with rotatable bonds. However, judicious substituent placement of vicinal dimethyl groups and a trisubstituted alkene sufficiently constrains C- and N-metalated nitriles to install quaternary stereocenters with excellent 1,2-induction. The structural differences between C- and N-metalated nitriles permit a rare series of chemoselective alkylations with bifunctional electrophiles. C-Magnesiated nitriles preferentially react with carbonyl electrophiles, whereas N-lithiated nitriles favor SN2 displacement of alkyl halides. The chemoselective alkylations potentially provide a strategy for late-stage alkylations of polyfunctional electrophiles en route to bioactive targets. In this Account, the bonding of metalated nitriles is summarized as a prelude to the different strategies for selectively preparing C- and N-metalated nitriles. With this background, the Account then transitions to applications in which C- or N-metalated nitriles allow complementary diastereoselectivity in alkylations and arylations, and regioselective alkylations and arylations, with acyclic and cyclic nitriles. In the latter sections, a series of regiodivergent cyclizations are described that provide access to cis- and trans-hydrindanes and decalins, structural motifs embedded within a plethora of natural products. The last section describes chemoselective alkylations and acylations of C- and N-metalated nitriles that offer the tantalizing possibility of selectively manipulating functional groups in bioactive medicinal leads without recourse to protecting groups. Collectively, the unusual reactivity profiles of C- and N-metalated nitriles provide new strategies for rapidly and selectively accessing valuable synthetic precursors.

Efficient Organocatalytic Construction of C4-Alkyl Substituted Piperidines and Their Application to the Synthesis of (+)-α-Skytanthine.

Chiral piperidines which contain an alkyl group at C4 positions are one of the important architectures because it is appeared in several natural products. An efficient protocol for the preparation of C4-alkyl substituted chiral piperidines using secondary amine catalyzed formal aza [3+3] cycloaddition reaction with aliphatic α,β-unsaturated aldehydes and thiomalonamate derivatives is reported. In our reaction system, thiomalonamate is an excellent nucleophile and the addition of suitable acid and its amount is an important factor for the acceleration effect in organocatalytic reaction. Furthermore, water and MeOH also have an acceleration effect. These efforts lead to only 0.1 mol % catalyst loading in multigram scale synthesis for suitable reaction time. In addition, the efficient enantioselective total synthesis of (+)-α-skytanthine by using our developed reaction as key step was achieved in total 15 % yield. All carbon and nitrogen units were introduced by one step with high enantioselectivity.

Regio- and Stereoselective Modification of Chiral α-Amino Ketones by Pd-Catalyzed Allylic Alkylation.

Chiral α-amino ketones are excellent nucleophiles for stereoselective palladium-catalyzed allylic alkylations. Both chiral as well as achiral allylic substrates can be applied, while the stereochemical outcome of the reaction is controlled by the chiral ketone enolate. The substituted amino ketones formed can be reduced stereoselectively, and up to five consecutive stereogenic centers can be obtained. This approach can be used for the synthesis of highly substituted piperidine derivatives.

An ionically tagged water-soluble artificial enzyme promotes the dephosphorylation reaction with nitroimidazole: enhanced ionic liquid effect and mechanism.

In this paper, we describe a novel synthesized ionically tagged water-soluble artificial enzyme (PI) that can efficiently cleave phosphate esters, with enhanced an ionic liquid effect through cooperative effects for the substrate activation and further nucleophilic reaction. The dephosphorylation reaction with PI was evaluated in the presence and absence of 2-methyl-4(5)-nitroimidazole, showing impressive rate enhancements of up to 2 × 10(6)-fold, ascribed to the imidazolide species known as excellent nucleophiles, and formed favorably at lower pH values in the presence of PI.

Zwitterionic carbene adducts and their carbene isomers

N-heterocyclic carbenes (NHC) and abnormal NHCs (aNHC) form the stable adducts,1and2, with X (CH2, SiH2, NH, PH, O, S), which are excellent nucleophiles at X.

(p-Cymene)RuCl2 ]2 : an efficient catalyst for highly regioselective allylic alkylations of chelated amino acid ester enolates.

Chelated amino acid ester enolates are excellent nucleophiles for ruthenium-catalyzed allylic alkylations. Although [Cp*Ru(MeCN)3 ]PF6 was found to be the most reactive catalyst investigated, with the resulting allyl complexes reacting at temperatures as low as -78 °C, unfortunately the process took place with only moderate regio- and diastereoselectivity. In contrast, [(p-cymene)RuCl2 ]2 allowed allylations to be performed with a high degree of regioretention. Secondary allyl carboxylates with a terminal double bond were found to be the most reactive substrates, giving rise to the branched amino acids with perfect regioretention and chirality transfer. In this case, no isomerization of the Ru-allyl complex formed in situ was observed, in contrast to the analogues palladium complexes. This isomerization-free protocol can also be used for the synthesis of (Z)-configured γ,δ-unsaturated amino acid derivatives, starting from (Z)-allylic substrates. Here, the more reactive phosphates were found to be superior to the carboxylates, providing the required amino acids in almost quantitative yield with perfect regio- and stereoretention. Therefore, the Ru-catalyzed allylation reactions are well positioned to overcome the drawbacks of Pd-catalyzed processes.

Creation of Highly Congested Quaternary Centers via Cu-catalyzed Conjugate Addition of Alkenyl Alanates to β-Substituted Cyclic Enones

Easily prepared alkenylalanates proved to be excellent nucleophiles for the creation of highly congested quaternary centers via copper-catalyzed conjugate addition. In addition, functionalized cis-decaline systems can now be prepared in a simple two-step sequence involving Cu-catalyzed conjugate addition with functionalized alkenylalanates.

Recent advances in stoichiometric phosphine-mediated organic synthetic reactions

Organic synthetic reactions mediated by tertiary phosphines have attracted much attention in the organic chemistry community in the past two decades. These reactions can be divided into two categories: phosphine-catalyzed and stoichiometric phosphine-mediated transformations. While the phosphine-catalyzed reactions mechanistically rely on the unique properties of tertiary phosphines such as excellent nucleophilicity and good leaving group ability, the stoichiometric transformations are usually driven by nucleophilicity and strong oxyphilicity of tertiary phosphines. Since tertiary phosphines represent an important class of versatile chemical reagents in organic synthesis, stoichiometric phosphine-mediated reactions have recently demonstrated their uniqueness and high efficiency in organic synthesis, particularly with respect to the construction of carbon–carbon and carbon–heteroatom bonds, and therefore have stimulated much research interest. In this review, recent advances in stoichiometric phosphine-mediated reactions primarily including olefinations and annulations are summarized.

Gated access to α-lithiated phenyltetrahydrofuran: functionalisation via direct lithiation of the parent oxygen heterocycle

Cycloreversion of α-lithiated phenyltetrahydrofuran was successfully tamed at -78 °C in a non-coordinating solvent in the presence of TMEDA. This anion showed excellent nucleophilicity and could be intercepted with a variety of structurally different electrophiles to give 2,2-disubstituted derivatives which can be further elaborated to γ-butyrolactones.

Mitsunobu Reaction of 1,2,3‐NH‐Triazoles: A Regio‐ and Stereoselective Approach to Functionalized Triazole Derivatives

Mitsunobu Reaction of 1,2,3‐NH‐Triazoles: A Regio‐ and Stereoselective Approach to Functionalized Triazole Derivatives

Dynamics of Thiol/Disulfide Exchange Within a Single Protein

The dynamics of thiol/disulfide exchange is essential to many cellular processes, such as oxidative folding in the endoplasmic reticulum. This group of reactions has usually been examined by bulk techniques, which are intrinsically limited when studying out-of-equilibrium reactions, characterizing transient intermediates, or inspecting the directionality of the reaction (regioselectivity). In this work, we present a single-molecule approach that allows a regiospecific description of the reaction pathways available for the reduction of a protein with two disulfide bonds. Two disulfides were introduced in the model protein I27; I272S-S. Single polyproteins based on I272S-S were mechanically extended using an atomic force microscope working in force-clamp mode and in the presence of reducing agents. In our experiments, the disulfides limit the mechanical extension of I272S-S, and the reduction events are detected as step-wise increases in the length of the polyprotein. Importantly, the magnitude of such steps depends on the number of amino acids sequestered by the disulfide being reduced, which allowed us to unambiguously assign specific steps to the cleavage of specific disulfide bonds. Cysteine residues resulting from the reduction of the first disulfide are excellent nucleophiles able to attack the remaining disulfide. This generates two additional intermediates and a total of four distinct reaction pathways. All of them can be unequivocally discriminated by our method. The frequency of appearance of the different pathways depends on the concentration of the reducing agent, as a consequence of the competition between inter- and intramolecular reactions. We observed that thiol/disulfide exchange reactions are highly regiospecific even within an unfolded polypeptide. Our results provide new insights into physiologically relevant reactions at an unprecedented level of detail.

ChemInform Abstract: Sulfones: New Reagents in Organocatalysis

AbstractReview: 76 refs.

Sulfones: new reagents in organocatalysis

The development of new asymmetric methodologies that afford different structures in an enantioselective fashion is one of the most exciting goals for chemists nowadays. In this subject, lately, the use of sulfones has become a fast growing field. From the works of Tan and Shibata until the last works of Palomo, sulfones have demonstrated their versatility and power in organocatalytic asymmetric reactions. Moreover, the easy removal of sulfones with Mg or Hg/Na makes this group a perfect choice to afford easily naked alkyls. Remarkably, bissulfones can be used as nucleophiles or electrophiles, being vinyl sulfones excellent electrophiles, while bismethylensulfones derivatives such as fluoro are excellent nucleophiles. This double possibility has been studied by several research groups, leading to new methodologies that allow obtaining formally simple alkylation in an enantioselective fashion, by using organocatalysis. The aim of this tutorial review is to summarize the last trends in the use of sulfones in organocatalytic processes, giving a complete scenario of these new reagents.

N-Trimethylsilyloxyenamines as new aldehyde enolate synthons: general, efficient and diastereoselective aldol reaction with ketals and acetals

(E)-N-trimethylsilyloxyenamines, easily accessible from aldonitrones, proved to be excellent nucleophiles in TMSOTf-induced diastereoselective aldol reaction, both with ketals and acetals, proceeding via an extended transition state and leading to a new aldol C-C-bond in the aldonitrone products, that can be readily hydrolysed to the corresponding aldehydes.

Azaanthraquinone assembly from N-propargylamino quinone via iodine-induced 6-endo-dig electrophilic cyclization

An efficient methodology taking advantage of the excellent nucleophilicity of aminoquinone to assemble the azaanthraquinone framework was developed via an iodine-induced 6-endo-dig electrophilic cyclization. Therefore, starting from N-propargylaminoquinones, various 3-iodo-1-azaanthraquinones were obtained in yields ranging from 45% to 90%. The metal-free protocol features facile installation of an iodine atom on the azaanthraquinone ring and benign functional group compatibility.

Dynamic Kinetic Asymmetric Allylation of Hydrazines and Hydroxylamines

Hydrazines and hydroxylamines have been found to be excellent nucleophiles for the palladium-catalyzed dynamic asymmetric allylic amination of vinyl epoxide, with good yields and enantioselectivities of up to 97% ee. This method is applicable to acyclic and heterocyclic amines and was applied toward a five-step synthesis of (R)-piperazic acid.