Highly regio- and stereoselective (3 + 2) annulation reaction of allenoates with 3-methyleneindolin-2-ones catalyzed by a planar chiral [2.2]paracyclophane-based bifunctional phosphine-phenol catalyst

A planar chiral [2.2]paracyclophane-based phosphine-phenol catalyst catalyzed the (3 + 2) annulation reaction of ethyl 2,3-butadienoate with 3-methyleneindolin-2-ones to produce 2,5-disubstituted cyclopentene-fused C3-spirooxindoles in high yields with high regio-, diastereo-, and enantioselectivities. This catalyst was suitable for reactions of not only benzylideneindolinones but also alkylideneindolinones, the chiral phosphine-catalyzed reactions of which have not yet been reported. Density functional theory calculations suggested that the formation of hydrogen bonds between the phenolic OH group of the catalyst and the allenoate carbonyl group, rather than between the OH group and the carbonyl group of indolinone, contributed to the formation of an efficient reaction space at the enantiodetermining step.

Cu-Catalyzed Asymmetric Kinetic Boron Conjugate Addition of γ-Substituted α,β-Unsaturated γ-Lactams

Chiral α,β-unsaturated γ-lactams bearing simple γ- substituents are found in biologically active molecules and natural products, however, their synthesis still remains difficult. Herein, we report an efficient kinetic resolution (KR) of γ-substituted α,β-unsaturated γ-lactams via a Cu-catalyzed asymmetric boron conjugate addition, which also leads to the efficient synthesis of chiral β-hydroxy-γ-lactams with β,γ-stereogenic carbon centers. The KR proceeded smoothly with a wide range of γ-alkyl or aryl substituted substrates including those bearing aromatic heterocycles and different N-protected substrates in up to 347 of s value. Their highly versatile transformations, synthetic utility in biologically active molecules, and inhibitory activities against cisplatin-sensitive ovarian cancer cell A2780 have also been demonstrated. Differing from the well-known mechanism involving Cu-B species in Cu-catalyzed boron conjugate additions, our mechanistic studies using density functional theory (DFT) calculations and experiments indicate that a Lewis acid CuI -catalyzed mechanism is the likely pathway in the catalytic reaction.

Recent Advances in Enantioselective Organocatalytic Reactions Enabled by NHCs Containing Triazolium Motifs

N-Heterocyclic carbenes (NHCs) containing triazolium motifs have emerged as a powerful tool in organocatalysis. Recently, various NHC pre-catalyst mediated organic transformations have been developed successfully. This article aims to compile the current state of knowledge on NHC-triazolium catalysed enantioselective name reactions and introduce newly developed catalytic methods. Furthermore, this review article framework provides an excellent opportunity to highlight some of the unique applications of these catalytic procedures in the natural product synthesis of biologically active compounds, notably the wide range of preparation of substituted chiral alcohols, and their derivatives. This article provides an overview of chiral NHC triazolium-catalyst libraries synthesis and their catalytic application in enantioselective reactions.

Convergent total synthesis of (+)-calcipotriol: A scalable, modular approach to vitamin D analogs

A convergent approach for the total synthesis of calcipotriol (brand name: Dovonex), a proven vitamin D analog used for the treatment of psoriasis, and medicinally relevant synthetic analogs is described. A complete approach, not wedded to semisynthesis, toward both the A-ring and CD-ring is reported. From a retrosynthetic standpoint, hidden symmetry within the decorated A-ring is disclosed, which allowed for scalable quantities of this advanced intermediate. In addition, a radical retrosynthetic approach is described, which highlights an electrochemical reductive coupling as well as an intramolecular hydrogen atom transfer Giese addition to establish the 6,5-transcarbon skeleton found in the vitamin D family. Finally, a late-stage decarboxylative cross-coupling approach allowed for the facile preparation of various C20-arylated derivatives that show promising biological activity in an initial bioassay.

Development of Methods to the Synthesis of β-Boryl Acyls, Imines and Nitriles

Organoboron compounds are highly important and versatile synthetic intermediates for the preparation of a wide range of organic molecules. Organoboron compounds have drawn significant attention among organic chemists due to their Lewis acidic property, non-toxicity, and commercial availability. Over the last several decades, there has been a substantial development of new organoboron compounds, useful in organic synthesis. Among all other organoboron compounds, β-boryl carbonyl compounds are the important ones. The β-boryl compounds have appeared as promising intermediates for various synthetic transformations. The 1,4-conjugate addition of diboron reagents to carbon-carbon double bond in the presence of different transition-metal catalysts has been extensively reported by various research groups across the globe. This mini-review outlines the numerous racemic as well as asymmetric β-borylation methods developed to date.

Tuning StackPhim Ligands: Applications in Enantioselective Borylation and Alkynylation

Here we present a new stack ligand with a modified imidazoline backbone prepared from cyclohexanediamine. This new stack ligand­, Cy-StackPhim, has been found to complement the parent StackPhim ligand in an enantioselective borylation reaction. Additionally, a correlation between the nature of substituents on the imidazoline ring and the substituents on the electrophile is also discussed.

Designing successful monodentate N-heterocyclic carbene ligands for asymmetric metal catalysis

Chiral ligands are of particular importance in asymmetric transition-metal catalysis. Although the development of effective chiral monodentate N-heterocyclic carbenes (NHCs) has been slow, a growing amount of papers have been published in recent years showing their unique efficiency as chiral ancillary ligands. Herein we provide an overview of NHC structures that accomplish high levels of enantioselectivity (≥90% ee) and give guidelines to their use and thoughts on the future of this field.

α-Aminoboronates: recent advances in their preparation and synthetic applications

α-Aminoboronic acids and their derivatives are useful as bioactive agents. Thus far, three compounds containing an α-aminoboronate motif have been approved by the Food and Drug Administration (FDA) as protease inhibitors, and more are currently undergoing clinical trials. In addition, α-aminoboronic acids and their derivatives have found applications in organic synthesis, e.g. as α-aminomethylation reagents for the synthesis of chiral nitrogen-containing molecules, as nucleophiles for preparing valuable vicinal amino alcohols, and as bis-nucleophiles in the construction of valuable small molecule scaffolds. This review summarizes new methodology for the preparation of α-aminoboronates, including highlights of asymmetric synthetic methods and mechanistic explanations of reactivity. Applications of α-aminoboronates as versatile synthetic building blocks are also discussed.

Configuration Sampling With Five-Membered Atropisomeric P,N-Ligands

Here we report a strategy for the systematic variation of atropisomeric C₁ -symmetric P,N ligands to incrementally change the position of the groups within the chiral pocket without modifying their steric parameters. More specifically, the effects of systematic modification of the nitrogen heterocycle in atropisomeric C₁ -symmetric stack ligands have been investigated in this study. The versatility and applicability of this approach has been demonstrated in mechanistically distinct catalytic enantioselective transformations, resulting in the identification of a P,N-ligand for a highly enantioselective synthesis of organoboranes.

Enantiopure planar chiral [2.2]paracyclophanes: Synthesis and applications in asymmetric organocatalysis

This short review focuses on enantiopure planar chiral [2.2]paracyclophanes (pCps), a fascinating class of molecules that possess an unusual three-dimensional core and intriguing physicochemical properties. In the first part of the review, different synthetic strategies for preparing optically active pCps are described. Although classical resolution methods based on the synthesis and separation of diastereoisomeric products still dominate the field, recent advances involving the kinetic resolution of racemic compounds and the desymmetrization of meso derivatives open up new possibilities to access enantiopure key intermediates on synthetically useful scales. Due to their advantageous properties including high configurational and chemical stability, [2.2]paracyclophanes are increasingly employed in various research fields, ranging from stereoselective synthesis to material sciences. The applications of [2.2]paracyclophanes in asymmetric organocatalysis are described in the second part of the review. While historically enantiopure pCps have been mainly employed by organic chemists as chiral ligands in transition-metal catalysis, these compounds can also be used as efficient catalysts in metal-free reactions and may inspire the development of new transformations in the near future.

Multifunctional chiral aminophosphines for enantiodivergent catalysis in a palladium-catalyzed allylic alkylation reaction

Trifunctional MAP-based chiral phosphines were tested as new ligands in a Pd-catalyzed asymmetric allylic alkylation, demonstrating fast and enantiodivergent catalysis. The palladium complexes of representative ligands by X-ray analysis revealed a novel mode of P,N-coordination of the ligand to the palladium center, which may contribute to switching the sense of the asymmetric induction via combined steric and tunable H-bonding interactions between the metal complex and the substrates.

Transition metal-free synthesis of alkyl pinacol boronates

This review systematically outlined the research in the area of transition metal free synthesis of alkyl pinacol boronates, which are versatile and important scaffolds to construct diverse organic compounds.

An NHC-CuCl functionalized metal-organic framework for catalyzing β-boration of α,β-unsaturated carbonyl compounds

A microporous metal-organic framework functionalized with in situ generated NHC-CuCl units (1) has been successfully synthesized from a novel imidazolium-containing ligand. In particular, the MOF 1 can catalyze β-boration of α,β-unsaturated carbonyl compounds, while the isoreticular version of 1 (1-im) modified with only imidazolium moiety cannot. This work demonstrates for the first time the heterogeneous catalysis of NHC-Cu(i)Cl within a MOF solid.

Recent Synthesis Developments of Organoboron Compounds via Metal-Free Catalytic Borylation of Alkynes and Alkenes

Diboron reagents have been traditionally regarded as "Lewis acids", which can react with simple Lewis base to create a significant nucleophilic character in one of boryl moieties. In particular, bis(pinacolato)diboron (B₂pin₂) reacts with simple Lewis bases, such as N-heterocyclic carbenes (NHCs), phosphines and alkoxides. This review focuses on the application of trivalent nucleophilic boryl synthon in the selective preparation of organoboron compounds, mainly through metal-free catalytic diboration and the β-boration reactions of alkynes and alkenes.

Cascade Reaction by Chemo- and Biocatalytic Approaches to Obtain Chiral Hydroxy Ketones and anti 1,3-Diols

A chemo- and biocatalytic cascade approach was applied for the stereoselective synthesis of hydroxy ketones and the corresponding 1,3-diols. A new class of tridentate N,N,O ligands was used with copper(II) complexes for the asymmetric β-borylation of α,β-unsaturated compounds. The complex containing ligand L5 emerged as the best performer, and it gave the organoborane derivatives with good ee values. The corresponding keto-alcohol compounds were then bioreduced by yeasts. The biotransformation set up with Rhodotorula rubra allowed (R)-keto-alcohols and (S,S)-diols to be obtained with up to 99 % ee and up to 99 % de in favor of the anti enantiomers.

Transition-metal-free B-B and B-interelement reactions with organic molecules

This review is a guided tour along the activation modes and reactivity of B-B, B-Si, B-N, B-S, B-Se and B-P reagents, in the absence of any transition metal complex. Here are disclosed the general concepts related to the homolytic and heterolytic cleavage of B-B and B-interelement bonds, as well as the generation of new C-B and C-interelement bonds, in a selective way. The greener consequences of these novel routes facilitate the gram scale preparations of target functionalised organic compounds. Intrinsic data about the suggested mechanisms and spectroscopic evidence that supports the innovative theories are provided along the review. Since this is a stimulating area of work that has emerged within the last decade, this overview serves as the basis to understand the new trends and hopefully to generate inspiration for future discoveries in the field.

Well-Defined Chiral Copper NHC Complex in the Asymmetric Conjugated β-Borylation and One-Pot Metathesis-Asymmetric β-Borylation Reactions

Highly stereoselective conjugate β-borylation, using a new chiral NHC-based copper catalyst, has been achieved. The chiral NHC copper complex was prepared in gram scale and showed high enantioselectivity and activity (up to 10 000 turnovers at 100 ppm of catalyst loading). This method was employed in the synthesis of a chiral β-borylated ester from simple unconjugated alkenes though an unprecedented one-pot cross metathesis-asymmetric borylation sequence.

Metal-free enantioselective addition of nucleophilic silicon to aromatic aldehydes catalyzed by a [2.2]paracyclophane-based N-heterocyclic carbene catalyst

A transition metal-free enantioselective 1,2-silylation of aromatic aldehydes to yield chiral α-hydroxysilanes was developed for the first time.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.

[2.2]Paracyclophane-based N-heterocyclic carbene as efficient catalyst or as ligand for copper catalyst for asymmetric α-silylation of N-tosylaldimines

A carbene precursor plays a dual role as both an organocatalyst and as a ligand for the copper catalyst in asymmetric silylation.

[2.2]Paracyclophane-based carbene–copper catalyst tuned by transannular electronic effects for asymmetric boration

The catalytic performance of [2.2]paracyclophane-based carbene–copper complexes in the asymmetric boration can be controlled by the electronic tuning of carbene ligands.