Pyro-Borates, Spiro-Borates, and Boroxinates of BINOL—Assembly, Structures, and Reactivity

Total Syntheses of Borolithochromes A, D and G

Total syntheses of borolithochromes A, D and G, red pigments isolated from fossils of Jurassic putative red alga Solenopora jurassica, have been achieved. The benzo[gh]tetraphene skeletons of the borate ligands in these substances were constructed using Diels-Alder reactions of aryl dienes with naphthoquinone, followed by intramolecular Corey-Chaykovsky reactions. Complexation of these ligands with trimethyl borate generated homocomplexes, which upon sequential O-demethylation produced borolithochromes A and G. In the route to borolithochrome D, a heterocomplex was prepared by stepwise complexation of the ligands with 2-(dimethylamino)ethyl dimethyl borate. The strategy devised to accomplish the first total synthesis of borolithochromes A, D and G should be applicable to the preparation of other borolithochromes as well as spiroborates possessing two fused polycyclic aromatic ligands.

"Homoleptic" Tetracoordinate Boron Compounds

"Homoleptic" tetracoordinate boron compounds, in which the central boron atom links to four identical atoms, are a special and important family of boron compounds. During the past decades, they have been extensively employed in inorganic, organic, macromolecular, and materials chemistry. Many of them exhibit a diverse range of outstanding properties, and therefore, the synthesis and application of those compounds have emerged as a hot research topic in modern boron chemistry. This review summarizes and discusses the "homoleptic" tetracoordinate boron compounds, which are organized according to the kinds of atoms coordinated to the central boron.

Copper(I)-Photocatalyzed Diastereoselective Aziridination of N-Sulfonyl Imines with Vinyl Azides: Application to Benzo[f][1,2,3]oxathiazepines Dioxides and Fused Isoxazolines

An in situ generated photoactive copper(I)-complex-catalyzed aziridination reaction of cyclic N-sulfonyl imines with α-aryl-substituted vinyl azides irradiated by blue-LEDs light is reported for the first time. This novel SET process represents a mild, sustainable, and pragmatic method for accessing synthetically resourceful sulfamidate-fused aziridines in acceptable chemical yields with excellent diastereoselectivities. Delightedly, pharmacologically attractive benzo[f][1,2,3]oxathiazepine dioxides and fused isoxazoline frameworks were achieved through our newly developed metal-free based ring-expansion techniques, highlighting the synthetic value of accessed aziridines. Finally, the possible mechanism for [2+1] aza-cyclization was presented based on the conduction of a series of control experiments.

A Lewis Acid-Controlled Enantiodivergent Epoxidation of Aldehydes

Two epoxidation catalysts, one of which consists of two VANOL ligands and an aluminum and the other that consists of two VANOL ligands and a boron, were compared. Both catalysts are highly effective in the catalytic asymmetric epoxidation of a variety of aromatic and aliphatic aldehydes with diazoacetamides, giving high yields and excellent asymmetric inductions. The aluminum catalyst is effective at 0 °C and the boron catalyst at -40 °C. Although both the aluminum and boron catalysts of (R)-VANOL give very high asymmetric inductions (up to 99% ee), they give opposite enantiomers of the epoxide. The mechanism, rate- and enantioselectivity-determining step, and origin of enantiodivergence are evaluated using density functional theory calculations.

Three-Component Synthesis of Di-Keto Aziridines and Highly Functionalized Alkenes from Sulfoxonium Ylides, Nitrosoarenes, and Alkynes

A catalyst-free one-pot three-component method of sulfoxonium ylides, nitrosoarenes, and alkynes for the synthesis of highly functionalized di-keto aziridines and alkenes is described. This strategy features the catalyst-free and additive-free approach, the employment of safe, more stable, and readily accessible sulfoxonium ylides, which bear a much wider substrate scope as starting materials. In terms of terminal alkynes, a cascade reaction of nitrone formation/1,3-diploar cycloaddition/Baldwin rearrangement is involved to afford a wide variety of di-keto aziridines. However, highly functionalized alkenes could be obtained instead of di-keto aziridines through the same nitrone formation/1,3-diploar cycloaddition and another different rearrangement reaction when internal alkynes are employed as starting materials.

Chiral BINOL-based borate counterions: from cautionary tale on anion stability to enantioselective Cu-catalyzed cyclopropanation

The chiral weakly coordinating (3,3'-biphenyl-BINOL)BF₂ anion can be generated by the reaction of the BINOL derivative with Cu(NCMe)₄BF₄. Structural analysis suggest the anion is weakly coordinating to Cu⁺. However, its use in cyclopropanation reactions leads to the rearrangement of the anion to create a chiral 3,3'-diphenyl-BINOL ligand that coordinates to copper. The latter suggests an important feature to consider when using weakly association anions, but can also be used to design simple chiral BINOL-based ligands for asymmetric cyclopropanation.

3-Arylaziridine-2-carboxylic Acid Derivatives and (3-Arylaziridin-2-yl)ketones: The Aziridination Approaches

Aziridination reactions represent a powerful tool in aziridine synthesis. Significant progress has been achieved in this field in the last decades, whereas highly functionalized aziridines including 3-arylated aziridine-2-carbonyl compounds play an important role in both medical and synthetic chemistry. For the reasons listed, in the current review we have focused on the ways to obtain 3-arylated aziridines and on the recent advances (mainly since the year 2000) in the methodology of the synthesis of these compounds via aziridination.

Synthesis, structures and catalytic activity of some BINOL based boronates and boronium salts

The BINOL supported chiral boronate ester [C₁₀H₁₂O₂BC₆F₅(THF)] [(R)-1], [C₁₀H₁₂O₂BC₆F₅(O[double bond, length as m-dash]PEt₃)] [(R)-3] and [C₁₀H₁₂O₂BC₆F₅]₂ [(R,R)-2] as well as the chiral boronium salts [C₁₀H₁₂O₂B(O[double bond, length as m-dash]PEt₃)₂]⁺[B(O₂C₁₀H₁₂)₂]^-, [(R)-6] and [C₁₀H₁₂O₂B(O[double bond, length as m-dash]SMe₂)₂]⁺[B(O₂C₁₀H₁₂)₂]^- [(R)-7] have been synthesized, characterized by NMR spectroscopy, and the solid state structures of [(R)-1], [(R,R)-2] and [(R)-3] determined. Chiral ester [(R)-1] was found to be a potent Lewis acid, similar to B(C₆F₅)₃, and capable of rapidly catalyzing the annulation of (R)-, (S)- and rac-styrene oxide with nitrone PhCH[double bond, length as m-dash]N(O)Me to trans-2-methyl-3,6-diphenyl-1,4,2-dioxazine (trans-11) with high regio- and diastereoslectivities.

Resolution of Vaulted Biaryl Ligands via Borate Esters of Quinine and Quinidine

Given the sudden and unexplained rise in the cost of (+)- and (-)-sparteine, an alternative method for the resolution of vaulted biaryls has been developed. This method involves the reaction of a racemic vaulted biaryl ligand with one equivalent of BH₃·SMe₂ and one equivalent of either quinine or quinidine. A precipitate then forms from the resulting mixture of diastereomeric borates as a result of differential solubilities. Hydrolysis of the precipitate then liberates the (S)-ligand in the case of quinine and the (R)-ligand in the case of quinidine, both with >99% ee. This method has been applied to 16 different vaulted biaryl ligands, including 10 whose preparation is described here for the first time. In addition, proof of principle has been demonstrated for the dynamic thermodynamic resolution of the vaulted biaryl ligands with this method in combination with a nonchiral copper(II) complex that can racemize the ligand.

Straightforward Approach toward Multifunctionalized Aziridines via Catalyst-Free Three-Component Reactions of α-Diazoesters, Nitrosoarenes, and Alkynes

A straightforward, catalyst-free atom- and step-economical approach for the synthesis of multisubstituted 1-arylaziridine-2-carboxylates via one-pot three-component reactions of α-diazoesters, nitrosoarenes, and alkynes has been described. This method could provide facile access to a variety of multifunctionalized aziridines in up to 91% yields under mild reaction conditions and features the catalyst-free strategy and use of cheap and readily accessible starting materials.

2,2'-Biphenol/B(OH)3 Catalyst System for Nazarov Cyclization

A novel catalyst system-a combination of the readily available 2,2'-biphenol with the inexpensive, nontoxic, and eco-friendly B(OH)₃-promoted the Nazarov cyclization of activated and inactivated divinyl ketones to afford the corresponding cyclopentenones up to 96% yield under, in a cis-selective manner. Compared with the conventional harsh conditions with hazardous reagents, user-friendly method was established with bench-stable and easy-to-handle reagents.

Four-coordinate triarylborane synthesis via cascade B-Cl/C-B cross-metathesis and C-H bond borylation

To develop a simple and efficient synthetic method for four-coordinate triarylboranes, we herein describe a tandem highly selective B-Cl/C-B cross-metathesis of two of the same or different arylboranes and C-H bond borylation to synthesize four-coordinate triarylboranes with a broad substrate scope. By switching substituent groups of the target molecules, different emission wavelengths can be achieved from 467 nm to 583 nm with aggregation-induced emission (AIE) properties.

Chiral Diol-Based Organocatalysts in Enantioselective Reactions

Organocatalysis has emerged as a powerful synthetic tool in organic chemistry in the last few decades. Among various classes of organocatalysis, chiral diol-based scaffolds, such as BINOLs, VANOLs, and tartaric acid derivatives, have been widely used to induce enantioselectivity due to the ability of the hydroxyls to coordinate with the Lewis acidic sites of reagents or substrates and create a chiral environment for the transformation. In this review, we will discuss the applications of these diol-based catalysts in different types of reactions, including the scopes of reactions and the modes of catalyst activation. In general, the axially chiral aryl diol BINOL and VANOL derivatives serve as the most competent catalyst for most examples, but examples of exclusive success using other scaffolds, herein, suggests that they should not be overlooked. Lastly, the examples, to date, are mainly from tartrate and biaryl diol catalysts, suggesting that innovation may be available from new diol scaffolds.