Atom-economic and stereoselective catalytic synthesis of fully substituted enol esters/carbonates of amides in acyclic systems enabled by boron Lewis acid catalysis

Carboacyloxylation of internal alkynes is emerging as a powerful and straightforward strategy for enol ester synthesis. However, the reported examples come with limitations, including the utilization of noble metal catalysts, the control of regio- and Z/E selectivity, and an application in the synthesis of enol carbonates. Herein, a boron Lewis acid-catalyzed intermolecular carboacyloxylation of ynamides with esters to access fully substituted acyclic enol esters in high yield with generally high Z/E selectivity (up to >96 : 4) is reported. Most importantly, readily available allylic carbonates are also compatible with this difunctionalization reaction, representing an atom-economic, catalytic and stereoselective protocol for the construction of acyclic β,β-disubstituted enol carbonates of amides for the first time. The application of the carboacyloxylation products to decarboxylative allylations provided a ready access to enantioenriched α-quaternary amides. Moreover, experimental studies and theoretical calculations were performed to illustrate the reaction mechanism and rationalize the stereochemistry.

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6 F5 )3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N2 release. Recently, tris(pentafluorophenyl)borane [B(C6 F5 )3 ] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C6 F5 )3 is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.

A Theoretical Study on the Borane-Catalyzed Reductive Amination of Aniline and Benzaldehyde with Dihydrogen: The Origins of Chemoselectivity

Density functional theory calculations are used in this study to investigate the product selectivity and mechanism of borane-catalyzed reductive aldehyde amination by a H₂ reducing agent. Knowing that different boranes yield different products, two typical boranes, (B(2,6-Cl₂C₆H₃)(p-HC₆F₄)₂ and B(C₆F₅)₃), are studied. Of the seven possible pathways of B(2,6-Cl₂C₆H₃)(p-HC₆F₄)₂-catalyzed aldehyde amination analyzed herein, four are favorable. Three of the four favorable pathways involve imine intermediates, and the fourth is a Lewis acid-base synergistic pathway that involves amine-alcohol condensation. As for the B(C₆F₅)₃ catalyst, it forms a highly stable Lewis adduct with aniline, which impedes the hydrogenation of imine. Therefore, the product of B(C₆F₅)₃-catalyzed reductive amination of benzaldehyde and aniline is an imine. The linear relationship between the charge on the boron atom in the Lewis acid and the relative energies of the Lewis adduct and H₂ splitting transition state indicates that this parameter determines product selectivity. Indeed, when the natural charge on boron is larger than 1, an amine is produced, whereas when the charge is less than 1, an imine is produced. Hence, the selectivity of products can be controlled by adjusting the natural charge of the boron atom in the Lewis acid catalyst.

Boron-Based Lewis Acid Catalysis: Challenges and Perspectives

In the last two decades, boron-based catalysis has been gaining increasing traction in the field of organic synthesis. The use of halogenated triarylboranes as main group Lewis acid catalysts is an attractive strategy. It has been applied in a growing number of transformations over the years, where they may perform comparably or even better than the gold standard catalysts. This review discusses methods of borane synthesis and cutting-edge boron-based Lewis acid catalysis, focusing especially on tris(pentafluorophenyl)-borane [B(C6F5)3], and other halogenated triarylboranes, highlighting how boron Lewis acids employed as catalysts can unlock a plethora of unprecedented chemical transformations or improve the efficiency of existing reactions.

Tris(pentafluorophenyl)borane catalyzed C-C and C-heteroatom bond formation

A series of boron based Lewis acids have been reported to date, but among them, tris(pentafluorophenyl)borane (BCF) has gained the most significant attention in the synthetic chemistry community. The viability of BCF as a potential Lewis acid catalyst has been vastly explored in organic and materials chemistry due to its thermal stability and commercial availability. Most explorations of BCF chemistry in organic synthesis has occurred in the last two decades and many new catalytic reactivities are currently under investigation. This review mainly focuses on recent reports from 2018 onwards and provides a concise knowledge to the readers about the role of BCF in metal-free catalysis. The review has mainly been categorized by different types of organic transformation mediated through BCF catalysis for the C-C and C-heteroatom bond formation.