Asymmetric Synthesis of Fluorinated Allenes by Rhodium-Catalyzed Enantioselective Alkylation/Defluorination of Propargyl Difluorides with Alkylzincs

Recent advances in the applications of gem-difluoromethylene alkynes

As a special class of alkynes, gem-difluoromethylene alkynes exhibit a variety of fascinating properties due to the presence of the gem-difluoro substitution. This substitution highlights the distinctive fluorine effects in influencing the chemoselectivity of reactions. As a result, chemical scientists have shown great interest and enthusiasm for investigating their reactions. In this review, we briefly summarize recent advances in transition metal-catalysed reactions of gem-difluoromethylene alkynes with multiple reaction pathways. Their mechanistic studies and challenges will be highlighted. The purpose of this review is to provide illustrations of elegant gem-difluoromethylene alkynes and thereby elicit further interest among synthetic chemists in developing innovative transformations of gem-difluoromethylene alkynes.

Ligand-controlled regiodivergent arylation of aryl(alkyl)alkynes and asymmetric synthesis of axially chiral 9-alkylidene-9,10-dihydroanthracenes

Transition metal-catalyzed addition of organometallics to aryl(alkyl)alkynes has been well known to proceed with the regioselectivity in forming a carbon-carbon bond at the alkyl-substituted carbon (β-addition). Herein, the reverse regiochemistry with high selectivity in giving 1,1-diarylalkenes (α-addition) was realized in the reaction of arylboronic acids with aryl(alkyl)alkynes by use of a rhodium catalyst coordinated with a chiral diene ligand, whereas the arylation of the same alkynes proceeded with the usual regioselectivity (β-addition) in the presence of a rhodium/DM-BINAP catalyst. The regioselectivity can be switched by the choice of ligands on the rhodium catalysts. This reverse regioselectivity also enabled the catalytic asymmetric synthesis of phoenix-like axially chiral alkylidene dihydroanthracenes with high enantioselectivity through an α-addition/1,4-migration/cyclization sequence.

Chemo-, regio-, and stereoselective tetrafunctionalization of fluoroalkynes enables divergent synthesis of 5-7-membered azacycles

Alkyne annulation has been widely used in organic synthesis for the construction of azacycles with unique structural and physicochemical properties. However, the analogous transformation of fluoroalkynes remains a challenge and has seen limited progress. Herein we report a 1,2,3,4-tetrafunctionalization of polyfluoroalkynes for the divergent construction of 5-7-membered (E)-1,2-difluorovinyl azacycles. The use of the fluorine atom as a detachable "activator" not only obviates the use of any transition metal catalysts and oxidizing reagents, but also ensures the [3-5 + 2]-annulation and defluorinative functionalization of fluoroalkynes with high chemo-, regio-, and stereoselectivities. This method exhibits a broad substrate scope, good functional group tolerance, and excellent scalability, providing a modular platform for accessing fluorinated skeletons of medicinal and biological interest. The late-stage modification of complex molecules, the multi-component 1,2-diamination of fluoroalkyne, and the synthesis of valuable organofluorides from the obtained products further highlight the real-world utility of this fluoroalkyne annulation technology.

A review of frustrated Lewis pair enabled monoselective C-F bond activation

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years. FLP mediated monoselective C-F bond activation is complementary to other solutions developed to address 'over-defluorination' and offers several advantages and unique opportunities. This perspective highlights some of these advantages and opportunities and places the development of FLP mediated C-F bond activation into the context of the wider effort to overcome 'over-defluorination'.

Synthetic Advantages of Defluorinative C-F Bond Functionalization

Much progress has been made in the development of methods to both create compounds that contain C-F bonds and to functionalize C-F bonds. As such, C-F bonds are becoming common and versatile synthetic functional handles. This review summarizes the advantages of defluorinative functionalization reactions for small molecule synthesis. The coverage is organized by the type of carbon framework the fluorine is attached to for mono- and polyfluorinated motifs. The main challenges, opportunities and advances of defluorinative functionalization are discussed for each class of organofluorine. Most of the text focuses on case studies that illustrate how defluorofunctionalization can improve routes to synthetic targets or how the properties of C-F bonds enable unique mechanisms and reactions. The broader goal is to showcase the opportunities for incorporating and exploiting C-F bonds in the design of synthetic routes, improvement of specific reactions and advent of new methods.

Nickel-catalysed asymmetric hydromonofluoromethylation of 1,3-enynes for enantioselective construction of monofluoromethyl-tethered chiral allenes

An unprecedented nickel-catalysed enantioselective hydromonofluoromethylation of 1,3-enynes is developed, allowing the diverse access to monofluoromethyl-tethered axially chiral allenes, including the challenging deuterated monofluoromethyl (CD2F)-tethered ones that are otherwise inaccessible. It represents the first asymmetric 1,4-hydrofunctionalization of 1,3-enynes using low-cost asymmetric nickel catalysis, thus opening a new avenue for the activation of 1,3-enynes in reaction development. The utility is further verified by its broad substrate scope, good functionality tolerance, mild conditions, and diversified product elaborations toward other valuable fluorinated structures. Mechanistic experiments and DFT calculations provide insights into the reaction mechanism and the origin of the enantioselectivity.

A Pd-catalyzed highly selective three-component protocol for trisubstituted allenes

Herein we report the first example of a Pd-catalyzed highly selective three-component reaction of alkynyl-1,4-diol dicarbonates, organoboronic acids, and malonate anions for the efficient synthesis of trisubstituted 2,3-allenyl malonates not readily available by the known protocols. The reaction demonstrates an excellent regio- and chemo-selectivity for both the oxidative addition referring to the two C-O bonds and the subsequent coupling with the nucleophile with a remarkable functional group compatibility. A series of control experiments confirm a unique mechanism involving β-O elimination forming alka-1,2,3-triene and the subsequent insertion of its terminal C[double bond, length as m-dash]C bond into the Ar-Pd bond.

Chiral Diene Ligands in Asymmetric Catalysis

Asymmetric catalysis has emerged as a general and powerful approach for constructing chiral compounds in an enantioselective manner. Hence, developing novel chiral ligands and catalysts that can effectively induce asymmetry in reactions is crucial in modern chemical synthesis. Among such chiral ligands and catalysts, chiral dienes and their metal complexes have received increased attention, and a great progress has been made over the past two decades. This review provides comprehensive and critical information on the essential aspects of chiral diene ligands and their importance in asymmetric catalysis. The literature covered ranges from August 2003 (when the first effective chiral diene ligand for asymmetric catalysis was reported) to October 2021. This review is divided into two parts. In the first part, the chiral diene ligands are categorized according to their structures, and their preparation methods are summarized. In the second part, their applications in asymmetric transformations are presented according to the reaction types.

Photo and copper dual catalysis for allene syntheses from propargylic derivatives via one-electron process

Different from the traditional two-electron oxidative addition-transmetalation-reductive elimination coupling strategy, visible light has been successfully integrated into transition metal-catalyzed coupling reaction of propargylic alcohol derivatives highly selectively forming allenenitriles: specifically speaking, visible light-mediated Cu-catalyzed cyanation of propargylic oxalates has been realized for the general, efficient, and exclusive syntheses of di-, tri, and tetra-substituted allenenitriles bearing various synthetically versatile functional groups. A set of mechanistic studies, including fluorescence quenching experiments, cyclic voltammetric measurements, radical trapping experiments, control experiments with different photocatalyst, and DFT calculation studies have proven that the current reaction proceeds via visible light-induced redox-neutral reductive quenching radical mechanism, which is a completely different approach as compared to the traditional transition metal-catalyzed two-electron oxidative addition processes.

Catalytic Asymmetric β-Oxygen Elimination

A catalytic enantioselective β-O-elimination reaction is reported in the form of a zirconium-catalyzed asymmetric opening of meso-ketene acetals. Furthermore, a regiodivergent β-O-elimination is demonstrated. The reaction proceeds under mild conditions, at low catalyst loadings, and produces chiral monoprotected cis-1,2-diols in good yield and enantiomeric excess. The combination with a Mitsunobu reaction or a one-pot hydroboration/Suzuki reaction sequence then gives access to additional diol and aminoalcohol building blocks. A stereochemical analysis supported by DFT calculations reveals that a high selectivity in the hydrozirconation step is also important for achieving high enantioselectivity, although it does not constitute the asymmetric step. This insight is crucial for the future development of related asymmetric β-elimination reactions.

Nickel-catalyzed reductive defluorination of iodo allylic gem-difluorides: allenyl monofluoride synthesis

As a potential fluorinated synthon, there have been only limited reports on fluorinated allene synthesis and applications due to concerns about their stability. Here, we developed a nickel-catalyzed reductive defluorination of iodo allyl gem-difluorides to afford allenyl monofluorides under mild conditions with good functional group tolerance, which were easily converted to other C-F bond compounds, such as alkyl and alkenyl fluorides. Preliminary mechanistic studies suggested that monofluoroallenes were yielded by β-F elimination of the alkenyl C-Ni intermediates from the oxidative addition of C-I bonds to a nickel(0) catalyst, while zinc regenerates the catalyst and closes the catalytic cycle.

Recent advances in the metal-catalyzed asymmetric synthesis of chiral allenes

Recent advances in the metal-catalyzed asymmetric synthesis of chiral allenes are summarized. This review is categorized based on the starting material, including alkynes, racemic allenes, and conjugated dienes.