Palladium-Catalyzed Double ipso-Defluoroetherification of Allylic gem-Difluorides with Phenols or Alcohols

Significant progress has been made in the synthesis of diverse ketals through a palladium-catalyzed reaction involving allylic gem-difluorides and various phenols or alcohols. This methodology facilitates double ipso-defluoroetherification of allylic gem-difluorides, resulting in high product yields with excellent regioselectivity. The reactions were conducted under mild conditions and exhibited outstanding tolerance to a wide range of functional groups. Notably, this approach overcomes the traditional limitation of palladium-catalyzed processes, where nucleophiles typically target the common C-1 position of allylic gem-difluorides, enabling selective functionalization at the rare C-3 position instead.

Base-Promoted Nucleophilic Phosphorylation of Benzyl Fluorides via C(sp3)-F Cleavage

Herein, a transition-metal-free phosphorylation of benzyl fluorides with P(O)-H compounds is disclosed. In the presence of tBuOK, various benzyl fluorides react with P(O)-H compounds to produce the corresponding benzyl phosphine oxides, phosphinates, and phosphonates in good to high yields. This base-promoted phosphorylation reaction offers a facile and general strategy for the construction of a C(sp3)-P bond.

Cross- and Multi-Coupling Reactions Using Monofluoroalkanes

Carbon-fluorine bonds are stable and have demonstrated sluggishness against various chemical manipulations. However, selective transformations of C-F bonds can be achieved by developing appropriate conditions as useful synthetic methods in organic chemistry. This review focuses on C-C bond formation at monofluorinated sp3 -hybridized carbons via C-F bond cleavage, including cross-coupling and multi-component coupling reactions. The C-F bond cleavage mechanisms on the sp3 -hybridized carbon centers can be primarily categorized into three types: Lewis acids promoted F atom elimination to generate carbocation intermediates; nucleophilic substitution with metal or carbon nucleophiles supported by the activation of C-F bonds by coordination of Lewis acids; and the cleavage of C-F bonds via a single electron transfer. The characteristic features of alkyl fluorides, in comparison with other (pseudo)halides as promising electrophilic coupling counterparts, are also discussed.

Examining the reactivity of tris(ortho-carboranyl)borane with Lewis bases and application in frustrated Lewis pair Si-H bond cleavage

Reactions of tris(ortho-carboranyl)borane with Lewis bases reveals only small bases bind. The tremendous bulk and Lewis acidity is leveraged in frustrated Lewis pair Si-H cleavage with a wider range of Lewis bases and greater efficacy than B(C₆F₅)₃.

Structure and reactivity studies of the aluminum analogue of Piers' borane [HAl(C6F5)2]3

The aluminum analogue of Piers' borane, [HAl(C₆F₅)₂]₃1, is prepared on a gram-scale. Density functional theory (DFT) calculations reveal 1 has a higher fluoride ion affinity (FIA) than Piers' borane, while the Al-H moiety proved to be a strong hydride donor, reacting with alcohol and terminal alkyne to give the corresponding dehydrogenative products 3 and 4. Hydroalumination product 5 was prepared via reaction of 1 with aldehyde. In addition, 1 catalyzes the hydrosilylation of alkynes and alkenes.

Tris(ortho-carboranyl)borane: An Isolable, Halogen-Free, Lewis Superacid

The synthesis of tris(ortho-carboranyl)borane (BoCb3 ), a single site neutral Lewis superacid, in one pot from commercially available materials is achieved. The high fluoride ion affinity (FIA) confirms its classification as a Lewis superacid and the Gutmann-Beckett method as well as adducts with Lewis bases indicate stronger Lewis acidity over the widely used fluorinated aryl boranes. The electron withdrawing effect of ortho-carborane and lack of pi-delocalization of the LUMO rationalize the unusually high Lewis acidity. Catalytic studies indicate that BoCb3 is a superior catalyst for promoting C-F bond functionalization reactions than tris(pentafluorophenyl)borane [B(C6 F5 )3 ].

A computational tool to accurately and quickly predict 19F NMR chemical shifts of molecules with fluorine-carbon and fluorine-boron bonds

We report the evaluation of density-functional-theory (DFT) based procedures for predicting ¹⁹F NMR chemical shifts at modest computational cost for a range of molecules with fluorine bonds, to be used as a tool for assisting the characterisation of reaction intermediates and products and as an aid to identifying mechanistic pathways. The results for a balanced learning set of molecules were then checked using two further testing sets, resulting in the recommendation of the ωB97XD/aug-cc-pvdz DFT method and basis set as having the best combination of accuracy and computational time, with a RMS error of 3.57 ppm. Cationic molecules calculated without counter-anion showed normal errors, whilst anionic molecules showed somewhat larger errors. The method was applied to the prediction of the conformationally averaged ¹⁹F chemical shifts of 2,2,3,3,4,4,5,5-octafluoropentan-1-ol, in which gauche stereoelectronic effects involving fluorine dominate and to determining the position of coordination equilibria of fluorinated boranes as an aid to verifying the relative energies of intermediate species involved in catalytic amidation reactions involving boron catalysts.

Ferrocene tethered boramidinate frustrated Lewis pairs: stepwise capture of CO2 and CO

The synthesis and reactivity of novel ferrocene tethered boramidinate frustrated Lewis pairs (FLPs), capable of the sequential capture of small molecules, is reported. Reactions of 1,1'-dicarbodiimidoferrocenes with different boranes provides access to metallocene tethered FLPs. The reactivity of the boramidinate moieties can be tuned by the nature of the carbodiimido substituents (alkyl vs. aryl) and the borane used in the reduction (9-borabicyclo[3.3.1]nonane [(C₈H₁₄)₂BH]₂vs. bis-pentafluorophenyl borane [(C₆F₅)₂BH]₂). The boramidinate FLP arms do not engage in intramolecular reactions, allowing for independent small molecule capture by each FLP. By careful synthetic control, sequential capture of different gaseous small molecules (CO₂ and CO or CO₂ and CNtBu) by the same bis(boramidinate)ferrocene molecule has been demonstrated.

Synthesis and reactivity of donor stabilized thionylium (SO2+) dications

Robust mono-, di-, and tridentate base-stabilized thionylium (SO2+) dications were synthesized from the treatment of SOCl¬¬2 with Me3SiO3SCF3 and pyridine-based ligands. Computational and experimental data are consistent with Lewis acidities...

Methodologies and Strategies for Selective Borylation of C-Het and C-C Bonds

Organoborons have emerged as versatile building blocks in organic synthesis to achieve molecular diversity and as carboxylic acid bioisosteres with broad applicability in drug discovery. Traditionally, these compounds are prepared by the substitution of Grignard/lithium reagents with electrophilic boron species and Brown hydroboration. Recent developments have provided new routes for the efficient preparation of organoborons by applying reactions using chemical feedstocks with leaving groups. As compared to the previous methods that used organic halides (I, Br, and Cl), the direct borylation of less reactive C-Het and C-C bonds has become highly important to get efficiency and functional-group compatibility. This Review aims to provide a comprehensive overview of this topic, including (1) C-F bond borylation, (2) C-O bond borylation, (3) C-S bond borylation, (4) C-N bond borylation, and (5) C-C bond borylation. Considerable attention is given to the strategies and mechanisms involved. We expect that this Review will inspire chemists to discover more efficient transformations to expand this field.

Twenty-five years of bis-pentafluorophenyl borane: a versatile reagent for catalyst and materials synthesis

Highlights of the extensive chemistry and applications of bis-pentafluorophenyl borane (“Piers’ borane”) from the 25 years since its first appearance are featured.

Selective synthesis of spirobiindanes, alkenyl chlorides, and monofluoroalkenes from unactivated gem-difluoroalkanes controlled by aluminum-based Lewis acids

The highly selective synthesis of spirobiindanes, alkenyl chlorides, and monofluoroalkenes via the cleavage of inert C(sp³)–F bonds in unactivated gem-difluoroalkanes using readily available and inexpensive aluminum-based Lewis acids of low toxicity is reported. The selectivity of this reaction can be controlled by modifying the substituents on the central aluminum atom of the promoter. An intramolecular cascade Friedel-Crafts alkylation of unactivated gem-difluorocarbons can be achieved using a stoichiometric amount of AlCl₃. The subsequent synthesis of alkenyl chlorides via F/Cl exchange followed by an elimination can be accomplished using AlEt₂Cl as a fluoride scavenger and halogen source. The defluorinative elimination of acyclic and cyclic gem-difluorocarbons to give monofluoroalkenes can be achieved using AlEt₃.

Activation of Saturated Fluorocarbons to Synthesize Spirobiindanes, Monofluoroalkenes, and Indane Derivatives

Fluorinated organic compounds are produced in abundance by the pharmaceutical and agrochemical industry, making such compounds attractive as building blocks for further functionalization. Unfortunately, activation of C(sp³)-F bond in saturated fluorocarbons, especially for aliphatic gem-difluoroalkanes, remains challenging. Here we describe the selective activation of inert C(sp³)-F bonds catalyzed by B(C₆F₅)₃. In hexafluoro-2-propanol (HFIP), chemically robust aliphatic gem-difluorides are converted in high yields to the corresponding substituted 2,2′,3,3′-tetrahydro-1,1′-spirobiindenes via a B(C₆F₅)₃-catalyzed intramolecular cascade Friedel-Crafts cyclization, not requiring a silicon-based trapping reagent. However, in the absence of a hydrogen-bonding donor solvent such as HFIP, the aliphatic gem-difluorides preferentially engage in a defluorination/elimination process that provides monofluorinated alkenes in good yields. Furthermore, a series of substituted 1-alkyl-2,3-dihydro-1H-indenes was obtained in high yield from the B(C₆F₅)₃-catalyzed defluorinative cyclization of aliphatic secondary monofluorides in HFIP. The protocol could inspire development of a new class of main-group Lewis acid-catalyzed C(sp³)-F bond activation in general unactivated fluorocarbons.

•

C(sp³)-F bond activation in general unactivated fluorocarbons

•

The activation of C(sp³)-F bonds in aliphatic gem-difluoroalkanes

•

The selective activation of inert C(sp³)-F bonds catalyzed by B(C₆F₅)₃

•

An intramolecular cascade defluorinative Friedel-Crafts cyclization

9-Borabicyclo[3.3.l]nonane-induced Friedel-Crafts benzylation of arenes with benzyl fluorides

Friedel-Crafts benzylation of arenes with benzyl fluorides using 9-borabicyclo[3.3.l]nonane (9-BBN) as a mediator has been developed. This provides a simple and cheap route to the activation of C-F bonds to synthesize 1,1-diarylmethanes in good to excellent yields (up to 98%) under mild conditions. Functional group tolerance and the mechanism are considered.