Reductive Difunctionalization of Aryl Alkenes with Sodium Metal and Reduction-Resistant Alkoxy-Substituted Electrophiles

Synthesis of 1,2-Bis- and 1,1,2-Tris-Borylalkanes under Transition Metal-Free and Solvent-Free Conditions

Multi(boronate) esters are useful building blocks in modern chemical synthesis. Herein, we have developed an efficient, transition metal- and solvent-free method for the regioselective boration of alkenes and alkynes. The alkali metal Lewis base (NaOMe)-mediated reactions, using bis(pinacolato)diboron (B2pin2) as the boron reagent, resulted in the diboration of alkenes at room temperature and the triboration of alkynes at 60 °C to produce synthetically useful alkyl 1,2-bis(boronate) esters and 1,1,2-tris(boronate) esters, respectively, in excellent yields and with high regioselectivity. This environmentally benign protocol demonstrates a broad substrate scope and good functional group tolerance for alkenes and alkynes. The gram-scale reaction further highlights the practical usefulness of the method. The proposed reaction pathway has been evaluated based on stoichiometric reactions and DFT studies.

Mechanochemical generation of aryl barium nucleophiles from unactivated barium metal

Organobarium reagents are of interest as homologues of the Grignard reagents based on organomagnesium compounds due to their unique reactivity as well as regio- and stereoselectivity. However, reactions involving organobarium reagents are less developed in comparison to reactions involving Grignard reagents due to the lack of a simple and economical synthetic method and their high reactivity. To the best of our knowledge, there is no established method for the direct synthesis of organobarium compounds from commercially available bulk barium metal and organic halides. So far, the generation of organobarium compounds usually requires the use of activated barium (Rieke barium), which significantly reduces the practical utility of organobarium reagents and hinders the development of new organobarium-mediated transformations. Here, we present a mechanochemical strategy based on ball-milling that facilitates the direct generation of various aryl barium nucleophiles from commercially available unactivated barium metal and aryl halides without complicated pre-activation processes. Our simple mechanochemical protocol allows the rapid development of novel carbon-silicon-bond-forming reactions with hydrosilanes mediated by aryl barium nucleophiles; importantly, these reactions are difficult to achieve using other Grignard-type carbon nucleophiles. To the best of our knowledge, this is the first example of a nucleophilic substitution reaction involving an aryl barium species. Furthermore, this mechanochemical strategy established the first example of a nucleophilic addition to a carbonyl compound involving an aryl barium nucleophile. Preliminary theoretical calculations using the artificial force-induced reaction (AFIR) method to reveal the reaction mechanism of the hydrosilane arylation are also described.

Reductive anti-Dizincation of Arylacetylenes

Arylacetylenes undergo anti-1,2-dizincation to afford trans-1,2-dizincioalkenes. The process employs sodium dispersion as a reducing agent and zinc chloride TMEDA complex as a reduction-resistant zinc electrophile. This reductive anti-dizincation contrasts with the conventional additive syn-dimetalation like silylzincation. The resulting dizincated alkenes undergo the cross-coupling to yield multi-substituted alkenes stereoselectively.

Sodium-Mediated Reductive C-C Bond Cleavage Assisted by Boryl Groups

In contrast to the well-established oxidative C=C double bond cleavage to give the corresponding carbonyl compounds, little is known about reductive C=C double bond cleavage. Here we report that C-C single bond cleavage in 1,2-diaryl-1,2-diborylethanes proceeds by reduction with sodium metal to yield α-boryl benzylsodium species. In combination with our previous reductive diboration of stilbenes, the overall transformation represents reductive cleavage of the C=C double bonds of stilbene to yield α-boryl-α-sodiated toluenes. This reductive two-step C=C double bond cleavage is applicable to ring-opening or ring-expansion reactions of polycyclic aromatic hydrocarbons.

New Frontiers in Organosodium Chemistry as Sustainable Alternatives to Organolithium Reagents

With their highly reactive respective C-Na and N-Na bonds, organosodium and sodium amide reagents could be viewed as obvious replacements or even superior reagents to the popular, widely utilised organolithiums. However, they have seen very limited applications in synthesis due mainly to poor solubility in common solvents and their limited stability. That notwithstanding in recent years there has been a surge of interest in bringing these sustainable metal reagents into the forefront of organometallics in synthesis. Showcasing the growth in utilisation of organosodium complexes within several areas of synthetic chemistry, this Minireview discusses promising new methods that have been recently reported with the goal of taming these powerful reagents. Special emphasis is placed on coordination and aggregation effects in these reagents which can impart profound changes in their solubility and reactivity. Differences in observed reactivity between more nucleophilic aryl and alkyl sodium reagents and the less nucleophilic but highly basic sodium amides are discussed along with current mechanistic understanding of their reactivities. Overall, this review aims to inspire growth in this exciting field of research to allow for the integration of organosodium complexes within common important synthetic transformations.

Multiply exo-Methylated Corannulenes

The curved π-conjugated surface of bowl-shaped corannulene has been multiply methylated to form exo-di-, -tetra-, and -hexamethylated corannulenes. The multimethylations became possible through in-situ iterative reduction/methylation sequences that involve the reduction of corannulenes using sodium to form the anionic corannulene species, and the subsequent SN 2 reaction of the anionic species with reduction-resistant dimethyl sulfate. X-ray diffraction analyses, NMR, MS, UV-Vis measurements, and DFT calculations have revealed the molecular structures of the multimethylated corannulenes and the sequence of the multimethylation. This work has the potential to contribute to the controlled synthesis and characterizations of multifunctionalized fullerenes.

Diphenyl Ditelluride: An Unconventional Reducing Agent in the Sulfonylative Cascade of Alkynyl Cyclohexadienones

Herein, we report a reductive hydrazo-sulfonylative difunctionalization cascade of alkynyl cyclohexadienones employing PhTeTePh as an uncommon reducing agent. Diphenyl ditelluride is a commercially available solid with a good solubility profile in most organic solvents, and this is the first report illustrating it as a reducing agent. The protocol afforded a variety of difunctionalized dihydrochromenones and dihydrobenzofuranones in good yields under relatively mild conditions. The reactions were scalable, and mechanistic studies were conducted to probe the reaction mechanism. Additionally, photophysical studies of the products were carried out, which revealed that they had significant absorption (400-450 nm) and emission (520-570 nm) in the visible region.

Taming Highly Unstable Radical Anions and 1,4-Organodilithiums by Flow Microreactors: Controlled Reductive Dimerization of Styrenes

The reduction of styrenes with lithium arenide in a flow microreactor leads to the instantaneous generation of highly unstable radical anions that subsequently dimerize to yield the corresponding 1,4-organodilithiums. A flow reactor with fast mixing is essential for this reductive dimerization as the efficiency and selectivity are low under batch conditions. A series of styrenes undergo dimerization, and the resulting 1,4-organodilithiums are trapped with various electrophiles. Trapping with divalent electrophiles affords precursors for useful yet less accessible cyclic structures, for example, siloles from dichlorosilanes. Thus, we highlight the power of single-electron reduction of unsaturated compounds in flow microreactors for organic synthesis.

Selective, Transition Metal-free 1,2-Diboration of Alkyl Halides, Tosylates, and Alcohols

Defunctionalization of readily available feedstocks to provide alkenes for the synthesis of multifunctional molecules represents an extremely useful process in organic synthesis. Herein, we describe a transition metal-free, simple and efficient strategy to access alkyl 1,2-bis(boronate esters) via regio- and diastereoselective diboration of secondary and tertiary alkyl halides (Br, Cl, I), tosylates, and alcohols. Control experiments demonstrated that the key to this high reactivity and selectivity is the addition of a combination of potassium iodide and N,N-dimethylacetamide (DMA). The practicality and industrial potential of this transformation are demonstrated by its operational simplicity, wide functional group tolerance, and the late-stage modification of complex molecules. From a drug discovery perspective, this synthetic method offers control of the position of diversification and diastereoselectivity in complex ring scaffolds, which would be especially useful in a lead optimization program.

Reductive Ring Opening of Arylcyclopropanecarboxamides Accompanied by Borylation and Enolate Formation

Treatment of arylcyclopropanecarboxamides with a sodium dispersion in the presence of methoxypinacolborane as a reduction-resistant electrophile leads to reductive cleavage of the cyclopropane ring followed by instant trapping with the boron electrophile to yield the enolates of γ-aryl-γ-borylalkanamides. The enolates react further with a different electrophile to yield the corresponding α-substituted amides with anti selectivity.

Facile Multiple Alkylations of C60 Fullerene

The reduction of fullerene (C₆₀) with sodium dispersion in the presence of an excess amount of dipropyl sulfate was found to yield highly propylated fullerene, C₆₀(nC₃H₇)_n (max. n = 24), and C₆₀(nC₃H₇)₂₀ was predominantly generated as determined by mass spectroscopy.

Reductive Cleavage of Propargylic Ethers with Alkali Metal: Application to the Synthesis of Allenylboronates

Treatment of propargylic ethers with sodium dispersion in the presence of lithium iodide results in the generation of the corresponding carbanion species via cleavage of the propargylic C-O bond. The anionic species react with trimethoxyborane to yield the allenylboronates including highly substituted ones that are difficult to synthesize.

Regioselective Silylations of Propargyl and Allyl Pivalates through Ca-Promoted Reductive C(sp3)-O Bond Cleavage

A practical protocol for the regioselective preparation of 3-phenylpropargylsilanes and 3-phenylallylsilanes in yields of 36-77 and 48-86%, respectively, from readily accessible 3-phenylpropargyl and 1-phenylallyl pivalates was developed through reductive C(sp³)-O bond cleavage. This method represents the first example of the direct application of vastly abundant calcium granules to a reductive coupling reaction. A broad range of propargylsilanes and allylsilanes are simply prepared using easy-to-handle pivalates and chlorotrimethylsilane under mild catalyst-free and additive-free conditions.

Sodium-Promoted Borylation of Polycyclic Aromatic Hydrocarbons

Sodium dispersion promotes the reductive borylation of polycyclic aromatic hydrocarbons (PAHs) with MeOBpin. Anthracenes and phenanthrenes are converted to the corresponding dearomatized diborylated products. The reductive diborylation of naphthalene-based small π-systems yields similar yet unstable products that are oxidized into formal C-H borylation products with unique regioselectivity. Pyrene is converted to 1-borylpyrene without the addition of an oxidant. The latter two reactions represent a new route to useful borylated PAHs that rivals C-X borylation and catalytic C-H borylation.

Halogen-sodium exchange enables efficient access to organosodium compounds

With sodium being the most abundant alkali metal on Earth, organosodium compounds are an attractive choice for sustainable chemical synthesis. However, organosodium compounds are rarely used-and are overshadowed by organolithium compounds-because of a lack of convenient and efficient preparation methods. Here we report a halogen-sodium exchange method to prepare a large variety of (hetero)aryl- and alkenylsodium compounds including tri- and tetrasodioarenes, many of them previously inaccessible by other methods. The key discovery is the use of a primary and bulky alkylsodium lacking β-hydrogens, which retards undesired reactions, such as Wurtz-Fittig coupling and β-hydrogen elimination, and enables efficient halogen-sodium exchange. The alkylsodium is readily prepared in situ from neopentyl chloride and an easy-to-handle sodium dispersion. We believe that the efficiency, generality, and convenience of the present method will contribute to the widespread use of organosodium in organic synthesis, ultimately contributing to the development of sustainable organic synthesis by rivalling the currently dominant organolithium reagents.

Recent Advances in the Use of Sodium Dispersion for Organic Synthesis

This short review describes the recent emergence of organosodium chemistry, motivated by the requirements of modern synthetic chemistry for sustainability, and powered by the use of sodium dispersion, a form of sodium that is commercially available, easy to handle, and has a large active surface area. We present recent methods for the preparation of organosodium compounds using sodium dispersion, and their applications to synthesis. Sodium amides and phosphides are also briefly discussed.

1 Introduction

2 Sodium Dispersion

3 Preparation of Organosodium Compounds

3.1 Two-Electron Reduction of Aryl Halides

3.2 Halogen–Sodium Exchange

3.3 Directed Metalation

3.4 Cleavage of C–C and C–Heteroatom Bonds

4 Synthetic Applications

4.1 Reduction in Combination with a Proton Source

4.1.1 Bouveault–Blanc Reduction

4.1.2 Birch Reduction

4.1.3 Reductive Deuteration

4.1.4 Chemoselective Cleavage of Amides and Nitriles

4.2 Difunctionalization of Alkenes and Alkynes

5 Sodium Amides and Phosphides

6 Conclusions and Outlook

Copper(ii)-catalyzed protoboration of allenes in aqueous media and open air

A copper(ii)-catalyzed internal protoboration of monosubstituted allenes efficiently occurs in water at room temperature and open air to generate 1,1-disubstituted vinyl boronic acid derivatives.

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.