Diborative Reduction of Alkynes to 1,2-Diboryl-1,2-Dimetalloalkanes: Its Application for the Synthesis of Diverse 1,2-Bis(boronate)s

Borylative Desymmetrization of Multifunctional Haloarenes assisted by Sodium Dispersion

Multiply halogenated aromatic compounds were selectively borylated by a boron alkoxide in the presence of sodium dispersion when the reaction was carried out at a low temperature, while multi-functionalization took place at an elevated temperature. The reaction of 1,4-dichlorobenzene with sodium dispersion (200-1200 mol%) in the presence of isopropyloxyboron pinacolate (120-240 mol%) afforded (4-chlorophenyl)boron pinacolate in up to 84 % yield. Formation of diborylated product hardly accompanied under the reaction conditions at -78 °C for 1 h.

A Mild and General trans-Diboration of Both Terminal and Internal Propargyl Alcohols

A practical and efficient trans-diboration of propargyl alcohols was accomplished using sodium hydride (NaH) as a base in N,N-dimethylformamide at room temperature. The mild reaction conditions demonstrate general applicability, facilitating the successful conversion of both terminal and internal propargyl alcohols with diverse structures and functional groups into highly functionalized alkenediboronates [4-borylated 1,2-oxaborol-2(5H)-oles]. The resulting products, which incorporate two boron groups, can be selectively activated and subjected to stepwise transformations, thereby providing an effective platform for synthesizing a wide range of structurally diverse trisubstituted alkenes.

Diastereoselective Homocoupling of Benzylic C(sp3)-H Bonds Enabled by Halogen Transfer

A transition-metal- and harsh-oxidant-free strategy for diastereoselective homocoupling of benzylic α-boryl carbanions has been developed. Central to this methodology is the ability of the halogen transfer reagent to seamlessly integrate halogenation and substitution within a compatible process. Additionally, this methodology is also applicable to the homocoupling of diarylmethanes and alkylheteroarenes. Substrates bearing oxidatively sensitive functional groups were well-tolerated. Preliminary studies suggest that the hydrogen bond between two boryl groups contributes to the high diastereoselectivities.

Phosphination of aryl/alkyl bromides via Mn-mediated reductive C-P coupling

Mn-mediated reductive cross-coupling of organic bromides with 2-bromo-1,3,2-diazaphospholene was developed for efficient construction of C-P bonds under mild conditions. Mechanistic studies suggested that bromides are activated by in situ formed bis-diazaphospholene via hybrid radical and polar mechanisms.

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.

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.

Palladium-Catalyzed Cross-Coupling Reactions of Borylated Alkenes for the Stereoselective Synthesis of Tetrasubstituted Double Bond

The stereoselective formation of tetrasubstituted alkenes remains one of the key goals of modern organic synthesis. In addition to other methods, the stereoselective synthesis of tetrasubstituted alkenes can be achieved by means of cross-coupling reactions of electrophilic and nucleophilic alkene templates. The use of electrophilic templates for the stereoselective synthesis of tetrasubstituted alkenes has previously been described. Therefore, the present review summarizes the procedures available for the stereoselective preparation of tetrasubstituted alkenes using stable and isolable nucleophilic templates.

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.

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-Mediated Dichotomic Borylation of Alkyne Carbonates: Stereoselective Access to (E)-1,2-Diborylated 1,3-Dienes versus Traceless Monoborylation Affording α-Hydroxyallenes

A mild copper-mediated protocol has been developed for borylation of alkynyl cyclic carbonates. Depending on the nature of the borylating reaction partner, either stereoselective diborylation of the propargylic surrogate takes place, providing convenient access to (E)-1,2-borylated 1,3-dienes, or traceless monoborylation occurs, which leads to α-hydroxyallenes as the principal product. The dichotomy in this borylation protocol has been scrutinized by several control experiments, illustrating that a relatively small change in the diboron(4) reagent allows for competitive alcohol-assisted protodemetalation to forge an α-hydroxyallene product under ambient conditions.

Diboron-controlled product selectivity switch in copper-catalyzed decarboxylative substitutions of alkynyl cyclic carbonates

DFT calculations were performed to explore the mechanisms, origins of diboron-controlled divergent product selectivity and stereoselectivity in the copper-catalyzed decarboxylative substitution of alkynyl cyclic carbonates.

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.

α-Borylalkyl radicals: their distinctive reactivity in modern organic synthesis

Organoborons are extremely important for synthetic organic chemistry; they can serve as advanced intermediates for a variety of transformations. Such a well-known transformation involves the loss of the boron moiety, creating alkyl radicals. Although these originally developed protocols for alkyl radical generation remain in active use today, in recent years their α-boryl carbon-centred radicals have been joined by a new array of radical generation strategies that offer a unique reactivity to forge a wider diversity of organoborons that often operate under mild and benign conditions. Herein, we will highlight the stability and reactivity of α-borylalkyl radicals and their remarkably recent advances in order to further utilise them for C-C and C-heteroatom bond formation. Their use for this purpose has been reported over the last decade in an attempt to guide the synthetic community. Various transition-metal and metal-free methods for their generation are presented, and more advanced photoredox approaches are discussed, mainly for the period of 2009-2019.

Synthesis of anti-vicinal diboronates from diarylethynes and B2pin2

Anti-vicinal diboronates were fabricated from easily available diarylethynes and B₂pin₂ via a base-catalyzed domino-borylation-protodeboronation (DBP) strategy under transition-metal-free conditions. Under the standard conditions, reactants with a range of different classes of functional groups on the rings, such as MeO, MeS, CF₃O, Me₂N, TMS, I, Br, Cl, F, and the thiophene ring, were tolerated. Downstream transformation of the vicinal diboronates provided a facile pathway for obtaining vicinal diols by mild oxidation with NaBO₃, and a new deuteriation technique was developed in order to acquire 1,2-diarylethanes-1,2-d₂ and 1,2-diarylethanes-1,1,2,2-d₄. The new deuteriation strategy developed in this study may provide a new research direction for deuteriation chemistry.