Amide-Directed C-H Sodiation by a Sodium Hydride/Iodide Composite

The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Base-Induced Dehydrogenative and Dearomative Transformation of 1-Naphthylmethylamines to 1,4-Dihydronaphthalene-1-carbonitriles

Solvothermal treatment of 1-naphthylmethylamine with potassium hydride (KH) or n-butyllithium (n-BuLi)-potassium t-butoxide (t-BuOK) in THF induces unusual two consecutive β-hydride eliminations to form 1-naphthonitrile and KH. The freshly generated KH is hydridic enough to undergo dearomative hydride addition to the resultant 1-naphthonitrile regioselectively at the C4 position to afford α-cyano benzylic carbanion, which could be functionalized by a series of electrophiles, liberating the corresponding 1,4-dihydronaphthalene-1-carbonitriles having a quaternary carbon center.

Sodium Isopropyl(trimethylsilyl)amide: A Stable and Highly Soluble Lithium Diisopropylamide Mimic

The preparation, structure, physical properties, and reactivities of sodium isopropyl(trimethylsilyl)amide (NaPTA) are described. The solubilities at room temperature range from n-heptane (0.55 M), n-hexane (0.60 M), toluene (0.65 M), MTBE (1.7 M), Et₃N (3.2 M), and THF (>6.0 M). The half-life to destruction in neat THF is >1 year at 25 °C and 7 days at 70 °C, which compares favorably to 2.5 months and 1.5 days, respectively, for LDA in neat THF. This study focuses on NaPTA in THF. ²⁹Si NMR spectroscopy shows exclusively a mixture of cis and trans stereoisomeric dimers in 0.10-12 M THF in hexane. Density functional theory (DFT) computations suggest that the pK_b is intermediate between dimeric sodium diisopropylamide (NaDA) and dimeric sodium hexamethyldisilazide (NaHMDS). Metalations of arenes, epoxides, ketones, hydrazones, alkenes, and alkyl halides show higher reactivities than LDA (k_NaPTA/LDA = 1-30). While the rates of arene metalation are high, the lower pK_b of NaPTA limits the substrates. Metalation of pseudoephedrate-based carboxamides to form disodiated Myers enolates solves several challenging technical problems.

Rhodium-Catalyzed (4+1) Cycloaddition between Benzocyclobutenones and Styrene-Type Alkenes

Herein, we describe a unique one-carbon ring-expansion strategy to access multi-substituted 2-indanones from benzocyclobutenones and styrene-type olefins. The use of a cationic "ligandless" rhodium catalyst was the key for both high reactivity and selectivity towards the (4+1) product. Broad functional group tolerance, a good substrate scope, and scalability have been demonstrated. Computation studies reveal that the origin of the (4+1) selectivity is due to a facile β-H elimination pathway that reduces the overall barrier of the turnover-limiting C-C reductive elimination step.

Stereoselective Synthesis of Cyclohexane Derivatives: Tandem Lithium Iodide Mediated Intramolecular Conjugate Addition of Thiols to α,β-Bisenones

A tandem intramolecular conjugate addition reaction was conducted with α,β-bisenones as selected Michael acceptors which were converted into 1,2,3-trisubstituted six-membered rings in the presence of activated sulfur nucleophiles. The products were obtained in good to excellent yields (maximum yield: 99%). Various substituted α,β-bisenones and sulfur nucleophiles were examined to understand the substrate scope of the reaction. Only one diastereomer was isolated, as lithium iodide mediated enolate trapping reactions improve the stereoselectivity of reactions involving 1,2,3-trisubstituted cyclohexanes.

Partial reductions of carboxylic acids and their derivatives to aldehydes

Recent advances in partial reductions of inert carboxylic acids and their derivatives to active aldehydes are reviewed.

Carbonyl 1,2-transposition through triflate-mediated α-amination

[Figure: see text].

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.

Continuous Flow Sodiation of Substituted Acrylonitriles, Alkenyl Sulfides and Acrylates

The sodiation of substituted acrylonitriles and alkenyl sulfides in a continuous flow set-up using NaDA (sodium diisopropylamide) in EtNMe2 or NaTMP (sodium 2,2,6,6-tetramethylpiperidide)⋅TMEDA in n-hexane provides sodiated acrylonitriles and alkenyl sulfides, which are subsequently trapped in batch with various electrophiles such as aldehydes, ketones, disulfides and allylic bromides affording functionalized acrylonitriles and alkenyl sulfides. This flow-procedure was successfully extended to other acrylates by using Barbier-type conditions.

Facile and practical hydrodehalogenations of organic halides enabled by calcium hydride and palladium chloride

For the first time, calcium hydride and palladium chloride were used to reduce a wide range of organic halides including aromatic bromides, aromatic chlorides, aromatic triflates, aliphatic bromides, aliphatic chlorides and trihalomethyl compounds.

Advances in regioselective functionalization of 9-phenanthrenols

Various approaches are discussed for regioselective functionalization and transformation of 9-phenanthrenol and its ether derivatives, which are readily available from phenanthrene in coal tar.

TFA-Catalyzed [3+2] Spiroannulation of Cyclobutanols: A Route to Spiro[cyclobuta[a]indene-7,1'-cyclobutane] Skeletons

A straightforward method for the synthesis of spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives from cyclobutanols has been developed via one-pot [3+2] spiroannulation. A series of new spiro[cyclobuta[a]indene-7,1'-cyclobutane] derivatives are facilely synthesized in good yields under mild reaction conditions.

LiCl-promoted amination of β-methoxy amides (γ-lactones)

An efficient and mild method has been developed for the amination of β-methoxy amides (γ-lactones) including natural products michelolide, costunolide and parthenolide derivatives by using lithium chloride in good yields. This reaction is applicable to a wide range of substrates with good functional group tolerance. Mechanism studies show that the reactions undergo a LiCl promoted MeOH elimination from the substrates to form the corresponding α,β-unsaturated intermediates followed by the Michael addition of amines.

The amination of β-methoxy amides (γ-lactones) including natural products michelolide, costunolide and parthenolide derivatives were first developed by using lithium chloride.

Synthesis of Phenacene-Helicene Hybrids by Directed Remote Metalation

Polycyclic aromatic hydrocarbons (PAHs) with six and seven rings were synthesized via directed metalation and cross-coupling of chrysenyl N,N-diethyl carboxamides with o-tolyl and methylnaphthalenyl derivatives. In the presence of competing ortho sites, the site selectivity in iodination of chrysenyl amides by directed ortho metalation (DoM) was influenced by the lithium base. The catalyst ligand bite angle was presumably important in the cross-coupling of sterically hindered bulky PAHs. Subsequent directed remote metalation of biaryls under standard conditions and at elevated temperatures afforded various fused six- and seven-ring PAHs, all in good yields and with fluorescent properties.

NaH-mediated direct C-H arylation in the presence of 1,10-phenanthroline

Transition-metal-free coupling of haloarenes with unactivated arenes has been developed in the presence of NaH and 1,10-phenanthroline. Various haloarenes bearing methyl, methoxy, halogen (fluoride, chloride, and bromide), cyano, trifluoromethyl, ester, and amide groups can be cross-coupled with unactivated arenes, or heteroarenes in this reaction.

Transition-Metal-Free Reductive Functionalization of Tertiary Carboxamides and Lactams for α-Branched Amine Synthesis

A new method for the synthesis of α-branched amines by reductive functionalization of tertiary carboxamides and lactams is described. The process relies on the efficient and controlled reduction of tertiary amides by a sodium hydride/sodium iodide composite, in situ treatment of the resulting anionic hemiaminal with trimethylsilyl chloride and subsequent coupling with nucleophilic reagents including Grignard reagents and tetrabutylammonium cyanide. The new method exhibits broad functional-group compatibility, operates under transition-metal-free reaction conditions, and is suitable for various synthetic applications on both sub-millimole and on multigram scales.

Stereo-controlled anti-hydromagnesiation of aryl alkynes by magnesium hydrides

A concise protocol for anti-hydromagnesiation of aryl alkynes was established using 1 : 1 molar combination of sodium hydride (NaH) and magnesium iodide (MgI₂) without the aid of any transition metal catalysts. The resulting alkenylmagnesium intermediates could be trapped with a series of electrophiles, thus providing facile accesses to stereochemically well-defined functionalized alkenes. Mechanistic studies by experimental and theoretical approaches imply that polar hydride addition from magnesium hydride (MgH₂) is responsible for the process.

Anti-hydromagnesiation of aryl alkynes was facilitated solely by magnesium hydride. The resulting alkenylmagnesium intermediates were functionalized with various electrophiles to afford stereochemically defined tri-substituted alkenes.

Pd(ii)-Catalyzed decarboxylative meta-C–H difluoromethylation

Palladium(ii)-catalyzed decarboxylative meta-C–H difluoromethylation reactions have been developed. Initial mechanistic studies disclosed that a migratory insertion would be involved in this meta-selective C–H functionalization.

Nucleophilic amination of methoxypyridines by a sodium hydride-iodide composite

A new protocol for nucleophilic amination of methoxypyridines and their derivatives was developed using sodium hydride (NaH) in the presence of lithium iodide (LiI). The method offers a concise access to various aminopyridines which are potentially of medicinal interest.

Electron Transfer Reactions: KO tBu (but not NaO tBu) Photoreduces Benzophenone under Activation by Visible Light

Long-standing controversial reports of electron transfer from KO tBu to benzophenone have been investigated and resolved. The mismatch in the oxidation potential of KO tBu (+0.10 V vs SCE in DMF) and the first reduction potential of benzophenone (of many values cited in the literature, the least negative value is -1.31 V vs SCE in DMF), preclude direct electron transfer. Experimental and computational results now establish that a complex is formed between the two reagents, with the potassium ion providing the linkage, which markedly shifts the absorption spectrum to provide a tail in the visible light region. Photoactivation at room temperature by irradiation at defined wavelength (365 or 400 nm), or even by winter daylight, leads to the development of the blue color of the potassium salt of benzophenone ketyl, whereas no reaction is observed when the reaction mixture is maintained in darkness. So, no electron transfer occurs in the ground state. However, when photoexcited, electron transfer occurs within a complex formed from benzophenone and KO tBu. TDDFT studies match experimental findings and also define the electronic transition within the complex as n → π*, originating on the butoxide oxygen. Computation and experiment also align in showing that this reaction is selective for KO tBu; no such effect occurs with NaO tBu, providing the first case where such alkali metal ion selectivity is rationalized in detail. Chemical evidence is provided for the photoactivated electron transfer from KO tBu to benzophenone: tert-butoxyl radicals are formed and undergo fragmentation to form (acetone and) methyl radicals, some of which are trapped by benzophenone. Likewise, when KOC(Et)₃ is used in place of KO tBu, then ethylation of benzophenone is seen. Further evidence of electron transfer was seen when the reaction was conducted in benzene, in the presence of p-iodotoluene; this triggered BHAS coupling to form 4-methylbiphenyl in 74% yield.

Dearylation of arylphosphine oxides using a sodium hydride–iodide composite

A new protocol for the dearylative functionalization of arylphosphine oxides was developed using NaH in the presence of LiI.

Radical Hydrodehalogenation of Aryl Bromides and Chlorides with Sodium Hydride and 1,4-Dioxane

A practical method for radical chain reduction of various aryl bromides and chlorides is introduced. The thermal process uses NaH and 1,4-dioxane as reagents and 1,10-phenanthroline as an initiator. Hydrodehalogenation can be combined with typical cyclization reactions, proving the nature of the radical mechanism. These chain reactions proceed by electron catalysis. DFT calculations and mechanistic studies support the suggested mechanism.

Nucleophilic Amination of Methoxy Arenes Promoted by a Sodium Hydride/Iodide Composite

A method for the nucleophilic amination of methoxy arenes was established by using sodium hydride (NaH) in the presence of lithium iodide (LiI). This method offers an efficient route to benzannulated nitrogen heterocycles. Mechanistic studies showed that the reaction proceeds through an unusual concerted nucleophilic aromatic substitution.