Bis(amino)cyclopropenylidenes as Organocatalysts for Acyl Anion and Extended Umpolung Reactions

Remote Fluorination of α,β-Unsaturated Carbonyls via Silyl Dienol Ethers

We report a general, regioselective, and metal free γ-fluorination of α,β-unsaturated carbonyls via silyl dienol ethers that are readily prepared from simple ketones and aldehydes. The transformation displays broad scope including 27 cyclic and acyclic siloxydienes providing γ-fluoro compounds in 28-91 % yield. Notably, the reported conditions are also suitable for the synthesis of challenging tertiary fluorides. The regioselectivity of the reaction was studied on a series of acyclic siloxydienes and was observed to be sensitive to the conformational flexibility of the substrate. Diversification of the γ-fluorocarbonyls demonstrates the promise of fluorine as a stereocontrol element.

Iron-Catalyzed Gamma-Gamma Dimerization of Siloxydienes

We report the oxidative dimerization reaction of siloxydienes derived from simple enones that creates a new gamma-gamma (γ-γ) C-C bond using catalytic iron and benzoyl peroxide as the terminal oxidant in acetonitrile solvent at ambient temperature. The reaction shows a broad substrate scope including cyclic and acyclic siloxydienes derived from ketones, aldehydes, and esters, which are converted to 1,8-dicarbonyl compounds under mild catalytic reaction conditions in 19-89 % yield across 30 examples. The method is suitable for the coupling of sterically demanding carbon centers, including the formation of vicinal quaternary centers. Conceptually, the dienol ether serves as a precursor to a conjugated radical cation, which undergoes highly site selective γ-dimerization reactions. The γ-γ dimerization strategy is applied to the synthesis of a bioactive analogue of honokiol.

Electrochemical chemoselective hydrogenation of 1,4-enediones with HFIP as the hydrogen donor: scalable access to 1,4-diketones

A straightforward electrochemical protocol for efficient hydrogenation of unsaturated CC bonds has been reported in an undivided cell. A series of versatile 1,4-diketones are smoothly generated under metal-free and external-reductant-free electrolytic conditions. Moreover, the tolerance of various functional groups and decagram-scale experiments have shown the practicability and potential applications of this methodology. Moreover, a range of heterocyclic compounds were easily prepared through follow-up procedures of 1,4-diketones.

Dimerization and ring-opening in bis(diisopropylamino)cyclopropenylidene (BAC) mediated by [U(NR2)3(CCPh)] (R = SiMe3)

Addition of 2 equiv. of bis(diisopropylamino)cyclopropenylidene (BAC) to [U(NR2)3(CCPh)] (1, R = SiMe3), in Et2O, results in formation of [cyclo-N(iPr)C(Me)2CH(NiPr2)C{CHC3(NiPr2)2}][U(NR2)2(N(SiMe3)SiMe2CH2)(CCPh)] (2) in moderate isolated yield. Complex 2 is the result of coupling and protonation of two BAC molecules, where complex 1 contributes the required proton. It was characterized by NMR spectroscopy and X-ray crystallography and represents a new mode of reactivity of the cyclopropenylidene fragment.

Palladium-catalyzed coupling of amides and cyclopropanols for the synthesis of γ-diketones

A palladium catalytic method has been developed for the coupling of amides and cyclopropanols to γ-diketones, through simultaneous C-N and C-C activation. Heteroatom ligand exchange and heteroatom-to-carbon ligation mode switching enable the achievement of molecular cross-coupling in an amide N-atom structural context-dependent manner, avoiding any stoichiometric organometallic reagent or base.

Synthesis of Novel 1,4-Diketone Derivatives and Their Further Cyclization

One of the important reactions to obtain a new carbon-carbon bond is the Stetter reaction, which is generally via a nucleophilic catalyst like cyanide or thiazolium-NHC catalysts. In particular, 1,4-diketones with very functional properties are obtained by the Stetter reaction with the intermolecular reaction of an aldehyde and an α,β-unsaturated ketone. In this study, we synthesized new derivatives (substituted arenoxy) of 1,4-diketone compounds (2a-2n) with useful features by a new version of the Stetter reaction method. In our work, arenoxy benzaldehyde derivatives with different structures as the Michael donor and methyl vinyl ketone as the Michael acceptor were used for the intermolecular Stetter reaction. The reaction was catalyzed by 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride (3b), using triethylamine for the basic medium and dimethyl sulfoxide as the solvent. As a result, some novel arenoxy-substituted 1,4-diketones were gained with good yields at room temperature within 24 h through an intermolecular Stetter reaction. In addition, new furan and pyrrole derivatives were prepared by performing the cyclization reaction with one of the obtained new diketone compounds.

Manganese(III) porphyrin-catalyzed regioselective dual functionalization of C(sp3)-H bonds: the transformation of arylalkanes to 1,4-diketones

The first, direct way from arylalkanes to 1,4-dicarbonyl compounds has been shown. It makes obtaining these useful products more accessible and cheaper. Our method is based on a one-pot reaction with excellent regioselectivity, mild conditions, and water as the main solvent. A plausible reaction mechanism has also been proposed.

An Alternative Method for the Synthesis of N-Pentafluorophenyl Triazolium Salts

A route for the synthesis of 1,2,4-triazolium salts via oxidation of a thione precursor is demonstrated. N-Pentafluorophenyl-substituted salts are produced in 20-63% overall yields. Isolation and purification of the azolium salts are simplified compared to the traditional synthetic route. Late-stage selection of the counterion allows the synthesis of a variety of salts from a parent thione. The salts have been compared in Stetter and cross-benzoin reactions with an appreciable counterion effect in both reactions.

Boron-Promoted Deprotonative Conjugate Addition: Geminal Diborons as Soft Pronucleophiles and Acyl Anion Equivalents

Conjugate addition of α-boron-stabilized carbanions is an underexplored reaction modality. Existing methods require deborylation of geminal di-/triboryl alkanes and/or the presence of additional activating groups. We report the 1,4-addition of α,α-diboryl carbanions generated via deprotonation of the corresponding geminal diborons. The methodology provided a general route to highly substituted and synthetically useful γ,γ-diboryl ketones. The development of geminal diborons as soft pronucleophiles also enabled their use as acyl anion equivalents via a one-pot tandem conjugate addition-oxidation sequence.

Three-component carboacylation of alkenes via cooperative nickelaphotoredox catalysis

Various commercially available acyl chlorides, aldehydes, and alkanes were exploited for versatile three-component 1,2-carboacylations of alkenes to forge two vicinal C–C bonds through the cooperative action of nickel and sodium decatungstate catalysis. A wealth of ketones with high levels of structural complexity was rapidly obtained via direct functionalization of C(sp²)/C(sp³)–H bonds in a modular manner. Furthermore, a regioselective late-stage modification of natural products showcased the practical utility of the strategy, generally featuring high resource economy and ample substrate scope.

Various commercially available acyl chlorides, aldehydes, and alkanes were exploited for versatile three-component 1,2-carboacylations of alkenes to forge two vicinal C–C bonds through the cooperative action of nickel and sodium decatungstate catalysis.

Direct 1,2-Dicarbonylation of Alkenes towards 1,4-Diketones via Photocatalysis

1,4-Dicarbonyl compounds are intriguing motifs and versatile precursors in numerous pharmaceutical molecules and bioactive natural compounds. Direct incorporation of two carbonyl groups into a double bond at both ends is straightforward, but also challenging. Represented herein is the first example of 1,2-dicarbonylation of alkenes by photocatalysis. Key to success is that N(n-Bu)₄ ⁺ not only associates with the alkyl anion to avoid protonation, but also activates the α-keto acid to undergo electrophilic addition. The α-keto acid is employed both for acyl generation and electrophilic addition. By tuning the reductive and electrophilic ability of the acyl precursor, unsymmetric 1,4-dicarbonylation is achieved for the first time. This metal-free, redox-neutral and regioselective 1,2-dicarbonylation of alkenes is executed by a photocatalyst for versatile substrates under extremely mild conditions and shows great potential in biomolecular and drug molecular derivatization.

Recent advances in the organocatalytic applications of cyclopropene- and cyclopropenium-based small molecules

The development of novel small molecule-based catalysts for organic transformations has increased noticeably in the last two decades. A very recent addition to this particular research area is cyclopropene- and cyclopropenium-based catalysts. At one point in time, particularly in the mid-20^th century, much attention was focused on the structural aspects and physical properties of cyclopropene-based compounds. However, a paradigm shift was observed in the late 20^th century, and the focus shifted to the synthetic utility of these compounds. In fact, a wide range of cyclopropene derivatives have been found serving as valuable synthons for the construction of carbocycles, heterocycles and other useful organic compounds. In the last few years, the catalytic applications of cyclopropene/cyclopropenium-based compounds have been uncovered and many synthetic protocols have been developed using cyclopropene-based compounds as organocatalysts. Therefore, the main objective of this review is to highlight recent developments in the catalytic applications of cyclopropene-based small molecules in different areas of organocatalysis such as phase-transfer catalysis (PTC), Brønsted base catalysis, hydrogen-bond donor catalysis, nucleophilic carbene catalysis, and electrophotocatalysis developed within the past two decades.

Synthesis of Unsymmetrical 1,4-Dicarbonyl Compounds by Photocatalytic Oxidative Radical Additions

Herein we report a photocatalytic oxidative radical addition reaction for the synthesis of unsymmetrical 1,4-dicarbonyl compounds. This reaction utilizes a desulfurization process to generate electrophilic radicals, which add to α-halogenated alkenes and undergo further oxidation to deliver 1,4-dicarbonyl compounds. This mild and highly efficient method provides a valuable alternative to known strategies.

Copper-Catalyzed Three-Component Reactions of α-Ketoaldehyde, 1,3-Dicarbonyl Compound, and Organic Boronic Acid in Water: A Route to 1,4-Diketones

A novel three-component reaction of α-ketoaldehydes, 1,3-dicarbonyl compounds, and organic boronic acids catalyzed by CuO in water has been developed to give a wide range of products containing 1,3/1,4-diketones. The method has some advantages such as the use of readily available starting materials, wide substrate scopes, excellent yields, gram-scale synthesis, and mild reaction conditions.

A Density Functional Theory Study on Et-BAC-Catalyzed 1,6-Conjugate Addition of p-Chlorobenzaldehyde to p-Quinone Methide for the Synthesis of α,α'-Diarylated Ketones

Umpolung-based organocatalysis has made a remarkable breakthrough in the field of synthetic organic chemistry. Among a plethora of umpolung catalysts, bis(amino)cyclopropenylidenes (BACs) have emerged as efficient organocatalysts with potential applications in synthesizing numerous essential organic moieties. In this study, a plausible mechanism for bis(diethylamino)cyclopropenylidene (Et-BAC)-catalyzed synthesis of α,α'-diarylated ketones has been established using the density functional theory (DFT) method. The proposed catalytic cycle of the studied reaction initiates with the nucleophilic interaction of Et-BAC with p-chlorobenzaldehyde to form a zwitterionic intermediate, which is then transformed to a reactive Breslow intermediate. The Breslow intermediate further undergoes a chemoselective and stereoselective 1,6-conjugate addition reaction with p-quinone methide to form a new C-C bond connection. Finally, the generated adduct undergoes a proton shift reaction with the assistance of both 8-diazabicyclo(5.4.0)undec-7-ene (DBU) and protonated DBU to yield the desired product. Conceptual DFT-derived reactivity indices and frontier molecular orbital theory analysis have been successfully utilized to unravel the role of Et-BAC in this studied reaction. In addition to Et-BAC, DBU and protonated DBU also play a very important role in lowering the activation energy barrier of proton transfer steps. This investigation will help in the rational designing of simple nonheterocyclic carbene-mediated novel organic transformations.

Bis(amino)cyclopropenium Ion as a Hydrogen-Bond Donor Catalyst for 1,6-Conjugate Addition Reactions

The catalytic application of the bis(amino)cyclopropenium ion has been investigated in conjugate addition reactions. The hydrogen atom, which is attached to the cyclopropene ring of bis(amino)cyclopropenium salts, is moderately acidic and can potentially serve as a hydrogen-bond donor catalyst in some organic transformations. This hypothesis has been successfully realized in the 1,6-conjugate addition reactions of p-quinone methides with various nucleophiles such as indole, 2-naphthol, thiols, phenols, and so forth. The spectroscopic studies (NMR and UV-vis) as well as the deuterium isotope labeling studies clearly revealed that the hydrogen atom (C-H) that is present in the cyclopropene ring of the catalyst is indeed solely responsible for catalyzing these transformations. In addition, these studies also strongly indicate that the C-H hydrogen of the cyclopropene ring activates the carbonyl group of the p-quinone methide through hydrogen bonding.

Photochemical Synthesis of 1,4-Dicarbonyl Bifluorene Compounds via Oxidative Radical Coupling Using TEMPO as the Oxygen Atom Donor

A visible-light-induced metal-free synthesis of 1,4-dicarbonyl compounds from alkyne-containing aryl iodides via photochemical C-I bond cleavage, intramolecular cyclization, oxidation, and intermolecular radical coupling sequence is reported. TEMPO was employed as the oxygen atom donor in this transformation. This protocol provided a new strategy for the synthesis of 1,4-dicarbonyl bifluorene compounds.

Enantioselective Stetter Reactions Catalyzed by Bis(amino)cyclopropenylidenes: Important Role for Water as an Additive

The first highly enantioselective intermolecular Stetter reaction using simple enones is reported. A series of novel chiral BAC structures were designed and prepared. They were tested in the Stetter reaction with simple aldehydes and enones. The products were generated in excellent yields and enantioselectivities (up to 94% ee). Surprisingly, a substoichiometric amount of water was crucial to obtain high enantioselectivities. Chiral BACs were also shown to catalyze 1,6-conjugate addition reactions with paraquinone methides enantioselectively.

An electrochemically controlled release of NHCs using iron bis(dithiolene) N-heterocyclic carbene complexes

A series of five coordinated [Fe(NHC)(S₂C₂R₂)₂] complexes were isolated and subjected to electrochemical reduction for the facile release of NHCs in the catalytic media.

N-Heterocyclic carbene-catalyzed radical ring-opening acylation of oxime esters with aldehydes

We report a radical cross-coupling of cycloketone oxime esters with aldehydes by N-heterocyclic carbene (NHC) organocatalysis. This protocol features easy operation, with no need for external redox reagent, and a broad functional group compatibility.

Gold(I)-Catalyzed Synthesis of Highly Substituted 1,4-Dicarbonyl Derivatives via Sulfonium [3,3]-Sigmatropic Rearrangement

An efficient and straightforward gold-catalyzed protocol for the synthesis of 2-substituted 4-oxo-4-arylbutanal derivatives from commercially available or easily accessible alkynes and vinylsulfoxide substrates has been developed. Extension of the methodology to the use of 1-cycloalkenyl sulfoxides allowed the facile synthesis of five-, six-, and seven-membered-ring cycloalkyl-1-one backbone. Subsequently, the tetrahydrocycloalkyl[b]pyrrole derivatives, which are found in many active pharmaceutical ingredients, were isolated in good yields. Mechanistic investigation highlighted a [3,3]-sigmatropic rearrangement of a sulfonium intermediate in this process.

Bis-aminocyclopropenylidene carbene borane catalyzed imine hydrogenation

Certain borenium cations supported by carbenes can function as hydrogenation catalysts for imines. While many carbenes have been explored, variation of the other groups on boron has been less common. We have investigated several carbene-borane adducts in an attempt to understand the ability of a bis-amino cyclopropenylidene (BAC) carbene dicyclohexylborane adduct to hydrogenate relatively sterically unhindered benzyl imines. As an additional variant, a BAC carbene adduct of diphenylborane was prepared. A convenient preparation of diphenylboron fluoride via a potassium fluoroborinate salt was employed in this chemistry. Reaction of diphenylboron fluoride with a BAC carbene afforded a modest yield of a carbene-fluoroborane adduct. Reaction between the fluoroborinate salt and a lithium tetrafluoroborate adduct of the carbene provided the adduct in much improved yield and cleanliness, and the product was structurally characterized. The fluoroborate could be converted to a boron hydride through fluoride-hydride exchange with dimethylchlorosilane. The boron hydride adduct was also structurally characterized. Unlike the BAC carbene dicyclohexylborane adduct, the BAC carbene diphenylborane adduct showed essentially no activity in hydrogenation of imines or enamines.

Photocatalytic Radical Aroylation of Unactivated Alkenes: Pathway to β-Functionalized 1,4-, 1,6-, and 1,7-Diketones

We report the development of a photocatalytic strategy for the synthesis of β-functionalized unsymmetrical 1,4-, 1-6 and 1,7-diketones from aroyl chlorides and unactivated alkenes at room temperature. The mild reaction conditions not only tolerate a wide range of functional groups and structural moieties, but also enable migration of a variety of distal groups including (hetero)arenes, nitrile, aldehyde, oxime-derivative, and alkene. The efficiency of chirality transfer, factors that control the distal-group migration, and synthesis of carbo- and heterocycles from the diketones are also described. Mechanistic studies suggest a reaction pathway involving a photocatalytic radical aroylation of unactivated alkenes followed by a distal-group migration, oxidation, and deprotonation to afford the desired diketones.

Visible-light-induced consecutive C-C bond fragmentation and formation for the synthesis of elusive unsymmetric 1,8-dicarbonyl compounds

Synthesis of the valuable unsymmetric 1,8-dicarbonyl compounds remains underexplored currently. Herein, we disclose a new strategy for the synthesis of 1,8-diketones through the coupling of cyclopropanols and cyanohydrins under visible-light irradiation. The protocol features a cascade of intriguing ring opening of cyclopropanols and remote cyano migration. The unfavorable addition of an alkyl radical to an electron-rich alkene is facilitated by the intramolecular cyanohydrin interception. A variety of multiply functionalized 1,8-diketones are furnished in useful yields. The products could be further transformed into other valuable compounds, manifesting the utility of this method.

Electronic and ligating properties of carbocyclic carbenes: A theoretical investigation

Carbocyclic carbenes (CCCs) are a class of nucleophilic carbenes which are very similar to N-heterocyclic carbenes (NHCs) in terms of their reactivity, but they do not contain a stabilizing heteroatom in their cyclic ring system. In this study, 17 representative known CCCs and 34 newly designed CCCs are evaluated using quantum chemical methods, and the results are compared in terms of their stability, nucleophilicity, and proton affinity (PA) parameters. The results are divided on the basis of ring size of the known and reported CCCs. The stability, nucleophilicity, PA, complexation energy, and bond strength-related parameters were estimated using M06/6-311++G(d,p) method. The results indicated that the CCCs known in the literature are strong σ-electron donating species and have considerable π-accepting properties. This study led to the design and identification of a few new CCCs with dimethylamine and diaminomethynyl substituents which can be singlet stable and are substantially nucleophilic. © 2018 Wiley Periodicals, Inc.

Oxazolium Salts as Organocatalysts for the Umpolung of Aldehydes

Oxazolium salts were successfully employed for the first time as organocatalysts for benzoin, Stetter, and redox esterification reactions. An N-mesityl oxazolium salt catalyzed homobenzoin reaction of aromatic, heteroaromatic, and aliphatic aldehydes delivered α-hydroxy ketones in high yields. This new type of catalyst proved remarkably effective for the Stetter reaction of challenging substrates such as β-alkyl-α,β-unsaturated ketones and electron-rich aromatic aldehydes in comparison to common thiazolium and triazolium salts.

Exploring bis-(amino)cyclopropenylidene as a non-covalent Brønsted base catalyst in conjugate addition reactions

The catalytic application of bis-(amino)cyclopropenylidene, as a non-covalent Brønsted base, in the 1,4- and 1,6-conjugate addition of carbon nucleophiles to enones andp-quinone methides, respectively, is described.

Metal-free benzannulation of 1,7-diynes towards unexpected 1-aroyl-2-naphthaldehydes and their application in fused aza-heterocyclic synthesis

A novel I₂-mediated benzannulation of 1,7-diyne-involved 1,4-oxo-migration was established, providing unexpected 1-aroyl-2-naphthaldehydes. The resulting 1-aroyl-2-naphthaldehydes were successfully applied in the synthesis of polycyclic isoindoles.

Palladium-catalyzed cross-coupling of enamides with sterically hindered α-bromocarbonyls

Palladium-catalyzed intermolecular alkylation of enamides with α-bromo carbonyls was developed. Under mild reaction conditions, various cyclic and acyclic enamides reacted well with α-bromo carbonyls to afford the corresponding multi-substituted alkene products in good yields. The coupling products could be converted into very useful γ-amino acid, δ-amino alcohol, 1,4-dicarbonyl and γ-lactam derivatives.

Gas-phase reactions of cyclopropenylidene with protonated alkyl amines

Reactions of c-C₃H₂ with protonated amines are driven by its high gas-phase basicity, forming proton-bound dimer as the first step.

Synthesis, characterization and properties of amino acid ionic liquids derived from the triaminocyclopropenium cation

Amino acid ionic liquids based on the triaminocyclopropenium cation form a variety of structural types depending on the amino acid. Dications were obtained with Lys, His, Arg, and Cys.

Aza-Morita–Baylis–Hillman reactions catalyzed by a cyclopropenylidene

Catalysis using a bis(dialkylamino)cyclopropenylidene (BAC) has been developed, which relies on a formal umpolung activation of Michael acceptor pro-nucleophiles.

An expedient synthesis of functionalized 1,4-diketone-derived compounds via silyloxyallyl cation intermediates

Herein we describe the synthesis of highly functionalized 1,4-diketones that are readily differentiated as monosilylenol ethers via direct capture of silyloxyallyl cations by silyl enolates under Brønsted acid catalysis.