Triarylborane Catalyzed Carbene Transfer Reactions Using Diazo Precursors

B(C6F5)3-Catalyzed Regiodivergent Thioetherifications of Alkenes via Thiiranium Intermediates: Experimental and Computational Insights

Precise control of selective alkene functionalization is a continuing challenge in the chemical community. In this study, we develop a substitution-controlled regiodivergent thioetherification of di- or trisubstituted alkenes using 10 mol % tris(pentafluorophenyl)borane [B(C6F5)3] as a catalyst and N-thiosuccinimide as a sulfenylating reagent. This metal-free borane catalyzed C-S bond forming method is utilized for a Csp2-H sulfenylation reaction to synthesize an array of diphenylvinylsulfide derivatives with good to excellent yields (25 examples, up to 91 % yield). Some of the products exhibit aggregation-induced emission luminogen properties used in organic light-emitting diodes (OLEDs), chemical sensors, and biological imaging units. Depending upon the starting alkene, Csp3-S sulfenylation products could also be generated regioselectively. A variety of allylic thioethers from α-alkyl substituted styrenes were isolable in good yields and selectivities (14 examples, up to 67 % yield). The DFT-supported mechanistic study confirms that the reaction proceeds via a thiiranium ion intermediate, where the regioselectivity and product formation is determined by the alkene substituents which influence the activation barriers and energy profiles. Diphenylvinylsulfide derivatives can also be efficiently transformed into a range of synthetically valuable compounds, including vinyl sulfoxides, vinyl sulfones, and vinyl sulfoximines.

BF3-Mediated C2-Amidation of Quinoline N-Oxides Employing Trifluorodiazoethane and Acetonitrile: Access to 2-N-(Trifluoroethyl)amidoquinolines

A Lewis acid-mediated C2-N-trifluoroethylamidation of quinolines, employing quinoline N-oxides, trifluorodiazoethane, and acetonitrile to forge a new class of N-(quinolin-2-yl)-N-(trifluoroethyl)acetamide is presented in this Letter. The reaction proceeds through a carbene generation/nitrile ylide formation/(3 + 2) cycloaddition/rearrangement cascade to furnish quinoline-2-N-(trifluoroethyl)acetamide derivatives in high yields.

Comparison of the Efficiency of B-O and B-C Bond Formation Pathways in Borane-Catalyzed Carbene Transfer Reactions Using α-Diazocarbonyl Precursors: A Combined Density Functional Theory and Machine Learning Study

Lewis acidic boranes, especially tris(pentafluorophenyl)borane [B(C6F5)3], have emerged as metal-free catalysts for carbene transfer reactions of α-diazocarbonyl compounds in a variety of functionalization reactions. The established mechanism for how borane facilitates carbene generation for these compounds in the scientific community is based on the formation of a B-O (C=O) intermediate (path O). Herein, we report an extensive DFT study that challenges the notion of a ubiquitous path O, revealing that B-C(=N=N) bond formation (path C) for certain diazocarbonyl substrates proves to be the preferred pathway. This study elucidates, through the introduction of 22 various substituents on each side of the α-diazocarbonyl backbone, how the electron-donating and -withdrawing properties of substituents influence the competition between these B-O and B-C pathways. To elucidate the impact of the electronic features of diazo substrates on the competition between the O and C pathways in the studied dataset, we employed a machine learning approach based on the Random Forest model. This analysis revealed that substrates with higher electron density on the diazo-attached carbon, lower electron density on the carbonyl carbon, and more stable HOMO orbitals tend to proceed via path C. Furthermore, this study not only demonstrates that borane efficiency in facilitating N2 release is greatly affected by the nature of substituents on both sides of the α-diazocarbonyl functionality but also shows that for some substrates, borane is incapable of catalyzing the release of molecular nitrogen.

Visible Light Induced B-H Bond Insertion Reaction with Diazo Compounds

A protocol induced by visible light for the direct insertion of α-carbonyl carbenes into the B-H bond of amine-borane adducts has been developed under conditions that are free of metal and photocatalyst. This approach provides a straightforward route to various organoboron compounds from diazo compounds and amine-borane adducts with moderate to good yields. Mechanistic investigations reveal that this photoinduced reaction proceeds through concerted carbene insertion into the B-H bond, and the photoinduced generation of free carbene from α-diazo esters may be the rate-determining step.

Magnetically Recyclable Borane Lewis Acid Catalyst for Hydrosilylation of Imines and Reductive Amination of Carbonyls

Fluorinated arylborane-based Lewis acid catalysts have shown remarkable activity and serve as ideal examples of transition metal-free catalysts for diverse organic transformations. However, their homogeneous nature poses challenges in terms of recyclability and separation from reaction mixtures. This work presents an efficient technique for the heterogenization of boron Lewis acid catalysts by anchoring Piers' borane to allyl-functionalized iron oxide. This catalyst demonstrates excellent activity in the hydrosilylation of imines and the reductive amination of carbonyls using various silanes as reducing agents under mild reaction conditions. The catalyst exhibits broad tolerance towards a wide range of functional substrates. Furthermore, it exhibits good recyclability and can be easily separated from the products using an external magnetic field. This work represents a significant advance in the development of sustainable heterogenous metal-free catalysts for organic transformations.

Borane-Catalyzed Coupling of Diazooxindoles and Difluoroenoxysilanes to Tetrasubstituted Monofluoroalkenes

A highly stereoselective coupling reaction of diazooxindoles with difluoroenoxysilanes catalyzed by Lewis acidic boranes has been developed. The reaction proceeded at ambient temperature under transition metal-free conditions with wide functional group tolerance. By using this simple procedure, a series of tetrasubstituted monofluoroalkenes can be accessed in good yield with high selectivity.

Chemodivergent phosphonylation of diazocarboxylates: light-on vs. light-off reactions

By tapping into the divergent reactivity of diazocarboxylates under thermal and photocatalytic conditions, we could develop chemodivergent phosphonylation protocols for α-diazocarboxylates with trialkyl phosphites. While the thermal reaction led to N-P bond formation affording phosphonylated hydrazones, the visible light-mediated reaction furnished phosphonylated aryl carboxylates through C-P bond formation. Both reactions are notable for their operational simplicity and mild conditions affording products in good yields without the requirement of a metal, base or photocatalyst.

Addition and NN bond cleavage of diazo-compounds by phosphino-phosphenium cations

The phosphino-phosphenium cation (PPC) [Ph3PPPh2][GaCl4] reacts as a frustrated Lewis pair to add across the NN bond of (tBuO2CN)2. In contrast, photolytical addition [Ph2ClPPPh2][GaCl4] to (RN)2 results in cleavage of the NN bond affording [Ph2P(μ-NR)2PPh2Cl][GaCl4] (R = Ph 2, C6H4Cl3). While the chloride of 2 is replaced with N3 or CN via reaction with Me3SiN3 or tBuNC respectively, reaction with (C6F5)2BH effects ring opening to give [HN(Ph)PPh2(μ-NPh)PPh2][GaCl4] 7. This reactivity demonstrates that PPCs behave as FLPs to effect either addition or cleavage of NN double bonds.

Synthesis of Hypervalent Iodine Diazo Compounds and Their Application in Organic Synthesis

Diazomethyl-substituted iodine(III) compounds with electron-withdrawing groups (EWG) connected to diazo methyl center were a type of donor-acceptor diazo compounds with potential reaction abilities similar to ordinary diazo compounds. Although several diazomethyl-substituted iodine(III) compounds were synthesized and used in the nucleophilic substitution reactions as early as 1994, the synthesis and application of new iodine(III) diazo compounds have only been reported to a certain extent in recent years. In the presence of rhodium catalyst, photocatalyst, or nucleophiles, diazomethyl-substituted iodine(III) compounds can be converted into rhodium-carbenes, diazomethyl radicals, ester radicals or nucleophilic intermediates, which can be used as key intermediates for the formation of chemical bonds. The aim of this review is to give an overview of diazomethyl-substituted iodine(III) compounds in organic synthesis.

Borane catalyzed transesterification of tert-butyl esters using α-aryl α-diazoesters

The B(C6F5)3-catalyzed transesterification of a series of 3-alkenyl-oxindoles and other unsaturated tert-butyl esters with aryl-diazo esters is reported. This protocol is facile and generally high yielding proceeding under mild conditions and is remarkably chemoselective leaving the CC bonds intact.

Oxidative cross-coupling of quinoxalinones with indoles enabled by acidochromism

An oxidative cross-coupling of quinoxalinones with indole derivatives via B(C₆F₅)₃·H₂O induced acidochromism of quinoxalinone derivatives was developed under mild and external photocatalyst-free conditions. The reaction shows excellent substrate scope, accommodating a wide range of functional groups. The usefulness of this strategy was demonstrated by the synthesis of the natural products Azacephalandole A and Cephalandole A in high yields. Moreover, the products are fluorophores showing prevalent fluorescence properties with a wide emission range and good relative quantum yields.

B(C6F5)3-catalyzed cyclopropanation of 3-alkenyl-oxindoles with diazomethanes

Spirocyclopropane-oxindoles are key motifs in biologically active compounds and are versatile synthetic intermediates. Herein, we report a metal-free, B(C₆F₅)₃ catalyzed cyclopropanation of 3-alkenyl-oxindoles with diazomethanes. This provides 25 variants of spirocyclopropane-oxindole derivatives. These spirocyclopropane-oxindole products were obtained in good to excellent yields (up to 99%) and high diastereoselectivities (up to 20 : 1 d.r.) under mild reaction conditions and could be performed on a gram scale.

B(C6F5)3-Catalyzed Diastereoselective and Divergent Reactions of Vinyldiazo Esters with Nitrones: Synthesis of Highly Functionalized Diazo Compounds

Herein we report a mild, transition-metal-free, highly diastereoselective Lewis acid catalyzed methodology toward the synthesis of isoxazolidine-based diazo compounds from the reaction between vinyldiazo esters and nitrones. Interestingly, the isoxazolidine products were identified to have contrasting diastereoselectivity to previously reported metal-catalyzed reactions. Furthermore, the same catalyst can be used with enol diazo esters, prompting the formation of Mukaiyama-Mannich products. These diazo products can then be further functionalized to afford benzo[b]azepine and pyrrolidinone derivatives.

Reactions of Frustrated Lewis Pairs with Chloro-Diazirines: Cleavage of N=N Double Bonds

The reactions of FLPs with diazomethanes leads to the rapid loss of N2 . In contrast, in this work, we reported reactions of phosphine/borane FLPs with chlorodiazirines which led to the reduction of the N=N double bond, affording linked phosphinimide/amidoborate zwitterions of the general form R3 PNC(Ar)NR'BX(C6 F5 )2 . A detailed DFT mechanistic study showed that these reactions proceed via FLP addition to the N=N bond, followed by subsequent group transfer reactions to nitrogen and capture of the halide anion.

Lewis Acid Assisted Brønsted Acid Catalysed Decarbonylation of Isocyanates: A Combined DFT and Experimental Study

An efficient and mild reaction protocol for the decarbonylation of isocyanates has been developed using catalytic amounts of Lewis acidic boranes. The electronic nature (electron withdrawing, electron neutral, and electron donating) and the position of the substituents (ortho/meta/para) bound to isocyanate controls the chain length and composition of the products formed in the reaction. Detailed DFT studies were undertaken to account for the formation of the mono/di-carboxamidation products and benzoxazolone compounds.

Visible-Light-Mediated (sp 3)Cα-H Functionalization of Ethers Enabled by Electron Donor-Acceptor Complex

A synthetically beneficial visible-light-mediated protocol has been disclosed to achieve C-H amination of readily available feedstocks cyclic and acyclic ethers. A rarely identified N-bromosuccinamide-tetrahydrofuran electron donor-acceptor complex served as an initiator to functionalize both α-diazoketones and dialkyl azodicarboxylates. This developed methodology gives an alternative and milder way to construct the C-N bond and can be explored for the formation of C-C bond to perform arylation and allylation reactions.

Cu-Catalyzed tandem cyclization and coupling of enynones with enaminones for multisubstituted furans & furano-pyrroles

A synthetic strategy that efficiently constructs complex molecular diversity in a few steps will always be embraced by organic chemists. Here, we report a cascade reaction of enynones with enaminones via carbene insertion and aryl migration to engineer distinctive multisubstituted furans with an all-carbon quaternary center, and could extend the protocol in the same pot towards furano-pyrrole bis-heterocycles. Heterogeneity of this protocol was proved with the upshot of divergent chemical space under a relatively mild reaction environment.

Exploiting the umpolung reactivity of diazo groups: direct access to triazolyl-azaarenes from azaarenes

The present work documents electrophilic substitution of azaarenes, mainly isoquinolines, with hypervalent iodine diazo reagents (HIDR) followed by formal [3+2]-dipolar cycloaddition in a tandem fashion. Other azaarenes viz. pyridines and phenanthridines too could be successfully used in the reaction. The methodology capitalizes on the umpolung nature of α-aryliodonio diazo compounds for installing a nucleophile, i.e. azaarene, at their α-position. Subsequent ylide formation and intramolecular 1,5-cyclization furnished 4,3-fused 1,2,4-triazolyl-azaarenes in good yields. The reaction is notable for its mild conditions, operational simplicity and fairly general scope.

Solvent-free, B(C6 F5 )3 -Catalyzed S-H Insertion of Thiophenols and Thiols with α-Diazoesters

Described herein is a B(C₆ F₅ )₃ -catalyzed S-H insertion reaction of thiophenols and thiols with α-diazoesters to access valuable α-thioesters. With the established protocol, an array of α-thioester products are generated in moderate to good yields with broad scope and functional group tolerance. In addition, this reaction maintains its high efficiency on gram scale and the product can be easily transformed into other useful motifs. This reaction proceeds under solvent-free conditions at room temperature, and generally finishes in twenty minutes upon magnet stirring, which offers an expedient way for synthesis of thioether-containing compounds.

Borane promoted aryl transfer reaction for the synthesis of α-aryl functionalised β-hydroxy and β-keto esters

The synthesis of a series of α-aryl or α-alkyl functionalised β-hydroxy and β-keto esters has been achieved by reacting α-diazoesters with boranes, and aldehydes, ketones, anhydrides, nitriles, esters or isocyanates. In a mild reaction protocol, 26 examples are presented in yields up to 73%.

Borane-catalyzed arylation of aryldiazoacetates with N,N-dialkylanilines

A selective arylation of donor-acceptor diazo compounds with aniline derivatives catalyzed by Lewis acidic boranes is developed. This simple reaction protocol provides an efficient method for the synthesis of diarylacetates under metal-free conditions.

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6 F5 )3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N2 release. Recently, tris(pentafluorophenyl)borane [B(C6 F5 )3 ] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C6 F5 )3 is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.

Catalyst-dependent chemoselective insertion of diazoalkanes into N-H\C-H\O-H\C-O bonds of 2-hydroxybenzothiazoles

The control of chemoselective insertions of diazoalkanes with 2-hydroxybenzothiazoles is challenging. Herein, the chemoselective N-H, O-H, C-O or C-H bond insertions of diazoalkanes with 2-hydroxybenzothiazoles are achieved using B(C6F5)3, Rh2(OAc)4...

Chemo- and regio-selective amidation of indoles with isocyanates using borane Lewis acids

A metal-free synthetic route using boranes has been developed for the amidation of indoles. A detailed mechanistic study was carried out to understand the reaction mechanism.

Recent advances in pyrazole synthesis employing diazo compounds and synthetic analogues

This review summarizes the recent developments in the construction of pyrazoles using diazo compounds, nitrile imines and their synthetic equivalents.