Cycloaddition reactions of enoldiazo compounds

Brønsted Acid Catalyzed Friedel-Crafts-Type Coupling and Dedinitrogenation Reactions of Vinyldiazo Compounds

The direct Friedel-Crafts-type coupling and dedinitrogenation reactions of vinyldiazo compounds with aromatic compounds using a metal-free strategy are described. This Brønsted acid catalyzed method is efficient for the formation of α-diazo β-carbocations (vinyldiazonium ions), vinyl carbocations, and allylic or homoallylic carbocation species via vinyldiazo compounds. By choosing suitable nucleophilic reagents to selectively capture these intermediates, both trisubstituted α,β-unsaturated esters, β-indole-substituted diazo esters, and dienes are obtained with good to high yields and selectivity. Experimental insights implicate a reaction mechanism involving the selective protonation of vinyldiazo compounds and the subsequent release of dinitrogen to form vinyl cations that undergo intramolecular 1,3- and 1,4- hydride transfer processes as well as fragmentation.

Rhodium-Catalyzed Sequential Cycloisomerization/Aldol Addition of Cyclopropene Carboxylic Acids with Isatins

A unique, rhodium-catalyzed reaction of cyclopropene carboxylic acids with isatins has been developed, which takes place through a trapping process of transient cyclic carboxylic oxonium ylides. This reaction tolerates a wide variety of cyclopropene carboxylic acids and N-protected or unprotected isatins and represents a new, direct strategy to access valuable γ-substituted γ-butenolide architectures containing an oxindole moiety in very high yields under mild reaction conditions.

Blue Light-promoted Carbene Transfer Reactions of Tosylhydrazones

Metal-free photochemical carbene-transfer reactions of tosylhydrazones were developed under blue light irradiation at room temperature. This reaction constructs C-X (X=C, N, O, S) bonds and cyclopropanes from readily available and stable starting materials.

Carbocation versus Carbene Controlled Chemoselectivity: DFT Study on Gold- and Silver-Catalyzed Alkylation/Cyclopropanation of Indoles with Vinyl Diazoesters

Density functional theory calculations were performed to investigate the catalyst-controlled selective functionalization of indoles with vinyl diazoacetates. The detailed reaction mechanism was established, and different roles of carboncation or carbene intermediates in defining an experimentally observed chemo- and regioselectivity were fully rationalized.

Copper-Catalyzed Asymmetric Reaction of Alkenyl Diynes with Styrenes by Formal [3 + 2] Cycloaddition via Cu-Containing All-Carbon 1,3-Dipoles: Access to Chiral Pyrrole-Fused Bridged [2.2.1] Skeletons

The generation of metal-containing 1,3-dipoles from metal carbenes represents a significant advance in 1,3-dipolar cycloaddition reactions. However, these transformations have so far been limited to reactions based on diazo compounds or triazoles as precursors. Herein, we disclose a copper-catalyzed enantioselective reaction of alkenyl N-propargyl ynamides with styrene derivatives by formal [3 + 2] cycloaddition via Cu-containing all-carbon 1,3-dipoles, which constitutes a novel way for the generation of metal-containing 1,3-dipoles via metal carbenes. This protocol allows the practical and atom-economical synthesis of valuable chiral pyrrole-fused bridged [2.2.1] skeletons in moderate to good yields (up to 90% yield) with excellent diastereoselectivities (dr > 50/1) and generally excellent enantioselectivities (up to >99% ee).

Rhodium-Catalyzed Enantioselective [4+2] Cycloadditions of Vinylcarbenes with Dienes

The reaction of 2-siloxycyclo-1,3-dienes with E-vinyldiazoacetates in the presence of the bulky chiral dirhodium tetracarboxylate catalyst, Rh₂ (R-p-PhTPCP)₄ results in an enantioselective [4+2] cycloaddition, in which three new stereogenic centers are formed. The [4+2] cycloadducts are generated as single diastereomers with high enantiocontrol (95-98 % ee). When the diene contains an additional stereogenic center, effective kinetic resolution can be achieved.

Recent progress on donor and donor-donor carbenes

Donor and donor-donor carbenes are two important kinds of carbenes, which have experienced tremendous growth in the past two decades. This review provides a comprehensive overview of the recent development of donor and donor-donor carbene chemistry. The development of this chemistry offers efficient protocols to construct a wide variety of C-C and C-X bonds in organic synthesis. This review is organized based on the different types of carbene precursors, including diazo compounds, hydrazones, enynones, cycloheptatrienes and cyclopropenes. The typical transformations, the reaction mechanisms, as well as their subsequent applications in the synthesis of complex natural products and bioactive molecules are discussed. Due to the rapidly increasing interest in this area, we believe that this review will provide a timely and comprehensive discussion of recent progress in donor and donor-donor carbene chemistry.

Regioselective Reversal Cyclization To Access either Eight-Membered Sulfur-Containing Heterocycle-Fused γ-Pyrones or 2-(1,4-Dithianyl)-4-pyrones from the Same Precursors

A regioselective reverse strategy for the construction of eight-membered sulfur-containing heterocycle-fused γ-pyrones and 2-(1,4-dithianyl)-4-pyrones starting from 2-diazo-γ-pyrones and dithioacetals was achieved for the first time. The process combines C-S bond formation via sulfur ylides and C-C bond formation via electron transfer to afford the target molecules in a facile manner with 100% regioselectivity and in excellent isolated yields (up to 90%).

Synthetic Strategies for Trapping the Elusive trans-Dirhodium(II,II) Formamidinate Isomer: Effects of Cis versus Trans Geometry on the Photophysical Properties

The cis- and trans-dirhodium(II,II) complexes cis-[Rh₂(μ-DTolF)₂(μ-np)(MeCN)₄][BF₄]₂ (1; DTolF = N,N'-di-p-tolylformamidinate and np = 1,8-naphthyridine), cis- and trans-[Rh₂(μ-DTolF)₂(μ-qxnp)(MeCN)₃][BF₄]₂ [2 and 3, respectively, where qxnp = 2-(1,8-naphthyridin-2-yl)quinoxaline], and trans-[Rh₂(μ-DTolF)₂(μ-qxnp)₂][BF₄]₂ (4) were synthesized and characterized. A new synthetic methodology was developed that consists of the sequential addition of π-accepting axially blocking ligands to favor formation of the first example of a bis-substituted formamidinate-bearing trans product. Isolation of the intermediates 2 and 3 provides insight into the mechanistic requirements for obtaining 4 and the cis analogue, cis-[Rh₂(μ-DTolF)₂(μ-qxnp)₂][BF₄]₂ (5). Density functional theory calculations provide support for the synthetic mechanism and proposed intermediates. The metal/ligand-to-ligand charge-transfer (ML-LCT) absorption maximum of the trans complex 4 at 832 nm is red-shifted by 1173 cm^-1 and exhibits shorter lifetimes of the ¹ML-LCT and ³ML-LCT excited states, 3 ps and 0.40 ns, respectively, compared to those of the cis analogue 5. The shorter excited-state lifetimes of 4 are attributed to the longer Rh-Rh bond of 2.4942(8) Å relative to that in 5, 2.4498(2) Å. A longer metal-metal bond reflects a decreased overlap of the Rh atoms, which leads to more accessible metal-centered excited states for radiationless deactivation. The ³ML-LCT excited states of 4 and 5 undergo reversible bimolecular charge transfer with the electron donor p-phenylenediamine when irradiated with low-energy light. These results indicate that trans isomers are a source of unexplored tunability for potential p-type semiconductor applications and, given their distinct geometric arrangement, constitute useful building blocks for supramolecular architectures with potentially interesting photophysical properties.

Tandem Carbenoid C-H Functionalization/Conia-ene Cyclization of N-Propargyl Indoles Generates Pyrroloindoles under Cooperative Rh(II)/Zn(II) Catalysis

The decomposition of diazodicarbonyl compounds in the presence of various metal catalysts has become a reliable method for the functionalization of indoles via carbenoid intermediates. Exploiting the nucleophilic reactivity of the in situ generated malonic ester product formed, we herein report a tandem C-H functionalization/Conia-ene cyclization of N-alkyne tethered indoles. This double functionalization of diazodicarbonyls generates a range of pyrrolo[1,2-a]-, pyrido[1,2-a]-, and azepino[1,2-a]indole products with good synthetic efficiency.

Visible light-induced photocatalytic C–H ethoxycarbonylmethylation of imidazoheterocycles with ethyl diazoacetate

A visible-light-promoted regioselective ethoxycarbonylmethylation of imidazoheterocycles has been developed using an α-diazoester via a radical pathway.

Cross coupling of sulfonyl radicals with silver-based carbenes: a simple approach to β-carbonyl arylsulfones

A radical–carbene coupling reaction of sulfonyl radicals and silver-based carbenes has been well established, which provides an efficient approach to various β-carbonyl arylsulfones.

Sulfoxonium ylides: simple compounds with chameleonic reactivity

Sulfoxonium ylides, manageable compounds with an appealing safety profile, adapt to reaction partners like chameleons to their environment, resulting in a variety of useful, and sometimes surprising and unique, reactions.

A Carbene-Extended ATRA Reaction

Atom-transfer radical addition (ATRA) reactions have gained a strong foothold in organic synthesis by virtue of their operational simplicity, synthetic versatility, and perfect atom economy. A rich chemical space can be accessed through clever combinations of the simple starting materials. Many variations of this general motif have been reported. However, the vast majority involve the addition of an organic halide across a C=C double bond, resulting in the formation of 1,2-bifunctional products. This report introduces a significant expansion of this general reactivity concept to give 1,3-bifunctional adducts through the combination of 1,1-ATRA to a carbenoid and 1,2-ATRA to an alkyne. Both processes operate under mild conditions (RT, 5 h) with the same commercial catalyst (CoBr2 , dppbz).

Synthesis of Isoxazolines and Oxazines by Electrochemical Intermolecular [2 + 1 + n] Annulation: Diazo Compounds Act as Radical Acceptors

Reported herein is an unprecedented synthesis of isoxazolines and oxazines through electrochemical intermolecular annulation of alkenes with tert-butyl nitrite, in which diazo compounds serve as radical acceptors. Notably, [2 + 1 + 2] and [2 + 1 + 3] annulations occur when styrenes and allylbenzenes are used as substrates, respectively. The latter reaction undergoes group migration to form more stable radical, manifesting radical route instead of conventional 1,3-dipolar cycloaddition occurs. Moreover, scale-up experiments suggest the potential application value of these transformations in industry.

Synthesis of Chiral Tetrasubstituted Azetidines from Donor-Acceptor Azetines via Asymmetric Copper(I)-Catalyzed Imido-Ylide [3+1]-Cycloaddition with Metallo-Enolcarbenes

The all-cis stereoisomers of tetrasubstituted azetidine-2-carboxylic acids and derivatives that possess three chiral centers have been prepared in high yield and stereocontrol from silyl-protected Z-γ-substituted enoldiazoacetates and imido-sulfur ylides by asymmetric [3+1]-cycloaddition using chiral sabox copper(I) catalysis followed by Pd/C catalytic hydrogenation. Hydrogenation of the chiral p-methoxybenzyl azetine-2-carboxylates occurs with both hydrogen addition to the C=C bond and hydrogenolysis of the ester.

Bis(imino)pyridine iron complexes for catalytic carbene transfer reactions

The bis(imino)pyridine iron complex, for the first time, is developed as an effective metal carbene catalyst for carbene transfer reactions of donor-acceptor diazo compounds. Its broad catalytic capability is demonstrated by a range of metal carbene reactions, from cyclopropanation, cyclopropenation, epoxidation, and Doyle-Kirmse reaction to O-H insertion, N-H insertion, and C-H insertion reactions. The asymmetric cyclopropanation of styrene and methyl phenyldiazoacetate was successfully achieved by the new chiral bis(imino)pyridine iron catalyst, which delivers a new gateway for the development of chiral iron catalysis for metal carbene reactions.

Gold-Catalyzed Bicyclic Annulations of 2-Alkynylbenzaldehydes with Vinyldiazo Carbonyls that Serve as Five-atom Building Units

This work reports gold-catalyzed bicyclic annulations of 2-alkynyl-1-carbonylbenzenes with vinyldiazo ketones that serve as five-atom building units. The importance of these reactions is to access 4,5-dihydro-benzo[g]indazoles, which form the structural cores of various bioactive molecules. According to our mechanistic analysis, we postulate initial [5+4]-cycloadditions between benzopyrilium intermediates and vinyldiazo ketones, followed by 6-π-electrocyclizations to achieve the excellent stereoselectivity.

Rhodium-Catalyzed Formal C-O Insertion in Carbene/Alkyne Metathesis Reactions: Synthesis of 3-Substituted 3 H-Indol-3-ols

An efficient and novel rhodium-catalyzed formal C-O insertion reaction of alkyne-tethered diazo compounds for the synthesis of 3 H-indol-3-ols is described. A type of donor/donor rhodium carbene generated in situ via a carbene/alkyne metathesis (CAM) process is the key intermediate and terminates in a unique transformation different from donor/acceptor carbenoids. In addition, ¹⁸O-labeling experiments indicate that intramolecular oxygen-atom transfer from the amide group to the carbon-carbon triple bond occurs during this transformation.

Catalytic asymmetric cycloaddition reactions of enoldiazo compounds

This review describes catalytic asymmetric cycloaddition reactions of silyl-protected enoldiazo compounds for the construction of highly functionalized carbo- and heterocycles which possess one or more chiral center(s). The enoldiazo compound or its derivative, donor-acceptor cyclopropene, form electrophilic vinylogous metal carbene intermediates that combine stepwise with nucleophilic dipolar reactants to form products from [3 + 1]-, [3 + 2]-, [3 + 3]-, [3 + 4]-, and [3 + 5]-cycloaddition, generally in high yield and with exceptional stereocontrol and regioselectivity.