The Mechanism of the Catalytic Functionalization of Haloalkanes by Carbene Insertion: An Experimental and Theoretical Study

Defluorinative 1,3-Dienylation of Fluoroalkyl N-Triftosylhydrazones with Homoallenols

A silver-catalyzed regioselective defluorinative 1,3-dienylation of trifluoromethyl phenyl N-triftosylhydrazones using homoallenols as 1,3-dienyl sources provides a variety of α-(di)fluoro-β-vinyl allyl ketones with excellent functional group tolerance in moderate to good yields. The reaction proceeds through a silver carbene-initiated sequential etherification and Claisen type [3,3]-sigmatropic rearrangement cascade. The synthetic utility of this protocol was demonstrated through the downstream synthetic elaboration toward diverse synthetically useful building blocks.

Introducing the Catalytic Amination of Silanes via Nitrene Insertion

The direct functionalization of Si–H bonds by the nitrene insertion methodology is described. A copper(I) complex bearing a trispyrazolylborate ligand catalyzes the transfer of a nitrene group from PhI=NTs to the Si–H bond of silanes, disilanes, and siloxanes, leading to the exclusive formation of Si–NH moieties in the first example of this transformation. The process tolerates other functionalities in the substrate such as several C–H bonds and alkyne and alkene moieties directly bonded to the silicon center. Density functional theory (DFT) calculations provide a mechanistic interpretation consisting of a Si–H homolytic cleavage and subsequent rebound to the Si-centered radical.

Dinuclear Silver Complexes in Catalysis

Over the past two decades, there has been a substantial increase in the number of synthetically useful transformations catalyzed by silver. Across the range of silver-catalyzed reactions that have been reported, dinuclear species often emerge as a common feature, either as the (pre-)catalysts themselves or as intermediates during catalysis. This Minireview explores the role of dinuclear silver complexes in homogeneous catalysis, which we hope will aid in the development of improved design principles for silver catalysts.

Silver-Catalyzed, Chemo- and Enantioselective Intramolecular Dearomatization of Indoles to Access Sterically Congested Azaspiro Frameworks

An asymmetric dearomatization of indoles bearing α-diazoacetamide functionalities was developed for synthesizing high-value spiro scaffolds. A silver phosphate chemoselectively catalyzed the sterically challenging dearomatization, whereas more typically used metal catalysts for carbene transfer reactions, such as a rhodium complex, were not effective and instead resulted in a Büchner ring expansion or cyclopropanation. Mechanistic studies indicated that the spirocyclization occurred through a silver-assisted asynchronous concerted process and not via a silver-carbene intermediate. Analyses based on natural bond orbital population and a distortion/interaction model indicated that the degree of C-Ag mutual interaction is crucial for achieving a high level of enantiocontrol. In addition, an oxidative disconnection of a C(sp³)-C(sp²) bond in the product provided unconventional access to the corresponding chiral spirooxindole.

How CuCl and CuCl2 Insert into C-N Bonds of Diazo Compounds: An Electronic Structure and Mechanistic Study

The transition-metal Cu catalysts CuCl and CuCl₂ have been widely employed to catalyze a series of chemical reactions with diazo compounds because of their high efficiency and selectivity. However, how to yield the active Cu carbene species from the Cu catalysts and diazo compounds still remains unclear. In this work, we performed a comprehensive theoretical investigation on the electronic structures of CuCl and CuCl₂ in solution. The results indicate that the most stable structures for CuCl and CuCl₂ are dimer and monomer, respectively. The C-N bond insertion of aryldiazoacetate by CuCl yields a stable bimetallic carbene species, which differs from the monometallic carbene generated from CuCl₂.

R. P, V. U, K. S. A polyamine dendritic polymer–copper complex: a reusable catalyst for the additive-free amination of aryl bromides, and iodides

Additive-free synthesis of aryl amines from aryl halides using a reusable water-soluble porphyrin-cored amine-functionalized dendritic polymer copper complex (PPECH–Amine–Cu) catalyst in aqueous media.

Silver-Catalyzed Asymmetric Insertion into Phenolic O-H Bonds using Aryl Diazoacetates and Theoretical Mechanistic Studies

An enantioselective insertion reaction of silver carbenes generated from donor-acceptor-substituted diazo compounds into the O-H bond of phenols was developed. A homobinuclear silver complex with a chiral phosphorous ligand was created in situ from AgNTf₂ and (S)-XylylBINAP (in a 2:1 mole ratio). Detailed mechanistic studies using combined experimental and computational techniques revealed that one silver atom center of the catalyst forms a silver carbene and another one works as a Lewis acid for the nucleophilic addition of a phenol. Two counter-anions, two water molecules, and two silver atoms cooperatively mediate the subsequent protonation event to lower the activation energy and control enantioselectivity, affording an array of valuable α-aryl-α-aryloxy esters.

Gold-catalyzed ethylene cyclopropanation

Ethylene can be directly converted into ethyl 1-cyclopropylcarboxylate upon reaction with ethyl diazoacetate (N₂CHCO₂Et, EDA) in the presence of catalytic amounts of IPrAuCl/NaBAr^F₄ (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene; BAr^F₄ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate).

Silver-catalyzed regioselective hydroamination of alkenyl diazoacetates to synthesize γ-amino acid equivalents

A simple protocol to directly access γ-amino acid derivatives by intermolecular regioselective hydroamination of trichloroethyl alkenyldiazoacetates with carbamate using a silver tetrafluoroborate catalyst is described. Density functional theory (DFT) calculations to analyze the reaction mechanism revealed that multiple attractive interactions occur in a transition state to promote the vinylogous addition of nitrogen nucleophiles.

Reaction Chemistry of Silver(I) Trifluoromethanesulfonate Complexes of Nitrogen-Confused C-Scorpionates

Two new C-scorpionate ligands with a bis(3,5-dimethylpyrazol-1-yl)methyl group bound to the 3 position of either an N-tosyl (^TsL*) or an N-H pyrazole (^HL*) ring have been prepared. The silver(I) complexes of these new ligands and the two previously reported analogous ligands with unsubstituted bis(pyrazol-1-yl)methyl groups (^TsL and ^HL) in both 1:1 and 2:1 ligand/metal ratios were investigated to explore the effects of ligand sterics on their physical and chemical properties. The structurally characterized derivatives of the type [Ag(L)₂](OTf) are four-coordinate, where the confused pyrazolyl is not bound to the metal. On the other hand, three 1:1 complexes [Ag(L)](OTf) had all pyrazolyls bound, while the μ-κ¹,κ¹-^TsL derivative had an unbound confused pyrazolyl. The molecularity of the latter four ranged from polymeric to dimeric to monomeric in the solid with increasing steric bulk of the ligand. The utility of these complexes in stoichiometric ligand-transfer reactions and in styrene aziridination was demonstrated. Thus, tricarbonylmanganese(I) complexes were prepared as kinetically inert models for comparative solution diffusion NMR studies. Also, [Fe(^HL)₂](OTf)₂ was prepared for similar reasons and to compare the effects of anion on spin-crossover properties.

Chemoselective Asymmetric Intramolecular Dearomatization of Phenols with α-Diazoacetamides Catalyzed by Silver Phosphate

We report asymmetric dearomatization of phenols using Ag carbenoids from α-diazoacetamides. The Ag catalyst promoted intramolecular dearomatization of phenols, whereas a Rh or Cu catalyst caused C-H insertion and a Büchner reaction. Studies indicated Ag carbenoids have a carbocation-like character, making their behavior and properties unique. Highly enantioselective transformations using Ag carbenoids have not been reported. We achieved a Ag carbenoid-mediated chemo- and highly enantioselective phenol dearomatization with substrate generality for the first time.

Gold/silver-catalyzed controllable regioselective vinylcarbene insertion into O–H bonds

The controllable regioselective vinylcarbene insertion into O–H bonds of 2-pyridones/benzyl alcohols has been realized.

An unusual bis-heteroscorpionate complex with anomalous ligands: [tris(3,4-dibromo-5-phenylpyrazolyl)hydroborato][hydrotris(3-neopentylpyrazolyl)borato]nickel(II)

Tridentate trispyrazolylborate (Tp) ligands, introduced by Trofimenko in 1966, have been widely utilized in metal coordination chemistry because of the relatively facile synthetic modification of their electronic and steric factors. The title heteroscorpionate, [Ni(C27H16BBr6N6)(C24H40BN6)], features one ligand, namely hydrotris(3-neopentylpyrazolyl)borate, that has previously displayed variable steric effects, and a brominated ligand, namely tris(3,4-dibromo-5-phenylpyrazolyl)hydroborate, that, atypical in trispyrazolylborate chemistry, coordinates such that the less bulky pyrazole substituent is oriented facing toward the metal ion. The potential molecular threefold symmetry in scorpionates can allow axial chirality. Although crystallized in the centrosymmetricP\overline{1} space group, a closer inspection of the structure of the title compound reveals axial diastereomers.

Amide-Directed Formation of Five-Coordinate Osmium Alkylidenes from Alkynes

The amide-directed synthesis of five-coordinate osmium alkylidene derivatives from alkynes is reported. These types of complexes, which have been elusive until now because of the tendency of osmium to give hydride alkylidyne species, are prepared by reaction of the dihydride OsH₂Cl₂(PⁱPr₃)₂ (1) with terminal alkynes containing a distal amide group. Complex 1 reacts with N-phenylhex-5-ynamide and N-phenylhepta-6-ynamide to give OsCl₂{=C(CH₃)(CH₂)_nNH(CO)Ph}(PⁱPr₃)₂ (n = 3 (2), 4 (3)). The relative position of carbonyl and NH groups in the organic substrates has no influence on the reaction. Thus, treatment of 1 with N-(pent-4-yn-1-yl)benzamide leads to OsCl₂{=C(CH₃)(CH₂)₃NHC(O)Ph}(PⁱPr₃)₂ (4). The new compounds are intermediate species in the cleavage of the C–C triple bond of the alkynes. Under mild conditions, they undergo the rupture of the C_α–CH₃ bond of the alkylidene, which comes from the alkyne triple bond, to afford six-coordinate hydride–alkylidyne derivatives. In dichloromethane, complex 2 gives a 10:7 mixture of OsHCl₂{≡C(CH₂)₃C(O)NHPh}(PⁱPr₃)₂ (5) and OsHCl₂{≡CCH(CH₃)(CH₂)₂C(O)NHPh}(PⁱPr₃)₂ (6). The first complex contains a linear separation between the alkylidyne C_α atom and the amide group, whereas the spacer is branched in the second complex. In contrast to the case for 2, complex 4 selectively affords OsHCl₂{≡C(CH₂)₃NHC(O)Ph}(PⁱPr₃)₂ (7). In spite of their instability, these compounds give the alkylidene–allene metathesis, being a useful entry to five-coordinate vinylidene complexes, including the dicarbon-disubstituted OsCl₂(=C=CMe₂)(PⁱPr₃)₂ (8) and the monosubstituted OsCl₂(=C=CHCy)(PⁱPr₃)₂ (9).

Gold and diazo reagents: a fruitful tool for developing molecular complexity

After a decade since its discovery and being yet at its sunrise, the use of gold catalysis for the transfer of carbene groups from diazo compounds is becoming a powerful synthetic tool. The main advances in the area and future directions are provided in this Feature Article.

The [3 + 3]-cycloaddition alternative for heterocycle syntheses: catalytically generated metalloenolcarbenes as dipolar adducts

The combination of two or more unsaturated structural units to form cyclic organic compounds is commonly referred to as cycloaddition, and the combination of two unsaturated structural units that forms a six-membered ring is formally either a [5 + 1]-, [4 + 2]-, [2 + 2 + 2]-, or [3 + 3]-cycloaddition. Occurring as concerted or stepwise processes, cycloaddition reactions are among the most useful synthetic constructions in organic chemistry. Of these transformations, the concerted [4 + 2]-cycloaddition, the Diels-Alder reaction, is by far the best known and most widely applied. However, although symmetry disallowed as a concerted process and lacking certifiable examples until recently, stepwise [3 + 3]-cycloadditions offer advantages for the synthesis of a substantial variety of heterocyclic compounds, and they are receiving considerable attention. In this Account, we present the development of stepwise [3 + 3]-cycloaddition reactions from virtual invisibility in the 1990s to a rapidly growing synthetic methodology today, involving organocatalysis or transition metal catalysis. With origins in organometallic or vinyliminium ion chemistry, this area has blossomed into a viable synthetic transformation for the construction of six-membered heterocyclic compounds containing one or more heteroatoms. The development of [3 + 3]-cycloaddition transformations has been achieved through identification of suitable and compatible reactive dipolar adducts and stable dipoles. The reactive dipolar species is an energetic dipolar intermediate that is optimally formed catalytically in the reaction. The stepwise process occurs with the reactive dipolar adduct reacting as an electrophile or as a nucleophile to form the first covalent bond, and this association provides entropic assistance for the construction of the second covalent bond and the overall formal [3 + 3]-cycloaddition. Organocatalysis is well developed for both inter- and intramolecular synthetic transformations, but the potential of transition metal catalysis for [3 + 3]-cycloaddition has only recently emerged. The key to the rapid development of the transition metal-based methodology has been recognition that certain catalytically generated vinylcarbenes are effective dipolar adducts for reactions with stable dipolar compounds, including aryl and vinyl ylides. In particular, metallo-enolcarbenes that are generated catalytically from conveniently prepared stable enoldiazoacetates or from donor-acceptor cyclopropenes are highly effective dipolar adducts for [3 + 3]-cycloaddition. The electron-donating oxygen of the silyl ether enhances electrophilic ring closure to the metal-bound carbon of the initial adduct from vinylogous addition, and this enhancement inhibits the alternative [3 + 2]-cycloaddition across the carbon-carbon double bond of the vinylcarbene. Catalytically generated metallo-enolcarbenes react under mild conditions with a broad spectrum of compatible stable dipoles, including nitrones, azomethine imines, ylides, and certain covalent precursors of stable dipoles, to form [3 + 3]-cycloaddition products having the β-ketoester functionality (in dihydrooxazines, tetrahydropyridazines, pyrazolidinone and pyraxole derivatives, dihydroquinolines, and quinolizidines, for example) in high yield. Two ways to access these metallo-enolcarbenes, either by dinitrogen extrusion from enoldiazoacetate esters or by rearrangement of donor-acceptor cyclopropenes, enhance the versatility of the process. The [3 + 3]-cycloaddition methodology is a complementary strategy to [4 + 2]-cycloaddition for the synthesis of heterocyclic compounds having six-membered rings. High levels of enantioselectivity are obtained with the use of chiral ligands on transition metal catalysts that include those on dirhodium(II) and silver(I).

Electrophilicity of α-oxo gold carbene intermediates: halogen abstractions from halogenated solvents leading to the formation of chloro/bromomethyl ketones

α-Oxo gold carbenes generated via intermolecular oxidation of terminal alkynes are shown to be highly electrophilic and can effectively abstract halogen from halogenated solvents such as 1,2-dichloroethane or 1,2-dibromoethane. Chloro/bromomethyl ketones are prepared in moderate efficiencies in one step using Ph(3)PAuNTf(2) as the catalyst and 8-methylquinoline N-oxide as the oxidant.