Mechanism of the Gold(I)-Catalyzed Rearrangement of Alkynyl Sulfoxides: A DFT Study

Controlling the selectivity of an intramolecular Friedel-Crafts alkylation with alkenes using selenium under mild conditions

An efficiently divergent intramolecular Friedel-Crafts alkylation by unactivated alkenes with seleniranium ion-controlled Markovnikov/anti-Markovnikov specificities under mild conditions has been investigated. 2-Benzoxepin, isochroman, and isochromene can be produced in one-pot procedures from the same substrate in high yields and with high regio- and stereospecificity. The products are challenging to access via 7-endo-trig carbocyclizations and by 7-endo-trig carbocyclization/rearrangement/6-exo-trig oxycyclization and 6-exo-trig carbocyclization/deselenenylation reaction sequences, respectively. Mechanistic experiments indicated that in addition to the stereospecific anti-addition processes of the cyclization reactions, the formation of a stable carbocation after ring opening of the seleniranium ion leads to an NPSP-mediated 7-endo-trig carbocyclization; the steric hindrance of the seleniranium intermediate controls the regioselectivity when using TPSCA at 60 °C, which promotes 6-exo-trig carbocyclization. Two distinct catalytic cycles were proposed, and the structures of transition states and products were identified by ab initio calculations and X-ray analyses.

Theoretical Insight into the Au(I)-Catalyzed Intermolecular Condensation of Homopropargyl Alcohols with Terminal Alkynes: Reactant Stoichiometric Ratio-Controlled Chemodivergence

The mechanisms and chemoselectivities on the Au(I)-catalyzed intermolecular condensation between homopropargyl alcohols and terminal alkynes were investigated by performing DFT calculations. The reaction was indicated to involve three stages: transformation of the homopropargyl alcohol (R₁) via intramolecular cyclization to the cyclic vinyl ether (R₁'), formation of the C-2-arylalkynyl cyclic ether (P₁) via hydroalkynylation of R₁' with phenylacetylene (R₂), and conversion from P₁ to 2,3-dihydro-oxepine (P₂). The results revealed the origin of the reaction divergence and rationalized the experimental observations that a 1:3 reactant stoichiometric ratio affords P₁ as the major product, whereas the 1:1.1 ratio results in P₂ in high yield. The reactant stoichiometric ratio-controlled divergent reactivity is attributed to different catalytic activities of the gold catalyst toward different reaction stages. In the 1:3 situation, the excess R₂ induces the Au catalyst toward its dimerization and/or hydration, inhibiting the conversion of P₁ to P₂ and resulting in product P₁. Without excess R₂, the Au catalysis follows a general cascade reaction, leading to product P₂. Theoretical results described a general strategy controlling the reaction divergence by a different reactant stoichiometric ratio. This strategy may be enlightening for chemists who are exploring various synthesis methods with high chemo-, regio-, and enantioselectivities.

Crystal-defect-induced facet-dependent electrocatalytic activity of 3D gold nanoflowers for the selective nanomolar detection of ascorbic acid

Understanding and exploring the decisive factors responsible for superlative catalytic efficiency is necessary to formulate active electrode materials for improved electrocatalysis and high-throughput sensing. This research demonstrates the ability of bud-shaped gold nanoflowers (AuNFs), intermediates in the bud-to-blossom gold nanoflower synthesis, to offer remarkable electrocatalytic efficiency in the oxidation of ascorbic acid (AA) at nanomolar concentrations. Multicomponent sensing in a single potential sweep is measured using differential pulse voltammetry while the kinetic parameters are estimated using electrochemical impedance spectroscopy. The outstanding catalytic activity of bud-structured AuNF [iAuNFp(Bud)/iGCp ≅ 100] compared with other bud-to-blossom intermediate nanostructures is explained by studying their structural transitions, charge distributions, crystalline patterns, and intrinsic irregularities/defects. Detailed microscopic analysis shows that density of crystal defects, such as edges, terraces, steps, ledges, kinks, and dislocation, plays a major role in producing the high catalytic efficiency. An associated ab initio simulation provides necessary support for the projected role of different crystal facets as selective catalytic sites. Density functional theory corroborates the appearance of inter- and intra-molecular hydrogen bonding within AA molecules to control the resultant fingerprint peak potentials at variable concentrations. Bud-structured AuNF facilitates AA detection at nanomolar levels in a multicomponent pathological sample.

Gold(i)-catalyzed cycloisomerization of ortho-(alkynyl) styrenes: DFT analysis of the crucial role of SbF6− in the elimination of protons

The mechanism, regioselectivity and role of SbF₆⁻ in the gold(i)-catalyzed reaction of o-(alkynyl) styrene are clarified through our DFT calculations.

Unveiling the mechanisms and secrets of chemoselectivities in Au(i)-catalyzed diazo-based couplings with aryl unsaturated aliphatic alcohols

The mechanisms and origins of the unsaturated-aliphatic-alcohols-modulated chemoselectivity of Au-catalyzed carbene transfers from diazo compounds are disclosed by DFT studies.

Theoretical insight into the zinc(ii)-catalyzed synthesis of 2-indolyltetrahydroquinolines from N-propargylanilines and indoles: cross-dehydrogenative coupling with temporally separated catalytic activity

An abnormal catalytic cascade reaction with temporally separated catalytic activity.

C-H Coupling Reactions Directed by Sulfoxides: Teaching an Old Functional Group New Tricks

Sulfoxides are classical functional groups for directing the stoichiometric metalation and functionalization of C-H bonds. In recent times, sulfoxides have been given a new lease on life owing to the development of modern synthetic methods that have arisen because of their unique reactivity. They have recently been used in catalytic C-H activation proceeding via coordination of an internal sulfoxide to a metal or through the action of an external sulfoxide ligand. Furthermore, sulfoxides are able to capture nucleophiles and electrophiles to give sulfonium salts, which subsequently enable the formation of C-C bonds at the expense of C-H bonds. This Review summarizes a renaissance period in the application of sulfoxides arising from their versatility in directing C-H functionalization.

Mechanism of the gold(i)-catalyzed synthesis of imidazo-pyrimidines and imidazo-pyrazines via [3 + 2] dipolar cycloaddition: a DFT study

The mechanisms of gold(i)-catalyzed [3 + 2] cycloaddition reactions of pyridinium N-(heteroaryl)-aminides with alkynes have been investigated using DFT calculations.

Regioselective functionalisation of dibenzothiophenes through gold-catalysed intermolecular alkyne oxyarylation

A protocol has been developed for direct Csp(3)-Csp(2) bond formation at the 4- and 6-positions of dibenzothiophenes using a gold(I) catalyst with terminal alkynes and dibenzothiophene-S-oxides. The sulfoxide acts as a traceless directing group to avoid the need to prefunctionalise at carbon. The iterative use of this protocol is possible and has been employed in the preparation of novel macrocyclic structures. In addition, a cascade process shows how oxyarylations can be combined with other processes resulting in complex, highly efficient transformations.

Theoretical investigation of the mechanism of gold(i)-catalyzed hydrothiolation of alkynes and alkenes with phenthiol

The mechanisms of gold-catalyzed hydrothiolation of alkynes and alkenes with phenthiol have been investigated using density functional theory calculations.

1,2-boryl migration empowers regiodivergent synthesis of borylated furans

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C2- and C3-borylated furans has been developed. It was found that the copper catalyst as well as the gold catalyst with more basic triflate counterion favor boryl migration toward C3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate affords C2-borylated furan via a formal 1,2-hydrogen shift.

[3,3]-Sigmatropic rearrangement versus carbene formation in gold-catalyzed transformations of alkynyl aryl sulfoxides: mechanistic studies and expanded reaction scope

Gold-catalyzed intramolecular oxidation of terminal alkynes with an arenesulfinyl group as the tethered oxidant is a reaction of high impact in gold chemistry, as it introduced to the field the highly valued concept of gold carbene generation via alkyne oxidation. The proposed intermediacy of α-oxo gold carbenes in these reactions, however, has never been substantiated. Detailed experimental studies suggest that the involvement of such reactive intermediates in the formation of dihydrobenzothiepinones is highly unlikely. Instead, a [3,3]-sigmatropic rearrangement of the initial cyclization intermediate offers a reaction path that can readily explain the high reaction efficiency and the lack of sulfonium formation. With internal alkyne substrates, however, the generation of a gold carbene species becomes competitive with the [3,3]-sigmatropic rearrangement. This reactive intermediate, nevertheless, does not proceed to afford the Friedel-Crafts-type cyclization product. Extensive density functional theory studies support the mechanistic conclusion that the cyclized product is formed via an intramolecular [3,3]-sigmatropic rearrangement instead of the previously proposed Friedel-Crafts-type cyclization. With the new mechanistic insight, the product scope of this versatile formation of mid-sized sulfur-containing cycloalkenones has been expanded readily to various dihydrobenzothiocinones, a tetrahydrobenzocyclononenone, and even those without the entanglement of a fused benzene ring. Besides gold, Hg(OTf)2 can be an effective catalyst, thereby offering a cheap alternative for this intramolecular redox reaction.

Coordination chemistry of gold catalysts in solution: a detailed NMR study

Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh(3), JohnPhos (L2), Xphos (L3), DTBP, IMes, IPr, dppf, S-tolBINAP (L8)] has been studied by NMR spectroscopy in solution at room temperature. Cationic or neutral mononuclear complexes LAuX (L=L2, L3, IMes, IPr; X=charged or neutral ligand) underwent simple ligand exchange without giving any higher coordinate complexes. For L2AuX the following ligand strength series was determined: MeOH≪hex-3-yne <MeCN≈OTf(-) ≪Me(2)S<2,6-lutidine<4-picoline<CF(3)CO(2)(-) ≈DMAP<TMTU<PPh(3) <OH(-) ≈Cl(-). Some heteroligand complexes DTBPAuX exist in solution in equilibrium with the corresponding symmetrical species. Binuclear complexes dppf(AuOTf)(2) and S-tolBINAP(AuOTf)(2) showed different behavior in exchange reactions with ligands depending on the ligand strength. Thus, PPh(3) causes abstraction of one gold atom to give mononuclear complexes LLAuPPh(3)(+) and (Ph(3)P)(n)Au(+), but other N and S ligands give ordinary dicationic species LL(AuNu)(2)(2+). In reactions with different bases, LAu(+) provided new oxonium ions whose chemistry was also studied: (DTBPAu)(3)O(+), (L2Au)(2)OH(+), (L2Au)(3)O(+), (L3Au)(2)OH(+), and (IMesAu)(2)OH(+). Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand- or base-assisted interconversions between (L2Au)(2)OH(+), (L2Au)(3)O(+), and L2AuOH are described. Reactions of dppf(AuOTf)(2) and S-tolBINAP(AuOTf)(2) with bases provided more interesting oxonium ions, whose molecular composition was found to be [dppf(Au)(2)](3)O(2)(2+), L8(Au)(2)OH(+), and [L8(Au)(2)](3)O(2)(2+), but their exact structure was not established. Several reactions between different oxonium species were conducted to observe mixed heteroligand oxonium species. Reaction of L2AuNCMe(+) with S(2-) was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined.