Substrate-Dependent Mechanisms for the Gold(I)-Catalyzed Cycloisomerization of Silyl-Tethered Enynes: A Computational Study

The gold­(I)-catalyzed alkenyl-, allyl-, and arylsilylation reactions of silyl-tethered enynes discovered by Murakami et al. provide efficient methods for the facile constructions of 1-silaindene derivatives. A comprehensive mechanistic DFT study of these reactions was carried out to better understand the experimental outcomes, and divergent and substrate-dependent mechanisms for the formations of 1-silaindene derivatives were uncovered based on the computational results. From cationic gold­(I) π-alkyne complexes, the <i>endo</i>-dig cyclization pathway may lead possibly to both C₂- and C₃-group-substituted (group = alkenyl, allyl, or aryl) 1-silaindene products, and the regioselectivity will be finally determined by the 1,2-group migration of the gold carbenoid intermediate. On the other hand, the <i>exo</i>-dig cyclization pathway leads only to C₃-group-substituted (group = alkenyl, allyl, or aryl) 1-silaindene, in which a notable promoting effect of the bistriflimide counterion on the rearrangement of the silyl cation intermediate was disclosed. The results reported herein provide insights into aspects of regioselective cyclization, silyl-involved skeletal rearrangements, chemoselective 1,2-migration in gold carbenoids, and the dramatic counterion effect in the reactions concerned

Catalyst-Controlled C–C σ Bond Cleavages in Metal Halide-Catalyzed Cycloisomerization of 3‑Acylcyclopropenes via a Formal 1,1-Halometalation Mechanism: Insights from Quantum Chemical Calculations

The ring-opening cycloisomerization reactions of cyclopropenyl ketones developed by S. Ma et al. [<i>J. Am. Chem. Soc.</i> <b>2003</b>, <i>125</i>, 12386–12387] provided an efficient method for the constructions of trisubstituted furans in which an elegant control of the regiochemistry was achieved by using CuI or PdCl₂ catalyst. In the current report we aimed at uncovering the origin of the divergent regiochemistry of the reactions with different metal halide catalysts using quantum chemical calculations. By comparing the energies of all possible pathways, we found that a novel mechanism involving a formal 1,1-halometalation is the energetically most favorable one. In this pathway, an organometallic intermediate is involved from addition of the metal atom and the halide ligand to the same sp² carbon of the cyclopropene moiety by sequential 1,5-addition and 1,5-rearrangement steps, and the furan product is finally formed via an asynchronous intramolecular substitution/metal halide elimination process. The initial 1,5-addition was found to be the rate- and regiochemistry-determining step. The calculations reproduced well the experimentally observed selectivity. By analyzing the divergence of the Pd­(II) and Cu­(I) halides using the distortion/interaction model, it was found that the interaction energy plays a more important role in determining the selectivity. The strong π-affinity of PdCl₂ enables its strong coordination with the C¹C² double bond in the TS, and the opening of the more substituted C¹–C³ single bond is favored. On the other hand, the harder Lewis acid CuI is more sensitive to the steric effect and the opening of the less substituted C²–C³ single bond thus becomes predominant

Mechanistic Understanding of the Divergent Reactivity of Cyclopropenes in Rh(III)-Catalyzed C–H Activation/Cycloaddition Reactions of <i>N</i>‑Phenoxyacetamide and <i>N</i>‑Pivaloxybenzamide

Density functional theory calculations were conducted to develop a mechanistic understanding of the Rh­(III)-catalyzed C–H activation/cycloaddition reactions of <i>N</i>-phenoxyacetamide and <i>N</i>-pivaloxybenzamide with cyclopropenes, and insights into the substrate-dependent chemoselectivity were provided. The results showed that the divergence originated from the different reactivity of the seven-membered rhodacycles from the insertion of cyclopropene into the Rh–C bond. In reactions of <i>N</i>-pivaloxybenzamide, such an intermediate undergoes the pivalate migration to form a cyclic Rh­(V)-nitrenoid intermediate in a reaction that is easier than the opening of the three-membered ring by β-carbon elimination, leading finally to a tricyclic product with retention of the cyclopropane moiety by facile reductive elimination. While similar Rh­(V)–nitrenoid species could also be possibly formed in Cp*Rh­(III)-catalyzed reactions of <i>N</i>-phenoxyacetamide, the β-carbon elimination occurs more easily from the corresponding seven-membered rhodacycle intermediate and the subsequent O–N bond cleavage gives rise to an unexpected dearomatized (<i>E</i>)-6-alkenylcyclohexa-2,4-dienone intermediate. The <i>E</i>/<i>Z</i> isomerization of this intermediate is required for the final cyclization to 2<i>H</i>-chromene, and interesting metal–ligand cooperative catalysis with Rh­(III) carboxylate was disclosed in the CC double bond rotation process

Noninnocent Counterion Effect on the Rearrangements of Cationic Intermediates in a Gold(I)-Catalyzed Alkenylsilylation Reaction

A mechanistic DFT study of the gold(I)-catalyzed alkenylsilylation reaction of a silyl-tethered 1,6-enyne system is reported. A novel pathway involving bistriflimide counterion-assisted rearrangements of carbocation and silyl cation intermediates corroborates the experimental observations. The results suggest the important role of the counterion in modulating the reactivity of cationic intermediates in gold catalysis

Computational Revisit to the β‑Carbon Elimination Step in Rh(III)-Catalyzed C–H Activation/Cycloaddition Reactions of <i>N</i>‑Phenoxyacetamide and Cyclopropenes

This computational study uncovered the origin of the contradicting results in two recent DFT studies of the Rh­(III)-catalyzed C–H activation/cycloaddition reactions between <i>N</i>-phenoxyacetamide and cyclopropenes. It was found that the β-carbon elimination of the tricyclic intermediate occurs very faciely via a conformer in which the opening of the three-membered ring is trans to the Cp* ligand so that the steric repulsion between the two moieties is avoided. Thus, the conclusions of our previous study were reconfirmed

Mechanistic Understanding of the Aryl-Dependent Ring Formations in Rh(III)-Catalyzed C–H Activation/Cycloaddition of Benzamides and Methylenecyclopropanes by DFT Calculations

The divergence between Rh­(III)-catalyzed C–H activation/cycloaddition of phenyl- and 2-furanyl-containing benzamides with methylenecyclopropanes (MCP) was studied by DFT calculations. Calculations found that the C–H activation via a CMD mechanism is the most difficult step of the reaction involving phenyl. In contrast, the C–H activation of the 2-furanyl-containing substrate is kinetically easier but the formed five-membered rhodacycle is relatively unstable, making the following MCP insertion more difficult. Thus, different KIE data was obtained in experiments. The MCP insertion forms a seven-membered-ring rhodacycle intermediate, from which the chemoselectivity of the whole reaction is determined by the competitive pivalate migration (path I) and β-C elimination (path II). While the β-C elimination is lower in energy when a furanylene is contained in the intermediate, a reversed preference of pivalate migration was predicted for the phenylene counterpart. Structural analysis suggested that the unfavorable β-C elimination in the phenylene case should be attributed to the obviously increased ring strain in the corresponding transition state, instead of the difference in electronic properties between the aryl groups. This accounts for why aryl-dependent chemoselectivity was observed. In addition, the results indicated that for both paths I and II the generation of a Rh­(V)–nitrenoid intermediate from pivalate migration is crucial for the final C–N bond formation. This explains why no reaction occurred when the N–OPiv moiety was replaced with an N–OMe group, as no Rh­(V) intermediate could be formed in this system

Reactivity of Alkynyl Metal Carbenoids: DFT Study on the Pt-Catalyzed Cyclopropanation of Propargyl Ester Containing 1,3-Diynes

DFT/M06 calculations were performed to investigate the mechanism of the Pt-catalyzed intermolecular cyclopropanation of propargyl ester containing diynes with styrene. The results show that the alkynyl Pt-carbenoid formed from proximal activation of the diyne is a more favorable productive intermediate for cyclopropanation, which occurs preferentially at the distal <i>sp</i>-hybridized carbon via an S_N2′-type olefin addition. Notably, the widely accepted [1,3]-metallotropic shift of such an alkynyl metal carbenoid is found to be energetically demanding

Reactivity of Arynes toward Functionalized Alkenes: Intermolecu-lar Alder-Ene vs. Addition Reactions

The selectivity between two different manifolds of reactions of arynes reacting with functionalized alkenes is described. Arynes generated from bis-1,3-diynes react with various trisubstituted and 1,1-disubstituted alkenes including methallyl amine, prenyl azide, and methacrylic acid, provide mainly addition products of the polar heteroatom functionalities over the Alder-ene prod-ucts of the alkene segment. The selectivity, however, intricately depends on the substituent pattern of the alkene. Except for the most reactive 2-propenyl group-containing aldehyde, -unsaturated aldehydes generally participated in an addition reaction, generating chromene derivatives

Reactivity of arynes toward functionalized alkenes: intermolecular Alder-ene vs. addition reactions

The selectivity between two different manifolds of reactions of arynes reacting with functionalized alkenes is described. Arynes generated from bis-1,3-diynes react with various trisubstituted and 1,1-disubstituted alkenes including methallyl amine, prenyl azide, and methacrylic acid, providing mainly addition products of the polar heteroatom functionalities over the Alder-ene products of the alkene segment. The selectivity, however, intricately depends on the substituent pattern of the alkene. Except for the most reactive 2-propenyl group-containing aldehyde, α,β-unsaturated aldehydes generally participated in an addition reaction, generating chromene derivatives

Visible-Light-Promoted Hydrogenation of Azobenzenes to Hydrazobenzenes with Thioacetic Acid as the Reductant

A catalyst- and metal-free hydrogenation of azobenzenes to hydrazobenzenes in the presence of thioacetic acid was achieved under visible light irradiation. The transformation was carried out under mild conditions in an air atmosphere at ambient temperature, generating a variety of hydrazobenzenes with yields up to 99%. The current process is compatible with a variety of substituents and is highly chemoselective for azo reduction when other unsaturated functionalities (carbonyl, alkenyl, alkynyl, etc.) are contained. Preliminary mechanistic study indicated that the transformation could be a radical process