Mechanistic Origin of Chemo- and Regioselectivity of Nickel-Catalyzed [3 + 2 + 2] Cyclization Reaction

Co-Catalyzed Asymmetric Intramolecular [3+2] Cycloaddition of Yne-Alkylidenecyclopropanes and its Reaction Mechanism

Developing new transition metal-catalyzed asymmetric cycloadditions for the synthesis of five-membered carbocycles (FMCs) is a research frontier in reaction development due to the ubiquitous presence of chiral FMCs in various functional molecules. Reported here is our discovery of a highly enantioselective intramolecular [3+2] cycloaddition of yne-alkylidenecyclopropanes (yne-ACPs) to bicyclo[3.3.0]octadiene and bicyclo[4.3.0]nonadiene molecules using a cheap Co catalyst and commercially available chiral ligand (S)-Xyl-BINAP. This reaction avoids the use of precious Pd and Rh catalysts, which are usually the choices for [3+2] reactions with ACPs. The enantiomeric excess in the present reaction can be up to 92 %. Cationic cobalt(I) species was suggested by experiments as the catalytic species. DFT calculations showed that this [3+2] reaction starts with oxidative cyclometallation of alkyne and ACP, followed by ring opening of the cyclopropyl (CP) group and reductive elimination to form the cycloadduct. This mechanism is different from previous [3+2] reactions of ACPs, which usually start from CP cleavage, not from oxidative cyclization.

Cycloaddition of cyclopropanes for the elaboration of medium-sized carbocycles

The stereocontrolled formation of medium-sized carbocycles is a major goal in modern organic chemistry due to their widespread occurrence in natural products and pharmaceutically active ingredients. One approach consists in the use of cycloaddition reactions which notably results in high selectivities and atom-economy. To this end, cyclopropanes are ideal substrates since they can provide readily functionalized three- or five-carbon synthons. Herein we report advances made in cycloaddition reactions of cyclopropanes towards the synthesis of medium-sized carbocycles via transition metal catalysis or Lewis acid catalysis.

Probing the origin of the stereoselectivity and enantioselectivity of cobalt-catalyzed [2 + 2] cyclization of ethylene and enynes

Theoretical calculations reveal the origin of the stereoselectivity and enantioselectivity of cobalt-catalyzed [2 + 2] cyclization of ethylene and enynes.

DFT Study of a Missing Piece in Brasilane-Type Structure Biosynthesis: An Unusual Skeletal Rearrangement

Brasilane-type sesquiterpenes have been known for a long time, but their biosynthetic pathways and mechanisms remain elusive. Recently, two groups independently characterized a Trichoderma terpene cyclase that produces trichobrasilenol, a brasilane-type sesquiterpene, and a plausible biosynthetic pathway was proposed based on isotopic labeling experiments. In the proposed mechanism, the characteristic brasilane-type 5/6 bicyclic skeleton is synthesized from a 5/7/3 tricyclic intermediate via a complicated concerted reaction, including six chemical events of C-C σ bond metathesis and rearrangements, ring-contraction, π bond formation, and regioselective hydroxylation. However, our density functional theory (DFT) calculations do not support this mechanism. On the basis of DFT calculations, we propose a new pathway for trichobrasilenol biosynthesis, involving a multistep carbocation cascade in which cyclopropylcarbinyl cations in equilibrium with homoallyl cations play a pivotal role. This pathway and mechanism is in good agreement with previous biosynthetic studies on brasilane-type compounds and related terpenoids, including isotope-labeling experiments and byproducts analysis.

Rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes: DFT analysis of origins of regio- and diastereo-selectivities

A unconventional reaction mechanism can be responsible for the regio- and diastereo-selectivities of rhodium-catalyzed ene-cycloisomerization of allylic-sulfide-tethered alkylidenecyclopropanes.

Rhodium-Catalyzed Enantioselective [2 + 2 + 1] Cycloaddition of 1,6-Enynes with Cyclopropylideneacetamides

It has been established that a cationic rhodium(I)/( R)-tol-BINAP or ( R)-BINAP complex catalyzes the enantioselective [2 + 2 + 1] cycloaddition of 1,6-enynes, possessing monosubstituted alkene units, with cyclopropylideneacetamides at room temperature through the elimination of ethylene to give bicyclic (cyclopent-2-en-1-ylidene)acetamides with high enantioselectivity.

Mechanistic insights into Pd(0)-catalyzed intermolecular and intramolecular hydroamination of methylenecyclopropanes: a computational study

The mechanisms of transition metal-catalyzed methylenecyclopropanes (MCPs)-involved reactions are rather complicated due to the diverse pathways for the activation of MCPs. Herein, computational studies were carried out to investigate the detailed mechanisms of Pd(0)-catalyzed intermolecular and intramolecular hydroamination of MCPs. The initial activation of the three-membered ring of MCPs readily occurs via the insertion of Pd(0) into the distal C-C bond, leading to a metallacyclobutane intermediate. The commonly proposed oxidative addition of amine/amide nucleophile (Nu-H) onto the Pd(0) center to afford a hydrido-Pd(ii) complex, however, is less favorable in comparison with the Pd(0)-mediated cleavage of the distal C-C bond of MCPs. Subsequently, for the Pd(0)-catalyzed intermolecular hydroamination of 1,1-diphenyl MCP with 2-pyrrolidone, it is more favorable for the C1 of the metallacyclobutane intermediate to undergo protonation to yield a π-allylpalladium intermediate, from which the final allylamine product is afforded via reductive elimination. For the Pd(0)-catalyzed intramolecular hydroamination of aniline-tethered MCP, the intramolecular nucleophilic attack of the amine moiety to C3/C4 of the corresponding metallacyclobutane intermediate is preferable to generate a cyclic intermediate. Subsequent proton transfer steps could occur to complete the hydroamination reaction. The different pKa values of the N-H bonds of amide/amine are mainly responsible for the mechanistic difference in the Pd(0)-catalyzed hydroamination of MCPs.

Mechanistic Study of Nickel-Catalyzed Reductive Coupling of Ynoates and Aldehydes

In this work, (1,5-hexadiene)Ni(SIPr) (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene) is used in place of Ni(COD)₂/SIPr·HBF₄/KO^tBu (COD = 1,5-cyclooctadiene) as a more robust catalyst for regioselective reductive coupling of ynoates and aldehydes with triethylsilane. The catalytic reaction of ethyl 3-(trimethylsilyl)propiolate and methyl 4-formylbenzoate shows first-order dependence on aldehyde and catalyst concentrations, inverse first-order dependence on [ynoate], and no dependence on [silane]. The kinetics data, coupled with deuterium-labeling experiments, support a mechanism involving dissociation of the ynoate from a catalytically dormant nickelacyclopentadiene intermediate prior to turnover-limiting formation of a catalytically active nickeladihydrofuran.

Fulvene Synthesis by Rhodium(I)-Catalyzed [2+2+1] Cycloaddition: Synthesis and Catalytic Activity of Tunable Cyclopentadienyl Rhodium(III) Complexes with Pendant Amides

It has been established that a Rh^I+ /segphos complex catalyzes the [2+2+1] cycloaddition of 1,6-diynes with cyclopropylideneacetamides to give substituted fulvenes in good yields. The reductive complexation of the product fulvenes with RhCl₃ in EtOH furnished the corresponding dinuclear cyclopentadienyl Rh^III complexes bearing a pendant amide moiety. These Rh^III complexes were highly active catalysts for oxidative annulation and cyclization through C(sp² )-H and C(sp³ )-H functionalization.

Computational Studies on Reaction Mechanism and Origins of Selectivities in Nickel-Catalyzed (2 + 2 + 2) Cycloadditions and Alkenylative Cyclizations of 1,6-Ene-Allenes and Alkenes

The reaction mechanism and origins of ligand-controlled selectivity, regioselectivity, and stereoselectivity of Ni-catalyzed (2 + 2 + 2) cycloadditions and alkenylative cyclizations of 1,6-ene-allenes and alkenes were studied by using density functional theory. The catalytic cycle involves intermolecular oxidative coupling and an intramolecular concerted 1,4-addition step to afford a stable metallacycloheptane intermediate; these steps determine both the regioselectivity and stereoselectivity. Subsequent C-C reductive elimination leads to the cyclohexane product, whereas the β-hydride elimination leads to the trans-diene product. The selectivity between (2 + 2 + 2) cycloadditions and alkenylative cyclizations is controlled by the ligand. Irrespective of the nature of the terminal substituents on the ene-allene and alkene, the P(o-tol)₃ ligand always favors the C-C reductive elimination, resulting in the cyclohexane product. On the other hand, the flexibility of the PBu₃ ligand means that electronic and steric factors play important roles. Electron-withdrawing groups such as CO₂Me in the ene-allene terminal substituent induce obvious substrate-ligand repulsion and destabilize the C-C reductive elimination, giving rise to the trans-diene product.

Computational Study of Rh-Catalyzed Carboacylation of Olefins: Ligand-Promoted Rhodacycle Isomerization Enables Regioselective C-C Bond Functionalization of Benzocyclobutenones

The mechanism, reactivity, regio- and enantioselectivity of the Rh-catalyzed carboacylation of benzocyclobutenones are investigated using density functional theory (DFT) calculations. The calculations indicate that the selective activation of the relatively unreactive C1-C2 bond in benzocyclobutenone is achieved via initial C1-C8 bond oxidative addition, followed by rhodacycle isomerization via decarbonylation and CO insertion. Analysis of different ligand steric parameters, ligand steric contour maps, and the computed activation barriers revealed the origin of the positive correlation between ligand bite angle and reactivity. The increase of reactivity with bulkier ligands is attributed to the release of ligand-substrate repulsions in the P-Rh-P plane during the rate-determining CO insertion step. The enantioselectivity in reactions with the (R)-SEGPHOS ligand is controlled by the steric repulsion between the C8 methylene group in the substrate and the equatorial phenyl group on the chiral ligand in the olefin migratory insertion step.

Nickel-Catalyzed Chemo-, Regio- and Diastereoselective Bond Formation through Proximal C-C Cleavage of Benzocyclobutenones

The first catalytic intermolecular proximal C1-C2 cleavage of benzocyclobutenones (BCB) without prior carbonyl activation or employing noble metals has been developed. This protocol operates at room temperature and is characterized by an exquisite chemo-, regio- and diastereoselectivity profile, constituting a unique platform for preparing an array of elusive carbocyclic skeletons.

A new protocol for nickel-catalysed regio- and stereoselective hydrocyanation of allenes

Regio- and stereoselective hydrocyanation under nickel catalysis is described.

Mechanism, reactivity, and selectivity of nickel-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes

Density functional theory (DFT) calculations with B3LYP and M06 functionals elucidated the reactivities of alkynes and Z/E selectivity of cyclodecatriene products in the Ni-catalyzed [4 + 4 + 2] cycloadditions of dienes and alkynes. The Ni-mediated oxidative cyclization of butadienes determines the Z/E selectivity. Only the oxidative cyclization of one s-cis to one s-trans butadiene is facile and exergonic, leading to the observed 1Z,4Z,8E-cyclodecatriene product. The same step with two s-cis or s-trans butadienes is either kinetically or thermodynamically unfavorable, and the 1Z,4E,8E- and 1Z,4Z,8Z-cyclodecatriene isomers are not observed in experiments. In addition, the competition between the desired cooligomerization and [2 + 2 + 2] cycloadditions of alkynes depends on the coordination of alkynes. With either electron-deficient alkynes or alkynes with free hydroxyl groups, the coordination of alkynes is stronger than that of dienes, and alkyne trimerization prevails. With alkyl-substituted alkynes, the generation of alkyne-coordinated nickel complex is much less favorable, and the [4 + 4 + 2] cycloaddition occurs.

Nickel-catalyzed intramolecular [3 + 2 + 2] cycloadditions of alkylidenecyclopropanes. A straightforward entry to fused 6,7,5-tricyclic systems

A highly diastereo- and chemoselective intramolecular nickel-catalyzed cycloaddition of alkene- and alkyne-tethered alkynylidenecyclopropanes is reported. The method constitutes the first fully intramolecular [3 + 2 + 2] alkylidenecyclopropropane cycloaddition occurring via a proximal cleavage of the cyclopropane and makes it possible to build relevant 6,7,5-tricyclic frameworks in a single-pot reaction. Importantly, the reaction outcome is highly dependent on the characteristics of the nickel ligands.