Mechanism and Selectivity in Rhodium-Catalyzed [7 + 2] Cycloaddition and Cyclopropanation/Cyclization of Allenylcyclopentane-alkynes: Metallacycle-Directed C(sp3)-C(sp3) vs C(sp3)-H Activation

Mechanism and Origins of Enantioselectivity of Cobalt-Catalyzed Intermolecular Hydroarylation/Cyclization of 1,6-Enynes with N-Pyridylindoles

Density functional theory calculations were performed to investigate the cobalt-catalyzed intermolecular hydroarylation/cyclization of 1,6-enynes with N-pyridylindoles. The computations reveal that the reaction begins with the oxidative cyclization of 1,6-enyne to afford the five-membered cobaltacycle, from which the metal-assisted σ-bond metathesis/C-C reductive elimination led to the final hydroarylation/cyclization product. The initial oxidative cyclization constitutes the rate-determining step of the overall reaction. The steric repulsion and π···π interaction were found to play a crucial role in dictating the experimentally observed enantioselectivity.

Mechanism and Origins of Enantioselectivity of Cobalt-Catalyzed Intermolecular Hydroacylation/Cyclization of 1,6-Enynes with Aldehydes

Density functional theory calculations were performed to investigate the cobalt-catalyzed intermolecular hydroacylation/cyclization of 1,6-enynes. The computations show that the initial oxidative cyclization constitutes the rate-determining step of the overall reaction....

Chemodivergent reactions

An important strategy for the efficient generation of diversity in molecular structures is the utilization of common starting materials in chemodivergent transformations. The most studied solutions for switching the chemoselectivity rely on the catalyst, ligand, additive, solvent, temperature, time, pressure, pH and even small modifications in the substrate. In this review article several processes have been selected such as inter- and intramolecular cyclizations, including carba-, oxa-, thia- and oxazacyclizations promoted mainly by Brønsted or Lewis acids, transition metals and organocatalysts, as well as radical reactions. Catalyst-controlled intra- and intermolecular cyclizations are mainly described to give five- and six-membered rings. Cycloaddition reactions involving (2+2), (3+2), (3+3), (4+1), (4+2), (5+2), (6+2) and (7+2) processes are useful reactions for the synthesis of cyclic systems using organocatalysts, metal catalysts and Lewis acid-controlled processes. Addition reactions mainly of carba- and heteronucleophiles to unsaturated conjugated substrates can give different adducts via metal catalyst-, Lewis acid- and solvent-dependent processes. Carbonylation reactions of amines and phenols are carried out via ligand-controlled transition metal-catalyzed multicomponent processes. Ring-opening reactions starting mainly from cyclopropanols, cyclopropenols and epoxides or aziridines are applied to the synthesis of acyclic versus cyclic products under catalyst-control mainly by Lewis acids. Chemodivergent reduction reactions are performed using dissolving metals, sodium borohydride or hydrogen transfer conditions under solvent control. Oxidation reactions include molecular oxygen under solvent control or using different dioxiranes, as well as chemodivergent palladium catalyzed cross-coupling reactions using boronic acids are applied to aromatic and allenic compounds. Other chemodivergent reactions such as alkylations and allylations under transition metal catalysis, dimerization of acetylenes, bromination of benzylic substrates, and A³-couplings are performed via catalyst- or reaction condition-dependent processes.

Substituent-dependent generation of tricyclic frameworks by the rhodium-catalyzed cycloisomerization of homopropargyl allene-alkynes: a theoretical study

Polycyclic compounds having biological activities can be modified by employing different substituents.

Enantioselective Rhodium-Catalyzed Cycloisomerization of 1,6-Allenynes to access 5/6-Fused Bicycle[4.3.0]nonadienes

Transition-metal-catalyzed cycloisomerization of 1,n-allenynes represents a powerful synthetic tool to rapidly assemble complex polycyclic skeletons from simple linear substrates. Nevertheless, there are no reports of the asymmetric version of these reactions. Moreover, most of these reactions proceed through a 6-endo-dig cyclization pathway, which preferentially delivers the distal product (via 5/5 rhodacyclic intermediate) rather than the proximal one (via 6/5 rhodacyclic intermediate). Herein, we report an enantioselective rhodium(I)-catalyzed cycloisomerization of 1,6-allenynes to provide the proximal product 5/6-fused bicycle[4.3.0]nonadienes in good yields and with excellent enantioselectivities. Remarkably, this chemistry works perfectly for 1,6-allenynes having a cyclic substituent within the allene component, thereby affording synthetically formidable tricyclic products with excellent enantioselectivities. Moreover, extensive DFT calculations suggest an uncommon pathway involving 5-exo-dig cycloisomerization, ring-expansion, rate-determining alkene isomerization involving C_sp3-H activation, C-C activation of the cyclobutene moiety and finally reductive elimination. Deuterium labeling experiments support the rate-determining step involving the C–H bond activation in this transformation.

Cycloisomerization of 1,n-allenynes allows to efficiently assembly polycyclic skeletons, however its asymmetric variant remains elusive. Here, the authors report the enantioselective Rh(I)-catalyzed cycloisomerization of 1,6-allenynes and disclose an uncommon 5-exo-dig pathway via DFT calculations.

Mechanistic Insight into the [4 + 2] Diels-Alder Cycloaddition over First Row d-Block Cation-Exchanged Faujasites

The Diels-Alder cycloaddition (DAC) is a powerful tool to construct C-C bonds. The DAC reaction can be accelerated in several ways, one of which is reactant confinement as observed in supramolecular complexes and Diels-Alderases. Another method is altering the frontier molecular orbitals (FMOs) of the reactants by using homogeneous transition-metal complexes whose active sites exhibit d-orbitals suitable for net-bonding orbital interactions with the substrates. Both features can be combined in first row d-block (TM) exchanged faujasite catalysts where the zeolite framework acts as a stabilizing ligand for the active site while confining the reactants. Herein, we report on a mechanistic and periodic DFT study on TM-(Cu(I), Cu(II), Zn(II), Ni(II), Cr(III), Sc(III), V(V))exchanged faujasites to elucidate the effect of d-shell filling on the DAC reaction between 2,5-dimethylfuran and ethylene. Two pathways were found: one being the concerted one-step and the other being the stepwise two-step pathway. A decrease in d-shell filling results in a concomitant increase in reactant activation as evidenced by increasingly narrow energy gaps and lower activation barriers. For models holding relatively small d-block cations, the zeolite framework was found to bias the DAC reaction toward an asynchronous one-step pathway instead of the two-step pathway. This work is an example of how the active site properties and the surrounding chemical environment influence the reaction mechanism of chemical transformations.

Rhodium(I)-Catalyzed Cycloisomerization of Homopropargylallene-Alkynes through C(sp3 )-C(sp) Bond Activation

Upon exposure to a catalytic amount of [RhCl(CO)₂ ]₂ in 1,4-dioxane, homopropargylallene-alkynes underwent a novel cycloisomerization accompanied by the migration of the alkyne moiety of the homopropargyl functional group to produce six/five/five tricyclic compounds in good yields. A plausible mechanism was proposed on the basis of an experiment with ¹³ C-labeled substrate. The resulting tricyclic derivatives were further converted into the corresponding bicyclo[3.3.0] skeletons with vicinal cis dihydroxy groups.

Mechanism, selectivity, and reactivity of iridium- and rhodium-catalyzed intermolecular ketone α-alkylation with unactivated olefinsviaan enamide directing strategy

DFT calculations were performed to investigate the title reaction, focusing on detailed reaction mechanism and origins of selectivity and reactivity.

Mechanism and origins of chemo- and regioselectivities of (NHC)NiH-catalyzed cross-hydroalkenylation of vinyl ethers with α-olefins: a computational study

Density functional theory calculations were performed to investigate the (NHC)NiH-catalyzed cross-hydroalkenylation of vinyl ethers with α-olefins.

Mechanism and origins of selectivity in rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes

DFT calculations were performed to investigate the rhodium-catalyzed intermolecular [3 + 2] cycloadditions of vinylaziridines with allenes.

Mechanism of iridium-catalysed branched-selective hydroarylation of vinyl ethers: a computational study

An unconventional modified Chalk–Harrod-type mechanism was revealed by DFT calculations for the iridium-catalysed branched-selective hydroarylation of vinyl ethers.