Theoretical Studies on Nickel-Catalyzed Cycloaddition of 3-Azetidinone with Alkynes

DFT studies on Ni-catalyzed intermolecular cycloaddition of diynes with methyleneaziridines

The mechanisms of nickel-catalyzed intermolecular cycloaddition of diynes with methyleneaziridines to form substituted pyrroles have been investigated with DFT calculations. The DFT results don't support the originally proposed mechanisms, which involve β-C elimination or α-C elimination. Detailed calculations revealed that the preferred catalytic cycle is a combination of the cod dissociative mechanism and the cod associative mechanism, which is comprised of four stages: oxidative addition, ligand substitution of the diyne by cod, alkyne insertion and reductive elimination. Each of the alkyne moieties of the diyne substrate has an important role: one alkyne moiety acts as the reactant and inserts into the Ni-C bond to form the cycle expansion complex; the other free alkyne moiety has an effect as a ligand coordinated to the Ni center to promote the oxidative addition step (rate-determining step). Since there is no free alkyne in the monoalkyne substrate to coordinate to the Ni center, the monoalkyne catalytic cycle is unfavorable because of the high energy barrier for the oxidative addition step.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.

Regioselective cycloaddition of potassium alkynyltrifluoroborates with 3-azetidinones and 3-oxetanone by nickel-catalysed C–C bond activation

The nickel-catalysed (4+2) cycloaddtion of potassium alkynyltrifluoroborates and 3-azetidinones and 3-oxetanone gives only one regioisomer for all alkyne substituents.

Fused-Ring Formation by an Intramolecular "Cut-and-Sew" Reaction between Cyclobutanones and Alkynes

The development of a catalytic intramolecular "cut-and-sew" transformation between cyclobutanones and alkynes to construct cyclohexenone-fused rings is described herein. The challenge arises from the need for selective coupling at the more sterically hindered proximal position, and can be addressed by using an electron-rich, but less bulky, phosphine ligand. The control experiment and 13 C-labelling study suggest that the reaction may start with cleavage of the less hindered distal C-C bond of cyclobutanones, followed by decarbonylation and CO reinsertion to enable Rh insertion at the more hindered proximal position.

Recent Methodologies That Exploit C-C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

In this review, synthetic and mechanistic aspects of key methodologies that exploit C-C single-bond cleavage of strained ring systems are highlighted. The focus is on transition-metal-catalyzed processes that are triggered by C-C bond activation and β-carbon elimination, with the review concentrating on developments from mid-2009 to mid-2016.

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.

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.