Mechanistic Study on Rh-Catalyzed Stereoselective CC/CH Activation oftert-Cyclobutanols

Recent advances in Rh(I)-catalyzed enantioselective C-H functionalization

Chiral carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds are pervasive and very essential in natural products, bioactive molecules, and functional materials, and their catalytic construction has emerged as one of the hottest research fields in synthetic organic chemistry. The last decade has witnessed vigorous progress in Rh(I)-catalyzed asymmetric C-H functionalization as a complement to Rh(II) and Rh(III) catalysis. This review aims to provide the most comprehensive and up-to-date summary covering the recent advances in Rh(I)-catalyzed C-H activation for asymmetric functionalization. In addition to the development of diverse reactions, chiral ligand design and mechanistic investigation (inner-sphere mechanism, outer-sphere mechanism, and 1,4-Rh migration) will also be highlighted.

Palladium- and nickel-catalyzed cascade enantioselective ring-opening/coupling reactions of cyclobutanones

The chemistry of small ring compounds is an intriguing subject in organic chemistry. As the smallest stable cyclic aliphatic ketones, cyclobutanones have garnered tremendous attention owing to their intrinsic high reactivity such as transition-metal catalyzed C-C bond cleavage. In this context, transition-metal catalyzed formal cycloaddition of cyclobutanones via a "cut and sew" strategy has gained marvelous advances. In contrast, an alternative reaction paradigm, i.e., transition-metal catalyzed ring-opening reactions of cyclobutanones, is still underdeveloped. This feature article aims to summarize our efforts in developing enantioselective palladium-catalyzed ring-opening/coupling reactions and recently emerging nickel-catalyzed ring-opening/reductive coupling reactions of cyclobutanones with a tethered aryl halide. The possible mechanisms are briefly showcased and the advantages and limitations of each strategy as well as their synthetic applications in the synthesis of natural products or bioactive compounds are presented.

Enantioselective Cleavage of Cyclobutanols Through Ir-Catalyzed C-C Bond Activation: Mechanistic and Synthetic Aspects

The Ir-catalyzed conversion of prochiral tert-cyclobutanols to β-methyl-substituted ketones proceeds under comparably mild conditions in toluene (45-110 °C) and is particularly suited for the enantioselective desymmetrization of β-oxy-substituted substrates to give products with a quaternary chirality center with up to 95 % ee using DTBM-SegPhos as a chiral ligand. Deuteration experiments and kinetic isotope effect measurements revealed major mechanistic differences to related RhI -catalyzed transformations. Supported by DFT calculations we propose the initial formation of an IrIII hydride intermediate, which then undergoes a β-C elimination (C-C bond activation) prior to reductive C-H elimination. The computational model also allows the prediction of the stereochemical outcome. The Ir-catalyzed cyclobutanol cleavage is broadly applicable but fails for substrates bearing strongly coordinating groups. The method is of particular value for the stereo-controlled synthesis of substituted chromanes related to the tocopherols and other natural products.

Enantioselective Synthesis of Chiral Indane Derivatives by Rhodium-Catalyzed Addition of Arylboron Reagents to Substituted Indenes

Rhodium-catalyzed asymmetric addition of arylboron reagents to indene derivatives proceeded to give 2-arylindanes in good yields with high enantioselectivity. Deuterium-labeling experiments indicated that the present reaction involved a 1,4-Rh shift from an initially formed benzylrhodium to an arylrhodium intermediate before protonation leading to the corresponding addition product. The asymmetric addition was also successful for acenaphthylene, which has a similar skeleton to indene, where it was found that the benzylrhodium intermediate underwent direct protonation without the 1,4-Rh shift.

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.

Rhodium-catalyzed asymmetric addition of arylboronic acids to 2H-chromenes leading to 3-arylchromane derivatives

The Rh-catalyzed enantioselective addition of arylboronic acids to 2H-chromenes proceeded to give 3-arylchromanes with high enantioselectivity.

Addition of arylstannanes to alkynes giving ortho-alkenylarylstannanes catalysed cooperatively by a rhodium complex and zinc chloride

The reaction of arylstannanes ArSnR3 with unfunctionalised alkynes was found to proceed in the presence of a rhodium catalyst and a catalytic amount of zinc chloride to give ortho-alkenylarylstannanes with high selectivity in high yields. The catalytic cycle is very unique, consisting of three transmetalation steps, from Sn to Rh, Rh to Zn, and Zn to Sn, in addition to arylrhodation of alkyne followed by 1,4-migration of Rh from 2-arylalkenyl carbon to ortho-alkenylaryl carbon.

Rhodium-catalyzed decarbonylative cycloadditions of 1H-indene-1,2,3-triones and alkynes via direct C–C bond activation: divergent synthesis of indenones and quinones

A one-step preparation method for indenone and quinone derivatives via rhodium-mediated [5 + 2 − 2] and [5 + 2 − 1] decarbonylative cycloadditions of 1H-indene-1,2,3-triones and alkynes has been achieved.

Rhodium(i)-catalysed skeletal reorganisation of benzofused spiro[3.3]heptanes via consecutive carbon–carbon bond cleavage

Benzofused spiro[3.3]heptane derivatives undergo skeletal reorganisations involving two consecutive carbon–carbon bond cleavages in the presence of rhodium(i) catalysts to afford substituted naphthalenes.

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.