Diversified Synthesis of Chiral Fluorinated Cyclobutane Derivatives Enabled by Regio- and Enantioselective Hydroboration.

The diversified synthesis of chiral fluorinated cyclobutane derivatives has remained a difficult task in synthetic chemistry. Herein, we present an approach for asymmetric hydroboration and formal hydrodefluorination of gem-difluorinated cyclobutenes through rhodium catalysis, providing chiral gem-difluorinated α-boryl cyclobutanes and monofluorinated cyclobutenes with excellent regio- and enantioselectivity, respectively. The key to the success of the two transformations relies on an efficient, mild and highly selective rhodium-catalyzed asymmetric hydroboration with HBPin (pinacolborane), in which the subsequent addition of a base, and a catalytic amount of palladium in some cases, results in the formation of formal hydrodefluorination products with the four-membered ring retained. The obtained chiral gem-difluorinated α-boryl cyclobutanes are versatile building blocks that provide a platform for the synthesis of enantioenriched fluorinated cyclobutane derivatives to a great diversity.

Nickel-Catalyzed Tunable Enantioconvergence and Kinetic Resolution in the Coupling of Tertiary Cyclobutenols with Arylboroxines.

We report that a nickel catalyst system with a modified 1,1'-spirobiindane-7,7'-diol-phosphoramidite (SPINOL) as the chiral ligand can enable the coupling of tertiary cyclobutenols and arylboroxines in an enantioconvergent manner, providing cyclobutenes with an all-carbon quaternary stereocenter in good yields (up to 84 % yield) with excellent enantioselectivities (up to >99 % ee). Moreover, the catalytic system can be applied in the kinetic resolution of cyclobutenols under slightly modified conditions, giving enantioenriched tertiary cyclobutenols with an s factor of up to >200. The reaction uses free hydroxyl groups as the leaving group without additional activation while the strained ring remains untouched. Preliminary mechanistic studies reveal that the inherent discrepant reactivity of the two enantiomers is the key to the controllable enantioconvergent and kinetic resolution process.

Directing Group-Free Formal Suzuki-Miyaura Coupling of Simple Ketones Enabled by Activation of Unstrained C-C Bonds.

The use of ketones as electrophiles to couple with arylboronic acid derivatives via C-C bond activation has become a significant progress in the area of Suzuki-Miyaura coupling (SMC) reaction, in which a permanent or temporary directing group is often required to promote the activation of the unstrained C-C bond via oxidative addition. Herein, we disclosed the first example of directing group free formal SMC reaction of simple ketones with arylboronates via Rh-catalyzed unstrained C-C bond activation. A wide range of simple ketones, including aryl alkyl ketones, diaryl ketones and aryl perfluoroalkyl ketones, can serve as electrophiles to participate in the SMC reaction with aryl or perfluoroalkyl as the leaving group. The key to the success of this reaction is by means of nucleophilic addition/ß-carbon elimination sequence that can activate the unstrained ketone carbonyl C-C bond without the assistance of directing group.

Selective Activation of Unstrained C(O)-C Bond in Ketone Suzuki-Miyaura Coupling Reaction Enabled by Hydride-Transfer Strategy.

A Rh(I)-catalyzed ketone Suzuki-Miyaura coupling reaction of benzylacetone with arylboronic acid is developed. Selective C(O)-C bond activation, which employs aminopyridine as a temporary directing group and ethyl vinyl ketone as a hydride acceptor, occurs on the alkyl chain containing a ß-position hydrogen. A series of acetophenone products were obtained in yields up to 75%.

Rhodium-catalyzed annulative coupling of N-aryl-2-aminopyridine and propargylic amine via selective C-C and C-H bond activation.

A Rh(iii)-catalyzed/Cu(ii)-mediated cascade reaction between N-aryl-2-aminopyridine and propargylic amine has been developed. Selective C(sp2)-H bond activation and C(sp)-C(sp3) cleavage occurred during the reaction, which was followed by a cyclization reaction to provide an unprecedented synthetic route to form 1,2-disubstituted indoles in yields up to 85%.

Rhodium-Catalyzed Pyridine N-Oxide Assisted Suzuki-Miyaura Coupling Reaction via C(O)-C Bond Activation.

A rhodium-catalyzed Suzuki-Miyaura coupling reaction via C(O)-C bond activation to form 2-benzoylpyridine N-oxide derivatives is reported. Both the C(O)-C(sp2) and C(O)-C(sp3) bond could be activated during the reaction with yields up to 92%. The N-oxide moiety could be employed as a traceless directing group, leading to free pyridine ketones.

Synthesis of 2-Arylindoles by Rhodium-Catalyzed/Copper-Mediated Annulative Coupling of N-Aryl-2-aminopyridines and Propargyl Alcohols via Selective C-H/C-C Activation.

A versatile rhodium-catalyzed/copper-mediated C-H/C-C activation and cascade annulation reaction was described to form 2-arylindole derivatives. Highly selective C-C bond cleavage of γ-substituted tert-propargyl alcohols occurred, together with pyridine-directed ortho C(sp2)-H bond activation, affording a series of 2-arylindoles with yields up to 90%. Subsequent derivations were smoothly conducted to access polyfunctionalized 2-arylindoles, illustrating the potential applications of this method.

Palladium-catalyzed C(carbonyl)-C bond cleavage of amides: a facile access to phenylcarbamate derivatives with alcohols.

A sulfur-containing auxiliary enabled palladium-catalyzed C(carbonyl)-C bond activation of amides was reported to form phenylcarbamate derivatives with alcohols. Both alkyl and benzyl alcohols could be employed well with yields up to 85%. Derivations from phenylcarbamates to ureas and thiocarbamates illustrated the potential applications of this sequential C-C cleavage/C-O coupling reaction.

Palladium-catalyzed direct arylation of phenols with aryl iodides.

An efficient protocol of palladium-catalyzed direct para-arylation of unfunctionalized phenols with aryl iodides under mild conditions was reported. A variety of substrates were applied in this reaction with yields up to 87%.

Copper-catalyzed formation of N,N-dimethyl benzamide from nitrile and DMF under an O2 atmosphere.

Amidation of nitrile with N,N-dimethylformamide (DMF) was catalyzed by Cu2O with 1,10-phenanthroline as a ligand under an oxygen atmosphere. A variety of N,N-dimethyl benzamides were obtained in yields up to 84%.