Rhodium(III)‐Catalyzed Aryl Borrowing Amination of Diaryl Methanols Containing Pyridine‐Directing Groups

Rhodium(III)-catalyzed three-component C(sp2)-H activation for the synthesis of amines

Rhodium-catalyzed three-component C-H bond activation of aromatics with amides and aldehydes to synthesize amines was established. The addition of copper was found to be essential to ensure the high reactivity. The mechanistic studies indicated that key intermediates formed by the transmetallization between rhodium and copper could further promote the addition between 2-(pyridin-2-yl)-phenyl-metal species and imines. A series of densely substituted amines could be conveniently prepared by this one-step, three-component procedure from commercially available substrates via C-H bond activation with water as the only by-product.

Transition-Metal-Catalyzed C-C Bond Formation from C-C Activation

ConspectusC-C single bonds are ubiquitous in organic compounds. The activation and subsequent functionalization of C-C single bonds provide a unique opportunity to synthesize conventionally inaccessible molecules through the rearrangement of carbon skeletons, often with a favorable atom and step economy. However, the C-C bonds are thermodynamically and kinetically inert. Consequently, the activation of C-C bonds is particularly attractive yet challenging in the field of organic chemistry. In the past decade, we sought to develop efficient strategies to carry out transition-metal-catalyzed diverse C-C cleavage/C-C forming reactions and to obtain some insights into the intrinsic reactivities of different C-C bonds. With our efforts, readily available alcohols, carboxylic acids, and ketones served as suitable substrates for the catalytic C-C coupling reactions, which are reviewed in this Account. In 2009, we observed a Ni-catalyzed cross coupling of aryl nitriles with arylboronic esters through C-CN cleavage. Encouraged by these results, we are interested in transition-metal-catalyzed C-C bond activation. Due to their broad availability, we then turned our attention to C-C cleavage of carboxylic acids. Rhodium-catalyzed decarbonylative coupling of carboxylic acids with (hetero)arenes was then achieved through oxidative addition of in situ formed, more reactive mixed anhydrides to Rh(I) without the need for oxidants that are commonly required for the decarboxylative coupling of carboxylic acids. Subsequently, the decarbonylation of more challenging unstrained aryl ketones was realized under Rh catalysis assisted by N-containing directing groups. Following this work, a group exchange of aryl ketones with carboxylic acids was achieved through 2-fold C-C bond cleavage. By employing the chelation strategy, Rh-catalyzed C-C bond activation of secondary benzyl alcohols was also accomplished through β-carbon elimination of the rhodium alcoholate intermediates. The competing oxidation of secondary alcohols to ketones via β-hydrogen elimination of the same intermediates was suppressed as thermodynamically favorable five-membered rhodacycles are formed after β-carbon elimination. Different types of transformations of alcohols, including the Heck-type reaction with alkenes, cross coupling with arylsilanes, and Grignard-type addition with aldehydes or imines, have been achieved, showing the great potential of secondary alcohols in the formation of C-C bonds. These C-C bond-forming reactions are complementary to traditional cross couplings of aryl halides with organometallic reagents. However, these transformations produce small molecules as byproducts. To improve the atom economy, we then investigated C-C bond transformations of strained-ring cyclic compounds. Ni-catalyzed intermolecular cyclization of benzocyclobutenones with alkynes was recently achieved via the uncommon cleavage of the C1-C8 bond by employing a removable blocking strategy. Rh-catalyzed intramolecular annulation of benzocyclobutenols with alkynes was also achieved. In summary, our developments demonstrate the great potential of transition-metal-catalyzed C-C bond activation for the formation of new C-C bonds. To further expand the synthetic utility of C-C bond activation, more efforts are required to expand the substrate scope and to achieve earth-abundant metal-catalyzed transformations.

Copper-Catalyzed Aza-Benzyl Transfer Michael Addition via C-C Bond Cleavage

A non-noble Cu-catalyzed transfer aza-benzyl Michael addition via the C-C bond cleavage of aza-benzyl alcohols has been disclosed. The unstrained C(sp³)-C(sp³) bond of an alcohol was selectively cleaved. This aza-benzyl transfer strategy provides a selective and environmentally benign approach for the C-alkylation of α,β-unsaturated carbonyl compounds that employs readily available alcohols as carbon nucleophiles and is characterized by a wide range of substrates and good to excellent yields.

Ruthenium(II)-Catalyzed Grignard-Type Nucleophilic Addition of C(sp2)-H Bonds to Unactivated Aldehydes

The Grignard-type nucleophilic addition of C(sp²)-H bonds to aldehydes catalyzed by high-oxidation-state transition metal complexes is limited to activated aldehydes. Herein, we report the first example of Grignard-type nucleophilic addition of C(sp²)-H bonds to unactivated aldehydes catalyzed by high-oxidation-state ruthenium(II). The reaction has mild reaction conditions and good functional group tolerance. The corresponding alcohol products are obtained in good to excellent yields.

Dimeric Manganese-Catalyzed Direct Nucleophilic Addition of C(sp2)-H Bonds to Inert Aldehydes

An efficient direct nucleophilic addition reaction of C(sp²)-H bonds to aldehydes catalyzed by a dimeric manganese has been developed. This reaction has a broad range of substrates, and high yields were also obtained with inert aliphatic aldehydes as substrates. A dimeric Mn₂(CO)₈Br₂ was proven to be a more efficient catalyst precursor than the monomeric Mn(CO)₅Br.

Sliver-Catalyzed 1,3-Aza-Benzyl Migration of Allyl Alcohol

Carbon migration of alkenyl alcohols has been recognized as an increasingly viable methodology in organic synthesis. Herein, we disclose a silver-catalyzed 1,3-aza-benzyl migration of allyl alcohols by utilizing chelation-assisted selective cleavage of an unstrained C(sp³)-C(sp³) bond. This approach provides an available, efficient, high atom-economic, and environmentally benign procedure, leading to alkylation products with broad substrate scopes and excellent yields. The migration proceeds via a one-pot, two-step process involving a free-state alkyl metal species.

Alkynyl Borrowing: Silver-Catalyzed Amination of Secondary Propargylic Alcohols via C(sp3)-C(sp) Bond Cleavage

The silver-catalyzed alkynyl borrowing amination of secondary propargyl alcohols via C(sp³)-C(sp) bond cleavage has been developed. This new strategy was based on the β-alkynyl elimination of propargyl alcohols and alkynyl as the borrowing subject. This alkynyl borrowing amination featured high atom economy, wide functional group tolerance, and high efficiency.

Copper-catalyzed transfer methylenation via C(sp3)–C(sp3) bond cleavage of alcohols

Transfer Methylenation: A copper-catalyzed transfer methylenation via the cleavage of unstrained C(sp3)-C(sp3) bonds is developted. This is a de novo report for transfer hydrocarbylation between alcohols and carbonyl compounds.