Jun Takaya

Hydrocarboxylation of Allenes with CO2 Catalyzed by Silyl Pincer-Type Palladium Complex

Tridentate PSiP pincer-type palladium complex-catalyzed hydrocarboxylation of allenes under carbon dioxide to give synthetically useful beta,gamma-unsaturated carboxylic acids was developed. This novel CO2-fixation reaction is thought to proceed through the catalytic generation of sigma-allyl palladium species via hydropalladation of allenes, followed by its regioselective nucleophilic addition to CO2 in the presence of an appropriate reducing agent. The reaction is successfully applied to various allenes bearing functional groups such as ester, carbamate, ketone, and alkene, showing high synthetic utility of this protocol.

Rhodium(I)-Catalyzed Direct Carboxylation of Arenes with CO2 via Chelation-Assisted C−H Bond Activation

Rh-catalyzed direct carboxylation of unactivated aryl C-H bond under atmospheric pressure of carbon dioxide was realized via chelation-assisted C-H activation for the first time. Variously substituted and functionalized 2-arylpyridines and 1-arylpyrazoles underwent the carboxylation in the presence of the rhodium catalyst and a stoichiometric methylating reagent, AlMe(2)(OMe), to give carboxylated products in good yields. The catalysis is proposed to consist of methylrhodium(I) species as the key intermediate, which undergoes C-H activation to afford rhodium(III), followed by reductive elimination of methane to give nucleophilic arylrhodium(I). This approach demonstrates promising application of C-H bond activation strategy in the field of carbon dioxide fixation.

Copper(I)-Catalyzed Carboxylation of Aryl- and Alkenylboronic Esters

The copper(I)-catalyzed carboxylation reaction of aryl- and alkenylboronic esters proceeded smoothly under CO(2) to give the corresponding carboxylic acid in good yield. This reaction showed wide generality with higher functional group tolerance compared to the corresponding Rh(I)-catalyzed reaction.

Palladium(II)-Catalyzed Direct Carboxylation of Alkenyl C–H Bonds with CO2

Pd-catalyzed direct carboxylation of alkenyl C-H bonds with carbon dioxide was realized for the first time. Treatment of 2-hydroxystyrenes and a catalytic amount of Pd(OAc)2 with Cs2CO3 under atmospheric pressure of CO2 afforded corresponding coumarins in good yield. Furthermore, isolation of the key alkenylpalladium intermediate via C-H bond cleavage was achieved. The reaction was proposed to undergo reversible nucleophilic addition of the alkenylpalladium intermediate to CO2.

Efficient one-to-one coupling of easily available 1,3-dienes with carbon dioxide.

An efficient one-to-one coupling reaction of atmospheric pressure carbon dioxide with 1,3-dienes is realized for the first time through PSiP-pincer type palladium-catalyzed hydrocarboxylation. The reaction is applicable to various 1,3-dienes including easily available chemical feedstock such as 1,3-butadiene and isoprene. This protocol affords a highly useful method for the synthesis of β,γ-unsaturated carboxylic acid derivatives from CO(2).

Efficient Synthesis of Diborylalkenes from Alkenes and Diboron by a New PSiP-Pincer Palladium-Catalyzed Dehydrogenative Borylation

The efficient synthesis of various diborylalkenes such as 1,1-, trans-1,2-, and cyclic 1,2-diborylalkenes from alkenes and diboron was achieved for the first time. Selective preparation of di- and monoborylalkenes was also realized by the appropriate choice of reaction conditions. The reaction was found to proceed via a new mechanism of dehydrogenative borylation through a monoborylpalladium complex bearing an anionic PSiP-pincer ligand as a key intermediate, which realized the efficient borylation without sacrificial hydroboration or hydrogenation of the alkene.

Two reversible σ-bond metathesis pathways for boron-palladium bond formation: selective synthesis of isomeric five-coordinate borylpalladium complexes.

Two reversible σ-bond metathesis pathways for B-B bond activation to give borylpalladium complexes are demonstrated in the reaction of η(2)-(Si-H)Pd(0) complexes with B(2)pin(2). These two pathways are connected by fluxional behavior of the Si-H bond and can be efficiently controlled by the appropriate choice of phosphine ligand, enabling the selective synthesis of two types of five-coordinate borylpalladium complexes.

Use of Formate Salts as a Hydride and a CO2 Source in PGeP-Palladium Complex-Catalyzed Hydrocarboxylation of Allenes

Use of formate salts as a hydride as well as a CO2 source was achieved in a PGeP-palladium complex-catalyzed hydrocarboxylation of allenes through a highly efficient decarboxylation-carboxylation process. This reaction proceeds under mild conditions and provides an alternative strategy for utilizing formate salts as a C1 source.

Silyl ligand mediated reversible β-hydrogen elimination and hydrometalation at palladium.

The mechanism and origin of the facile β-hydrogen elimination and hydrometalation of a palladium complex bearing a phenylene-bridged PSiP pincer ligand are clarified. Experimental and theoretical studies demonstrate a new mechanism for β-hydrogen elimination and hydrometalation mediated by a silyl ligand at palladium, which enables direct interconversion between an ethylpalladium(II) complex and an η(2) -(Si-H)palladium(0) complex without formation of a square-planar palladium(II) hydride intermediate. The flexibility of the PSiP pincer ligand enables it to act as an efficient scaffold to deliver the hydrogen atom as a hydride ligand.

Synthesis, Structure, and Catalysis of Palladium Complexes Bearing a Group 13 Metalloligand: Remarkable Effect of an Aluminum-Metalloligand in Hydrosilylation of CO2

Efficient synthesis and catalysis of a series of palladium complexes having a group 13 metalloligand (Al, Ga, In) are reported utilizing 6,6″-bis(phosphino)terpyridine as a new scaffold for Pd-E bonds (E = Al, Ga, In). Systematic investigation revealed unique characteristics of the Al-metalloligand in both structure and reactivity, which exhibited the highest catalytic activity for hydrosilylation of CO2 ever reported (TOF = 19 300 h-1). This study demonstrated fine-tuning of catalyst activity by the precisely designed metalloligand is a promising approach for new catalyst development in synthetic organometallic chemistry.