Aerobic Alcohol Oxidation with Cationic Palladium Complexes:  Insights into Catalyst Design and Decomposition

Abstract

Aerobic alcohol oxidation catalyzed by the newly synthesized dimeric [(neocuproine)Pd(µ-OAc)] 2 -(OTf) 2 (1; neocuproine ) 2,9-dimethyl-1,10-phenanthroline) proceeds under exceptionally mild conditions (room temperature, ambient air) compared to those required for the previously reported monomeric diacetate analogue (neocuproine)Pd(OAc) 2 (2) and the monomeric ditriflate analogue (neocuproine)Pd-(MeCN) 2 (OTf) 2 (3). An unprecedented initial turnover frequency (TOF i ) for such mild conditions (78 (Pd atom) -1 h -1 ) is observed with catalyst 1; however, competitive oxidation of a methyl group on the ligand to a carboxylate group results in catalyst inactivation. During the study of 1, we isolated for the first time [(neocuproine)Pd(µ-OH)] 2 (OTf) 2 (4), a possible intermediate in the catalytic cycle, and we report its crystal structure and catalytic activity herein.

Rhodium Derivatives of Peroxoboronic Acids and Peroxoboric Acid: Formation of Metallatrioxaborolanes from an η2‐Peroxo Complex

“…[3] h 2 -Peroxo palladium species might also play a key role in the catalytic homocoupling of aryl boronic acids or aryl boronic esters in the presence of air.…”

“…Although 2 is insoluble in most organic solvents, treatment with methanol resulted in alcoholysis, and peroxo complex 1 and B(OMe) 3 were generated (Scheme 1). If the reaction is performed with the isotopologue 2', the corresponding 18 O 2 isotopologue trans-[Rh(h 2 - 18 O 2 )(4-C 5 F 4 N)-(CNtBu)(PEt 3 ) 2 ] (1') is formed as the sole organometallic product.…”

“…To the best of our knowledge, these acids do not exist in pure form and have not been identified spectroscopically, and metal derivatives thereof are also without precedent. 3 , but the generation of peroxoboronic acid B(OH) 2 (OOH) in low concentration has also been deduced by evaluation of equilibrium constants. [27] Considering the capability of, for instance, sodium perborate Na 2 [B 2 (OH) 4 (m-OO) 2 ] as oxidizing agent as well as rhodadioxolanes as oxygenating agents, [1e, 28, 29] our results should stimulate further studies on the properties of neutral metallaperborates and -perboronates to elucidate their stability and their potential in oxidation and oxygenation reactions.…”

Stepwise Oxygenation of Pinacolborane by a Rhodiumperoxo Complex: Detection of an Intermediate Metal Borate and Perborate

“…[1] One of the essential objectives for understanding the mechanistic details is to study the properties of peroxo compounds, which often play a key role in these processes. [1][2][3] h 2 -Peroxo complexes are crucial intermediates in the palladium-catalyzed homocoupling of aryl boronic acids or aryl boronic esters in the presence of O 2 . [4,5] Thus, the palladium(II)-h 2 -peroxo complex [Pd(h 2 -O 2 )(PPh 3 ) 2 ] is transformed in the presence of ArB(OH) 2 (Ar = 4-Z-C 6 H 4 , where Z = MeO, H, CN) into peroxoborates such as [PdAr{h 1 -OOB(OH) 2 }(PPh 3 ) 2 ], but the latter species have not been identified.…”

“…[4,5] Thus, the palladium(II)-h 2 -peroxo complex [Pd(h 2 -O 2 )(PPh 3 ) 2 ] is transformed in the presence of ArB(OH) 2 (Ar = 4-Z-C 6 H 4 , where Z = MeO, H, CN) into peroxoborates such as [PdAr{h 1 -OOB(OH) 2 }(PPh 3 ) 2 ], but the latter species have not been identified. [4] It has been suggested that the hydroxo complex [PdAr(OH)(PPh 3 ) 2 ] as well as the putative HOOB(OH) 2 species are generated in the presence of water. Furthermore, it is presumed that the perboronic acid immediately hydrolyzes to H 2 O 2 and B(OH) 3 .…”