Transmetalation of boronic acids and their derivatives: mechanistic elucidation and relevance to catalysis

The Suzuki-Miyaura reaction (the cross-coupling reaction of boronic acids with organic halides catalysed by Pd complexes) has been recognised as a useful synthetic organic reaction that forms a C(sp²)-C(sp²) bond. The catalytic cycle of the reaction involves the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. It migrates the aryl and alkenyl groups of boronic acid to Pd and produces a Pd-C bond. Many studies have investigated the mechanism of transmetalation. They elucidated the mechanism of the organometallic reaction and its role as a fundamental step in catalytic reactions. This perspective reviews studies on the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. Emphasis was laid on the structures and chemical properties of the intermediate Pd complexes and the effects of OH^- on the pathways of the catalytic Suzuki-Miyaura reaction. The reactions of arylboronic acids with Rh(I)-OH complexes were investigated, which are relevant to the mechanism of Rh-catalysed addition of aryl boronic acids to enones and aldehydes. Recent studies on the transmetalation of boronic acids with other late transition metals such as Fe(II), Co(I), Pt(II), Au(III), and Au(I) are presented with the related catalytic reactions and their utilisation in the synthesis of aromatic molecules and π-conjugated materials.

Diruthenium aryl compounds - tuning of electrochemical responses and solubility

Reported herein are the two new series of diruthenium aryl compounds: Ru₂(DiMeOap)₄(Ar) (1a-6a) (DiMeOap = 2-(3,5-dimethoxyanilino)pyridinate) and Ru₂(m-ⁱPrOap)₄(Ar) (1b-5b) (m-ⁱPrOap = 2-(3-iso-propoxyanilino)pyridinate), prepared through the lithium-halogen exchange reaction with a variety of aryl halides (Ar = C₆H₄-4-NMe₂ (1), C₆H₄-4-^tBu (2), C₆H₄-4-OMe (3), C₆H₃-3,5-(OMe)₂ (4), C₆H₄-4-CF₃ (5), C₆H₅ (6)). The molecular structures of these compounds were established with X-ray diffraction studies. Additionally, these compounds were characterized using electronic absorption and voltammetric techniques. Compounds 1a-6a and 1b-5b are all in the Ru₂⁵⁺ oxidation state, with a ground state configuration of σ²π⁴δ²(π*δ*)³ (S = 3/2). Use of the modified ap ligands (ap') resulted in moderate increases of product yield when compared to the unsubstituted Ru₂(ap)₄(Ar) (ap = 2-anilinopyridinate) series. Comparisons of the electrochemical properties of 1a-6a and 1b-5b against the Ru₂(ap')Cl starting material reveals the addition of the aryl ligand cathodically shifted the Ru₂^6+/5+ oxidation and Ru₂^5+/4+ reduction potentials. These oxidation and reductions potentials are also strongly dependent on the p-substituent of the axial aryl ligands.

Dirhodium(II)/Xantphos-Catalyzed Relay Carbene Insertion and Allylic Alkylation Process: Reaction Development and Mechanistic Insights

Although dirhodium-catalyzed multicomponent reactions of diazo compounds, nucleophiles and electrophiles have achieved great advance in organic synthesis, the introduction of allylic moiety as the third component via allylic metal intermediate remains a formidable challenge in this area. Herein, an attractive three-component reaction of readily accessible amines, diazo compounds, and allylic compounds enabled by a novel dirhodium(II)/Xantphos catalysis is disclosed, affording various architecturally complex and functionally diverse α-quaternary α-amino acid derivatives in good yields with high atom and step economy. Mechanistic studies indicate that the transformation is achieved through a relay dirhodium(II)-catalyzed carbene insertion and allylic alkylation process, in which the catalytic properties of dirhodium are effectively modified by the coordination with Xantphos, leading to good activity in the catalytic allylic alkylation process.

Axial Ligand Coordination to the C-H Amination Catalyst Rh2(esp)2: A Structural and Spectroscopic Study

The compound Rh2(esp)2 (esp = α,α,α',α'-tetramethyl-1,3-benzenediproponoate) is the most generally effective catalyst for nitrenoid amination of C-H bonds. However, much of its fundamental coordination chemistry is unknown. In this work, we study the effects of axial ligand coordination to the catalyst Rh2(esp)2. We report here crystal structures, cyclic voltammetry, UV-vis, IR, Raman, and (1)H NMR spectra for the complexes Rh2(esp)2L2 where L = pyridine, 3-picoline, 2,6-lutidine, acetonitrile, and methanol. The compounds all show well-defined π* → σ* electronic transitions in the 16500 to 20500 cm(-1) range, and Rh-Rh stretching vibrations in the range from 304 to 322 cm(-1). Taking these data into account we find that the strength of axial ligand binding to Rh2(esp)2 increases in the series CH3OH ∼ 2,6-lutidine < CH3CN < 3-methylpyridine ∼ pyridine. Quasi-reversible Rh2(4+/5+) redox waves are only obtained when either acetonitrile or no axial ligand is present. In the presence of pyridines, irreversible oxidation waves are observed, suggesting that these ligands destabilize the Rh2 complex under oxidative conditions.