Phosphoramidate-Assisted Alkyne Activation: Probing the Mechanism of Proton Shuttling in a N,O-Chelated Cp*Ir(III) Complex

The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(μ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

Metal-Ligand-Anion Cooperation in C-H Bond Formation at Platinum(II)

Thermolysis of [^H(BPI)Pt(CH₃)][OTf] (BPI = 1,3-bis(2-(4-tert-butyl)pyridylimino)isoindole) to release methane and form (BPI)Pt(OTf) is reported. Kinetic, mechanistic, and computational studies point to an unusual anion-assisted pathway that obviates the need for a higher oxidation state intermediate to couple the metal-bound methyl group with the ligand-bound hydrogen. Leveraging this insight, a triflimide derivative of the (BPI)Pt complex was shown to activate benzene, highlighting the role of the counteranion in controlling the activity of these complexes.

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes

The cooperative action of the acetate ligand, the 2-pyridyl sulfonyl (SO2Py) directing group on the alkyne substrate, and the palladium catalyst has been shown to be crucial for controlling reactivity, regioselectivity, and stereoselectivity in the acetoxylation of unsymmetrical internal alkynes under mild reaction conditions. The corresponding alkenyl acetates were obtained in good yields with complete levels of β-regioselectivity and anti-acetoxypalladation stereocontrol. Experimental and computational analyses provide insight into the reasons behind this delicate interplay between the ligand, directing group, and the metal in the reaction mechanism. In fact, these studies unveil the multiple important roles of the acetate ligand in the coordination sphere at the Pd center: (i) it brings the acetic acid reagent into close proximity to the metal to allow the simultaneous activation of the alkyne and the acetic acid, (ii) it serves as an inner-sphere base while enhancing the nucleophilicity of the acid, and (iii) it acts as an intramolecular acid to facilitate protodemetalation and regeneration of the catalyst. Further insight into the origin of the observed regiocontrol is provided by the mapping of potential energy profiles and distortion-interaction analysis.

Recent Advances in the Synthesis and Reactivity of Transition Metal σ-Borane/Borate Complexes

The coordination of an element-element σ bond to a transition metal (TM) is both a fundamentally intriguing binding mode and of critical importance to metal-mediated bond activation mechanisms and catalysis, particularly the hotly contested field of C-H activation. TM σ complexes of dihydrogen (i.e., H-H) and silanes (H-SiR₃) have been extensively studied, the latter being of interest as models for the (generally unstable and unisolable) σ complexes of alkanes (i.e., H-CR₃). TM σ complexes of hydroboranes and hydroborates (i.e., H-BR₂, H-BR₃, (H-)₂BR₂) are somewhat less well studied but similarly have relevance to catalytic borylation reactions that are of high current interest to organic synthesis. Our two research groups have made significant contributions to elaborating the family of σ-borane/-borate complexes using two distinct approaches: while the Ghosh group generally starts from hydrogen-rich tetracoordinate boron species such as borates, the Braunschweig group starts from hypovalent and/or hypocoordinate boron building blocks. Through these two approaches, a wide range of species containing one or two σ-bound B-H ligands have been prepared, some with additional chelating donor sites. Over the past 2 years, the body of work on σ-borane/-borate complexes from our two research groups has significantly expanded, with a combined nine published articles in 2019-2020 alone. Very recent work from the Braunschweig group has led to the synthesis of the first bis(σ)-borane complexes of group 6 metals, as well as the synthesis of a series of novel bis(σ)-borane and bis(σ)-borate complexes of ruthenium and iridium, the former being useful precursors for pentacoordinate borylene complexes of Ru. Recent work from the Ghosh group has uncovered a remarkable diversity of structures with σ(B-H)-bound ligands from the combination of borohydrides and nitrogen/chalcogen-containing groups and heterocycles. These reactions, while in some cases producing conventional scorpionate-type chelating products, more frequently undergo fascinating rearrangements with unpredictable outcomes. This Account aims to highlight this recent acceleration of research progress in this area, particularly the distinct but related approaches of-and complexes produced by-our two research groups, in addition to relevant works from other groups where appropriate.

Five-Membered Ruthenacycles: Ligand-Assisted Alkyne Insertion into 1,3-N,S-Chelated Ruthenium Borate Species

Building upon previous work, the chemistry of [(η⁶ -p-cymene)Ru{P(OMe)₂ OR}Cl₂ ], (R=H or Me) has been extended with [H₂ B(mbz)₂ ]^- (mbz=2-mercaptobenzothiazolyl) using different Ru precursors and borate ligands. As a result, a series of 1,3-N,S-chelated ruthenium borate complexes, for example, [(κ² -N,S-L)PR₃ Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (2 a-d and 2 a'-d'; R=Ph, Cy, OMe or OPh and L=C₅ H₄ NS or C₇ H₄ NS₂ ) and [Ru{κ³ -H,S,S'-H₂ B(L)₂ }₂ ], (3: L=C₅ H₄ NS, 3': L=C₇ H₄ NS₂ ) were isolated upon treatment of [(η⁶ -p-cymene)RuCl₂ PR₃ ], 1 a-d (R=Ph, Cy, OMe or OPh) with [H₂ B(mp)₂ ]^- or [H₂ B(mbz)₂ ]^- ligands (mp=2-mercaptopyridyl). All the Ru borate complexes, 2 a-d and 2 a'-d' are stabilized by phosphine/phosphite and hemilabile N,S-chelating ligands. Treatment of these Ru borate species, 2 a'-c' with various terminal alkynes yielded two different types of five-membered ruthenacycle species, namely [PR₃ {C₇ H₄ S₂ -(E)-N-C=CH(R')}Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (4-4'; R=Ph and R'=CO₂ Me or C₆ H₄ NO₂ ; L=C₇ H₄ NS₂ ) and [PR₃ {C₇ H₄ NS-(E)-S-C=CH(R')}Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (5-5', 6 and 7; R=Ph, Cy or OMe and R'=CO₂ Me or C₆ H₄ NO₂ ; L=C₇ H₄ NS₂ ). All these five-membered ruthenacycle species contain an exocyclic C=C moiety, presumably formed by the insertion of a terminal alkyne into the Ru-N and Ru-S bonds. The new species have been characterized spectroscopically and the structures were further confirmed by single-crystal X-ray diffraction analysis. Theoretical studies and chemical-bonding analyses established that charge transfer occurs from phosphorus to ruthenium center following the trend PCy₃ <PPh₃ <P(OPh)₃ <P(OMe)₃ .