Reactivity of Scandium Terminal Imido Complex toward Boranes: C(sp3)–H Bond Borylation and B–O Bond Cleavage

Terminal dysprosium and holmium organoimides

Terminal rare-earth-metal imide complexes TptBu,MeLn(NC6H3iPr2-2,6)(dmap) of the mid-late rare-earth elements dysprosium and holmium were synthesized via double methane elimination of Lewis acid stabilized dialkyl precursors TptBu,MeLnMe(GaMe4) with primary aniline derivative H2NC6H3iPr2-2,6 (H2NAriPr). Exploiting the weaker Ln-CH3⋯[GaMe3] interaction compared to the aluminium congener, addition of the aniline derivative leads to the mixed methyl/anilido species TptBu,MeLnMe(HNAriPr) which readily eliminate methane after being exposed to the Lewis base DMAP ([double bond, length as m-dash]N,N-dimethyl-4-aminopyridine). Under the same conditions, [AlMe3]-stabilized dimethyl rare-earth-metal complexes transform immediately to Lewis acid bridged imides TptBu,MeLn(μ2-NC6H3Me2-2,6)(μ2-Me)AlMe2 (Ln = Dy, Ho). DMAP/THF donor exchange is accomplished by treatment of TptBu,MeLn(NC6H3iPr2-2,6)(dmap) with 9-BBN in THF while the terminal imides readily insert carbon dioxide to afford carbamate complexes.

First-Row d-Block Element-Catalyzed Carbon-Boron Bond Formation and Related Processes

Organoboron reagents represent a unique class of compounds because of their utility in modern synthetic organic chemistry, often affording unprecedented reactivity. The transformation of the carbon-boron bond into a carbon-X (X = C, N, and O) bond in a stereocontrolled fashion has become invaluable in medicinal chemistry, agrochemistry, and natural products chemistry as well as materials science. Over the past decade, first-row d-block transition metals have become increasingly widely used as catalysts for the formation of a carbon-boron bond, a transformation traditionally catalyzed by expensive precious metals. This recent focus on alternative transition metals has enabled growth in fundamental methods in organoboron chemistry. This review surveys the current state-of-the-art in the use of first-row d-block element-based catalysts for the formation of carbon-boron bonds.

Chemoselectivity for B-O and B-H Bond Cleavage by Pincer-Type Phosphorus Compounds: Theoretical and Experimental Studies

Selective cleavage of the B-O bond or B-H bond in HBpin can be achieved by adjusting the pincer ligand of a phosphorus(III) compound guided by a combination of theoretical prediction and experimental verification. Theoretical calculations reveal that a pincer-type phosphorus compound with an [ONO]^3- ligand reacts with HBpin, leading to cleavage of the stronger B-O bonds (ΔG°^⧧ = 23.2 kcal mol^-1) rather than the weaker B-H bond (ΔG^°⧧ = 26.4 kcal mol^-1). A pincer-type phosphorus compound with a [NNN]^3- ligand reacts with HBpin, leading to the weaker B-H bond cleavage (ΔG°^⧧ = 16.2 kcal mol^-1) rather than cleavage of the stronger B-O bond (ΔG°^⧧ = 33.0 kcal mol^-1). The theoretical prediction for B-O bond cleavage was verified experimentally, and the final products were characterized by NMR, HRMS, and single-crystal X-ray diffraction. The chemoselectivity of B-O bond cleavage was also observed in the presence of B-C or B-B bonds in borane substrates.

Low-Symmetry β-Diketimine Aryloxide Rare-Earth Complexes: Flexible, Reactive, and Selective

We report the synthesis, characterization, and reactivity of a new low-symmetry β-diketimine featuring a pendant amino(methyl)phenol donor and its corresponding heteroleptic rare-earth (RE) complexes. This includes the first structurally characterized examples of alcoholysis and insertion from an isolated RE^III amide in a β-diketimine framework. The flexible methylene linkage leads to RE^III complexes with tunable dynamic solution behavior that defines their stoichiometric and catalytic reactivity. The addition of a strong neutral donor ligand, tricyclohexylphosphine oxide, suppresses a prevalent catalyst degradation pathway (base-promoted elimination) and dramatically enhances the catalyst performance in the stereospecific ring-opening polymerization of rac-β-butyrolactone. Our results further demonstrate the importance of ligand reorganization in the stoichiometric and catalytic activity of RE^III ions.

Rare-earth metal and actinide organoimide chemistry

Elaborate synthesis schemes pave the way to f-element and group 3 complexes with multiply bonded imido ligands displaying intriguing reactivity.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Scandium Terminal Imido Chemistry

Research into transition metal complexes bearing multiply bonded main-group ligands has developed into a thriving and fruitful field over the past half century. These complexes, featuring terminal M═E/M≡E (M = transition metal; E = main-group element) multiple bonds, exhibit unique structural properties as well as rich reactivity, which render them attractive targets for inorganic/organometallic chemists as well as indispensable tools for organic/catalytic chemists. This fact has been highlighted by their widespread applications in organic synthesis, for example, as olefin metathesis catalysts. In the ongoing renaissance of transition metal-ligand multiple-bonding chemistry, there have been reports of M═E/M≡E interactions for the majority of the metallic elements of the periodic table, even some actinide metals. In stark contrast, the largest subgroup of the periodic table, rare-earth metals (Ln = Sc, Y, and lanthanides), have been excluded from this upsurge. Indeed, the synthesis of terminal Ln═E/Ln≡E multiple-bonding species lagged behind that of the transition metal and actinide congeners for decades. Although these species had been pursued since the discovery of a rare-earth metal bridging imide in 1991, such a terminal (nonpincer/bridging hapticities) Ln═E/Ln≡E bond species was not obtained until 2010. The scarcity is mainly attributed to the energy mismatch between the frontier orbitals of the metal and the ligand atoms. This renders the putative terminal Ln═E/Ln≡E bonds extremely reactive, thus resulting in the formation of aggregates and/or reaction with the ligand/environment, quenching the multiple-bond character. In 2010, the stalemate was broken by the isolation and structural characterization of the first rare-earth metal terminal imide-a scandium terminal imide-by our group. The double-bond character of the Sc═N bond was unequivocally confirmed by single-crystal X-ray diffraction. Theoretical investigations revealed the presence of two p-d π bonds between the scandium ion and the nitrogen atom of the imido ligand and showed that the dianionic [NR]^2- imido ligand acts as a 2σ,4π electron donor. Subsequent studies of the scandium terminal imides revealed highly versatile and intriguing reactivity of the Sc═N bond. This included cycloaddition toward various unsaturated bonds, C-H/Si-H/B-H bond activations and catalytic hydrosilylation, dehydrofluorination of fluoro-substituted benzenes/alkanes, CO₂ and H₂ activations, activation of elemental selenium, coordination with other transition metal halides, etc. Since our initial success in 2010, and with contributions from us and across the community, this young, vibrant research field has rapidly flourished into one of the most active frontiers of rare-earth metal chemistry. The prospect of extending Ln═N chemistry to other rare-earth metals and/or different metal oxidation states, as well as exploiting their stoichiometric and catalytic reactivities, continues to attract research effort. Herein we present an account of our investigations into scandium terminal imido chemistry as a timely summary, in the hope that our studies will be of interest to this readership.

Lewis acid catalysis: regioselective hydroboration of alkynes and alkenes promoted by scandium triflate

An unprecedented scandium-catalysed hydroboration of alkynes and alkenes has been achieved with HBpin in presence of catalytic amount of NaBHEt₃.