A zirconium-catalyzed hydroaminoalkylation of alkynes to access α,β,γ-substituted allylic amines in an atomeconomic fashion is reported. The reaction is compatible with N-(trimethylsilyl)benzylamine and a variety of N-benzylaniline substrates, with the latter giving the allylic amine as the sole organic product. Various internal alkynes with electron-withdrawing and electron-donating substituents were tolerated. Model intermediates of the reaction were synthesized and structurally characterized. Stoichiometric studies on key intermediates revealed that the open coordination sphere at zirconium, imparted by the tethered bis(ureate) ligand, is crucial for the coordination of neutral donors. These complexes may serve as models for the inner-sphere protonolysis reactions required for catalytic turnover.

A thorough experimental examination of a series of half-sandwich Ru indenyl complexes [Ru­(η5-indenyl)­(PPh2)­(L)­(PPh3) (L = PPh2H, CO, NCPh)] in the catalytic hydrophosphination of tert-butyl acrylate by diphenylphosphine provides valuable lessons for the design of active and robust catalysts for this important P–C bond-forming reaction. Evidence for each fundamental step in the relevant catalytic cycles was gathered from reaction monitoring (1H and 31P NMR), kinetic analyses, stoichiometric control reactions, and the isolation and spectroscopic identification of key intermediates, catalyst deactivation products, and off-cycle byproducts. For L = PPh2H, two distinct catalytic cycles each rely on the outer-sphere, conjugate addition of the Ru–PPh2 ligand at the electron-deficient alkene. The cycles differ in their P–H activation steps (intra- vs intermolecular) but are connected by a common resting state [Ru­(η5-indenyl)­(PPh2)P 2, where P is the hydrophosphination product Ph2PCH2CH2CO2Bu t ]. The complex with L = CO is inert to substitution by PPh2H, which precludes one of the two conjugate addition catalytic cycles. This catalyst provides critical evidence for the conjugate addition step in the form of a spectroscopically identified phospha-enolate intermediate, a long-lived species that participates in competing, off-cycle alkene oligomerization. Nitrile lability allows the complex with L = NCPh to access the same two conjugate addition cycles observed for the complex with L = PPh2H. However, the “free” benzonitrile both inhibits catalysis and participates in the formation of a deactivation product containing the 1-azaallyl fragment, which has been isolated and crystallographically characterized. Collectively, these results indicate a surprising complexity that can arise from a simple mechanistic premise for metal-mediated hydrophosphination, and demonstrate a variety of impacts of ancillary ligands on catalysis. They highlight design features that allowed us to develop a half-sandwich Ru Cp* catalyst [Ru­(η5-Cp*)­(PPh2)­(PPh2H)2] that exhibits a 30-fold increase in hydrophosphination activity.

Efforts to synthesize a low-coordinate zirconaaziridine complex supported by a bis­(ureate) ligand resulted in the formation of bridging aminoalkylidene 2. Complex 2 is a rare example of an aminoalkylidene complex featuring a μ­(η2:η2)-N,C binding mode. This complex was fully characterized by single-crystal X-ray diffraction, NMR spectroscopy, and LIFDI-MS. Analysis of the natural localized molecular orbitals (NLMOs) showed a σ-bonding interaction between the alkylidene carbon and each zirconium center with significant delocalization of the Zr1–C1 σ bond into Zr2, thereby explaining the significant differences in the Zr–C bond lengths observed in the solid state. A solution-phase equilibrium between complex 2 and the putative zirconaaziridine species was supported by evaluating the reactivity of 2 in alkyne hydroaminoalkylation. The reaction of diphenylacetylene with complex 2 produced a five-membered metallacycle product, a key intermediate in alkyne hydroaminoalkylation. This reaction was greatly accelerated in the presence of pyridine, suggesting that the presence of neutral donors is essential to favor the formation of reactive zirconaaziridine complexes. In addition, the catalytic activity of complex 2 in the benchmark hydroaminoalkylation reaction of diphenylacetylene with N-benzylaniline was established.

The chemistry of group 4 elements has historically been dominated by the use of metallocene complexes in stoichiometric transformations. While this area continues to be widely explored, the development of non-cyclopentadienyl-based ligands has substantially contributed to the increase in applicability of group 4 metals in catalysis during the last 15 years. In addition to their application in polymerization catalysis, zirconium complexes supported by nitrogen-based anionic ligands have been useful as catalysts for a variety of E–H functionalization reactions. Two particular zirconium systems, (1) bis(ureate)-supported zirconium complexes reported by the Schafer group and (2) tripodal triamidoamine-supported zirconium complexes employed by the Waterman group, promote a variety of E–E′ bond-forming catalytic processes upon E/E′–H activation. The former system has been exploited for catalytic hydroamination, hydroaminoalkylation, and hydroalkynylation (alkyne dimerization) reactions, and the latter system has been used in hydrophosphination and dehydrocoupling reactions. This Perspective focuses on the bountiful reactivity of these catalytic systems with an emphasis on mechanistic insights of these transformations gained from a combination of kinetic analyses, isolation of reaction intermediates, stoichiometric reactivity studies, and computational calculations. The insights generated from this approach have revealed a series of features that enable catalytic E–E′ bond formation and that can contribute to guided efforts in early transition-metal ligand design. For the zirconium bis(ureate) system, the expanded coordination sphere promoted by the multidentate ligand facilitates the coordination of neutral amine donors that are essential for realizing innersphere E–H bond formation in hydrofunctionalization catalysis. For the triamidoamine-supported zirconium complexes, the noninnocent tripodal ligand mediates E–H bond formation/activation during catalysis. For both zirconium systems, the highly ionic nature of the chelating ligands has been shown to induce significant polarization of reactive Zr–E bonds (E = N, C, P). This bond polarization translates into exceptionally reactive Zr–E bonds, akin to those of rare earth metals, enabling σ-bond insertion reactions for E–E′ bond formation. The goal of this Perspective is to highlight examples where compelling evidence has been gathered demonstrating ligand design effects to promote zirconium catalysis. Lessons learned from the featured zirconium systems aim to highlight ligand design features that will advance new directions in early transition-metal catalysis.

Highly conjugated perinones were evaluated as proligands with [Ru(η 6 -arene)Cl 2 ] 2 precursors (arene = p-cymene or benzene). None of them, except itaco-perinone (IP) bearing one exocyclic methylene group, were able to form coordination compounds. Expected η 1 -coordination through the lone pair of the nitrogen or oxygen atoms of the perinone did not occur. Instead a deprotonation reaction involving the exocyclic methylene was observed and the corresponding [Ru(η 6 -arene)- [a] 3494 (η 3 -IP)Cl] complex was isolated in moderate yields. Mechanistic studies revealed that the base-promoted isomerization of itacoperinone to citraco-perinone prevented higher yields in the synthesis of the allylic complexes. Additionally, it was observed that IP can dimerize through the exocyclic methylene group, indicating high reactivity of this carbon-carbon double bond. Electronic absorption and emission properties of the perinones and organometallic compounds were studied.