Tetranuclear Strontium and Barium Siloxide/Amide Clusters in Metal-Ligand Cooperative Catalysis

Ytterbium(II) Complex-Catalyzed Selective Single and Double Hydrophosphination of 1,3-Enynes

1,3-Enynes with conjugated alkene and alkyne moieties are attractive building blocks in synthetic chemistry. However, neither 4,1-hydrophosphination nor dihydrophosphination of 1,3-enynes has been reported. In this paper, the divalent ytterbium and calcium amide complexes supported by silaimine-functionalized cyclopentadienyl ligands (C5Me4-Si(L)=NR) were developed, which successfully catalyzed the efficient single and double hydrophosphination of 1,3-enynes with diarylphosphines. The hydrophosphination reactions selectively produced homoallenyl phosphines and (E)-propenylene diphosphines, respectively. This work demonstrated the potential of hemilabile silaimine-Cp ligands in the supporting the efficient and selective rare- and alkaline-earth catalysts.

Alpha-metalated N,N-dimethylbenzylamine rare-earth metal complexes and their catalytic applications

This perspective summarizes our group's extensive research in the realm of organometallic lanthanide complexes, while also placing the catalytic reactions supported by these species within the context of known lanthanide catalysis worldwide, with a specific focus on phosphorus-based catalytic reactions such as intermolecular hydrophosphination and hydrophosphinylation. α-Metalated N,N-dimethylbenzylamine ligands have been utilized to generate homoleptic lanthanide complexes, which have subsequently proven to be highly active lanthanum-based catalysts. The main goal of our research program has been to enhance the catalytic efficiency of lanthanum-based complexes, which began with initial successes in the stoichiometric synthesis of organometallic lanthanide complexes and utilization of these species in catalytic hydrophosphination reactions. Not only have these species supported traditional lanthanide catalysis, such as the hydrophosphination of heterocumulenes like carbodiimides, isocyanates, and isothiocyanates, but they have also been effective for a plethora of catalytic reactions tested thus far, including the hydrophosphinylation and hydrophosphorylation of nitriles, hydrophosphination and hydrophosphinylation of alkynes and alkenes, and the heterodehydrocoupling of silanes and amines. Each of these catalytic transformations is meritorious in its own right, offering new synthetic routes to generate organic scaffolds with enhanced functionality while concurrently minimizing both waste generation and energy consumption. Objectives: We aim for the research summary presented herein to inspire and encourage other researchers to investigate f-element based stoichiometric and catalytic reactions. Our efforts in this field began with the recognition that potassium salts of benzyldimethylamine preferred deprotonation at the α-position, rather than the ortho-position, and we wondered if this regiochemistry would be retained in the formation of lanthanide complexes. The pursuit of this simple idea led first to a series of structurally fascinating homoleptic organometallic lanthanide complexes with surprisingly good stability. Fundamental studies of the protonolysis chemistry of these complexes ultimately revealed highly versatile lanthanide-based precatalysts that have propelled a catalytic investigation spanning more than a decade. We anticipate that this summative perspective will animate the synthetic as well as biological communities to consider La(DMBA)3-based catalytic methods in the synthesis of functionalized organic scaffolds as an atom-economic, convenient, and efficient methodology. Ultimately, we envision our work making a positive impact on the advancement of novel chemical transformations and contributing to progress in various fields of science and technology.

Exploring Electrophilic Hydrophosphination via Metal Phosphenium Intermediates

Two Mo(0) phosphenium complexes containing ancillary secondary phosphine ligands have been investigated with respect to their ability to participate in electrophilic addition at unsaturated substrates and subsequent P-H hydride transfer to "quench" the resulting carbocations. These studies provide stoichiometric "proof of concept" for a proposed new metal-catalyzed electrophilic hydrophosphination mechanism. The more strongly Lewis acidic phosphenium complex, [Mo(CO)4(PR2H)(PR2)]+ (R=Ph, Tolp), cleanly hydrophosphinates 1,1-diphenylethylene, benzophenone, and ethylene, while other substrates react rapidly to give products resulting from competing electrophilic processes. A less Lewis acidic complex, [Mo(CO)3(PR2H)2(PR2)]+, generally reacts more slowly but participates in clean hydrophosphination of a wider range of unsaturated substrates, including styrene, indene, 1-hexene, and cyclohexanone, in addition to 1,1-diphenylethylene, benzophenone, and ethylene. Mechanistic studies are described, including stoichiometric control reactions and computational and kinetic analyses, which probe whether the observed P-H addition actually does occur by the proposed electrophilic mechanism, and whether hydridic P-H transfer in this system is intra- or intermolecular. Preliminary reactivity studies indicate challenges that must be addressed to exploit these promising results in catalysis.

s-Block Metal Catalysts for the Hydroboration of Unsaturated Bonds

The addition of a B-H bond to an unsaturated bond (polarized or unpolarized) is a powerful and atom-economic tool for the synthesis of organoboranes. In recent years, s-block organometallics have appeared as alternative catalysts to transition-metal complexes, which traditionally catalyze the hydroboration of unsaturated bonds. Because of the recent and rapid development in the field of hydroboration of unsaturated bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca, Sr, Ba) metals, we provide a detailed and updated comprehensive review that covers the synthesis, reactivity, and application of s-block metal catalysts in the hydroboration of polarized as well as unsaturated carbon-carbon bonds. Moreover, we describe the main reaction mechanisms, providing valuable insight into the reactivity of the s-block metal catalysts. Finally, we compare these s-block metal complexes with other redox-neutral catalytic systems based on p-block metals including aluminum complexes and f-block metal complexes of lanthanides and early actinides. In this review, we aim to provide a comprehensive, authoritative, and critical assessment of the state of the art within this highly interesting research area.

Recent advances in the carbon-phosphorus (C-P) bond formation from unsaturated compounds by s- and p-block metals

Researchers around the globe have witnessed several breakthroughs in s- and p-block metal chemistry. Over the past few years, several applications in catalysis associated with these main group metals have been established, and owing to their abundance and low cost and they have proved to be essential alternatives to transition metal catalysts. In this review, we present a detailed discussion on the catalytic addition of P-H bonds from various phosphine reagents to multiple bonds of unsaturated substrates for the synthesis of organophosphorus compounds with C-P bonds promoted by various s- and p-block metal catalysts, as published in the last decade.

Calcium catalyzed enantioselective intramolecular alkene hydroamination with chiral C2-symmetric bis-amide ligands

The chiral building block (R)-(+)-2,2'-diamino-1,1'-binaphthyl, (R)-BINAM, which is often used as backbone in privileged enantioselective catalysts, was converted to a series of N-substituted proligands R1-H2 (R = CH2tBu, C(H)Ph2, PPh2, dibenzosuberane, 8-quinoline). After double deprotonation with strong Mg or Ca bases, a series of alkaline earth (Ae) metal catalysts R1-Ae·(THF)n was obtained. Crystal structures of these C2-symmetric catalysts have been analyzed by quadrant models which show that the ligands with C(H)Ph2, dibenzosuberane and 8-quinoline substituents should give the best steric discrimination for the enantioselective intramolecular alkene hydroamination (IAH) of the aminoalkenes H2C[double bond, length as m-dash]CHCH2CR'2CH2NH2 (CR'2 = CPh2, CCy or CMe2). The dianionic R12- ligand in R1-Ae·(THF)n functions as reagent that deprotonates the aminoalkene substrate, while the monoanionic (R1-H)- ligand formed in this reaction functions as a chiral spectator ligand that controls the enantioselectivity of the ring closure reaction. Depending on the substituent R in the BINAM ligand, full cyclization of aminoalkenes to chiral pyrrolidine products as fast as 5 minutes was observed. Product analysis furnished enantioselectivities up to 57% ee, which marks the highest enantioselectivity reported for Ca catalyzed IAH. Higher selectivities are impeded by double protonation of the R12- ligand leading to complete loss of chiral information in the catalytically active species.

On the molecular architectures of siloxane coordination compounds: (re-)investigating the coordination of the cyclodimethylsiloxanes Dn(n= 5–8) towards alkali metal ions

Cyclodimethylsiloxanes have been used as complex ligands and silyl-ether bonding towards various cations is explored stepwise. Compounds with unprecedented coordination modes as well as novel molecular architectures are obtained and characterized.

Alkaline Earth Metal Template (Cross-)Coupling Reactions with Hybrid Disila-Crown Ether Analogues

Alkaline earth metal iodides were used as templates for the synthesis of novel silicon-based ligands. Siloxane moieties were (cross-)coupled and ion-specific, silicon-rich crown ether analogues were obtained. The reaction of 1,2,7,8-tetrasila[12]crown-4 (I) and 1,2-disila[9]crown-3 (II) with MgI2 yielded exclusively [Mg(1,2,7,8-tetrasila[12]crown-4)I2 ] (1). The larger Ca2+ ion was then employed for cross-coupling of I and II and yielded the complex [Ca(1,2,7,8-tetrasila[15]crown-5)I2 ] (2). Cross-coupling of I and 1,2,4,5-tetrasila[9]crown-3 (III) with SrI2 enables the synthesis of the silicon-dominant 1,2,4,5,10,11-hexasila[15]crown-5 ether complex of SrI2 (3). Further, the compounds [Sr(1,2,10,11-tetrasila[18]crown-6)I2 ] (4), [Sr(1,2,13,14-tetrasila[24]crown-8)I2 ] (5), and [Sr(1,2,13,14-tetrasila-dibenzo[24]crown-8)I2 ] (6) were obtained by coupling I, 1,2-disila[12]crown-4 (IV) or 1,2-disila-benzo[12]crown-4 (V), respectively. Using various anions, the (cross-)coupled ligands were also observed in an X-ray structure within the mentioned complexes. These template-assisted (cross-)couplings of various ligands are the first of their kind and a novel method to obtain macrocycles and/or their metal complexes to be established. Further, the Si-O bond activations presented herein might be of importance for silane or even organic functionalization.

Not Non-Coordinating at All: Coordination Compounds of the Cyclodimethylsiloxanes Dn (D = Me2SiO; n = 6, 7) and Group 2 Metal Cations

We present the coordination chemistry of the cyclodimethylsiloxanes D₆ and D₇ toward alkaline earth metal salts. The coordination chemistry of these macrocycles toward alkaline earth metals has been unprecedented to date, and we could show that these ligands coordinate better than previously thought. Direct reaction of alkaline earth metal salts with these ligands yields stable complexes even with a relatively strongly coordinating iodide anion. A handful of counterintuitive coordination compounds could be characterized by single-crystal X-ray diffraction analysis. Quantum chemical calculations of suited Born-Haber cycles showed that these complexes are indeed stable, for Mg²⁺ and Ca²⁺ even with iodide employed as the anion and for Sr²⁺ and Ba²⁺ in the presence of GaI₃.