Z-Selective Alkene Isomerization by High-Spin Cobalt(II) Complexes

Cobalt-catalyzed regioselective stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes

The first transition metal-catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of (E/Z)-dienes was effectively achieved with excellent E-selectivities using a cobalt catalyst.

We report the first stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes using catalysts generated from bench-stable Co(acac)₂ and phosphine ligands. A wide range of E/Z-dienes underwent this Markovnikov 1,2-hydrosilylation in a stereoconvergent manner, affording (E)-allylsilanes in high isolated yields with high stereoselectivities (E/Z = >99 : 1) and high regioselectivities (b/l up to > 99 : 1). Mechanistic studies revealed that this stereoconvergence stems from a σ–π–σ isomerization of an allylcobalt species generated by the 1,4-hydrometalation of Z-dienes. In addition, a cobalt catalyst that can only catalyze the hydrosilylation of the E-isomer of an (E/Z)-diene was identified, which allows the separation of the (Z)-isomer from an isomeric mixture of (E/Z)-dienes. Furthermore, asymmetric hydrosilylation of (E)-1-aryl-1,3-dienes was studied with Co(acac)₂/(R)-difluorphos and good enantioselectivities (er up to 90 : 10) were obtained.

Preparation of 2-Amino-5-homoallylfurans via Palladium-Catalyzed Tandem Cycloisomerization/Heck-Type Coupling of Homoallenyl Amides with Allyltrialkylsilanes

The direct access to 2-amino-5-homoallylfurans has been realized by a palladium-catalyzed tandem cycloisomerization/Heck-type coupling between homoallenyl amides and allyltrialkylsilanes, using a novel DDQ/MnO₂ combination as the efficient oxidant. The reaction exclusively affords γ-allylation products in good to excellent yields with broad substrate scope under exceptionally mild reaction conditions. It represents one of the rare examples of the Pd-catalyzed intermolecular Heck-type coupling of allytrialkylsilanes terminated by β-silyl elimination, thus complementing traditional allylation methods because of the excellent γ-selectivity.

Solid-state molecular organometallic chemistry. Single-crystal to single-crystal reactivity and catalysis with light hydrocarbon substrates

Solid-state molecular organometallic catalysis (SMOM-cat): synthetic routes, unique structural motifs, mobility in the solid-state and very active gas/solid isomerization catalysts.

Single-crystal to single-crystal solid/gas reactivity and catalysis starting from the precursor sigma-alkane complex [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] (NBA = norbornane; Ar^F = 3,5-(CF₃)₂C₆H₃) is reported. By adding ethene, propene and 1-butene to this precursor in solid/gas reactions the resulting alkene complexes [Rh(Cy₂PCH₂CH₂PCy₂)(alkene)_x][BAr^F₄] are formed. The ethene (x = 2) complex, [Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F₄]-Oct, has been characterized in the solid-state (single-crystal X-ray diffraction) and by solution and solid-state NMR spectroscopy. Rapid, low temperature recrystallization using solution methods results in a different crystalline modification, [Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F₄]-Hex, that has a hexagonal microporous structure (P6₃22). The propene complex (x = 1) [Rh(Cy₂PCH₂CH₂PCy₂)(propene)][BAr^F₄] is characterized as having a π-bound alkene with a supporting γ-agostic Rh···H₃C interaction at low temperature by single-crystal X-ray diffraction, variable temperature solution and solid-state NMR spectroscopy, as well as periodic density functional theory (DFT) calculations. A fluxional process occurs in both the solid-state and solution that is proposed to proceed via a tautomeric allyl-hydride. Gas/solid catalytic isomerization of d₃-propene, H₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHCD₃, using [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] scrambles the D-label into all possible positions of the propene, as shown by isotopic perturbation of equilibrium measurements for the agostic interaction. Periodic DFT calculations show a low barrier to H/D exchange (10.9 kcal mol^–1, PBE-D3 level), and GIPAW chemical shift calculations guide the assignment of the experimental data. When synthesized using solution routes a bis-propene complex, [Rh(Cy₂PCH₂CH₂PCy₂)(propene)₂][BAr^F₄], is formed. [Rh(Cy₂PCH₂CH₂PCy₂)(butene)][BAr^F₄] (x = 1) is characterized as having 2-butene bound as the cis-isomer and a single Rh···H₃C agostic interaction. In the solid-state two low-energy fluxional processes are proposed. The first is a simple libration of the 2-butene that exchanges the agostic interaction, and the second is a butene isomerization process that proceeds via an allyl-hydride intermediate with a low computed barrier of 14.5 kcal mol^–1. [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] and the polymorphs of [Rh(Cy₂PCH₂CH₂PCy₂)(ethene)₂][BAr^F₄] are shown to be effective in solid-state molecular organometallic catalysis (SMOM-Cat) for the isomerization of 1-butene to a mixture of cis- and trans-2-butene at 298 K and 1 atm, and studies suggest that catalysis is likely dominated by surface-active species. [Rh(Cy₂PCH₂CH₂PCy₂)(η²η²-NBA)][BAr^F₄] is also shown to catalyze the transfer dehydrogenation of butane to 2-butene at 298 K using ethene as the sacrificial acceptor.

Isomerization of N-Allyl Amides To Form Geometrically Defined Di-, Tri-, and Tetrasubstituted Enamides

Enamides represent bioactive pharmacophores in various natural products, and have become increasingly common reagents for asymmetric incorporation of nitrogen functionality. Yet the synthesis of the requisite geometrically defined enamides remains problematic, especially for highly substituted and Z-enamides. Herein we wish to report a general atom economic method for the isomerization of a broad range of N-allyl amides to form Z-di-, tri-, and tetrasubstituted enamides with exceptional geometric selectivity. This report represents the first examples of a catalytic isomerization of N-allyl amides to form nonpropenyl disubstituted, tri- and tetrasubstituted enamides with excellent geometric control. Applications of these geometrically defined enamides toward the synthesis of cis vicinal amino alcohols and tetrasubstituted α-borylamido complexes are discussed.

Long-Range Olefin Isomerization Catalyzed by Palladium(0) Nanoparticles

Long-range olefin isomerization of 2-alkenylbenzoic acid derivatives going through two to five sp³-carbon atoms to give (E)-alkenes was achieved with palladium(0) nanoparticles. The substrate scope of this reaction includes carboxylic acid, ester, and primary to tertiary amides and tolerates various substituents on the benzene ring. This isomerization reaction was catalyzed by recyclable Pd(0) nanoparticles, prepared in situ from PdCl₂ and characterized by X-ray powder diffraction and scanning electron microscopy analyses. ¹H NMR studies and kinetic modeling supported a stepwise process. This new process was applied to synthesize a natural dihydroisocoumarin with good efficiency.

Mode of Activation of Cobalt(II) Amides for Catalytic Hydrosilylation of Alkenes with Tertiary Silanes

Cobalt(II) complexes capable of catalyzing alkene hydrosilylation in the absence of external activators are rarely known, and their activation mode has remained poorly understood. We present here that cobalt(II) amide complexes, [Co(N(SiMe₃)₂)₂] and its NHC adducts [(NHC)Co(N(SiMe₃)₂)₂] (NHC = N-heterocyclic carbene), are effective catalysts for the hydrosilylation of alkenes with tertiary silanes. Mechanistic studies revealed that cobalt(II) amides can react with hydrosilane to form cobalt(I) species, silylamide, and hydrogen, which serves as the entry to the genuine catalytically active species, presumably cobalt(I) species, for the cobalt-catalyzed hydrosilylation reaction.

Cyclotrimerisation of isocyanates catalysed by low-coordinate Mn(ii) and Fe(ii) m-terphenyl complexes

Isocyanurates can be synthesised through the cyclotrimerisation of primary isocyanates catalysed by low-coordinate manganese(ii) and iron(ii) m-terphenyl complexes under mild conditions.

β-Diketiminate complexes of the first row transition metals: applications in catalysis

Although β-diketiminate complexes have been widely explored in stoichiometric studies, their use as catalysts is largely underdeveloped.

Mn-, Fe-, and Co-Catalyzed Radical Hydrofunctionalizations of Olefins

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

Highly Isospecific Polymerization of Silyl-Protected ω-Alkenols Using an [OSSO]-Type Bis(phenolato) Dichloro Zirconium(IV) Precatalyst

The coordination polymerization of silyl-protected ω-alkenols such as ω-alken-α-oxytriisopropylsilanes 1 provides poly(ω-alkenyl-α-oxytriisopropylsilalne)s with a highly isospecific microstructure ([mmmm] > 95%) when a combination of [OSSO]-type bis(phenolato) dichloro zirconium(IV) complex 2 and dried methylaluminoxane is used as the precatalyst and activator, respectively. The resulting siloxy-substituted polymers could be efficiently transformed into the corresponding functionalized polyolefins, which contained up to 90% acetyl groups and ≈7% hydroxy groups in the terminal side chains.

A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

The influence of ligands on the spin state of a metal ion is of central importance for bioinorganic chemistry, and the production of base-metal catalysts for synthesis applications. Complexes derived from [Fe(bpp)2 ](2+) (bpp=2,6-di{pyrazol-1-yl}pyridine) can be high-spin, low-spin, or spin-crossover (SCO) active depending on the ligand substituents. Plots of the SCO midpoint temperature (T1/2 ) in solution vs. the relevant Hammett parameter show that the low-spin state of the complex is stabilized by electron-withdrawing pyridyl ("X") substituents, but also by electron-donating pyrazolyl ("Y") substituents. Moreover, when a subset of complexes with halogeno X or Y substituents is considered, the two sets of compounds instead show identical trends of a small reduction in T1/2 for increasing substituent electronegativity. DFT calculations reproduce these disparate trends, which arise from competing influences of pyridyl and pyrazolyl ligand substituents on Fe-L σ and π bonding.

Low coordinate iron derivatives stabilized by a β-diketiminate mimic. Synthesis and coordination chemistry of enamidophosphinimine scaffolds to generate diiron dinitrogen complexes

The reduction of the iron enamido-phosphinimine complex under N₂ leads to dinitrogen activation and cleavage of the phosphinimine linkage to generate a di-iron complex with a bridging imido moiety.

Zr-based metal–organic frameworks: design, synthesis, structure, and applications

This review summarizes the advances in the study of Zr-based metal–organic frameworks in terms of their design, synthesis, structure, and potential applications.

Aminoquinoline-assisted vinylic C–H arylation of unsubstituted acrylamide for the selective synthesis of Z olefins

A method for Pd-catalyzed, aminoquinoline-directed arylation of vinylic C–H bonds with aryl iodides has been developed. This reaction represents a rare example of Pd-catalyzed vinylic C–H functionalization of unsubstituted acrylamide, allowing for preparation of Z-olefins.

Rapid, Regioconvergent, Solvent-Free Alkene Hydrosilylation with a Cobalt Catalyst

Alkene hydrosilylation is typically performed with Pt catalysts, but inexpensive base-metal catalysts would be preferred. We report a Co catalyst for anti-Markovnikov alkene hydrosilylation that can be used without added solvent at low temperatures with low loadings, and can be generated in situ from an air-stable precursor that is simple to synthesize from low-cost, commercially available materials. In addition, a mixture of Co catalysts performs a tandem catalytic alkene isomerization/hydrosilylation reaction that converts multiple isomers of hexene to the same terminal product. This regioconvergent reaction uses isomerization as a benefit rather than a hindrance.

A Two-Coordinate Cobalt(II) Imido Complex with NHC Ligation: Synthesis, Structure, and Reactivity

The synthesis, structural characterization, and reactivity of the first two-coordinate cobalt complex featuring a metal-element multiple bond [(IPr)Co(NDmp)] (4; IPr=1,3-bis(2',6'-diisopropylphenyl)imidazole-2-ylidene; Dmp=2,6-dimesitylphenyl) is reported. Complex 4 was prepared from the reaction of [(IPr)Co(η(2) -vtms)2 ] (vtms=vinyltrimethylsilane) with DmpN3 . An X-ray diffraction study revealed its linear C-Co-N core and a short Co-N distance (1.691(6) Å). Spectroscopic characterization and calculation studies indicated the high-spin nature of 4 and the multiple-bond character of the Co-N bond. Complex 4 effected group-transfer reactions to CO and ethylene to form isocyanide and imine, respectively. It also facilitated E-H (E=C, Si) σ-bond activation of terminal alkyne and hydrosilanes to produce the corresponding cobalt(II) alkynyl and cobalt(II) hydride complexes as 1,2-addition products.

A Bio-Inspired, Catalytic E → Z Isomerization of Activated Olefins

Herein, Nature's flavin-mediated activation of complex (poly)enes has been translated to a small molecule paradigm culminating in a highly (Z)-selective, catalytic isomerization of activated olefins using (-)-riboflavin (up to 99:1 Z/E). In contrast to the prominent Z → E isomerization of the natural system, it was possible to invert the directionality of the isomerization (E → Z) by simultaneously truncating the retinal scaffold, and introducing a third olefin substituent to augment A1,3-strain upon isomerization. Consequently, conjugation is reduced in the product chromophore leading to a substrate/product combination with discrete photophysical signatures. The operationally simple isomerization protocol has been applied to a variety of enone-derived substrates and showcased in the preparation of the medically relevant 4-substituted coumarin scaffold. A correlation of sensitizer triplet energy (ET) and reaction efficiency, together with the study of additive effects and mechanistic probes, is consistent with a triplet energy transfer mechanism.

Tuning steric and electronic effects in transition-metal β-diketiminate complexes

β-Diketiminates are widely used supporting ligands for building a range of metal complexes with different oxidation states, structures, and reactivities. This Perspective summarizes the steric and electronic influences of ligand substituents on these complexes, with an eye toward informing the design of new complexes with optimized properties. The backbone and N-aryl substituents can give significant steric effects on structure, reactivity and selectivity of reactions. The electron density on the metal can be tuned by installation of electron withdrawing or donating groups on the β-diketiminate ligand as well. Examples are shown from throughout the transition metal series to demonstrate different types of effects attributable to systematic variation of β-diketiminate ligands.

Distinctive Reaction Pathways at Base Metals in High-Spin Organometallic Catalysts

Inexpensive "base" metals are more affordable and sustainable than precious metals and also offer opportunities to discover new mechanisms for selective catalytic reactions. Base metal complexes can have high-spin electronic configurations that are rare in precious metal complexes. This Account describes some concepts relevant to high-spin organometallic complexes, focusing on our recent work with β-diketiminate complexes of iron and cobalt. Even though high-spin organometallic complexes have some unfamiliar spectroscopic properties, they can be studied using NMR spectroscopy as well as techniques that focus on the magnetism brought about by the unpaired electrons. Understanding the mechanisms of reactions using these complexes can be complicated, because complexes with a high-spin electronic configuration may need to change spin states to avoid high barriers for reaction. These spin-state changes can be rapid, and the ability of an excited spin state to "cut through" the barrier for a reaction can lead to spin acceleration. These concepts, originally developed by Poli, Shaik, Schwarz, and Harvey, are applied here to the fundamental organometallic reaction of β-hydride elimination (BHE). Experimentally validated density-functional calculations show spin acceleration in BHE using three-coordinate iron(II) and cobalt(II) complexes. A square-planar transition state is particularly beneficial for accelerating BHE when a high-spin iron(II) complex goes from an S = 2 ground state to an S = 1 transition state or when a high-spin cobalt(II) complex goes from an S = 3/2 ground state to an S = 1/2 transition state. The relative energies of the spin states can be controlled with the choice of the supporting ligand. Using an appropriate ligand, isomerization of 1-alkenes to their Z-2 isomers can be catalyzed in high yields using the cobalt(II) alkyl complexes as catalysts. Though an earlier paper attributed the regioselectivity and stereoselectivity to the preferred geometry of the BHE step, the results of isotope labeling experiments suggest that the selectivity may actually come from the alkene exchange step (again with spin acceleration). In general, the use of multiple intersecting spin states is envisioned as a profitable strategy for bringing about low reaction barriers and high selectivity in catalytic reactions. This effort requires high-accuracy computational models as well as ligand design that gives nearby spin states with appropriate geometries.