Pd-, Ni-, Fe-, and Co-Catalyzed Cross-Couplings Using Functionalized Zn-, Mg-, Fe-, and In-Organometallics

Synthesis of Sterically Encumbered Alkaline-Earth Metal Amides Applying the In Situ Grignard Reagent Formation

Magnesium and calcium are too inert to deprotonate amines directly. For the synthesis of bulky amides alternative strategies are required and in the past, N-bound trialkylsilyl groups have been used to ease metalation reactions. The in situ Grignard reagent formation in stirred suspensions of magnesium or calcium with hydryl halide and imine in THF allows the synthesis of a plethora of amides with bulky silyl-free substituents. Ball milling protocols partially favor competitive side reactions such as aza-pinacol coupling reactions. Calcium is the advantageous choice for the in situ Grignard reagent formation and subsequent addition onto the imines yielding bulky calcium bis(amides) whereas the stronger reducing heavier alkaline-earth metals strontium and barium are less selective and hence, the aza-pinacol coupling reaction becomes competitive. Exemplary, the solid-state molecular structures of [(Et₂ O)Mg(N(Ph)(CHPh₂ )₂ ] and [(Et₂ O)₂ Ca(N(Ph)(CHPh₂ )₂ ] have been determined.

Manganese-catalyzed hydrogenation, dehydrogenation, and hydroelementation reactions

The emerging field of organometallic catalysis has shifted towards research on Earth-abundant transition metals due to their ready availability, economic advantage, and novel properties. In this case, manganese, the third most abundant transition-metal in the Earth's crust, has emerged as one of the leading competitors. Accordingly, a large number of molecularly-defined Mn-complexes has been synthesized and employed for hydrogenation, dehydrogenation, and hydroelementation reactions. In this regard, catalyst design is based on three pillars, namely, metal-ligand bifunctionality, ligand hemilability, and redox activity. Indeed, the developed catalysts not only differ in the number of chelating atoms they possess but also their working principles, thereby leading to different turnover numbers for product molecules. Hence, the critical assessment of molecularly defined manganese catalysts in terms of chelating atoms, reaction conditions, mechanistic pathway, and product turnover number is significant. Herein, we analyze manganese complexes for their catalytic activity, versatility to allow multiple transformations and their routes to convert substrates to target molecules. This article will also be helpful to get significant insight into ligand design, thereby aiding catalysis design.

Intermediates and Reactivity in Iron-Catalyzed Cross-Couplings of Alkynyl Grignards with Alkyl Halides

Iron-catalyzed cross-coupling reactions using alkynyl nucleophiles represent an attractive approach for the incorporation of alkynyl moieties into organic molecules. In the present study, a multitechnique approach combining inorganic spectroscopic methods, inorganic synthesis, and reaction studies is applied to iron-SciOPP catalyzed alkynyl-alkyl cross-couplings, providing the first detailed insight into the effects of variation from sp2- to sp-hybridized nucleophiles on iron speciation and reactivity. Reaction studies demonstrate that reaction of FeBr2(SciOPP) with 1 equiv (triisopropylsilyl)ethynylmagnesium bromide (TIPS-CC-MgBr) leads to a distribution of mono-, bis-, and tris-alkynylated iron(II)-SciOPP species due to rapid alkynyl ligand redistribution. While solvents such as THF promote these complex redistribution pathways, nonpolar solvents such as toluene enable increased stabilization of these iron species and further enabled assessment of their reactivity with electrophile. While the tris-alkynylated iron(II)-SciOPP species was found to be unreactive with the cycloheptyl bromide electrophile over the average turnover time of catalysis, the in situ formed neutral mono- and bis-alkynylated iron(II)-SciOPP complexes are consumed upon reaction with the electrophile with concomitant generation of cross-coupled product at catalytically relevant rates, indicating the ability of one or both of these species to react selectively with the electrophile. The nature of the reaction solvent and Grignard reagent addition rate were found to have broader implications in overall reaction selectivity, reaction rate, and accessibility of off-cycle iron(I)-SciOPP species. Additionally, the effects of steric substitution of the alkynyl Grignard reagent on catalytic performance were investigated. Fundamental insight into iron speciation and reactivity with alkynyl nucleophiles reported herein provides an essential foundation for the continued development of this important class of reactions.

Oxidative C-C Bond Formation Reactivity of Organometallic Ni(II), Ni(III), and Ni(IV) Complexes

The use of the tridentate ligand 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn) and the cyclic alkyl/aryl C-donor ligand -CH₂CMe₂-o-C₆H₄- (cycloneophyl) allows for the synthesis of isolable organometallic Ni^II, Ni^III, and Ni^IV complexes. Surprisingly, the five-coordinate Ni^III complex is stable both in solution and the solid state, and exhibits limited C-C bond formation reactivity. Oxidation by one electron of this Ni^III species generates a six-coordinate Ni^IV complex, with an acetonitrile molecule bound to Ni. Interestingly, illumination of the Ni^IV complex with blue LEDs results in rapid formation of the cyclic C-C product at room temperature. This reactivity has important implications for the recently developed dual Ni/photoredox catalytic systems proposed to involve high-valent organometallic Ni intermediates. Additional reactivity studies show the corresponding Ni^II species undergoes oxidative addition with alkyl halides, as well as rapid oxidation by O₂, to generate detectable Ni^III and/or Ni^IV intermediates and followed by C-C bond formation.

Isolated Organometallic Nickel(III) and Nickel(IV) Complexes Relevant to Carbon-Carbon Bond Formation Reactions

Nickel-catalyzed cross-coupling reactions are experiencing a dramatic resurgence in recent years given their ability to employ a wider range of electrophiles as well as promote stereospecific or stereoselective transformations. In contrast to the extensively studied Pd catalysts that generally employ diamagnetic intermediates, Ni systems can more easily access various oxidation states including odd-electron configurations. For example, organometallic Ni^III intermediates with aryl and/or alkyl ligands are commonly proposed as the active intermediates in cross-coupling reactions. Herein, we report the first isolated Ni^III-dialkyl complex and show that this species is involved in stoichiometric and catalytic C-C bond formation reactions. Interestingly, the rate of C-C bond formation from a Ni^III center is enhanced in the presence of an oxidant, suggesting the involvement of transient Ni^IV species. Indeed, such a Ni^IV species was observed and characterized spectroscopically for a nickelacycle system. Overall, these studies suggest that both Ni^III and Ni^IV species could play an important role in a range of Ni-catalyzed cross-coupling reactions, especially those involving alkyl substrates.

Recent advances in iron-catalysed cross coupling reactions and their mechanistic underpinning

Advances in iron-catalysed cross coupling from 2010–2015 are critically reviewed.

Stable bis(trifluoromethyl)nickel(III) complexes

Organometallic Ni(III) intermediates have been proposed in several Nickel-catalyzed cross-coupling reactions, yet no isolated bis(hydrocarbyl)Ni(III) complexes have been reported to date. Herein we report the synthesis and detailed characterization of stable organometallic Ni(III) complexes that contain two trifluoromethyl ligands and are supported by tetradentate N-donor ligands (R)N4 (R = Me or tBu). Interestingly, the corresponding Ni(II) precursors undergo facile oxidation, including aerobic oxidation, to generate uncommonly stable organometallic Ni(III) complexes that exhibit limited reactivity.

Synthesis of highly functionalized diaryl ethers by copper-mediated O-arylation of phenols using trivalent arylbismuth reagents

Highly functionalized diaryl ethers were prepared by copper(II) acetate mediated O-arylation reaction of phenols using trivalent organobismuthanes. The reaction is performed under simple conditions and tolerates a wide diversity of functional groups on the phenol and on the organobismuth reagent. Substoichiometric amounts of catalyst can be used by performing the reaction under an oxygen atmosphere. The N-arylation of pyridones is also reported.

Aryl ether as a Negishi coupling partner: an approach for constructing C-C bonds under mild conditions

An etheric Negishi coupling: The first cross-coupling reaction between aryl alkyl ethers and dianion-type zincate reagents to afford biaryl compounds through selective cleavage of the etheric C(sp(2))-O bond was developed. Dianion-type zincates showed excellent reactivity toward the aromatic ethers under mild conditions, with good functional group compatibility (see scheme).