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We describe the development of an efficient method for the olefination of hydrazones and oximes. The key design approach that enables this transformation is tuning of the energy/polarity of C=N π-bonds by employing heteroatom functionalities (NR 2 , OR). The resulting hydrazones or oximes facilitate olefination with ruthenium alkylidenes. Through this approach, we show that air-stable, commercially available ruthenium alkylidenes provide access to functionalized alkenes (20 examples) in ring-closing reactions with yields up to 88 %., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Catalytic olefin metathesis based on the second- and third-row transition metals has become one of the most powerful transformations in modern organic chemistry. The shift to first-row metals to produce fine and commodity chemicals would be an important achievement to complement existing methods with inexpensive and greener alternatives. In addition, those systems can offer unusual reactivity based on the unique electronic structure of the base metals. In this Minireview, we summarize the progress of the development of alkylidenes and metallacycles of first-row transition metals from scandium to nickel capable of performing cycloaddition and cycloreversion steps, crucial reactions in olefin metathesis. In addition, we will discuss systems capable of performing olefin metathesis; however, the nature of active species is not yet known., (© 2021 Wiley-VCH GmbH.)

Despite their excellent selectivities and activities, Mo-and W-based catalysts for olefin metathesis have not gained the same widespread use as Ru-based systems, mainly due to their inherent air sensitivity. Herein, we describe the synthesis of air-stable cationic-at-metal molybdenum and tungsten imido alkylidene NHC nitrile complexes. They catalyze olefin metathesis reactions of substrates containing functional groups such as (thio-) esters, (thio-) ethers and alcohols without the need for prior activation, for example, by a Lewis acid. The presence of a nitrile ligand was found to be essential for their stability towards air, while no decrease in activity and productivity could be observed upon coordination of a nitrile. Variations of the imido and anionic ligand revealed that alkoxide complexes with electron-withdrawing imido ligands offer the highest reactivities and excellent stability compared to analogous triflate and halide complexes., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Transition-metal alkylidenes have exhibited wide applications in organometallic chemistry and synthetic organic chemistry, however, cyclic Schrock-carbene-like bis-alkylidenes of group 4 metals with a four-electron donor from an alkylidene have not been reported. Herein, the synthesis and characterization of five-membered cyclic bis-alkylidenes of titanium (4 a,b) and zirconium (5 a,b) are reported, as the first well-defined group 4 metallacyclopentatrienes, by two-electron reduction of their corresponding titana- and zirconacyclopentadienes. DFT analyses of 4 a show a four-electron donor (σ-donation and π-donation) from an alkylidene carbon to the metal center. The reaction of 4 a with N,N'-diisopropylcarbodiimide (DIC) leads to the [2+2]-cycloaddition product 6. Compound 4 a reacted with CO, affording the oxycyclopentadienyl titanium complex 7. These reactivities demonstrate the multiple metal-carbon bond character. The reactions of 4 a or 5 a with cyclooctatetraene (COT) or azobenzene afforded sandwich titanium complex 8 or diphenylhydrazine-coordinated zirconacyclopentadiene 9, respectively, which exhibit two-electron reductive ability., (© 2020 Wiley-VCH GmbH.)

Facile α-H elimination from tetrakis(trimethylsilylmethyl)titanium precursors to give adducts of (alkylidene)bis(alkyl)titanium complexes is induced by light alkali metal amides of the NNNN-type macrocyclic anionic ligand Me 3 TACD [(Me 3 TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane]. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH 2 group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by the reaction with benzophenone and terminal acetylenes., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

This article outlines the fundamental concept to molybdenum imido, tungsten imido and tungsten oxo alkylidene N-heterocyclic carbene (NHC) catalysts for olefin metathesis. Starting from Mo(NR)(CHCMe 2 R')(OTf) 2 ⋅DME (R=alkyl, aryl; R'=CH 3 , Ph; OTf=CF 3 SO 3 ; DME=1,2-dimethoxyethane), W(NR)(CHCMe 2 Ph)X 2 (X=pyrrolide, OR'', SiPh 3 ) and W(O)Cl 2 (CHCMe 2 Ph)(PMe 2 Ph) 2 , respectively, the neutral, pentacoordinated 16-valence electron (VE) complexes became accessible. These serve as precursors for the corresponding cationic, tetracoordinated 14-VE catalysts. This class of olefin metathesis catalysts based on group 6 metal alkylidene NHCs is characterized by a broad structural variability, high activity, high productivity and, in tailored systems, high functional group tolerance even vs. protic groups including alcohols and carboxylic acids. Also, high regio- and stereoselectivity is achieved, both in ring-opening metathesis polymerization (ROMP) and in the cyclopolymerization of α,ω-diynes. Fundamentals concerning the influence of the imido/oxo ligand, the different NHCs and the anionic ligands on catalyst performance and stability as well as reaction pathways are summarized. Also, the concepts for catalyst immobilization, whether via the NHC or by substitution of an alkoxide, are presented. Finally, special features such as the use of betaine-type anionic ligands (alkoxides and sulfonates), which offer access to ionic catalysts for use biphasic reaction setups, are briefly outlined., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Unprecedented silyl-phosphino-carbene complexes of uranium(IV) are presented, where before all covalent actinide-carbon double bonds were stabilised by phosphorus(V) substituents or restricted to matrix isolation experiments. Conversion of [U(BIPM TMS )(Cl)(μ-Cl) 2 Li(THF) 2 ] (1, BIPM TMS =C(PPh 2 NSiMe 3 ) 2 ) into [U(BIPM TMS )(Cl){CH(Ph)(SiMe 3 )}] (2), and addition of [Li{CH(SiMe 3 )(PPh 2 )}(THF)]/Me 2 NCH 2 CH 2 NMe 2 (TMEDA) gave [U{C(SiMe 3 )(PPh 2 )}(BIPM TMS )(μ-Cl)Li(TMEDA)(μ-TMEDA) 0.5 ] 2 (3) by α-hydrogen abstraction. Addition of 2,2,2-cryptand or two equivalents of 4-N,N-dimethylaminopyridine (DMAP) to 3 gave [U{C(SiMe 3 )(PPh 2 )}(BIPM TMS )(Cl)][Li(2,2,2-cryptand)] (4) or [U{C(SiMe 3 )(PPh 2 )}(BIPM TMS )(DMAP) 2 ] (5). The characterisation data for 3-5 suggest that whilst there is evidence for 3-centre P-C-U π-bonding character, the U=C double bond component is dominant in each case. These U=C bonds are the closest to a true uranium alkylidene yet outside of matrix isolation experiments., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Alkylidene-bridged scandium-copper/silver heterobimetallic complexes were synthesized and structurally characterized. The complexes contain different Sc-C and M-C (M=Cu I , Ag I ) bonds. The reactivity of the scandium-copper heterobimetallic complex was also studied, which reveals that the heterobimetallic complex is a reaction intermediate for the transmetalation of akylidene group from Sc III to Cu I . The scandium-copper heterobimetallic complex also undergoes an addition reaction with CO, resulting in the formation of a new C=C double bond. DFT calculations were used to study the bonding and the subsequent reactivity with CO of the scandium-copper heterobimetallic complex. It clearly demonstrates a cooperative effect between the two metal centers through the formation of a direct Sc⋅⋅⋅Cu interaction that drives the reactivity with CO., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The stable cationic iridacyclopentenylidene [Tp(Me2)Ir(=CHC(Me)=C(Me)CH2(NCMe)]PF6 (A; Tp(Me2)=hydrotris(3,5-dimethylpyrazolyl)borate) has been obtained by α-hydride abstraction from the iridacyclopent-2-ene [Tp(Me2)Ir(CH2C(Me)=C(Me)CH2)(NCMe)]. Complex A exhibits Brønsted-Lowry acidity at the Ir-CH2 and proximal (relative to Ir-CH2 ) methyl sites. The coordination of an extra molecule of acetonitrile to the iridium center initiates the reversible isomerization of the chelating carbon chain of A to the monodentate butadienyl ligand of complex [Tp(Me2)Ir(CH=C(Me)C(Me)=CH2)(NCMe)2]PF6, which is capable to engage in a water-promoted C-C coupling with the MeCN co-ligands. The product is an aesthetically appealing bicyclic structure that resembles the hydrocarbon barrelene., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We synthesized the first N-heterocyclic carbene (NHC) complexes of Schrock's molybdenum imido alkylidene bis(triflate) complexes. Unlike existing bis(triflate) complexes, the novel 16-electron complexes represent metathesis active, functional-group-tolerant catalysts. Single-crystal X-ray structures of two representatives of this novel class of Schrock catalysts are presented and reactivity is discussed in view of their structural peculiarities. In the presence of monomer (substrate), these catalysts form cationic species and can be employed in ring-closing metathesis (RCM), ring-opening metathesis polymerization (ROMP), as well as in the cyclopolymerization of α,ω-diynes. Monomers containing functional groups, which are not tolerated by the existing variations of Schrock's catalyst, e.g., sec-amine, hydroxy, and carboxylic acid moieties, can be used. These catalysts therefore hold great promise in both organic and polymer chemistry, where they allow for the use of protic monomers., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)