Olefination and group transfer reactions of an electron deficient tantalum methylidene complex

A Vanadium Methylidene

Examples of stable 3d transition metal methylidene complexes are extremely rare. Here we report an isolable and stable vanadium methylidene complex, [(PNP)V(=NAr)(=CH2)] (PNP = N[2-PiPr2-4-methylphenyl]-, Ar = 2,6-iPr2C6H3), via H atom transfer (HAT) from [(PNP)V(NHAr)(CH3)] or [(PNP)V(=NAr)(CH3)] using two or one equivalents of the TEMPO radical (TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxyl), respectively. Alternatively, the vanadium methylidene moiety can also be formed via the treatment of transient [(PNP)V=NAr] with the Wittig reagent, H2CPPh3. Structural and spectroscopic analysis, including 13C enriched labeling of the methylidene ligand, unequivocally confirmed the terminal nature of a rare 3d methylidene complex, featuring a V=CH2 bond distance of 1.908(2) Å and a highly downfield 13C NMR spectral shift at 298 ppm. In the absence of the ylide, intermediate [(PNP)V=NAr] activates dinitrogen to form an end-on bridging N2 complex, [(PNP)V(=NAr)]2(μ2-η1:η1-N2), having a singlet ground state. Complex [(PNP)V(=NAr)(=CH2)] reacts with H3COTf to form [(PNP)V(=NAr)(OTf)], accompanied by the release of ethylene as evidenced by 1H NMR spectroscopy, and reactivity studies suggest a β-hydride elimination pathway.

Mechanism of Coupling of Methylidene to Ethylene Ligands in Dimetallic Assemblies; Computational Investigation of a Model for a Key Step in Catalytic C1 Chemistry

Methylidene complexes often couple to ethylene complexes, but the mechanistic insight is scant. The path by which two cations [(η⁵-C₅H₅)Re(NO)(PPh₃)(═CH₂)]⁺ (5⁺) transform (CH₂Cl₂/acetonitrile) to [(η⁵-C₅H₅)Re(NO)(PPh₃)(H₂C═CH₂)]⁺ (6⁺) and [(η⁵-C₅H₅)Re(NO)(PPh₃)(NCCH₃)]⁺ is studied by density functional theory. Experiments provide a number of constraints such as the second-order rate in 5⁺; no prior ligand dissociation/exchange; a faster reaction of (S)-5⁺ with (S)-5⁺ than with (R)-5⁺ ("enantiomer self-recognition"). Although dirhenium dications with Re(μ-CH₂)₂Re cores represent energy minima, they are not accessible by 2 + 2 cycloadditions of 5⁺. Transition states leading to ReCH₂CH₂Re linkages are prohibitively high in energy. However, 5⁺ can give non-covalent S_Re/S_Re or S_Re/R_Re dimers with π interactions between the PPh₃ ligands but long ReCH₂···H₂CRe and H₂CRe···H₂CRe distances (3.073-3.095 Å and 3.878-4.529 Å, respectively). In rate-determining steps, these afford [(η⁵-C₅H₅)Re(NO)(PPh₃)(μ-η²:η²-H₂C···CH₂)(Ph₃P)(ON)Re(η⁵-C₅H₅)]²⁺ (13²⁺), in which one rhenium binds the bridging ethylene more tightly than the other (2.115-2.098 vs 2.431-2.486 Å to the centroid). In the S_Re/R_Re adduct, Dewar-Chatt-Duncanson optimization leads to unfavorable PPh₃/PPh₃ contacts. Ligand interactions are further dissected in the preceding transition states via component analyses, and ΔΔG^‡ (1.2 kcal/mol, CH₂Cl₂) favors the S_Re/S_Re pathway, in accordance with the experiment. Acetonitrile then displaces 6⁺ from the more weakly bound rhenium of 13²⁺. The formation of similar μ-H₂C···CH₂ intermediates is found to be rate-determining for varied coordinatively saturated M═CH₂ species [M = Fe(d⁶)/Re(d⁴)/Ta(d²)], establishing generality and enhancing relevancy to catalytic CH₄ and CO/H₂ chemistry.

Metallo-Wittig chemistry of an alkylidene to form a terminal titanium oxo complex

The synthesis and characterization of a terminal titanium oxo complex generated by alkylidene benzophenone cross-metathesis is reported.

Binary role of an ylide in formation of a terminal methylidene complex of niobium

The methylidene complex can be prepared readily in high yield from the reaction of [(Ar′O)₂Nb(CH₃)₂Cl] (Ar′ = (2,6-CHPh₂)₂-4-^tBu-C₆H₂) and two equivalents of H₂CPPh₃. The nucleophilic methylidene of [(Ar′O)₂NbCH₂(CH₃)(H₂CPPh₃)] can undergo Wittig-like or protonation chemistry to form structurally similar molecules.

Solvents for ring-closing metathesis reactions

A study of the influence of eight diverse solvents on a Grubbs II-catalysed ring-closing metathesis (RCM) reaction reveals a complex dependence of the different reaction steps on the solvent and suggests acetic acid as a useful solvent for RCM reactions.