Organometallic Chemistry of Transition Metal Alkylidyne Complexes Centered at Metathesis Reactions

Transition metals form a variety of alkylidyne complexes with either a d0 metal center (high-valent) or a non-d0 metal center (low-valent). One of the most interesting properties of alkylidyne complexes is that they can undergo or mediate metathesis reactions. The most well-studied metathesis reactions are alkyne metathesis involving high-valent alkylidynes. High-valent alkylidynes can also undergo metathesis reactions with heterotriple bonded species such as NCR, PCR, and NNR+. Metathesis reactions involving low-valent alkylidynes are less known. Highly efficient alkyne metathesis catalysts have been developed based on Mo­(VI) and W­(VI) alkylidynes. Catalytic cross-metathesis of nitriles with alkynes has also been achieved with M­(VI) (M = W, Mo) alkylidyne or nitrido complexes. The metathesis activity of alkylidyne complexes is sensitively dependent on metals, supporting ligands and substituents of alkylidynes. Beyond metathesis, metal alkylidynes can also promote other reactions including alkyne polymerization. The remaining shortcomings and opportunities in the field are assessed

Understanding Nonplanarity in Metallabenzene Complexes

The nonplanarity found in metallabenzene complexes has been investigated theoretically via density functional theory (DFT) calculations. A metallabenzene has four occupied π molecular orbitals (8 π electrons) instead of three that benzene has. Our electronic structure analyses show that the extra occupied π molecular orbital, which is the highest occupied molecular orbital (HOMO) in many metallabenzenes, has antibonding interactions between the metal center and the metal-bonded ring-carbon atoms, providing the electronic driving force toward nonplanarity. Calculations indicate that the electronic driving force toward nonplanarity, however, is relatively small. Therefore, other factors such as steric effects also play important roles in determining the planarity of these metallabenzene complexes. In this paper, how the various electronic and steric factors interplay has been discussed

Theoretical Studies on Coupling Reactions of Carbon Dioxide with Alkynes Mediated by Nickel(0) Complexes

A computational study with the Becke3LYP DFT functional theory was carried out on Ni(0)-mediated coupling reactions of both terminal and internal alkynes with CO2. We studied the mechanism for the formation of the five-membered metallacyclic intermediates in order to understand the regioselectivity. The steric and electronic factors that determine the regioselectivity have been discussed. The calculations indicate that electronic factors nicely explain the trend observed in the barriers calculated for the coupling reactions of CO2 with the three terminal alkyne substrates having substituents with different electronic properties, but steric factors are dominant in the regioselectivity for the reaction of a given terminal alkyne substrate. For silyl-substituted internal alkynes, both electronic and steric effects favor the formation of compounds in which CO2 couples with the silyl-substituted carbon

Theoretical Studies on O-Insertion Reactions of Nitrous Oxide with Ruthenium Hydride Complexes

DFT calculations have been carried out to study the mechanism of the N2O O-insertion into the Ru−H bonds of ruthenium hydride complexes (dmpe)2Ru(H)(X) (X = OH, H). The reaction pathways for the formation of the monoinsertion product (dmpe)2Ru(H)(OH) and the bis(hydroxo) complex (dmpe)2Ru(OH)(OH), which were obtained directly from the reactions of N2O with the ruthenium hydride complexes, have been investigated in detail. Focus has been made to understand how the kinetically inert N2O is activated by the hydride complexes. It is found that N2O is activated through the hydride ligand nucleophilically attacking the terminal nitrogen of N2O followed by coordination of the activated N2O via the O-end

DFT Studies on the Mechanism of Reactions between N₂O and Cp₂M(η²-alkyne) (M = Ti, Zr)

DFT calculations have been carried out to study the activation of N2O by the transition-metal alkyne complexes Cp2M(η2-alkyne) (M = Ti, Zr). The mechanism for the formation of the five-membered metallacyclic complexes Cp2M(RCCR′NN(O)), which were obtained directly from the reactions of N2O with the metal alkyne complexes, and the conversion of the five-membered metallacyclic complexes Cp2M(RCCR′NN(O)) via N2 loss to the oxametallacyclobutene complexes Cp2M(RCCR′O) have been investigated in detail. An effort has been made to understand how the kinetically inert N2O can be activated. We concluded that N2O is best activated by metal fragments that possess high capability of π-back-bonding interactions with the π* orbitals of N2O

Theoretical Investigation of Alkyne Metathesis Catalyzed by W/Mo Alkylidyne Complexes

In this paper, the mechanism of alkyne metathesis catalyzed by W/Mo alkylidyne complexes has been theoretically investigated with the aid of density functional theory calculations. Calculations on various model alkylidyne complexes M(⋮CMe)(OR)3 (M = W, Mo; R = Me, CH2F), W(⋮CMe)(NMe2)3, and W(⋮CMe)(Cl)3 allow us to examine the factors that influence the reaction barriers. In the reaction mechanism, metallacyclobutadienes are initially formed from a ring-closing step between alkynes and alkylidyne complexes. A ring-opening step then gives the metathesis products. The factors that determine the metathesis reaction barriers have been examined. The reaction paths leading to the formation of Cp complexes, a possible path deactivating catalytic activity, were also studied

O-Abstraction Reactions of Nitrous Oxide with Cp₂Ti(II) and Other Middle Transition Metal Complexes

DFT calculations have been carried out to study the detailed mechanisms of the O-abstraction reaction of N2O with Cp2Ti(II). The reaction is initiated by coordination of N2O to Cp2Ti via the N-end to form a linear N2O-coordinated species Cp2Ti(N2O), from which the metal center transfers one of its metal d electrons to one π* orbital of the N2O ligand and gives a bent N2O-coordinated intermediate Cp2Ti←NN−O. The intermediate then reacts barrierlessly with another molecule of Cp2Ti to form an N2O-bridged intermediate Cp2Ti←NN−O−TiCp2, from which the singly oxo-bridged product (Cp2Ti)2O is formed with a release of N2. Reactions of N2O with other middle transition metal complexes have also been calculated and discussed. General mechanisms for O-abstraction reactions of N2O with early and middle transition metal complexes have been provided

Palladium-Catalyzed Regioselective Cyclopropanating Allenylation of (2,3-Butadienyl)malonates with Propargylic Carbonates and Their Application to Synthesize Cyclopentenones

An efficient and highly regioselective route to synthesize polysubstituted 1,3,4-alkatrien-2-yl cyclopropane derivatives via Pd(0)-catalyzed highly regioselective coupling cyclization of (2,3-butadienyl)malonate or bis(phenylsulfonyl)methane with propargylic carbonates was reported. The reaction proceeded smoothly under neutral conditions to afford the products in 73−96% yields. The products may be efficiently converted to cyclopentenone derivatives via a catalytic Pauson−Khand reaction under ambient conditions

Palladium-Catalyzed Decarboxylation of Allenyl 3-Oxoalkanoates: An Efficient Synthesis of 3,4-Allenyl Ketones

An efficient synthesis of 3,4-allenyl ketones via the Pd-catalyzed decarboxylative coupling of the readily available 3-oxoalkanoates is reported. The C–C bond forming reaction occurs under mild conditions producing CO₂ as the only byproduct

Synthesis of the Chiral Triphosphine (<i>S</i>,<i>S</i>)-PhP(CH₂CHMeCH₂PPh₂)₂ and Its Metal Complexes

The chiral triphosphine ligand (S,S)-PhP(CH2CHMeCH2PPh2)2, ttp*, was synthesized by the reaction of (S)-Ph2PCH2CHMeCH2Cl and PhPH2 in the presence of LDA. Reactions of ttp* with RuCl2(PPh3)3, [RhCl(COD)]2, and CoCl2 produced RuCl2(ttp*), RhCl(ttp*), and CoCl2(ttp*), respectively. These compounds were characterized by elemental analysis and multinuclear NMR spectroscopy. The structure of RhCl(ttp*) has been determined by X-ray diffraction