Alkylidene-metals

It is known from previous work that diazoalkanes can form carbene(alkylidene)-metal complexes [17, 18], cf. eq. (5). It is thus reasonable to assume that a metal-carbene 9 is formed from the (phosphine oxide-stabilized) species CH3Re 02) (eq. (5)). High oxidation-state metal carbene complexes have ample precedent, especially through the work of Schrock et al. [19], Isolation of type-8 species may be facilitated by sterically more-demanding auxiliary groups (e. g., C5H5 in place of CH3) or, by using heterocyclic carbenes of pronounced Lewis basicity (e. g., 1.3-imidazolin-2-ylidene [20]). 

Alkylidyne-metal complexes have traditionally been divided into two categories, according to the oxidation state of the metals, in a manner directly analogous to the classification of the very large number of known alkylidene-metal species (19a,b). Hence Fischer-type alkylidyne complexes involve metals in low oxidation states, while Schrock-type complexes generally involve more electropositive metals with higher oxidation states (13). However, the properties of some of the numerous carbyne-metal complexes that have been characterized since the early days have in many cases blurred the distinction between the two classes (12a). 

Reactivity characteristic of alkylidene complexes of tantalum is that the a-carbon is susceptible to electrophilic attack, in contrast to the electron-deficient a-carbon of Fischer-type carbene complexes of group 6 transition metals [62]. Based on this unique property of the alkylidene metal-carbon double bond, a range of new types of reactions has been developed. The discovery of the alkylidene complexes of tantalum was a key to understanding the mechanism of olefin metathesis, and they continue to play important roles in C—H bond activation, alkyne polymerization, and ring-opening metathesis polymerization. 

Many metal-catalyzed reactions of propargylic esters proceed as if alkylidene-metal carbenoids are involved. In the Pd-catalyzed reaction such species add to norbomenes/ norbomadienes to give ring expansion products. 

Acetylenes also react with alkylidene complexes to afford a new carbene derivative formed by inserting the triple bond into the alkylidene-metal bond . A metallacyclo-butene is the intermediate species ... 

Two different methodologies are available for the proton-mediated formation of dimetal complexes from the compounds 1 or 2. In the first, mixtures containing one of the reagents 1 or 2 together with one of the neutral alkylidyne(cyclopentadienyl)metal complexes 3 are protonated. Protonation of the salts 1 or 2 generates an electronically and coordinatively unsaturated alkylidene-metal species, which can bind a molecule of type 3 to generate a metal-metal bond. This process is formally similar to the... 

Dimetal complexes may equally well be obtained from the reagents 1,2 or 3 alone, by treatment with 0.5 molar equivalents of acid. With this stoicheiometry a mixtxue is effectively created upon protonation, containing alkylidyne- and alkylidene-metal species in equal proportions. The validity of this methodology was first demonstrated in 1985 during protonation studies... 

Alkene metathesis is a catalytic reaction that has brought revolutions during the last 15 years, not only in catalysis and organic synthesis but also in polymer and material science. This is due to the discovery of the catalytic mechanism based on metal-caibene by Chauvin [1] and of well-defined, efficient catalysts from 1990 based on coordinatively unsaturated alkylidene-metal complexes mainly derivatives of molybdenum by Schrock [2] and of ruthenium (Ru) by Gmbbs [3]. The increasing importance of alkene metathesis and its catalysts by the scientific community has led to the award of 2005 chemistry Nobel Prize to the main pioneers in this field, Chauvin, Gmbbs, and Schrock [1-3]. 

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