Alkylidene-alkene complexes

Detection of a rare alkylidene-alkene complex is possible by NMR spectioscopy using 22. The metallacyclobutane remains undetected (the ROMP propagation rate is higher than that of initiation). These ideas are supported by the kinetics studies of norbomene derivatives using 22.290... 

Section III of the review is concerned with alkylidene and alkylidyne halocarbonyl complexes, and a review on recent advances in the chemistry of metal-carbon triple bonds by Mayr and Hoffmeister in 1991 contains material of interest.6 Section IV is concerned with halocarbonyl alkyne and alkene complexes described from 1987 to mid-1995, because an exten-... 

The trityl cation (EbjC" ") is a commonly used reagent to effect the abstraction of a hydride from eoordinated ligands. For example, the preparation of metal alkylidene and alkene complexes by a- and jS-hydride abstraction from metal alkyls using trityl salts is a well-established synthetic method, Eqs 35 and 36. 

The ring-opening metathesis polymerization (ROMP, cf. Section 2.3.3) of strained-ring cyclic alkenes has attracted considerable attention in recent years due to the discovery that well-characterized metallacyclobutane [24] and metal alkylidene [25] complexes catalyze the living polymerization of monomers such as norbomene. Unfortunately, these catalysts often sulfer deactivating side reactions... 

Recycled solutions of 2 initiated ROMP as quickly as the recycled Ru(III) solutions, closer examination of which revealed NMR resonances identical to those of the alkene protons in 2 [25]. It was therefore suggested that a key step in the initiation process using Ru(III) was the in situ formation of a Ru(II)-alkene complex [27]. Current evidence supports the disproportionation of the Ru(III) species to form Ru(II) and Ru(IV) species, followed by formation of a Ru(II)-alkene complex [25]. The equilibrium constant for disproportionation is small, accounting for the poor initiation efficiency of the Ru(III) systems [30]. An alternative, the disproportionation of an equilibrium amount of Ru(III)-alkene complex to a Ru(II)-alkene complex and a Ru(IV) species, is unlikely since Ru(III)-alkene complexes are generally unstable. Formation of a ruthenium alkylidene, the requi-... 

Olefin metathesis is a catalytic process in which alkenes are converted into new products via the rupture and reformation of carbon-carbon double bonds. The key step in this process is the 2 + 2 reaction between an olefin and a transition metal alkylidene (carbene) complex, generating an unstable metallacyclobutane intermediate. This intermediate can either revert to the starting material, or open productively to regenerate a metal carbene and produce a new olefin (Eq. 1). 

When it became clear that carbene complexes might well be involved, van Rijn (1977) presented other kinetic models, based on alkene and alkylidene surface complexes as the reaction intermediates. The derived rate expressions were tested with data from the literature and it was shown that these models described the data as well as if not better than the Langmuir-Hinshelwood model. 

A significant development for the selective synthesis of alkenes makes use of alkene metathesis. Metathesis, as applied to two alkenes, refers to the transposition of the alkene carbon atoms, such that two new alkenes are formed (2.110). The reaction is catalysed by various transition-metal alkylidene (carbene) complexes, particularly those based on ruthenium or molybdenum. The ruthenium catalyst 84, developed by Grubbs, is the most popular, being more stable and more tolerant of many functional groups (although less reactive) than the Schrock molybdenum catalyst 85. More recently, ruthenium complexes such as 86, which have similar stability and resistance to oxygen and moisture as complex 84, have been found to be highly active metathesis catalysts. 

It should be noted that while a number iridapyrroles exist (Section II-E) their protonation does not universally afford isolable alkylidenes, particularly those complexes comprising an alkyl ligand in place of the hydride, which typically afford chelating Ti-alkene complexes. 

Organotransition metal chemistry. Alkyl and alkylidene derivatives. Complexes of alkenes and alkynes. 

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]. 

Detailed kinetic and stereochemical experiments have been made on the isomerization of an alkylidene to an alkene complex (Scheme 10). The reactions as shown are highly stereospecific and the kinetics of the last step have given AH = 20.4 1.4 kcal mol and AS = -27 0.3 cal K mol . The 1,2-hydrogen shift seems to occur spontaneously but probably not via a single transition state. ... 

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