Alkylidene complexes reactivity

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

The reactivity of a remarkable electronically unsaturated tantalum methyli-dene complex, [p-MeCgH4C(NSiMe3)2]2Ta( = CH2)CH3, has been investigated. Electrophilic addition and olefination reactions of the Ta = CH2 functionality were reported. The alkylidene complex participates in group-transfer reactions not observed in sterically similar but electronically saturated analogs. Reactions with substrates containing unsaturated C-X (X = C, N, O) bonds yield [Ta] = X compounds and vinylated organic products. Scheme 117 shows the reaction with pyridine N-oxide, which leads to formation of a tantalum 0x0 complex. ... 

Alkylidene complexes are generally considered to be reactive intermediates but the actual surface organometallic species have never been fully characterized. However, the synthesis of silica-supported tantalum(V) carbene complexes and their characterization have been reported.332... 

Although transition metal alkylidene complexes, i.e., carbene complexes containing only hydrogen or carbon-based substituents, were first recognized over 15 years ago, it is only relatively recently that Ru, Os, and Ir alkylidene complexes have been characterized. Neutral and cationic complexes of these Group 8 metals are known for both metal electron configurations d8 and d6. The synthesis, structural properties, and reactivity of these compounds are discussed in this section. 

The reactions of other reactive zero-valent Group 8 substrates M(CO)2(PPh3)3 (M = Ru, Os), RuH(NO)(PPh3)3, and Ir(NO)(PPh3)3 with RCHN2 (R = H, Me,p-tolyl, C02Et) failed to yield carbene complex products, indicating that very specific properties of the substrate complex are required for successful isolation of alkylidene complexes. 

The characteristic reactivity of neutral dg alkylidene complexes of Ru, Os, and Ir is with electrophilic reagents. The osmium methylene 47 reacts with the widest range of electrophiles, the most significant reactions being summarized in Scheme 2. 

Removal of a product (e.g., ethylene, Eq. 2) from the system could dramatically alter the course and/or rate of a desired metathesis reaction, since ethylene reacts with an alkylidene complex to form a methylene (M=CH2) complex, which is the most reactive (and also the least stable) of the alkylidene complexes. Of... 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

Today, however, carbene complexes covering a broad range of different reactivities have been prepared. Often it is no longer possible to predict whether a carbene complex will behave as an electrophile or as a nucleophile. Thus, a reactivity-based nomenclature would be difficult to apply consistently. For this reason in this book compounds with a carbon-metal double bond will be called carbene complexes or alkylidene complexes , terms not associated with any specific chemical behavior. 

Mo and W alkylidene complexes 4, the so-called Schrock carbenes, have explosively evolved the polymerization chemistry of substituted acetylenes. Although the preparation of these catalysts is relatively difficult because of their low stability, in other words, high reactivity, they elegantly act as living polymerization catalysts for substituted... 

A special note about nomenclature is in order. Both the terms carbene and alkylidene complex have come into use, and attempts have been made to use these terms to differentiate various types of reactivity of these species. However, the term carbene complex is used throughout this chapter, primarily because of the emphasis on cyclopropanation reactivity. 

A study has been made of the reactivity of high-oxidation state tantalum-alkylidene complexes stabilized by the aryldiamine ligand [C6H3(CH2NMe2)2-2, 6] , and an unusual rearrangement involving the unique activation of an aryl C—H bond by the alkylidenetantalum moiety has been observed.531... 

In order to investigate this point more fully, the rates of reaction of the two complexes with ethyl vinyl ether (EVE) were studied. This alkene was chosen as it is rather reactive towards ruthenium alkylidene complexes and forms an inert alkoxyalkylidene product in an essentially irreversible manner. This alkene, therefore, should rapidly capture any nascent complex from which a Cy3P ligand has dissociated (27 and 30 in Scheme 12.21). The two complexes displayed very different kinetics. The rate of reaction of the first generation pro-catalyst complex 24c with EVE was found to be dependent on EVE concentration (over a range of 30-120 equivalents of EVE) and did not reach pseudo-first-order conditions... 

Further investigation of the equilibrium between titanacyclobutene and titanium vinyl alkylidene complexes, as discussed in Section 2.12.6.1.4, was reported recently <2007CEJ4074>, along with the incorporation of this reactivity pattern into the synthesis of conjugated dienes, homoallylic alcohols, vinylcyclopropanes, and phosphacyclobutenes from y-chloroallyl sulfides and a source of titanocene(ll). 

Further synthetic utility can be introduced by changing the metal in the above alkylidene complexes from tungsten to molybdenum [52]. Molybdenum which is less oxophilic than tungsten will tolerate monomers containing mildly reactive functionalities such as esters [52a] and nitriles [52b] without appreciable catalyst deactivation during the polymerization reaction. For example, the polymerization of e rfo,endo-5,6-dicarbomethoxynorbornene (DCNBE) with Mo(CHtBu)(NAr) (OtBu)2 (Ar = 2,6-diisopropylphenyI) (35a) has been reported to give the cone-... 

The reactions of carbyne complexes will be discussed, albeit more briefly, along the lines developed for alkylidene complexes, to emphasize the parallels. Once again, as with alkylidenes, early generalizations suggested two classes of complexes, Fischer-type and Schrock-type, which have subsequently been recognized as extremes in a tunable reactivity continuum. Furthermore, within the chemistry of tungsten, Fischer-type... 

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