Alkylidene complexes chemistry

The presence of a heteroatom at the alkylidene carbon atom moves properties of metal-alkylidene complexes to the borderline of Fischer carbene chemistry.69 The... 

We found no evidence for isomerization of the a,6-dialkoxy-alkylidene complexes CpFe(C0)L[C(0Et)CH20R] (9,, 15) to alkylated carboalkoxymethyl salts CpFe(C0)L(CH2C(OR)(OEt) ) under ambient conditions. Indeed the solution chemistry of a,6-dialkoxyethylidene complexes parallels that of a-alkoxyethylidene compounds CpFe(C0)L[C(0R)CH3]+, in that 9,14,15 quantitatively revert to 5-7 upon treatment with excess io3i3e in CH2C12. We did, however, prepare samples of the ethylated carboalkoxymethyl... 

The chemistry of alkylidene and alkylidyne complexes of early transition metals was developed by Schrock and co-workers and these complexes turned out to be of crucial importance to alkene and alkyne metathesis. Initially their research focused on tantalum complexes of the type CpTaCEIE, which after a-elimination (Figure 16.6) led to alkylidene complexes Cp(R)Cl2Ta=CHR [11]. 

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... 

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... 

The synthesis of d° Alkylidene complexes by Schrock demonstrated that carbene complexes could be isolated that were electronically similar to those postulated to be involved in the alkene metathesis reaction see Schrock-type Carbene Complexes). Eventually, this pioneering work led to the synthesis of a class of compounds that are among the most active catalysts known for metathesis chemistry. The first observation that a d° carbene complex was involved in metathesis chemistry was when Tebbe showed that the Ti complex (2) would catalyze the degenerate metathesis of... 

Surface chemistry of the alkyl/alkylidene complexes has been studied by C CPMAS NMR spectroscopy. Cp2Ta(= CH2)( CH3) is converted predominantly to a stable cation-hke Cp2Ta( CH3)2+ species on partially dehydroxylated Si02 (PDS) presumably via electrophilic addition of a proton from the PDS surface. The formation of cation-like Cp2Ta( CH3)2+ and CpTa( CH3)3+ is observed when Cp2Ta( CH3)3 and CpTa( CH3)4 are chemisorbed on PDS and dehydroxylated Si02 (DS),... 

Evidence points to initial electron transfer between (Re)-CH2R and Ph3C+ followed by H transfer to PhsC. For further chemistry of these alkylidene complexes, see Section 8.1.3. 

The chemistry of transition metal carbene complexes has been examined with an eye to applications in organic synthesis ever since their discovery by Fischer in 1964, and the growth in the number of useful applications has been exponential with tirne. " There are two types of transition metal carbene complexes those which have electrophilic carbene carbons and which are typified by the pentacarbonylchro-mium complex (1), and those which have nucleophilic carbene carbons and which are typified by the biscyclopentadienyltitanium complex (2). Complexes (1) and (2) are often referred to as carbene and alkylidene complexes, respectively. This review will be limited to the chemistry of electrophilic carbene complexes of the Fischer type. The chemistry of the nucleophilic alkylidene complexes will be covered in Chapter 9.3, this volume. ... 

Alkene metathesis catalysis involves intermediates in which a transition metal is multiply bonded to carbon. These species are often referred to as nucleophilic caibenes when the carbon atom is negatively polarized. A more functional description is to name these compounds as alkylidene complexes, since they react to transfer an alkylidene moiety from a transition metid to a substrate carbon atom. Previous sections of this chapter have focused on a common example of this chemistry the process of metathesis that involves transition metal mediated interaction of carbon-carbon multiple bonds. 

Although early transition metal-alkylidene complexes have rather well-defined chemistry involving the nucleophilicity of Ccarbene, mid- to late transition metal alkylidenes and Fischer carbene complexes sometimes have a different chemistry associated with them. In this section, we will look at some of this chemistry, some of which has significant impact on modern organic synthesis methodology. We will compare the chemistry of these complexes with that of traditional Fischer carbene complexes. 

Alkoxide ligands play an important spectator role in the chemistry of metal-carbon multiple bonds. Schrock and coworkers have shown that niobium and tantalum alkylidene complexes are active toward the alkene metathesis reaction. One of the terminating steps involves a j8-hydrogen abstraction from either the intermediate metallacycle or the alkylidene ligand. In each case the -hydrogen elimination is followed by reductive elimination. The net effect is a [1,2] H-atom shift, as shown in equations (73) and (74), and a breakdown in the catalytic cycle. Replacing Cl by OR ligands suppresses these side reactions and improves the efficiency of the alkylidene catalysts. ... 

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