Alkylidenes hydride complexes

These results at least demonstrate that ethylene can be polymerized by an alkylidene hydride catalyst, probably by forming a metallacyclobutane hydride intermediate. The extent to which this is relevant to the more classical Ziegler-Natta polymerization systems (27) is unknown. Recent results in lutetium chemistry (28), where alkylidene hydride complexes are thought to be unlikely, provide strong evidence for the classical mechanism. 

The stoichiometric interaction of an enyne and [RuCl(PCy3)(pcymene)]B(Ar )4 XVIIIa containing a bulky non-coordinating anion B(ArF)4 showed by NMR at —30 ° C the formation of the alkenyl alkylidene ruthenium complex and acrolein. This formation could be understood by the initial formation of a vinylidene intermediate and transfer of a hydride from the oxygen a-carbon atom to the electrophilic vinylidene carbon, as a retroene reaction step (Scheme 8.13) [54]. 

The reaction of organometalhc compounds with O2 may produce more or less stable dioxygen complexes. An early and unambiguous example of this kind of transformation was provided in the report by van Asselt et al. of the isolation of a series of stable peroxo alkyl complexes of the type Cp Ta( -02)R (R = Me, Et, Pr, Bn, Ph) [5]. As shown in Scheme 1, O2 presumably oxidatively adds to the 16-electron fragments Cp 2TaR, which are in rapid equihbrium with the 18-electron olefin hydrides or alkylidene hydrides. 

The reduction behaviour of the alkylidene adduct of a cobalt-dithiolene complex (423) has been examined548 and the study has shown that, when the alkylidene-bridged structure (423) is reduced by one electron, it isomerizes rapidly and quantitatively to the ylide form (424). This represents the first example of reversible isomerization of the metal-carbon bond in a cobaltadithiolene complex. A surprising cis- to tra .s-dihydride isomerization which is unprecedented for 18-electron six-coordinate complexes has been observed549 in an octahedral iridium-c7.y-di hydride complex. 

In some cases, hydrogenation of the alkylidenes and alkylidynes reduces the metal-carbon multiple bonds to single bonds. The alkyhdene hgand in (29) is converted to an alkyl gronp when exposed to H2, leading to the formation of an interesting tantalum hthium bridging hydride complex. 

Finally, the iridium carbene complex 580, obtained from the double C-H activation of 2-ethylphenol by Tp Ir(C6H5)2(N2) (211), illustrates the rare incidence of an equilibrium between alkylidene hydride and alkene hydride complexes. The alkylidene forms in admixture with ca. 5% of the alkene hydride isomer 581, illustrating a preference for the a- over hydrogen in the second activation step. However, in isolation 581 is observed to re-establish the same equilibrium mixture (i.e. 20 1 580 581, Scheme 61) a rare example of a metal-alkene converting to a metal-alkylidene, the reverse reaction being more typical. 

In most cases, the immediate product of a-hydrogen elimination is not observed. Instead, reductive elimination of a hydride and an alkyl group to form an alkane frequently occius (Equation 10.30). In other cases, a carbene forms from a dialkyl complex of a d metal that cannot accommodate the additional valency required to form an alkylidene and a hydride ligand. In these cases, an alternative four-center pathway involving the transition state shown in Equation 10.31 that does not involve formation of a metal-hydride complex is followed. When the valency of the metal allows either pathway to occur, it is difficult to distinguish between the a-elimination and a-hydrogen abstraction pathways. ... 

Carbene complexes have also been shown to undergo a-hydrogen elimination, in this case to form alkylidyne complexes. This reaction is much less common than the reaction of alkyl complexes to form alkylidenes, but a few examples are well documented. Two examples of this transformation are shown in Equations 10.39 and 10.40. Schrock reported the first direct observation of this transformation (Equation 10.39). ° In this first example, an isolated carbene complex converted to an alkylidyne hydride complex upon abstraction of a chloride ligand with trimethylaluminum. In a second example, a double C-H activation process by two sequential a-hydrogen elimination reactions converts the starting tungsten-methyl complex in Equation 10.40 into a methylidyne complex. ... 

Addition of carbene complexes to mixtures of vinylsilanes and olefins does not necessarily mean that their transformations proceed exclusively by the carbene mechanism since alkylidene complexes can decompose to form catalytically active hydride species [23]. Conversely, alkylidene complexes can be formed in the presence of hydride complexes [24]. 

Alkylidenation Several attempts have been made to prepare titanium alkylidene complex 17. The reaction of titanocene dichloride with triethylaluminum does not afford an alkylidene-bridged complex similar to the Tebbe reagent, probably due to the presence of a jS-hydrogen, and produces instead the aluminotitanium hydride... 

Common for alkyls that lack (3 hydrogens, this is the reverse of 1,1 insertion (e.g., Eq. 7.14). (3 elimination being impossible, L M-Me can only undergo an a elimination to give L M(=CH2)H. While any (3 process gives an alkene, a stable species that can dissociate from the metal, an alkylidene ligand from an a elimination is unstable in the free state and cannot dissociate. Methylene hydride complexes are therefore rarely seen because they are thermodynamically unstable with... 

The synthesis and X-ray structural determination of a stable Ir111 hydride/alkylidene complex, (165), has been reported, in which the tridentate N3 ligand is TpMe2. 9 The complex undergoes reversible hydride migration onto the electrophilic carbene atom, as shown in reaction Scheme 20. 

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