Metallacyde

In 1978, Tebbe and co-workers reported the formation of the metallacyde 4, commonly referred to as the Tebbe reagent, by the reaction of two equivalents of trimethylaluminum with titanocene dichloride. The expulsion of dimethylaluminum chloride by the action of a Lewis base affords the titanocene-methylidene 5 (Scheme 14.4) [8]. 

In this case the reaction takes place with other C-C multiple bonds rather than with a nucleophile (substrates of type 25). Cydometallation of the Jt-bonds leads either to metallacyde 26 or 27 (Scheme 15.4). Then different pathways are possible, for example /3-hydride elimination and reductive elimination are known. 

Again one suspects metallacydes 10 as intermediates that either insert CO or undergo a reductive elimination immediately. 

Considering the mechanistic rationales of the transition metal-catalyzed enyne cycloisomerization, different catalytic pathways have been proposed, depending on the reaction conditions and the choice of metal catalyst [3-5, 45], Complexation of the transition metal to alkene or alkyne moieties can activate one or both of them. Depending on the manner of formation of the intermediates, three major mechanisms have been proposed. The simultaneous coordination of both unsaturated bonds to the transition metal led to the formation of metallacydes, which is the most common pathway in transition metal-catalyzed cycloisomerization reactions. Hydrometalation of the alkyne led to the corresponding vinylmetal species, which reacts in turn with olefins via carbometalation. The last possible pathway involves the formation of a Jt-allyl complex which could further react with the alkyne moiety. The Jt-allyl complex could be formed either with a functional group at the allylic position or via direct C-H activation. Here the three major pathways will be discussed in a generalized form to illustrate the mechanisms (Scheme 8). 

MetaUacycle formation has also been observed in bis-Cp complexes. Heating Cp 2UR[P(Si(CH3)3)2] (R = Cl [146840-37-1], CH3 [146840-39-3]) results in the metaUation of the phosphido ligand. These complexes are structurally similar to the group 4 and 6 transition-metal metallacyde complexes, but show a dramatically reduced reactivity. 

Another class of heteroleptic alkyl complexes contains 7t-donating ancillary ligands such as RU[N(Si(CH3)3)2]3 (R = CH3, H, BH ). The hydride species can be converted into the methyl species via reaction with BuLi and CH3Br. The methyl compound has exhibited insertion chemistry with small molecules including aldehydes, ketones, nitriles, and isocyanides (206). Stable metallacyde compounds are also known, ie,... 

Suggs isolated and characterized the 5-membered metallacyde. See Suggs JW (1978) J Am Chem Soc 100 640... 

Figure 4 Possible orientations of the bottom-bound metallacyde relative to the NHC ligand...

It is expected that ligand-induced asymmetry with respect to the two faces of the metallacyde will have the most discernible influence on the syn-anti preference of disubstituted metallacycles. These effects will only be evident, however, if ligand rotations with respect to the metallacyde plane are slower than either productive... 

Substitution Reactions. As already mentioned (Eq. (3)) [39], bis-cyclometallated complexes of Pt(II) react very specifically with HCl, with elimination of one of the metallacyde-forming ligands and with formation of a chloro-bridged dimer. This is the method applied for the preparation of the bis-heteroleptic complexes. 

Pt(II) to the alkyne of the substrate likely triggers all these events. The cydoisomerization might undergo a metallacydic intermediate that proceeds to eliminate /3-H. The formation of cydopropanes is presumably succeeded via alkenyl platinum carbene followed by platina(IV)cyclobutane intermediates. The extension using formal metathesis of the enynes includes two transformations, the formation of 1,3-diene moieties and the stereoselective tetrasubstituted alkene derivatives via O C allyl shift, both leading to diverse structural motifs and serving as the key step in the total synthesis of bioactive targets (Scheme 83). ... 

Dating from the original discoveiy from Reppel on the cyclooligomerization of acetylene, nickel-catal5 ed multicomponent cydoadditions have attracted considerable attention (see also Houben-Weyl, VoL E18, pp 987,993).l l Metallacydes have been proposed as important intermediates in most classes of cyclotrimerizations. The mechanism is likely to involve initial oxidative cydization to a five-membered metaUacyde, followed by insertion of a third unsaturated component, and finally reductive elimination to afford six-membered ring produds (Scheme 46). 

The example in Scheme 6.16.4 shows the association of the neutral ligand ethylene to the Ni-hydride catalyst applied in the SHOP process, which carries an o-diphenylphosphinebenzoic add ligand (Vogt, 2002). In addition, the first step in the Cr-metallacyde mechanism, the addition of two neutral ethylene molecules to the Cr prior to the oxidative coupling step, is a ligand assodation step. 

Scheme 10.8 Proposed mechanism for formation of dinickel(l) compound 17 from metallacydic Ni(ll) carboxylate "nickelalactone" and dppm.

Scheme 3.10 Metallacydic mechanisms for selective ethylene trimerization to 1-hexene proposed by Manyik et al. (1977) (path (a)), Briggs (1989) (path (b)), and Hessen (Deckers et al., 2001, 2002) (path (c)).

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