Metallacydes mechanism

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 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)).

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). 

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). 

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

---