Metallacydic intermediate

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). ... 

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). 

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). 

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