Catalytic Addition of Carbo-Nucleophiles

All the examples described in the previous section involve the addition of hetero-nudeophiles, such as alcohols and amines, to the vinylidene carbon. Addition of 

Reactions of 5-siloxy-5-en-l-ynes are summarized in Table 5.2. In every case the reaction proceeds at rt in the presence of H2O to give cyclized p,y-unsaturated ketones in good yield without isomerization of the double bond, even using only 10 mol% W(CO)5(thf). The most characteristic feature of this reaction is that the endo mode of cyclization occurs more readily than in other related cyclization reactions such as HgCl2-mediated reactions. 

Although the exact reaction mechanisms and origin of endo versus exo selectivity remain to be clarified, these reactions should find wide use in the 

Electrocydization is another major mode of reaction utilizing the vinylidene complexes of Group 6 metals. It should be noted that the first example of this type of reaction was reported by Merlic et al. using a ruthenium complex employing nonaromatic diene-yne substrates [26]. 

In order to carry out this reaction by a one-pot procedure, we examined the same complexation reaction in the presence ofaketene acetal, which led us to the discovery of the novel reaction pathways described below [32]. Thus, treatment of o-ethynylphenyl ketone 104 with a catalytic amount of W(CO)5(thf) in the presence qf4equiv of 1,1-diethoxyethylene at rt gave a novel polycyclic compound 120 in good yield as a single stereoisomer. Both vinyl ethers and ketene acetals can be employed as the electron- 

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