Allenylidene Diels-Alder reactions

An unusual Diels-Alder cycloaddition involving the Cp=Cy bond has been described. The reaction took place by treatment of the electron-deficient allenylidene moiety in complex [RuCp(=C=C=CPh2)(CO)(P -Pr3)][BF4] (46) with a 20-fold excess of isoprene at room temperature affording the cycloadduct 90 (Scheme 33) [287]. This Diels-Alder cycloaddition in which the allenylidene moiety acts as a dienophile was completely regioselective, only the Cp=Cy bond of the allenylidene skeleton being implicated. Furthermore, it was also regioselective with regard to the orientation of the diene with the exclusive attack of C(l) and C(4) carbons at the Cp and Cy positions, respectively. Allenylidene 46 also underwent Diels-Alder reactions with cyclopentadiene and cyclohexadiene to afford the... 

The allenylidene-ruthenium system also appeared as effieient catalyst precursors for enyne metathesis and they were applied to the preparation of fluorinated cyclic amino esters with a 1,3-diene structure allowing Diels-Alder reactions (Scheme 20) [43],... 

Scheme 20. Amino esters by eonsecutive enyne metathesis/Diels—Alder reaction. 4.2 Allenylidene-ruthenium and polymerization...

Esteruelas and coworkers reported the stoichiometric Diels-Alder type addition of dienes to the Cp-Cy double bond of allenylidene complexes to give the corresponding substituted vinylidene complexes (Equation 7.7) [33]. The results of this stoichiometric reaction prompted us to investigate the diruthenium complex-catalyzed allenylidene-ene reaction between alkenes and the Cp-Cy double bond of an allenylidene moiety. Results of inter- and intramolecular allenylidene-ene reactions providing novel coupling products between alkynes and alkenes are described in this section [34]. 

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