Ruthenium-Catalyzed Synthesis of Alkylidenecyclobutenes via Head-to-Head Dimerization of Propargylic Alcohols and Cyclobutadiene-Ruthenium Intermediates

• Published in: Chemistry : a European journal 2005-02, Vol.11 (4), p.1312-1324
• Main Authors: Le Paih, Jacques, Dérien, Sylvie, Demerseman, Bernard, Bruneau, Christian, Dixneuf, Pierre H., Toupet, Loïc, Dazinger, Georg, Kirchner, Karl
• Format: Article
• Language: English
• Subjects: alkylidenecyclobutenes; alkynes; cyclodimerization; propargylic alcohols; ruthenium
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200400899

The reaction of propargylic alcohols with carboxylic acid, or phenol derivatives, in the presence of the precatalyst [RuCl(cod)(C5Me5)] leads selectively to a variety of alkylidenecyclobutenes through head‐to‐head dimerization of propargylic alcohol. The first step is the formation of a cyclobutadiene–ruthenium intermediate resulting from the head‐to‐head coupling of two molecules of propargylic alcohol. On protonation with strong acids (HPF6, HBF4) dehydration of the cyclobutadiene complex leads to formation of an alkylidenecyclobutenyl–ruthenium complex. The X‐ray structure of one such complex, [RuCl(C5Me5)(η4‐R′CCH‐‐CH‐‐CCR2)] (R′=cyclohexen‐1‐yl, CR2=cyclohexylidene) has been determined. Carboxylate is added at the less substituted carbon of the cyclic allylic ligand. DFT/B3 LYP calculations confirm that the intermediate arising from head‐to‐head coupling of alkyne to the RuClCp* species yields the cyclobutadiene–ruthenium complex more easily with propargylic alcohol than with acetylene. An original synthetic method is reported that involves the successive formation of a cyclobutadiene intermediate and a novel type of alkylidenecyclobutenyl–ruthenium complex with an unusual mode of coordination. Ruthenium‐catalyzed head‐to‐head cyclodimerization of propargylic alcohols leads to alkylidenecyclobutenes by addition of carboxylic acids or phenols (see scheme).