A new synthetic route to dihydrobiphenylenes has been developed. The process involves a mild Ru(II) -catalyzed [2+2+2] dimerization of ortho-alkenylarylacetylenes or its more versatile variant, the Ru-catalyzed [2+2+2] cycloaddition of ortho-ethynylstyrenes with alkynes. Mechanistic aspects of this [2+2+2] cycloaddition are discussed.
"Formal" and standard RuII-catalyzed [2+2+2] cycloaddition of 1,6-diynes to alkenes gave bicyclic 1,3-cyclohexadienes in relatively good yields. When terminal 1,6-diynes 1 were used, two isomeric bicyclic 1,3-cyclohexadienes 4 or 6 were obtained, depending on the acyclic or cyclic nature of the alkene partner. When unsymmetrical substituted 1,6-diynes 7 were used, the reaction with acyclic alkenes took place regio- and stereoselectively to afford bicyclic 1,3-cyclohexadienes 8. A cascade process that behaves as a "formal" RuII-catalyzed [2+2+2] cycloaddition explained these results. Initially, a Ru-catalyzed linear coupling of 1,6-diynes 1 and 7 with acyclic alkenes occurs to give open 1,3,5-trienes of type 3, which after a thermal disrotatory 6e(-) pi-electrocyclization led to the final 1,3-cyclohexadienes 4 and 8. When disubstituted 1,6-diyne 10 was used with electron-deficient alkenes, new exo-methylene cyclohexadienes 12 arose from a competitive reaction pathway.
The Tp Me2 -containing hydride-iridafurans 2a,b (Tp Me2 = hydrotris(3,5dimethylpyrazolyl)borate) cleanly reacted with ethylene to give the bicyclic derivatives 6a,b. Formation of the latter complexes is a reversible process and it is proposed to occur by an electrocyclic ring closure that takes place between C2H4 and the 16eunsaturated intermediates A resulting from hydride migration to the -carbon of the metallacycle. Similar reactions were observed with a variety of alkynes RCCR (R = H, Ph, CO2Me) and R´CCH (R´= CO2Me, Ph, CMe3) with the regioselectivity observed for the latter substrates depending on the nature of R´. In the case of Me3SiCCH the structure of an 2 unexpected byproduct indicates that an alkyne-vinylidene rearrangement has taken place on the metal coordination sphere during the reaction and this observation suggests that in the mechanism of all these coupling processes the corresponding -adducts are active intermediates. Finally, complexes 2a,b reacted with H2 to give products derived from the hydrogenation of their alkenyl arms.
The reaction of Tp Me2 Ir(C 2 H 4 ) 2 (Tp Me2 =hydrotris-(3,5-dimethylpyrazolyl )borate) with 1 equiv of DMAD (DMAD= dimethyl acetylenedicarboxylate) and 1 equiv of an aromatic aldehyde cleanly yields iridafuran derivatives, characterized by spectroscopy and X-ray analysis. These derivatives experience a stepwise selective hydrolysis-decarboxylation of the CO 2 Me group located at the β-carbon of the metallacycle, which proceeds through two isolable intermediates: a water adduct of an iridaenolate and an iridium carboxylate bicyclic species.
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