Alkyne/Alkene Coupling Reactions Leading to 1, 3-Diene and 2, 4-Pentadienyl Complexes of Rhenium(III). Activation of Dichloro(η5-pentamethylcyclopentadienyl)(η2-alkyne)-rhenlum(III) by Brønsted Acid Catalysis

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Abstract

Alkyne/alkene coupling reactions leading to 1, 3-diene- and 2, 4-pentadienyl-type complexes 9d-1 and 10a-d, respectively, of rhenium(III), arise from activation of dichloro(η5-pentamethylcyclo-pentadienyl)(η2-alkyne)rhenium(III) (2a-d) by catalytic amounts of Bronsted acids. These latter alkyne complexes of rhenium exhibit not the slightest reactivity toward alkenes if acids are absent. Isotopic labeling and trapping experiments suggest that initial HC1 elimination from the starting alkyne complexes 2a-d is promoted by trace amounts of strong Bronsted acid. This mechanistic key step is also supported by the observed product distribution that depends on the nature of the alkyne and alkene substituents as well as on the geometry around the metal center. In addition, derivatives of 2a-d without chloride ligands (e.g., the glycolato complex 5) do not allow alkyne/alkene coupling reactions. The new 1, 3-diene and 2, 4-pentadienyl complexes of trivalent rhenium were characterized by their NMR and mass spectra as well as by two X-ray structure analyses. (η5-C5Me5)ReCl24-(E, E')-3, 4-dimethylhexa-2, 4-diene) (9g, Me = CH3) crystallizes in the monoclinic space group P21/c, with a = 859.4 (1) pm, b = 2745.4 (4) pm, c = 855.5 (1) pm, β = 117.73 (1)°, and Z = 4. Pertinent bond distances are Re-C(2) = 222.0 (3), Re-C(3) = 236.4 (3), C(2)-C(3) = 143.8 (4), and C(3)-C(4) = 139.6 (4) pm. The dihedral angle between the C(2)/Re/C(5) and C(1…8)-planes is 95.8 (1)°. These data are in agreement with the description of this compound as a η4-s-cis-diene complex. (η6-C5Me5)ReCl(η5-(E, E)-3, 4-diphenylhexa-2, 4-dien-6-yl) (10d) crystallizes also in the monoclinic space group P21/c, with a = 1605.2 (2) pm, b = 1022.7 (3) pm c = 1567.6 (1) pm, β = 113.35 (1)°, and Z = 4. Pertinent bond distances are Re-C(1) = 242.1 (6), Re-C(2***6) = 218.3 (3), C(l)-C(2) = 153.1 (4), C(2)-C(3) = 143.6 (4), C(3)-C(4) = 149.0 (3), C(4)-C(5) = 141.9 (4), and C(5)-C(6) = 144.5 (3) pm. Due to this specific ligand, the 2, 4-pentadienyl skeleton defined by atoms C(2***6) is not coplanar, thus contrasting the common 2, 4-pentadienyl ligands typical of the so-called “open ferrocenes”.

Original languageEnglish
Pages (from-to)2821-2831
Number of pages11
JournalOrganometallics
Volume8
Issue number12
DOIs
StatePublished - Dec 1989

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