Metal-ligand cooperative C-O bond cleavage of propargylic alcohol with protic pyrazole complexes of ruthenium

摘要

The C-O bond cleavage of a propargylic alcohol, 1,1-dimethyl-3-phenylprop-2-yn-1-ol (3), by metal-ligand cooperation was investigated. The reactions of a naphthalene complex [CpRu(eta(6)-C10H8)]PF6 (Cp = eta(5)-C5H5) with 5-R-3-(pyrid-2-yl)pyrazoles (R-LH; R = Ph, CF3) in acetonitrile afforded the starting metal-ligand bifunctional ruthenium complexes [CpRu(R-LH)(MeCN)]PF6 (1a, R = Ph; 1b, R = CF3) featuring an N-N chelate protic pyrazole. The treatment of the CF3-substituted pyrazole complex 1b with 3 in 1,2-dichloroethane at 50 degrees C resulted in the formation of the eta(3)-butadienyl complex 5. Meanwhile, the corresponding reaction of the phenylpyrazole complex 1a in 1,4-dioxane at 90 degrees C gave the N-allenylmethylpyrazole complex 6. The C-O and C-H bond cleavage of 3 in these reactions can be ascribed to the metal-ligand cooperation: initial formation of an eta(3)-propargyl complex assisted by NHMIDLINE HORIZONTAL ELLIPSISO hydrogen bonding and following C-H deprotonation by the neighboring pyrazolato ligand. On the other hand, in boiling methanol, the protic pyrazole complex 1a catalyzed the C-O bond cleavage of 3, which resulted in catalytic isomerization of 3 to an alpha,beta-unsaturated enone, 3-methyl-1-phenylbut-2-en-1-one (8). The control experiments using non-protic and isocyanide ruthenium complexes indicated that both a labile nitrile ligand on the metal and the protic cooperating ligand are crucial for the catalysis.