Silver(I) Interactions with Ketones. Site of Complexation with Acetophenones and Effectiveness as a Lewis Acid Catalyst

Article abstract of DOI:10.1021/jo00187a034

Aromatic ketones present three possible sites for complexation of Ag(1+): the oxygen lone pair, the ? electrons of the carbonyl bond, and the ? electrons of the aromatic ring.Upfield shifts of (13)C chemical shifts of meta and para carbons of acetophenone in the presence of silver nitrate showed that Ag(1+) complexes with the aromatic ring moiety in water.This contrasts with previous results in methylene chloride in which the carbonyl group is not hydrogen bonded to solvent and acts as an n donor toward Ag(1+).In the solid state, an X-ray structure determination of (p-methylacetophenone)2AgBF4 showed that Ag(1+) was tetracoordinated to two carbonyl oxygens (2.36 Angstroem) and to two aromatic rings of different ketone molecules (2.55- and 2.72-Angstroem distances to meta and ortho carbons, respectively).Thus in the solid state, acetophenone acts both as an n and ? donor.The Ag-O bond was shorter than most Ag-O bonds and appears to contribute more to the stabilization of the complex than Ag(1+) interactions with the benzene rings.An sp² hybridization at oxygen was indicated by an AgOC angle of 137 deg and the fact that Ag(1+) is only 6.2 deg above the plane of the carbonyl bond.Although Ag(1+) catalyzed an aldol condensation of acetophenone in 1,2-dichloroethane at 70-80 deg C, rates of hydrogen exchange for acetone in deuterated water at 44 deg C showed no catalytic activity of AgNO3 or LiNO3.