Snapshot of a chelation-assisted C-H/alkyne coupling:: A ruthenium complex caught in the act of C-C bond formation

The complex Ru(CO)(2)(PPh3)(3) (1) reacts with o-(diphenylphosphanyl)benzaldehyde within 2 min at 20 degrees C to give the hydrido acyl complex Ru(H){P(C6H5)(2)(C6H4)C(O)}(CO)(2)(PPh3) (2) via selective activation of the C-H bond of the aldehyde function. Further addition of diphenylacetylene (110 degrees C, 3 h) affords the novel complex Ru{P(C6H5)(2)(C6H4)C(O)(C6H5)CCH(C6H5)}(CO){P(C6H5)(3)} (3), incorporating the newly assembled o-(diphenylphosphanyl)phenyl (E)-stilbenyl ketone ligand. The alpha,beta-unsaturated ketone moiety of the latter is bound to the metal in an eta(4) coordination mode involving both a side-on coordination of the carbonyl group and a more classical eta(2) linkage of the olefinic bond. Both complexes were fully characterized by spectroscopic methods and by X-ray diffraction. The whole reaction sequence provides a valuable experimental model for the chelation-assisted hydroacylation of an alkyne with a tethered aldehyde.