Ruthenium Catalyzed Reactions of Aldehydes: Molecular Structures of Ruthenium Complexes with Aldehyde Ligands

Abstract

[RuCl(H)(CO)(PPh3)3] (2) was found to catalyze, in the presence of H2C=CHSiMe3(3), the trimerisation of aldehydes RCHO [R=Et (4a), i-Bu (4b)] yielding 1,3,5-trioxanes (5) and the aldol condensation yielding α,β-unsaturated aldehydes (6). When (4a) was used as a reactant, from these reaction mixtures, the ruthenium complex [RuCl2(CO)(PPh3)2(i-BuCHO-κO)] (7) having the aldol condensation product as the ligand crystallized. In the analogous reaction with (4b), the complex [RuCl2(CO)(PPh3)2(i-BuCHO-κO)] (8) with the aldehyde as ligand was obtained. The constitution of these complexes was established by single-crystal X-ray diffraction measurements. The ruthenium centers are octahedrally coordinated having the aldehyde and the carbonyl ligand in mutually trans positions (coordination index: OC-6-12). The aldehydes are monodentately coordinated via the carbonyl oxygen atom (κO). The coordination induced elongations of the C=O double bonds [1.242(4) A (7), 1.234(4) A (8)] indicate an activation of the aldehydes. Furthermore, the Ru-CO bond lengths [1.842(4) A (7), 1.823(4) A (8)] exhibit a relatively low trans influence of the aldehyde ligands. The formation of the complexes (7) and (8) give an indication that the Lewis acidity of the ruthenium center is of importance for aldehyde activation in the catalytic process.