Selective Hydrogenation of α,ß-Unsaturated Aldehydes and Ketones by Air-Stable Ruthenium NNS Complexes.

The selective hydrogenation of the carbonyl functionality of α,ß-unsaturated aldehydes and ketones is catalysed by ruthenium dichloride complexes bearing a tridentate NNS ligand as well as triphenylphosphine. The tridentate ligand backbone is flexible, as evidenced by the equilibrium observed in solution between the cis- and trans-isomers of the dichloride precatalysts, as well as crystal structures of several of these complexes. The complexes are activated by base in the presence of hydrogen and readily hydrogenate carbonyl functionalities under mild conditions. Despite the activation by base, side reactions are negligible, even for aldehyde substrates, because of the low amount of base. Thus, the corresponding allylic alcohols can be isolated in very good yields on a 10-25 mmol scale. Turnover numbers up to 200 000 were achieved.

Arene C(sp(2))-H Metalation at Ni(II) Modeled with a Reactive PONCPh Ligand.

Coordination of the reactive phosphinitopyridylphenyl PONCPh ligand L(H) to NiBr2 initially yields paramagnetic brown NiBr2(L(H)) (1), but addition of triethylamine results in fast and facile cyclometalation at Ni(II), giving NiBr(κ(3)-P,N,C-L) (2) as well-defined species. This is a rare example of direct cyclometalation at Ni(II) from a C-H bond in a ligand structure other than encumbering ligands (e.g., ECE pincers). Diamagnetic yellow complex 2 reacts instantaneously with HBF4 to give purple [NiBr(κ(3)-P,N-L(H))]BF4 (3). A very unusual (an)agostic Ni(CPh-H) interaction in the solid-state structure of 3 was unequivocally demonstrated using single-crystal X-ray crystallography and was interpreted by density functional theory calculations (quantum theory of atoms in molecules and electron localization function analysis). These compounds may be viewed as models for key intermediates in the Ni-catalyzed C-H functionalization of arenes.