Synthesis of New Chiral P,S Hybrid Ligands and Their Use in Asymmetric Hydrosilylation of Ketones.

Synthesis of Novel (P,S) Ligands Based on Chiral Nonracemic Episulfides. Use in Asymmetric Hydrogenation

“…The coordination geometry around the metal center is a slightly distorted square-plane with a P−Pd−S angle of 87.84(7)°. This angle is typical of neutral Pd(II) complexes 7b,20 and smaller than that of cationic (P,S)Pd(II) allyl complexes described by Togni and Pregosin (94.6−96.9°). 7a,e,g,h Both the Pd−S and Pd−P bond lengths at 2.271(2) and 2.221(2) Å are typical, , but the two Pd−Cl distances, 2.309(2) Å and 2.377(2) Å, differ significantly. The latter effect is attributed to the larger trans influence of phosphorus versus sulfur.…”

“…Reports on catalytic reactions using the latter class are scarce partially owing to fear of metal poisoning by sulfur. The use of these ligands has been reported for the carbonylation of methanol,7m the reduction of ketones by hydrogen transfer (ee up to 20%), 7i,l and the hydroformylation of styrene 7i. Pregosin has synthesized and structurally characterized a number of metal complexes bearing chiral P,S-donor ligands.…”

“…Pregosin has synthesized and structurally characterized a number of metal complexes bearing chiral P,S-donor ligands. In particular, palladium complexes with ligands such as 1 and 2 were shown to carry out asymmetric allylic alkylation. 7a,c,d

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“…14% ee was achieved. Most recently, Achiwa has reported the use of (P,S) ligands in asymmetric hydrosilylation of ketones (ee up to 57%) 7p. In these ligand systems it is difficult to tune electronic and steric effects.…”

Cationic Iridium Complexes with Chiral Dithioether Ligands: Synthesis, Characterisation and Reactivity under Hydrogenation Conditions

“…[1,2] In particular, complexes containing thioether mixed-donor ligands such as the P,S-, [3][4][5][6][7][8] N,S- [9][10][11][12][13][14][15][16][17] and O,S- [18][19][20][21][22] donors have been successfully applied in asymmetric hydrogenation, [5,6,23] allylic substitutions [7][8][9][10]18] and enantioselective hydrosilylation of ketones. [4,23] In contrast, catalysts with chiral bidentate thioether ligands have been scarcely investigated, [24][25][26][27][28][29][30][31][32] although some have shown promising catalytic properties. In fact, we have recently shown that metal complexes with chiral dithioether ligands afford up to 81 % ee in palladium-catalysed asymmetric allylic substitutions [24] and 68 % ee in iridium-catalysed asymmetric hydrogenation reactions.…”