Phosphinooxazolines carrying 4-hydroxybenzyl and 4-methoxybenzyl substituents exhibit contrasting behavior in Pd- and Ir-catalyzed allylic alkylations. Whereas catalysts with the methoxy-contg. li ...

[reaction: see text] Phosphinooxazolines carrying (1-hydroxy-1-phenyl)methyl and (1-methoxy-1-phenyl)methyl substituents in the 4 position of the oxazoline ring exhibit contrasting behavior in Pd- and Ir-catalyzed allylic alkylations. Whereas catalysts with the methoxy-containing ligand generally provide products with high ee's, use of catalysts prepared from the hydroxy-containing ligand results in products with low ee's or even racemates. DFT calculations suggest the presence of a hydrogen bond with Pd(0) as the proton acceptor in the hydroxy-containing olefin-Pd(0) complexes, which induces a conformational change in the ligand, leading to different stereoselectivity.

[reaction: see text] Terminal acetylenes containing hydroxy and carboxylic acid groups were subjected to Sonogashira coupling with 4-bromo-2,6-bis[(R)-4-phenyloxazolin-2-yl]pyridine and the resulting pybox derivatives were immobilized on Tentagel resins. Ytterbium(III) chloride complexes of the polymeric ligands catalyzed the addition of trimethylsilyl cyanide to benzaldehyde with 80-81% ee. The ligands were reused more than 30 times without any loss in selectivity or activity, and the metal complexes could be recovered and reused at least four times, although with slightly decreasing activity.

(R,R)-Threoninol was prepared in one step from L-threonine in 86% isolated yield. The product was regioselectively transformed into 2,2-bis[(4R,5R)-4-(hydroxymethyl)-5-methyl-1,3-oxazolin-2-yl]propane in 42% isolated yield.

Chiral pyridinooxazoline, quinolinooxazoline, bis(oxazolino)pyridine (pybox), and bisoxazoline (box) derivatives containing crown ether residues were prepared. Some of the ligands were assessed in substrate binding studies and in palladium catalyzed allylic alkylations.

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First to fourth generation dendritic substituents based on 2,2-bis(hydroxymethyl)propionic acid and (1R,2S,5R)-menthoxyacetic acid were attached to 2-(hydroxymethyl)pyridinooxazoline and bis[4-(hydroxymethyl)oxazoline] compounds. The new ligands obtained were assessed in palladium-catalyzed allylic alkylations. The first type of ligands exhibited enantioselectivity similar to that of a benzoyl ester derivative, whereas the latter type of ligands afforded products with higher selectivity than the analogous benzoyl ester. The activity of the dendritic catalysts decreased with increasing generation.

Ru-catalyzed hydrogen transfer from propan-2-ol to acetophenone under microwave conditions using monotosylated (R,R)-diphenylethylenediamine as the chiral source afforded (R)-1-phenylethanol in >90% yield and 82% e.e. within 9 min, while use of ephedrine or norephedrine gave the same compound in high yield with 70 and 46% e.e., respectively. t-Butylphenylketone was reduced to (R)-2,2-dimethyl-1-phenyl-1-propanol under the same conditions in close to quantitative yield, although with low enantioselectivity.

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