Science of Synthesis

Jautze, S.; Peters, R., Science of Synthesis: Stereoselective Synthesis, (2011) 3, 461.

The rhodium(I)/2,2′-bis(diphenylphosphino)-1,1′-binaphthyl catalyst is not as efficient for catalytic asymmetric isomerization of allylic alcohols ‌54‌, to provide aldehydes ‌56‌, as it is for the corresponding conversion of allylic amines to enamines (see Section ‌3.10.2.1.1‌) affording only moderate yields and enantioselectivity.[‌47‌] The merit of the planar chiral bidentate phosphaferrocene ligand ‌55‌ (Ar1 = Ph) for this type of isomerization has been investigated.[‌48‌] Promising results with this ligand were further improved upon with the closely related variant ‌55‌ (Ar1 = 2-Tol).[‌49‌] Both E- and Z-configured allylic alcohols have been employed in this procedure, in which the latter provide slightly inferior results (‌Scheme 15‌). While most substrates contain an aromatic substituent (R1 or R2) this is not essential for good enantioselectivity [e.g., reaction of alcohol ‌54‌ (R1 = Cy; R2 = Me)], and substituents with different electronic effects do not significantly impact the outcome. It is noteworthy that the air-stable catalyst can be partly recycled by precipitation with pentane. Furthermore, the reaction has been conducted without problems on a 1-gram scale using 1 mol% of the catalyst. Deuterium-labeling experiments verified the intramolecular 1,3-hydrogen migration pathway and revealed that the catalyst preferentially activates one of the enantiotopic C1-hydrogens in an analogous manner to the allylic amines.

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References