The direct functionalization of aromatic compounds, via Friedel-Crafts alkylation reactions with alcohols, is one of the cornerstones in organic chemistry. The present emerging area deals with the recent advances in the use of pi-activated alcohols in the catalytic and stereoselective construction of benzylic stereocenters.
A new class of neutral cyclometalated iridium(III) complexes with enantiomerically pure C(1)-symmetric phenol-oxazolines (3a,b) have been synthetized in high yields and fully characterized. Resolution of the corresponding Δ(R) and Λ(R) or Δ(S) and Λ(S) isomers was easily achieved by conventional flash chromatography. The corresponding Δ and Λ helicities have been confirmed by CD spectroscopy and X-ray crystallography. Regarding the absorption and luminescence properties with unpolarized light, no significant difference between Δ and Λ isomers has been observed. A strong blue luminescence is observed for deaerated solutions of complexes 5a and 5b in CH(3)CN.
Alcohol versus alcohol: A highly stereocontrolled synthesis of substituted morpholines is realized by means of gold‐catalyzed dehydrative allylic cyclization of diols (see scheme for one example; segphos = 5,5′‐bis[di(3,5‐di‐tert‐butyl‐4‐methyoxyphenyl)phosphine]‐4,4′‐bi‐1,3‐benzodioxole). The present methodology represents one of the few examples of enantioselective gold‐catalyzed transformations involving unactivated alkenes.
A 1:2 mixture of [(S)-2](AuCl)2 [(S)-2 = (S)-3,5-t-Bu-4-MeO-MeOBIPHEP] and AgBF4 catalyzes the stereoconvergent, intermolecular enantioselective hydroamination of chiral, racemic 1,3-disubstituted allenes with N-unsubstituted carbamates to form N-allylic carbamates in good yield, with high regio- and diastereoselectivity, and up to 92% ee.
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