Efficient amide-directed catalytic asymmetric hydroboration.

ABSTRACT

A series of acyclic beta,gamma-unsaturated amides are shown to undergo highly regio- (>95%) and enantioselective (93-99% ee) rhodium-catalyzed hydroboration with pinacolborane (PinBH) using simple chiral monophosphite or phosphoramidite ligands in combination with Rh(nbd)2BF4. The most effective ligands identified are phosphoramidite 4, derived from BINOL and N-methylaniline, and phosphite 5c, prepared from the (4'-tert-butyl)phenyl TADDOL analogue and phenol. For example, (E)-3-hexenoic acid phenylamide ((E)-1) undergoes rhodium-catalyzed hydroboration with PinBH (0.5 mol % Rh(nbd)2BF4, 1.1 mol % BINOL-derived phosphoramidite 4, THF, 40 degrees C, 2 h) affording an intermediate boronate ester which after oxidation with basic hydrogen peroxide gives the beta-hydroxy amide, (S)-3-hydroxyhexanoic acid phenylamide ((S)-3), in good yield (80%) and high enantiomeric purity (99% ee). Isomeric disubstituted (E)- and (Z)-alkenes give nearly identical results, and a trisubstituted alkene substrate is also shown to undergo efficient hydroboration (97% ee). Moderate catalyst loading (0.5 mol %) and reaction temperatures in 25-40 degrees C range are generally effective. N-Phenyl amides are generally more efficient than the corresponding N-benzyl or N,N-dibenzyl analogues. Pinacolborane is found to be more efficient than catecholborane.