Efficient amide-directed catalytic asymmetric hydroboration

Sean M. Smith, Nathan C. Thacker, James M. Takacs

Research output: Contribution to journalArticlepeer-review

92 Scopus citations

Abstract

A series of acyclic β,γ-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 phosphoramidite4, THF, 40 °C, 2 h) affording an intermediate boronate ester which after oxidation with basic hydrogen peroxide gives the β-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 °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.

Original languageEnglish (US)
Pages (from-to)3734-3735
Number of pages2
JournalJournal of the American Chemical Society
Volume130
Issue number12
DOIs
StatePublished - Mar 26 2008

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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