A mechanistic investigation of the asymmetric Meerwein-Schmidt-Ponndorf- Verley reduction catalyzed by BINOL/AlMe3 - Structure, kinetics, and enantioselectivity

Christopher R. Graves, Hongying Zhou, Charlotte L. Stern, Son Binh T. Nguyen*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

30 Scopus citations

Abstract

(Chemical Equation Presented) The kinetics of the Al-catalyzed asymmetric Meerwein-Schmidt-Ponndorf-Verley (MSPV) reduction are presented. Structural identification of the catalytic precursor formed in situ between (S)-2,2′-dihydroxy-1,1′-binapthyl ((S)-BINOL), AlMe3, and 2-propanol was established through 1H and 27Al NMR spectroscopies, and APCIMS. All experimental evidence points toward the formation of a BINOL-chelated, pentacoordinate aluminum species in solution. Ligand-accelerated catalysis was confirmed for the phenolate/AlMe 3/2-propanol system. The rate law for the catalytic reaction was determined to be nearly unimolecular dependent on aluminum, zero-order dependent on substrate, and inversely dependent on 2-propanol. At the low catalyst loading employed in the BINOL/AlMe3 system, the inherent reversibility of the MSPV reaction does not affect product yield or enantiomeric excess over time. Systematic ligand studies imply that while a tetrahedral geometry around the aluminum center may result in the most active MSPV reduction catalysts, the enantioselectivity of the reaction is enhanced when the aluminum center allows for a 2-point coordination of the substrate to achieve a pentacoordinate geometry with the fifth ligand weakly coordinated to the axial site of a pseudo square pyramid.

Original languageEnglish (US)
Pages (from-to)9121-9133
Number of pages13
JournalJournal of Organic Chemistry
Volume72
Issue number24
DOIs
StatePublished - Nov 23 2007

ASJC Scopus subject areas

  • Organic Chemistry

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