Iron borohydride pincer complexes for the efficient hydrogenation of ketones under mild, base-free conditions: Synthesis and mechanistic insight

Robert Langer, Mark A. Iron, Leonid Konstantinovski, Yael Diskin-Posner, Gregory Leitus, Yehoshoa Ben-David, David Milstein

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146 Citations (Scopus)

Abstract

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η1-BH 4)] (1) catalyzes the base-free hydrogenation of ketones to their corresponding alcohols employing only 4.1atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH3 scavenger) resulted in a mixture of trans-[(iPr-PNP)Fe(H)2(CO)] (4a) and cis-[(iPr-PNP)Fe(H) 2(CO)] (4b). The dihydrido complexes 4a and 4b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron-catalyzed hydrogenation of ketones involves alcohol-assisted aromatization of the dearomatized complex [(iPr-PNP*)Fe(H)(CO)] (7) to initially give the Fe0 complex [(iPr-PNP)Fe(CO)] (21) and subsequently [(iPr-PNP)Fe(CO)(EtOH)] (38). Concerted coordination of acetophenone and dual hydrogen-atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr-PNP*)Fe(CO)(EtO)(MeCH(OH) Ph)] (32). The catalyst is regenerated by release of 1-phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.

Original languageEnglish
Pages (from-to)7196-7209
Number of pages14
JournalChemistry - A European Journal
Volume18
Issue number23
DOIs
Publication statusPublished - Jun 4 2012

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Keywords

  • density functional calculations
  • homogeneous catalysis
  • hydrogenation
  • iron
  • pincer ligands

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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