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

Research output: Contribution to journalArticle

138 Citations (Scopus)

Abstract

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η 1-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 BH 3 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 Fe 0 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

Fingerprint

Borohydrides
Carbon Monoxide
Ketones
Hydrogenation
Iron
Alcohols
Ligands
Aromatization
Atoms
Hydrogen
Proton transfer
Aniline
Discrete Fourier transforms
Ethanol
Nuclear magnetic resonance
Catalysts
Carbon
Oxygen

Keywords

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

ASJC Scopus subject areas

  • Chemistry(all)

Cite this

Iron borohydride pincer complexes for the efficient hydrogenation of ketones under mild, base-free conditions : Synthesis and mechanistic insight. / Langer, Robert; Iron, Mark A.; Konstantinovski, Leonid; Diskin-Posner, Yael; Leitus, Gregory; Ben-David, Yehoshoa; Milstein, David.

In: Chemistry - A European Journal, Vol. 18, No. 23, 04.06.2012, p. 7196-7209.

Research output: Contribution to journalArticle

Langer, Robert ; Iron, Mark A. ; Konstantinovski, Leonid ; Diskin-Posner, Yael ; Leitus, Gregory ; Ben-David, Yehoshoa ; Milstein, David. / Iron borohydride pincer complexes for the efficient hydrogenation of ketones under mild, base-free conditions : Synthesis and mechanistic insight. In: Chemistry - A European Journal. 2012 ; Vol. 18, No. 23. pp. 7196-7209.
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AB - The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η 1-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 BH 3 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 Fe 0 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.

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KW - pincer ligands

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