Premium
Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild, Base‐Free Conditions: Synthesis and Mechanistic Insight
Author(s) -
Langer Robert,
Iron Mark A.,
Konstantinovski Leonid,
DiskinPosner Yael,
Leitus Gregory,
BenDavid Yehoshoa,
Milstein David
Publication year - 2012
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201200159
Subject(s) - chemistry , acetophenone , pincer movement , medicinal chemistry , transfer hydrogenation , ketone , ligand (biochemistry) , catalysis , benzaldehyde , pincer ligand , borohydride , photochemistry , organic chemistry , ruthenium , biochemistry , receptor
The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [( i Pr‐PNP)Fe(H)(CO)(η 1 ‐BH 4 )] ( 1 ) catalyzes the base‐free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm 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 ‐[( i Pr‐PNP)Fe(H) 2 (CO)] ( 4 a ) and cis ‐[( i Pr‐PNP)Fe(H) 2 (CO)] ( 4 b ). The dihydrido complexes 4 a and 4 b 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 [( i Pr‐PNP*)Fe(H)(CO)] ( 7 ) to initially give the Fe 0 complex [( i Pr‐PNP)Fe(CO)] ( 21 ) and subsequently [( i Pr‐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 [( i Pr‐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.