Synthesis of [Bis(hexamethylene)cyclopentadienone]iron Tricarbonyl and its Application to the Catalytic Reduction of C=O Bonds

Abstract

Herein, we report the synthesis of [bis(hexamethylene)cyclopentadienone]iron tricarbonyl (1 b) by the reaction of cyclooctyne with Fe(CO)5 and the investigation of its catalytic properties in C=O bond reduction. As a result of the peculiar reactivity of cyclooctyne, 1 b was formed in good yield (56 %) by intermolecular cyclative carbonylation/complexation with Fe(CO)5. Compound 1 b was characterized fully and its crystal structure was determined by using XRD. Catalytic tests revealed that, upon in situ activation with Me3NO, 1 b promotes the hydrogenation of ketones, aldehydes, and activated esters as well as the transfer hydrogenation of ketones and shows a higher activity than the classical “Knölker complex” (1 a). Studies on the hydrogenation kinetics in the presence of 1 a and 1 b (respectively) suggest that this difference in activity is probably caused by the better stability of the 1 b‐derived complex than that of the in situ generated Knölker–Casey catalyst.

Manganese Complexes for (De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron Catalysts

“…It could be shown that Knölker′s iron complex ( Fe‐1 ) chemoselectively hydrogenates ketones at room temperature and a low H 2 pressure of only 3 bar . Diverse publications using Knölker′s iron complex followed . More active (pre‐)catalysts based on Fe‐2 a and Fe‐2 b were introduced by Milstein and co‐workers and contain 2,6‐lutidine‐based P,N,P ligands .…”

Recent Catalytic Applications of (Cyclopentadienone)iron Complexes

“…Besides the advancements that have been made in recent years employing the Knölker's complexes ( 1a , 2a ) or their thermally activable derivatives (“Funk”s complexes' such as 3aa and 3ab ), attempts were also made to improve the catalytic activity of CICs by modifying the cyclopentadienone ligand's substitution. In 2017, our research group reported a cyclooctene‐derived CIC ( 1b , Scheme ) displaying higher catalytic activity compared to 1a in the CH of C=O bonds . Several ketones, aldehydes and a trifluoroacetate ester were reduced in high yield.…”

“…The highly active CIC 1b , developed by our research group, and its applications in the CH of carbonyl compounds (A), in the CTH of imines (B), and in the reductive amination of carbonyl compounds (C) . DIPEA = N , N ‐diisopropylethylamine.…”

“…Kinetics of acetophenone CH promoted by pre‐catalyst 1a (■) and 1b (♦) activated with Me 3 NO. Reproduced from ref …”

Multi‐Step Reactions Involving Iron‐Catalysed Reduction and Hydrogen Borrowing Reactions

“…Direct reductive amination (DRA) can be also performed in hydrogen transfer conditions with iron‐based catalysts. The modified Knölker catalyst, [bis(hexamethylene)cyclo‐pentadienone] iron tricarbonyl complex 25 (5 mol‐%) in combination with Me 3 NO (5 mol‐%) can promote the transfer hydrogenative amination of aldehydes and ketones in the presence of 3 Å molecular sieves in 60–99 % yields when the reaction was performed at 100 °C for 18 h (Scheme ) , . Noticeably the reaction was more efficient with aromatic aldehydes (93–99 %) than with alkanals (60–90 %).…”