Cyclopentadienone Iron Alcohol Complexes: Synthesis, Reactivity, and Implications for the Mechanism of Iron-Catalyzed Hydrogenation of Aldehydes

Abstract

Cyclopentadienone iron alcohol complexes generated from the reactions of [2,5-(SiMe 3 ) 2 -3,4-(CH 2 ) 4 (η 5 -C 4 COH)]Fe(CO) 2 H (3) and aldehydes were characterized by 1 H NMR, 13 C NMR, and IR spectroscopy. The benzyl alcohol complex [2,5-(SiMe 3 ) 2 -3,4-(CH 2 ) 4 (η 5 -C 4 C=O)]Fe (CO) 2 (HOCH 2 C 6 H 5 ) (6-H) was also characterized by X-ray crystallography. These alcohol complexes are thermally unstable and prone to dissociate the coordinated alcohols. The alcohol ligand is easily replaced by other ligands such as PhCN, pyridine, and PPh 3 . Dissociation of the alcohol ligand in the presence of H 2 leads to the formation of iron hydride 3. The reduction of aldehydes by 3 was carried out in the presence of both potential intermolecular and intramolecular traps. The reaction of 3 with PhCHO in the presence of 4-methylbenzyl alcohol as a potential intermolecular trapping agent initially produced only iron complex 6-H of the newly formed benzyl alcohol. However, the reaction of 3 with 4-(HOCD 2 )C 6 H 4 CHO (11-d 2 ), which possesses a potential intramolecular alcohol trapping agent, afforded two alcohol complexes, one with the newly formed alcohol coordinated to iron and the other with the trapping alcohol coordinated. The intramolecular trapping experiments support a mechanism involving concerted transfer of a proton from OH and hydride from Fe of 3 to aldehydes. The kinetics and mechanism of the hydrogenation of benzaldehyde catalyzed by 3 are presented.Alcohol complexes play important roles in transition metal catalyzed reactions. They serve as catalyst precursors by dissociating the alcohol ligand and creating vacant coordination sites for substrate binding. They have been proposed as essential intermediates in aerobic alcohol oxidations, i silane alcoholysis, ii and ionic hydrogenation of carbonyl groups. iii Alcohol complexes have been suggested as possible intermediates in the hydrogenation of ketones and aldehydes catalyzed by Shvo's diruthenium hydride complex 1-S iv and its tolyl analog 1. However, all attempts to prepare or spectroscopically observe these ruthenium alcohol complexes have been unsuccessful. v We succeeded in synthesizing related cationic hydroxycyclopentadienyl ruthenium alcohol complexes, vi which dissociate alcohol rapidly at low temperature. Very few alcohol complexes have been crystallographically characterized; iiia,iiie,vii all of the structurally characterized alcohol complexes are cationic compounds.Ligand-metal bifunctional hydrogenation catalysis is dramatically changing the face of reduction chemistry. viii These transition metal catalysts contain electronically coupled casey@chem.wisc.edu. Supporting Information Available: General experimental information, experimental procedures, synthesis of alcohol complexes, rates of substitution of coordinated alcohols by PhCN, rates of hydrogenation of aldehydes catalyzed by 3, X-ray crystallographic data for {2,5-(SiMe 3 ) 2 -3,4-[(CH 2 ) 4 ](η 4 -C 4 CO)}Fe(CO) 2 (HOCH 2 Ph) (6-H). This material is available free of cha...

Iron/Brønsted Acid Catalyzed Asymmetric Hydrogenation: Mechanism and Selectivity‐Determining Interactions

“…[20] Also for iron-mediated aldehyde and ketone hydrogenation, the formed alcohol product has been proposed to coordinate to the iron complex. [15,36,35] In our calculations, coordination of the amine is highly favourable (by around 14 kcal/mol), making the relative energy of the Fe(0)-N complex comparable to that of the resting state, Fe(II)-H,OH. Hence, in presence of amine, H2 addition and splitting is only slightly exergonic (-2.3 kcal/mol, B3LYP-D2, Scheme 1B).…”

“…[14,15,22] The hydrogenation reaction comprises two essential parts: i) formation of the active catalyst species and ii) hydrogenation of the substrate. Experimental and computational results indicate that these two steps energetically and mechanistically might be very different in presence and absence of a phosphoric acid.…”

“…[ 12 , 13 ] Casey and Guan reported symmetric hydrogenation of aldehydes, ketones, and imines with Knölker's iron complex, which possesses a cyclopentadienone ligand. [14,15] Berkessel has reported a related iron cyclopentadienone complex with a chiral phosphoramidite ligand, which was applied in the asymmetric reduction of ketones. [16] This type of iron cyclopentadienone catalysts can be seen as cheap and relatively easily accessible derivatives of the original ruthenium-based Shvo's catalyst (for a recent review see [17]).…”

Iron catalysed direct alkylation of amines with alcohols

“…Although promising, the initial results using common organic solvents indicated low conversion of 1a or low selectivity towards 3aa and analysis of the reaction mixtures identified insufficient imine reduction as the key problem. We reasoned that most probably a weakly coordinating ethereal solvent is needed to stabilize the key iron intermediates to enhance the imine reduction step 27 . In addition, imine formation might be facilitated in solvents, which have limited miscibility with water.…”

Alcohol Dehydrogenation with a Dual Site Ruthenium, Boron Catalyst Occurs at Ruthenium

“…Such a strategy is used prolifically in ligand, metal bifunctional catalysts, such as those pioneered by Noyori [18] and Shvo [19][20][21][22] (Figure 1). In these systems, a hydrogen bond is used to position a protic substrate with a C-H bond in the proximity of a ruthenium center, from which the hydrogen group is abstracted.…”