Substituent Effects and Mechanistic Insights on the Catalytic Activities of (Tetraarylcyclopentadienone)iron Carbonyl Compounds in Transfer Hydrogenations and Dehydrogenations

Abstract

(Cyclopentadienone)­iron carbonyl compounds are catalytically active in carbonyl/imine reductions, alcohol oxidations, and borrowing hydrogen reactions, but the effect of cyclopentadienone electronics on their activity is not well established. A series of (tetraarylcyclopentadienone)­iron tricarbonyl compounds with varied electron densities on the cyclopentadienone were prepared, and their activities in transfer hydrogenations and dehydrogenations were explored. Additionally, mechanistic studies, including kinetic isotope effect experiments and modifications to substrate electronics, were undertaken to gain insights into catalyst resting states and turnover-limiting steps of these reactions. As the cyclopentadienone electron density increased, both the transfer hydrogenation and dehydrogenation rates increased. A catalytically relevant, trimethylamine-ligated iron compound was isolated and characterized and was observed in solution under both transfer hydrogenation and dehydrogenation conditions. Importantly, it was catalytically active in both reactions. Kinetic isotope effect data and initial rates in transfer hydrogenation reactions with 4′-substituted acetophenones provided evidence that hydrogen transfer from the catalyst to the carbonyl substrate occurred during the turnover-limiting step, and NMR spectroscopy supports the trimethylamine adduct as an off-cycle resting state and the (hydroxycyclopentadienyl)iron hydride as an on-cycle resting state. In transfer dehydrogenations of alcohols, the use of electronically modified benzylic alcohols provided evidence that the turnover-limiting step involves the transfer of hydrogen from the alcohol substrate to the catalyst. The trimethylamine-ligated compound was proposed as the primary catalyst resting state in dehydrogenations.

Cyclopentadienone Iron Complex-Catalyzed Hydrogenation of Ketones: The Influence of the Charge-Tag on Catalytic Performance

“…Such effects have been described before by Renaud, Poater, and co-workers, who have found that an increase in electron density in the cyclopentadienone ring leads to increased catalytic performance. , Furthermore, it was reported that introducing an oxygen atom in the β-position to the cyclopentadienone ring as a five-membered cyclic ether moiety leads to lower yields in reductive amination reactions compared to Fe TMS (CO) 3 , indicative that the presence of an electronegative heteroatom in this position leads to lower catalytic performance . Funk et al have also very recently reported the variation of substituents on the aryl groups in tetraphenylcyclopentadienone-based iron complexes, where they have shown that electron-donating groups lead to higher reaction rates and electron-withdrawing groups lead to lower reaction rates a compound featuring a positively charged ammonium moiety in the β-position to the cyclopentadienone ring, only indicate small changes in chemical shift and do not allow for unambiguous exclusion of inductive effects being responsible for the lower catalytic activity when a nitrogen atom is present in the β-position (the spectra are shown in Figure S169 in the Supporting Information).…”

Ruthenium Complexes with NNN-Pincer Ligands for N-Methylation of Amines Using Methanol

“…Among the Ru complexes, Ru3 bearing the electron-donor group OMe showed the best catalytic activity, furnishing product 4a in 94% yield. This may be attributed to the strong electron-donating ability of the OMe group, which enhances the hydricity of the metal-hydride intermediate, thereby improving the catalytic efficiency in the BH process. , By increasing the reaction time from 12 to 18 h, the yield was only slightly improved to 95%. Shortening the reaction time to 6 h significantly reduced the yield of 4a to 42% (Table , entries 12 and 13).…”

CO to Isonitrile Substitution in Iron Cyclopentadienone Complexes: A Class of Active Iron Catalysts for Borrowing Hydrogen Strategies

“…These reactions, using Me 3 NO as a metal-complex activating reagent, were problematic concerning the compatibility with acidic functions. Indeed, it has been shown that the chemical activation of the precatalyst leads to the formation of [LFe­(CO) 2 (NMe 3 )]-type species, releasing the trimethylamine base in solution to generate the active species [LFe­(CO) 2 ] . However, initial attempts at avoiding the use of Me 3 NO activation through photoactivation required lights with low wavelengths, providing moderate yield (47%), due to the partial decomposition of the iron complex under strong UV irradiation while this also resulted in moderate ee (69%) .…”