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

(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.


Figure S1. Transfer dehydrogenation of 4-phenyl-2-butanol with and without excess trimethylamine
Conversion (%) vs. time (h) for the transfer dehydrogenation of 4-phenyl-2-butanol under typical reaction conditions (dashed orange line) or under an atmosphere of trimethylamine (solid blue line) using 7-MeO (2.5 mol %).Conversions were determined by GC relative to biphenyl.

Figure S2. Transfer hydrogenation of acetophenone with and without excess trimethylamine
Conversion (%) vs. time (h) for the transfer hydrogenation of acetophenone under typical reaction conditions (dashed orange line) or under an atmosphere of trimethylamine (solid blue line) using 7-MeO (2 mol %).Conversions were determined by GC relative to biphenyl.

Hammett plot data processing
The transfer hydrogenation and dehydrogenation reactions were run as described in the Experimental section.Plots of conversion vs. time were made using the first few points (linear region at low conversions) for each individual run, and the data was fit with a linear trendline using Microsoft Excel.The slope of the line was used as the initial rate, and the average of the initial rates for each set of replicates was calculated.The average initial rate for the unsubstituted catalyst or substrate (e.g., 6-H, acetophenone, 1-phenylethanol) was defined as kH, and the rates of substituted catalysts or substrates were called k. Errors in the average initial rates were calculated as either the difference from the individual initial rates to the average (when comparing two runs) or one standard deviation (when comparing more than two runs).These errors were propagated through the data processing.

Kinetic isotope effect data processing in acetophenone transfer hydrogenations
Transfer hydrogenation reactions of acetophenone in isopropanol and isopropanol-d8 were run as described in the "Transfer hydrogenations monitored over <24 h" in the Experimental section.Plots of conversion vs. time were made using the first few points (linear region at low conversions) for each individual run (Figure S1), and the linear parts of the data were fit with a linear trendline using Microsoft Excel (Figure S2).The slope of the line was used as the initial rate, and the average of the initial rates for each pair of replicates was calculated.The average initial rate for the reaction run in isopropanol was defined as kH, and the average initial rate for the reaction run in isopropanol-d8 was defined as kD.Errors in the average initial rates were calculated as the difference from the individual initial rates to the average.These errors were propagated through the data processing.

NMR Spectra
All 1 H and 13 C{ 1 H} NMR spectra were recorded at ambient temperature at 400 MHz and 100 MHz, respectively, on a Bruker Avance Neo 400 MHz FT-NMR spectrometer unless otherwise noted.Chemical shifts are reported in parts per million (ppm) relative to tetramethylsilane (TMS) for spectra taken in CDCl3. 1 H NMR spectra taken in acetone-d6 and benzene-d6 used the residual solvent peaks, 2.05 ppm and 7.16 ppm, respectively, as references.Samples were prepared in 0.7 mL of solvent unless otherwise noted.

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Figure S27. 1 H NMR spectrum (400 MHz, 1:1 benzene-d6 and isopropanol) of transfer hydrogenation of 2-butanone with 6-Me + Me3NO mimicking the conditions at 75% conversion (see Experimental Section for details).Spectrum was taken at rt after 5 minutes at rt.No peaks were observed up to 15 ppm or down to -30 ppm other than those shown.

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Figure S28. 1 H NMR spectrum (400 MHz, 1:1 benzene-d6 and isopropanol) of transfer hydrogenation of 2-butanone with 6-Me + Me3NO mimicking the conditions at 75% conversion (see Experimental Section for details).Spectrum was taken at 65 °C after 15 minutes at 65 °C.No peaks were observed up to 15 ppm or down to -30 ppm other than those shown.