Stereoselective Alkyne Hydrogenation by using a Simple Iron Catalyst

Abstract

The stereoselective hydrogenation of alkynes constitutes one of the key approaches for the construction of stereodefined alkenes. The majority of conventional methods utilize noble and toxic metal catalysts. This study concerns a simple catalyst comprised of the commercial chemicals iron(II) acetylacetonate and diisobutylaluminum hydride, which enables the Z‐selective semihydrogenation of alkynes under near ambient conditions (1–3 bar H2, 30 °C, 5 mol % [Fe]). Neither an elaborate catalyst preparation nor addition of ligands is required. Mechanistic studies (kinetic poisoning, X‐ray absorption spectroscopy, TEM) strongly indicate the operation of small iron clusters and particle catalysts.

Stereoselective Chromium‐Catalyzed Semi‐Hydrogenation of Alkynes

“…The comparison with the XAS spectrum of a bulk Cr(0) foil (Figure 2, gray curve) demonstrates that reduction by the co‐catalyst DIBAL−H does not exclusively take place at Cr under the catalytic conditions. In agreement with earlier observations in the closely related Fe(acac) 2 ‐catalyzed semi‐hydrogenation, [5a] we postulate the operation of ligand‐centered reduction events. The XANES curvature of the reduced species (with 6 equiv.…”

“…Stoichiometric reaction of tris(benzoyl‐acetonato)chromium(III), Cr(bzac) 3 , and DIBAL−H afforded mostly ligand reduction by hydride addition and deoxygenation (Scheme 2b). In comparison with our earlier report of an iron‐catalyzed semi‐hydrogenation protocol, [5a] the extent of ligand‐centered reduction is higher for Cr, which appears to reflect the inherent redox potentials and Lewis acid character. The reaction order of the semi‐hydrogenation was determined to be ∼1 in [Cr] catalyst under the catalytic reaction conditions and slightly higher (approaching 2.5) at lower catalyst concentrations (see the ESI).…”

“…Importantly, no Cr−Cr pairs of potential clusters could be observed under the catalytic reaction conditions. In combination with the XANES results, we therefore postulate that the major reduction by the co‐catalyst DIBAL−H occurs at the acac ligands in solution phase [5a] …”

“…The continuous band shifts – and the fact that the most active Cr catalyst involves 3 equiv. DIBAL−H ‐ may be indicative of the presence of multiple equilibrating species among which the fully reduced species is not the active catalyst [5a,19] . The comparison with the XAS spectrum of a bulk Cr(0) foil (Figure 2, gray curve) demonstrates that reduction by the co‐catalyst DIBAL−H does not exclusively take place at Cr under the catalytic conditions.…”

“…We have recently reported the use of co‐catalytic iron(II)‐bis(acetylacetonate) and DIBAL−H in alkyne semi‐hydrogenations [5a] . A brief survey of 3d transition metal salts revealed distinct activities in the model reaction of 1‐phenyl‐1‐propyne under low H 2 pressure (2 bar): While Ni(acac) 2 and Co(acac) 2 6ek; underwent complete hydrogenations to the alkanes, chemoselective hydrogenation to the alkenes with high Z ‐stereo‐control was observed with Fe(acac) 2 [5a] and Cr(acac) 3 (Table 1, entries 1–4). To the best of our knowledge, the latter protocol constitutes the first application of a simple Cr salt/metal hydride co‐catalyst to efficient alkyne semi‐hydrogenations.…”