Homogeneous vs. heterogeneous: mechanistic insights into iron group metal-catalyzed reductions from poisoning experiments

Abstract

Iron group catalysts constitute a promising alternative to well-established noble metal catalysts in reduction reactions. This review advocates the use of kinetic poisoning experiments to distinguish between homotopic and heterotopic mechanisms.

Evidence for the “cocktail” nature of platinum-catalyzed alkyne and alkene hydrosilylation reactions

“…The split test, or hot filtration, is often used in studies on the leaching of active metal particles. 75 In the present study, the homogeneous path made the major contribution to the studied catalytic systems since the conversion of 1a in the solution was 80-98% (Table 1, entry 2).…”

Manganese‐Catalyzed Hydroborations with Broad Scope

“…A first indication may be derived from selective catalyst poisoning studies. [36] The reductive conditions of the hydroboration reactions with basic pre‐catalyst Mn(hmds) 2 and the active hydride reagent HBpin may facilitate reduction events at the Mn catalyst to produce Mn 0 species in the absence of strong ligands. However, the operation of pre‐catalyst reduction was not observed (no H 2 evolution, no diborane formation, no pinacol formation with aldehydes).…”

“…The clear distinction between homotopic and heterotopic catalysis mechanisms is not trivial, as sensitive and insightful analytical tools that operate under the reaction conditions are rare. A first indication may be derived from selective catalyst poisoning studies [36] . The reductive conditions of the hydroboration reactions with basic pre‐catalyst Mn(hmds) 2 and the active hydride reagent HBpin may facilitate reduction events at the Mn catalyst to produce Mn 0 species in the absence of strong ligands.…”

Stereoselective Chromium‐Catalyzed Semi‐Hydrogenation of Alkynes

“…The strongly reducing conditions (DIBAL−H, H 2 ) and the absence of stabilizing ligands may facilitate catalyst reduction to a low‐valent or naked metal species which ultimately may nucleate and grow to larger particles. However, the clear distinction between homogeneous and heterogeneous metal catalysts is not trivial, [17] yet reaction progress analyses and kinetic poisoning experiments can provide useful insight [18] . The semi‐hydrogenation of 1‐phenyl‐1‐propyne with 10 mol% [Cr] under standard conditions displayed quite effective catalyst inhibitions when 5 mol% trimethylphosphite, P(OMe) 3 , or trimethylphosphine, PMe 3 , respectively, were added after 30 min reaction.…”