Heterogeneous or Homogeneous Catalysis? Mechanistic Studies of the Rhodium-Catalyzed Dehydrocoupling of Amine-Borane and Phosphine-Borane Adducts

Abstract

In depth, comparative studies on the catalytic dehydrocoupling of the amine-borane adduct Me(2)NH.BH(3) (to form [Me(2)N-BH(2)](2)) and the phosphine-borane adduct Ph(2)PH.BH(3) (to form Ph(2)PH-BH(2)-PPh(2)-BH(3)) with a variety of Rh (pre)catalysts such as [[Rh(1,5-cod)(micro-Cl)](2)], Rh/Al(2)O(3), Rh(colloid)/[Oct(4)N]Cl, and [Rh(1,5-cod)(2)]OTf have been performed in order to determine whether the dehydrocoupling proceeds by a homogeneous or heterogeneous mechanism. The results obtained suggest that the catalytic dehydrocoupling of Me(2)NH.BH(3) is heterogeneous in nature involving Rh(0) colloids, while that of Ph(2)PH.BH(3) proceeds by a homogeneous mechanism even when starting with Rh(0) precursors such as Rh/Al(2)O(3). The catalytic dehydrocoupling reactions are thought to proceed by different mechanisms due to a combination of factors such as (i) the greater reducing strength of amine-borane adducts, (ii) the increased ease of dissociation of phosphine-borane adducts, and (iii) phosphine ligation and/or poisoning of active catalytic sites on metal colloids.

Evidence that SCS Pincer Pd(II) Complexes are only Precatalysts in Heck Catalysis and the Implications for Catalyst Recovery and Reuse

“…Following the work of Finke [14,17] and others, [25] for catalysis by in situ formed metal nanoparticles, the rate is expected to be limited by the formation of the catalytic nanoparticles. In this case, the rate of product formation will follow the kinetic model for metal-particle formation.…”

Frustrierte Lewis‐Paare: metallfreie Wasserstoffaktivierung und mehr

“…[9] Auch ist festzuhalten, dass für viele Phosphin-Boran-Addukte R 2 PH(BH 3 ) bekannt ist, dass sie thermisch induziert oder katalytisch H 2 unter Bildung cyclischer und polymerer Phosphinoborane eliminieren. [10] Diese bemerkenswerte Beobachtung lieferte das erste übergangsmetallfreie "frustrierte Lewis-Paar", das reversibel Wasserstoff aufnimmt und wieder abgibt. Interessanterweise erwies sich die verwandte Verbindung 10 b als stabil bis 150 8C, was zeigt, dass das 2,4,6-Me 3 C 6 H 2 -Derivat die geeignete Balance zwischen der Phosphonium-Acidität und dem Hydridcharakter des BH-Bausteins für die leichte Abgabe und Aufnahme von H 2 hält.…”

Highly Selective Hydrogenation of Carbon–Carbon Multiple Bonds Catalyzed by the Cation [(C6Me6)2Ru2(PPh2)H2]+: Molecular Structure of [(C6Me6)2Ru2(PPh2)(CHCHPh)H]+, a Possible Intermediate in the Case of Phenylacetylene Hydrogenation

“…As one of "Halperns rules" says that "if you can isolate it, it is probably not the catalyst", we decided to investigate the mechanistic aspects of this catalytic reaction with [1]A C H T U N G T R E N N U N G [BF 4 ] by Finkes method [12,13] by studying catalyst recycling and reproducibility, the kinetics of the reaction, and by catalyst-poisoning experiments with Hg 0 or strong coordinating ligands, such as 1,10-phenanthroline [14a] and triphenylphosphane. [17] Catalyst-recycling and reproducibility studies: 4 ] was used to hydrogenate cyclohexene at 50 bar H 2 and 40 8C in ethanol. The main product recovered after the catalytic reaction (more than 90 %) was intact Table 2.…”