Mechanism for Reduction Catalysis by Metal Oxo:  Hydrosilation of Organic Carbonyl Groups Catalyzed by a Rhenium(V) Oxo Complex

Abstract

The rhenium oxo complex [Re(O)(hoz)2][TFPB], 1 (where hoz = 2-(2'-hydroxyphenyl)-2-oxazoline(-) and TFPB = tetrakis(pentafluorophenyl)borate) catalyzes the hydrosilation of aldehydes and ketones under ambient temperature and atmosphere. The major organic product is the protected alcohol as silyl ether. Isolated yields range from 86 to 57%. The reaction requires low catalyst loading (0.1 mol %) and proceeds smoothly in CH2Cl2 as well as neat without solvent. In the latter condition, the catalyst precipitates at the end of reaction, allowing easy separation and catalyst recycling. Re(O)(hoz)(H), 3, was prepared, and its involvement in an ionic hydrosilation mechanism was evaluated. Complex 3 was found to be less hydridic than Et3SiH, refuting its participation in catalysis. A viable mechanism that is consistent with experimental findings, rate measurements, and kinetic isotope effects (Et3SiH/Et3SiD = 1.3 and benzaldehyde-H/benzaldehyde-D = 1.0) is proposed. Organosilane is activated via eta2-coordination to rhenium, and the organic carbonyl adds across the coordinated Si-H bond [2 + 2] to afford the organic reduction product.

Catalyzing Aldehyde Hydrosilylation with a Molybdenum(VI) Complex: A Density Functional Theory Study

“…In an earlier computational study, Lin and co-workers [17] were unable to locate a transition state for the metathesis mechanism proposed by Abu-Omar and co-workers; [11] as a result, we did not consider this possibility. Because the Mo VI complex is so electron deficient, chlorine loss is an unlikely possibility, and dissociative pathways were not taken into account.…”

“…+ [11] (hoz: 2-(2'-hydroxyphenyl)oxazoline(À)) and Re 2 O 7 [10] in the case of Re, and [MoCl 2 O 2 ] in the case of Mo. [12] In the latter, replacement of Cl by other alkyl, aryl, cyclopentadienyl, O, or S ligands clearly diminishes the catalytic activity and thus requires higher temperatures for the reaction to occur.…”

“…[11] These results included the confirmation of the low hydricity of the corresponding hydride [ReH-…”

“…[9] Further studies by some of us, as well as by AbuOmars group, showed that these reactions can be catalyzed by a wide range of molecular oxo complexes of Re 3 + metal cores with a variety of classical and organometallic ligands. [10,11] Abstract: A C H T U N G T R E N N U N G [MoCl 2 O 2 ] catalyzes the hydrosilylation reaction of aldehydes and ketones, as well as the reduction of other related groups, in apparent contrast to its known behavior as an oxidation catalyst. In this work, the mechanism of this reaction is studied by means of density functional theory calculations using the B3LYP functional complemented by experimental data.…”

Reduction of carbon dioxide and organic carbonyls by hydrosilanes catalysed by the perrhenate anion

“…3,37,[44][45][46][47][48][49][50][51][52][53][54] High oxidation state transition metal oxo complexes 55 have been studied in CO 2 reduction chemistry and have been shown to react to form metal carbonates where, in the case of molybdate, the resulting carbonate undergoes hydrosilylation to formate and silylated molybdate, albeit stoichiometrically. [56][57][58][59] Furthermore, rheniumIJV) mono and dioxo complexes act as catalysts for the hydrosilylation of organic carbonyls, [60][61][62][63][64][65] but this activity is not generally seen for the parent perrhenate, ReO 4…”