Kinetic Study of the Hydrosilylation of Acetophenone by [Rh(cod)Cl]₂/(R)-BINAP

Abstract

The [Rh(cod)Cl]2/(R)-BINAP catalyzed hydrosilylation reaction between acetophenone and H2SiPh2, H2SiEt2, H2SiBu2, HSiBu3, HSiPh3, and HSi(p-F3CC6H4)3 in benzene-d 6 at 63 °C has been studied by 1H NMR spectroscopy, GC/MS, and H/D exchange experiments with acetophenone-d 6. The reactions afford varying amounts of PhCH(OSiZ3)Me (3), PhC(OSiZ3)CH2 (4), and PhCH(OH)Me (5). The product distribution of the reaction is dependent on the order of mixing for the secondary silanes but independent of the order of mixing for tertiary silanes. The product distributions and initial rates of reaction of the tertiary silanes, which react more slowly than the secondary silanes, are dependent on the stereoelectronic properties of the silane, with HSi(p-F3CC6H4)3 being 10 times more reactive than HSiPh3. The reaction involving HSiBu3 is first order in the initial concentration of [Rh(cod)Cl]2/(R)-BINAP as well as the concentration of HSiBu3 but shows a saturation effect at high concentrations of acetophenone. In the earliest phases of the reaction, the coordinated cyclooctadiene is liberated and converted to cyclooctane and cyclooctene. The requisite hydrogens for the hydrogenation process come mainly from HSiBu3. The products 4 and 5 appear to be formed independently of the formation of 3. The catalysts for the formation of 4 and 5 appeared to decay, thereby abruptly terminating the formation of these materials after about 40% of the acetophenone had been consumed. Simulation of the kinetic results, based on the Ojima mechanism, qualitatively fits the production of 3.

Multiple Reaction Pathways in Rhodium‐Catalyzed Hydrosilylations of Ketones

“…[52] The latter was converted into cyclooctane by hydrogenation involving the silane; it generated a Si À Si compound as a side product. The activation of norbornadiene by direct reaction with the silane is also possible.…”

Rhodium(I) Complexes of New Ferrocenyl Benzimidazol‐2‐ylidene Ligands: The Importance of the Chelating Effect for Ketone Hydrosilylation Catalysis

“…The higher catalytic activity observed at higher ketone concentrations agrees with previous reports of a saturation effect: the reaction was found to be first order with respect to the ketone at low concentrations, but eventually became independent of the ketone concentration at high concentrations. [27][28][29] The mechanism of this catalytic process has been addressed by many authors. Although a few variations have been discussed, the most commonly accepted mechanism remains that originally proposed, [27,[30][31][32] which is based on a rate-determining Si-H oxidative addition pro-Scheme 7.…”

“…[27][28][29] The mechanism of this catalytic process has been addressed by many authors. Although a few variations have been discussed, the most commonly accepted mechanism remains that originally proposed, [27,[30][31][32] which is based on a rate-determining Si-H oxidative addition pro-Scheme 7. Proposed mechanism for the formation of saturated and unsaturated hydrosilylation products.…”

“…Interestingly, the observation by Kolb and Hetflejs of a saturation effect (while working with a cationic system) was interpreted as indicative of reversible complexation of the ketone prior to the rate-determining silane addition, [28] but Giering later discarded this variant when he could properly model, on the basis of the other mechanism, the kinetic results that he obtained with a neutral system, which also included a saturation effect in the ketone. [27] However, the data fittings were described as unsatisfactory unless additional processes of catalyst decomposition were included at various places in the catalytic cycle. We believe that the interpretation offered by Kolb and Hetflejs is reasonable and probably applies to our system, for the following reasons.…”

“…The cod ligand is probably lost rapidly from the coordination sphere by the hydrosilylation or hydrogenation processes, as also observed or proposed in other cases. [27,[32][33][34] The resulting [(NHCFcPPh 2 )Rh] + system is then likely to be coordinated by two solvent molecules, but the lability of these should easily allow ketone coordination, at a rate faster than that of the rate-determining silane oxidative addition step. Thus, the rate-determining silane oxidative addition may preferentially occur on the ketone-substituted complex, as shown in Scheme 7, when the ketone concentration is high, whereas the order of events may be inverted (pathway indicated with dashed arrows in Scheme 7) at low ketone concentrations.…”