Reduction of carbonyl compounds via hydrosilylation

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…These steps are important to the Chalk-Harrod and modified Chalk-Harrod mechanisms for olefin hydrosilation reactions that were first elucidated for platinum catalysts in the 1960's. [17] A closely related mechanism for rhodium-catalyzed ketone hydrosilations was proposed by Ojima in 1975. [18] These mechanisms allow for efficient catalysis, but the requirement of two-electron oxidative addition/reductive elimination steps has largely limited the range of available catalysts to platinum group metals.…”

“…[17] A closely related mechanism for rhodium-catalyzed ketone hydrosilations was proposed by Ojima in 1975. [18] These mechanisms allow for efficient catalysis, but the requirement of two-electron oxidative addition/reductive elimination steps has largely limited the range of available catalysts to platinum group metals. These catalysts are expensive and have additional drawbacks such as a tendency to engage in β-hydride eliminations that can lead to the formation of undesired side-products (e.g., those derived from olefin isomerization).…”

“…As depicted in Scheme 1, oxidative addition of the Si-H bond is part of the Chalk-Harrod and modified Chalk-Harrod mechanisms for olefin hydrosilation, as well as the closely related Ojima mechanism for ketone hydrosilation (essentially the modified Chalk-Harrod cycle applied to carbonyl substrates). [17,18] This provides a wider range of mechanistic possibilities for transition metal catalyzed hydrosilations than for main group Lewis acids. As a result, it is more difficult to clearly demonstrate when the electrophilic Si-H activation mechanism operates for transition metal catalysts.…”

“…Aside from this difference, the catalytic cycles have usually been depicted as essentially the same as that shown in Scheme 20 regardless of the catalyst used. One exception to this catalytic cycle was implicated by a DFT study of alcoholysis reactions using the cationic iron catalyst [CpFe(PPh3)(CO)] + (18,Scheme 22). [57c] This computational investigation suggested that backside attack of …”

“…The Toste mechanistic proposal was considered in this system, but was ruled out since Si-H activation across the rhenium oxo group was not observed in stoichiometric reactions. Additionally, an 18 O labeling study of silane hydrolysis mediated by this catalyst indicated that the oxo group did not participate in this related transformation. [66] Support for an electrophilic η 2 -H-SiR3 hydrosilation pathway was found by reaction of a hydride derivative of 25 (25-H) with an adduct of benzaldehyde and the Et3Si + cation (Scheme 28).…”