Theoretical Study of POCOP-Pincer Iridium(III)/Iron(II)
Hydride Catalyzed Hydrosilylation of Carbonyl Compounds: Hydride Not
Involved in the Iridium(III) System but Involved in the Iron(II) System
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Abstract
The
catalytic hydrosilylation of carbonyl compounds by two POCOP-pincer
transition-metal hydrides, (POCOP)Ir(H)(acetone)<sup>+</sup> (<b>1A-acetone</b>) and (POCOP)Fe(H)(PMe<sub>3</sub>)<sub>2</sub> (<b>1B</b>) (POCOP = 2,6-bis(dibutyl-/diisopropylphosphinito)phenyl),
was theoretically investigated to determine the underlying reaction
mechanism. Several plausible mechanisms were analyzed using density
functional theory calculations. The <b>1A-acetone</b>-catalyzed
hydrosilylation of carbonyl compounds proceeds via the ionic hydrosilylation
pathway, which is initiated by the nucleophilic attack of the η<sup>1</sup>-silane metal adduct by carbonyl substrate. This attack results
in the heterolytic cleavage of the Si–H bond and the generation
of a siloxy carbenium ion paired with a neutral iridium dihydride,
[(POCOP)Ir(H)<sub>2</sub>][R<sub>3</sub>SiOCHR′]<sup>+</sup>, followed by transfer of hydride from the metal center to the siloxy
carbenium ion to yield the silyl ether product. The activation energy
of the turnover-limiting step was calculated as ∼15.2 kcal/mol.
This value is energetically more favorable than those of other pathways
by as much as 22.6 kcal/mol. The most energetically favorable process
for the hydrosilylation of carbonyl compound catalyzed by POCOP-pincer
iron hydride <b>1B</b> was determined as the carbonyl precoordination
pathway, which involves the initial coordination of the carbonyl substrate
to the metal center and subsequent migratory insertion into the M–H
bond to give the alkoxide intermediate. This intermediate then undergoes
M–O/Si–H σ-bond metathesis to yield the silyl
ether product. The ionic hydrosilylation pathway requires an activation
energy that is ∼30.0 kcal/mol higher than that of the carbonyl
precoordination pathway. Our calculation results indicate that the
hydride moiety is not involved in the POCOP-pincer iridium(III) hydride <b>1A-acetone</b>-catalyzed hydrosilylation of carbonyl compounds
but is involved in the POCOP-pincer iron(II) hydride <b>1B-</b>catalyzed process