Mono(phosphine)palladium(0)
complexes, [Pd(η2:η2-C6H10O)(PR3)] (1), catalyze hydrosilylation
of electron-deficient conjugated dienes with HSiPh3. Hydrosilylation
of methyl penta-2,4-dienoate with HSiPh3 catalyzed by [Pd(η2:η2-C6H10O)(PR3)] (R = Me (1a), Et (1b), OEt (1d), O
i
Pr (1e)) proceeds
to give the 1,2-E product in quantitative yield with
exclusive Markovnikov selectivity. In contrast, their triphenylphosphine
and -phosphite analogues, [Pd(η2:η2-C6H10O)(PR3)] (R = Ph (1f), OPh (1g)), mainly produce the 1,4-Z product (1,2-E/1,4-Z = 3/7). The
regioselectivity in the hydrosilylation of methyl 2,4-pentadienoate
is also controlled by organosilanes. Mechanistic studies suggest that
the reaction using a compact and basic mono-phosphorus complex proceeds
by the Chalk–Harrod mechanism involving the reductive elimination
from an (η3-allyl)(silyl)palladium(II). A poor electron-donating
mono-phosphorus ligand such as P(OPh)3 destabilizes a Pd(II)
species and promotes direct reductive elimination to give the 1,4-Z product. On the other hand, a compact and electron-donating
phosphorus ligand, such as PMe3, PEt3, and P(OMe)3, favors to give the more stable (η3-allyl)(silyl)palladium(II)
intermediate by the allyl rotation, followed by reductive elimination
to give the 1,2-E product.
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