Enone as a Process Aid for the Highly Efficient Synthesis of Karstedt’s Catalyst: Probing the Mechanism of Dissolution of Platinum(II) Chloride

Homogeneous platinum complexes such as Speier’s
catalyst
and Karstedt’s catalyst are some of the most commonly used
catalysts for hydrosilylation reactions. The synthesis of Karstedt’s
catalyst from anhydrous PtCl2 requires the presence of
a polar solvent (methyl ethyl ketone/MEK) and divinyltetramethyldisiloxane
(dvtms) as the reagent. Despite being practiced over several decades,
the reaction suffers from several limitations such as moderate conversion
(80–85%), long reaction time (8–10 h), and thermal decomposition
of the catalyst over longer periods. Through an approach that relies
mostly on mechanistic insights and systematic investigation of all
reaction parameters, we identified that presoaking or milling PtCl2 in MEK at room temperature led to the formation of crystalline
Pt6Cl12·(MEK)1.5 (characterized
by powder X-ray diffraction), which drastically improved the reaction
conversion (4 h, 99% conversion of PtCl2). As our understanding
of the mechanism of this reaction improved, we discovered that small
amounts of PtCl2(enone) complexes (isolated and fully characterized
by X-ray crystallography) were formed in situ from
the preheated mixture of PtCl2 and MEK in the absence of
dvtms. These enone compounds were likely formed via aldol condensation
of MEK, followed by a dehydration reaction. We have since found that
these β,γ-enones are superb process additives and can
be independently added (as low as 1 wt %) to improve the reaction
rate (<4 h) and conversion (>98% conversion of PtCl2). Computational studies further suggest that enones behave as phase-transfer
additives. Once MEK disrupts the PtCl2 lattice, enones
facilitate the dissolution process by complexing with the individual
molecular PtCl2 moieties, thus stabilizing them in the
homogeneous phase. In addition, the calculated energy landscape suggests
that once the solid PtCl2 is brought into the homogeneous
liquid phase, the formation of Karstedt’s catalyst itself
is energetically downhill, overcoming only moderate activation barriers
for a Pt(II) to Pt(0) reduction process.