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 center dot(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 ss,gamma-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.