Platinum(II) Di-ω-alkenyl Complexes as “Slow-Release” Precatalysts for Heat-Triggered Olefin Hydrosilylation

Abstract

We describe the synthesis, characterization, and catalytic hydrosilylation activity of platinum­(II) di-ω-alkenyl compounds of stoichiometry PtR2, where R = CH2SiMe2(vinyl) (1) or CH2SiMe2(allyl) (2), and their adducts with 1,5-cyclooctadiene (COD), dibenzo­[a,e]­cyclooctatetraene (DBCOT), and norbornadiene (NBD), which can be considered as slow-release sources of the reactive compounds 1 and 2. At loadings of 0.5 × 10–6–5 × 10–6 mol %, 1-COD is an active hydrosilylation catalyst that exhibits heat-triggered latency: no hydrosilylation activity occurs toward many olefin substrates even after several hours at 20 °C, but turnover numbers as high as 200000 are seen after 4 h at 50 °C, with excellent selectivity for formation of the anti-Markovnikov product. Activation of the PtII precatalyst occurs via three steps: slow dissociation of COD from 1-COD to form 1, rapid reaction of 1 with silane, and elimination of both ω-alkenyl ligands to form Pt0 species. The latent catalytic behavior, the high turnover number, and the high anti-Markovnikov selectivity are a result of the slow release of 1 from 1-COD at room temperature, so that the concentration of Pt0 during the initial stages of the catalysis is negligible. As a result, formation of colloidal Pt, which is known to cause side reactions, is minimized, and the amounts of side products are very small and comparable to those seen for platinum(0) carbene catalysts. The latent reaction kinetics and high turnover numbers seen for 1-COD after thermal triggering make this compound a potentially useful precatalyst for injection molding or solvent-free hydrosilylation applications.

Cyclobutadiene platinum complex as a new type of precatalyst for hydrosilylation of alkenes and alkynes

“…High activity at ambient temperature distinguishes 1 from many other platinum complexes, which are normally inactive below 40°C and often require heating up to 80–110°C. [ 18 ] The reaction can be conveniently carried out without protection of the inert atmosphere. The product was simply isolated by passing the crude mixture through a short pad of silica gel and evaporating the solvent.…”

PtII−N‐Heterocyclic Carbene Complexes in Solvent‐Free Alkene Hydrosilylation

“…[12] Furthermore, much less attention has been devoted to Pt(II) hydrosilylation pre-catalysts. The existing literature reports suggest not only Pt(0)/Pt(II), [13] but also Pt(II)/Pt(IV) [14] variations of the widely accepted Chalk-Harrod mechanism, [15] and to the best of our knowledge the hypothesized Pt(II) ! Pt(0) reduction in the pre-catalyst activation step has not yet been supported by convincing scientific evidence.…”

“…Chemistry-A European Journal 1 H, 13 C and 195 Pt NMR spectroscopic studies Nuclear magnetic resonance spectroscopy, especially 13 C NMR, is an invaluable tool for understanding and quantifying the unique properties of NHC ligands. [22] A characteristic feature in 1 H NMR spectra of the dimethyl sulfide complexes are very distinct platinum satellites associated with S(CH 3 ) 2 proton signals at around 2.00-2.50 ppm.…”

“…[22] A characteristic feature in 1 H NMR spectra of the dimethyl sulfide complexes are very distinct platinum satellites associated with S(CH 3 ) 2 proton signals at around 2.00-2.50 ppm. Despite the fact that both short-and long-range PtÀ C couplings are clearly visible in the 13 C NMR spectra of Pt(0) [Pt(NHC)(dvtms)] complexes, [10d] the Pt satellites could not be easily detected in the carbon spectra of our Pt(II)À NHC compounds. The most interesting peak in the 13 C NMR spectra, the C2 carbene signal, provides insightful information on the new pre-catalysts.…”

“…Despite the fact that both short-and long-range PtÀ C couplings are clearly visible in the 13 C NMR spectra of Pt(0) [Pt(NHC)(dvtms)] complexes, [10d] the Pt satellites could not be easily detected in the carbon spectra of our Pt(II)À NHC compounds. The most interesting peak in the 13 C NMR spectra, the C2 carbene signal, provides insightful information on the new pre-catalysts. [22b] These values in Table 2 vary from 139. for neutral Pt(II) tetracoordinate complexes, [23] and the values for trans-[Pt(NHC)(DMS)Cl 2 ] complexes vary from À 3325 ppm for 3 ab to À 3232 ppm for 3 ac.…”