No abstract

A series of platinum metal silyl hydride complexes, cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ = SiPh2H, SiEt2H, SiPh3, SiEt3, SiMe2(OSiHMe2), Si(OSiMe3)2Me, SiMe2(CH2CHCH2), SiMe2Et, SiMe2[OCH2C(Me)CH2], Si(OMe)2(CH2CHCH2), SiPh2(OSiPh2H)), have been prepared in solution by reaction of Pt(PCy3)2 with the appropriate silane, HSiR2R‘. The complex cis-Pt(PCy3)2(H)(HSiPh2) (1-cis) has been characterized by X-ray crystallography at −100 °C. The platinum center exhibits a distorted-square-planar geometry with angles P(1)−Pt−P(2) = 113.55(3)°, P(1)−Pt−Si = 146.83(3)°, and P(2)−Pt−Si = 99.37(3)°. The reaction of Pt(PCy3)2 with chlorinated hydrosilanes at −78 °C yields the analogous complexes cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ = SiMe2Cl, SiMeCl2, SiCl3), which isomerize to their trans isomers on warming to room temperature. The complex 1-cis and several analogues convert to the trans isomers photochemically at room temperature. Ready silane exchange is demonstrated by the reaction of HSiPh3 with cis-Pt(PCy3)2(D)(SiPh3) and by the reaction of H2SiPh2 with cis-Pt(PCy3)2(H)(SiPh3). These experiments also revealed the relative thermodynamic stability of some of the platinum silyl complexes, of which the most stable was cis-Pt(PCy3)2(H)(SiPh2H). NMR spectroscopy demonstrates that the inequivalent phosphine ligands of the cis isomers undergo intramolecular mutual exchange on the NMR time scale. In competition with this process, the complexes undergo reversible reductive elimination of silane. Analysis of the NMR spectra yields the thermodynamic data for dissociation of silanes for SiR2R‘ = SiPh3, SiMe2Et. Rate constants for phosphine exchange were calculated via line-shape analysis of 1H NMR spectra. Rate constants for reductive elimination of silane in cis-Pt(PCy3)2(H)(SiR2R‘) (SiR2R‘ = SiPh2H, SiMe2Et, SiPh3) were calculated via 1H EXSY measurements. The three distinct reaction pathways, photochemical cis−trans isomerization, intramolecular thermal phosphine site exchange, and reductive elimination, are shown to involve three distinct transition states. The transition states for the independent processes of phosphine site exchange and for reductive elimination must retain substantial Pt−H and Pt−Si interactions, while there is also significant Si−H bond formation. This situation can therefore be described as involving Pt(η2-H−SiR3) interactions.

Results are presented of a survey of fermented foods and beverages sold in the United Kingdom for levels of ethyl carbamate (urethane) carried out to expand the range of food types sold in the United Kingdom for which data regarding ethyl carbamate are available. Samples were analyzed by in-house validated methods, which included measurement uncertainty estimates. The samples comprised 75 fermented liquids (beers, wines, fortified wines, spirits, liqueurs, soy sauces, and vinegars) and 25 fermented solid foods (cheeses, yogurts, soybean products, sauerkraut, yeast extract, olives, and Christmas pudding). Ethyl carbamate was not detected in the beers or the cider. Wines contained between 11 and 24 microg/kg and sake between 81 and 164 microg/kg. Fortified wines contained ethyl carbamate at levels between 14 and 60 microg/kg. Only two of five liqueurs contained ethyl carbamate. Most soy sauces and vinegars did not contain ethyl carbamate. No ethyl carbamate was detected in cheeses, yogurts, olives, or soybean-based products. Single samples of sauerkraut, yeast extract, and Christmas pudding contained low levels (29, 41, and 20 microg/kg ethyl carbamate, respectively).