Isoprene hydrosilation

The Pt(0) catalyst did not cause hydrosilation of internal olefins such as 2-hexene or cyclohexene, but it was effective with conjugated diolefins. With isoprene, even in excess, it produced considerable amounts of diadduct as well as 1,2-monoadduct. (This is anomalous, as will be discussed later in this review.) The principal products are shown in Eq. (8) ... 

The activity of Ziegler-type systems such as M(acac) -AlEt3 (M = Cr, Mn, Fe, Co, or Ni acac = acetylacetonate) was examined with 1-olefins and triethyl- or triethoxysilanes (55). Systems with nickel or cobalt showed low activity for hydrosilation but isomerized the olefin and were reduced to the metal. The study was extended to dienes and acetylenes (56). Isoprene gave the same products with these catalysts as are made with chloroplatinic acid. Penta-1,3-diene with Pt gave l-methylbut-2-en-ylsilanes. The Ziegler catalysts gave mainly penta-2-enylsilanes... 

The hydrosilation of butadiene, isoprene and other simple conjugated dienes with platinum catalysts has not been intensively studied. It usually occurs by 1,4 addition and only at elevated temperatures (10). Although palladium catalysts are practically ineffective to hydrosilate an alkene, they are very active under mild conditions with conjugated dienes to give two principal products, sometimes in 100% yields. With triethoxy silane, butadiene, and PdCl2(PhC=N)2 (5.5 x 103 moles/mole) at 22°C for 24... 

Co2Rh2(CO)i2] and [Co3Rh(CO)i2] catalyze the hydrosilation of isoprene, cyclohexanone, cyclohexenone, and 1-alkynes, the regioseleetive inter- or intramolecular (Eq. 5) silylformylation of alkynes and 1-alkynals, and the silylcarbo-cyclization of enynes, diynes, and alkynals. These synthetically important reactions have been reviewed recently. 

The use of the cobalt triad carbonyls as catalysts continues to provide many papers for this report. Publications cover the silylformylation of 1-Hexyne catalyzed by diodium-cobalt carbonyl clusters the formation of hydroxycarbene cobalt carbonyl derivatives, the use of rhodium cluster carbonyls in the water-gas shift reaction Rh4(CO) 2> and Co3Rh(CO)] 2 catalysts for the hydrosilation of isoprene, cyclohexanone and cyclohexenone catalytic reduction of NO by CO and the carbonylation of unsaturated compounds The chemistry of iridium carbonyl cluster complexes has been extended by making use of capping reactions with HgCl2and Au(PPh3)Q... 

Unlike the iron triad complexes, cobalt triad compounds, particularly those of Rh and Ir are much more active in hydrosilation catalysis, although the cobalt carbonyl Co2(CO)g has played a very important role in furthering understanding of the catalytic cycle with transition metals. Recently, a simple cobalt salt, CoBr2, in conjunction with added Bu"P, Znl2 and BU4NBH4 was shown to afford 1,4-hydrosilated isoprene (silicon at C-4) in 90% isolated yield, where most metal catalysts derived from Ru, Rh, Pd and Pt usually lead to a mixture of regioisomers. ... 

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