Rearrangement, 1,2-alkyl alkenes, metal catalyzed

As mentioned above, we planned to obtain optically pure styrenyl ethers through Zr-catalyzed kinetic resolution [5] subsequent metal-catalyzed rearrangement would afford optically pure chromenes. However, as shown in Scheme 11, the recovered starting material (40) was obtained with <10% ee (at 60% conversion) upon treatment with 10 mol% (,R)-(EBTHI)Zr-binol (3b) and five equivalents of EtMgCl (70°C, THF). We conjectured that, since the (EBT-HI)Zr-catalyzed reaction provides efficient resolution only when asymmetric alkylation occurs at the cyclic alkene site, competitive reaction at the styrenyl terminal olefin renders the resolution process ineffective. Analysis of the H NMR spectrum of the unpurified reaction mixture supported this contention. Indeed, as shown in Scheme 11, catalytic resolution of disubstituted styrene 49... 

The mechanisms of all metal-catalyzed hydrosilations are thought to be very similar. The pathway probably involves an adduct composed of the silane, the alkene, and the metal. Transfer of the silicon to the carbon is believed to occur after the 7r-bonded olefin rearranges to a a complex. Whereas the mechanism displayed in the following scheme involves olefin insertion into Pt-H, equally possible is insertion into Pt-Si followed by reductive elimination of the alkyl silane. 

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