Allylation with substituted vinylthionium ions from SnCl4 ionisation of 1,3- and 3,3-bis(alkyl/phenylthio) propenes

Add time:08/04/2019    Source:sciencedirect.com

α- and γ-substituted vinylthionium ions from SnCl4 ionisation of a range of substituted 1,3- and 3,3-bis (alkyl/phenylthio) propenes allylate enol allyl ethers in good yield. Levels of regioselectivity are sterically dependent and in the case of methyl as γ-substituent may be controlled by the steric bulk of the sulfur substituent. As with the Pummerer methodology, γ-addition generally gave (E)-vinyl sulfides exclusively implying the intermediacy of a ‘free’ vinylthionium ion. In appropriate cases, the allylated products may be hydrolysed and cyclised to the Robinson annulation products, desulfurised to the olefin or eliminated to the diene. Allyltrimethylsilane and indole are also allylated in good yield. Vinylthionium ions with a γ-phenylthio or methoxy group also undergo the reaction. In the methoxy case the reaction is highly sensitive to the number of equivalents of Lewis acid used and found to be truly catalytic in TiCl4. A mechanistic rationale for this is presented. The study concludes with a pentannulation sequence involving intramolecular capture of a vinylthionium ion generated by the methodology.

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