Organofluorosilicates in Organic Synthesis. XVI. Synthesis of Organopentafluorosilicates via the Diels-Alder, Ene, and Friedel-Crafts Reaction. Their Transformations to Organic Halides and Alcohols

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Abstract
Described herein are several representative examples of cleavage reactions of otherwise hardly accessible organopentafluorosilicates which are obtainable via the Diels-Alder reaction, ene reaction, and the Friedel-Crafts reaction of vinyl-, ethynyl-, or allyl-trichlorosilanes. The Diels-Alder reaction between vinyltrichlorosilane and o-xylylene generated in situ by debromination of α,α′-dibromo-o-xylene afforded (1,2,3,4-tetrahydro-2-naphthyl)trichlorosilane which was converted to the corresponding silicate. The silicate reacted with NBS and MCPBA to give the respective bromide and alcohol. The Diels-Alder reaction between ethynyltrichlorosilane and methyl coumalate gave a mixture of meta and para isomers of methyl (trichlorosilyl)benzoate in the ratio of 55/45, the silicate of which reacted with NBS to give two positional isomers of methyl bromobenzoate. The electronic effect of the methoxycarbonyl group on the Si–C bond cleavage was small. The silicate derived from 4-octenyltrichlorosilane which was obtained by the ene reaction between vinyltrichlorosilane and 1-hexene afforded, upon treatment with NBS, 4-octenyl bromide in the ratio of E⁄Z=83⁄17. The sterically crowded neophylsilicate obtainable via the Friedel-Crafts reaction of 2-methyl-1-propenyltrichlorosilane with benzene underwent the cleavage reaction with NBS in methanol. Neophyl bromide was formed with CuBr2 in a low yield, no product being formed arising from rearrangement of the neophyl radical.