Synthesis of 1-Silabenzo[ d , e ]isochromanes via Electrophilic Aromatic Substitution of Aldehydes Activated by Silylium Ion.

A strong Lewis acid silylium ion was utilized for dehydrogenative annulation between dialkyl(1-naphthyl)silanes 1 and aldehydes 2 . Silane 1a was reacted with [Ph 3 C][B(C 6 F 5 ) 4 ] in the presence of 2,6-di- tert -butyl-4-methylpyridine and aldehydes 2 to afford the annulation product, 1-silabenzo[ d , e ]isochromanes 3, in moderate isolated yields. The annulation occurred only at the 8-position on the 1-naphthyl group. The silylium ion-promoted hydrosilylation proceeded competitively to afford silyl ethers 4 via the same intermediates, silylcarboxonium ions, in the dehydrogenative annulation. The ratio of 3 and 4 was affected by solvents and the electronic properties of aromatic aldehydes; for example, the use of less polar solvents and that of benzaldehydes with an electron-withdrawing group at the para -position predominantly yielded 3 . This annulation reaction was applicable to aldehydes bearing a heteroaromatic group and aliphatic alkyl groups. Judging from these results, both the formation of silylcarboxonium ions by in situ- generated silylium ions and the electrophilic aromatic substitution are important for this annulation reaction.