Regioselective and Stereospecific Dehydrogenative Annulation Utilizing Silylium Ion-Activated Alkenes

Abstract

Treatment of dialkylbenzylsilanes (1) with trityl tetrakis(pentafluorophenyl)borate (TPFPB) afforded the corresponding silylium ions in equilibrium with their intra- or intermolecular π-complexes, which underwent dehydrogenative annulation with various alkenes to form 1,2,3,4-tetrahydro-2-silanaphthalenes (4) in up to 82% isolated yield. Sterically bulkier substituents on the silicon atom tended to increase the yield of cyclic products 4. The annulation products retained the stereochemistry in cases of the reactions using internal alkenes. The use of diisopropyl(1-naphthyl)silane (2) instead of 1 also resulted in annulation to obtain the 2,3-dihydro-1-sila-1H-phenalene derivatives 6. Electrophilic aromatic substitution at the 8-position was predominant, despite the two potentially reactive positions on the naphthyl group. The steric hindrance of the naphthyl group prevented addition of the cis-alkene to the silylium ion, which would considerably decrease yields of the desired products from 2 compared to those from 1.

Chiral Memory in Silyl-Pyridinium and Quinolinium Cations

“…Silylium ions are extremely reactive species that are generally difficult to isolate and characterize, unless they are stabilized by coordination with a solvent or through an external or internal Lewis base (LB) . This exacerbated reactivity has long delayed their isolation compared to carbocations, isolated and characterized nearly 100 years earlier .…”

“…However, these silicon species are only of synthetic interest if they are previously pacified and their reactivity controlled. The work of Siegel, Müller, Oestreich, and others has shown that the interaction between the cationic silylated center and Lewis basic substituents is able to control this reactivity and to modulate the Lewis acidity of silicon . Very recent studies have demonstrated that the addition of basic and chiral ligands around the silyl cation not only stabilizes the latter but also allows chirality transfer with satisfactory enantioselectivity levels in Diels–Alder reactions .…”

Transition-Metal-Free Photoinduced Intramolecular Annulation of 2,3-Di(hetero)arylchromen-4-one

“…Given our interest in the development of direct C–H functionalization and photoinduced annulation as well as the existence of limited literature reports on dehydrogenative annulation, which was thought to require transition metals and/or oxidative additives, , in this letter, we report an efficient transition-metal-free photoinduced intramolecular dehydrogenative annulation of 4 H -chromen-4-ones for the synthesis of benzo­[ c ]­furo­[2,3- a ]­xanthenone derivatives that does not require catalysts or additives (Scheme b).…”

Intramolecular Chain Hydrosilylation of Alkynylphenylsilanes Using a Silyl Cation as a Chain Carrier

“…We have extended silyl cation chemistry to the synthesis of silacyclic compounds and reported the synthesis of 1,2-dihydro-2-silanaphthalenes using alkynes [25,26]. The intramolecular addition of a silyl cation toward a C-C triple bond rather than an intermolecular one is expected to lead to a benzosilole by an appropriate reaction of the resulting ethenyl carbocation.…”