Synthesis of Stable 2-Silanaphthalenes and Their Aromaticity

Stable neutral silaaromatic compounds, 2-silanaphthalenes (1a; R = Tbt, 1b; R = Bbt), were synthesized by taking advantage of extremely bulky and efficient steric protection groups, 2,4,6-trisbis(trimethylsilyl)methylphenyl (Tbt) and 2,6-bisbis(trimethylsilyl)methyl-4-tris(trimethylsilyl)methylphenyl (Bbt). 2-Silanaphthalenes 1a,b were isolated as colorless crystalline compounds, and the structure of Tbt-substituted 1a was determined by X-ray crystallographic analysis. All 1H, 13C, and 29Si NMR signals of the silanaphthalene ring of 1 were in good agreement with those calculated; downfield-shifted 1H signals indicating aromatization were observed. Refined NICS (nucleus-independent chemical shift) calculations which separate the σ and π contributions revealed the presence of comparably large ring current effects in the two rings of 2-silanaphthalene. The aromatic stabilization energies (ASEs) of 1-, 2-, and 9-silanaphthalenes, computed by a novel isomerization method, are almost the same as naphthalene. UV−vis and Raman spectra, and the computed (B3LYP/6-311+G(d,p)) structure of 2-silanaphthalene were similar to those of naphthalene, although characteristic changes resulting from the replacement of carbon to silicon were observed. 2-Silanaphthalene 1a is still highly reactive due to its polarized SiC moiety, but was relatively stable toward air in the solid state and showed no complex formation with THF in solution. Considering all experimental and theoretical results, the 2-silanaphthalenes were concluded to have considerable aromatic character, closely approaching that of naphthalene itself.