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-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methy]phenyl (Bbt). 2-Silanaphthalenes 1a,b were isolated as colorless crystalline compounds, and the structure of Tbt-substituted la was determined by X-ray crystallographic analysis. All H-1, C-13, and Si-29 NMR signals of the silanaphthalene ring of 1 were in good agreement with those calculated; dowmfield-shifted H-1 signals indicating aromatization were observed. Refined NICS (nucleus-independent chemical shift) calculations which separate the sigma and pi 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 la is still highly reactive due to its polarized Si=C moiety, but was relatively stable toward air in the solid stale and showed no complex formation with TNF in solution. Considering all experimental and theoretical results, the 2-silanaphthalenes were concluded to have considerable aromatic character, closely approaching that of naphthalene itself.