Synthesis, stereochemistry and chiroptical properties of naphthylphenyl-substituted optically active oligosilanes with α,ω-chiral silicon centers

Novel naphthylphenyl-substituted optically active oligosilanes with α,ω-chiral silicon centers were synthesized. The absolute configuration of (1R,2R)-1,2-dimethyl-1,2-di(1-naphthyl)-1,2-diphenyldisilane (R,R)-3, one of the oligosilanes, was determined by X-ray diffraction, which gave a direct evidence for the retention and inversion stereochemistry of attacking silylanion and attacked chlorosilane. The intense π–π interaction of aryl substituents on chiral silicon centers enhanced by σ–π conjugation with oligosilane unit made it possible to clearly assign the absolute configuration and stable conformation of the optically active oligosilanes. The disilane (R,R)-3 showed positive exciton chirality originated from π–π interactions of 1Bb,Np transition bands of two naphthyl chromophores on the adjacent silicon atoms. Contrary to this, (1R,3R)-1,3-di(1-naphthyl)-1,3-diphenyl-2-trimethylsilyl-1,2,3-trimethyltrisilane (R,R)-5, having silylene spacer with bulky trimethylsilyl group as branched substituent between two chiral centers, showed positive exciton chirality by interaction between red-shifted 1La,Ph and 1Bb,Np on the same silicon atom. (1R,3R)-1,3-Di(1-naphthyl)-1,3-diphenyl-1,2,2,3-tetramethyltrisilane (R,R)-4, which has dimethylsilylene group between two chiral silicon centers, showed two exciton chiralities, namely, positive chirality by 1La,Ph and 1Bb,Np on the same silicon atom, and negative chirality by two 1Bb,Np on the adjacent silicon atoms. The intensified negative Cotton effect at 234 nm was caused by overlapping of Cotton effects of two exciton chiralities.