Optical activity polysilanes

Optically Active Polysilanes with Diastereomeric Helidties Containing Opposite Screw Senses and Different Screw Pitches... 

Chart 4.2 Wurtz-type reduction preparation of optically active polysilanes. 

This knowledge and understanding may be helpful to characterize local conformations of other optically active polysilanes in solution. For example, poly(methyl-(-)-(3-pinanylsilane) [(+)-7 Mw = 10,200] prepared by Shinohara and co-workers.281 showed a bisignate CD band at 280 and 303 nm, associated with a broad UV absorption at 300 nm in chloroform at 15°C. Since the spectroscopic features are quite similar to those of i,28d-28e it is possible that the main chain in 7 may contain diastereomeric helical motifs with opposite screw senses and different screw pitches. 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

Similarly, use of the chiral anionic initiator (+) or (—) potassium menthoxide with the same masked disilene afforded a polysilane with a PSS helical conformation. Optically active polysilanes are discussed further in Section 3.11.6. 

Chiral amines have also been used to prepare the quaternary ammonium salts, forming optically active polysilanes (see Section 3.11.6.1.3).173... 

A novel approach to aggregate control was described by Terao et al., who confined optically active polysilanes inside poly(ureaurethane) microcapsules and then induced aggregation at 20 and — 78 °C by addition of the polysilane-containing microcapsules to THF or ethanol. 5... 

Fig. 2 Comparison of UV absorption spectra of four optically active polysilanes in THF at 30 °C Poly methyl-(S)-2-methylbutylsilane (1, a of 0.59), poly n-hexyl-(S)-4-methylpentylsilane (2, a of 0.75), poly n-hexyl-(S)-3-methylpentylsilane (3, a of 0.92), poly n-hexyl-(S)-2-methyl-butylsilane (4-S, a of 1.25)...

Fujiki M (2003) Switching handedness in optically active polysilane. J Organomet Chem 685(1-2) 15-34... 

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