Optically Active Organotin Compounds Derived from β-Pinene. The Quest for Chiral Polystannanes.

series of enantiomerically pure bis(myrtanyl)tin compounds, cis-Myr 2SnX 2( 1, X = Ph; 3, X = Cl; 5, X = H) and trans-Myr 2SnX 2( 2, X = Ph; 4, X = Cl; 6, X = H) were prepared starting from (–)-1S-β-pinene and characterized by multinuclear NMR spectroscopy and in case of 3and 4also by X-ray crystallography. The reduction of 3and 4with Mg selectively produced pentastannane rings, cyclo-( cis-Myr 2Sn) 5( 7) and cyclo-( trans-Myr 2Sn) 5( 8), while the dehydropolymerization of 5and 6produced a mixture of polystannanes, poly( cis-Myr 2Sn) n( 9) or poly( trans-Myr 2Sn) n( 10), and oligomers that could not be separated. The pentastannane rings gave rise to UV absorptions at λ max219 and 217 nm, which were assigned to σ → σ* transitions of the Sn−Sn bonds. Due to the increased σ-electron delocalization, the polystannanes 9and 10show red-shifted UV absorptions at λ max416 and 417 nm. The CD spectra of the polystannanes 9and 10reveal a positive cotton effect at λ max422 and 425 nm, consistent with the idea that the chiral information is transferred from the UV-inactive myrtanyl groups to the polymer backbone, which most likely adopts a helical conformation with the right-handed screw sense being in excess. [ABSTRACT FROM AUTHOR]