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

series of enantiomerically pure bis(myrtanyl)tin compounds, cis-Myr2SnX2 (1, X = Ph; 3, X = Cl; 5, X = H) and trans-Myr2SnX2 (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 3 and 4 also by X-ray crystallography. The reduction of 3 and 4 with Mg selectively produced pentastannane rings, cyclo-(cis-Myr2Sn)5 (7) and cyclo-(trans-Myr2Sn)5 (8), while the dehydropolymerization of 5 and 6 produced a mixture of polystannanes, poly(cis-Myr2Sn)n (9) or poly(trans-Myr2Sn)n (10), and oligomers that could not be separated. The pentastannane rings gave rise to UV absorptions at λmax 219 and 217 nm, which were assigned to σ → σ* transitions of the Sn−Sn bonds. Due to the increased σ-electron delocalization, the polystannanes 9 and 10 show red-shifted UV absorptions at λmax 416 and 417 nm. The CD spectra of the polystannanes 9 and 10 reveal a positive cotton effect at λmax 422 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.