Allyl- and allyl-like-tin derivatives. 3. Crystal structures and (solution) conformations of some cyclohex-2-enyl- and cyclohept-2-enylstannanes

Abstract

The crystal structures of the allylic stannanes cyclohex-2-enyl- (1) and cyclohept-2-enyltriphenylstannanes (2) have been determined, and in both, the coordination about tin is tetrahedral, although the enlargement of the zC(13)-Sn-C(19) angle in both 1 (114.4°) and 2 (117.0°) may be in response to a short intramolecular contact between one phenyl ring and the cycloalkene moiety. The Sn-C(phenyl) bond lengths are normal (2.144 ± 0.005 A), but the allyl bond (Sn-C(19) = 2.185 ± 0.005 A) is longer (ca. 0.04-0.05 A) than the normal Sn-C (aliphatic) bond length (2.13 ± 0.01A) and may be associated with a hyperconjugative σC-Sn -πEffect. In 1, the (CH)Sn group formally occupies a quasi-equatorial orientation with a Sn-C(19)-C(20)-C(21) torsion angle of 121° which permits substantial σ-π interaction and may best accommodate other interactions in the crystal. Given the other values of bond and torsion angles, the actual distinction between quasi-axial and quasi-equatorial orientations with respect to double bond interaction with δ is a fine one. The molecular geometry of 2 is a slightly distorted chair arrangement with the (CH)Sn group clearly occupying a quasi-axial position with a Sn-C(19)-C(25)-C(24) torsion angle of 90°, the most favored for σ-π interaction. However, this “outward splaying” of the C-Sn bond is probably also associated with relief of interactions between Sn(CH) and the regions of C(21) and C(23). Solid-state (MAS) C NMR spectra have been recorded of both 1 and 2, and although only one set of signals was observed for each, Sn-C couplings were not located. H and C NMR spectra for 1 and 2 (in solution) have been obtained, and for 1 there is now a preponderance of the conformer with an axially disposed tin group, whereas for 2, the solution conformation is adequately described as a distorted chair arrangement with a quasi-axial Sn(CH) group, as identified in the crystal.