Rock around the ring: An experimental and theoretical study of the molecular dynamics of stannyltriphospholes with chiral tin substituents

The reaction of diphenyltin dichloride with enantiopure Grignard reagents prepared in situ leads to diphenyltin chlorides 7a-e, which contain chiral organyl substituents. The 2-bornyl derivative 7a forms a mixture of exo and endo isomers and the (-)-menthyl derivative 7b yields a mixture of epimers, whereas the cis- and trans-myrtanyl complexes 7c and 7d and the m-(2-bornyl-2-ene)phenyl species 7e form only one enantiomer as the tin atom is separated from the next stereogenic centre by a methylene or m-phenylene group. The target chirally modified 1-(triorganylstannyl)-1,2,4-triphospholes 2a-e are accessible from 7a-e by treatment of the latter with the salt (3,5-di-tert-butyl-1,2,4-triphospholyl)-sodium (Na5) in good yield. As for 7c-e, complexes 2c-e exhibit enantiopure organyl tin substituents that combine with the planar chiral 1,2,4-triphosphole moiety to form diastereomers. X-ray diffraction studies on crystalline phases of 2a,b,d and 7b,c confirm the stereochemical interpretations of their spectroscopic data with the help of absolute structure parameter determinations. Dynamic NMR spectroscopy experiments with these compounds demonstrate for the first time a facile stannyl group shift between the two adjacent P atoms (P1 and P2) of the heterocycles that includes ring in-version through a planar transition state (T-i). In line with the NMR spectroscopic results for 2a-e, density functional calculations with the simplified model compound (trimethylstannyl)-1,2,4-triphosphole (9) point to 1-(trimethylstannyl)-1, 2,4-triphosphole (9a) as the global energetic minimum for 9 and an almost isoenergetic 1,2-shift and ring inversion as the main epimerisation processes of the two enantiomers of 9a. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).