SYNTHESIS AND CHARACTERIZATION OF CHIRAL (-)-MENTHYLTIN(IV) COMPOUNDS - X-RAY STRUCTURE OF TERT-BUTYL-8-(DIMETHYLAMINO)NAPHTHYL-(-)-MENTHYLTIN HYDRIDE

A series of optically active (-)-menthyltin(IV) derivatives with tin directly bound to a chiral carbon atom has been synthesized and characterized. The Grignard reagent derived from (-)-menthyl chloride stereospecifically reacts with trimethyltin chloride to afford (-)-menthyltrimethyltin (1). 1 is derivatized by selective bromodemetalation, alkylation, and hydrogenation, yielding (-)-menthyldimethyltin bromide (2), tert-butyl-(-)-menthyldimethyltin (3), tert-butyl-(-)-menthyltin dibromide (4), and each prochiral at the tin center as well as tert-butyl-8-(dimethylamino)naphthyl-(-)-menthyltin bromide (5) and hydride (6), in which an additional center of asymmetry has been created at the tin atom. By means of multinuclear NMR investigations, the conformation of the optically active ligand in 1-6 was determined, the coordination behavior of the potentially bidentate aminonaphthyl ligand in 5 and 6, resulting in a trigonal bipyramidal (5) or a distorted tetrahedral (6) structure in solution, was established, and for 5 and 6, the ratio of epimers differing in the absolute configuration at the tin center was determined to be 45:55 and 40:60, respectively. The single-crystal X-ray structural analysis of 6 revealed a pair of R(Sn)/S(Sn) epimers. 6 crystallizes monoclinically, space group P2(1), with a = 17.786 (8) angstrom, b = 10.890 (4) angstrom, c = 13.700 (8) angstrom, beta = 108.0 (4)-degrees, and Z = 4. The structure was solved from 4449 observed reflections with I greater-than-or-equal-to 3-sigma(I) and refined to a final R factor of 0.028. The coordination geometry around tin for the two molecules in the asymmetric unit is that of a trigonal bipyramidal-like monocapped tetrahedron showing a very weak Sn-N interaction with a Sn-N distance of 2.931 (3) (molecule A) or 2.885 (3) angstrom (molecule B). The Sn-H bond lengths are 1.52 (5) and 1.63 (5) angstrom, respectively.