SYNTHESIS AND CHARACTERIZATION OF CHIRAL (-)-MENTHYLTIN(IV) COMPOUNDS CONTAINING POTENTIALLY C,N-CHELATING LIGANDS - X-RAY STRUCTURE OF (RSN)-[8-(DIMETHYLAMINO)NAPHTHYL]-(-)-MENTHYLMETHYLTIN BROMIDE

Starting from (-)-menthyltrimethyltin, Sn-chiral (-)-menthyltin(IV) derivatives bearing potentially bidentate (dimethylamino)organo ligands of different flexibility have been synthesized and characterized. (-)-Menthylmethyltin dibromide (1), obtained by dihalogenation, is monoalkylated by the respective lithium reagent to give [3-(dimethylamino)propyl]-, [2-[dimethylamino)methyl]phenyl]-, and [8-(dimethylamino)naphthyl]-(-)-menthylmethyltin bromide (2-4). Reduction yields the corresponding tin hydrides (5-7). For comparison, the non-donor-atom substituted (-)-menthylmethylnaphthyltin bromide (8) and hydride (9) have been prepared accordingly. Polarimetric and multinuclear NMR investigations show the structure and configurational stability of 2-7 in solution reflecting the increasing rigidity of their potentially C,N-chelating ligands: The tin center in 2-4 is rendered pentacoordinate as a result of intramolecular Sn-N coordination. The stability of the five-membered chelate ring with regard to a Sn-N dissociation/association process taking place and the configurational stability of the tin center increase likewise. While an epimerization process of 2 occurs fast on the NMR time scale, slow inversion of the configuration at the tin atom can be observed on a freshly prepared solution of crystalline, diastereomerically pure 3 or 4. The Lewis acidity of the tin center in 5-7 is too low for effecting Sn-N coordination. The geometric constraint of the aminoorgano ligand in 6 and 7, however, causes a distortion of the tetrahedral structure into a beginning trigonal bipyramidal structure. The epimer ratios of the ''doubly chiral'' organotin compounds in solution are found to differ from 1. The X-ray structural investigation of 4 reveals an absolute configuration of R(Sn) for both molecules in the asymmetric unit: 4 crystallizes monoclinically, space group P2(1), with a = 9.259(5) angstrom, b = 20.615(9) angstrom, c = 12.964(7) angstrom, beta = 105.40(5)-degrees, and Z = 4. The structure was solved from 3498 reflections with I > 2sigma(I) and refined to a final R factor of 0.044, showing a distorted trigonal bipyramidal coordination geometry at the tin center.