Surface organometallic chemistry of tin:: Reactivity of two chiral organostannics, ((-)-menthyl)Me3Sn and ((-)-menthyl)Me2SnH, toward silica

Two chiral organostannics, the tetraalkyl ((-)-menthyl)Me3Sn (1) and the hydride ((-)menthyl)Me2SnH (2), were synthesized. Their reactions with the surface of partially dehydroxylated silica were followed by analysis of evolved gases, IR, and C-13 and Sn-119 MAS NMR spectroscopic characterizations of the grafted organometallics. With both organostannics, a well-defined chiral surface organometallic compound, drop Si-O-Sn((-)-menthyl)Me-2 (3), is obtained from the reaction of 1 mol of organotin complex with 1 mol of silica surface hydroxyl groups, with formation of 1 mol of methane (from 1) or dihydrogen (from 2). Side reactions leading to nonchiral species, such as drop Si-O-SnMe3 (4) or involving two surface silanol groups for one organotin molecule, giving for instance (drop Si-O)(2)Sn((-)-menthyl)Me (5), also occur limiting thus the selectivity of the grafting reaction toward 3. The amount of grafted tin and the nature of the grafted tin species depend on the type of organostannic and on temperature. When using the tetraalkyl ((-)-menthyl)Me3Sn as a precursor, temperatures higher than 100 degrees C are required in order to graft tin onto the silica surface. Although the amount of grafted tin increases with temperature, the selectivity of the reaction toward 3 remains constant at 85% for temperatures lower than 160 degrees C and decreases at higher temperatures. With ((-)-menthyl)Me2SnH, tin grafting occurs already at room temperature, but a temperature of 80 degrees C allows one to graft high tin amounts (4-6 wt % Sn) within a reasonable time, with 95-100% of the grafted tin present as 3 on the silica surface. The reaction is less selective at 100 degrees C. The chiral surface organometallic species 3 is thermally stable up to 160-200 degrees C.