EXO-NIDO-MONOTHIORHODACARBORANES AND EXO-NIDO-MONOPHOSPHINORHODACARBORANES - SYNTHESIS, REACTIVITY, AND CATALYTIC PROPERTIES IN ALKENE HYDROGENATION

Reaction of Cs[7-SR-8-R'-7,8-C2B9H10] (R = Ph, R' = Me; R, R' = Ph; R = Et, R' = Me) with [RhCl(PPh3)(3)] in toluene-ethanol (8:1) affords [ Rh(7-SR-8-R'-7,8-C2B9H10)(PPh3)(2)]. Reaction of tetraalkylammonium sa lts of [7-PR2-8-R'-7,8-C2B9H10](-) (R = Ph, R' = H; R = Ph, R' = Me; R , R' Ph; R = Et, R' = Me; R = Et, R' = Ph; R = Pr-i, R' = Me) with [Rh Cl-(PPh3)(3)] in ethanol affords [Rh(7-PR2-8-R'-7,8-C2B9H10)(PPh3)(2)] . The structure of [Rh(7-PPh2-8-H-7,8-C2B9H10)(PPh3)(2)] has been dete rmined by crystallographic studies. Rh(I) has a normal four-coordinate d square-planar geometry. The carborane ligand is bonded to the metal by means of a P-Rh and a B(11)-H-Rh bond. Two PPh3 ligands fulfill the coordination sphere of the metal. The spectroscopic data indicate ana logous structures for the rest of the complexes. [Rh(7-SPh-8-Me-7,8-C2 B9H10)(PPh3)(2)] reacts with L = PPh3, PMePh2, and PEt3 to give the sa lts [RhL4][7-SPh-8-Me-7,8-C2B9H10]. [Rh(7-PPh2-8-H-7,8-C2B9H10)(PPh3)( 2)] does not react with PPh3, but it does react with L = PMePh2, PMe2P h, and PEt3 to give different substitution products depending on the c omplex:phosphine ratio: [Rh(7-PPh2-8-H-7,8-C2B9H10)(PPh3)L] at the rat io 1:1, [Rh(7-PPh2-8-H-7,8-C2B9H10)L-2] at the ratio 1:2, and [RhL4][7 -PPh2-8-H-7,8-C2B9H10] at ratios higher than 1:3. Both the thioether a nd the phosphino families catalyze the hydrogenation of 1-hexene to n- hexane. The thioether family is an active catalyst at P = 1 bar and T = 25 degrees C, whereas the phosphino family requires higher temperatu res and pressures. The hydrogenation of 2-hexenes proceeds much more s lowly than for 1-hexene. Both catalytic systems are recoverable upon c ompletion of reaction. [Rh(7-PPh2-8-H-7,8-C2B9H10)(PPh3)(2)] and [Rh(7 -PPh2-8-Me-7,8-C(2)B9H(10))(PPh3)(2)] catalyze the hydrogenation of th e antibiotic precursor methacycline to doxycycline with high yield and very high diastereoselectivity (ca. 100%) for the pharmacologically i mportant molecule. The other possible isomer epi-doxycycline was not o bserved in any case.