Forced exo-nido rhoda and ruthenacarboranes as catalyst precursors: a review

Forced exo-nide rhoda and ruthenacarboranes containing monothio and monopho sphinocarboranes have been tested as catalyst precursors in different catal ytic reactions. The catalyst precursors employed were [Rh(7-SR-8-R'-7,8-C2B 9H10)(PPh3)(2)] (R=Ph, Et; R'=Ph, Me), [Rh(7-PR2-X-R'-7,8-C2B9H10)(PPh3)2] (R=Ph, Et, Pr-i; R'=H, Me), [Rh(7-PPh2-8-Me-7,8-C2B9H10)(cod)], [Rh(7-SR-8- R'-7, 8-C2B9H10)(cod)], [RuX(7-PR2-8-R'-7,8-C2B9H10)(PPh3)(2)] (X = Cl, H; R = Ph; R' = H, Me, Ph) and [RuCl(7-SR-8-R'-7,8-C2B9H10)(PPh3)(2) (R=Ph, Et ; R'= Me, Ph). These complexes are obtained by the reaction of the tetramet hylammonium or cesium salt of the nide ligand with Rh(I) or Ru(II) complexe s incorporating ancillary ligands. Although two molecular structures are po ssible, the close and the exo-nido, only the exo-nido tautomer is generally formed. The cluster is coordinated to the metal through the S or P atom an d one or two B-H-M interactions, depending on the metal. These exo-nido rho da and ruthenacarboranes have been shown to catalyze in very good yield the hydrogenation of terminal alkenes but they are not active in the hydrogena tion of internal alkenes. Both rhoda-monothio and monophosphinocarboranes p resent comparable activity at P = 45 bar and T = 66 degreesC, in the hydrog enation and isomerization of I-hexene. However, while the monothioether pre cursors are active at P = 1 atm and T = 25 degreesC, the monophosphino exhi bited a very low activity. Ruthenamonophosphinocarboranes are also active i n the hydrogenation of l-hexene, with a higher selectivity that the respect ive rhodacarboranes. On the other hand, [Rh(7-PPh2-8-R'-7,8-C2B9H10)(PPh3)( 2) (R' = H, Me) catalyze the hydrogenation of methacycline to doxycycline w ith high yield (ca. 100%) and very high diastereoselectivity, ruthenacarbor anes are not active. All these complexes are recoverable after completion o f the catalytic reaction. These exo-nino rhoda and ruthenacarboranes displa yed a very low activity in the hydrogenation of internal alkenes, however, the close species [closo-3-(C8H13)-1-SR-2-R'-3,2,1-RhC2B9H9] (R = Ph; R' = Me, Ph) obtained from [Rh(7-SR-8-R'-7,8-C2B9H10)(cod)] were very efficient catalysts in the hydrogenation of cyclohexene exhibiting higher activity th an the parent exo-nido isomers. In addition to hydrogenation, exo-nino rhod a and ruthenamonothio and monophosphinocarboranes have also been tested as catalyst precursors in the insertion of carbenes to C=C and O-H bonds. The rhodamonophosphinocarboranes exhibited a high activity and similar stereose lectivity for the cyclopropanation of olefines (80-90%) and represent the f irst example of Rh(I) cyclopropanation catalysts. Furthermore, ruthenacarbo ranes are excellent cyclopropanation catalysts for activated olefins such a s styrene and their derivatives while the cyclopropane yields were lower fo r cyclic olefins and terminal linear monoolefines (C) 2000 Elsevier Science B.V. All rights reserved.