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

Forced exo-nido rhoda and ruthenacarboranes containing monothio and monophosphinocarboranes have been tested as catalyst precursors in different catalytic reactions. The catalyst precursors employed were [Rh(7-SR-8-R′-7,8-C2B9H10)(PPh3)2] (R=Ph, Et; R’=Ph, Me), [Rh(7-PR2-8-R′-7,8-C2B9H10)(PPh3)2] (R=Ph, Et, iPr; 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 tetramethylammonium or cesium salt of the nido ligand with Rh(I) or Ru(II) complexes incorporating ancillary ligands. Although two molecular structures are possible, the closo 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 and one or two B–H–M interactions, depending on the metal. These exo-nido rhoda and ruthenacarboranes have been shown to catalyze in very good yield the hydrogenation of terminal alkenes but they are not active in the hydrogenation of internal alkenes. Both rhoda-monothio and monophosphinocarboranes present comparable activity at P=45 bar and T=66°C, in the hydrogenation and isomerization of 1-hexene. However, while the monothioether precursors are active at P=1 atm and T=25°C, the monophosphino exhibited a very low activity. Ruthenamonophosphinocarboranes are also active in the hydrogenation of 1-hexene, with a higher selectivity that the respective 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 with high yield (ca. 100%) and very high diastereoselectivity, ruthenacarboranes are not active. All these complexes are recoverable after completion of the catalytic reaction. These exo-nido rhoda and ruthenacarboranes displayed a very low activity in the hydrogenation of internal alkenes, however, the closo 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 than the parent exo-nido isomers. In addition to hydrogenation, exo-nido rhoda 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 stereoselectivity for the cyclopropanation of olefines (80–90%) and represent the first example of Rh(I) cyclopropanation catalysts. Furthermore, ruthenacarboranes are excellent cyclopropanation catalysts for activated olefins such as styrene and their derivatives while the cyclopropane yields were lower for cyclic olefins and terminal linear monoolefines