[73482-96-9] · C32H60O8Rh2 · Dirhodium(II) Tetraoctanoate · (MW 778.74)
(nonpolar solvent-soluble catalyst for carbenoid reactions of diazo compounds)
Solubility: sol MeOH, MeCN, Et2O, CH2Cl2, toluene, hexane, pentane.
Form Supplied in: green solid; commercially available.
Preparative Method: prepared by ligand substitution from Rh2(OAc)4.1
Handling, Storage, and Precautions: air stable, weakly hygroscopic; stored in desiccator.
Because of its solubility in organic solvents, dirhodium(II) tetraoctanoate, Rh2(oct)4, has seen wide use as an alternative for Dirhodium(II) Tetraacetate as a catalyst for reactions of diazo compounds. However, although Rh2(oct)4 is initially soluble in relatively nonpolar organic solvents, whereas Rh2(OAc)4 is not, there are few other advantages to its use as a laboratory scale catalyst. Thus Rh2(OAc)4 can be chromatographically separated on silica or alumina from reaction products, but Rh2(oct)4 is often resistant to chromatographic separation.
Uses of Rh2(oct)4 for catalytic reactions with diazo compounds parallel those of Rh2(OAc)4.2,3 Although product yields are higher with Rh2(OAc)4, use of Rh2(oct)4 at 0 °C in dichloromethane causes exceptionally high diastereocontrol (97% de) for the intermolecular cyclopropanation of styrene by the (R)-(-)-pantolactone ester of trans-2-diazo-4-phenyl-3-butenoate (eq 1).4 Here temperature seems to be important since in refluxing dichloromethane both Rh2(OAc)4 and Rh2(oct)4 gave this cyclopropane product with a diastereomeric excess of 88 ± 1%.
Temperature control in Rh2(oct)4-catalyzed reactions of a vinyldiazomethane (Scheme 1) in CH2Cl2 (0 and 40 °C) or benzene (80 °C) affords selective entry to intramolecular aromatic cycloaddition (0 °C), [3 + 4] annulation (40 °C), and [3 + 2] annulation (80 °C).5
Use of Rh2(oct)4 in pentane significantly influences product control in catalytic carbenoid reactions. Cyclopropanation occurs to the exclusion of [3 + 2] annulation in reactions of vinyldiazomethanes with vinyl ethers (eq 2); with Rh2(OAc)4 in dichloromethane, both products are formed.6 Intramolecular reactions of diazo ketones show a similar solvent dependence with systems designed for tandem cyclopropenation/vinylcarbene formation/cyclopropenation versus hydrogen migration (eq 3).7 In other cases the influence of solvent on selectivity is not as great,8,9 but for the synthesis of tropanes the use of Rh2(oct)4 in pentane has advantages not demonstrated by Rh2(OAc)4 in dichloromethane (eq 4).10
Michael P. Doyle
Trinity University, San Antonio, TX, USA