[31126-95-1] · C8F12O8Rh2 · Dirhodium(II) Tetrakis(trifluoroacetate) · (MW 657.90)
(catalyst for carbenoid reactions of diazo compounds1)
Physical Data: l 610 nm, ε 210 (CH2Cl2).2
Solubility: sol MeCN, DMSO, pyridine; slightly sol dichloromethane, benzene.
Form Supplied in: anhydrous form is green; hydrate is blue.
Preparative Method: from Dirhodium(II) Tetraacetate by ligand displacement in refluxing trifluoroacetic acid containing trifluoroacetic anhydride.3
Handling, Storage, and Precautions: air stable, very hygroscopic; stored in desiccator.
Dirhodium(II) tetrakis(trifluoroacetate), Rh2(tfa)4, like the corresponding Dirhodium(II) Tetrakis(perfluorobutyrate), Rh2(pfb)4, forms p-complexes with alkenes.2,4 Equilibrium constants for alkene coordination are 2-3 times less than those for Rh2(pfb)4, and they are relatively insensitive to steric influences. Coordination causes substantial changes in chemical shifts for vinyl protons.4 A determination of chirality has been made through an axial bis-alkene complex of Rh2(tfa)4.5 Nitroxide and phosphine complexes have also been characterized.6,7
Fluoroalkanoate complexes of dirhodium(II) were first employed to effect intermolecular carbon-hydrogen insertion reactions (eq 1).8,9 Although insertion product yields are relatively high, selectivity is low. A similar low level of selectivity occurs in intermolecular cyclopropanation of alkenes10,11 and dienes.12 However, relative to Rh2(tfa)4, Rh2(pfb)4 exhibits even lower levels of stereocontrol and regiocontrol in cyclopropanation reactions.
Both reactivity and solubility in weakly polar solvents render Rh2(pfb)4 superior to Rh2(tfa)4 for metal carbene transformations.
The cationic character of the intermediate metal carbene formed from a vinyldiazo compound is evident in the formation of the bicyclo[2.2.1]heptene system in eq 2.13 Here, use of Rh2(tfa)4 favors this carbocation addition product even in pentane, whereas use of Dirhodium(II) Tetraacetate in pentane reduces the yield of this product to only 5%. Electrophilic substitution products have also been reported from Rh2(tfa)4-catalyzed reactions of vinyldiazomethane derivatives with N-(methoxycarbonyl)pyrrole.14 Overall Rh2(tfa)4 is not as effective as Rh2(pfb)4, nor is its selectivity in reactions of metal carbenes significantly different.
Michael P. Doyle
Trinity University, San Antonio, TX, USA