[73755-28-9] · C16F28O8Rh2 · Dirhodium(II) Tetrakis(perfluorobutyrate) · (MW 1057.98)
Physical Data: l 626 nm, ε 248 (CH2Cl2).4
Solubility: sol MeCN, DMSO, pyridine, benzene, CH2Cl2, alkenes.
Form Supplied in: anhydrous form is green; hydrate is blue.
Preparative Methods: from Dirhodium(II) Tetraacetate by ligand displacement in refluxing perfluorobutyric acid containing perfluorobutyric anhydride.5
Handling, Storage, and Precautions: air stable, very hygroscopic; stored in desiccator.
Dirhodium(II) tetrakis(perfluorobutyrate), Rh2(pfb)4, and Dirhodium(II) Tetrakis(trifluoroacetate), Rh2(tfa)4, are unique among dirhodium(II) carboxylates and carboxamides in their ability to form association complexes with alkenes and alkynes in solution.2,4 Equilibrium constants range from 70 M-1 (styrene) to 932 M-1 (2-methoxypropene). However, alkene coordination with Rh2(pfb)4 has no effect on relative reactivity or selectivity in cyclopropanation reactions with ethyl diazoacetate. Being more soluble in weakly polar solvents than is Rh2(tfa)4, removal of Rh2(pfb)4 from reaction solutions by simple chromatography on silica is more difficult.
The strongly electron-withdrawing perfluorobutyrate ligands render Rh2(pfb)4 significantly more electrophilic than Dirhodium(II) Tetraacetate or Dirhodium(II) Tetraacetamide. As a result, reactivity towards diazo compounds is substantially increased, relative to Rh2(OAc)4, and stereoselectivity and regioselectivity in cyclopropanation and carbon-hydrogen insertion reactions are greatly diminished (e.g. eqs 1-3).6 Regioselectivity for C-H insertion into primary and secondary carbon-hydrogen bonds (eq 3) is nearly statistical, demonstrating that relative reactivities for insertion are essentially equal.7-11
Dirhodium(II) perfluorobutyrate has proven to be highly selective in competitive intramolecular metal carbene transformations.12 In contrast to results with Dirhodium(II) Tetra(caprolactam), the use of Rh2(pfb)4 favors aromatic substitution over cyclopropanation (eq 4),13 insertion over cyclopropanation (eq 5),13 and aromatic substitution over carbonyl ylide generation (eq 6).14 Product yields are high in each case. The order of reactivity for metal carbenes generated from Rh2(pfb)4 is aromatic substitution > tertiary C-H insertion > cyclopropanation &AApprox; aromatic cycloaddition > secondary C-H insertion, and the rate differences between them are as much as 100-fold.13
Additional applications of Rh2(pfb)4 to metal carbene transformations include intramolecular O-H insertion (eq 7),14 where intramolecular cyclopropanation would also be a viable pathway. For intramolecular cyclopropenation reactions of diazo ketones which undergo subsequent vinylcarbene formation, use of Rh2(pfb)4 leads to different products than those from dirhodium(II) octanoate.15 In contrast to the use of rhodium(II) octanoate in pentane which effects exclusive cyclopropanation, Rh2(pfb)4 in CH2Cl2 promotes [3 + 2] annulation in reactions of a vinyldiazomethane with vinyl ethers (eq 8),16 and this result further exemplifies the influence of the highly electrophilic Rh2(pfb)4 on selectivity.
Cyclopropenes undergo facile rearrangements catalyzed by Rh2(pfb)4 to yield products that are structurally different from those obtained with the use of copper, platinum, or silver catalysts.17 The selectivity achieved with Rh2(pfb)4 is remarkable (Scheme 1), and, as suggested by eq 9, the involvement of Rh2(pfb)4 is consistent with the generation of a vinylcarbene intermediate.
Hydrosilylation of 1-alkenes is catalyzed by Rh2(pfb)4 under mild conditions.18 The mode of addition determines the products that are formed. When the alkene is added to triethylsilane in the presence of Rh2(pfb)4,
normal hydrosilylation (eq 10) occurs; reversed addition causes the formation of vinyl- or allylsilanes (eq 11). Alkene isomerization catalyzed by the combination of Rh2(pfb)4 with triethylsilanes has also been reported.18
Hydrosilylation of 1-alkynes catalyzed by Rh2(pfb)4 forms either vinylsilanes (trans addition) or allylsilanes in moderate to high isolated yields, dependent on the mode of addition.19 Addition of triethylsilane to 1-alkynes in dichloromethane containing Rh2(pfb)4 results in the formation of vinylsilanes, whereas addition of the alkynes to triethylsilane produces allylsilanes (eq 12) in high yield. Product dependence on the mode of addition is associated with organosilane coordination with Rh2(pfb)4. Organosilane alcoholysis is also catalyzed by Rh2(pfb)4 (eq 13);20 primary alcohols react with triethylsilane approximately five times faster than do secondary alcohols.
Rh2(pfb)4 is a highly effective catalyst for silylation of terminal alkynes in reactions performed at atmospheric pressure or at 10 atm CO pressure and at or below rt.3 At atmospheric CO pressure, addition of the alkyne to the organosilane is critical to the success of this transformation (eq 14), which is characterized by virtually complete regioselectivity and (Z/E) selectivity that is greater than 10:1, often reaching >30:1. Dirhodium(II) Tetraacetate is relatively ineffective as a catalyst for silylcarbonylation. The advantages of Rh2(pfb)4 as a catalyst for silylcarbonylation of alkynes lie in the mild conditions employed, high catalysts turnovers, and exceptional (E/Z) ratios.
Michael P. Doyle
Trinity University, San Antonio, TX, USA