Rhodium(II) acetate is the chemical compound with the formula Rh₂(AcO)₄, where AcO^- is the acetate group (CH₃CO₂). This emerald green powder is a catalyst for cyclopropanation of alkenes. It is also used as catalyst for insertion into C-H and X-H bonds (X = N/S/O) and for ylide formation for organic syntheses.

Preparation

Rhodium(II) acetate is usually prepared by the heating hydrated rhodium(III) chloride in acetic acid (CH₃COOH): Rhodium(II) acetate dimer undergoes ligand exchange, the replacement of the acetate group by other carboxylates and related groups.

Rh₂(OAc)₄ + 4 HO₂Y → Rh₂(O₂Y)₄ + 4 HOAc

Structure and Properties

The structure of rhodium(II) acetate freatures a pair of rhodium atoms, each with octahedral molecular geometry, defined by four acetate oxygen atoms, a water ligand, and a Rh-Rh bond (2.39 Å.. Copper(II) acetate and chromium(II) acetate adopt similar structures.

Chemical Properties

The application of dirhodium tetraacetate to organic synthesis was pioneered by Teyssie and co-workers. A extensive library of successful transformations rapidly evolved, ranging from Rh(II)-catalyzed OH and NH insertions to cyclopropanation of olefins and aromatic systems. Nowadays, it is used mainly as a catalyst. It can help distinguish between ribonucleosides and deoxynucleosides by binding selectively to ribonucleosides at their 2' and 3' OH groups. Rhodium(II) acetate dimer, compared to copper(II) acetate, is more reactive and useful in differentiating ribonucleosides and deoxynucleosides because it is soluble in aqueous solution like water whereas copper(II) acetate only dissolves in non-aqueous solution.

Selected catalytic reactions

1. Cyclopropanation

through the decomposition of diazocarbonyl compounds, the intra- and inter­molecular cyclopropanation reactions occurs.

2. Aromatic cycloaddition

Rhodium acetate is a very efficient catalyst for two-component ­cycloaddition as well as three-component 1,3-dipolar cycloaddition reactions.

3. C-H insertion

Rh(II)-catalyzed regioselective intramolecular and regiospecific intermolecular C-H insertion into aliphatic and aromatic C-H bonds is a useful method for the synthesis of a ­diverse range of organic compounds.

4. Oxidation of alcohols

Allylic and benzylic alcohols were oxidized to the corresponding carbonyl compounds using tert-butyl hydroperoxide in ­stoichiometric amounts and Rh₂(OAc)₄ as catalyst in dichloromethane at ambient temperature.

5. X-H insertion (X = N/S/O)

Rh(II) carbenoid reacts with amines, alcohols or thiols to yield the product of a formal intra- or intermolecular X-H bond (X = N/O/S) insertion via the formation of an ylide intermediate.

References