Dirhodium(II) Tetraacetate

Rh2(O2CMe)4

[15956-28-2]  · C8H12O8Rh2  · Dirhodium(II) Tetraacetate  · (MW 442.02)

(catalyst for carbenoid reactions of diazo compounds,1 hydroboration of alkenes and alkynes,2 hydrocarbon oxidation3)

Physical Data: l 590 nm, ε 210 (EtOH); l 552 nm, ε 235 (MeCN).4 IR -n (CO2) 1585 cm-1.5

Solubility: sol MeOH, acetic acid, MeCN, acetone; slightly sol toluene; insol Et2O, 1,2-dichloroethane, CH2Cl2.

Form Supplied in: emerald-green solid for anhydrous form.

Preparative Method: prepared from RhCl3.xH2O.6

Handling, Storage, and Precautions: air stable, moderately hygroscopic; stored in desiccator. Removal of axially coordinated solvent can be achieved in vacuum oven at 60-80 °C.

Metal Carbene Transformations.

When used in amounts as low as 0.05 mol %,7 Rh2(OAc)4 serves as a highly effective and efficient catalyst for dinitrogen extrusion from diazo carbonyl compounds and subsequent metal carbene directed alkene1,8 and alkyne9 addition reactions, s-bond insertion reactions,1,10 ylide generation,1,11 and carbene coupling processes,12 among others. With diazoacetates and diazo ketones, catalytic reactions are generally performed in dichloromethane at rt. The diazo compound is added at such a rate so as to minimize its concentration in the reaction solution.6 The less reactive a-diazo-b-carbonyl alkanoates, including diazomalonates and diazoacetoacetates, and related diazo dicarbonyl compounds require refluxing 1,2-dichloroethane or refluxing benzene for efficient catalyst turnovers. In laboratory scale reactions, Rh2(OAc)4 is removed from the reaction solution by filtering through a short column of silica or alumina, or the product(s) are distilled directly from the catalyst-containing mixture. (Rhodium acetate undergoes thermal decomposition at temperatures near 250 °C.)

Cyclopropanation/Cyclopropenation.

Its catalytic uses for intermolecular cyclopropanation and cyclopropenation reactions, discovered only recently by Teyssié and co-workers,9,13 have been the standard through which mechanistic14 and synthetic (selectivity)8,15 understanding of metal carbene addition reactions have been derived.1 Intermolecular addition reactions of diazoacetates with alkenes occur at 25 °C in high yield. Their stereoselectivities and regioselectivities16 are lower than those achieved with the use of Dirhodium(II) Tetraacetamide,8 but high selectivities are observed with vinylcarbenoid addition (eq 1)17 and nitrocarbenoid addition18 to alkenes. Cyclopropene formation occurs with a broad selection of alkynes (eq 2),19a but not phenylacetylene.9,19 Neither cyclopropanation nor cyclopropenation occurs readily with a,b-unsaturated carbonyl compounds or nitriles.20

Intramolecular analogs of these reactions suggest the overall viability of Rh2(OAc)4 catalysis (eqs 3 and 4),21-23 although for alkyne addition the presumed cyclopropene intermediate is unstable and undergoes reactions characteristic of vinylcarbenoid species,22,23 of which the most frequently encountered involve rearrangement to furans. The intramolecular Büchner reaction of aryl diazoketones (eq 5)24 is, formally, cyclopropanation of an aromatic ring, for which Rh2(OAc)4 is especially suitable;24 the diazoamide analog of this transformation is particularly facile.25

Carbon-Hydrogen Insertion.

The utility of Rh2(OAc)4 as a catalyst for metal carbene transformations is most evident in its ability to effect insertion into unactivated carbon-hydrogen bonds.1,10 Preference for the formation of five-membered rings is pronounced (eq 6),26 and selectivity for insertion into tertiary C-H bonds is usually greater than for insertion into secondary C-H bonds, and primary C-H bonds are the least reactive.27 Although these controlling influences are pervasive, an increasing number of examples suggest that the factors which control regioselectivity in these reactions are complex (e.g. eq 7).28

Heteroatom activation of adjacent C-H bonds29 has made possible the construction of b-lactam derivatives from diazoacetoacetamides (eq 8)30 and of b-lactones from selected diazomalonates.31 With N-benzyl derivatives, the presence or absence of an acyl group uniquely defines the course of the catalytic reaction (eq 9; R = t-Bu; S = H, Me, OMe, Br).32 Both electronic and conformational (steric) effects are responsible for the selectivity in these reactions.

Intermolecular variants of carbon-hydrogen insertion reactions are generally of limited value because of lower reactivity and selectivity.1 Insertion into vinylic or alkynic C-H bonds is not competitive with cyclopropanation or cyclopropenation under ordinary circumstances. The so-called allylic C-H insertion reaction, commonly observed in copper-catalyzed reactions of diazomalonates or b-diazo-a-keto esters,33 is not common with Rh2(OAc)4 catalysis.14b

In contrast to the paucity of examples for C-H insertion into vinylic or alkynic C-H bonds, intramolecular insertion into aromatic C-H bonds is well documented.1 These reactions are most pronounced when the aromatic ring is activated for substitution by oxygen34 or nitrogen35 (e.g. eqs 10 and 11),34,35a and they are more suitably described as aromatic substitution reactions than as C-H insertion reactions. Aryl-substituted diazo ketones,36 diazoacetates,34 diazoacetoacetates,34 and their corresponding amide derivatives35 are all effective. The formation of a five-membered ring is preferred, but, with suitable structural demands in the diazo carbonyl reactant, six-membered ring formation occurs.37

Heteroatom-Hydrogen Insertion.

One of the most important applications of metal carbene chemistry has been in the syntheses of penems, exemplified in eq 12, which is key step in the total synthesis of the carbapenem thienamycin.38 This reaction has become the method of choice for the synthesis of bicyclic b-lactams from 2-azetidinones substituted through the 4-position to a diazocarbonyl group.39

Oxygen-hydrogen insertion (with water) is a common undesirable side reaction in Rh2(OAc)4 catalyzed transformations, but its synthetic importance is evident in intramolecular processes. Cyclization via O-H insertion occurs readily and in high yield for five-, six-, and seven-membered ring formation (eq 13),40 but C-H insertion becomes competitive in attempts to effect eight-membered ring formation. Intermolecular processes have also been examined,41 but they have more limited usefulness.

Thiol insertion also occurs in both intermolecular and intramolecular transformations,42,43 but these reactions are more difficult to perform because of the facile coordination of thiols and sulfides with Rh2(OAc)4. Intermolecular silicon-hydrogen insertion provides a convenient methodology for the synthesis of a-silyl esters and ketones (eq 14).44 Overall, Rh2(OAc)4 is generally suitable, and often the catalyst of choice, for heteroatom-hydrogen insertion reactions of diazo esters and ketones.43

Ylide Generation.

Metal carbenes produced by Rh2(OAc)4-catalyzed dinitrogen extrusion from diazo compounds are electrophilic.1 Reactions of these reactive intermediates, which resemble metal-stabilized carbocations, with Lewis bases constitute a generally effective methodology for ylide generation (eq 15).11 The relative reactivity of Lewis bases towards ylide generation follows the expected order of basicity for carbon-heteroatom compounds. Ylide products are further transformed by insertion, sigmatropic rearrangement, or dipolar addition reactions, dependent on the design of the ylide. Heteroatom-hydrogen insertion is reasonably regarded as an ylide transformation.

The relative reactivity for ylide generation, determined from subsequent [2,3]-sigmatropic rearrangement of the initially formed ylide in competition with cyclopropanation (eq 16),45,46 shows that ylide formation increases in the order RI > RBr > RCl, R3N > R2O, and R2S > R2O. Allyl substituents facilitate ylide generation (eq 17),46,47 and their influences on relative reactivities suggest that the formation of the metal-stabilized ylide and metal dissociation (eq 15) are equilibrium processes.

Rhodium(II) acetate has two axial coordination sites at which reactions with diazo compounds occur. Strongly coordinating compounds, either reactants or solvents, occupy these sites and inhibit electrophilic addition to diazo compounds. Consequently, whereas reactions with diazo compounds that possess chloride, bromide, iodide, or ether functional groups take place at rt, those with sulfide or amine functional groups require higher temperatures for dinitrogen extrusion.

Stable sulfonium ylides have been produced by intermolecular reactions of thiophenes with dimethyl diazomalonate, catalyzed by Rh2(OAc)4 (eq 18),48 as well as by intramolecular reactions (eq 19).49 The formation of four- to seven-membered rings has been possible,49,50 but C-H insertion is competitive with seven-membered ring ylide generation. The stability of the ylide is dependent on the sulfur substituent (Ph > PhCH2 > allyl) as well as on ring size. Stable sulfoxonium ylides have also been formed in Rh2(OAc)4-catalyzed reactions.51

Use of the [2,3]-sigmatropic rearrangement of sulfonium ylides for ring enlargement has made possible the construction of medium ring compounds (eq 20).47 b-Elimination is a competing process and becomes the favored transformation with the proper stereoelectronic arrangement in the reactant ylide (e.g. eq 21).52 An intramolecular ring contraction methodology is also effectively promoted through sulfonium ylide generation with Rh2(OAc)4 catalysis (e.g. eq 22).53 Sulfur ylides derived from Rh2(OAc)4-catalyzed reactions of diazo compounds have played important roles in b-lactam antibiotic syntheses.54-56

Nitrogen ylide formation is not reported as extensively as is sulfur ylide generation, but sufficient examples exist to suggest its versatility. The synthesis of allenes by [2,3]-sigmatropic rearrangement (eq 23)57 of prop-2-yn-1-yl-dimethylammonium ylides exemplifies the potential diversity of its applications. A general methodology for oxazole synthesis has been developed through the use of nitrile ylides (eq 24).58 Additional examples that suggest the advantages of the catalytic route to ylide generation are reported elsewhere.11

Relative to cyclopropanation (of alkenes) or cyclopropenation (of alkynes), oxonium ylide formation is generally disfavored in Rh2(OAc)4-catalyzed reactions of diazo compounds. Notable exceptions include those described in eq 17 and in intramolecular transformations. Cyclobutanone formation is the outcome of Rh2(OAc)4-catalyzed dinitrogen extrusion from 4-alkoxydiazo ketones (eq 25).59 Allyl ethers offer a pathway to [2,3]-sigmatropic rearrangement products, including those leading to medium ring ethers (eq 26),60,61 and tetrahydrofuran-3-ones have been prepared by a carbon-oxygen insertion methodology involving ylide intermediates,62 but few other successful demonstrations of oxonium ylide generation have been reported.

Dirhodium(II) tetraacetate has been shown to have superior capabilities for carbonyl ylide generation with diazo ketones,11 especially in intramolecular reactions. The carbonyl ylide, when generated in the presence of selected dipolarophiles, readily undergoes 1,3-dipolar addition (e.g. eqs 27 and 28)63,64 either intermolecularly (Dimethyl Acetylenedicarboxylate, N-Phenylmaleimide, diethyl fumarate, aldehydes) or intramolecularly. Regioselectivity is predictable by frontier molecular orbital interactions.64b Carbonyl ylide generation with carboxylate esters is rare,30b but with amides, carbamates, and imides, carbonyl ylide formation is well documented (e.g. eq 29).11,65

Selectivity in Metal Carbene Transformations.

The high reactivity of catalytically generated metal carbenes often limits their selectivity when more than one site for addition and/or insertion exists in a molecule. In this regard, Rh2(OAc)4 is often less selective for metal carbene transformations than is either Dirhodium(II) Tetrakis(perfluorobutyrate) or rhodium(II) carboxamides (either Dirhodium(II) Tetraacetamide or Dirhodium(II) Tetra(caprolactam)).8,28,66 In addition, use of Rh2(OAc)4 can lead to lower product yields than does use of alternative dirhodium(II) catalysts. Nevertheless, for most transformations where there is one preferred site for metal carbene reaction, Rh2(OAc)4 remains the catalyst of choice.

High diastereocontrol complements exceptional stereocontrol for geometrical isomer formation in the cyclopropanation of styrene with vinyldiazocarboxylates that possess the pantolactone chiral auxiliary (eq 30).67 However, with pantolactone diazoacetate in the cyclopropanation of styrene, a 76:24 trans:cis product mixture forms in only 72% yield and 28-30% diastereomeric excess (de).68 Similarly, in competitive reactions between cyclopropanation and C-H insertion, use of Rh2(OAc)4 provides very little selectivity (eq 31),66 but dirhodium(II) tetrakis(perfluorobutyrate) promotes exclusive C-H insertion and use of dirhodium tetra(caprolactam) leads to exclusive cyclopropanation. A variety of examples of high and low selectivities has been reported for Rh2(OAc)4-catalyzed reactions,1 and no simple explanation has emerged that will account for all of the results. Predictions are often empirical, but changing the ligands of the catalyst often provides a dirhodium(II) compound with the desired selectivity enhancement.

Catalytic Hydroboration of Alkenes and Alkynes.

The use of Rh2(OAc)4 to catalyze the hydroboration of alkenes and alkynes with Catecholborane represents a novel application of this transition metal compound.2 In laboratory scale reactions, 0.5 mol % Rh2(OAc)4 is effective. Selectivity is dependent on the alkene (eq 32) and often differs from that found with Chlorotris(triphenylphosphine)rhodium(I) (Wilkinson's catalyst).69

The hydroboration of alkynes is also promoted by rhodium acetate. However, since Rh2(OAc)4 is transformed during hydroboration, the nature of the active catalyst is at present unknown. The combination of catecholborane and Rh2(OAc)4, both in catalytic amounts, catalyzes isomerization of terminal alkenes.2

Autooxidation of Alkenes.

In the presence of catalytic amounts of Rh2(OAc)4, cyclohexadienes undergo aromatization (eq 33), and dienes undergo oxidative cleavage (eq 34),3a at atmospheric pressure. p-Cymene is also reported to be oxidized to the tertiary alcohol. Hydroperoxide decomposition to alcohol and dioxygen, catalyzed by Rh2(OAc)4, has been demonstrated.

Other Catalytic Reactions.

Hydrogenation activity for Rh2(OAc)4 has been reported,70 but these results could not be repeated.71 Hydrosilylation of alkenes is catalyzed by Rh2(OAc)4, but reactions are much less efficient than those catalyzed by dirhodium(II) tetrakis(perfluorobutyrate).71 The same is true of silylcarbonylation reactions with alkynes,72 and Rh2(OAc)4 is not a hydroformylation catalyst.


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Michael P. Doyle

Trinity University, San Antonio, TX, USA



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