Rhodium II acetate

The reaction of thiocarbonyl compounds with diazoalkanes (alkyl, aryl substituted) frequently gives good to excellent yields of thiiranes. The mechanism may involve addition of a carbene across the thiocarbonyl group, especially in the presence of rhodium(II) acetate... 

There are several examples of intramolecular reactions of monocyclic /3-lactams with carbenes or carbenoids most of these involve formation of olivanic acid or clavulanic acid derivatives. Thus treatment of the diazo compound (106) with rhodium(II) acetate in benzene under reflux gives (107), an intermediate in the synthesis of thienamycin (80H(14)1305, 80TL2783). 

The rhodium(II) acetate catalyzed reaction of 2-(3-oxo-2-diazobutyryl)-1,2,3,4-tetrahydroisoquinoline in boiling toluene yielded 2-methyl-4,6,7,l 16-tetra-hydro[l, 3]oxazino[2,3-n]-isoquinoline-4-one in 72% yield (99TL8269). 

The dinuclear rhodium(II) acetate is described in section 2.8.2 the dinuclear structure is retained on one-electron oxidation, but when ozone is used as the oxidant, a compound with a trinuclear Rh30 core is formed, analogous to those formed by Fe, Cr, Mn and Ru. (It can also be made directly from RhCl3.)... 

Much of the early work into the rhodium(II)-catalysed formation of oxazoles from diazocarbonyl compounds was pioneered by the group of Helquist. They first reported, in 1986, the rhodium(II) acetate catalysed reaction of dimethyl diazomalonate with nitriles.<86TL5559, 93T5445, 960S(74)229> A range of nitriles was screened, including aromatic, alkyl and vinyl derivatives with unsaturated nitriles, cyclopropanation was found to be a competing reaction (Table 3). 

A series of catalysts was also screened in the reaction with benzonitrile to give methyl 2-phenyl-5-methoxyoxazole-4-carboxylate, with rhodium(II) acetate being the most effective (Table 4).<93T5445>... 

The role of the rhodium is probably two-fold. Initially due to its Lewis acidity it reversibly forms a complex with the nitrile nitriles are known to complex to the free axial coordination sites in rhodium(II) carboxylates as evidenced by the change of colour upon addition of a nitrile to a solution of rhodium(II) acetate, and by X-ray crystallography. Secondly the metal catalyses the decomposition of the diazocarbonyl compound to give a transient metallocarbene which reacts with the nitrile to give a nitrile ylide intermediate. Whether the nitrile ylide is metal bound or not is unclear. 

The isoxazolopyrylium date 14 is generated by the action of rhodium(II) acetate on the diazo compound 13 in the presence of dimethyl acetylenedicarboxylate the cycloadduct 15 is isolated <96JCS1035>. 

SCHEME 7.21 Synthesis of a stable quinone methide using rhodium (II) acetate in methanol.84,93 The 13C label is designated with an asterisk ( ). 

The search for catalysts which are able to reverse the ratio of cyclopropane diastereomers in favor of the thermodynamically less stable isomer has met with only moderate success to date. Rh(II) pivalate and some ring-substituted Rh(II) benzoates induce cw-selectivity in the production of permethric acid esters 77,98 99 contrary to rhodium(II) acetate, which gives a 1 1 mixture 74,77,98), and some copper catalysts 98) (Scheme 10). 

Rhodium(II) acetate was found to be much more superior to copper catalysts in catalyzing reactions between thiophenes and diazoesters or diazoketones 246 K The outcome of the reaction depends on the particular diazo compound 246> With /-butyl diazoacetate, high-yield cydopropanation takes place, yielding 6-eco-substituted thiabicyclohexene 262. Dimethyl or diethyl diazomalonate, upon Rh2(OAc)4-catalysis at room temperature, furnish stable thiophenium bis(alkoxycarbonyl)methanides 263, but exclusively the corresponding carbene dimer upon heating. In contrast, only 2-thienylmalonate (36 %) and carbene dimer were obtained upon heating the reactants for 8 days in the presence of Cul P(OEt)3. The Rh(II)-promoted ylide formation... 

The use of rhodium(II) acetate in carbenoid chemistry has also been extended to promoting intramolecular C/H insertion reactions of ketocarbenoids 277,280,280 ,). From the a-diazo-P-ketoester 305, highly functionalized cyclopentane 306 could thus be constructed in acceptable yields by regiospecific insertion into an unactivated... 

Rhodium(II) acetate complexes of formula [Rh2(OAc)4] have been used as hydrogenation catalysts [20, 21]. The reaction seems to proceed only at one of the rhodium atoms of the dimeric species [20]. Protonated solutions of the dimeric acetate complex in the presence of stabilizing ligands have been reported as effective catalysts for the reduction of alkenes and alkynes [21]. 

A method for the preparation of olefins from primary amines is shown in equation 120. Treatment of 2-(4-bromophenyl)ethylamine (358) with acetic acid, acetic anhydride and sodium nitrite generates the nitroso amide 359, which decomposes to 4-bromostyrene in the presence of rhodium(II) acetate. The procedure is thus a mild, non-basic alternative to the classical Hofmann elimination of amines396,397. 

The catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). 

Before turning to specific results we will have a look at the properties of rhodium(II) acetates/carboxamidates as catalysts for reactions with diazocompounds as the substrates via carbenoid intermediates. Rhodium(II) has a d7 electron configuration, forming the lantern type dimers with bridging carboxylates. The single electrons in the respective dz2 orbitals form an electron... 

A mixture of rhodium II) acetate (228 mg, 0.516 mmol), the imidazolidinone (1.70 g, 6.15 mmol), and dry chlorobenzene (20 mL) is heated under reflux for 18 h in a flask fitted with a Soxhlet extraction apparatus into which a thimble is placed containing an oven-dried mixture of sodium carbonate and sand (2 1, 5 g). The progress of the ligand-exchange reaction can be monitored by HPLC (p-Bondapak-CN column, methanol). The resulting blue solution is concentrated under reduced pressure, and the residue is purified by column chromatography (reversed phase silica, Bakerbond Cyano 40 mm prep. LC packing, methanol). 

The different synthetic applications of acceptor-substituted carbene complexes will be discussed in the following sections. The reactions have been ordered according to their mechanism. Because electrophilic carbene complexes can undergo several different types of reaction, elaborate substrates might be transformed with little chemoselectivity. For instance, the phenylalanine-derived diazoamide shown in Figure 4.5 undergoes simultaneous intramolecular C-H insertion into both benzylic positions, intramolecular cyclopropanation of one phenyl group, and hydride abstraction when treated with rhodium(II) acetate. 

Fig. 4.5. Rhodium(II) acetate-catalyzed decomposition of amino acid-derived diazoamides...

Silanes can react with acceptor-substituted carbene complexes to yield products resulting from Si-H bond insertion [695,1168-1171]. This reaction has not, however, been extensively used in organic synthesis. Transition metal-catalyzed decomposition of the 2-diazo-2-phenylacetic ester of pantolactone (3-hydroxy-4,4-dimethyltetrahydro-2-furanone) in the presence of dimethyl(phenyl)silane leads to the a-silylester with 80% de (67% yield [991]). Similarly, vinyldiazoacetic esters of pantolactone react with silanes in the presence of rhodium(II) acetate to yield a-silylesters with up to 70% de [956]. 

An example of the vinylogous reactivity is the reaction of 52 with cyclopentadiene (Tab. 14.9) [77]. Rhodium(II) acetate-catalyzed decomposition of 52 in dichloro-methane, yields a 2 1 mixture of the bicyclic system 53 derived from the [3-1-4] cycloaddition, and the bicyclo[2.2.1]heptene 54 resulting from electrophihc attack at the vinylic position followed by ring closure. When Rh2(TFA)4 is used as the catalyst, bicy-clo[2.2.1]heptene 54 becomes the dominant product, while the reactivity of the vinyl terminus is suppressed using a hydrocarbon solvent as observed in the Rh2(OOct)4-cat-alyzed reaction in pentane, which affords a 50 1 ratio of products favoring the [3-1-4] cycloadduct 53. 

Despite this promising beginning, and its growing use for the generation of electrophilic carbenes [5, 6], it was not until many years later that rhodium(II) was used generally for the formation of 1,3-dipoles. Padwa and Stull reported the use of rhodium(II) acetate [Rh2(OAc)4] in the successful formation of a six-membered ring carbonyl ylide (Scheme 19.2) [21]. This work was quickly followed by the use of rhodium(II) for the generation of... 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

The perfluoroacetamide catalysts, rhodium(II) trifluoroacetamidate [Rh2(tfm)4] and rhodium(II) perfluorobutyramidate [Rh2(pfbm)4], are interesting hybrid molecules that combine the features of the amidate and perfluorinated ligands. In early studies, these catalysts were shown to prefer insertion over cycloaddition [30]. They also demonstrated a preference for oxindole formation via aromatic C-H insertion [31], even over other potential reactions [86]. In still another example, rhodium(II) perfluorobutyramidate showed a preference for aromatic C-H insertion over pyridinium ylide formation, in the synthesis of an indole nucleus [32]. Despite this demonstrated propensity for aromatic insertion, the perfluorobutyramidate was shown to be an efficient catalyst for the generation of isomtinchnones [33]. The chemoselectivity of this catalyst was further demonstrated in the cycloaddition with ethyl vinyl ethers [87] and its application to diversity-oriented synthesis [88]. However, it was demonstrated that while diazo imides do form isomtinchnones under these conditions, the selectivity was completely reversed from that observed with rhodium(II) acetate [89, 90]. 

Principal Compounds Rhodium trichloride rhodium trioxide rhodium (II) acetate rhodium nitrate rhodium potassium sulfate rhodium sulfate rhodium sulfite... 

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