Rhodium perfluorobutyrate

In a careful study of rhodium catalysts for the decomposition of a-diazo imide 450, Padwa and co-workers found that perfluorinated ligands greatly favor isomunchnone formation, whereas acetate leads to the generation of a six-membered carbonyl ylide. Thus 450 is converted to isomunchnone 451 with either rhodium perfluorobutyroamidate (Rh2(pfm)4), rhodium perfluorobutyrate (Rh2(pfb)4), or rhodium trifluoroacetate (Rh2(tfa)4) but is converted to 454 with Rh2(OAc)4 (Scheme 4.17). Neither 1,3-dipole can be isolated, but isomunchnone... 

Mejia-Oneto and Padwa have described the rhodium(II) perfluorobutyrate-cata-lyzed decomposition of an a-diazo ketoamide precursor (Scheme 6.78) [163], Micro-wave heating of a solution of the diazo compound in benzene with a catalytic... 

The rhodium(II)-catalyzed reaction of propargyl compounds 169 and diazo compounds 170 gave corresponding functionalized allenes 171 together with cydopro-penes 172 (Scheme 3.87) [126]. Rh2(pfb)4, where pfb represents perfluorobutyrate, was found to be an excellent catalyst for preparing the allenes 171. An analogous rhodium(II) complex, Rh2(OAc)4, afforded mainly 172 with only a trace amount of 171 (<5%). 

The development of the first alkyne silylformylation reaction was reported in 1989 by Matsuda [27]. Alkynes were treated with Me2PhSiH and Et3N with 1 mol% Rh4(CO)i2 under CO pressure to produce yS-silyl-a,/ -unsaturated aldehydes (Scheme 5.20). A second report from Ojima detailed the development of rhodium-cobalt mixed metal clusters as effective catalysts for alkyne silylformylation [28]. Shortly thereafter, Doyle reported that rhodium(II) perfluorobutyrate was a highly efficient and selective catalyst for alkyne silylformylation under remarkably mild reaction conditions (0°C, 1 atm CO) [29]. In all these reports, terminal alkynes react regiospedfically with attachment of the silane to the unsubstituted end of the alkyne. The reaction is often (but not always) stereospecific, producing the cis-product preferentially. 

Carbonyl ylides possess versatile reactivities, among which the 1,3-dipolar cycloaddition is the most common and important reaction. The reaction sequence of ylide formation and then 1,3-dipolar cycloaddition can occur in either inter- or intramolecular manner. When the reaction occurs intermolecularly, the overall reaction is a one-pot three-eomponent process leading to oxygen-containing five-membered cyclic compounds, as demonstrated by the example shown in Scheme 8. A mixture of diazo ester 64, benzaldehyde, and dimethyl maleate, upon heating to reflux in CH2CI2 in the presence of 1 mol% rhodium(ii) perfluorobutyrate [Rh2(pfb)4], yields tetrahedrofuran derivative 65 in 49% yield as single diastereomer. " ... 

Thus changing the ligands on dirhodium(II) can provide a switch which, in some cases, can turn competitive transformations on or ofT146. Other examples include the use of dirhodium(II) carboxamides to promote cyclopropanation and suppress aromatic cycloaddition146. For example, catalytic decomposition of diazoketone 105 with dirhodium(II) caprolactamate [Rh2(cap)4] provides only cyclopropanation product 106. In contrast, dirhodium(II) perfluorobutyrate [Rh2(pfb)4] or dirhodium(II)triphenylacetate [Rh2(tpa)4] gave the aromatic cycloaddition product 107 exclusively (equation 100)l46 148. Although we have already seen that rhodium(II) acetate catalysed decomposition of diazoketone 59, which bears both aromatic and olefinic functionalities, afforded stable norcaradiene 60 (equation 70)105, the rhodium(II) acetate catalysed carbenoid transformation within an acyclic system (108) showed no chemoselectivity (equation 101). However, when dirhodi-um(II) carboxamides were employed as catalysts for this type of transformation, only cyclopropanation product 109 was obtained (equation 101). ... 

Rhodium(ll) perfluorobutyrate, Rh2(pfb)4. This reagent is obtained as a bright yellow-green solid by transesterification of Rh2(0Ac)4 with perfluorobutyric acid and the anhydride. 

Alcoholysis of R3SiH.1 Rhodium(II) perfluorobutyrate is more effective than Rh2(OAc)4 as a catalyst for reaction of primary or secondary alcohols with trialkyl-silanes at 25° to form silyl ethers. Tertiary alcohols are inert under these conditions. Selective reactions with only primary alcohols can be realized with r-butyldimethyl-silane but not with dimethylphenylsilane. 

In contrast to hydroformylation of olefin derivatives, the addition of carbon monoxide and trialkylsilane to alkynes gives carbon-centered silanes exclusively when catalyzed by, for example, rhodium and Rh-Co carbonyl clusters [153, 154] and Rh2(pfb)4 (pfb = perfluorobutyrate) (eq. (13) [153]). 

In the laboratory of A. Padwa, a novel synthetic approach to the fully functionalized core of lysergic acid was developed utilizing an intramolecular isomunchone cycloaddition pathway. The key cycloaddition precursor diazo imide was prepared using the standard Regitz diazo tranter conditions. The diazo imide then was heated with catalytic amouts of rhodium(ll)-perfluorobutyrate in dichloromethane to afford the desired cycloadduct as a single diastereomer and in excellent yield. The only reason the authors were not able to complete the total synthesis of lysergic acid was that they could not affect the isomerization of the double bond between the two six-membered rings. 

Experimental evidence for the thermal isomerization from a substituted diazirine to the corresponding diazoalkane was also reported by Doyle et al. (1989 a). They used diazirines as stable diazoalkane precursors which, with the help of the catalyst rhodium(ii) perfluorobutyrate, undergo carbenoid-type reactions. An example is reaction (5-22). Less competition is observed from side reactions that were dominant... 

The outcome of the reaction of triethylsilane with terminal acetylenes in the presence of rhodium(II) perfluorobutyrate depends on the order in which the reagents are added. Thus,... 

Other workers have found that ylide formation is favoured by using rhodium(II) perfluorobutyrate as the catalyst. 

Equation (37) proceeds faster in a mixed water-perfluorobutyric acid solvent compared to the reaction in H2O alone (62). Moreover, the reaction can also be applicable to ethane and longer-chain alkanes, but C-C bond cleavage occurs. The details of the reaction mechanism are still unknown. In addition, replacing the rhodium catalyst by Pd-Cu results in the formation of a methanol derivative even under a CO atmosphere (see eq. (21)) (41). 

There are several examples of catalyzed aromatic cycloadditions leading to heterocyclic systems. The rhodium(II) acetate-catalyzed intramolecular Buchner reactions of iV-benzyldiazoacetamides 64a/b afford azabicyclo[5.3.0]decatrienes 66a/b in excellent yields. In contrast, the N-methyl derivative 64c gives 66c in moderate yield. Use of rhodium(II) perfluorobutyrate (Rh2(pfb)4) in place of rhodium(II) acetate increases the yield to 54%. Unlike its carbon counterpart, dihydroazulenone 29a (vide supra), 66a is insensitive to either trifluoroacetic acid or boron trifluoride etherate, even in excess, and the unrearranged reactant is recovered intact even after prolonged treatment at room temperature. 

Triphenylcyclopropene was transformed into 2,3-diphenylindene in high yield when treated with rhodium(II) perfluorobutyrate dimer, Rh2(pfb) (Scheme 2.89) [143]. Zeise s dimer also catalyzed the reaction [144]. 

The known tricyclic olefin 193 was oxidatively ring opened at the olefinic ring to give an indoline derivative which was transformed to the starting prerequisite diazo imide 194. The rhodium-catalyzed reaction of 194 proceeded smoothly, using rhodium(II) perfluorobutyrate as the catalyst, to give the cycloadduct 195 as the exclusive product in 93% yield. The conversion of the cycloadduct 195 to methyl paspalate was undertaken by treating 195 with... 

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