Rhodium trifluoroacetates

The selective C-H functionalization of alkanes has been a long-standing goal of the organometallic community. The intermolecular C-H insertion chemistry has arguably become, over the last decade, one of the most efficient processes for such a functionalization. Although there are several early papers on the C-H functionalization of alkanes by ethyl diazoacetate,7 -74 the reaction was not considered initially to be of broad synthetic utility.46 An illustrative example from the early literature is the reaction of ethyl diazoacetate with 2-methylbutane, catalyzed by rhodium trifluoroacetate, in which all four possible products were formed (Equation (3)).72,73 The product ratio could be influenced by the nature of the catalyst but the formation of mixtures could not be effectively controlled. 

The chiral diazo ester 27 was cychzed [16] with four commonly used rhodium car-boxylate catalysts (Tab. 16.2), wherein the rhodium pivalate [20] (entry 4) was most efficient for forming the cyclopentanes, and the rhodium trifluoroacetate (entry 1) was optimum for forming alkenes [21]. Furthermore, it was demonstrated that the yield of the cyclization and the diastereoselectivity could be improved at lower temperature using the pivalate-derived catalyst (entry 5). 

A reactive carbenoid would have an early transition state, favoring /9-H elimination over 1,5-insertion. We expect that the increased proportion of eHmination observed with rhodium trifluoroacetate (entry 5) is due to the electron-withdrawing nature of the ligand, which makes the carbene carbon more electron-deficient and thus more reactive. 

Rhodium pivalate dirhodium tetrakis /i-(2,2-dimethylpropanato-0 0 )] was synthesized by heating commercially available rhodium trifluoroacetate in 8 equiv pivalic acid for 24 h followed by removal of excess acid by heating under vacuum. The cmde catalyst was purified by flash chromatogra-... 

A similar transformation was observed with the rhodium trifluoroacetate catalyzed decomposition of diazo ketones in the presence of benzene (Scheme 32).130 The cycloheptatrienes (147) formed in this case were acid labile and could be readily rearranged to benzyl ketones (148) on treatment with TFA. The reaction was effective even when the side chain contained reactive halogen and cyclopropyl functionality, but competing intramolecular reactions occurred with benzyl diazomethyl ketone. A more exotic example of this reaction is the rhodium(ll) trifluoroacetate catalyzed decomposition of the diazopenicillinate (149) in the presence of anisole, which resulted in the formation of two cycloheptatriene derivatives (150) and (151) (equation 35).m... 

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... 

The problems endemic to the thermal and photochemical Buchner reactions were solved comprehensively in 1980 when rhodium(II) catalysts were introduced. The measurement of improvement using Rh(II) catalysts can be appreciated by comparing the thermal reaction of ethyl diazoacetate with anisole (35% yield, seven products) with its rhodium trifluoroacetate-catalyzed counterpart (83% yield, three products 19-21). The methoxy substituent clearly exerts a directive effect in favor of the 4-methoxy isomer 19, and all the products are kinetically controlled unconjugated esters. In general, the rhodium(II)-catalyzed decomposition of alkyl diazoacetates in the presence of a large excess of aromatic substrates at room temperature affords kinetically controlled cycloheptatrienyl esters in excellent yield. 

Reaction of the cyclopentadienyl rhodium and iridium tris(acetone) complexes with indole leads to the species 118 (M = Rh, Ir) [77JCS(D)1654 79JCS(D)1531]. None of these compounds deprotonates easily in acetone, but the iridium complex loses a proton in reaction with bases (Na2C03 in water, r-BuOK in acetone) to form the ri -indolyl complex 119. This reaction is easily reversed in the presence of small amounts of trifluoroacetic acid. 

In a direct competition between 1,2- and 1,5-insertion into methylene C —H bonds, the relative proportion of products depends on the rhodium carboxylate employed. Rhodium(II) pivalate is the most efficient catalyst so far found for the cyclization of methyl 2-diazo-10-undecenoate. In contrast, rhodiumfll) trifluoroacetate gives a 52 48 ratio of cyclic 5/acyclic 6 products. 

For cyclopropanation of very electron-rich alkenes such as vinyl ethers copper(II) trifluoroacetate, copper(II) hexafluoroacetylacetonate or rhodium(II) acetate are the catalysts of choice. Copper trifluoroacetate catalysed cyclopropanation of vinyldia-zomethane with dihydropyran gives the corresponding vinyl cyclopropane adduct in low yield (equation 17). In contrast, catalytic decomposition of phenyldiazomethane in the presence of various vinyl ethers results in high-yield phenylcyclopropane formation (equations 18 and 19)27. 

Carbenoid transformations involving competition between intramolecular cyclopropa-nation and /8-hydride elimination have been investigated149. The chemoselectivity of these catalytic transformations can be effectively controlled by the choice of catalyst. Rhodium(II) trifluoroacetate catalysed decomposition of diazoketone 111 proceeds cleanly to give only enone 112. However, rhodium(II) acetate or bis-(iV-t-butylsalicyladiminato) copper(II) cu(TBs)2 provides exclusively cyclopropanation product 113 (equation 102)149. 

Metal-containing compounds, Rhodium Compounds (Continued) Hydridotetrakis(triphenylphosphine)-rhodium(I), 144 Rhodium(II) acetate, 226, 266 Rhodium(II) carboxylates, 226, 266 Rhodium(II) trifluoroacetate, 266 Tetra-jx3-carbonyldodecacarbonylhexa-rhodium, 152, 288... 

Novotny found that neat trifluoroacetic acid could be hydrogenated to 2,2,2-tri-fluoroethanol in the presence of rhodium or iridium catalyst under much milder con-... 

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