Rhodium carboxylates

The most common oxidatiou states and corresponding electronic configurations of rhodium are +1 which is usually square planar although some five coordinate complexes are known, and +3 (t7 ) which is usually octahedral. Dimeric rhodium carboxylates are +2 (t/) complexes. Compounds iu oxidatiou states —1 to +6 (t5 ) exist. Significant iudustrial appHcatious iuclude rhodium-catalyzed carbouylatiou of methanol to acetic acid and acetic anhydride, and hydroformylation of propene to -butyraldehyde. Enantioselective catalytic reduction has also been demonstrated. 

Figure 2.38 The effect of varying the relative energies of the metal and ligand orbitals upon the final molecular orbital scheme for a dimeric rhodium carboxylate. (Reprinted from Coord. Chem. Rev., 50, 109, 1983, with kind permission from Elsevier Science S.A., P.O. Box 564,...

Rhodium, tetrakis(trimethylphosphine)-reactions, 4, 926 Rhodium carboxylates, 4,903 chemotherapy, 4, 903 Rhodium complexes, 4. 901 acetylacetone synthesis, 2, 376 alkylperoxo... 

In addition, this methodology was extended to the cyclopropanation of a series of alkenes with phenyldiazomethane, giving rise to the corresponding cyclopropanes with high yields, diastereo- and enantioselectivities, as shown in Scheme 6.9. It was shown that the diastereoselectivity of these reactions was not greatly altered by the type of rhodium carboxylate catalyst that was used. 

Rhodium carboxylates have been found to be effective catalysts for intramolecular C—H insertion reactions of a-diazo ketones and esters.215 In flexible systems, five-membered rings are formed in preference to six-membered ones. Insertion into methine hydrogen is preferred to a methylene hydrogen. Intramolecular insertion can be competitive with intramolecular addition. Product ratios can to some extent be controlled by the specific rhodium catalyst that is used.216 In the example shown, insertion is the exclusive reaction with Rh2(02CC4F9)4, whereas only addition occurs with Rh2(caprolactamate)4, which indicates that the more electrophilic carbenoids favor insertion. 

As corroborated by deuterium labeling studies, the catalytic mechanism likely involves oxidative dimerization of acetylene to form a rhodacyclopen-tadiene [113] followed by carbonyl insertion [114,115]. Protonolytic cleavage of the resulting oxarhodacycloheptadiene by the Bronsted acid co-catalyst gives rise to a vinyl rhodium carboxylate, which upon hydrogenolysis through a six-centered transition structure and subsequent C - H reductive elimina-... 

For leading references to the preparation of other rhodium carboxylates, see Felthouse, T. R. Prog. Inorg. Chem. 1982, 29, 73. 

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. 

With the long chain a-diazo ketone. 6-diazo-7-tridecanone, 1,5-insertion could proceed with placement of the carbonyl outside the ring, or included in the ring. In fact, only the product 7, from the first of these two cyclization modes, is observed67. The alternative cyclopentane 9 is not formed. As with the a-diazo ester, the relative proportion of 1,2- and 1,5-products depends on the rhodium carboxylate employed. Throughout these studies, it has been observed that the olefin 8, obtained from 1,2-elimination, is cleanly Z-configured67 68. 

No report involving incorporation of rhodium into a quadruple bond has been published. For the sake of completeness the dimeric rhodium carboxylates are noted here. The Rh2(02CCH3)4 dimer can be conveniently synthesized from rhodium(III) chloride and sodium acetate (215), although routes from... 

The intermolecular C-H insertion of alkanes is very chemoselective, as is illustrated by the reaction with 2-methylbutane [4]. The only C-H transformation observed occurs at the methine site to form 21 in 68 % ee. This result is very different from the rhodium carboxylate-catalyzed reactions of ethyl diazoacetate with 2-methylbutane, which gives rise to all four C-H insertion products [16], Improved regioselectivity has recently been achieved in the intermolecular C-H insertion chemistry of ethyl diazoacetate by using either copper [17] or silver [18] scorpionate catalysts, but enantioselective versions of these reactions are not known. 

Figure 6.1. Rhodium-carboxylate catalysts for C-N bond formation.

The addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). 

The reaction of diazoacetic esters with alkenes is stereospecific and anti-isomers are produced preferentially. However, the anti/syn ratio is markedly dependent on the catalyst utilized. In order to prepare the three-membered adducts in better yields, with higher stereoselectivity, or with higher optical yields, extensive investigations have been conducted. Among the various catalysts examined, rhodium carboxylates have been found to be very efficient (equation 66) The reaction is relatively insensitive to steric... 

The synthesis of substituted chromanones 369 via a C—H insertion reaction of a-diazo ketones 370 has demonstrated that high levels of enantiose-lectivity are attainable through the use of chiral rhodium carboxylates (92CC823). Treating diazo ketone 370 (R = CH=CH2, R = H) with Rh2[(S)(-l-)BINAP]4 leads enantioselectively to the cis isomer of chroma-none 369 (92TL5983). 

Hubert in 1976 reported that rhodium acetate efficiently catalyzes diazo insertion into an alkene, to give the cyclopropane. In 1979, Southgate and Ponsford reported that rhodium acetate also catalyzes diazo insertion into a C—H bond. Prompted by these studies, Wenkert then demonstrated that cyclization of (58) to (59) proceeded much more efficiently with the rhodium carboxylates than it had with copper salt catalysis (equation 23). ... 

Concurrently, Noels had reported that rhodium carboxylates smoothly catalyze the intermolecular C—H insertion of ethyl diazoacetate into alkanes. Following up on this report, Taber demonstrated that the open chain a-diazo 3-keto ester (60) cyclizes smoothly under rhodium acetate catalysis to give the corresponding cyclopentane (61 equation 24). In contrast to the copper-mediated cyclization cited above (equation 22), the six-membered ring product is not observed. The insertion shows significant electronic selectivity. Although there is a 3 1 statistical preference for methyl C—H, only the methylene C—H insertion product (61) is observed (equation 24). 

Intramolecular C—H insertion, on the other hand, is already a practical alternative for the constmction of cyclobutanols, P-lactams - and of cyclopentane-containing targets. With regard to the latter, diazo transfer can be effected on a large scale with the inexpensive methanesulfonyl azide. The rhodium carboxylate catalysts are effective at very low concentration (<1 mol %) and can easily be recovered from the reaction mixture, if desired. ... 

In the rhodium carboxylate catalyzed process, the transition state leading to C—insertion is highly ordered. Rhodium-mediated C—insertion has further been shown to proceed with retention of absolute configuration. It may likely be possible, therefore, to design enantiomerically pure ligands for rhodium that would direct the absolute course of insertion into a target methylene. 

Carbenoid insertions. The species generated from decomposition of diazo compounds in the presence of rhodium carboxylates are capable of inserting into various X-H bonds. Thus, N-C bond formation has been exploited for the preparation of precursors of indoles, oxazoles, and peptides. ... 

An approach to imidazolones started from polymer-bound a-diazo-p-ketoester 33, which was transformed to intermediate 35 by treatment with urea 34 in the presence of a rhodium carboxylate catalyst (Scheme 10) [76]. Treatment of the resin-bound insertion product 35 with 10% TFA at room temperature afforded the resin-bound imidazolone 36 within 1 h. The polymer-bound imidazolone could then be cleaved by transesterification to give esters 37 or by a diversity building amidation reaction to provide amides 38. After preparative TLC, the products were obtained in yields of 19-84% (14 examples). 

The chiral diazo ester 29 was cyclized with four commonly used rhodium carboxylate catalysts (Table 2). It was found as before that rhodium pivalate... 

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