Rhodium pivalate

An alternative strategy for selective intermolecular G-H insertions has been the use of rhodium carbenoid systems that are more stable than the conventional carbenoids derived from ethyl diazoacetate. Garbenoids derived from aryldiazoacetates and vinyldiazoacetates, so-called donor/acceptor-substituted carbenoids, have been found to display a very different reactivity profile compared to the traditional carbenoids.44 A clear example of this effect is the rhodium pivalate-catalyzed G-H insertion into cyclohexane.77 The reaction with ethyl diazoacetate gave the product only in 10% yield, while the parallel reaction with ethyl phenyldiazoacetate gave the product in 94% yield (Equation (10)). In the first case, carbene dimerization was the dominant reaction, while this was not observed with the donor/acceptor-substituted carbenoids. 

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). 

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-... 

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

The [3+4] annulation approach to the hydroazulenes is achieved with high asymmetric induction (greater than 90% de) by using (/ )-pantolactone as a chiral auxiliary (Table 7). The nature of the catalyst has a considerable effect on the level of asymmetric induction. A sterically crowded catalyst, such as rhodium pivalate, results in much lower diastereoselectivity than rhodium(II) acetate or rho-dium(II) hexanoate. Consequently, even though the enantiomers of rhodium(II) mandelate exhibit double stereodifferentiation with the (/ )-pantolactone auxiliary (entries 5,6), both catalysts are bulky and result iinferior asynunetric induction compared to that obtained with an uncrowded achiral catalyst (entries 1-3). 

Rhodium(II) pivalate has also been recommended for the cyclopropanation of vinyl halides with ethyl diazoacetate 78). As Table 8 shows, yields with this catalyst are far higher and reaction conditions milder than with copper. Failures are noted,... 

Table 8. Cyclopropanation of vinyl halides with ethyl diazoacetate in the presence of rhodium(Il) pivalate (Rhpiv) or copper...

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). 

Rh(II) pivalate is, however, still not efficient in producing more of the syn than of the anti isomer of ethyl bicyclo[4.1.0]heptane-7-carboxylate from cyclohexene and ethyl diazoacetate 87 98>. It needs a rhodium(III) porphyrin 47 to be successful in this case... 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates 286), rhodium(II) pivalate or some other carboxylates 287,288 and rhodium-(III) porphyrins 287 > proved to be well suited (Tables 19 and 20). In the era of copper catalysts, this reaction type ranked as a quite uncommon process 14), mainly because the yields were low, even in the absence of other functional groups in the substrate which would be more susceptible to carbenoid attack. For example, CuS04(CuCl)-catalyzed decomposition of ethyl diazoacetate in a large excess of cyclohexane was reported to give 24% (15%) of C/H insertion, but 40% (61 %) of the two carbene dimers 289). 

Mejla-Oneto and Padwa have explored intramolecular [3+2] cycloaddition reactions of push-pull dipoles across heteroaromatic jr-systems induced by microwave irradiation [465]. The push-pull dipoles were generated from the rhodium(II)-cata-lyzed reaction of a diazo imide precursor containing a tethered heteroaromatic ring. In the example shown in Scheme 6.276, microwave heating of a solution of the diazo imide precursor in dry benzene in the presence of a catalytic amount of rhodium I) pivalate and 4 A molecular sieves for 2 h at 70 °C produced a transient cyclic carbonyl ylide dipole, which spontaneously underwent cydoaddition across the tethered benzofuran Jt-system to form a pentacyclic structure related to alkaloids of the vindoline type. 

Scheme 2.57 Rhodium-catalyzed enantioselective 1,6-addition of aryltitanium reagents to 3-alkynyl-2-cycloalkenones 180 (ee values referto the corresponding allenic enol pivalates).

The observed first-order rate constants, Kobs (s ) for the reaction of the rhodium] 11) complexes with diazo ester 34 varied over a range of > 10, in which the pivalate catalyst (entry 3) was almost two orders of magnitude faster than any of the other catalysts studied. The carboxamidate catalysts (entries 8-10) were slower than all the carboxy-lates, while the bridged phosphine catalyst (entry 7) behaved like most of the other car-boxy late s. 

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. 

The reaction of rhodium(II) pivalate dimer, Rh2(02CCMe3)4, with 1,4-benzo-quinone (BQ) in hexane gave a chain complex, [Rh2(02CCMe3)4 BQ] , where the rhodium(II) pivalate dimers are connected by the bifunctional ligation of the p-quinone through its carbonyl oxygen or C = C double bond, i.e., the chain structure consists of two kinds of dimer units 963 and 964 [235-237] ... 

Z)-l-Trimethylsilyl-l-alkenes.1 The a-trimethylsilyl diazoalkanes (2), prepared by reaction of primary halides (1) with the lithium anion of trimethylsilyl diazomethane, decompose on treatment with rhodium(II) pivalate [superior in this case to rhodium(II) acetate] to (Z)-l-trimethylsilyl-1-alkenes. 

Cyclopropanation. Rhodium(II) pivalate is superior to copper catalysts for the cyclopropanation of l,l-dihalo-4-methylpenta-l,3-dienes (1) with ethyl diazoacetate. The yield is higher than that obtained with copper catalysts (48%), and the cis/trans ratio is considerably higher. Similar contrathermodynamic product ratios were observed with other substrates. 

Diazo ester/rhodium(II) carboxylate combinations other than EDA/Rh2(OAc)4 have been tested It turned out that the solubility of the rhodium(II) carboxylate greatly influenced the efficiency of cyclopropanation. For the reaction of monoolefins with ethyl diazoacetate, markedly higher yields than with Rh(II) acetate were obtained with the better soluble rhodium(II) butanoate and rhodium(II) pivalate, the latter one being soluble even in pentane. However, only poor yields resulted from the use of rhodium(Il) trifluoroacetate, even though this compound is readily soluble, Rh CCFjCOO), in contrast to the other rhodium(II) carboxylates, is able to form 1 1 complexes with olefins particularly with electron-rich ones thus, competition of olefin and diazo compound for the only available coordination site at the metal atom could be responsible for the reduced catalytic action of Rh2(CF3COO)4 (as will be seen in Section 4.1, this complex is an excellent catalyst for cyclopropanation of aromatic substrates). The diazoester substituent also has some influence on the yields. Increasing yields were obtained in the series methyl ester, ethyl ester, n-butyl... 

Similar to the intramolecular insertion into an unactivated C—H bond, the intermolecular version of this reaction meets with greatly improved yields when rhodium carbenes are involved. For the insertion of an alkoxycarbonylcarbene fragment into C—H bonds of acyclic alkanes and cycloalkanes, rhodium(II) perfluorocarb-oxylates rhodium(II) pivalate or some other carboxylates and rhodium-... 

TroponesReaction of vinyldiazomcthanes (1) with l-methoxy-l-(trimethyl-silyloxy)butadiene (2) catalyzed by rhodium(H) acetate or rhodium(ll) pivalate results in [3+4]cycloaddition via cyclopropanation/Cope rearrangement to form a cyclohcptadiene (3). Short exposure of 3 to citric acid followed by oxidation (DDQ) provides the... 

Synthesis of (+)-griseofulvin (3).5 The key step in a recent asymmetric synthesis of this antifungal reagent is the decomposition of the diazo ketone 1 with rhodium(ll) pivalate to provide 2 in 62% yield as the only isolablc product. 

The alkaloid ( + )-retronecine (883, Scheme 129) is structurally similar to ( + )-heliotridine (850), with the exception that the stereocenter at C-7 is of opposite configuration. The basic approach to its synthesis involves a carbenoid displacement similar to that in the previous scheme. The acetyl protecting group of the common intermediate 875 (R=SPh) is changed to a TBS group, and the benzoate is converted to pivalate. Carbenoid displacement with dibenzyl a-diazomalonate in the presence of rhodium acetate gives 879. Reductive desulfurization... 

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