Arylation Rhodium carboxylates

The benzylic position of an alkylbcnzene can be brominated by reaction with jV-bromosuccinimide, and the entire side chain can be degraded to a carboxyl group by oxidation with aqueous KMnCfy Although aromatic rings are less reactive than isolated alkene double bonds, they can be reduced to cyclohexanes by hydrogenation over a platinum or rhodium catalyst. In addition, aryl alkyl ketones are reduced to alkylbenzenes by hydrogenation over a platinum catalyst. 

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. 

Hashimoto and co-workers (55) reported that generation of ylide 152 from aryl ester 151 in the presence of a chiral rhodium complex Rh2(S-PTTL)4, a chiral phthalimide substimted carboxylate, followed by cycloaddition with DMAD, led to the formation of adduct 153 in good yield and in 74% enantiomeric excess (ee). 

RADICAL ANION ARYLATION, 58, 134 Raney nickle, 57, 19, 58, 114, 116 Raney nickel,W-2, 56, 16, 74 Reduction, carboxyl groups, 56,83 Reductive alkylation, 56, 52 Resolution of amines, 55, 80, 83 Rexyn 201, 55,4 Rhodium(III) oxide, 57,1 Ring contraction, 56. 107... 

In 2006, Ukai et al. proposed an interesting alternative with a rhodium(I)-catalyzed carboxylation of aryl- and alkenylboronic esters proceeding under mild conditions, and leaving ancillary reactive functional groups such as carbonyl- and cyano unreacted [51] (Scheme 5.13). Considering that organoboronic esters are easily available, and that various functional groups are tolerated, this reaction appeared to be particularly useful for the preparation of functionalized arylcarboxylic acids, such as benzoic and cinnamic acid derivatives. 

The ferrocenylbisphosphines 8f—h bearing the amino pendant side chain are unique ligands that effect the rhodium-catalyzed asymmetric hydrogenation of tetrasubstituted olefms 57 (Scheme 2-43) [61]. Thus, the hydrogenation of a-aryl-acrylic acid 57a in the presence of a cationic rhodium catalyst coordinated with 8g gives a quantitative yield of carboxylic acid 58a with 98.4% ee. Other tetra-... 

Ghlorotris(triphenylphosphine) rhodium causes the decarbonylation of aroyl to aryl chlorides in excellent yields at relatively low temp. At higher temp., only catalytic amounts of the reagent are required. — E A mixture of henzoyl chloride, the above Rh-compound, and benzene heated 15 min. at 80° chlorobenzene. Y almost 100%. F. e. s. J. Blum, Tetrah. Let. 1966, 1605 chlorides from sulfonic acid chlorides s. Tetrah. Let. 1966, 3041 also ethylene derivatives from carboxylic acid chlorides s. J. Tsuji and K. Ohno, Am. Soc. 88, 3452 (1966) Tetrah. Let. 1966, 4713. 

An early report on rhodium-catalysed Mizoroki-Heck-type arylations of alkene 20 using two alkenyl bromides was disclosed by Chiusoli et al [28], Thus, catalytic amounts of [RhCl(PPh3)3] (84) allowed for conversion of potassium 3-butenoate (20) (Scheme 10.28), a substrate that can potentially direct the rhodium catalyst through coordination of its carboxylate group. 

There has been a review of palladium-catalysed carbonylative coupling reactions of aryl halides with carbon nucleophiles in the presence of carbon monoxide. It has been shown that rhodium is an efficient catalyst for the homocoupling reaction of arylzinc compounds in the presence of 1 atm of carbon monoxide to give diaryl ketones. Under similar conditions, palladium and nickel catalysts yield biaryls. The beneficial catalysis by rhodium is likely to derive from the ease of migration of the aryl ligand to carbon monoxide in the rhodium(III) intermediate. A rhodium catalyst has also been used in the formation of indole-3-carboxylates by reaction of indoles with alcohols in the presence of carbon monoxide. The catalytic cycle. Scheme 5, is likely to involve metallation of the indole at the 3-position, followed... 

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