Rhodium catalysts rearrangement

The view has been expressed that a primarily formed ylide may be responsible for both the insertion and the cyclopropanation products 230 246,249). In fact, ylide 263 rearranges intramolecularly to the 2-thienylmalonate at the temperature applied for the Cul P(OEt)3 catalyzed reaction between thiophene and the diazomalonic ester 250) this readily accounts for the different outcome of the latter reaction and the Rh2(OAc)4-catalyzed reaction at room temperature. Alternatively, it was found that 2,5-dichlorothiophenium bis(methoxycarbonyl)methanide, in the presence of copper or rhodium catalysts, undergoes typical carben(oid) reactions intermole-cularly 251,252) whether this has any bearing on the formation of 262 or 265, is not known, however. 

Rh2(OAc)4-catalyzed decomposition of 2-diazocyclohexane-l,3-dione 380a or its 5,5-dimethyl derivate 380b in the presence of an aryl iodide leads to an iodonium ylide 381 355). The mild reaction conditions unique to the rhodium catalyst are essential to the successful isolation of the ylide which rearranges to 382 under the more forcing conditions required upon copper catalysis (copper bronze, Cu(acac)2, CuCl2) 355). 

The aza-[2,3] Wittig rearrangement of aziridines is an excellent method for the synthesis of substituted piperidines. The analogous reaction of an epoxide has recently been examined <06TL7281>. Reaction of divinyl epoxide 48 with /-butyl diazo acetate provides the ylide intermediate 49, which then undergoes the [2,3] Wittig rearrangement to 50, Several catalysts were examined as catalysts for the formation of 49. It is noteworthy that the copper catalyst performed much better than the more widely used rhodium catalysts. 

The coordination of the alkyne to the rhodium catalyst allows the carborhodation of the triple bond to afford the vinylrhodium intermediate 47 (Scheme 14). The rearrangement of this organometallic compound into the 2-(alkenyl)phenylrhodium intermediate 48 is evidenced by one deuterium incorporation resulting from the deuter-iolysis of the Rh-C bond. The addition of the phenylrhodium intermediate 45 must occur before its hydrolysis with water. The 2-(alkenyl)phenylrhodium intermediate 45, generated by the phenylrhodation of an alkyne followed by... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

A review about the rearrangement and cycloaddition of carbonyl ylides generated from a-diazo compounds is available <2001CSR50>. Enantioselective intramolecular cyclopropanations of allyl 2-diazo-3-silanyloxybut-3-enoates to yield cyclopropyl 7-butyrolactones have been investigated with a variety of chiral rhodium catalysts. The best results were obtained with Rh2(PTTL)4, where enantioselectivity culminated at 89% ee (Equation 99) <2005TA2007>. 

Doyle has shown that the rhodium-catalyzed reaction of allylic sulfides and amines with ethyl diazoacetate produced smoothly the products of 3,2-rearrangement. In contrast with the copper-catalyzed reaction, allylic amines can be used and the yields are good to high (Scheme 44) virtually no cyclopro-panation is observed. These observations demonstrate the superiority of rhodium catalysts compared with either copper ones or the use of light. 

The cycloprop-2-enecarboxylates 1 rearranged in refluxing benzene in the presence of a rhodium catalyst to produce methylenecyclopentanes 2, apparently by formation of a metal-carbenoid of a ring-opened vinylcarbene, and insertion of this into a 5,6-related C-H bond. ... 

Rhodium(II) complexes, e.g.. Rh2(OAc)4, are the mildest and most efficient and versatile catalysts for cyclopropanation, especially with diazo carbonyl reagents5 la 1 31 8S. Side reactions are C-H-insertion and Wolff rearrangement. Intensive mechanistic investigations and a comparison with other metal catalysts have been described10 2. Rhodium catalysts have frequently been used in stereoselective and asymmetric cyclopropanation12. 

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