Rhodium catalysis cyclization

Rhodium catalysis in an aqueous-organic biphasic system was highly effective for intramolecular [2+2+2] cyclotrimerization. It has been shown that the use of a biphasic system could control the concentration of an organic hydrophobic substrate in the aqueous phase, thus increasing the reaction selectivity. The intramolecular cyclization for... 

Allylamines cyclize readily with a dicobalt octacarbonyl catalyst (equation 55).1,2 Rhodium catalysis generally allows the carbonylative cyclization to be carried out under milder conditions.86 Application of this reaction to unsaturated amides yields the corresponding imides, the best yields arising when R1 = H and R2 = allyl (equation 56).I>2... 

Although the mechanism in rhodium catalysis is stiU unclear, it is believed that the reaction initiates through an oxidative cyclization of a diene with a low-valent rhodium species to produce a rhodacycle, which undergoes p-hydride elimination to form the common intermediate for both exo- and endo-observed products (Scheme 7.40) [59]. 

Aromatization of enediynes with catalytic insertion of C-H bond—In the case of the enediynes 3.718 bearing long alkyl substituents terminating one alkyne branch, the cycloaromatization occurs with radical insertion into a C-H bond of the alkyl group (Scheme 3.80) [257, 263]. The route taken by catalysis with ruthenium differs from that with rhodium [257]. In the rhodium system, cyclization is initiated by a rhodium-vinylidene intermediate which forms OTe i2-diradical naphthalene intermediate A. In the case of ruthenium, the cyclization comprises primary of the formation of a ruthenium-Ti-alkyne, which forms para-dirsidicsil B that converts to the product 3.719. 

While palladium, ruthenium, and rhodium are the most common metal catalysts used to facilitate Alder-ene cyclization, a few successful examples of catalysis using different metals have been published. Both of the references reviewed in this section demonstrate chemistry that is novel and complimentary to the patterns of reactivity exhibited by late transition metals in the Alder-ene cyclization. 

The reaction of a-diazocarbonyl compounds with nitriles produces 1,3-oxazoles under thermal (362,363) and photochemical (363) conditions. Catalysis by Lewis acids (364,365), or copper salts (366), and rhodium complexes (367) is usually much more effective. This latter transformation can be regarded as a formal [3 + 2] cycloaddition of the ketocarbene dipole across the C=N bond. More than likely, the reaction occurs in a stepwise manner. A nitrilium ylide (319) (Scheme 8.79) that undergoes 1,5-cyclization to form the 1,3-oxazole ring has been proposed as the key intermediate. 

It was then demonstrated38 that cyclization of 3 proceeded much more efficiently with rhodium acetate than with copper salt catalysis (no cyclization with copper sulfate). 

Chromium alkenyl Fischer carbenes have been shown to undergo a 3 + 2-cyclization with allenes under Rh(I) catalysis and a CO atmosphere, yielding 2-alkylidenecyclo-pentanone (54) after acidic hydrolysis.46 Reactions with electron-rich allenes are carried out with a neutral rhodium complex whereas electron-poor allenes require a... 

Exposure of the (cycloalkenyl)methyl carbamates 304 to iodosylbenzene in the presence or absence of Rh2(OAc)4 gives the tricyclic aziridines 305 (Scheme 88) (02OL2137). Reactions of 305 with nucleophiles, facilitated with tosic acid or lithium perchlorate, proceed with cleavage of the C-N edge bond and afford the tf 7 z-spirooxazolidinones 306. Intramolecular aziridination of the indolyl carbamate 307 with DAIB, on the other hand, requires Rh(II)-catalysis and leads directly to the acetoxy-substituted YDi-spirooxazolidinone 308 (Scheme 88) (02OL2137). When iodosylbenzene is used instead of DAIB and alcohols are available in the reaction medium, alkoxy-substituted syn-spirooxazolidinones 309 are obtained. Whereas the conversion of 304 to 305 appears to proceed by direct cyclization of intermediate iminoiodanes, the production of 308 from 307 was attributed to the intervention of a rhodium nitrene, which collapses to 308 through zwitterionic intermediates (02OL2137). 

FIGURE 8 Molecular clips equipped with phosphines or phosphites used for host-guest catalysis in rhodium-catalyzed hydrogenation and hydroformylation. At the bottom are depicted the peculiar tandem cyclizations, found only for allyldihydr-oxybenzenes in the host molecule as the catalyst (37,32). (For a color version of this figure, the reader is referred to the Web version of this chapter.)... 

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). 

Other elecuon-withdrawing groups are compatible with both diazo transfer and cyclization. Both the -keto sulfone (65) and the -keto phosphonate (67) have been cyclized using rhodium acetate catalysis (equations 25 and 26). The cyclized keto phosphonate (68) can be further reacted with formaldehyde to make the a-alkylidenecyclopentanone (69 equation 26). ... 

All efforts to affect the requisite cyclization however proved futile. When rhodium acetate in benzene was used as a catalyst, C—H insertion of the resulting carbenoid occurred to furnish cyclobutanone 285, which was not isolated, but underwent ring opening to yield tetrahydro-P-carboline 286. The use of catalytic copper(I) triflate, protic add catalysis, and photolysis conditions were also explored, but the desired product was not obtained. 

A nice addition to this work was made by Ellman and coworkers [100], who reported an intramolecular cyclization of aromatic ketones using Cp Rh (C2H3SiMe3)2 as catalyst (Scheme 19.69). This rhodium-catalyzed reaction was more efficient with respect to ruthenium catalysis (about 20% yield with RUH2 (CO)(PPh3)3), but its scope was limited to substrates bearing noniso-merizable olefins. 

A rhodium catalyst was also proposed for the intramolecular cyclization of imidazoles and oxazoles, which was previously shown to proceed under ruthenium catalysis (Scheme 19.61) [90bj. An enantioselective version was later reported... 

The intramolecular Buchner reaction of aryl diazoketones has been carried out using both copper(I) and rhodium(II) catalysts. For example, 1-diazo-4-phenylbutan-2-one 27a cyclizes in bromobenzene with copper(I) chloride catalysis, furnishing 3,4-dihydroazulen-l(2//)-one 30 in 50% yield after purification by chromatography over alumina. Trienone 30 is not the primary cyclization product, and the less conjugated isomeric trienone 29a is first produced, but contact with alumina causes isomerization to 30. The yield of this cyclization is further improved when rhodium(II) acetate is used as the catalyst instead of copper(I) chloride. Thus a catalytic amount of rhodium(II) acetate brings about the nearly quantitative conversion of 27a to 29a within minutes in hot dichloromethane. Compound 29a isomerizes to 30 on treatment with triethylamine, and rearranges to 2-tetralone 31a when exposed to silica gel or acid. 

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