Rhodium-catalyzed Cyclizations

A quinoline-tethered NHC-rhodium complex was used as catalyst for the [3 + 2] cycloaddition of diphenylcyclopropenone and internal alkynes (Equation (8.11)). In the presence of 2mol% of 23, the desired pentenone derivatives were isolated in up to 86% yield. Also, this catalyst tolerated functional substituents such as ketone, ester or hydroxyl groups. 

Dinuclear rhodium(II) complexes were tested as catalysts for intramolecular C-H insertions of diazo-acetamides. In this study, the reactivity of axially monocoordinated [(NHC)Rh2(OAc)4j complexes was found to be different 

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Graening, T, Friedrichsen, W., Lex, J., Schmalz, H.G. (2002) FacUe Construction of the Colchicine Skeleton by a Rhodium-Catalyzed Cyclization/Cycloaddition Cascade. Angewandte Chemie International Edition, 41, 1524-1526. 

Padwa and co-workers employed a rhodium-catalyzed [3+2] cycloaddition reaction to generate a number of 3-hydroxy-2-pyridones, including the tricyclic 146, obtained using fV-phenylmaleimide 145 as the dipolarophile. The rhodium-catalyzed cyclization of 143 generates an isomiinchnone intermediate 144, which undergoes the cycloaddition (Scheme 11) <1997JOC438>. 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

For mechanistic studies of the rhodium-catalyzed cyclization of 4-alkenals to cyclopen-tanones, see (a) Campbell, R. E., Jr. ... 

For preliminary mechanistic studies of the rhodium-catalyzed cyclization of 4-alkynals... 

Padwa utilized this rhodium-catalyzed cyclization of diazothiocarbonyl compounds to great effect in the generation and trapping of thioisomiinchnones (60,61). For example, thioisomiinchnone (101) is readily prepared (and isolated) from the reaction of 2-thiopyrrolidinone and diketene, followed by diazo transfer and rhodium induced cyclization (Scheme 10.18) (60). Trapping with NPM afforded cycloadduct 102. 

Padwa and Prein (105,106) applied chiral, but racemic, isomiinchnone dipoles in diastereoselective 1,3-dipolar cycloadditions. The carbonyl ylide related isomiinch-none derivative rac-70 was obtained from the rhodium-catalyzed cyclization of diazo-derivative rac-69 (Scheme 12.24) (105). The reactions of the in situ formed dipole with a series of alkenes was described and in particular the reaction with maleic acid derivatives 71a-c gave rise to reaction with high selectivities. The tetracyclic products 72a-c were all obtained in good yield with high endo/ exo and diastereofacial selectivities. In another paper by the same authors, the reactions of racemic isomilnchnones having an exo-cyclic chirality was described (106). 

While the detailed mechanism of these rhodium-catalyzed cyclizations is not known, a working hypothesis that accommodates all of the observations to date is as follows. The diazo ketone can be considered to be a stabilized ylide, 14. Association of the Lewis acidic LUMO of the rhodium(II) carboxylate with the locally electron-rich ylide yields 15. Loss of nitrogen would then give the highly electrophilic intermediate 16. In nondonating solvents, the richest source of electron density available to this reactive species is the remote C—H bond. Complexation with the electron density in this bond gives 17, which collapses to the cyclopentanone product. 

Conformational effects are clearly important in the cyclization of alkyl amides. It was observed60 that the proportion of products 16/17 + 18 from the cyclization of amide 15 did not change with changing electronic demands of the catalyst. Apparently, rhodium-catalyzed cyclization is faster than amide rotation. 

A powerful feature of intramolecular C-H insertion is the inherent ability to transform an acyclic ternary stereogenic center into a cyclic quaternary stereogenic center26. It has been demonstrated that the rhodium-catalyzed cyclization of methyl (tf)-2-diazo-6-(4-methylphenyl)-3-oxohep-tanoate to cyclopentanone 6 indeed proceeds with retention of absolute configuration61. The absolute configuration of the latter was confirmed by conversion to the sesquiterpene (+ )-a-cu-parenone. 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/hydrosilylation of enynes initiated by oxidative addition of the H-Si bond of the hydrosilane to form the Rh(iii) silyl hydride complex If (Scheme 7). Silylmetallation of the G=G bond of the enyne coupled with coordination of the pendant G=G bond could form... 

Shibata and co-workers have reported an effective protocol for the cyclization/hydrosilylation of functionalized eneallenes catalyzed by mononuclear rhodium carbonyl complexes.For example, reaction of tosylamide 13 (X = NTs, R = Me) with triethoxysilane catalyzed by Rh(acac)(GO)2 in toluene at 60 °G gave protected pyrrolidine 14 in 82% yield with >20 1 diastereoselectivity and with exclusive delivery of the silane to the G=G bond of the eneallene (Equation (10)). Whereas trimethoxysilane gave results comparable to those obtained with triethoxysilane, employment of dimethylphenylsilane or a trialkylsilane led to significantly diminished yields of 14. Although effective rhodium-catalyzed cyclization/hydrosilylation was restricted to eneallenes that possessed terminal disubstitution of the allene moiety, the protocol tolerated both alkyl and aryl substitution on the terminal alkyne carbon atom and was applicable to the synthesis of cyclopentanes, pyrrolidines, and tetrahydrofurans (Equation (10)). 

Rhodium-catalyzed cyclization/silylation/carbonylation of 1,6-diynes was proposed to occur via a-migratory insertion of CO into the Rh-C bond of dialkylidene cyclopentane intermediate Ik to form acylrhodium alkene intermediate Ilk (Scheme S-endo-[5-M gX2itory insertion of the silylated alkylidene moiety into the Rh-G bond of Ilk would form... 

Carbacephalosporins The ketene-imine cyclization described above has been extended to a synthesis of a chiral carbacepham (4). This synthesis uses a dihy-droanisole group as the equivalent of a p-keto ester. Thus the azetidinone 1, obtained in 80% yield by the above route, was reduced and acylated in situ to provide 2. Ozonization followed by a rhodium-catalyzed cyclization of an a-diazo-P-keto ester provides 3, which is a useful intermediate to various substituted car-bacephams such as 4. 

Rhodium-catalyzed cyclization and carbonylation of 2-ethynylbenzylalcohols 1168 under water-gas shift reaction conditions leads to isochroman-3-ones (Equation 455) <1998TL5061>. 

In the same vein, Schmalz has proposed a facile construction of the colchicine skeleton by a rhodium-catalyzed cyclization/cycloaddition cascade [56]. A TMS group has to be introduced on the alkyne moiety of 66 in order to avoid participation of the relatively acidic alkynyl hydrogen atom in undesired proton transfers. The resulting 6,7,7 of 67a and 67b architecture was assembled in a remarkably diastereoselective manner (14 1) and in satisfactory yield (Scheme 30). 

DICARBOXYLATE. A rhodium-catalyzed cyclization of acetylenes with a-diazocarbonyls provides a nice route to ETHYL 2-METHYL-5-PHENYL-... 

The rhodium-catalyzed cyclization of cycloalkyldiazoacetates 1135 gives mixtures of cis- and //ms-fused lactones when 3.61 is used to generate the chiral catalyst. Recently, Doyle and coworkers found that cis-fused lactones 11.36 could be obtained with a high selectivity when the catalyst was generated from the methyl l-acetylimidazolidin-2-one-4-carboxylate 11.37 [1672],... 

The effect of the solvent on the product selectivity for the rhodium-catalyzed cyclization of an allylamine under a CO/H2 atmosphere (Scheme 70) is such that carbonylation in conventional organic solvents forms a cyclic amide 41, whereas the reaction in SCCO2 preferentially produces reduced five-membered cyclic amine 40. In SCCO2, the amino group of the starting material is transformed to the carbamic... 

In 2006, the group of Artok showed that 5-aryl-2(5H)-furanones could be prepared in moderate to good yields by a rhodium-catalyzed carbonylative arylation of internal alkynes with aryl boronic acids (Scheme 1.9a) [22]. a,P-Unsaturated ketones (chal-cone derivatives) were formed as the major product when some TFA (trifluoroacetic acid) was added under the same reaction conditions [23a]. By varying the catalytic system, indanones could be produced as the main product [23b]. The chemical behavior of terminal alkynes is different, and either a,P-unsaturated ketones or furans starting from propargylic alcohols can be achieved (Scheme 1.9b) [24, 25]. In the case of vinyl ketones, 1,4-diketones were obtained by rhodium-catalyzed coupling of arylboronic acids in the presence of 20-40 bar of CO [26]. In 2007, Chatani demonstrated that indenones could be accessed by a carbonylative rhodium-catalyzed cyclization of alkynes with 2-bromophenylboronic adds (Scheme 1.9c) [27]. Here, the key intermediate is a vinylrhodium(I) spedes that is formed by transmetaUation of RhCl with 2-bromophenylboronic acid followed by insertion of... 

Davies and coworkers discovered that pyridines can be produced by rhodium-catalyzed cyclization of carbenoids and isoxazoles as well [74], Highly functionalized pyridines and 1,4-dihydropyridines could be produced by this procedure in good yields (Scheme 3.35). For the reaction mechanism, the reaction proceeds through an initial carbenoid induced ring expansion of isoxazoles and then followed by a rearrange-ment/tautomerization/oxidation sequence. 

An enantioselective version of this rhodium-catalyzed cyclization reaction with a TangPhos ligand was developed by the authors at a later date. It was hypothesized that the highly electron-rich ligand was so efficient at promoting asymmetric induction because of its ability to create a vacant coordinating site on the metal center through partial dissociation. ... 

Marder TB, Chan DM-T, Fultz WC, Calabrese JC, Milstein D (1987) Mechanistic studies of the rhodium-catalyzed cyclization of am-alkynoic acids to alkylidene lactones. Cristal structure of two iridium model catalytic intermediates. J Chem Soc Chem Commun 1885-1887... 

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