Cyclopropane catalysts, rhodium complexes

Chelucci et al. [41] synthesized further chiral terpyridines derived from (-)-yd-pinene, (-i-)-camphor, and (-l-)-2-carene and tested their ability to chelate copper or rhodium for the asymmetric cyclopropanation of styrene. The copper catalysts were poorly efficient and selective in this reaction. The corresponding rhodium complexes led to the best result (64% ee) with the ligand derived from (-l-)-2-carene (ligand 33 in Scheme 17). 

Metal-Catalyzed. Cyclopropanation. Carbene addition reactions can be catalyzed by several transition metal complexes. Most of the synthetic work has been done using copper or rhodium complexes and we focus on these. The copper-catalyzed decomposition of diazo compounds is a useful reaction for formation of substituted cyclopropanes.188 The reaction has been carried out with several copper salts,189 and both Cu(I) and Cu(II) triflate are useful.190 Several Cu(II)salen complexes, such as the (V-f-butyl derivative, which is called Cu(TBS)2, have become popular catalysts.191... 

The preparation of cyclopropanes by intermolecular cyclopropanation with acceptor-substituted carbene complexes is one of the most important C-C-bond-forming reactions. Several reviews [995,1072-1074,1076,1077,1081] and monographs have appeared. In recent decades chemists have focused on stereoselective intermolecular cyclopropanations, and several useful catalyst have been developed for this purpose. Complexes which catalyze intermolecular cyclopropanations with high enantiose-lectivity include copper complexes [1025,1026,1028,1029,1031,1373,1398-1400], cobalt complexes [1033-1035], ruthenium porphyrin complexes [1041,1042,1230], C2-symmetric ruthenium complexes [948,1044,1045], and different types of rhodium complexes [955,998,999,1002-1004,1010,1062,1353,1401-1405], Particularly efficient catalysts for intermolecular cyclopropanation are C2-symmetric cop-per(I) complexes, as those shown in Figure 4.20. These complexes enable the formation of enantiomerically enriched cyclopropanes with enantiomeric excesses greater than 99%. Illustrative examples of intermolecular cyclopropanations are listed in Table 4.24. 

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... 

The use of chiral additives with a rhodium complex also leads to cyclopropanes enantioselectively. An important chiral rhodium species is Rh2(5-DOSP)4, which leads to cyclopropanes with excellent enantioselectivity in carbene cyclopro-panation reactions. Asymmetric, intramolecular cyclopropanation reactions have been reported. The copper catalyzed diazoester cyclopropanation was reported in an ionic liquid. ° It is noted that the reaction of a diazoester with a chiral dirhodium catalyst leads to p-lactones with modest enantioselectivity Phosphonate esters have been incorporated into the diazo compound... 

So far, while there is a relative abundance of synthetically useful cyclopropana-tion catalysts, all of them provide a mixture of diastereomers with the anti product predominating. Thus, a catalyst able to provide optically active syn cyclopropyl esters would constitute a useful complement to existing methodology. Rhodium complexes of bulky porphyrins ( chiral fortress porphyrins) have been developed for this purpose [27]. The porphyrin ligands bear chiralbinaphthyl groups appended directly to the meso positions. Their rhodium(III) complexes provide predominantly the syn cyclopropane with diazoesters, with very good stereoselectivity in some cases. However, the enantioselectivities observed are modest. 

Rhodium-based catalysis suffers from the high cost of the metal and quite often from a lack of stereoselectivity. This justifies the search for alternative catalysts. In this context, ruthenium-based catalysts look rather attractive nowadays, although still poorly documented. Recently, diruthenium(II,II) tetracarboxylates [42], polymeric and dimeric diruthenium(I,I) dicarboxylates [43], ruthenacarbor-ane clusters [44], and hydride and silyl ruthenium complexes [45 a] and Ru porphyrins [45 b] have been introduced as efficient cyclopropanation catalysts, superior to the Ru(II,III) complex Ru2(OAc)4Cl investigated earlier [7]. In terms of efficiency, electrophilicity, regio- and (partly) stereoselectivity, the most efficient ruthenium-based catalysts compare rather well with the rhodium(II) carboxylates. The ruthenium systems tested so far seem to display a slightly lower level of activity but are somewhat more discriminating in competitive reactions, which apparently could be due to the formation of less electrophilic carbenoid species. This point is probably related to the observation that some ruthenium complexes competitively catalyze both olefin cyclopropanation and olefin metathesis [46], which is at variance with what is observed with the rhodium catalysts. 

For enantioselective intramolecular cyclopropanation with chiral catalysts, tetrakis[(5)- and (7 )-5-methoxycarbonyl-2-pyrrolidonato]dirhodium 30 and related rhodium complexes such as 31 and 32 (only the S -enantiomer is shown) appear the best choices at this time. With the appropriate catalyst, impressive enantiomeric excesses have been obtained for intramolecular cyclopropanations of unsaturated diazoacetates and diazoacetamides, as is illustrated by the following examples (see also Houben-Weyl, Vol. E21c, p3234). 

The direct transfer of carbene from diazocompounds to olefins catalyzed by transition metals is the most straightforward synthesis of cyclopropanes [3,4]. Reactions of diazoesters with olefins have been studied using complexes of several transition metals as catalysts. In most cases trans-isomers are preferably obtained, but the selectivity depends on the nature of the complex. In general the highest trans-selectivity is obtained with copper catalysts and it is reduced with palladium and rhodium complexes. Therefore, the rhodium mesotetraphenylporphyrin (RhTPPI) [5] and [(r 5-C5H5)Fe(CO)2(THF)]BF4 [6] are the only catalysts leading to a preference for the cis-isomer in the reaction of ethyl diazoacetate with styrene. 

Mechanistic studies of rhodium porphyrins as cyclopropanation catalysts have resulted in the spectroscopic identification of several potential intermediates in the reaction of ethyl diazoacetate with olefins, including a diazoniumfethoxy-carbonyl)methyl-rhodium complex formed by electrophilic addition of the rhodium center to the a-C atom of ethyl diazoacetate [29]. It is not known if analogous intermediates are also formed in analogous reactions of copper catalysts. However, the initial part of the catalytic cycle leading to the metal carbene intermediate is not of primary concern for the enantioselective reactions described herein. It is the second part, the reaction of the metal-carbene complex with the substrate, that is the enantioselective step. 

Among other examples of catalysed asymmetric cyclopropanation using rhodium (II) complexes are those involving Kodadek s chiral wall and chiral fortress porphyrins [26, 27], e.g., IRh(l -BNPP)4 in Fig. 8. These unique designs provide high turnover numbers (>1,800) and relatively high diastereoselectivi-ties (>70 30, cis trans),hut enantiocontrol in cyclopropanation with EDA was at best moderate (<60% ee). The Rh2(4S-IBAZ)4 catalyst (Fig. 4) exerts comparable diastereocontrol and significantly better enantioselectivity. 

Rhodium complexes generated from A-functionalized (S)-proline 3.60 [933, 934, 935] or from methyl 2-pyrrolidone-5-carboxylates 3.61 [936, 937, 938] catalyze the cyclopropanation of alkenes by diazoesters or -ketones. Diastereoisomeric mixtures of Z- and E-cydopropylesters or -ketones are usually formed, but only the Z-esters exhibit an interesting enantioselectivity. However, if intramolecular cyclopropanation of allyl diazoacetates is performed with ligand 3.61, a single isomer is formed with an excellent enantiomeric excess [936,937], The same catalyst also provides satisfactory results in the cyclopropanation of alkynes by menthyl diazoacetate [937, 939] or in the intramolecular insertion of diazoesters into C-H bonds [940]. 

Simkhovich L, Mahammed A, Goldberg I, Gross Z (2001) Synthesis and characterization of germanium, tin, phosphorous, iron and rhodium complexes of tris(pentafluorophenyl)corrole, and the utilization of the iron and rhodium corroles as cyclopropanation catalysts. Chem Eur J 7 1041-1055... 

A critical factor for the undoubtedly most interesting group of catalysts for the reactions of carbenoids, the rhodium complexes, is the price of rhodium. In 1993, it was US 1500 per Troy ounce ( 50 per g), i.e., fourteen times higher than that of copper. Therefore, soluble rhodium carboxylates of terminally functionalized poly(ethenecarboxylic acids) have been developed recently (Bergbreiter et al., 1991 Doyle et al., 1992a). They are effective and recoverable cyclopropanation catalysts. 

The cyclopropanation with diazo compounds via decomposition is amenable to asymmetric induction using chiral metal chelates. Rhodium complexes of 3(S)-phthalimido-2-piperidinone and Al-(arenesulfonyl)proline 44 are typical. The latter catalyst is suitable for generating alkenylcarbenoids. For intramolecular reactions, the cognate complex 45 and the semicorrin-copper 46 are effective. 

Enantioselective C-H insertion reactions have been successfully performed by various rhodium catalysts over a broad range of substrates in both an intramolecular and an intermolecular manner. Of the many examples reported, a few are highhghted here. McKervey and coworkers have obtained good diastereoselectivity and enantioselectivity in the C-H insertion reaction of compound (9.91) catalysed by rhodium complex (9.92). Intramolecular cyclopropanation is not competitive in this case, since the alkene moiety is too remote. 

The usual cyclopropanation reagent, diazomethane and diazoacetates, react smoothly with alkenes in the presence of palladium-catalyst to form cyclopropanes. However, it should be noted that rhodium complexes serve in most cases as superior catalyst. Cyclopropanes can be formed from alkenes (dienes) and nucleophiles using oxidative palladium catalysis (Scheme 5-175). In this case, copper(II) salts regenerate the palladium(II) complexes. The vinyl cyclopropanes produced undergo subsequent vinylcyclopropane-cyclopentane rearrangement to form bicycles. ... 

Diazoalkanes were readily decomposed and underwent [2+l]-type cycloaddition to alkenes in the presence of catalytic amounts of rhodium complexes. Rh2(OAc)4 is the simplest complex for the reaction. For example, fluoro-cyclopropane 134 was prepared by the carbene addition to fluoroalkene 133 (Scheme 1.64) [108]. A copper catalyst also catalyzed the addition reaction. 

The rhodium complex of A -heterocyclic carbene 216 was developed as a new catalyst for the cyclopropanation of... 

Ruthenium complexes serve as catalysts for the cyclopropanation in a manner similar to rhodium complexes. For example, the mthenium complex of bisoxiazohnyl thiophene 260 was examined for asymmetric cyclopropanation (Scheme 1.125) [182]. PyBOX-mthenium catalyst 261 promoted the asymmetric cyclopropanation of diazoacetate and good tran -selectivity was observed (Scheme 1.126) [183]. The cycloadduct was converted to BMS-505130 262, a potential serotonin reuptake inhibitor. 

The dirhodium complexes of the general structure 7.64 and 7.65 have been found to be very effective. Note that in these complexes the oxidation state of rhodium is 2+, and there is a single metal-metal bond between the two metal atoms. A copper (I) complex such as 7.66 having a chiral bis-oxazoline ligand is an early example of a stereoselective cyclopropanation catalyst. In all these complexes L is a weakly coordinating ligand, usually a solvent molecule. 

Both rhodium and osmium porphyrins are active for the cyclopropanation of alkenes. The higher activity of the rhodium porphyrin catalysts can possibly be attributed to a more reactive, cationic carbene intermediate, which so far has defied isolation. The neutral osmium carbene complexes are less active as catalysts but the mono- and bis-carbene complexes can be isolated as a result. 

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