Cyclopropanation catalysts

Enantioselective Cyclopropanation. Enantioselective versions of both copper and rhodium cyclopropanation catalysts are available. The copper-imine class of catalysts is enantioselective when chiral imines are used. Some of the chiral ligands that have been utilized in conjunction with copper salts are shown in Scheme 10.10. 

Another remarkable property of iodorhodium(III) porphyrins is their ability to decompose excess diazo compound, thereby initiating carbene transfer reactions 398). This observation led to the use of iodorhodium(III) me.vo-tetraarylporphyrins as cyclopropanation catalysts with enhanced syn anti selectivity (see Sect. 2.2.3) s7, i°o) as wep as catalysts for carbenoid insertion into aliphatic C—H bonds, whereby an unusually high affinity for primary C—H bonds was achieved (see Sect. 6.1)287). These selectivities, unapproached by any other transition metal catalyst,... 

A remarkable complex (33) with a C2-symmetric semicorrin ligand has been recently developed by Pfaltz and coworkers.64 A copper(II) complex was used as a procatalyst, but (33) was shown to be the active cyclopropanation catalyst. As shown in Table 3, this complex resulted in spectacular enantioselecti-vities in the range of 92-97% ee. Once again, the (15,35,4/ )-menthyl group attenuated the selectivity. Unfortunately, even though respectable yields were obtained with dienes and styrenes, the reaction with 1-heptene was rather inefficient. 

Callot and co-workers established in 1982 that iodorhodium(III) porphyrin complexes could be used as cyclopropanation catalysts with diazo esters and alkenes with c/.s-disubstituted alkenes these catalysts provide preferential production of cis(syn) disubsdtutcd cyclopropancs (syn/anti up to 3.3 with 1,4-cyclohexadiene) [72], More recently, chiral porphyrins have been designed and prepared by Kodadek and co-workers [73], and their iodorhodium(lll) complexes have been examined for asymmetric induction in catalytic cyclopropanation reactions [74,751. The intent here has been to affix chiral attachments onto the four porphyrin positions that are occupied in tetraphenylporphyrin by a phenyl group. Iodorhodium(III) catalysts with chiral binaphthyl (27, called chiral wall porphyrin [74]) and the structurally analogous chiral pyrenyl-naphthyl (28,... 

Even before Aratani introduced his chiral salicylaldimine copper catalysts, Nakamura and Otsuka reported in 1974 that chiral bis(a-camphorquinonedioximato)cobalt(II) (29) and related complexes were effective enantioselective cyclopropanation catalysts [76], and they more fully described the preparation, characteristics, and uses of these catalysts in 1978 [77], Optical yields as high as 88% were achieved for the cyclopropanation of styrene with neopentyl diazoacetate, and chemical yields greater than 90% were obtained in several cases. 

The C2-symmetric 2,6-bis(2-oxazolin-2-yl)pyridine (pybox) ligand was originally applied with Rh for enantioselective hydrosilylation of ketones [79], but Nishiyama, Itoh, and co-workers have used the chiral pybox ligands with Ru(II) as an effective cyclopropanation catalyst 31 [80]. The advantages in the use of this catalyst are the high enantiocontrol in product formation (>95 % ee) and the exceptional diastereocontrol for production of the trans-cyclopropane isomer (>92 8) in reactions of diazoacetates with monosubstituted olefins. Electronic influences from 4-substituents of pyridine in 31 affect relative reactivity (p = +1.53) and enantioselectivity, but not diastereoselectivity [81]. The disadvantage in the use of these catalysts, at least for synthetic purposes, is their sluggish reactivity. In fact, the stability of the intermediate metal carbene has allowed their isolation in two cases [82]. 

In 2001, a modified procedure for sulfur ylide-catalyzed epoxidation, aziridination and cyclopropanation was introduced by Aggarwal and co-workers that utilized the generation of the diazo compounds in situ from tosyl hydrazone salts at 40 °C in the presence of a phase-transfer catalyst [46, 79]. (For experimental details see Chapter 14.12.1). Using this modified protocol, sulfide 4 was shown to be effective for epoxidation and aziridination (see Sections 10.2.1.4 and 10.3), but was not an effective cyclopropanation catalyst (see Table 10.3). Sulfide 28 was tried instead as it had been shown in achiral studies [96] that six-membered sulfides were more effective than five-membered analogues. This change gave rise to... 

Sulfur, tellurium and nitrogen ylide-catalyzed cyclopropanations have been developed. Enantioenriched cyclopropanes can be synthesized in good yield and moderate to excellent diastereoselectivity. These catalysts are complementary to metal-based cyclopropanation catalysts, and have the potential for further development. 

Although many chiral cyclopropanation catalysts are known, this class of complexes is superior for the alkenes containing vinyl, phenyl, or alkoxycarbonyl groups. Some relevant examples are shown in eq 1-5. In eq 5, the enantiomeric excess of the product is not known due to the absence of enantiomerically pure isomer. The absolute configuration is not known. 

The role of Z11I2 is that an equimolar quantity of the compound drives the Schlenk equilibrium from the reagent bis(iodomethyl)zinc to (iodomethyl)zinc iodide, which is the actual cyclopropanation catalyst and has high reactivity and stereoselectivity [50c,52], The structure of the active catalyst, Zn-bis(sulfonamide) complex XXIV, was characterized by NMR analysis and X-ray study of the structure of its bipyri-dyl complex 66 (Sch. 28) [53]. The Zn-bis(sulfonamide) complex XXIV aggregates in solution and functions as a divalent Lewis acid. 

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. 

In 1981, Noels reported that rhodium(II) carboxylates, originally developed as cyclopropanation catalysts, smoothly catalyze the addition of ethyl diazoacetate (21 equation 9) to a variety of alkanes. " While some differentiation between possible sites of insertion is observed, selectivity is not as high for this carbenoid process as it is for the free radical processes illustrated above. 

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. 

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... 

Treatment of [Rh Cl)2bis(oxozaline)] with AgOTf yields [Rh (Cl)(Tf)bis (oxozaline)], which can be used as an olefin cyclopropanation catalyst. Although the catalytic precursor does not contain a Rh—C bond, the carbene-transfer reaction is highly likely to involve organometaUic intermediates. Catalyst... 

Cross-metathesis reactions are useful for the production of fine chemicals such as synthetic perfumes, prostaglandin intermediates, and insect pheromones. An example of the last is the cross-metathesis of ethyl oleate with 5-decene in the presence of a MoCl5/Si02/Me4Sn catalyst at 90 °C [14], or a Mo03/Si02/cyclopropane catalyst at 50 °C [16], resulting in a cisitrans mixture of ethyl 9-tetradecenoate, an insect pheromone precursor (Eq. 12). 

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 work of Nakamura (Tatsuno et al., 1974 Nakamura et al., 1978a, 1978b) is not only interesting because he used a cobalt complex (8.161) as cyclopropanation catalyst, but also because of enantiomeric selectivity. The latter was high when using dienes and styrene as substrate (8-73), but low with simple alkenes. 

Recent interest in the development of environmentally benign synthesis has driven the development of the polymer-bound cyclopropanation catalyst 92... 

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