Catalyst, olefin cyclopropanation

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

In considering catalyzed olefin-cyclopropane interconversions, an important question arises concerning thermodynamic control and the tendency (or lack thereof) to attain a state of equilibrium for the system. Mango (74) has recently estimated the expected relative amounts of ethylene and cyclopropane for various reaction conditions and concluded that the reported results were contrary to thermodynamic expectation. In particular, the vigorous formation of ethylene from cyclopropane (16) at -78°C was stated to be especially unfavored. On the basis of various reported observations and considerations, Mango concluded that a reaction scheme such as that in Eq. (26) above (assuming no influence of catalyst) was not appropriate, because the proper relative amounts of cyclopropanes and olefins just do not occur. However, it can be argued that the role of the catalyst is in fact an important element in the equilibration scheme, for the proposed metal-carbene and [M ] species in Eq. (26) are neither equivalent nor freely interconverted under normal reaction conditions. Consequently, all the reaction pathways are not simultaneously accessible with ease, as seen in the published literature, and the expected equilibria do not really have an opportunity for attainment. In such a case, absence of thermodynamic control should not a priori deny the validity of Eq. (26). 

Various approaches to epoxide also show promise for the preparation of chiral aziridines. Identification of the Cu(I) complex as the most effective catalyst for this process has raised the possibility that aziridination might share fundamental mechanistic features with olefin cyclopropanation.115 Similar to cyclo-propanation, in which the generally accepted mechanism involves a discrete Cu-carbenoid intermediate, copper-catalyzed aziridation might proceed via a discrete Cu-nitrenoid intermediate as well. 

Dinuclear Rh(II) compounds are another class of effective catalysts (227). Electrophilic carbenes formed from diazo ketones and dimeric Rh(II) carboxylates undergo olefin cyclopropanation. Chiral Rh(II) carboxamides also serve as catalysts for enantioselective cyclopropanation (Scheme 95) (228). The catalysts have four bridging amide ligands, and... 

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. 

Diazo ester/rhodium(II) carboxylate combinations other than EDA/Rh2(OAc)4 have been tested It turned out that the solubility of the rhodium(II) carboxylate greatly influenced the efficiency of cyclopropanation. For the reaction of monoolefins with ethyl diazoacetate, markedly higher yields than with Rh(II) acetate were obtained with the better soluble rhodium(II) butanoate and rhodium(II) pivalate, the latter one being soluble even in pentane. However, only poor yields resulted from the use of rhodium(Il) trifluoroacetate, even though this compound is readily soluble, Rh CCFjCOO), in contrast to the other rhodium(II) carboxylates, is able to form 1 1 complexes with olefins particularly with electron-rich ones thus, competition of olefin and diazo compound for the only available coordination site at the metal atom could be responsible for the reduced catalytic action of Rh2(CF3COO)4 (as will be seen in Section 4.1, this complex is an excellent catalyst for cyclopropanation of aromatic substrates). The diazoester substituent also has some influence on the yields. Increasing yields were obtained in the series methyl ester, ethyl ester, n-butyl... 

Cycloheptatriene, as an example of a conjugated triene, is mainly cyclopropanated at an outer double bond (Scheme 6). This is true for Rh2(OAc)4, Cu(OTf)2 and Pd(OAc)2, but the highest yield is obtained again with the rhodium catalyst Twofold cyclopropanation occurs to only a minor extent, as long as an excess of olefin is applied. With equal amounts of diazo ester and cycloheptatriene, double cyclopropanation increases and even traces of the triply cyclopropanated triene are found with Rhj(OAc and CuCOTflj. This behavior essentially parallels the earlier... 

Cyclopropanation of C=C bonds by carbenoids derived from diazoesters usually occurs stereospecifically with respect to the configuration of the olefin. This has been confirmed for cyclopropanation with copper s.si.eo.ss) palladium and rhodium catalysts However, cyclopropanation of cw-Dj-styrene with ethyl diazoacetate in the presence of a (l,2-dioximato)cobalt(II) complex occurs with considerable geometrical isomerization Furthermore, CuCl-catalyzed cyclopropanation of cis-2-butene with co-diazoacetophenone gives a mixture of the cis- and rrans-1,2-dimethylcyclopropanes... 

There are examples of all metals from groups 8 to 11 to catalyze the transfer of a carbene group from a diazo compound to organic substrates. One of the most studied transformation is the olefin cyclopropanation reaction, " for which the use of Tp ML catalysts has provided valuable improvement. Thus, the diastereoselectivity of this reaction, that usually leads to mixtures of both cis and trans isomers, was directed toward the d.y-cyclopropane with the complex Tp Cu(thf) (hydrotris [3-mesitylpyrazolyl]borate) as the catalyst, affording a 98 2 cisdrans mixture with styrene (Scheme 5) and ethyl diazoacetate (EDA) as the carbene source. Other olefins were also cyclopropanated with the preferential formation of the cis isomer. The catalysts can be prepared in situ by mixing a Cu(I) source and the MTp salt. Also, the Tp Cu(NCMe) complex has been employed as catalyst in a fluorous phase for the styrene cyclopropanation reaction. ... 

With regard to the use of heterogeneous catalysts, copper bronze is a traditional catalyst in cyclopropanation reactions [7] and the use of zeolite CuNaX in the reactions of ethyl diazoacetate with several olefins has been described [8]. 

It has been reported that it is possible to fix the preformed copper poly(pyrazolyl)borate complexes Cu(Tp ) and Cu(pzTp) on silica gel and use them under heterogeneous conditions as catalysts for the olefin cyclopropanation reaction. The catalytic activity is similar to that found in homogeneous conditions as a consequence of a ligand-to-support interaction that likely involve the hydroxyl groups of the silica gel surface and the borohydride B-H or the nitrogen atom of a pyrazolyl ring.537... 

The [Cu(Bp)] system has been employed to investigate kinetics of the ethyl diazoacetate decomposition reaction in the presence or absence of olefin. The available data have allowed the proposition for a copper-mediated olefin cyclopropanation reaction. It has been proposed that the real catalyst is a 14-electron species, independent of the nature of the ligand bonded to the copper center.50... 

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... 

Early Copper Catalyst. Since a study of copper dust catalyzed decomposition of ethyl diazoacetate (EDA) in 1906 was reported (10), copper bronze catalyzed cyclopropanations with diazo compoimd decomposition have been developed gradually. Although the insoluble copper has been a popular catalyst for cyclopropanation of olefins for more than six decades (11), their many limitations, such as high catalyst loading, elevated temperature, and moderate yield of cyclopropane, have generally not been surmounted. 

Apart from cyclopropenation, catalytic aziridination with nitrene transfer to olefins is generally considered an analogue reaction of metal-catalyzed carbene-transferred cyclopropanation. Aziridination and cyclopropanation are proposed to share fundamental mechanistic features. Many of the catalysts that were successfully applied in aziridination are also efficient catalysts for cyclopropanation. For... 

The use of diazo compounds has also attracted some attention. NHC-Au catalysts, and especially those bearing the IPr and IMes ligands, mediated the decomposition of diazo species, leading to organogold carbenes, which could be observed in the gas phase. Once formed, the NHC-Au carbene could participate in a variety of reactions that include insertions into X-H bonds (X = C, N, Buchner reactions, ° and olefin cyclopropanation. ... 

Although copper complexes are widely used as catalysts for cyclopropanation of alkenes with alkyl diazoacetates they are often poor catalysts for reactions with substituted olefins. Rhodium(ii) carboxylates, however, are highly efficient for this reaction (Equation 7). The soluble carboxylates, such as the butanoate and pivalate, are particularly effective catalysts giving excellent yields for a variety of substituted monoenes and dienes. It is interesting to note that the oxidation state of the rhodium plays an important role as can be concluded from the fact that only very low yields of cyclopropanation products are obtained in the presence of Rh oi Rh complexes. 

Epoxidation of aldehydes and ketones is the most profound utility of the Corey-Chaykovsky reaction. As noted in section 1.1.1, for an a,P-unsaturated carbonyl compound, 1 adds preferentially to the olefin to provide the cyclopropane derivative. On the other hand, the more reactive 2 generally undergoes the methylene transfer to the carbonyl, giving rise to the corresponding epoxide. For instance, treatment of P-ionone (26) with 2, derived from trimethylsulfonium chloride and NaOH in the presence of a phase-transfer catalyst Et4BnNCl, gave rise to vinyl epoxide 27 exclusively. ... 

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