Carbene transfer reactions from copper complexes

As already mentioned for rhodium carbene complexes, proof of the existence of electrophilic metal carbenoids relies on indirect evidence, and insight into the nature of intermediates is obtained mostly through reactivity-selectivity relationships and/or comparison with stable Fischer-type metal carbene complexes. A particularly puzzling point is the relevance of metallacyclobutanes as intermediates in cyclopropane formation. The subject is still a matter of debate in the literature. Even if some metallacyclobutanes have been shown to yield cyclopropanes by reductive elimination [15], the intermediacy of metallacyclobutanes in carbene transfer reactions is in most cases borne out neither by direct observation nor by clear-cut mechanistic studies and such a reaction pathway is probably not a general one. Formation of a metallacyclobu-tane requires coordination both of the olefin and of the carbene to the metal center. In many cases, all available evidence points to direct reaction of the metal carbenes with alkenes without prior olefin coordination. Further, it has been proposed that, at least in the context of rhodium carbenoid insertions into C-H bonds, partial release of free carbenes from metal carbene complexes occurs [16]. Of course this does not exclude the possibility that metallacyclobutanes play a pivotal role in some catalyst systems, especially in copper-and palladium-catalyzed reactions. 

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). 

A very interesting amido functionahsed carbene was prepared by Legault et al. [116] from A-mesitylimidazole and 0-(2,4-dinitrophenyl)hydroxylamine, an electrophilic ami-nation reagent [117]. The exo-amino group is subsequently acylated to afford a zwitterionic amido functionalised carbene (see Figure 4.38). Reaction with silver(l) acetate and sodium carbonate [a rare variant of the silver(I) oxide method] yields the silver(l) carbene complex as a dimer with a Ag-Ag bond. The silver(l) carbene complex can be used as a carbene transfer reagent to synthesise the homoleptic monomeric copper(Il) carbene complex. 

Cyclopropanation of olefins is currently performed by direct transition metal-catalyzed carbene transfer from a diazo compound to the olefin. Dirhodium(II) carboxylates and carboxamidates have proved to be the catalysts of choice. Other rhodium compounds, such as Rh (CO),6, Rh2(BF4)4, and rhodium(III) porphyrins, have been also investigated, but did not show better reactivity, while rhodium(I) compounds have never been successful [66]. Other complexes containing copper or ruthenium have been tested in cyclopropanation reactions, but have never shown better reactivity or selectivity than rhodinm(II) compounds [67]. 

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. 

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