Diazo compounds, electrophilic carbene complex reactions

The reaction of Ru(TMP) with ethyl diazoacetate yielded a carbene complex, e.g. Ru(CHC02Et)(TMP) [316], An excess of the diazo compound led to catalytic formation of cis- and trans diethyl maleate in an unexpected ratio of 15 1. The nucleophilic ethyl diazoacetate is proposed to attack the electrophilic carbene complex and produce an intermediate betaine-line species which eliminates both Ru(TMP) and N2 to form the maleates. Similar reactions were observed with Os(TTP) complexes [313a], These reactions are reminiscent of the above-mentioned lability of a putative methyleneruthenium porphyrin. 

Rh and Pd-catalysed Reactions of Diazo Compounds via Electrophilic Carbene Complexes... 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

Remarkably, mononitrosyl iron(—II) complexes displayed great potential in the activation of diazo compormds and carbene-transfer reactions [102]. Generally, the activation of diazo compound can be realized by electrophilic transition metal complexes. However, according to the concept of Umpoirmg [103], the electron-rich, nucleophilic iron(—II) compound 31 is expected to react with diazo compounds of electron-poor carbenes, such as ethyl diazoacetate (Scheme 42). At first the iron center would add the C=N bond of the diazo compound followed by release of N2 and formation of the electrophilic iron carbene moiety. The nitrosyl group in such transformations is assumed to support as an ancillary ligand the N2 release by pulling electron density to the iron center. 

It is well established that the rhodium-carbene species generated upon activation of diazo compounds by rhodium complexes can undergo insertion into a X—H bond (X= C, Si, O, N) to form a new C—X bond under mild conditions (Scheme 3.58). This reaction involves formation of an onium yUde intermediate and subseqnent proton transfer [156]. The high reactivity of these onium ylide species, which can be trapped by an electrophile prior to the proton transfer leading to a second bond formation. 

Diazocarbonyl compounds, especially diazo ketones and diazo esters [19], are the most suitable substrates for metal carbene transformations catalyzed by Cu or Rh compounds. Diazoalkanes are less useful owing to more pronounced carbene dimer formation that competes with, for example, cyclopropanation [7]. This competing reaction occurs by electrophilic addition of the metal-stabilized carbocation to the diazo compound followed by dinitrogen loss and formation of the alkene product that occurs with regeneration of the catalytically active metal complex (Eq. 5.5) [201. 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

For example, in attempts to realize benzannelation reactions, alkyloxy aryl carbene complexes of manganese failed to react with alkynes even in refluxing toluene, and the starting compounds could be recovered [4]. The documented low reactivity of the Mn as opposed to Cr and Mo carbene complexes may in part explain why the electrophilic carbene C-atom and the nucleophilic diazo C-atom tolerate each other in the same molecule. Besides, the bulky substituents at the silicon atom protect it fi"om being attacked by nucleophiles leading to desilylation as reported for trimethylsilyl substituted Cr carbene complexes [5]. 

The metal-carbene complexes postulated as intermediates in transition metal-catalyzed reactions of diazo compounds are electrophilic species (especially if they are derived from a-diazocarbonyl compounds). Accordingly, electron-rich olefins are the most suitable substrates for copper-catalyzed cyclopropanations, whereas electron-poor substrates such as a,P-unsaturated carbonyl compounds in general are not sufficiently reactive. 

The understanding of this catalysis started in 1952, shortly after the concept of carbenes was introduced (see Sect. 8.1). Yates postulated that transition-metal catalysts react with diazo compounds by formation of transient electrophilic metal carbenes, because that complex can be depicted as a metal-stabilized carbocation (8.104). Doyle (1986 a) proposed the catalytic cycle (8-46) for the formation of the carbenoid 8.104 and its reaction with an electron-rich substrate S . The reagent S is, first of all, an alkene in cyclopropanation, but can also belong to other groups of compounds, to be discussed later in this section. 

Many rhodium(II) complexes are excellent catalysts for metal-carbenoid-mediated enantioselective C-H insertion reactions [101]. In 2002, computational studies by Nakamura and co-workers suggested the dirhodium tetracarboxylate catalyzed diazo compounds insertion reaction to alkanes C-H bonds proceed through a three-centered hydride-transfer-like transition state (Fig. 25) [102]. Only one rhodium atom of the catalyst is involved in the formation of rhodium carbene intermediate, while the other rhodium atom served as a mobile ligand, which enhanced the electrophilicity of the first one and facilitate the cleavage of rhodium-carbon bond. In this case, the metal-metal bond constitutes a special example of Lewis acid activation of Lewis acidic transition-metal catalyst. 

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