Nucleophilic metal-carbene

Based on a detailed investigation, it was concluded that the exceptional ability of the molybdenum compounds to promote cyclopropanation of electron-poor alkenes is not caused by intermediate nucleophilic metal carbenes, as one might assume at first glance. Rather, they seem to interfere with the reaction sequence of the uncatalyzed formation of 2-pyrazolines (Scheme 18) by preventing the 1-pyrazoline - 2-pyrazoline tautomerization from occurring. Thereby, the 1-pyrazoline has the opportunity to decompose purely thermally to cyclopropanes and formal vinylic C—H insertion products. This assumption is supported by the following facts a) Neither Mo(CO)6 nor Mo2(OAc)4 influence the rate of [3 + 2] cycloaddition of the diazocarbonyl compound to the alkene. b) Decomposition of ethyl diazoacetate is only weakly accelerated by the molybdenum compounds, c) The latter do not affect the decomposition rate of and product distribution from independently synthesized, representative 1-pyrazolines, and 2-pyrazolines are not at all decomposed in their presence at the given reaction temperature. 

The picture is quite different for nucleophilic metal-carbene complexes. Here, contributing structures 10 and 13 seem to make the most contribution to the overall structure. Support for this observation comes from temperature-dependent NMR measurements8 of the M-C rotational barriers of various Ta-carbene complexes. The values obtained range from 12 to 21 kcal/mol, and seem to indicate considerable double bond character (structure 10). 

A decade after Fischer s synthesis of [(CO)5W=C(CH3)(OCH3)] the first example of another class of transition metal carbene complexes was introduced by Schrock, which subsequently have been named after him. His synthesis of [((CH3)3CCH2)3Ta=CHC(CH3)3] [11] was described above and unlike the Fischer-type carbenes it did not have a stabilizing substituent at the carbene ligand, which leads to a completely different behaviour of these complexes compared to the Fischer-type complexes. While the reactions of Fischer-type carbenes can be described as electrophilic, Schrock-type carbene complexes (or transition metal alkylidenes) show nucleophilicity. Also the oxidation state of the metal is generally different, as Schrock-type carbene complexes usually consist of a transition metal in a high oxidation state. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

The reaction of isocyanide complexes with nucleophiles gives metal-carbene complexes [49], which constitute an important branch of organometallic chemistry and are effective catalyst systems for a variety of processes [50, 51]. 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

Exclusive O/H insertion takes place in the Rh2(OAc)4-catalyzed reaction of diethyl diazomalonate with a,(J-unsaturated y-hydroxyesters 167 a-c163). This is not surprising in view of the reluctance of electrophilic metal carbenes to add to electron-poor double bonds (see Sect. 2.3.2). However, the more electron-rich double bond of p-methoxybenzyl clavulanate 168 also cannot compete with the O—H function for the same carbenoid 164). The steric situation at the trisubstituted double bonds of 167 and 168 may be reason enough to render an attack there highly unfavorable as compared to the easily accessible O—H function, no matter how nucleophilic the double bond is. 

The EfZ ratio of stilbenes obtained in the Rh2(OAc)4-catalyzed reaction was independent of catalyst concentration in the range given in Table 22 357). This fact differs from the copper-catalyzed decomposition of ethyl diazoacetate, where the ratio diethyl fumarate diethyl maleate was found to depend on the concentration of the catalyst, requiring two competing mechanistic pathways to be taken into account 365), The preference for the Z-stilbene upon C ClO -or rhodium-catalyzed decomposition of aryldiazomethanes may be explained by the mechanism given in Scheme 39. Nucleophilic attack of the diazoalkane at the presumed metal carbene leads to two epimeric diazonium intermediates 385, the sterically less encumbered of which yields the Z-stilbene after C/C rotation 357,358). Thus, steric effects, favoring 385a over 385 b, ultimately cause the preferred formation of the thermodynamically less stable cis-stilbene. 

Some of these approaches were attempted by Grubbs et al. 143 171. In later studies [45, 46], the phenoxyimine ligands used in their initial study were replaced with nucleophilic heterocyclic carbene (NHC) ligands with the objective of pushing more electrons into the metal center to reduce the tendency of a last-inserted acrylate... 

Such a protonated carbonyl should be more susceptible to nucleophilic hydride attack to give 26. Repetition of the process could then yield a metal carbene species and water ... 

To summarize briefly, our approach involves initial attack by a relatively nucleophilic metal hydride on coordinated CO. Such reactivity has been demonstrated repeatedly for main-group metal hydrides perhaps the most elegantly worked-out system involves CpRe(C0)2(N0)+ (Cp = Tl-C H ) which, under varying conditions, can be converted to an entire range of products containing CO at different stages of reduction, including formyl, carbene, hydroxymethyl and methyl species (Scheme l). Reactions lead-... 

Transition metal carbene complexes have broadly been classified into Fischer-type and Schrock-type carbene complexes. The former, typically low-valent, 18-electron complexes with strong 7t-acceptors at the metal, are electrophilic at the carbene carbon atom (C ). On the other hand, Schrock-type carbene complexes are usually high-valent complexes with fewer than 18 valence electrons, and without n-accepting ligands. Schrock-type carbene complexes generally behave as carbon nucleophiles (Figure 1.4). 

Mechanistically similar to the reaction of nucleophilic metallates with a-haloimines is their reaction with amides. In this case formation of the carbene complex requires treatment with a silyl chloride (Figure 2.8) [42,125,126]. 

We have previously stated that an ylide could be considered the coupling product of a singlet carbene with a nucleophile. Therefore, it seems logical that the reaction of a metallic carbene with a nucleophile would give a metal bonded ylide and, in fact, this is a quite useful method to prepare metallated ylides. Even more, in some cases coordinated ylides have been used as masked caibenes [85]. Complexes (26) (Scheme 9, M = Cr, W), which contain a pyridinium yhde, are conveniently prepared by reaction of the corresponding carbenes [(CO)5M=C(OEt) R] with 1,2- or 1,4-dihydropyridines. During the reaction an unprecedented hydride... 

All synthetic methods described up to now (ligand displacement, acac or halo-methyl precursors, metal-bonded carbene + nucleophile, metal-bonded nucleophile + carbene) result in a metal-bonded ylide through the Ca atom. The reactivity of ylides toward metallic systems is, however, greater than anticipated and other reaction pathways could compete with simple C-bonding. 

The attack of nucleophiles on unsaturated ligands or functional groups bonded to metallic centers, exemplified in Scheme 9 (reaction of metallic carbenes with phosphines or pyridines) or in Scheme 15 (Wittig reaction) can be extended to a wide variety of reagents. Two main groups of reactions can be considered (1) those in which the nucleophile is an ylide and (2) those in which the nucleophile is a phosphine (and less commonly other nucleophiles). Usually these reactions give metallated ylides (type III), that is, species in which the ylide substituents are metallic centers. 

It is well known that metal carbenes can be classified as Fisher and Schrock carbenes. The classification is mainly based on the n electron density distribution on the M = C moiety (Scheme 4.2). On the basis of the n electron density distribution, carbene complexes of the Fisher-type (E) are normally electrophilic at the carbene carbon while carbene complexes of the Schrock-type (F) are nucleophilic at the carbene carbon. Similarly, metal vinylidenes could also be classified into the two types Fisher-type (G) and Schrock-type (H). The majority of isolated metal vinylidenes belong to the Fisher-type. On the basis of the 7t electron density distribution shown in... 

There is a variety of reagents that undergo carbene-related transformations that do not fit into the categories of nucleophilic and electrophilic metal carbenes described earlier. Those that are the most versatile for organic synthesis, like the Tebbe... 

Lewis bases (B ) that can occupy the open coordination site inhibit catalytic activity. The electrophilic nature of the metal carbene is seen in its subsequent reactions with nucleophiles (S ), which occur with the transfer of the carbene entity from the metal to a nucleophile without ever having generated or transformed an actual free carbene. 

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