Nitrogen-substituted carbene complexes

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

N-Substituted carbene complexes show y(CN) absorption in the 1470-1620 cm 1 range of the IR spectrum. These data are consistent with the crystallographic evidence for substantial carbon-nitrogen multiple bonding in these compounds. 

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

The reaction of acceptor-substituted carbene complexes with alcohols to yield ethers is a valuable alternative to other etherification reactions [1152,1209-1211], This reaction generally proceeds faster than cyclopropanation [1176], As in other transformations with electrophilic carbene complexes, the reaction conditions are mild and well-suited to base- or acid-sensitive substrates [1212], As an illustrative example, Experimental Procedure 4.2.4 describes the carbene-mediated etherification of a serine derivative. This type of substrate is very difficult to etherify under basic conditions (e.g. NaH, alkyl halide [1213]), because of an intramolecular hydrogen-bond between the nitrogen-bound hydrogen and the hydroxy group. Further, upon treatment with bases serine ethers readily eliminate alkoxide to give acrylates. With the aid of electrophilic carbene complexes, however, acceptable yields of 0-alkylated serine derivatives can be obtained. 

Nitrogen often appears attached directly to carbon in Fischer-type carbene complexes. Equations 10.823 and 10.924 provide two examples of N-substituted carbene complex synthesis. The first procedure involves attack by the amide on one of the carbonyls (analogous to equation 10.6) followed by alkylation. 

The links between formation of an iron-alkyl complex and irreversible destruction of the heme moiety have not been forged, but model studies with diaryl- and carbethoxy-substituted carbene complexes suggest that the halogenated carbenes may shift to form a bond with a nitrogen of the porphyrin. The resulting A -haloaIkyl adduct are likely to undergo water-dependent hydrolysis and might therefore not be detected by the methods used to isolate other A -alkyl porphyrins. However, the formation of alternative reactive species that attack the protein or the heme cannot be ruled out. 

Alkoxy-substituted carbene complexes serve as valuable precursors for a wide variety of metal-carbene complexes as the carbene carbon atom is very susceptible to nucleophilic attack and the alkoxy group is a better leaving group than the entering nitrogen, sulfur, or carbon nucleophiles. 

The use of metal-carbene complexes as an amino-protecting group in peptide synthesis has been reported by Fischer and Weiss (1973). The free amino group of an amino acid ester readily displaces the alkoxy group of an alkoxy-substituted carbene complex. The nitrogen atom of the resulting N-substituted carbene complex is nonbasic and nonnucleophilic because it bears a partial positive charge. The low reactivity of the amino-substituted carbene complex allows a series of reactions to be carried out to construct a peptide chain. Finally, the completed peptide chain can be removed from the carbene complex by treatment with trifluoroacetic acid at 20°. Scheme 16 illustrates the variety of reactions that can be carried out in the presence of the metal-carbene functionality. 

Diamino-substituted complexes of type 37 were first obtained by Fischer et al. [12] in two steps via the 1,2-addition-elimination product 34 from di-methylamine and 35 (Scheme 6). The (3-aminoallenylidene)chromium complexes 36, which can be prepared either from 33 [47,48] or directly from 35 [33], can also be transformed to l,3-bis(dialkylamino)-substituted complexes of type 37 (e.g., R2=z Pr) by treatment with dimethylamine in excellent yields [33]. Although the complex 37 is accessible by further reaction of the complex 34 with dimethylamine, and 34 itself stems from the reaction of 35 with dimethylamine, the direct transformation of 33 to 37 could not be achieved [12]. In spite of this, heterocyclic carbene complexes with two nitrogens were obtained by reactions of alkynylcarbene complexes 35 with hydrazine [49] and 1,3-diamines [50]. 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

When planning reactions of thiocarbonyl compounds with electrophilic carbene complexes it should be taken into aceount that thiocarbonyl compounds can undergo uncatalyzed 1,3-dipolar cycloaddition with acceptor-substituted diazomethanes to yield 1,3,4-thiadiazoles. These can either be stable or eliminate nitrogen to yield thiiranes or other products similar to those resulting from thiocarbonyl ylides [1338]. 

As discussed above, dialkylamino carbene complexes result in the formation of indenes due to the increased donor ability of the nitrogen compared to the oxygen heteroatom. The formation of benzannulation products is favored, however, if the electron density at nitrogen is lowered by substitution with electron-withdrawing acyl groups [33]. The example in Scheme 12 demonstrates the effect. 

The (cyclopropylamino)carbene complex 15 reacted with diphenylalkyne to give a nitrogen ylide, which was converted via thermal A -cyclopropyl to C-cyclopropyl rearrangement and decomplexation to a cyclopropyl-substituted lactam. 

Recently Adams in reviewing [13] the metal cluster complexes containing heteroatom-substituted carbene ligands, attributed the absence of bridging aminocarbene derivatives (A) to the destabilizing effect of the strong ji-donation from the lone pair of the nitrogen atom to the empty p-orbital on the carbene carbon atom. 

Figure 5.1 Metal complexes comprising the classical imidazol-2-ylidene ligand (A) and representative non-classical carbene ligands (B-N), including normal carbenes (B-E), abnormal carbenes (F-I), remote carbenes (E, G, I), cyclic alkyl(amino)carbenes (J), acyclic carbenes (K, L, M) and amino(ylide)-carbenes (N). Substituted nitrogen centres may be replaced by oxygen or sulfur. The M=C bond representation— while strongly over-emphasizing the differences in the nature of the metal-carbon bond in these non-classical carbene complexes— was used to accentuate normal and abnormal bonding.

Silyl-substituted diazomethanes are regarded as a silylcarbene precursor or as a diazomethane equivalent, as they coordinate, after loss of nitrogen, to transition metals to generate metal-carbene complexes and participate in a variety of metal-catalyzed reactions and the silyl substituent is readily lost in many cases after the total transformation. For example, nickel-catalyzed cycloaddition reactions of dienynes proceed with incorporation of a silylmethyl group to give seven-membered carbocycles having an allylsilane functionality (Scheme 3-69). ... 

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