Metal carbene chelate

Figure 5.6 Conformational flexibility of transition metal carbene chelate complexes.

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, formally speaking a [3 + 2] cycloaddition between the aldehyde and a ketocarbene, resembles the dihydrofuran formation from 57 a or similar a-diazoketones and alkenes (see Sect. 2.3.1). For that reaction type, 2-diazo-l,3-dicarbonyl compounds and ethyl diazopyruvate 56 were found to be suited equally well. This similarity pertains also to the reactivity towards carbonyl functions 1,3-dioxole-4-carboxylates are also obtained by copper chelate catalyzed decomposition of 56 in the presence of aliphatic and aromatic aldehydes as well as enolizable ketones 276). No such products were reported for the catalyzed decomposition of ethyl diazoacetate in the presence of the same ketones 271,272). The reasons for the different reactivity of ethoxycarbonylcarbene and a-ketocarbenes (or the respective metal carbenes) have only been speculated upon so far 276). 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

Reaction of (butadiene)ZrCp2 (31/32), and substituted Cp variants, with a wide range of metal-carbonyl complexes, generates the chelated metal-carbene complexes 163 (equation 22)163. The crystal structure of a number of these complexes has been determined... 

It has generally been assumed that in olefin metathesis reactions the olefin first coordinates to the metal carbene complex, en route to the formation of the intermediate metallacyclobutane complex, and that after cleavage of this intermediate the newly formed double bond is temporarily coordinated to the metal centre. A number of stable metal-carbene-olefin complexes are known see elsewhere116,117 for earlier references. They are mostly stabilized by chelation of the olefin and/or by heteroatom substituents on the carbene, although some have been prepared which enjoy neither of these modes of stabilization118,119. 

Functionalised carbenes can anchor free carbenes to the metal site, introduce hemilabil-ity, provide a means to immobilise transition metal carbene catalysts, introduce chirality, provide a chelate ligand or bridge two metal centres. NHC can be attached to carbohydrates and camphor, derived from amino acids and purines, and they can be used as organocata-lysts mimicking vitamin B1 or as weak solvent donors in lanthanide chemistry. There are many possibilities which are still only scarcely explored. 

Note The coordination behaviour (bridging versus chelating) of transition metal bis-carbene complexes can be dependent on the oxidation state of the metal centre. Chelating coordination is favoured by the higher oxidation state. 

Several methodologies for the preparation of metal-carbene complexes have been developed (Scheme 2.153). In the best case, imidazolium salts are submitted directly to a solution of a metal complex. The active catalyst is formed under catalytic conditions. Mandatory removal of the acidic proton and subsequent formation of the carbene is carried out in situ and can be promoted by a basic ligand already present in the metal salt. This task can be fulfilled, for example, by acetate in the palladium precursor, which is basic enough to remove the proton and to form the carbene-metal complex. Also, counter-ions of imidazolium salts can participate in the coordination reaction of the newly formed carbene complex by the displacement of less chelating ligands from the metal center [2]. 

The list of rare earth-element multiple bonds is still missing terminal metal carbenes. However, CaveU (Apama et al., 2000), Le Hoch (Cantat et al., 2005), and Liddle (Mills et al., 2010 Chart 4) synthesized formal rare earth carbene complexes by incorporating the carbene in the framework of a chelating bis(iminophosphorano)methylene (N2C ) or bis (diphenylthiophosphinoyl)methylene (S2C ) ligand. Also, P. Arnold reported numerous examples of interesting reactivity of f-block complexes supported by A -heterocyclic carbenes (NHCs Chart 4 Arnold and Casely, 2009 Liddle and Arnold, 2005). 

Carbene Complexes.— Protonation of the chelated iron-carbene complex (59) in the presence of ethylene yields (60), the first example of a metal-carbene-olefin complex of the type postulated as intermediates in olefin metathesis reactions. Hydride abstraction from (61) affords (62). The new carbene complexes (63), (64), and (65) have been prepared by treatment of [Fe(jj-C5H5)(SnPh3)-(CO)(CS)l, [Fe( -C6H6) C(S)OPh (CO)3], and [Fe( -CS3) P(OMe)3 (CO)3] with ethylenediamine, methylfluorosulphonate (followed by methanol and PF e ions),... 

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