Carbonyls, metal Nucleophilic attack

The formation of imine or Schiff base metal complexes can be effected very readily by arranging for reaction of the amine with the carbonyl compound to take place in the presence of the metal ion. Nucleophilic attack of the amine on the carbonyl group is enhanced by polarization of the carbonyl group by coordination of the oxygen atom to the metal ion. Furthermore, the reaction is reversible unless steps are taken to effect dehydration of the intermediate hydroxy amine, and coordination of the product imine is an important factor in the promotion of the reaction by suitable metal ions. 

In a large number of carbonyls nucleophilic attack seems more likely at the carbonyl-carbon than at the metal. There is often good evidence from products for such attack at the ligand, and these examples are therefore discussed in a later section on reactions of co-ordinated ligands (p. 301). 

Recently, metal-containing carbonyl ylides have attracted much attention as a class of powerful dipoles for 1,3-dipolar cycloaddition reactions to assemble heterocyclic compounds, which was first reported by Iwasawa et al. in 2001 [20]. The in situ generation of metal-containing carbonyl ylides involves the electrophilic activation of an alkyne by a transition metal toward nucleophilic attack of the carbonyl group. One representative example for the synthesis of naphthalene derivatives is shown in... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The reactions of nucleophilic reagents with cationic and uncharged metal carbonyl complexes have received much attention in the past, and it is not surprising that these studies have now been extended to isocyanide metal complexes. Different products in these reactions can arise by three general routes these include ligand substitution, reactions involving attack at a ligand, and reduction of the metal complex. All have been observed in reactions with metal isocyanide complexes. 

It has been noted (Section II,B,1) that reactions between transition metal carbonyl anions and silicon halides often fail to produce species containing silicon-transition metal bonds, and that such failure has been ascribed to nucleophilic attack by carbonyl oxygen. It is therefore interesting that compounds containing Si—O—C—transition metal linkages have recently been isolated from such reactions [Eqs. (105) (R = Me, Ph) 183) and (106)... 

The Reformatsky reaction is a classical reaction in which metallic zinc, an a-haloester, and a carbonyl compound react to give a (i-hydroxyester.162 The zinc and a-haloester react to form an organozinc reagent. Because the carboxylate group can stabilize the carbanionic center, the product is essentially the zinc enolate of the dehalogenated ester.163 The enolate effects nucleophilic attack on the carbonyl group. 

Studies of the base-hydrolysis mechanism for hydrolysis of technetium complexes have further been expanded to an octahedral tris(acetylacetonato)techne-tium(III) [30], Although a large number of studies dealing with base hydrolysis of octahedral metal(III) complexes have been published [31], the mechanism of the tris(acetylacetonato)metal complex is still unclear. The second-order base hydrolysis of the cationic complex tris(acetylacetonato)silicon(IV) takes place by nucleophilic attack of hydroxide ion at carbonyl groups, followed by acetylacetone liberation, and finally silicon dioxide production [32], The kinetic runs were followed spectrophotometrically by the disappearance of the absorbance at 505 nm for Tc(acac)3. The rate law has the following equation ... 

Fischer carbenes characteristically contain a number of electron-withdrawing carbonyl ligands while the typical Ru, Os, or Ir carbene complexes described above frequently contain several cr-donor ligands. The metal centers in these former compounds, then, are rather electron-deficient, with nucleophilic attack at Ca being a favorable reaction. 

The logical basis for employing metal carbonyls as catalysts would be the CO activation through coordination which facilitates nucleophilic attack by water or OH" (6). The key step then may be the formation of a hydroxy-carbonyl species followed by 6-hydrogen elimination reaction (eq. 2,3). Another important elemental re-... 

Decarbonylation of the acyl is likely to be metal-assisted (Ru11) giving rise to a Ru carbonyl, which is subsequently decarbony-lated by nucleophilic attack by nBu3P. This phosphine can displace coordinated carbonyl, as exemplified by reaction 3 ... 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

Nesmeyanov has provided interesting examples of apparent intramolecular nucleophilic attack by amine on carbonyl ligands (37). Angelici (38,39) has demonstrated that amine attack on cationic metal carbonyl complexes is a general reaction resulting in the formation of carbamoyl complexes ... 

However, while ruthenium carbonyl was found to decompose the formate ion in basic media, the rate was slower (<0.1 mmol trimethyl ammonium formate to H2 and C02 per hour) than the rate of the water-gas shift reaction (>0.4 mmol H2/hr) at 5 atm CO and 100 °C. Increasing CO pressure decreased the formate decomposition rate. However, it was observed that increasing the CO pressure from 5 atm CO to 50 atm increased the H2 production rate to 10 mmol/hr. They proposed, in a similar manner to Pettit et al.,34 a mechanism that involved nucleophilic attack by amine (instead of hydroxide). Activation of the metal carbonyl (e.g., Ru3(CO) 2 cluster to Ru(CO)5) was suggested to be favored by dilution, increases in CO pressure, or, in the case of Group VIb metal carbonyl complexes, photolytic promotion. The mechanism is shown below in Scheme 9 ... 

A somewhat related problem is the nature of the bridged carbonyl group between two metal centers. Obvious correlations with organic ketonic behavior in general provide difficulties. In general nucleophilic attack by OR- (R = H or Me) does appear to occur at the carbon center. For R = Me, stable M- C02R complexes may often be isolated, but for R = H, transfer of hydrogen to the metal with elimination of C02 occurs readily, to yield the hydridocarbonyl. 

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