Nucleophilic reactions Carbonyl ligands

It has been pointed out that the types of solvents which are used here, are not generally such as would enter into strong association with the substrate. The molecularity of the substitution reaction may then stand more chance of being an operational concept. Amongst the binary carbonyls, the only systems which have been extensively studied have been nickel tetracarbonyl and the hexacarbonyls of group VI. For the former, the observation of a first-order rate is at least consistent with a rate-determining dissociation of one carbonyl ligand followed by reaction of the intermediate with whichever nucleophile should be available. 

These reactions involve activation of the carbonyl group by the Lewis acid. A nucleophile, either a ligand from the Lewis acid or the solvent, assists in the desilylation step. 

Irreversible inhibition is probably due to the alkylation of a histidine residue.43 Chymotrypsin is selectively inactivated with no or poor inhibition of human leukocyte elastase (HLE) with a major difference the inactivation of HLE is transient.42,43 The calculated intrinsic reactivity of the coumarin derivatives, using a model of a nucleophilic reaction between the ligand and the methanol-water pair, indicates that the inhibitor potency cannot be explained solely by differences in the reactivity of the lactonic carbonyl group toward the nucleophilic attack 43 Studies on pyridyl esters of 6-(chloromethyl)-2-oxo-2//-1 -benzopyran-3-carboxylic acid (5 and 6, Fig. 11.5) and related structures having various substituents at the 6-position (7, Fig. 11.5) revealed that compounds 5 and 6 are powerful inhibitors of human leukocyte elastase and a-chymotrypsin thrombin is inhibited in some cases whereas trypsin is not inhibited.21... 

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 nucleophilic reaction of hydroxide with carbonyl ligands of transition metal complexes,... 

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 ... 

It now appears that at least two mechanisms exist for the base-promoted homogeneous water gas shift reaction, differing in the method of hydride formation. The "associative mechanism", first proposed by Pettit and co-workers (1,4), involves nucleophilic attack on a carbonyl ligand and it has two variations. 

The classic Hieber-base reaction 16 is that of a hydroxide with metal carbonyls, which proceeds by nucleophilic attack of the hydroxide at a carbon atom of a carbonyl ligand to give a carboxy group or consequently carbon dioxide and a metal hydride.17 Metal carbonyls are catalysts for the water-gas shift reaction.18 Pentacarbonyl(tetrafluoroborato)rhenium reacts with alkali hydroxide in a similar way however, due to the coordinatively unsaturated nature of the [Re(CO)5]+ group polynuclear compounds are formed.15... 

Reaction of coordinated CO with less active nucleophiles can take place when the carbonyl ligand is sufficiently activated. The manner in which this activation occurs is by a reduction in the backbonding interaction (60) that may be achieved when the CO-bound metal ion is in a higher oxidation state. 

The reaction is very sensitive to acid concentration and the ratio of [H+]/[H20] the rate is maximized at approximate values of 2 M and 0.1, respectively (100). In the context of CO activation, the strongly acidic medium may lead to the formation of a kinetically significant species in which the carbonyl ligand is protonated and thus further activated to nucleophilic attack by water. The resultant species must lose protons to form the hydroxycarbonyl intermediate (18) which in turn may have to lose H+ to decarboxylate. In extremely strong acid, these deprotonation steps may be inhibited, thus offering a possible explanation for the reduced rate of catalysis when [H+] is greater than ca. 3 M and [H+ ]/[H20] is greater than 0.2. 

Although transesterification and transamidation reactions of simple carbonyl ligands are usually associated with the involvement of a co-ordinated nucleophile, there are some well-documented processes which unambiguously involve attack by an external nucleophile upon a co-ordinated electrophile. Dithiocarbamate complexes contain a chelated. S, 5"-bonded dithioamide ligand and undergo facile transamination reactions upon treatment of the copper complexes with amines (Fig. 3-22). 

Under conditions of low methyl/iridium ratios, in media with low levels of water and ionic iodide, the major species in solution was found to be Ir(CO)3I, and the carbonylation reaction was found to be inhibited by increasing CO pressure. In separate experiments involving reaction of this complex with methyl iodide, the product was found to be the dicarbonyl iridium(III) species, Ir(CH3XCO)2I2. Apparently, the presence of three carbonyl ligands on the iridium(I) center is sufficient to completely inhibit any nucleophilic behavior by the tricarbonyl complex. Prior dissociation of one of the carbonyl ligands produces a metal complex capable of this type of reactivity. 

It is useful to consider the reactions of carbonyl metallates separately, since their reactivity is generally concerned with the nucleophilic metal centre and will be discussed below. Simple ligand substitution reactions have already been discussed above, as have redox processes that provide access to carbonyl metallates through reduction of the metal centre. These redox or ligand addition/elimination processes are in principle no different from those encountered for classical ligands. We will now consider reactions in which the carbonyl ligand itself enters directly into the reaction and emerges transformed. 

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