Reactions of Co-ordinated Carbonyl Compounds with Nucleophiles

The first class is that in which the nucleophile reacts with the electrophilic carbon atom of a carbonyl group to generate a new tetrahedral centre (Fig. 3-1). 

The fate of this tetrahedral intermediate is dependent upon the nature of the substituents R and X and upon the incoming nucleophile. If X is a better leaving group than Nu, the overall reaction is a nucleophilic substitution of X by Nu, with the resultant loss of X (Fig. 3-2). This is the mechanism which is typically adopted in the alkaline hydrolysis of esters. 

Metals and Ligand Reactivity, New Edition. Edwin C. Constable Copyright 1996 VCH Verlagsgesellschaft mbH, Weinheim ISBN 3-527-29278-0 

In contrast, if X is not a better leaving group than Nu, then either Nir is lost (with no overall resulting reaction) or a substituted alcohol may be formed by reaction of the tetrahedral anion with a proton (Fig. 3-3). In many cases the alcohol may then undergo further reactions. 

The second type of reaction which is commonly associated with carbonyl compounds involves the generation of a nucleophilic enol or enolate ion. Although the conversion of a ketone to the tautomeric enol does not necessarily involve any other species, the generation of an enolate requires a base (Fig. 3-4). In this latter reaction, the putative nucleophile may act as a general base. 

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Reactions of Co-ordinated Carbonyl Compounds with Nucleophiles... 

The position of the equilibrium between imine and carbonyl may be perturbed by interaction with a metal ion. We saw in Chapter 2 how back-donation of electrons from suitable orbitals of a metal ion may stabilise an imine by occupancy of the jc level. It is possible to form very simple imines which cannot usually be obtained as the free ligands by conducting the condensation of amine and carbonyl compounds in the presence of a metal ion. Reactions which result in the formation of imines are considered in this chapter even in cases where there is no evidence for prior co-ordination of the amine nucleophile to a metal centre. Although low yields of the free ligand may be obtained from the metal-free reaction, the ease of isolation of the metal complex, combined with the higher yields, make the metal-directed procedure the method of choice in many cases. An example is presented in Fig. 5-47. In the absence of a metal ion, only low yields of the diimine are obtained from the reaction of diacetyl with methylamine. When the reaction is conducted in the presence of iron(n) salts, the iron(n) complex of the diimine (5.23) is obtained in good yield. 

Many of the reactions which have been reported for carbonyl compounds (hydration, alcoholysis, aminolysis, etc.) are also shown by co-ordinated nitrile ligands. The reaction of a nucleophile with a nitrile is a two-stage process, which may involve one or two molecules of the incoming nucleophile (Fig. 4-6). 

The reactions of cyclopropenium ions with nucleophiles have led to several interesting observations. In the carbohydrate area, diphenylcyclopropenium perchlorate has been employed in an improved oligosaccharide synthesis based on the glycosylation of sugar diphenylcyclopropenyl ethers. With certain cyclopenta-dienyl-co-ordinated metal carbonyls (Mo, W), tri-t-butylcyclopropenium tetrafluoro-borate affords products derived from electrophilic attack of the cyclopropenium ion on the co-ordinated cyclopentadienyl anion, while for the analogous co-ordinated iron species attack at a carbonyl ligand occurs. With the cyclopentadienyl or indenyl anion, cyclopropenium ion (225) yields the tripolar mesomeric compounds (227), and... 

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