Protection and Activation by Coordination

1 Protection and Activation of Alkenes by the Coordination of Iron Carbonyls 

The carbonyls Fe(CO)5 and [CpFe(CO)2]+ (2) form stable cationic complexes with alkenes, which are used for both protection and activation of alkenes [1]. [CpFe(CO)2]+ (2 abbreviated as Fp+) is prepared by the reaction of cyclopentadienyl anion (1) with Fe(CO)5, followed by oxidative cleavage with bromine, and used for the protection of alkenes. The electron density of the double bond is decreased by the coordination of [CpFe(CO)2]+ and hence this bond is activated to nucleophilic attacks. Introduction of nucleophiles, such as the carbon nucleophile of malonate, to cyclopentene becomes possible via the formation of the complex 3, and the stable tftmv-er-alkyliron complex 4 of cyclopentane is prepared. The vinyl ether complex 6 is obtained easily from the a-bromoacetal 5, and reacts with an enolate of ketone 7 as an 

Cyclobutadiene (26) is antiaromatic and its isolation is not possible. Flowever, it can be stabilized by -coordination of Fp+ to one of the double bonds to give 27, and the uncomplexed double bond in 27 undergoes Diels-Alder reaction with cyclopentadiene to give 28 [4]. As described in Section 9.2, cyclobutadiene (26) can be stabilized as a diene by the )/4-coordination of Fe(CO)3. 

Fe(CO)3 forms stable tf complexes of conjugated dienes, and acts as a useful protecting group for dienes, preventing reactions normally associated with carbon- 

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The methylene group of a-amino acid metal complexes such as the copper(II) glycinato complex is activated to some extent by the polarizing influence of the metal.46,47 Consequently, carban-ion-type reactions can be carried out at this position, while the amino and carboxyl groups are protected by coordination. These reactions cannot be performed on either the free ligand or the conjugate base. 

The synthetic utility of reactions of coordinated ligands is an important and varied subject. It is based on the enhancement in reactivity of organic ligands as a consequence of metal coordination. For example, the metal can act as a super add and cause enhanced nucleophilic attack on coordinated carbonyl and imine ligands. The metal ion can also enable the ligand itself to act as a nucleophile, sometimes by direct activation, sometimes by protecting other parts of the ligand and sometimes by a combination of both. 

In addition to benzene rings, cycloheptatriene is activated or protected by forming the stable if complex 300. An example of the strong stabilization effected by coordination is shown by isolation of the optically active l,3,5-cycloheptatrien-3-ols 301, 304 and 305 as their enol forms. l,3,5-Cycloheptatrien-3-ol was isolated as complex 301 by hydrolysis of silyl enol ether 300. The triene system is stabilized by coordination,... 

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