Electrophilic Attack on Coordinated Double Bonds

The reactivity of olefins toward electrophilic reagents is modified considerably on coordination to a transition metal. Cycloctatetraene, for example, is usually polymerized by electrophilic reagents. However, tricarbonyl(cyclo-octatetraene)iron undergoes a facile formylation in reasonable yield. The coordinated formyl compound XIX can be used in further synthetic transformations, as shown below, and the tricarbonyliron group readily removed by oxidation with ceric ion [Eq. (55) (Johnson el al., 1969, 1971)J. 

The same principle has been used by Gill el al. (1969) to prepare substituted azepines [Eq. (57)]. Introduction of substituents into the azepine nucleus was not previously possible. 

A metal atom can lead to significant changes in the reaction pattern toward electrophilic reagents. For example, tricarbonyliron derivatives of cyclo-heptatriene and cyclooctatetraene undergo facile, unprecedented, 1,3-cycloaddition reactions with electrophiles such as hexafluoroacetone and tetra-cyanoethylene. Removal of the tricarbonyliron group can lead to useful products [Eq. (58) (Green et al., 1973)]. 

Paquette (1974) has used a 1,3-cycIoaddition reaction followed by oxidative removal of the tricarbonyliron group in a ready preparation of triquinacene derivatives from cyclooctatetraene [Eq. (59)]. 

Relatively little work has been done with simple coordinated dienes and electrophilic reagents in organic synthesis. Butadiene, as its tricarbonyliron derivative, undergoes Friedel-Crafts acylation much more readily than does benzene 2 3800). The product arises from attack by the acyl cation on the same side of the molecule as the iron atom [Eq. (60) (Greaves e/ ai, 1969, 1974 Graf and Lillya, 1972)] and at an outer position only. 

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B. Electrophilic Attack on Coordinated Double Bond Systems... 

C-C bond formation resulting from electrophilic attack on coordinated double-bond systems is a useful synthetic reaction (Section lll,B). The formation of C-X bonds by electrophilic attack of X, however, is restricted essentially to or D. Electrophiles such as Br or NOj usually lead to oxidative cleavage of the metal-olefin complex, although NBS (iV-bromosuccinimide) has been reported to act as a hydride acceptor in some cases (Khand et al., 1968, 1969). 

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