Cyclohexadienyltricarbonyliron salts

Pyrrole forms a few complexes in which it is -bonded as C H N. These include bis(pyrrolyl)iron, Fe(C4H4N)2, which is isoelectronic with ferrocene, and pyrrolyltricarbonylmanganese. 

Cyclohexadienyl cations are well known as intermediates in the electrophilic substitution of benzene. Indeed benzene is protonated by superacids to give solutions containing the ion, which can be characterized spectroscopically. These ions are, however, far too reactive to be isolated as salts, and as they are generated only in acidic media, their reactions with nucleophiles cannot be studied. In contrast, cyclohexadienyltricarbonyliron hexafluorophosphate is a stable yellow solid which can be stored indefinitely at room temperature and can even be recrystallized from water. This is an especially striking example of the stabilization of a reactive organic species by coordination to a transition element. 

Nucleophilic reagents include R Cd, NaCH(C02Me)2, R0 , R2NH, activated aromatics such as 1,3-dimethoxybenzene, allylSiMCj, BH and CN . Lithium alkyls in dichloromethane at low temperatures give good results, but not in diethyl ether. Tertiary amines and phosphines form quaternary salts. 

The 2-methoxycyclohexadienyl complexes are particularly useful in synthesis. The methoxy substituent directs nucleophiles to the remote end of the dienyl system. This directive effect is strong enough to overcome the steric hindrance of a methyl substituent at the 5-position, although a mixture of products is obtained with the 5-ethyl complex. 

Spirocyclic compounds have been prepared by making use of the regioselectiv-ity of these reactions. Dienyl complexes were constructed which contained latent nucleophiles in the side chain which were released on treatment with a mild base. Similar reactions have also been used to introduce angular substituents into bicyclic systems. 

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The complexes of cyclic dienes, such as 1,3-cyclohexadienes, are precursors to / -cyclohexadienyltricarbonyliron salts, which have been applied in origanic synthesis (p. 303). 1,4-Cyclohexadienes, available from the Birch reduction of arenes, are converted into the 1,3-diene complexes. The isomerization occurs through a transient f/ -allyliron hydride (p. 363). 

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