Cationic iron complexes enamines

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

These Fp-alkene complexes react with a variety of other nucleophiles,3 5 including water, alcohols, amines,3 6 phosphines, and thiols as well as carbon nucleophiles (enamines, organocuprates, enolates),3 2,3l8 and dialkyl cadmium reagents.3 9 Diene complexes such as 487 were converted to the corresponding cationic complexes (488), for example, and reaction with malonate gave 489. The iron complex was removed with trimethylamine N-oxide (Me N-O) to give 490.320... 

If iron carbonyl is used to generate the oxyallyl cation, the iron supplies two extra electrons in the complex 90 which now adds23 to mono-enes to give cyclopentanones 91. We shall see more examples of transition metal complexes altering selectivity later in the book. To get a cyclopentenone 92 we need an extra degree of unsaturation, best provided by using the enamine of a ketone instead of a simple alkene. 

Early applications of iron cationic species resulted from the facile reaction of a, a -dibromo-(and tetrabromo) acetone with Fe2(CO)9 with formation of the cation (A). Such cationic complexes can be efficiently trapped by olefin with formation of five-membered ketones. The reaction with conjugated dienes affords a straightforward access to tropones. Furan is a particularly good acceptor of these cationic complexes whereas their addition to enamines constitutes a potentially useful pathway to prostaglandins (cyclopentenones being thus obtained). 

Cationic alkene-Fp complexes are rather stable and easier to handle than their neutral alkene(tetracarbonyl)iron congeners. Due to their positive charge, they are inert towards electrophiles and, thus, can be employed as protecting groups for olefins. Bromination and hydrogenation of other double bonds in the molecule leaves them unaffected. On the other hand they readily react with various nucleophiles such as enamines, enolates, silyl enol ethers, phosphanes, thiols, and amines to give alkyl-Fp... 

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