Iron allenyl complexes

Other variations of the [3 + 2] cycloaddition chemistry involving either SO2 or a S2O equivalent have been explored (Scheme 47). Iron allenyl complexes undergo reaction with SO2 to give either vinyl iron (105) or (106), depending on the substitution pattern. A propargylic iron complex reacts in a similar manner to give (107), but reacts with a cyclic thiosulfinate ester to give (108). 

The work reported by Eaton and coworkers can be summarized as reactions where an allene system in conjugation with a further unsaturated functionality reacts with carbon monoxide in the presence of an iron-carbonyl complex such as Fe(CO)5 under photochemical and thermal conditions when Fe2(CO)9 is used. When diallenes are used (X=R2C=C, Scheme 9.24), five-membered carbocyclic products are obtained [51, 52], whereas when allenyl ketones (X = O) are applied, five-membered lactones are generated [53, 54]. The use of allenylimines (X = NR) leads to five-membered lactams under these conditions [55]. 

C(E)R] This method was extended by Wojcicki and co-workers 110,111) to utilize coordinatively unsaturated metal fragments as electrophiles, whereby new synthetic methods for homo- and heterobinuclear and trinuclear metal-/u--allenyl complexes were developed. The difference between this approach and that of Stone and co-workers (89,90) is that the unsaturation is contained entirely within the ligand, which, as a result of this, often rearranges. Complex 28 was obtained in high yields from the reaction of the iron-propargyl precursor and diiron nonacarbonyl (111). 

In contrast to ruthenium and osmium, the reactivity of iron allenylidenes remains almost unexplored. Only the behavior of the cationic diphenylallenylidene-Fe(II) derivative frans-[FeBr(=C=C=CPh2)(depe)2]" has been studied in detail. Thus, it has been found that this complex reacts exclusively at Cy with both neutral (amines, phosphines) and anionic (H , MeO , CN ) nucleophiles [105-107]. This behavior contrasts with that of the neutral Fe(0) derivative [Fe =C=C=C(f-Bu)2 (CO)5] which undergoes PPhs-attack at Co- to afford the zwitterionic phosphonio-allenyl species [Fe C(PPh3)=C=C(f-Bu)2 (CO)5] [104]. 

The analogous reactions on ( / -propargyl)Fp and (/3-oxoaUcyl)Fp complexes afford related complexes (79n) and (79o), respectively (Scheme 38). Cationic allenyl iron complexes related to (79o) can also be obtained by protonation of the propargylic species with HBF4 or by transfer of the Fp cation via reagents (79a-c). ... 

Nucleophilic addition to the related cationic jj -allenyl iron complexes is generally a high-yielding process that produces vinyhc iron complexes (Scheme 43). Double bond isomerizations to give the internal alkene can be effected by column chromatography or catalytic amounts of acid. ... 

Finally, nucleophilic addition of alcohols at the a carbon of allenyls coordinated on binuclear, phosphido-bridged, iron complexes followed by insertion of a carbonyl, leads to coordinated unsaturated esters (Fig. 11). 

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