Acyclic pentadienyl ligands

The reactivity of acyclic (pentadienyl)iron cations with carbon and heteroatom nucleophiles has been examined. Nucleophilic attack may occur on either the cisoid or transoid conformation of the ligand. In general, it has been found that attack by weak nucleophiles (water, alcohols, weakly basic... 

Addition of amines and phosphines to acyclic (ry -pentadienyOiron cations is reversible in some cases. In a representative example (Scheme 4), kinetically controlled nucleophilic attack by phosphine occurs on the more abundant cisoid cation to generate an ,Z-diene 47. Since significant steric interactions exist between the phosphine nucleophile and substituents present on the pentadienyl ligand, nucleophilic attack in this case is reversible. At higher temperatures and longer reaction times, the thermodynamically more stable , -diene 48 is formed via nucleopilic attack on the less abundant transoid form of the (ry -pentadienyOiron cation. Amine nucleophiles add regioselectively to the unsubstituted terminus of the (ry -pentadienyOiron cation. 

Over the past 20 years, there has been a growing interest in the use of acyclic (or open) ( 7 -pentadienyl)iron cations 1 (Scheme 1) as synthetic tools for G-C bond formation. Complexation of Fe(CO)3 to a diene distinguishes between two enantiotopic faces of the ligand and directs diastereoselective bond formation at unsaturated centers adjacent to the coordinated diene. In addition, the electron-donor ability of the carbonyliron group allows for the generation of cationic centers adjacent to the coordinated diene. ... 

The regioselectivity of nucleophilic attack for the formation of G-G bonds depends on the nature of the nucleophile as well as the substituents attached to the dienyl ligand. Reactions of 1,2-disubstituted 42a and 42b and 1,4-disubstituted 43a and 43b cations with methanol, triphenylphosphine, lithium dimethylcuprate, and sodium dimethylmalonate nucleophiles have been investigated. It was found that cations 42a and 42b direct nucleophilic attack regiospecifically to the unsubstituted C5 position whereas cations 43a and 43b undergo G1 addition, although in a less predictable manner (Scheme 3). Weak nucleophiles such as methanol, allyltrimethylsilane, furan, and allylic organozirconiums are found to attack exclusively at the less substituted pentadienyl terminus and on the less stable, but more reactive, transoid form. Malonate anion attack on 1-substituted acyclic (77 -pentadienyl)-... 

Two close L2X analogs are cyclohexadienyl 5.23 and pentadienyl 5.24. In 5.23, the uncomplexed ring CH2 is bent 30-40° out of the ligand plane. Pentadienyl, being acyclic, is more easily able to shuttle back and forth between the r rj, and t structures. 

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