Studies on the reactivity of tricarbonyl diene and dienyl iron complexes
The use of organoiron complexes has been recognized as a powerful tool in synthetic methodology due to their ability to react with nucleophiles and electrophiles in regioselective or stereoselective fashion. The reversible addition of phosphines to certain (pentadienyl)Fe(CO)$\sb3$ has been observed. Steric interactions between the phosphine and other substituents on the dienyl moiety may destabilize the phosphonium salt such that phosphine dissociation can occur at an observable rate. This reversible process could be utilized for the generation of optically pure dienyl phosphonium salts via desymmetrization. Reactions of the Fe(CO)$\sb2$PPh$\sb3$ complexes pentadienyl cations (24) with "soft" nucleophiles (i.e. malonate anion, NaBH$\sb3$CN) proceeds via attack at the substituted pentadienyl terminus in a highly regioselective fashion. In contrast, reaction of the Fe(CO)$\sb3$ complexes cation (3a) proceeds via attack at both of the pentadienyl termini (C1, C5) and at the internal positions (C2, C4). The difference is rationalized on the basis of the "reactivity/selectivity" principle. Thus cations 24, which is more stable/less reactive, reacts with greater selectivity than does cation 3a which is less stable/more reactive. Tricarbonyl iron complexes, which have been found to be unstable under oxidation conditions in certain cases, have been found to be reasonably stable to sodium periodate. Thus glycol-diene iron complexes may be cleaved under NaIO$\sb4$ conditions. The reactivity of glycol tricarbonyl iron complexes toward to sodium periodate depends upon the flexibility of the conformation of the complexes. Only if the two adjacent hydroxide groups which are conformationally flexible to coordinate the periodate can cleavage occur.
"Studies on the reactivity of tricarbonyl diene and dienyl iron complexes"
(January 1, 1996).
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