Polyene and polyenyl complexes

Mann, B.E. (1986) Fluxionality of polyene and polyenyl complexes. Chem. Soc. Revs., 15, 125. 

In general, coordination complexes of CO2, SO2, NO and other oxygen-containing ligands have strong bands. Polyene and polyenyl complexes, in general, do not give characteristic absorptions. 

In polyene and polyenyl complexes, carbons directly attached to the metal tend to be more shielded on binding, and a coordination shift (i.e., relative to the free ligand) of 25 ppm to high field is common. Metal nuclei with I = Vi show coupling to the metal in H and spectra. Diastereotopy also apphes in spectra and is seen for the diastereotopic F-Me carbons in 10.2. 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

Direct electrophilic attack on the exo side of a coordinated diene is also possible, even to complexes of modestly electron-rich metal centers. For example, Friedel-Crafts acetylation of (cyclohexa-l,3-diene)Fe(CO)3 gives principally the exo isomer. However, such direct exo attack occurs more commoiJy on the uncoordinated portion of a partially coordinated polyene or polyenyl ligand (recall that uncoordinated olefins and dienes are generally more nucleophilic than coordinated ones). An example of such exo attack on the uncoordinated portion of a pentadienyl ligand is shown in Equation 12.74. In ttiis case, coordination of a CO ligand is thought to displace one or more carbons of the dienyl ir-system from the iron center to trigger exo acylation. Similar results have been reported for electrophilic attack on the uncoordinated portion of the polyene system in [(C )Fe(CO)3]". ... 

Green-Davies-MIngos Rules It is not unusual for a single complex to have several polyene or polyenyl ligands, in which case we often see selective attack at one site of one ligand only. Green, Davies and Mingos noticed certain patterns in these reactions and from them devised a set of rules that usually allow us to predict the site of addition. 

Davies et al. have developed a succinct set of guidelines to predict the most favorable position of nucleophilic attack on electronically saturated cationic metal complexes beating more than one unsaturated hydrocarbon ligand. In essence, the so-called DGM rules state that (i) polyenes (L -type ligands) are more reactive than polyenyls (L, -type ligands see also Scheme 11) (ii) open or acyclic ligands react before closed or cyclic ligands and (iii) addition to... 

The combination of Birch reduction and electrophilic abstraction, followed by nucleophilic attack on the resulting cation, generates a modified coordinated diene complex, as shown in Equation 12.72. Decomplexation and hydrolysis yields the enone product. Alternatively, additions of nucleophiles and electrophiles have been conducted in reverse, and one such sequence is shown in Equation 12,73. In this case, nucleophilic attack on a coordinated polyene leads to a polyenyl system that is susceptible to hydride abstraction. Abstraction of the hydride restores the polyene system and allows for a second nucleophilic attack. Flydrolysis, lactonization of the half acid, and oxidative decomplexation gives the final free lactone. The product of this sequence contains the two nucleophiles cis to each other because they are delivered to the dienyl fragment on the side anti to the metal center. 

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