Nucleophilic Addition to Polyenes and Polyenyls

The nucleophilic addition to polyene (e.g., alkene, diene, arene) and polyenyl (e.g., allyl, cyclopentadienyl) ligands has contributed significantly to the numerous and unique applications organometallic reagents have in organic synthesis. The catalytic oxidation of olefins to carbonyl compounds (e.g., the palladium-mediated conversion... 

The orbital coefficients obtained from Hiickel calculations predict the terminal position to be the most reactive one, while the AMI model predicts the Cl and C3 positions to be competitive. In polyenes, this is true for the addition of nucleophilic as well as electrophilic radicals, as HOMO and LUMO coefficients are basically identical. Both theoretical methods agree, however, in predicting the Cl position to be considerably more reactive as compared to the C2 position. It must be remembered in this context that FMO-based reactivity predictions are only relevant in kinetically controlled reactions. Under thermodynamic control, the most stable adduct will be formed which, for the case of polyenyl radicals, will most likely be the radical obtained by addition to the C1 position. 

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. 

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. 

They destroy thermally labile structural defects. These defects are tertiary ehloride, internal allylie chloride, and polyenyl chloride groups. Their destruction involves the nucleophilic displacement of chloride and possibly the addition of the thiols to polyene double bonds (as in reaction 2, for example). 

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