Nucleophilic Addition to Polyene and Polyenyl Ligands

In the vast majority of cases, the nucleophile adds to the face of the polyene opposite to the metal. Since the metal is likely to have bound to the least hindered face of the free polyene, we may therefore see a selective attack of the nucleophile on what was the more hindered face in the free polyene this is often useful in organic synthetic appUcations. 

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  

Rule 1 Polyenes (even or L ligands) react before polyenyls (odd or L X ligands). 

Rule 2 Open ligands react before closed. 

Rule 3 Open polyenes terminal addition in all cases. Open polyenyls usually terminal attack, but nonterminal if L M is electron donating. 

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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... 

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... 

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