Metallation of Dienes and Polyenes

Conjugated and homoconjugated dienes can also be easily metallated by superbases at the activated allylic position, to give rise to pentadienyl type organopotassium species. Their conformation has been the object of careful NMR and 

The simplest member of the series, the 2,4-pentadienyl metallo species, has been shown by NMR studies,132 133 to adopt either a W-shape or a U-shape conformation with the metal being lithium or potassium, respectively, when both are dissolved in tetrahydrofuran.134 136 The W-form is adopted by the 2,4-pentadienylpotassium when the organometallic species is generated in liquid ammonia137 or in paraffinic suspension.138 This has also been confirmed by the stereochemistry of the products formed after reaction with electrophiles.134,135,138,139 

The situation becomes more complicated if the pentadienyl moiety carries substituents but it has been found that the potassium species prefers again the U-shape when alkyl substituents are located in position l-,140 2-,138 and 2 4 i38,i4i wjjj[e W-shape is adopted by the 3-substitued member.138 

Understandably, stereoselective reactions with electrophiles become less realistic under circumstances in which a large number of conformations are possible. Nevertheless, some applications have been reported21, 2 concerning the use of pentadienyl metallic species. 

Heptatrienylpotassium compounds are formed when a triene having an allylic hydrogen atom is deprotonated with superbasic reagents. The number of possible conformations increases but the zigzag band form seems to prevail 

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