Polyenes into Metal-Carbon Bonds

Insertions of Polyenes into Metal-Carbon Bonds 

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All carbon-carbon bonds in the skeleton have 50% double bond character. This fact was later confirmed by X-ray diffraction studies. A simple free-electron model calculation shows that there is no energy gap between the valence and conduction bands and that the limit of the first UV-visible transition for an infinite chain is zero. Thus a simple free-electron model correctly reproduces the first UV transition with a metallic extrapolation for the infinite system. Conversely, in the polyene series, CH2=CH-(CH=CH) -CH=CH2, he had to disturb the constant potential using a sinusoidal potential in order to cover the experimental trends. The role of the sinusoidal potential is to take into account the structural bond alternation between bond lengths of single- and double-bond character. When applied to the infinite system, in this type of disturbed free-electron model or Hiickel-type theory, a non-zero energy gap is obtained (about 1.90 eV in Kuhn s calculation), as illustrated in Fig. 36.9. 

Based on the diene substrates and the reaction conditions, two distinct mechanisms can be followed to induce alkene activation and consequent cycloisomerization. The first mechanism invokes the hydrometallation of an alkene, followed by its subsequent carbometallation into the second one, terminated by a p-hydride elimination. The second mechanism implies the formation of a it-allylic complex either by its prior or later to the cycloisomerization formation (Scheme 7.35). Dienes possessing heteroatom functionality in the allylic position is able to provide it-allylic complex with a metal prior its cyclization. On the other hand, polyenes bearing 1,3-dienes in their system are commonly producing a 1,3-metal complex, which is readily cycloisomerize to give a ii-allyl metal complex after the formation of the new carbon—carbon bonds. 

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