Allyls pentadienyls

The C-H bond dissociation energies (BDEs) decrease in the sequence vinylic phenylic > primary > secondary > tertiary > allylic > pentadienylic = benzylic considerably (Table 6.3). 

Fig. 1. Comparison of modes of bonding of allyl, pentadienyl, and cyclopentadienyl ligands. For pentadienyl the symbols refer to the shape of the group the W and U conformations resemble the letter symbols, the S sickle.

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

Whereas NMR spectroscopy is generally applied to the diamagnetic allyl, pentadienyl, or cyclopentadienyl cations or anions, this technique is generally not applicable to the analogous radical species, although CIDNP (chemically induced dynamic nuclear polarization) has been observed for allyl140 and pentadienyl radical species139. More useful for the radicals, naturally, is EPR spectroscopy. 

TT-Bonded ligands can be closed shell, 2n electron donors (n = 1 — 3) such as alkenes, allenes, alkynes, arenes, and polyenes these have been termed even polyene ligands. They may also be open shell, odd polyene ligands such as allyl, pentadienyl, cyclopropenyl, and cyclopentadienyl, which are formally viewed as anionic hgands, donating 2n electrons (n = 1 - 3). 

Metallations are more useful synthetically when the substrate contains only one reactive site. The syntheses of benzyl-, allyl-, pentadienyl- and more delocalized group-IA organometallics and of group-IA acetylides are best performed by reaction (a). These preparations are not complicated by secondary coupling reactions or by contamination of the reaction mixture with other metals. Metallations of halogen-containing substrates at low T give carbenoids which are difficult to prepare by other routes. 

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