Dienes, hybridization hydrogen

We have just seen that a conjugated diene is more stable than an isolated diene. Now we need to see why a cumulated diene is less stable than an isolated diene. Cumulated dienes are unlike other dienes in that the central carbon is sp hybridized since it has two tt bonds. In contrast, all the double-bonded carbons of isolated dienes and conjugated dienes are sp hybridized. The sp hybridization gives the cumulated dienes unique properties. For example, the —AH° for hydrogenation of allene is similar to the —AH° for the hydrogenation of propyne, a compound with two sp hybridized carbons. 

Given the need for photoactivation, an often encountered additional requirement for substrates participating in DPM reactions is the presence of a strong chromophore as provided, for example, by the presence of a phenyl group on at least one of the double bonds. In acyclic 1,4-dienes, the central sp -hybridized carbon normally needs to be tetrasubstituted otherwise, competing 1,2-hydrogen shifts can occur. This requirement does not apply, however, to those cyclic substrates where isomerization would lead to an anti-Bredt olefin (as would be the case for the conversion of barrelene to semibullvalene, 6 7, shown in Scheme 9.2). 

The same authors pointed out, however, that the infrared stretching frequencies of the C—H bonds were more like those associated with sp rather than with sp hybridized carbon-hydrogen bonds. In view of the unknown effect the Fe(CO)3 grouping might have on the local magnetic field in the diene moiety they considered the infrared evidence, favoring (IV), to be more diagnostic. 

There is little evidence to distinguish between A, B or C. X-ray studies of a number of diene iron tricarbonyl complexes have been made.f A comparison of the observed bond distances between the terminal carbons of diene systems and the adjacent carbons of a substituent group (1-44-1 49 A) with the calculated distances for an s/ hydridized terminal carbon (1-48 A) and an sp hybridized carbon atom (1-52-1-54 A) slightly favours structures A and C [16]. The C -H coupling constants of butadiene iron tricarbonyl are more consistent with sp hybridized carbons than with hybridization [16a]. The data also suggests that there is a small rotation of the CH2 hydrogens out of the C4-plane. 

---