Relative Stabilities of Dienes

Which is the most stable arrangement of double bonds in an alkadiene—isolated, conjugated, or cumulated  

The cumulated double bonds of an allenic system are of relatively high energy. The heat of hydrogenation of allene is more than twice that of propene. 

By the same reasoning, hydrogenation of the terminal double bond in the conjugated diene (i )-l,3-pentadiene releases only 111 kJ/mol (26.5 kcal/mol) when it is hydrogenated to (/ )-2-pentene. Hydrogenation of the terminal double bond in the conjugated diene 

Sample Solution (a) p-Springene has three isolated double bonds and a pair of conjugated double bonds  

Isolated double bonds are separated from other double bonds by at least one sp -hybridized carbon. Conjugated double bonds are joined by a single bond. 

Alkadienes are named according to the lUPAC rules by replacing the -ane ending of an alkane with -adiene and locating the position of each double bond by number. Compounds with three carbon-carbon double bonds are called alkatrienes and named accordingly, those with four double bonds are alkatetraenes, and so on. 

Heats of hydrogenation are used to assess the stabilities of isolated versus conjugated double bonds. Comparing the measured heats of hydrogenation (solid lines) of the four compounds shown gives the values shown by the dashed lines for the heats of hydrogenation of the terminal double bond of 1,4-pentadiene and ( )-l,3-pentadiene. A conjugated double bond is approximately 15 kJ/mol more stable than an isolated double bond. 

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The endo exo selectivity for the Lewis acid-catalyzed carbo-Diels-Alder reaction of butadiene and acrolein deserves a special attention. The relative stability of endo over exo in the transition state accounts for the selectivity in the Diels-Alder cycloadduct. The Lewis acid induces a strong polarization of the dienophile FMOs and change their energies (see Fig. 8.2) giving rise to better interactions with the diene, and for this reason, the role of the possible secondary-orbital interaction must be considered. Another possibility is the [4 + 3] interaction suggested by Singleton... 

We start with a discussion of allene (propadiene), the simplest diene of all. Its gas phase enthalpy of formation is 190.5 1.2 kJmol-1. We wish to compare this quantity with that of related monoenes. The first comparison addresses the relative stability of one and two double bonds in a 3-carbon chain. Conceptually, this may be expressed as the enthalpy of the formal reaction 9... 

Mechanism B was originally proposed for the palladium-catalysed reaction [208], which is characterised by a high trans cis ratio. In this mechanism, conformational interconversion of adsorbed species does not occur and the trans cis ratio is a direct reflection of the relative surface concentrations of the anti- and syn-diadsorbed buta-1 3-diene, which are dependent upon the nature of the sites available at the surface and upon the relative stabilities of the two conformers. In addition to palladium, mechanism B has been proposed, along with a contribution from mechanism A,... 

Because penta-1,2-diene has a larger heat of hydrogenation than penta-1,4-diene, we conclude that the cumulated double bonds of allenes are less stable than isolated double bonds and much less stable than conjugated double bonds. Figure 15-1 summarizes the relative stability of isolated, conjugated, and cumulated dienes and compares them with alkynes. 

With acyclic dienes, the success of the cycloaddition is dependent on the relative stability of the cisoid form of the diene and on the steric hindrance affecting the trifluoromethyl-sub-stituted carbon. Examples arc given by the reactions of 1, 2, and 4. ... 

Two major factors are likely to be responsible for the relative stability of (5-rrani-i7 -diene)zirconocene and -hafnocene complexes. It seems that the characteristic bonding features of the bent metallocene unit thermodynamically favor coordination of the 5-trans over the i-cis conjugated-diene... 

The molecular structures produced are influenced by (1) the relative stabilities of the two conformations about the double bond in the terminal unit of the macroion (2) the conformation of the diene monomer when it adds to the macroion and (3) in the case of 1,4 addition, the relative rales of isomerization between cis and trans terminal units compared to the rates at which new monomers are added. 

The relative stability of conjugated and isolated dienes can also be determined by comparing their heats of hydrogenation. 

The relative stability of conjugated dienes versus isolated dienes was first discussed in Section 16.9. 

Although zirconium is only one out of over 50 potentially usable metals in this class (including the lanthanides and actinides), virtually all synthetic applications of hydrometallation with transition metals involve zirconium Why is this so The primary reason derives from the near requirement of a d -metal center for hydrometallation of a general alkene or alkyne. For later transition metals, hydrometallation to give a stable organometallic product can usually be achieved only for special cases—conjugated dienes, alkenes with electronegative substituents, etc. This is due to the relative stability of the ti -complex, as discussed previously. 

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