1.4- Pentadiene stability

The groups at the termini of the 1,4-pentadiene system also affect the efficiency and direction of the the di-7c-methane reaction. The general trend is that cyclization oceurs at the diene terminus that best stabilizes radical character. Thus, a terminus substituted with aryl groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus ... 

The effect of conjugation on bond stability is revealed by comparing 1,3-pentadiene and 1,4-pentadiene. Figure 10-43 shows that both have eight C—bonds, two C—C bonds, and two C C bonds. The only significant... 

Does the delocalized tt system of 1,3-pen tadiene stabilize the molecule relative to 1,4-pentadiene Each molecule reacts with two molecules of H2 to give a common product, pentane. Thus, a comparison of their energies of hydrogenation allows us to determine the relative stabilities of two delocalized tt bonds compared with two isolated tt bonds. The hydrogenation of 1,4-pentadiene releases 252 kJ/mol, but the hydrogenation of... 

Conjugation affords tram A,3-pentadiene an extra stability of 15 kJ mol 1 => conjugated dienes are more stable than isolated dienes. 

Delocalization of the odd electron into extended n systems results in considerable radical stabilization. The C—H BDE at C3 of propene is reduced by 13 kcal/mol relative to that of ethane. That the stabilization effect in the allyl radical is due primarily to delocalization in the n system is shown by the fact that the rotational barrier for allyl is 9 kcal/mol greater than that for ethyl. Extending the conjugated system has a nearly additive effect, and the C—H BDE at C3 of 1,4-pentadiene is 10 kcal/mol smaller than that of propene. Delocalization of the odd electron in the benzyl radical results in about one-half of the electron density residing at the benzylic carbon, and the C—H BDE of the methyl group in toluene is the same as that in propene. 

In a proton NMR experiment in which 1,4-pentadiene was added to a solution of HNi[P(OMe)3]4, it was possible to watch the isomerization of 1,4- to 1,3-pentadiene, followed by formation of l,3-dimethyl-7t-allyl complexes (53). The observation of 7t-allyl products in the reaction of the hydride with the conjugated diene, but not in the ff-alkyl intermediates involved in isomerization, illustrates the much greater stability of zr-allyl complexes of nickel compared to tr-alkyls, a feature which is also observed in the hydrocyanation reactions. 

Many other organochromium compounds have since been synthesized and found to be active, including those with chromium exhibiting every valence up to Cr (IV). Chromocene is a well-studied example of an active divalent compound (52-55). Pentadiene-Cr(II) (56) is another, along with allyl-Cr(II) (52, 57). Allyl-Cr(III) is also active (52, 57-61). -Stabilized alkyls of Cr(II) and Cr(IV) such as trimethylsilylmethyl-Cr(IV), which also polymerizes ethylene when supported on an oxide carrier, have been synthesized and tested in this laboratory (57,62). All these organochromium catalysts are comparable in activity to the Cr(VI)/silica standard. 

The formation of 1,2-dimethylcyclobutene (Formula 385) in the vapor phase irradiation of 2,3-dimethyl-l,3-butadiene (Formula 384) is not quenched by oxygen or nitric oxide (169). Addition of inert vapor (diethyl ether) increased the quantum efficiency in this reaction (169). The inert vapor presumably removes excess vibrational energy from the product cyclobutene thus stabilizing the product (169). Rate studies on the cis- and Jrans-isomers of 1,3-pentadiene in solution indicate that the iraras-isomer is the only source of 3-methylcyclobutene (169). The photoisomerization to 3-methylcyclobutene is faster than photoisomerization of trans- to m-l,3-pentadiene (169). 

A particularly strong type of resonance stabilization is found for those compounds which form an aromatic ring upon removal of a proton. The enhanced aromatic stability of the conjugate base translates into a large increase in acidity of the acid. Whereas the doubly ally lie proton of 1,4-pentadiene is predicted to have a pKa % 40 due to resonance stabilization of the anion, the doubly allylic proton in cyclopentadiene has a pKa = 16 because the resulting anion produces an aromatic jt system. 

This group covers polymeric peroxides of indeterminate structure rather than polyfunctional macromolecules of known structure. These usually arise from autoxidation of susceptible monomers and are of very limited stability or explosive. Polymeric peroxide species described as hazardous include those derived from butadiene (highly explosive) isoprene, dimethylbutadiene (both strongly explosive) 1,5-p-menthadiene, 1,3-cyclohexadiene (both explode at 110°C) methyl methacrylate, vinyl acetate, styrene (all explode above 40°C) diethyl ether (extremely explosive even below 100°C ) and 1,1-diphenylethylene, cyclo-pentadiene (both explode on heating). 

Bond strengths give a measure of the thermodynamic stability and LUMO levels a measure of the kinetic stability (with respect to nucleophilic attacks). The parallelism between thermodynamic and kinetic stabilities is frequent, but not compulsory. Thus 1,3-pentadiene, which is stabilized by conjugation, is more reactive than 1, 4-pentadiene. 

With this model, we need only apply the method already used to derive the selection rules for electrocyclic reactions (p. 53). From the Coulson equations, we can deduce that in the in conrotatory cyclization of pentadiene, the MO generates a destabilizing C5-C4 secondary interaction, a stabilizing and Fg a destabilizing interaction. The absolute values of these contributions rise steadily because the terminal coefficients increase from Fg to Fg. Therefore, the sign of their sum is given by the HOMO contribution. If R is an attractor, the HOMO is Fg and rotation inwards is favored. If R is a donor, the HOMO is 4T and rotation inwards is disfavored. As the Coulson equations are valid only for polyenes, these conclusions are correct insofar as R can be modeled by a carbon 2p orbital. It follows that the Rondan-Houk theory works better for conjugative than for saturated substituents. 

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