Homoconjugated olefins

The homoconjugated olefins in NBD also undergo cycloadditions of the [2 + 2 + n] type which are summarized following with some representative examples shown in Scheme 2. 

If conjugation contributes more stabilization energy for two double bonds than homoconjugation for an olefin and cyclopropane, then it should not surprise anyone that aromaticity provides more stabilization than homoaromaticity62. A simple comparison is between the ca 9 kJ mol"1 increase in enthalpy of formation on going from (Z)-2-butene to cz s-1,2-dimethylcycIopropane and the [202 - (82.6 1.7)] = 119 kJ mol"1 on going from... 

We now discuss the related series wherein there are three double bonds or cyclopropanes. How does homoconjugation involving cyclopropanes compare to conjugation involving olefins Paralleling the nearly constant 57 kJmol" difference between the enthalpies of formation of ethylene (1, X = H) and butadiene (5), and butadiene (5) and ( )-hexatriene (6b), the first related difference of cyclopropane (2, X = H) and bicyclopropyl (8) is (75.8 3.6) kJmol We lack the enthalpy of formation data of either cis- or ra 5 -l,L 2, l"-tercycIopropyl (9a or 9b) and so the second related difference cannot be found directly from experimental measurements. However, the enthalpies of formation of their I -methyl derivatives have been reported" 34a = cw(l), (134.1 1.2), cw(g), 181.0 trans( ), (131.9 1.2), 34b = trans, 179.2 kJ mol. The first approximation of the enthalpy of formation assumes the change of enthalpies of formation upon demethylation... 

Variations to the above scheme can of course be envisaged, particularly in terms of the intervention of HX as the assisting entity for the intermediate concerted complex (as in some AdgS mechanisms proposed for the addition of Br(Ansted acids to olefins), and of the formation of homoconjugated Lewis acid anions of the type Mt2X2 +i. 

Danishefsky and co-workers have used activated cyclopropanes to promote intramolecular alkylation of an amino function, followed by lactam formation to provide a new entry to the pyrrolizidine ring system.20 This route is outlined in Scheme 1. Cyclopropanation of the phthalimido-olefin (19) was achieved with dimethyldiazomalonate in the presence of copper bronze. Treatment of the cyclopropane derivative (20) with hydrazine released the amine, which gave the lactam ester (21) in quantitative yield. This corresponds to intramolecular homoconjugate addition entirely in the spiro mode.21 It seems reasonable that the first step in this process is internal alkylation of the amine by the activated cyclopropane. The alternative... 

Electrophilic attack by trityl cation on olefin complexes cani also occur to abstract a hydride from the allylic position. This process converts a neutral olefin complex into a cationic allyl complex and has been conducted with complexes that are less strongly backbonding than the complexes described in the preceding two paragraphs. One example of such an allylic hydride abstraction from a rhenium-propene complex is shown in Equation 12.69. An early example to form a homoconjugated system is shown in Equation 12.70. ... 

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