Resonance integral cycloadditions

In contrast, when ot,P-unsaturated multiple bond systems act as dienophiles in concerted [4+2] cycloaddition reactions, they react across the C=C double bond Periselectivity as well as regiochemistry are explained on the basis of the size of the orbital coefficients and the resonance integrals [25S]... 

Reactivity may also depend on the C(l)-C(4) distance of the diene63. In a concerted [2 + 4] cycloaddition reaction, overlap has to be achieved between the lobes of the n orbital at C(l) and C(2) of the dienophile, lying about 1.3 A apart, and the lobes of the TT-orbital at C(l) and C(4) of the diene, lying about 3 A apart. This means that the shorter the C(l)-C(4) distance is, the more efficient the overlap will be in the transition state. This is translated into a higher resonance integral fi. It was shown for a series of equally substituted rigid 1,3-dienes that their reactivity in the cycloaddition reactions... 

The frontier orbital — second-order perturbation concept (Sustmann and Trill, 1972) led to an equation for the energy gain resulting from formation of the two new o bonds in the cycloaddition, shown in a simplified form in (6-15). The vj/ terms, A i, and AEu correspond to those in Figure 6-1. Ai and An include the overlap between the occupied and unoccupied orbitals and the resonance integral. 

A NMR analysis of the reaction product resulting from the thermolysis of 8 in the presence of ( )-2-deuteriostyrene indicated an equal ratio of the cis and trans isomers of 2,5-dideuterio-l,l-dimethyl-3,4-diphenylgermacyclopentane. In addition, integration of the resonances for the ring protons indicated that the deuterium label was partially scrambled. It was concluded that the scrambling occurred during the cycloaddition, since... 

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