Biradical, perfect

Photostationary cis/trans ratios vary with sensitizer structure in a manner not yet perfectly understood, and both the time required for attainment of a photostationary state and actual flash spectroscopic quenching rates indicate that energy transfer to diphenylcyclopropane is quite inefficient. Such should be the case if nonvertical energy transfer with the production of a biradical is occurring. When care is taken so that only the sensitizer absorbs light, only cis-trans isomerization is observed. Direct excitation of the cyclopropane produces 1,3-diphenyl-propene and 1-phenylindane as well.298... 

FIGURE 5.2 A VB mixing diagram for the formation of the symmetry-adapted states of 02 from the biradical (1, 1 ) and perfectly paired (2, 2 ) structures. 

The regioselectivity of intramolecular [2 + 2]-photocydoaddition reactions is predictable if five-membered ring formation is possible in the formation of biradicals of type C or C (rule of five, vide supra). If five-membered ring formation is not feasible, then six-membered rings are most readily formed. The facial diastereo-selectivity is efficiently controlled by a stereogenic center in the cyclopentenone if the intramolecular alkene is attached via a tether to this stereogenic center. The key step 16 —> 17 in the stereoselective synthesis of (—)-incarvilline (18) illustrates the point (Scheme 6.7) [28]. The side chain attached to C-4 in the cyclopentenone 16 carries the terminal alkene, which reacts intramolecularly with perfect regio- and diastereoselectivity to cyclobutane 17. 

This is in perfect harmony with the theory presented in the first section. In fact, a simple qualitative application would allow an a priori deduction of the chemical structure of the O3 molecule, since the biradical is the only nondegenerate symmetric structure possible. 

Figure 4.19. Wave functions and energy levels of a perfect biradical (center), constructed from the most localized orbitals x nnd Xi> tind from the most delocalized orbitals < > and (right) (adapted from BonaCiC-Kouteck et al., 1987).

First of all, we consider the significance of the presence of C-, +, the coefficient of the in-phase combination of the two hole-pair functions in the So wave function, in Equation 4.12. In the simple model for a perfect biradical, this in-phase combination is exactly equal to the wave function of the Sj state, and it does not enter into those of the So and S, states at all. Thus, in this approximation. So does not spin-orbit couple to the triplet. The same is true in weakly heterosymmetric biradicaloids (0 < 5 < S ), in which the in-phase hole-pair character is shared by S, and Sj, but not Sg, and the former two spin-orbit couple to T, but So does not. In strongly heterosymmetric... 

Figure 6.12. The effect of diagonal inieraciions on the energy of the degenerate localized nonhonding orbitals of a cyclobiitadiene-likc perfect biradical.

Xi - Xi + Xt)- The most stable MO , = j( f, + Xi Xi X ) >s neglected in this example it is always doubly occupied and forms a nonpolar-izable core. At square or nearly square geometries the system is a perfect biradical or a homosymmetric biradicaloid, respectively, and the energy ordering of the three singlet states may be obtained from Figure 4.19 or 4.20 ... 

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