Biradical axial

To explain the high degree of memory of chirality, it was proposed that there is a high activation barrier for the rotation around the central C-N single bond of the 1,7-biradical intermediate. Then the axial chirality can be preserved during the whole process. 

Biradicals for which K h = K are referred to as axial hiradicals and in these. So and S, are degenerate. This condition is normally enforced by the presence of a threefold or higher axis of symmetry (e.g., in O2 and the pentagonal cyclopentadienyl cation), but alone, this is not sufficient (e.g., square cyclobutadiene is almost a pair biradical). 

Pyrolysis of a methyl-labeled derivative of 1,2,6-heptatriene, namely, optically active i -5-methyl- -l,2,6-octatriene gave a 4 1 mixture of E and Z-4-methyl-3-methylene-l,5-octatriene, but the E isomer was racemized to the extent of 32% while the starting material was optically stable.This data is consistent with closure to two stereoisomeric, chair-like 1,4-biradicals in an 84 16 ratio (due to equatorial/ axial preferences) which undergo ring flip and cleavage (Scheme 8.50). 

The photo-dehydro-Diels-Alder reaction provides an access to 1-phenylnaphthale-nes, 1,1-binaphthyls, A-heterocyclic biaryls, and naphthalenophanes. The PDDA reaction proceeds through a multistage mechanism with biradicals and cycloallenes as intermediates. Again, the intramolecular photo-dehydro-Diels-Alder reaction of diyne (65) produced strained axially chiral (l,5)naphthalenophanes (66) in moderate yields (Scheme 20). " ... 

Perfect biradicals in which Kj = AB that 5[x]2 and 5[z]S are degenerate, are called axial hiradicals, since this situation is frequently reached in the presence of a rotation symmetry axis of order three or more (e.g., the O2 molecule). This situation implies an equivalence of the localized orbitals A, B and the delocalized "real" orbitals A, B with respect to electron repulsion [cf. equation (32)]. It also implies an equivalence of two of the molecular symmetry axes, and thus, E 0. 

In axial biradicals, the value of 2Kj is at its maximum relative to 2Kj, and the singlet-triplet splitting implied by the simple model is so large that the effects of electrons of the fixed core are usually not capable of inverting the order of the lowest singlet and triplet states. Many of the known ground-state triplet biradicals are of this type (e.g., methylnitrene). 

In photochemical applications, where minima in the second lowest singlet potential energy surface (S ) are sought, this is an unproductive type of perturbation of a perfect biradical, and tends to be avoided by photochemical paths. If there is an Sq - Si degeneracy, it occurs at the geometry of the perfect biradical, and only if the biradical is of the axial type. 

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