Benzvalene-like intermediates

The LST is a very quick and cheap way to generate a guess that may be close enough to the saddle point to refine with more elaborate methods. However, the method may fail to locate the correct saddle point. For example, in the rearrangement of l,2-C2B4Hg to l,6-C2B4Hg, the LST/QST method located a sagging saw horse saddle point, which was later shown- to be an artifact. The reaction actually involved formation of a benzvalene-like intermediate with saddle points to and from the intermediate. ... 

If two DSD steps occur in concert, a reaction path can be constructed in which a C2 axis is preserved between reactants and products (Figure 4). In this case, the transformation becomes orbitally allowed. The principle is illustrated with the transformation of BeHe " through a benzvalene-like intermediate into the octahedral product. The process has been referred to as a local bond rotation because it can be visualized as the local rotation of a bonded pair of atoms around an axis passing through the midpoint of the bond between the two atoms and through the rest of the molecule. 

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